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Superconductor digital electronics
Abstract
The objective of these notes is to offer a brief review of the history of superconductor digital electronics, and discuss prospects of its future development. Due to length restrictions, many important technical contributions could not be mentioned at all – with sincere apologies to their authors. Though an attempt has been made to give an unbiased review of the most important work all over the world, a special emphasis on the efforts in the former Soviet Union, which had not been discussed much in literature, and in which the author of this text took an active part, seemed excusable. Another important qualification is that the author phased out his own research in the field about 10 years ago, so that the last parts of the notes, devoted to present-time and future work, should be viewed as not much more than remarks by an (interested) outsider.
... Superconducting devices such as Josephson junction and SQUID are typically fast (hundreds of giga-hertz operating frequency) and highly energy-efficient (atto-joule switching energy) and hence, have been utilized in different applications [15]. However, due to the absence of resistance, superconducting devices suffer from convenient gating mechanism and voltage control [16]. ...
... Our FE-SQUID-based classification system achieves up to 17% better accuracy than existing room temperature memristor and CMOSbased approaches. Considering the typical operating speed of a SQUID (>100 GHz [15]), our FE-SQUID-based memory cells will consume less than 5 atto-Joules of energy (on average) for 1-bit MVM operation. The cooling cost is not a concern for our target application (QC systems), as the qubits are already cooled to cryogenic temperature. ...
... FE-SQUID is fully compatible with the existing commercial process technology used for fabricating single-flux quantum (SFQ) circuits [33]. Besides the QC systems, our proposed in-memory computing technique can also be directly adopted for space electronics, and cryogenic highperformance computing systems [15]. [29] Fashion [13] Memristive 69 [29] 80.5 75.2 [30] IMDB Sentiment [31] Memristive 84.3 [31] 86.8 CMOS 81 [32] Emotion [14] Software 81 [14] 92.5 ...
... In addition to silicon semiconductor technology, a new trend toward superconducting devices for advanced computing is becoming evident. Superconducting circuits can realize qubits for quantum information processing [56,57], neurons for neuromorphic computing [58][59][60][61], and logic gates for digital computing [62][63][64]. Operation at low temperature is required to maintain superconductivity. Niobium is a primary material for superconducting information-processing devices (as well as large magnets) for several reasons: (1) niobium is a superconductor with energy gap significantly larger than the temperature of the liquid-gas phase transition of helium that is used for cooling; (2) niobium is plentiful, perhaps especially where it will ultimately be used [65]; (3) tunneling barriers can be formed on Nb, enabling Josephson junctions [66,67]; (4) the electronic properties of niobium and related materials enable myriad superconducting devices [68,69]; and (5) like Si, Nb can be integrated with many other materials for large-scale manufacturing of complex circuits. ...
... Superconducting circuits are promising for quantum information processing [56,57] as well as neural computing [58][59][60][61]. While superconducting circuits can perform digital logic [62][63][64], silicon is likely to reign supreme in this domain, possibly into the asymptotic future of technology. Digital superconducting circuits may find the most relevance as a means to interface with and control superconducting qubits or neurons [95]. ...
... JJs consist of two superconducting leads separated by a thin tunneling barrier [66,67]. Much like transistors have particular properties that make them uniquely capable of digital computation [51], JJs are exceptionally capable of information processing [56][57][58][61][62][63][64] and other technological functions [68,69]. One important property is the ability of a JJ to produce small, quantized pulses of magnetic flux (fluxons) [67][68][69] that can represent various types of information. ...
If the parameters defining the physics of our universe departed from their present values, the observed rich structure and complexity would not be supported. This article considers whether similar fine-tuning of parameters applies to technology. The anthropic principle is one means of explaining the observed values of the parameters. This principle constrains physical theories to allow for our existence, yet the principle does not apply to the existence of technology. Cosmological natural selection has been proposed as an alternative to anthropic reasoning. Within this framework, fine-tuning results from selection of universes capable of prolific reproduction. It was originally proposed that reproduction occurs through singularities resulting from supernovae, and subsequently argued that life may facilitate the production of the singularities that become offspring universes. Here I argue technology is necessary for production of singularities by living beings, and ask whether the physics of our universe has been selected to simultaneously enable stars, intelligent life, and technology capable of creating progeny. Specific technologies appear implausibly equipped to perform tasks necessary for production of singularities, potentially indicating fine-tuning through cosmological natural selection. These technologies include silicon electronics, superconductors, and the cryogenic infrastructure enabled by the thermodynamic properties of liquid helium. Numerical studies are proposed to determine regions of physical parameter space in which the constraints of stars, life, and technology are simultaneously satisfied. If this overlapping parameter range is small, we should be surprised that physics allows technology to exist alongside us. The tests do not call for new astrophysical or cosmological observations. Only computer simulations of well-understood condensed matter systems are required.
... In recent years, the growing demand of fast electronics promoted the thrive of applications based on Josephson vortices, i.e., solitons [1][2][3] . Nonetheless, the effective interplay between thermal transport and soliton dynamics is far from being fully explored. ...
... Although little is known about caloritronics effects in these systems, LJJs are still nowadays an active research field, both theoretically 3, 26-34 and experimentally 2,[35][36][37][38][39][40][41][42] . In our work, we explore theoretically the effects of a sinusoidal magnetic drive on the thermal transport across a temperature-biased LJJ (see Fig. 1). ...
Since its recent foundation, phase-coherent caloritronics has sparkled continuous interest giving rise to numerous concrete applications. This research field deals with the coherent manipulation of heat currents in mesoscopic superconducting devices by mastering the Josephson phase difference. Here, we introduce a new generation of devices for fast caloritronics able to control local heat power and temperature through manipulation of Josephson vortices, i.e., solitons. Although most salient features concerning Josephson vortices in long Josephson junctions were comprehensively hitherto explored, little is known about soliton-sustained coherent thermal transport. We demonstrate that the soliton configuration determines the temperature profile in the junction, so that, in correspondence of each magnetically induced soliton, both the flowing thermal power and the temperature significantly enhance. Finally, we thoroughly discuss a fast solitonic Josephson heat oscillator, whose frequency is in tune with the oscillation frequency of the magnetic drive. Notably, the proposed heat oscillator can effectively find application as a tunable thermal source for nanoscale heat engines and coherent thermal machines.
... Low-temperature superconducting single-flux quantum (SFQ) logic employs magnetic fluxons (Josephson vortices) in Josephson transmission lines (JTLs) as basic bits for classical computation [12,13]. Superconducting SFQ electronics offers a possibility of implementing an extremely fast, up to 500 GHz clock speed, lowtemperature classical computer [12,13]. ...
... Low-temperature superconducting single-flux quantum (SFQ) logic employs magnetic fluxons (Josephson vortices) in Josephson transmission lines (JTLs) as basic bits for classical computation [12,13]. Superconducting SFQ electronics offers a possibility of implementing an extremely fast, up to 500 GHz clock speed, lowtemperature classical computer [12,13]. SFQ logic can be optimized to be used at millikelvin temperatures [14,15] and be also modified to allow for reversible operation [16,17]. ...
Quantum computing using superconducting circuits underwent rapid development in the last decade. This field has propelled from quantum manipulation of single two-level systems to complex designs employing multiple coupled qubits allowing one to execute simple quantum algorithms. On the way to a practical quantum computer, a need for scalable interfaces between classical circuits and the quantum counterparts has arisen. Low-temperature superconducting single-flux quantum (SFQ) logic employs magnetic fluxons in Josephson transmission lines (JTLs) as basic bits for classical computation. Here, we report on an experiment implementing a direct link between SFQ electronics and a superconducting qubit. We demonstrate a readout of the state of a flux qubit through a frequency shift of a single fluxon oscillating in a JTL. The energy spectrum of the flux qubit is measured using this technique. The demonstrated approach may open ways to future full-scale integration of solid-state quantum computers with digital SFQ electronics.
... The idea of superconducting electronics (SCEs) was first put forward in 1950s with the effort of building magnetic fieldmodulated superconducting wires. 1,2 The requirement of extremely low temperature was one of the major roadblocks for the superconducting devices to be used in practical applications which now becomes an advantage, thanks to the recent interests in superconducting qubit-based quantum computers and space electronics. [3][4][5][6] Superconducting logic circuits can significantly improve the performance of the control processor in the quantum computers and the aircrafts for space exploration. ...
... SCE is considered as one of the most promising alternatives to CMOS technology, thanks to their high speed and low power operation. 2,8 SCE has also been used in radio frequency receivers, high-end computing, and so on. 9,10 Over the last few decades, several superconducting logic devices 1,11-20 have been introduced. ...
The recent progress in quantum computing and space exploration led to a surge in interest in cryogenic electronics. Superconducting devices such as Josephson junction, Josephson field effect transistor, cryotron, and superconducting quantum interference device (SQUID) are traditionally used to build cryogenic logic gates. However, due to the superconducting nature, gate-voltage-based control of these devices is extremely difficult. Even more challenging is to cascade the logic gates because most of these devices require current bias for their operation. Therefore, these devices are not as convenient as the semiconducting transistors to design logic gates. Here, to overcome these challenges, we propose a ferroelectric SQUID (FeSQUID) based voltage-controlled logic gates. FeSQUID exhibits two different critical current levels for two different voltage-switchable polarization states of the ferroelectric. We utilize the polarization-dependent (hence, voltage-controllable) superconducting to resistive switching of FeSQUID to design Boolean logic gates such as Copy, NOT, AND, and OR gates. The operations of these gates are verified using a Verilog-A-based compact model of FeSQUID. Finally, to demonstrate the fanning out capability of FeSQUID-based logic family, we simulate a two-input XOR gate using FeSQUID-based NOT, AND, and OR gates. Together with the ongoing progress on FeSQUID-based non-volatile memory, our designed FeSQUID-based logic family will enable all FeSQUID-based cryogenic computer, ensuring minimum mismatch between logic and memory blocks in terms of speed, power consumption, and fabrication process.
... The idea of superconducting electronics (SCE) was first put forward in 1950s with the effort of building magnetic field-modulated superconducting wires 1,2 . The requirement of extremely low temperature was one of the major roadblocks for the superconducting devices to be used in practical applications which now becomes an advantage, thanks to the recent interests in superconducting qubit-based quantum computers and space electronics [3][4][5][6] . ...
... To solve this issue, several beyond-CMOS alternatives are being explored. SCE is considered as one of the most promising alternatives to CMOS technology, thanks to their high speed and low power operation 2,8 . SCE has also been used in radio frequency receivers, high-end computing, and so on 9,10 . ...
The recent progress in quantum computing and space exploration led to a surge in interest in cryogenic electronics. Superconducting devices such as Josephson junction, Josephson field effect transistor, cryotron, and superconducting quantum interference device (SQUID) are traditionally used to build cryogenic logic gates. However, due to the superconducting nature, gate-voltage-based control of these devices is extremely difficult. Even more challenging is to cascade the logic gates because most of these devices require current bias for their operation. Therefore, these devices are not as convenient as the semiconducting transistors to design logic gates. Here, to overcome these challenges, we propose a ferroelectric SQUID (FeSQUID) based voltage-controlled logic gates. FeSQUID exhibits two different critical current levels for two different voltage-switchable polarization states of the ferroelectric. We utilize the polarization-dependent (hence, voltage-controllable) superconducting to resistive switching of FeSQUID to design Boolean logic gates such as Copy, NOT, AND, and OR gates. The operations of these gates are verified using a Verilog-A-based compact model of FeSQUID. Finally, to demonstrate the fanning out capability of FeSQUID-based logic family, we simulate a 2-input XOR gate using FeSQUID-based NOT, AND, and OR gates. Together with the ongoing progress on FeSQUID-based non-volatile memory, our designed FeSQUID-based logic family will enable all-FeSQUID based cryogenic computer, ensure minimum mismatch between logic and memory blocks in terms of speed, power consumption, and fabrication process.
... Switching cells on JJs can be viewed as building blocks for different superconducting devices. Josephson comparators are the main structural elements of modern superconducting electronics [4][5][6]. First Josephson comparators were based on two tunnel junctions with different critical currents. One of the junctions acted as a generator of short strobe pulses, while the other acted as a comparator [7]. ...
... It means that, in this limit, the first term is ignored in Eqs. (5) and (6) due to β << 1 , 2 . ...
In this study, we carried out the analysis of the resolution time of Josephson junction (JJ) balanced compara-tors with anharmonic current-phase relation (CPR). An expression for the time resolution was obtained in the case of linearly growing strobe current pulse driven via the junction. It is shown that the resolution time of balanced comparators is improved by the renormalized critical current of JJ with anharmonic CPR.
... The achievements in Josephson-junction (JJ) technology have made it possible to develop a variety of sensors for detecting ultralow magnetic fields and weak electromagnetic radiation; they have also enabled the fabrication, testing, and application of ultrafast digital rapid single flux quantum (RSFQ) circuits as well as the design of large-scale integrated circuits for signal processing and general purpose computing. 4,5 It is clear that the Josephson effect, the 50th anniversary of which was celebrated in 2012, remains one of the most spectacular manifestations of quantum mechanics in all of experimental science. At its most fundamental level the Josephson effect is nothing more than the electronic analogue of interference phenomena in optical physics. ...
... 11 Simple sinusoidal form of CPR (1) was widely used to study the dynamics and ultimate performance of analogous and digital devices based on JJ up to recent time. [4][5][6][7][8][9][10][11][12][13][14] Above mentioned reviews [8][9][10][11][12] have been devoted to theoretical basis for the study of CPR in different Josephson structures. Results of these studies reveal fundamental physical mechanisms for control and experimental investigation of CPR. ...
V dannom obzore rassmatrivaetsya dinamicheskie svoystva Dzhozefsonovskix perexodov s angarmonicheskim toko-fazovim sootnosheniem.
... In this section we present a series of experiments, performed on niobium and vanadium superconducting weak-link devices, aimed at extending the range of materials suitable for gated-superconductor applications at elemental superconductors with critical temperature higher than the liquid helium temperature ∼ 4.2 K. The presented results demonstrate the possibility to implement gate-controlled all-metallic superconducting electronics [16] compatible with industrial standards. ...
... From a technological point of view, the unconventional field-effect promises to be suitable for a wide range of applications. We already demonstrated the rectification properties of a Dayem bridge system and more complex devices such as gate-controlled radiation detector [52][53][54], signal routers and computational systems [16,55] are at reach. ...
It was recently demonstrated the possibility to tune, through the application of a controlgate voltage, the superconducting properties of mesoscopic devices based on Bardeen-Cooper-Schrieffer metals. In spite of the several experimental evidences obtained on different materialsand geometries, a description of the microscopic mechanism at the basis of such unconventionaleffect has not been provided yet. This work discuss the technological potential of gate control ofsuperconductivity in metallic superconductors and revises the experimental results which provideinformation regarding a possible thermal origin of the effect: in the first place, we review experimentsperformed on high critical temperature elemental superconductors (niobium and vanadium) andshow how devices based on these materials can be exploited to realize basic electronic tools such as,e. g., a half-wave rectifier. In a second part, we discuss the origin of the gating effect by showingthe gate-driven suppression of the supercurrent in a suspended titanium wire and by providing acomparison between thermal and electric switching current probability distributions. Furthermore,we discuss the cold field-emission of electrons from the gate by means of finite element simulationsand compare the results with experimental data. In our view, the presented data provide further astrong indication regarding the unlikelihood of thermal origin of the gating effect.
... Superconductor electronics (SCE) deals with electronic circuits based on elements that are superconducting below their critical temperature (T c ) and exhibits unique characteristics and performances, which are unrivaled by conventional semiconductor counterparts. 1,2 SCE relies on the quantum properties of superconductors such as Josephson effects, 3,4 magnetic flux quantization, 5,6 and extremely low-power absorption in both DC and AC fields up to the superconducting gap frequency (f D ¼ 2D=h, where D and h are the superconducting gap and Planck's constant, respectively). For practical SCE, the superconducting material of choice is niobium (Nb): a Bardeen-Cooper-Schrieffer (BCS) metal that has the highest T c ($9:2 K) and f D ($770 GHz) among elemental superconductors, 1 which is therefore suitable for circuit operation at temperatures around $4 K. ...
... 19 All these ports are based on the so-called EF-Tron, i.e., the electrostatically gated counterpart of the nanocryotron (nTron). 6,28,29 The latter is a device where an injection current is used to control the supercurrent flowing in a metallic channel. S-GCTs based on Al, 25 Ti, 17 and V 19 have already been demonstrated, but no implementation with Nb was reported so far. ...
We report on the realization of Nb-based all-metallic Dayem nano-bridge gate-controlled transistors (Nb-GCTs). These Josephson devices operate up to a temperature of ∼3 K and exhibit full suppression of the supercurrent thanks to the application of a control gate voltage. The dependence of the kinetic inductance and of the transconductance on gate voltage promises a performance already on par with so far realized metallic Josephson transistors and leads us to foresee the implementation of a superconducting digital logic based on the Nb-GCT. We conclude by showing the practical realization of a scheme implementing an all-metallic gate-tunable half-wave rectifier to be used for either superconducting electronics or photon detection applications.
... The key elements of single quantum logic (SFQ) circuits are Nb/Al/AlOx/Nb Josephson Junctions (JJ) and Nb-inductors, which are extremely promising for high-performance computing [1], [2], [3], [4] particularly as classical coprocessors for control and error tracking of large-scale quantum arrays [5], [6], [7], [8]. In addition, Nb/Al/Al/AlOx/Nb structures are the base of high-frequency receivers [9], parametric amplifiers [10], voltage standards [11], [12], and highly sensitive magnetic field sensors [13], [14]. ...
Superconducting single quantum logic integrated circuits traditionally exploit magnetron sputtered niobium thin films on silicon oxide substrates. The sputtering depends on multiple process parameters, which dramatically affect mechanical, electrical, and cryogenic properties of Nb thin films. In this work, we focus on the comprehensive relationship study between 200-nm Nb film characteristics and their intrinsic stress. It is shown that there is a critical value of the working pressure pcritical at the fixed sputtering power above which stress in the film relaxes whereas the film properties degrade significantly. Below pcritical one can control intrinsic stress in the wide range from -400 MPa to +600 MPa maintaining perfect film surface with a 0.8 nm roughness (Rq), electrical resistivity less than 20 uOhm*cm, critical superconducting transition temperature above 8.9 K and residual resistance ratio over 6.4. We suggest a modified kinetic model to predict Nb films stress with the linear dependence of high-energy parameters on the working pressure replaced with an exponential one, which allowed reduction of the approximation error from 20 to 8%.
... Thus, solitons in the context of LJJs are usually referred to as fluxons or Josephson vortices. Measured for the first time more than 40 years ago [6,7], LJJs are still nowadays an active research field [8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26]. Indeed, the fact that a single topologically protected excitation, i.e., a flux quantum, can be moved and controlled by bias currents, created by the magnetic field, manipulated through shape engineering [9,[27][28][29][30], or pinned by inhomogeneities [31,32], naturally stimulated a profusion of ideas and applications. ...
We explore the coherent thermal transport sustained by solitons through a long Josephson junction, as a thermal gradient across the system is established. We observe that a soliton causes the heat current through the system to increase. Correspondingly, the junction warms up in correspondence of the soliton, with temperature peaks up to, e.g., approximately 56 mK for a realistic Nb-based proposed setup at a bath temperature Tbath = 4.2 K. The thermal effects on the dynamics of the soliton are also discussed. Markedly, this system inherits the topological robustness of the solitons. In view of these results, the proposed device can effectively find an application as a superconducting thermal router in which the thermal transport can be locally mastered through solitonic excitations, which positions can be externally controlled through a magnetic field and a bias current.
... RAM should have short write/erase times. For the target clock frequency of 100 GHz [43], it should be below 10 ps. For vortex-based electronics, the switching time is determined by the vortex time of flight, ...
The lack of dense random-access memory is one of the main obstacles to the development of digital superconducting computers. It has been suggested that AVRAM cells, based on the storage of a single Abrikosov vortex—the smallest quantized object in superconductors—can enable drastic miniaturization to the nanometer scale. In this work, we present the numerical modeling of such cells using time-dependent Ginzburg–Landau equations. The cell represents a fluxonic quantum dot containing a small superconducting island, an asymmetric notch for the vortex entrance, a guiding track, and a vortex trap. We determine the optimal geometrical parameters for operation at zero magnetic field and the conditions for controllable vortex manipulation by short current pulses. We report ultrafast vortex motion with velocities more than an order of magnitude faster than those expected for macroscopic superconductors. This phenomenon is attributed to strong interactions with the edges of a mesoscopic island, combined with the nonlinear reduction of flux-flow viscosity due to the nonequilibrium effects in the track. Our results show that such cells can be scaled down to sizes comparable to the London penetration depth, ∼100 nm, and can enable ultrafast switching on the picosecond scale with ultralow energy per operation, ∼10−19 J.
... Cryogenic supercomputing has shown to be more energy efficient and faster than classical counterparts, still with room to improve [2]. In [3,4], advantages and challenges of superconductor Josephson junction circuits are explained. ...
In this chapter, a pulse-based method to facilitate equilibrium transitions in a superconductor memory cell is developed. In a cryogenic environment, non-dissipative storage of non-binary memory states has the potential to drastically increase energy efficiency per operation and decrease the number of memory cells per unit area. Memory cells in consideration are fabricated with Josephson junctions, which are superconducting devices analogous to the classical bit. Dynamics of a memory cell are identical to an array of coupled nonlinear oscillators, which have no known analytic solution. Despite this difficulty, a novel closed-form control technique is developed to design pulses that force stable state transitions in a memory cell. Simulation results are shown to verify the proposed solution.
... [1][2][3][4] This technology has several interesting applications, such as low energy consumption and extremely high-frequency capabilities. [5][6][7][8][9][10] Additionally, superconducting electronics is a useful platform for studying fundamental physics related to condensed matter. [11][12][13][14][15][16] An example of this is the Supercurrent Diode Effect (SDE), which is the property of superconducting two-terminal devices to transfer dissipationless current with different magnitudes in two directions. ...
The development of superconducting electronics requires careful characterization of the components that makeup electronic circuits. Superconducting weak links are the building blocks of most superconducting electronic components and are characterized by highly nonlinear current-to-phase relations (CPRs), which are often not perfectly known. Recent research has found that the Josephson diode effect (JDE) can be related to the high harmonic content of the current-to-phase relation of the weak links embedded in superconducting interferometers. This makes the JDE a natural tool for exploring the harmonic content of weak links beyond single-harmonic CPR. In this study, we present the theoretical model and experimental characterization of a double-loop superconducting quantum interference device (DL-SQUID) that embeds all-metallic superconductor-normal metal-superconductor junctions. The proposed device exhibits the JDE due to the interference of the supercurrents of three weak links in parallel, and this feature can be adjusted through two magnetic fluxes, which act as experimental knobs. We carry out a theoretical study of the device in terms of the relative weight of the interferometer arms and the experimental characterization concerning flux tunability and temperature.
... Развитие современных технологий неразрывно связано с разработкой новых функциональных элементов, многие из которых являются наноразмерными, что определяет их уникальные свойства. Примером могут служить массивы наноразмерных магнитных элементовпаттернированные среды для сверхплотной записи информации [1]; фотонные кристаллы на основе упорядоченных массивов квантовых точек [2]; наноразмерные элементы полупроводниковой электроники и разрабатываемые новые элементы криогенной электроники [3]; оптические однофотонные детекторы на основе сверхпроводящих нанопроводников [4]; элементы с применением джозефсоновских переходов [5]; биологические датчики отдельных молекул и многие другие. ...
Using the focused ion beam probe method, cross-section sample of a single functional device of micron dimensions were cut out for STEM and TEM studies. The use of analytical methods of transmission electron microscopy made it possible to obtain accurate data on the geometric parameters of nanoscale functional devices, the phase and elemental composition of functional element material, as well as on the concentration of free electrons at the Fermi level in the nanoelement material.
... Superconducting electronics could be used to solve the speed and power issues faced by CMOS technology 19 due to the faster and more energy-efficient operation offered by superconducting devices (Fig. 1b). ...
Cryogenic data storage technology is of use in superconducting single-flux quantum electronics and quantum computing. However, the lack of compatible cryogenic memory technology, which can operate at temperatures of 4 K (or lower), hinders the development of practical and scalable systems. Here we examine the development of cryogenic memory technologies. We explore three areas of memory technology: cryogenic non-superconducting memories (including those based on charge and resistance), superconducting memories (including those based on Josephson junctions, superconducting quantum interference devices and superconducting memristors) and hybrid memories (which use both superconducting and non-superconducting technologies). We consider the key challenges involved in the integration of such memories with single-flux quantum circuits and quantum computers. We also provide a comparison of the capabilities of the different technologies in the context of the requirements of superconducting electronics and quantum computing. This Review examines the development of cryogenic memory technologies—including non-superconducting memories, superconducting memories and hybrid memories—and their potential application in superconducting single-flux quantum circuits and quantum computers.
... A key component for microwave signal sensing under a weak power regime is Josephson junction. The Josephson junctions are important building blocks of superconducting quantum electronics [23,24] , and are widely used in ultrasensitive superconducting devices and systems for quantum computing. A pair of currents can only be generated on both sides of the Josephson junction by tunneling through a weak barrier. ...
We provide an overview of the recent progresses on the system architecture design and performance prediction for microwave signal detection under weak signal intensity regime, up to quantumized level. The technique roadmap includes two perspectives, the opto-electro-mechanical (OEM) and superconducting devices. For the former one, we first overview the concept of OEM, and then introduce the signal detection based on capacitive-opto-electro-mechanical systems and piezo- opto-electro-mechanical systems. For the latter one, we first overview the concept and architecture of Josephson junction, and then introduce the signal detection based on superconducting Hanbury Brown-Twiss (HBT) experiments and Λ energy-level splitting system. Besides, we review the microwave detection based on Rydberg atom system. We believe that this overview can provide a guidance for future transmission limit, signal processing, detection device fabrication and real experiments.
... Superconductor-based technologies are promising for exaflopscale supercomputing, big-data processing, artificial intelligence, and neuromorphic computing [1][2][3][4][5][6][7]. The highlight features of superconducting data processing techniques, for example, RSFQ logic [1][2][3][4][5][6][7][8][9], are the high speed and unprecedental energy efficiency [2,3,[10][11][12][13]. Superconducting spintronics is a branch of superconducting electronics, the key components of which are thin-film magnetic Josephson junctions (MJJs), which include layers of superconductors (S), ferromagnets (F) and insulators (I) [1][2][3]14,15]. ...
A series of Pd 1− x Fe x alloy epitaxial films ( x = 0, 0.038, 0.062, and 0.080), a material promising for superconducting spintronics, was prepared and studied with ultrafast optical and magneto-optical laser spectroscopy in a wide temperature range of 4–300 K. It was found that the transition to the ferromagnetic state causes a qualitative change of both the reflectivity and the magneto-optical Kerr effect transients. A nanoscale magnetic inhomogeneity of the ferromagnet/paramagnet type inherent in the palladium-rich Pd 1− x Fe x alloys reveals itself through the occurrence of a relatively slow, 10–25 ps, photoinduced demagnetization component following a subpicosecond one; the former vanishes at low temperatures only in the x = 0.080 sample. We argue that the 10 ps timescale demagnetization originates most probably from the diffusive transport of d electrons under the condition of nanoscale magnetic inhomogeneities. The low-temperature fraction of the residual paramagnetic phase can be deduced from the magnitude of the slow reflectivity relaxation component. It is estimated as ≈30% for x = 0.038 and ≈15% for x = 0.062 films. The minimal iron content ensuring the magnetic homogeneity of the ferromagnetic state in the Pd 1− x Fe x alloy at low temperatures is about 7–8 atom %.
... For example, Josephson Junction spiking neurons operating at up to 50 GHz range have been demonstrated in Segall et al. (2017). The historical challenges of such an approach include inferior fabrication technology (which may finally change given the enormous investments in superconductor quantum computing), the lowtemperature operation that limits its applications, and the lack of efficient analog memory circuits (Likharev, 2012). The photonic spiking neural networks (e.g., Feldmann et al., 2019) and hybrid superconductor/optoelectronic neuromorphic circuits (Buckley et al., 2017) share the same challenges of the already discussed photonic neuromorphic inference approaches. ...
In this community review report, we discuss applications and techniques for fast machine learning (ML) in science—the concept of integrating powerful ML methods into the real-time experimental data processing loop to accelerate scientific discovery. The material for the report builds on two workshops held by the Fast ML for Science community and covers three main areas: applications for fast ML across a number of scientific domains; techniques for training and implementing performant and resource-efficient ML algorithms; and computing architectures, platforms, and technologies for deploying these algorithms. We also present overlapping challenges across the multiple scientific domains where common solutions can be found. This community report is intended to give plenty of examples and inspiration for scientific discovery through integrated and accelerated ML solutions. This is followed by a high-level overview and organization of technical advances, including an abundance of pointers to source material, which can enable these breakthroughs.
... Superconducting spintronics is one of the technologies promising for exaflop-scale supercomputing, big-data processing, and artificial intelligence [1][2][3][4][5]. The highlighting features of the superconducting spintronic systems, for example, RSFQ-logic [1][2][3][4][5][6][7], are the high speed and unprecedental energy efficiency [2,3,[8][9][10][11]. The important components here are thin-film magnetic Josephson junctions (MJJ), which include layers of superconductors (S) and ferromagnets (F) [1][2][3]. ...
A series of Pd 1- x Fe x alloy epitaxial films ( x = 0, 0.038, 0.062, and 0.080), a material promising for superconducting spintronics, was prepared and studied with ultrafast optical and magnetooptical laser spectroscopies in a wide 4 – 300 K temperature range. It was found that the transition to the ferromagnetic state causes the qualitative modification of both the reflectivity and the magnetooptical Kerr effect transients. Nanoscale magnetic inhomogeneity of the ferromagnet/paramagnet type inherent in the palladium-rich Pd 1‑ x Fe x alloys reveals itself in an occurrence of a relatively slow, 10-25 ps, photoinduced demagnetization component following a subpicosecond one; the former vanishes at low temperatures only in the x = 0.080 sample. We argue that the 10-ps timescale demagnetization originates most probably from the d -electron diffusive transport in the conditions of the nanometer-scale magnetic inhomogeneity. The low-temperature amount of the residual paramagnetic phase can be deduced from the magnitude of the slow reflectivity relaxation component, and is estimated as ~ 30% for x = 0.038 and ~ 15% for x = 0.062 films. The minimal iron content ensuring the magnetic homogeneity of the ferromagnetic state in Pd 1- x Fe x alloy at low temperatures is about 7-8 at.%.
... The PQ is an rf superconducting quantum interference device (SQUID) in which the critical current of the Josephson junction is modulated by applying a magnetic flux so that the potential energy shape changes gradually. Likharev explored reversible computing based on the PQ [23], but PQ-based circuits have never been demonstrated, for several reasons [24]. ...
The adiabatic quantum-flux-parametron (AQFP) is an energy-efficient superconductor logic element based on the quantum flux parametron. AQFP circuits can operate with energy dissipation near the thermodynamic and quantum limits by maximizing the energy efficiency of adiabatic switching. We have established the design methodology for AQFP logic and developed various energy-efficient systems using AQFP logic, such as a low-power microprocessor, reversible computer, single-photon image sensor, and stochastic electronics. We have thus demonstrated the feasibility of the wide application of AQFP logic in future information and communications technology. In this paper, we present a tutorial review on AQFP logic to provide insights into AQFP circuit technology as an introduction to this research field. We describe the historical background, operating principle, design methodology, and recent progress of AQFP logic.
... Recently, to reduce the size, mass, and energy consumption of electronic techniques, electrophysical properties of different nanoscale contacts are being investigated to be used as active and passive elements in electric circuits and to replace traditional elements (see, for example, [1,2]). New directions of electronics based on exotic materials are being developed to create a new generation of electronic devices and appliances, such as superconductor [3][4][5], organic [6], single-electronics [7,8], spintronics [9] and others. In this respect, one of the most promising nanomaterials for such nanocontacts is graphene, which is an allotropic modification of carbon in the form of a twodimensional monatomic layer [10,11]. ...
In this work, within the framework of density functional theory combined with the method of nonequilibrium Green’s functions the density of states, transmission spectrum, current-voltage characteristics, and differential conductivity of Li-intercalated graphene (LiC6) have been determined. It is shown that in the energy range of -1.3÷-1.05 eV the quasiparticle transport through the nanostructure is disable. The features of IV- and dI/dV-characteristics of LiC6 in the form of decreasing of resistance in the range of -0.4÷0.4 V were revealed, and in the interval of 0.4÷1.4 V formation of negative differential resistance area, related to scattering of quasiparticles. It is established, that LiC6 nanodevice has 12÷13 ballistic channels and has the maximum amount of conductance 12÷13 G 0 , where G o is the conductance quantum.
... Cryogenic memories are also crucial for superconducting SFQ electronics. To solve the speed and power issues faced by CMOS technology 24 , superconducting electronics is an attractive solution, thanks to the faster and more energy-efficient operations offered by superconducting devices such as JJs and SQUIDs. Figure 1(b) depicts the significant reduction in power requirement and increase in speed with the lowering of operating temperature. ...
The surging interest in quantum computing, space electronics, and superconducting circuits has led to new, recent developments in cryogenic data storage technology. Quantum computers and superconducting single flux quantum (SFQ) electronics promise the ability to solve the challenges (such as currently unsolvable problems and speed-power issues) faced by their conventional and semiconductor counterparts, respectively. But, one of the major bottlenecks for implementing scalable quantum computers and practical SFQ circuits is to find a suitable and compatible cryogenic memory that can operate at 4 Kelvin (or lower) temperatures. Cryogenic memory is also critically important in space-based applications. Different memory technologies, such as non-superconducting memories like charge-based and resistance-based memories, Josephson junction-based superconducting memories, and hybrid (utilizing both non-superconducting and superconducting technologies) memories, are being explored to find a suitable storage system for the promising cryogenic applications. Here we review the variants of cryogenic memory technologies and scrutinize the challenges associated with these technologies and discuss future research prospects.
... For example, Josephson Junction spiking neurons operating at up to 50 GHz range have been demonstrated in Ref. [612]. The historical challenges of such an approach include inferior fabrication technology (which may finally change given the enormous investments in superconductor quantum computing), the low-temperature operation that limits its applications, and the lack of efficient analog memory circuits [634]. The photonic spiking neural networks (e.g., Ref. [608]) and hybrid superconductor / optoelectronic neuromorphic circuits [609] share the same challenges of the already discussed photonic neuromorphic inference approaches. ...
In this community review report, we discuss applications and techniques for fast machine learning (ML) in science -- the concept of integrating power ML methods into the real-time experimental data processing loop to accelerate scientific discovery. The material for the report builds on two workshops held by the Fast ML for Science community and covers three main areas: applications for fast ML across a number of scientific domains; techniques for training and implementing performant and resource-efficient ML algorithms; and computing architectures, platforms, and technologies for deploying these algorithms. We also present overlapping challenges across the multiple scientific domains where common solutions can be found. This community report is intended to give plenty of examples and inspiration for scientific discovery through integrated and accelerated ML solutions. This is followed by a high-level overview and organization of technical advances, including an abundance of pointers to source material, which can enable these breakthroughs.
... Currently, superconducting devices of quantum electronics are actively developing [1,2]. It is expected that they will soon be able to challenge conventional semiconductor technologies in several areas of quantum technologies. ...
... From a technological point of view, the unconventional field effect promises to be suitable for a wide range of applications. We already demonstrated that the rectification properties of a Dayem bridge system and more complex devices such as gatecontrolled radiation detector [65][66][67], signal routers, and computational systems [25,68] are within reach. Moreover, the superconducting gate-controlled transistors are suitable for high-current application thanks to the highly isolated gate, which is a fundamental requirement for high-power applications such as for power metal-oxide-semiconductor field-effect transistors (MOSFETs) and insulated-gate bipolar transistors (IGBTs). ...
The possibility to tune, through the application of a control gate voltage, the superconducting properties of mesoscopic devices based on Bardeen–Cooper–Schrieffer metals was recently demonstrated. Despite the extensive experimental evidence obtained on different materials and geometries, a description of the microscopic mechanism at the basis of such an unconventional effect has not been provided yet. This work discusses the technological potential of gate control of superconductivity in metallic superconductors and revises the experimental results, which provide information regarding a possible thermal origin of the effect: first, we review experiments performed on high-critical-temperature elemental superconductors (niobium and vanadium) and show how devices based on these materials can be exploited to realize basic electronic tools, such as a half-wave rectifier. Second, we discuss the origin of the gating effect by showing gate-driven suppression of the supercurrent in a suspended titanium wire and by providing a comparison between thermal and electric switching current probability distributions. Furthermore, we discuss the cold field-emission of electrons from the gate employing finite element simulations and compare the results with experimental data. In our view, the presented data provide a strong indication regarding the unlikelihood of the thermal origin of the gating effect.
... Underlying physics should allow such elements to overcome the constraints inherent to the complementary metal-oxide-semiconductor (CMOS) electronics. Superconducting logic devices are candidates for supercomputer applications due to their high operating frequency and ultralow energy consumption [6][7][8][9][10]. The conventional Josephson junction [11,12], in which the superconducting current flows due to the phase difference between two superconducting electrodes, is the basic element of superconducting circuits such as single-flux quantum logic (SFQ) [13,14], quantum flux parametron [15,16], reciprocal quantum logic [17,18], and adiabatic superconductor logic [19]. ...
A thin-film superconductor(S)/ferromagnet(F) F1/S/F2-type Pd0.96Fe0.04(20 nm)/VN(30 nm)/Pd0.92Fe0.08(12 nm) heteroepitaxial structure was synthesized on (001)-oriented single-crystal MgO substrate utilizing a combination of the reactive magnetron sputtering and the molecular-beam epitaxy techniques in ultrahigh vacuum conditions. The reference VN film, Pd0.96Fe0.04/VN, and VN/Pd0.92Fe0.08 bilayers were grown in one run with the target sample. In-situ low-energy electron diffraction and ex-situ X-ray diffraction investigations approved that all the Pd1−xFex and VN layers in the series grew epitaxial in a cube-on-cube mode. Electric resistance measurements demonstrated sharp transitions to the superconducting state with the critical temperature reducing gradually from 7.7 to 5.4 K in the sequence of the VN film, Pd0.96Fe0.04/VN, VN/Pd0.92Fe0.08, and Pd0.96Fe0.04/VN/Pd0.92Fe0.08 heterostructures due to the superconductor/ferromagnet proximity effect. Transition width increased in the same sequence from 21 to 40 mK. Magnetoresistance studies of the trilayer Pd0.96Fe0.04/VN/Pd0.92Fe0.08 sample revealed a superconducting spin-valve effect upon switching between the parallel and antiparallel magnetic configurations, and anomalies associated with the magnetic moment reversals of the ferromagnetic Pd0.92Fe0.08 and Pd0.96Fe0.04 alloy layers. The moderate critical temperature suppression and manifestations of superconducting spin-valve properties make this kind of material promising for superconducting spintronics applications.
... Forefront in the high-end supercomputing is associated with superconducting Josephson-junction technology [7][8][9][10], which offers up to two orders increase in the clock frequency (100 GHz and beyond) and six orders reduction in the energy dissipation per bit operationincredible gain against present semiconductor processors [11,12]. Requirement of the cryogenic cooling may seem complicated; however, it should not be a scare, thanks to the development of close-cycle refrigeration. ...
Thin epitaxial films of the palladium-rich Pd1−xFex alloy were synthesized and extensively studied as a tunable ferromagnetic material for superconducting spintronics. The (001)-oriented MgO single-crystal substrate and the composition range of x = 0.01–0.07 were chosen to support the epitaxial growth and provide the films with magnetic properties spanning from very soft ferromagnet for memory applications to intermediately soft and moderately hard for the programmable logic and circuit biasing, respectively. Dependences of the saturation magnetization, Curie temperature and three magnetic anisotropy constants on the iron content x were obtained for the first time from the analyses of the magnetometry and ferromagnetic resonance data. The experimental results were discussed based on existing theories of dilute ferromagnetic alloys. Simulation of the hysteresis loops within the Stoner-Wohlfarth model indicates the predominant coherent magnetic moment rotation at cryogenic temperatures. The obtained results were compiled in a database of magnetic properties of a palladium-iron alloy in a single-crystal thin-film form considered as a material for superconducting spintronics.
... The high speeds and low energy dissipation of its switching 46 has motivated the investigation of many electronic devices However, the prospect of rapid single flux quantum (RSFQ) 50 technology based on point junctions is still unclear [11]. Many 51 devices are based on thermal effects in the junction. ...
The dynamics of the fluxon in the Josephson junction is studied. The dielectric layer of the junction has a variable thickness. It is shown that the modified area of the junction acts on the fluxon as a potential barrier. The relation between the critical bias current and the thickness of the dielectric layer is analytically and numerically determined.
... Since its invention, rapid single-flux quantum (RSFQ) logic [1,2] based on superconducting digital electronics has been seriously considered as an alternative to semiconductor electronics for supercomputing applications [3][4][5]. Merging it with magnetism [6][7][8] has given a birth to superconducting spintronics [9,10]. The latter concept was implemented in the US Cryogenic Computing Complexity (C3) Program [11][12][13] with the goal "to demonstrate a small-scale computer based on superconducting logic and cryogenic memory that is energyefficient, scalable and able to solve interesting problems", opening prospects of reaching 100 PFLOPS/s with about 200 kW of electric power consumption including the cryogenic cooling. ...
Single-layer vanadium nitride (VN) and bilayer Pd0.96Fe0.04/VN and VN/Pd0.92Fe0.08 thin-film heterostructures for possible spintronics applications were synthesized on (001)-oriented single-crystalline magnesium oxide (MgO) substrates utilizing a four-chamber ultrahigh vacuum deposition and analysis system. The VN layers were reactively magnetron sputtered from a metallic vanadium target in Ar/N2 plasma, while the Pd1-
x
Fe
x
layers were deposited by co-evaporation of metallic Pd and Fe pellets from calibrated effusion cells in a molecular beam epitaxy chamber. The VN stoichiometry and Pd1-
x
Fe
x
composition were controlled by X-ray photoelectron spectroscopy. In situ low-energy electron diffraction and ex situ X-ray diffraction show that the 30 nm thick single-layer VN as well as the double-layer VN(30 nm)/Pd0.92Fe0.08(12 nm) and Pd0.96Fe0.04(20 nm)/VN(30 nm) structures have grown cube-on-cube epitaxially. Electric resistance measurements demonstrate a metallic-type temperature dependence for the VN film with a small residual resistivity of 9 μΩ·cm at 10 K, indicating high purity and structural quality of the film. The transition to the superconducting state was observed at 7.7 K for the VN film, at 7.2 K for the Pd0.96Fe0.04/VN structure and at 6.1 K for the VN/Pd0.92Fe0.08 structure with the critical temperature decreasing due to the proximity effect. Contrary to expectations, all transitions were very sharp with the width ranging from 25 mK for the VN film to 50 mK for the VN/Pd0.92Fe0.08 structure. We propose epitaxial single-crystalline thin films of VN and heteroepitaxial Pd1-
x
Fe
x
/VN and VN/Pd1-
x
Fe
x
(x ≤ 0.08) structures grown on MgO(001) as the materials of a choice for the improvement of superconducting magnetic random access memory characteristics.
... However, even relatively small changes in their size can lead to a significant increase in access delay and JJ count [28] making them impractical for real-world applications. closer at the fundamentals of this technology as well as its main differences from CMOS [16,17]. ...
Digital communication between temperature stages is a critical part of superconducting electronics systems. Specifically, enabling clean, low-loss communication between single flux quantum (SFQ) circuits at 4 K and CMOS circuitry and memory at higher temperature stages (such as 50 K) can allow for significantly more system memory than what current superconducting devices and memories allow. An amplifier for SFQ pulses, such as a four-junction logic (4JL) gate, must also be included at the beginning of the data link. We have simulated such a data link in Pathwave ADS, which uses four-junction logic (4JL) gate characteristics that were initially simulated in JoSIM. ANSYS HFSS was used to generate the S-parameters used in ADS. Eye diagrams generated from the ADS simulation results are analyzed. The eye diagrams from the data link give an eye width of 77.2 ps and an eye height of 2.16 mV in the worst case, compared to an eye width of 94.3 ps and an eye height of 4.24 mV from the 4JL gate. These simulations will enable future hardware implementations of SFQ to CMOS memory data links.
In conventional metal-oxide semiconductor (CMOS) electronics, the logic state of a device is set by a gate voltage (VG). The superconducting equivalent of such effect had remained unknown until it was recently shown that a VG can tune the superconducting current (supercurrent) flowing through a nanoconstriction in a superconductor. This gate-controlled supercurrent (GCS) can lead to superconducting logics like CMOS logics, but with lower energy dissipation. The physical mechanism underlying the GCS, however, remains under debate. In this review article, we illustrate the main mechanisms proposed for the GCS, and the material and device parameters that mostly affect it based on the evidence reported. We conclude that different mechanisms are at play in the different studies reported so far. We then outline studies that can help answer open questions on the effect and achieve control over it, which is key for applications. We finally give insights into the impact that the GCS can have toward high-performance computing with low-energy dissipation and quantum technologies.
The increasing complexity of modern single flux quantum (SFQ) circuits has increased the importance of flux trapping and trapped magnetic fields within SFQ systems. This trapped flux reduces margins while damaging the operability of superconductive circuits. In this chapter, an area-efficient stripline topology is introduced to prevent flux from being trapped within striplines. The topology is composed of coupled narrow lines rather than wide striplines. The topology uses a fingered narrow line configuration. The fingered narrow line topology enhances the scalability of SFQ systems while not requiring additional area. The topology decreases the length of the striplines by exploiting the mutual inductance between narrow parallel lines. The topology requires less area while preventing flux from being trapped within wide superconductive striplines. Due to the stripline configuration, residual current is eliminated in VLSI complexity SFQ circuits. The fingered narrow line topology also reduces coupling capacitance between striplines. The topology is compatible with automated routing of large-scale SFQ integrated circuits.
The past two decades have seen an explosion of work on Josephson junctions containing ferromagnetic materials. Such junctions are under consideration for applications in digital superconducting logic and memory. In the presence of the exchange field, spin–singlet Cooper pairs from conventional superconductors undergo rapid phase oscillations as they propagate through a ferromagnetic material. As a result, the ground-state phase difference across a ferromagnetic Josephson junction oscillates between 0 and π as a function of the thickness of the ferromagnetic material. π-junctions have been proposed as circuit elements in superconducting digital logic and in certain qubit designs for quantum computing. If a junction contains two or more ferromagnetic layers whose relative magnetization directions can be controlled by a small applied magnetic field, then the junction can serve as the foundation for a memory cell. Success in all of those applications requires careful choices of ferromagnetic materials. Often, materials that optimize magnetic properties do not optimize supercurrent propagation, and vice versa. In this review, we discuss the significant progress that has been made in identifying and testing a wide range of ferromagnetic materials in Josephson junctions over the past two decades. The review concentrates on ferromagnetic metals, partly because eventual industrial applications of ferromagnetic Josephson junctions will most likely start with metallic ferromagnets (either in all metal junctions or junctions containing an insulating layer). We will briefly mention work on non-metallic barriers, including ferromagnetic insulators, and some of the exciting work on spin–triplet supercurrent in junctions containing non-collinear magnetic inhomogeneity.
The quantum-dot cellular automata (QCA) is popular nanotechnology to process at deep sub-micron levels. In recent years, in QCA technology, numerous multi-layer circuits of adders and subtractors have been developed. However, little attention has been made to the QCA circuit’s instantiation of the subtractors schemes. This paper gives a three-layered subtractor with simple access to inputs and outputs as an essential block in QCA technology. This design was created, optimized, and simulated using QCADesigner-E. The results revealed that the suggested proposal effectively reached a higher level of efficiency, speed, and cost, owing primarily to the use of the three layers’ design. Also, this architecture offers a platform to access the input and output lines more easily. The multi-layer crossover technique is used to build this design. According to the simulation findings, the suggested subtractor in QCA technology employs 22 QCA cells. The simulation findings demonstrated that the proposed design surpasses the majority of previous results in the literature.
By modeling the propagation of a seed layer with various crystal orientations, this study explores the influence of process variations on grain formation with the help of a physics-based process simulator. Grain boundaries allow easy diffusion of foreign atoms through the lattice, which causes Al to move inside the Nb bottom electrode layer and more interestingly, O to penetrate through the Al layer during oxidation and create a barrier with non-uniform thickness. In addition to thickness variations, the grain structure exhibited by Nb and Al can cause significant suppression of supercurrent at the boundaries depending on the degree of lattice mismatch, impurity deposition, etc. This work details the process simulation of grain boundary formation and aims to provide geometrical models that may be used in the simulation of device performance to account for process-induced variations.
The dynamics of objects of various physical origins in barrier structures of superconductors, magnetic materials, quantum crystals and other solids is determined by the rate of fluctuation decay of metastable states in intermediate minima of a potential barrier. In a low-temperature region important for many nanotechnological applications, thermal fluctuations are frozen and are replaced by quantum fluctuations, which lead to a specific phase transition in the dynamics. The character of this transition in various systems is an analog of second-order or first-order transitions. The transition temperature is dependent on the degree of metastability and can be controlled by external load. This dependence is calculated for an extended nanosystem in an oblique periodic relief like a “washboard” in a wide range of varying the load, and the results generalize the results known before.
As part of the intense effort toward identifying platforms in which Majorana bound states can be realized and manipulated to perform qubit operations, we propose a topological Josephson junction architecture that achieves these capabilities and which can be experimentally implemented. The platform uses conventional superconducting electrodes deposited on a topological insulator film to form networks of proximity-coupled lateral Josephson junctions. Magnetic fields threading the network of junction barriers create Josephson vortices that host Majorana bound states localized in the junction where the local phase difference is an odd multiple of π, i.e. attached to the cores of the Josephson vortices. This enables us to manipulate the Majorana states by moving the Josephson vortices, achieving functionality exclusive to these systems in contrast to others, such as those composed of topological superconductor nanowires. We describe protocols for: (1) braiding localized Majorana states by exchange, (2) controlling the separation and hence the coupling of adjacent localized Majorana states to effect non-Abelian rotations via hybridization of the Majorana modes, and (3) reading out changes in the non-local parity correlations induced by such operations. These schemes utilize current pulses and local magnetic field pulses to control the location of vortices, and measurements of the Josephson current–phase relation to reveal the presence of the Majorana bound states. Finally, we present brief discussions of readout schemes and viable experimental settings for realizing the platform.
In February 2014 Time magazine announced to the world that the first quantum computer had been put in use. One key component of this computer is the “Josephson-junction,” a superconducting device, based on completely different scientific and technological principles with respect to semiconductors. The origin of superconductors dates back to the 1960s, to a large-scale 20-year long IBM project aimed at building ultrafast computers. We present a detailed study of the relationship between Science and Technology making use of the theoretical tools of presumptive anomaly and technological paradigms: superconductors were developed while the semiconductors revolution was in full swing. We adopt a historiographical approach—using a snowballing technique to sift through the relevant literature from various epistemological domains and technical publications—to extract theoretically robust insights from a narrative which concerns great scientific advancements, technological leaps forward and business-driven innovation. The study we present shows how technological advancements, business dynamics, and policy intertwine.
The paper deals with the properties of octagraphene nanoribbon determined in terms of the local spin density approximation and nonequilibrium Green’s function, namely transmission spectra, current–voltage characteristics (I–V curves), and differential conductivity of the zigzag-edged octagraphene nanoribbon obtained after the removal of carbon atoms from its center and saturation with hydrogen atoms. The I–V curves are characterized by a section with negative differential resistance caused by the resonant-tunneling of quasiparticles. The dI/dV curve shows similar changes. It is found that in the zigzag-edged octagraphene nanoribbon (with the network consisting of hexagons and a carbon bridge), spin-down quasiparticles are blocked in the energy range from –1.65 to –0.5 eV. This behavior of the transmission spectrum allows octagraphene nanoribbons to be applied in creation of energy spin filters. It is shown that the spin-polarized current in zigzag-edged octagraphene nanoribbon (with the network consisting of pentagons and a carbon bridge) with spin-up quasiparticles significantly exceeds the current in that with spin-up and spin-down quasiparticles. This effect allows the selection of spin-up quasiparticles at a certain voltage applied. The obtained results can be useful in the new developments of spintronic devices.
Recently, we demonstrated a rapid-single-flux-quantum NOT gate comprising a toggle storage loop. In this paper, we present our design and operation of a NOR gate that is a straightforward extension of the NOT gate by attaching a confluence buffer. Parameter margins wider than ±28% were confirmed in simulation. Functional tests using Nb integrated circuits demonstrated correct NOR operation with a bias margin of ±21%.
Rapid Single-Flux-Quantum (RSFQ) logic, based on the representation of digital bits by single quanta of magnetic flux in superconducting loops, may combine several-hundred-GHz speed with extremely low power dissipation (close to 10-18 Joule/bit) and very simple fabrication technology. The drawbacks of this technology include the necessity of deep (liquid-helium-level) cooling of RSFQ circuits and the rudimentary level of the currently available fabrication and testing facilities. The objective of this paper is to review RSFQ device physics and also discuss in brief the prospects of future development of this technology in the light of the tradeoff between its advantages and handicaps.
Multiplexed superconducting quantum interference device (SQUID) readout systems are critical for measuring large arrays of superconducting transition-edge sensors (TES). We demonstrate a code-division SQUID multiplexing (CDM) architecture that is modulated by Walsh codes. Measurements and simulations of a prototype multiplexer show that this modulation scheme is not degraded by SQUID-noise aliasing, suppresses parasitic pickup, and has low levels of crosstalk. These properties enable this architecture to scale to large TES arrays. Furthermore, CDM modulation suppresses the 1/f knee in the noise to below 20 mHz, suggesting the use of this circuit for low-frequency-noise mitigation in more general SQUID applications.
A relation is established between the critical current Ic at which the vortex structure in a one-dimensional, long (width W⪢λJ) Josephson junction begins to move and the intensity H of the external magnetic field, for various geometries of the current electrodes. Considered are cases where the external current flows along the junction edges, into one point inside the junction, into several points inside the junction, or uniformly over the entire junction. It is shown that in the last two cases Ic may be much larger than the critical current 2JcλJ corresponding to the first critical field intensity for such a junction.
The difference between the phases of superconducting order parameter plays in
superconducting circuits the role similar to that played by the electrostatic
potential difference required to drive a current in conventional circuits. This
fundamental property can be altered by inserting in a superconducting circuit a
particular type of weak link, the so-called Josephson -junction having
inverted current-phase relation and enabling a shift of the phase by . We
demonstrate the operation of three superconducting circuits -- two of them are
classical and one quantum -- which all utilize such -phase shifters
realized using superconductor-ferromagnet-superconductor sandwich technology.
The classical circuits are based on single-flux-quantum cells, which are shown
to be scalable and compatible with conventional niobium-based superconducting
electronics. The quantum circuit is a -phase biased qubit, for which we
observe coherent Rabi oscillations and compare the measured coherence time with
that of conventional superconducting phase qubits.
We have implemented a simple circuit of the rapid single‐flux‐quantum (RSFQ) logic family using a single‐layer YBa 2 Cu 3 O 7-x thin‐film structure with 14 in‐plane Josephson junctions formed by direct electron beam writing. The circuit includes two dc/SFQ converters, two Josephson transmission lines, a complete RS SFQ flip‐flop, and an SFQ/dc converter (readout SQUID). Low‐frequency testing has shown that the dc‐current‐biased circuit operates correctly and reliably at T∼30 K, a few degrees below the effective critical temperature of the junctions. Prospects for a further increase of the operation temperature and implementation of more complex RSFQ circuits are discussed in brief. © 1995 American Institute of Physics.
We have carried out measurements of bit error rate (BER) of Rapid
Single-Flux-Quantum (RSFQ) XOR gates with various nominal dc power
supply voltages (from 0.1 V to 1.0 mV), operating at speeds up to 25
GHz. (For these gates, implemented using HYPRES' standard, 3.5-μm, 10
μA/μm2 Nb-trilayer process, this speed is close to
maximum.) A special on-chip RSFQ test circuit allowed high-speed
measurements of BER in the range from 10-9 to
10-13 to be carried out. As a result of these experiments, a
new type of thermal-fluctuation-induced digital errors in RSFQ circuits
has been identified. These “timing” errors arise at high
speed due to time jitter of data and clock pulses. We have developed a
simple theory of these errors which allows a fair description of the
experimental data. The theory shows that in some cases the timing errors
may be an important factor limiting speed performance of RSFQ circuitry.
Nevertheless, our XOR gates could operate at 25 GHz with BER below 10
-13 at the standard temperature (4.2 K) at any dc power
supply voltage in our range. For the lowest voltage (0.1 Volt) the
calculated static power dissipation in the gate was as low as 23
nanowatts, lower than the unavoidable dynamic dissipation (43 nanowatts)
We have designed and demonstrated an Arithmetic-Logic Unit (ALU) based on RSFQ technology as a required step toward building an 8-bit RSFQ processor datapath. The circuit was designed and fabricated with HYPRES' standard 4.5 kA/cm2 process. The target clock frequency of the ALU is 20 GHz. In this paper, we present the design and functionality (low-speed) test results of the 8-bit ALU.
We present a novel, resistor-free approach to dc biasing of RSFQ circuits, known as Energy-efficient RSFQ (ERSFQ). This biasing scheme does not dissipate energy in the static (non-active) mode, and dissipates orders of magnitude less power than traditional RSFQ while operating. Using this approach, we have designed, fabricated and successfully tested at low and high speed a D flip-flop with complementary outputs and several static frequency dividers. We present the method, demonstrate experimental results, and discuss future implementations of ERSFQ.
Figures of merit connecting processing capabilities with power dissipated (OpS/Watt, Joule/bit, etc.) are becoming dominant factors in choosing technologies for implementing the next generation of computing and communication network systems. Superconductivity is viewed as a technology capable of achieving higher energy efficiencies than other technologies. Static power dissipation of standard RSFQ logic, associated with dc bias resistors, is responsible for most of the circuit power dissipation. In this paper, we review and compare different superconductor digital technology approaches and logic families addressing this problem. We present a novel energy-efficient single flux quantum logic family, ERSFQ/eSFQ. We also discuss energy-efficient approaches for output data interface and overall cryosystem design.
Superconductor digital receiver systems of increasing functionality, modularity and user-friendliness have been developed. The modular design methodology ensures that within its input-output and heat load capacity, the system can be reconfigured to perform a different function by changing the chip module and by reprogramming FPGA-based digital signal processors. One of the systems (ADR-004), originally equipped with a 10 × 10 mm2 channelizing receiver chip for signals intelligence application, was reconfigured with a 5 × 5 mm2 1.1-GHz bandpass ADC chip to perform world's first multi-net Link-16 demonstration at a U.S. Navy facility. Substantial improvements in system integration have been obtained in each successive generation of digital-RF receiver systems. The latest (third) generation system (ADR-005), hosting a 5 × 5 mm2 7.5-GHz bandpass ADC chip and an FPGA channelizer, successfully repeated the over-the-air SATCOM demonstration performed previously using a 1-cm2 single-chip bandpass digital receiver with an on-chip superconductor channelizer. This system ran error-free for over 12 hours with and without a low-noise amplifier. To our knowledge, this is the first time an X-band SATCOM receiver has been operated without analog amplification and down-conversion in a military application.
Digital superconductor electronics has been experiencing rapid maturation with the emergence of smaller-scale, lower-cost communications applications which became the major technology drivers. These applications are primarily in the area of wireless communications, radar, and surveillance as well as in imaging and sensor systems. In these areas, the fundamental advantages of superconductivity translate into system benefits through novel Digital-RF architectures with direct digitization of wide band, high frequency radio frequency (RF) signals. At the same time the availability of relatively small 4K cryocoolers has lowered the foremost market barrier for cryogenically-cooled digital electronic systems. Recently, we have achieved a major breakthrough in the development, demonstration, and successful delivery of the cryocooled superconductor digital-RF receivers directly digitizing signals in a broad range from kilohertz to gigahertz. These essentially hybrid-technology systems combine a variety of superconductor and semiconductor technologies packaged with two-stage commercial cryocoolers: cryogenic Nb mixed-signal and digital circuits based on Rapid Single Flux Quantum (RSFQ) technology, room-temperature amplifiers, FPGA processing and control circuitry. The demonstrated cryocooled digital-RF systems are the world's first and fastest directly digitizing receivers operating with live satellite signals in X-band and performing signal acquisition in HF to L-band at ˜30GHz clock frequencies.
The Josephson microprocessor is a good example for exhibiting typical performance of digital circuits with Josephson devices. Although present Josephson microprocessors are not large, they can be operated over ten times faster than similar semiconductor microprocessors. This kind of performance is due largely to the reliability of Josephson junction technology. The niobium Josephson junction (Nb/AlOx/Nb) has served as the base upon which high-speed digital circuits have been developed because of its uniform, stable characteristics. Using niobium junctions, we can fabricate microprocessors having a few thousand gates and memory of a few kilobits. Interface circuits to adapt the Josephson circuit signal to semiconductor circuits are also feasible. Josephson microprocessor development, as described in this paper, relies mainly on the technology for niobium junction fabrication, the high-speed logic gate family, and the power system. An experimental cryogenic system for a prototype Josephson computer is also described.
This review covers experimental results and theoretical ideas on the properties of superconducting weak links, i.e., weak electrical contacts between superconducting electrodes which exhibit direct (non-tunnel-type) conductivity. When the dimensions of such weak links are sufficiently small, the Josephson effect is observed in them, in other words, a single-valued and 2π -periodic relationship exists between the supercurrent Is and the phase difference σ of the electrodes. With increasing dimensions, this relationship has a tendency to deviate gradually from the Josephson behavior. This deviation varies, depending on whether the weak link material is a superconductor or a normal metal. The various known types of weak links are described, and special mention is made of those weak links which are most suitable for physical investigations and have various practical applications. The data on the nonstationary (ac) processes in weak links, when the phase difference varies with time, are analyzed. In conclusion the existing concepts about the processes in weak links are briefly summarized and the most urgent outstanding problems are outlined.
The structure and tunneling behavior of Nb/Al multilayer films is reported. Using similar deposition methods we also show that a few monolayers of Al strongly modifies the oxidation behavior of Nb and the characteristics of tunnel junctions formed on such surfaces. This has been deduced from measurements of the oxidation rate and the tunneling characteristics over a wide voltage range with the Nb in both normal and superconducting states. The improvement in characteristics resulting from the Al overlayer allows tunneling spectroscopy of Nb without having to allow for a proximity effect on the surface.
A time-to-pulse height converter was studied. With the clipping lines used, the converter has a linear region of 0.5 x 10-8 sec and a variation in delay of 2.5 x 10-11 sec corresponds to a variation in amplitude of 1%.
This Letter reports the measurement of an upper bound of 0.8 nsec for the time of transition of Sn&sngbnd;SnOx&sngbnd;Sn Josephson junctions from the pair tunneling (V = 0) to the single-particle tunneling state (V = 2Δ: 1mV at 1.7°K). The threshold is extremely sharp. There is no observed delay in the transition after the threshold is exceeded, and there is no change in the observed risetime of the signal with changes in driving current amplitude or risetime. The magnetic field behavior of these junctions is the same as that observed in the dc case. The combination of high speed, sharp threshold, and strong magnetic field dependence are desirable properties for logic elements.
The use of SQUIDs enables the ultimate switching speeds and low power dissipation of Josephson devices to be achieved in digital logic systems. In addition, unique circuit capabilities become apparent which can simplify the logic circuits. These concepts will be applied to fast analog to digital conversion. A continuous signal is applied to a single-junction SQUID and converted to a pulse train of flux quanta. These quanta are counted in a scalar consisting of a linear array of coupled DC SQUIDs. Scaling is achieved by utilizing the bistable nature of a double-junction SQUID biased at the half-flux point. In this way fast, single-flux quantum logic can be achieved.
A high-speed performance of a Josephson MVTL (modified variable threshold logic) OR gate was demonstrated. This fast operation was achieved by miniaturizing the gate. An MVTL gate fabricated with Nb/AlOx/Nb Josephson junctions, SiO2 insulators and Mo resistors is shown. The Josephson critical current, Ic, is proportional to the junction size, so when the size is reduced, the increased Ic spread is crucial in operating many junctions at the same bias current. The maximum-to-minimum spread in Ic for 100 gates connected in series was +or-6% of the mean. This small spread was achieved by reducing the thickness of the upper Nb electrode of the electron junction from 90 nm to 60 nm. The average current of Ic was 8800 A/cm2, and the measured operating margin of a single gate was +or-32%. At the highest bias level, the average gate delay was 1.5 ps/gate.
(Previously cited in issue 23, p. 3338, Accession no. A74-44368)
This book provides coverage of the essential elements of superconductivity theory. It illustrates how principles of superconductivity are applied to electrical devices and circuits.
The program PSCAN (personal superconductor circuit analyser) is briefly described. It uses the nodal Josephson phase approach which automatically accounts for peculiarities of superconductor circuits such as magnetic flux quantization and quantum interference. The program is able to perform five major types of analyses: transient, I-V curve, threshold curve, operation margin and time-delay.
Fundamental limitations on the energy dissipated during one elementary logical operation are discussed. A model of a real physical device (parametric quantron) based on the Josephson effect in superconductors is used throughout the discussion. This device is shown to be physically reversible, and moreover it can serve as the clementary cell of a logically reversible computer, both these properties being necessary to achieve the fundamental limits of energy dissipation. These limits due to classical and quantum statistics are shown to lie well below the earlier estimates,k
B T and , respectively.
A new Josephson junction logic device working on the principle of parametron and operated entirely on dc flux for input, output and excitation is discussed in this article. Computer simulation of the device operation with clock signal of 100psec shows that the device is behaving very well as a logic element with very low power dissipation. The device's parameters and noise problems such as thermal noise and quantum tunnelling noise are also discussed. Promising results are obtained from the analysis.
A possibility of physically and logically reversible processing of digital information in Josephson-junction circuits of a reasonable complexity has been considered. As example, a 8-bit 1024-point fast convolver has been designed on the basis of a two-dimentional quasi-uniform array of sim 250times30,000 parametric quantrons. This completely reversible conveyer device can operate with the estimated rate of at least sim10^{9} numbers per second, which corresponds to sim10^{14} binary logical operations per second. At this rate the power dissipation in the whole device can still be as low as ∼30nW. A new mode of the parametric quantron operation with larger parameter margins is also described.
By numerical calculations of the differential‐difference sine‐Gordon equation, we have discussed the discrete Josephson‐junction transmission lines which are constructed of a series of small‐area Josephson junctions connected by superconducting strips. It is shown that the discrete Josephson lines containing D lines, N lines, T turning points, and S turning points are elementarily characterized by the discreteness parameter (2πLI c /Φ 0 )1/2. On the discrete Josephson logic circuits there exists a region of forbidden propagation in the (2πLI c /Φ 0 )1/2‐γ (bias‐current parameter) plane for single flux quanta. A single flux quantum can be stuffed in a small area of the discrete Josephson logic circuits. The discrete circuits can be conveniently and easily linked to each other, in a practical fabrication of a Josephson network.
New logic circuits are designed which employ as information bits the flux‐quantum vortices occurring in Josephson junctions of extended dimensions. It is shown that Josephson lines can be interconnected in certain direct ways so that complete logic capability can be achieved with networks of Josephson lines alone.
Preparation of high quality all‐refractory Josephson tunnel junctions based on Nb/Al‐oxide‐Nb and Nb/Al‐oxide‐Al/Nb structures is reported. Critical currents up to 1300 A/cm2 and V m values up to 35 mV were obtained. The specific capacitance of these junctions is 0.06±0.02 pF/μm2. Junctions were fabricated using standard photolithography and a new plasma etching process coupled with anodization of Nb.
Experiments with latching and nonlatching quantum interference logic devices are described. The device is a three‐junction interferometer, switching with control currents of ≪0.5 mA. Simple logic functions have been performed with dc‐powered nonlatching devices having a power dissipation of ≪40 nW in continuous operation.
This paper reports the design and test results of a single-flux-quantum (SFQ) bit-serial adder, which we designed with a target-clock frequency of 100 GHz, to investigate several techniques for producing ultra-high-speed computations using SFQ circuits. The bit-serial adder was designed based on a new cell library developed for the ISTEC Advanced Process, where the critical current density and McCumber-Stewart parameters of Josephson junctions were increased to 10 kA/cm2 and 2.0, respectively, to obtain higher operating frequencies. In addition, we adopted a circuit-design technique based on state transitions excluding a feedback loop in a typical bit-serial adder, and redesigned the NOR gate with the McCumber-Stewart parameter increased to 4.0 to improve performance. As a result, we experimentally obtained a sufficient dc bias margin of ±18% from low frequencies to 60 GHz, and verified the correctness of operations up to 93 GHz. We also demonstrated that the introduction of a higher bias voltage or large inductors in series with bias resistors is effective for achieving faster operation.
NbN-TaN-Nb(Ti)N non-hysteretic SNS Josephson junctions with TaXN barriers tuned through nitrogen content at metal-insulator transition have been produced in a reliable way with characteristic voltage RNIC above 1.5 mV at 9 K. Barriers are typically 7-10 nm thick and Jc in the range 5 to 25 kA/cm-2 making feasible LSI RSFQ ADC circuit integration by using the lithographic tools of the Leti C-MOS platform. Junction tri-layers, NbTiN ground-plane and wiring levels have been reactively sputtered and optimized at 300°C in a reproducible way on 8-inch oxidized silicon wafers. A deep UV stepper combined with SF6 gas mixtures RIE has been used to achieve low spread lithography in junction diameters of 716 nm (3σ ~ 40 nm), of 465 nm (3σ ~ 84 nm) across the 8-inch. A well controlled CMP planarization process has been developed on thick PE-CVD silica layers deposited on each superconducting nitride layer. On-line FIB-SEM observations and electrical characterizations associated to the development of a technology test vehicle demonstrate the feasibility of the complete process. The reliable production of nitrides LSI RSFQ circuits such as ADCs and integrated imagers operating in a relaxed cryogenic environment in the 6-10 K temperature range is foreseen to open new telecoms and medical applications. Moreover, a new MgO-AlN-MgO layered insulating barrier junction is shown to operate above 10 K and will be developed for analog front-end circuit parts.
The first laboratory prototype of a superconductor
analog-to-digital converter (ADC) with on-chip fast digital signal
processing has been designed and tested at internal clock frequencies up
to 7 GHz. Estimated accuracy of the prototype is about 12 bit for 7 MHz
signal bandwidth; at lower frequencies the dynamic range increases at
least as fast as the reciprocal bandwidth. The single-chip (5×5 mm
2) device contains 1778 Josephson junctions and dissipates
power at about 0.5 mW
The usual general-purpose computing automaton (e.g., a Turing machine) is logically irreversible—its transition function lacks a single-valued inverse. Here it is shown that such machines may be made logically reversible at every step, while retaining their simplicity and their ability to do general computations. This result is of great physical interest because it makes plausible the existence of thermodynamically reversible computers which could perform useful computations at useful speed while dissipating considerably less than kT of energy per logical step. In the first stage of its computation the logically reversible automaton parallels the corresponding irreversible automaton, except that it saves all intermediate results, thereby avoiding the irreversible operation of erasure. The second stage consists of printing out the desired output. The third stage then reversibly disposes of all the undesired intermediate results by retracing the steps of the first stage in backward order (a process which is only possible because the first stage has been carried out reversibly), thereby restoring the machine (except for the now-written output tape) to its original condition. The final machine configuration thus contains the desired output and a reconstructed copy of the input, but no other undesired data. The foregoing results are demonstrated explicitly using a type of three-tape Turing machine. The biosynthesis of messenger RNA is discussed as a physical example of reversible computation.
Several models, both static and dynamic, for calculating the maximum dc Josephson current versus external magnetic field “characteristic” of tunnel junctions of various geometries are described. The static models have the advantage of short computation times; the dynamic models, although slower, yield additional information on circuit-switching times. The dependence of the characteristics on the junction shape is described. The accuracy of a one-dimensional approximation used in the models has been tested experimentally, and theoretically against a two-dimensional approximation. In both cases, the one-dimensional treatment yields quantitatively good agreement provided the inductance of the junction is included in the model. Junctions defined by an oxide “window” of sinusoidal shape (maximum and minimum width at the center and ends, respectively) have a characteristic in which the side lobes are almost completely suppressed. The characteristics of a wide variety of junction shapes, including interferometers having two or more junctions, have been accurately calculated, permitting the design of devices for specific purposes.
We present a technology based on Nb/NbxSi1-x/Nb junctions, with barriers near the metal-insulator transition, for applications in superconducting electronics (SCE) as an alternative to Nb/AlOx/Nb tunnel junctions. Josephson junctions with co-sputtered amorphous Nb-Si barriers can be made with a wide variety of electrical properties: critical current density (Jc), capacitance (C), and normal resistance (Rn) can be reliably selected within wide ranges by choosing both the barrier thickness and Nb concentration. Nonhysteretic Nb/NbxSi1-x/Nb junctions with IcRn products greater than 1mV, where Ic is the critical current, and Jc values near 100kA/cm2 have been fabricated and are promising for superconductive digital electronics. These barriers have thicknesses of several nanometers; this improves fabrication reproducibility and junction uniformity, both of which are necessary for complex digital circuits. Recent improvements to our deposition system have allowed us to obtain better uniformity across the wafer.
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The energy dissipation in a proposed digital device in which discrete degrees of freedom are used to represent digital information (a "single-electron parametron") was analyzed. If the switching speed is not too high, the device may operate reversibly (adiabatically), and the energy dissipation ℰ per bit may be much less than the thermal energy scale kBT (where kB is Boltzmann's constant and T is temperature). The energy-time product ℰtau is, however, much greater than Planck's constant Planck's over 2pi, at least in the standard "orthodox" model of single-electron tunneling that was used in these calculations.
The paradigm of massively parallel processing and message-passing
concurrent processes has culminated in the Federal HPCC goal of
achieving teraflops-scale computing. Detailed studies of important
applications have demonstrated that critical needs of mission-drives
agencies and major scientific and engineering challenges require
substantially more computing capability i.e., petaflops-scale computing.
The first workshop on Enabling Technologies for Peta(FL)ops Computing in
February, 1996 provided a first examination of the potential
opportunities and challenges to the future achievement of petaflops
computing systems. Subsequent workshops have addressed hardware,
architecture, and software issues. The Petaflops Architecture WorkShop
(PAWS) in April, 1996 concentrated on alternative architecture
approaches to petaflops computer systems development. The Petaflops
System Software Summer Study (PetaSoft) in June, 1996 focused on the
fundamental problem of system software, its requirements, and approaches
to supporting petaflops computing. An important element in both
workshops was a set of sponsored studies in near-petaflops architecture
initiated by NSF in cooperation with NASA and DARPA. The eight studies
proposed a number of approaches to petaflops systems architecture
including software. Parallelism, bandwidth, and latency were considered
the principal challenges to effective petaflops computing and required
solutions involving all levels of hardware, software, and algorithms
techniques. This paper provides a perspective of some of the key
questions, critical issues, findings, and recommendations pertinent to
petaflops architecture from the two petaflops systems workshops
The following is a brief description of the basic principles and applications of the parametron, which is a digital computer element invented by the author in 1954. A parametron element is essentially a resonant circuit with a nonlinear reactive element which oscillates at one-half the driving frequency. The oscillation is used to represent a binary digit by the choice between two stationary phases ¿ radians apart. The basic principle of logical circuits using the parametron is explained, and research on and applications of parametrons in Japan are described.
The study of nonlinearities in nature suitable for computer use has led to the cryotron, a device based on the destruction of superconductivity by a magnetic field. The cryotron, in its simplest form, consists of a straight piece of wire about one inch long with a single-layer control winding wound over it. Current in the control winding creates a magnetic field which causes the central wire to change from its superconducting state to its normal state. The device has current gain, that is, a small current can control a larger current; it has power gain so that cryotrons can be interconnected in logical networks as active elements. The device is also small, light, easily fabricated, and dissipates very little power.
A superconducting storage device is proposed in which a Josephson tunnelling junction, switched by a control film strip, replaces the cryotron in a one-cryotron-per-bit storage cell. The device should be fast, work at 4.2° K and require a simple technology, which must, however, yield reproducible thin Josephson tunnelling layers.
To achieve larger scale and higher speed single flux quantum (SFQ) circuits, we have been developing a 10-kA/cm2 Nb multi-layer fabrication process composed of more than six pla- narized Nb layers, an Nb/AlOx /Nb junction layer, a Mo resistor layer, and SiO2 insulator layers. To evaluate reliability of the fabrication process, we have designed superconducting random access memories (RAMs) with four different memory capacities: 256, IK, 4 K, and 16 K bits. Although the circuit configuration of these RAMs is almost the same as that of previously developed ones that have conventional latching devices, we modified the circuit parameters and layout design based on specifications of the new fabrication process. We have obtained operations for the 256-bit RAM with a bit yield of 100%, the lK-bit RAM with a bit yield of 99.8%, and the 4K-bit RAM with a bit yield of 96.7%. The number of defects in the 4K-bit RAM was estimated to be approximately 10. We confirmed that evaluations using the RAMs were effective at detecting defects due to the fabrication process.
We have developed an advanced fabrication process for fabricating Nb integrated circuits with up to nine planarized Nb layers, and with critical current density of Josephson junctions of 10 kA/cm2. We have continued to improve this advanced process. For nine-layer integration, we readjusted film thickness of Nb and SiO2 layers in order to reduce the strain of films and substrate. Total film thickness of the nine-Nb layered structure was about 3 mum; this was kept nearly as thin as that of the six-Nb-layered structure. The resulting thinner SiO2 layers enabled narrower passive transmission line wiring, which had the advantage of smaller occupation area. The room temperature measurement of process monitoring patterns is useful for screening defective wafers in the middle step of the process. For higher circuit reliability, we modified fabrication processes such as junction planarization. As a result, the reliability of SiO2 insulation between an upper and a lower Nb wire adjacent to a Josephson junction was improved.
HYPRES has developed new fabrication processes for higher critical current density Josephson junctions (JJs). These processes incorporate an additional anodization step for junction insulation, which enables fabrication of junctions down to submicron sizes. A set of new processing tools has been employed, including a high density (ICP) plasma etching of niobium and aluminum, and low temperature plasma-enhanced chemical vapor deposition of interlayer dielectric (SiO2) from a TEOS source. A set of new parametric control monitor (PCM) test chips has been designed and implemented. Results of electric and SEM characterization of JJ's, wiring, and contact-hole etching are presented. The critical current spreads and shunt resistance uniformity along with the effects of junction shape are discussed. The critical current 1σ spreads of 1.2% have been achieved for the 4.5 kA/cm2 process.