F. C. Wellstood

University of Maryland, College Park | UMD, UMCP, University of Maryland College Park · Department of Physics

We have fabricated and characterized asymmetric gap-engineered junctions and transmon devices. To create Josephson junctions with asymmetric gaps, Ti was used to proximitize and lower the superconducting gap of the Al counter-electrode. DC IV measurements of these small, proximitized Josephson junctions show a reduced gap and larger excess current...

Low-loss superconducting rf devices are required when used for quantum computation. Here, we present a series of measurements and simulations showing that conducting losses in the packaging of our superconducting resonator devices affect the maximum achievable internal quality factors (Qi) for a series of thin-film Al quarter-wave resonators with f...

Low-loss superconducting microwave devices are required for quantum computation. Here, we present a series of measurements and simulations showing that conducting losses in the packaging of our superconducting resonator devices affect the maximum achievable internal quality factors (Qi) for a series of thin-film Al quarter-wave resonators with fund...

We have fabricated and characterized asymmetric gap-engineered junctions and transmon devices. To create Josephson junctions with asymmetric gaps, Ti was used to proximitize and lower the superconducting gap of the Al counter-electrode. DC IV measurements of these small, proximitized Josephson junctions show a reduced gap and larger excess current...

To test the contribution of non-equilibrium quasiparticles to qubit relaxation, we have repeatedly measured the relaxation time T_1 in Al/AlOx/Al transmons with electrodes that have different superconducting gaps. In one device, the first layer electrode was formed by thermal evaporation of nominally pure Al, while the counter-electrode was formed...

To test the contribution of non-equilibrium quasiparticles to qubit relaxation, we have repeatedly measured the relaxation time T1 in Al/AlOx/Al transmons with electrodes that have different superconducting gaps. In one device, the first layer electrode was formed by thermal evaporation of nominally pure Al, while the counter-electrode was formed b...

We report scanning tunneling microscopy results on 25- and 50-nm-thick films of superconducting TiN that show Andreev tunneling behavior at 0.5 K. At most locations on the topographically rough surfaces, we observe tip-sample current-voltage characteristics with a clear superconducting gap, as expected for superconductor-normal (S-N) tunneling thro...

We demonstrate improvements to the cooling power of broad bandwidth (10 GHz) microwave attenuators designed for operation at temperatures below 100 mK. By interleaving 9-μm thick conducting copper heatsinks in between 10-μm long, 70-nm thick resistive nichrome elements, the electrical heat generated in the nichrome elements is conducted more readil...

We have embedded two fixed-frequency Al/AlOx/Al transmons, with ground-to-excited transition frequencies at 6.0714 and 6.7543 GHz, in a single three-dimensional Al cavity with a fundamental mode at 7.7463 GHz. Strong coupling between the cavity and each transmon results in an effective qubit-qubit coupling strength of 26 MHz and a −1 MHz dispersive...

We have embedded two fixed-frequency Al/AlO$_{\textrm{x}}$/Al transmons, with ground-to-excited transition frequencies at 6.0714 GHz and 6.7543 GHz, in a single 3D Al cavity with a fundamental mode at 7.7463 GHz. Strong coupling between the cavity and each transmon results in an effective qubit-qubit coupling strength of 26 MHz and a -1 MHz dispers...

We demonstrate improvements to the cooling power of broad bandwidth (10 GHz) microwave attenuators designed for operation at temperatures below 100 mK. By interleaving 9-$\mu$m thick conducting copper heatsinks in between 10-$\mu$m long, 70-nm thick resistive nichrome elements, the electrical heat generated in the nichrome elements is conducted mor...

We examine coherent population trapping in a circuit-QED system consisting of an Al/AlOx/Al transmon qubit embedded in an Al three-dimensional cavity. By engineering the dissipation rate of the cavity to be much larger than that of the qubit and continuously pumping a two-photon process to an excited state of the cavity-qubit system, we are able to...

We have modified a dual-tip scanning tunneling microscope (STM) by electrically connecting the tips together with a short (3 mm) strip of flexible 25 μm thick Nb foil. For simultaneous topographic imaging with both tips, we moved each tip to within tunneling distance z of a surface and modulated one tip's z-piezo at 5 kHz and the other at 10 kHz. T...

To reduce the level of thermally generated electrical noise transmitted to superconducting quantum devices operating at 20 mK, we have developed thin-film microwave power attenuators operating from 1 to 10 GHz. The 20 dB and 30 dB attenuators are built on a quartz substrate and use 75 nm thick films of nichrome for dissipative components and 0.001...

The deterministic generation of non-classical states of light, including squeezed states, Fock states and Bell states, plays an important role in quantum information processing and exploration of the physics of quantum entanglement. Preparation of these non-classical states in resonators is non-trivial due to their inherent harmonicity. Here we use...

Supplementary Figures, Supplementary Tables and Supplementary References

Material-based two-level systems (TLSs), appearing as defects in low-temperature devices including superconducting qubits and photon detectors, are difficult to characterize. In this study we apply a uniform dc-electric field across a film to tune the energies of TLSs within. The film is embedded in a superconducting resonator such that it forms a...

We have illuminated a thin-film superconducting Al lumped-element microwave resonator with 780 nm light and observed the resonator quality factor and resonance frequency as a function of illumination and microwave power in the 20 to 300 mK temperature range. The optically induced microwave loss increases with increasing illumination but decreases w...

Atomic three-level Λ systems dressed by two coherent electromagnetic fields can exhibit coherent population trapping and electromagnetically induced transparency (EIT) due to quantum interference. By addressing the combined qubit–cavity states of a superconducting transmon qubit in a three-dimensional copper cavity with two microwave drives we esta...

We measure photon-occupancy in a thin-film superconducting lumped element resonator coupled to a transmon qubit at 20$\,$mK and find a nonlinear dependence on the applied microwave power. The transmon-resonator system was operated in the strong dispersive regime, where the ac Stark shift ($2\chi$) due to a single microwave photon present in the res...

We observe photon-assisted Cooper-pair tunneling in an atomic-scale Josephson junction formed between a superconducting Nb tip and a superconducting Nb sample in a scanning tunneling microscope (STM) at 30 mK. High-resolution tunneling spectroscopy data show a zero-bias conduction peak and other sharp subgap peaks from coupling of the STM junction...

Random tunneling two-level systems (TLSs) in dielectrics have been of interest recently because they adversely affect the performance of superconducting qubits. The coupling of TLSs to qubits has allowed individual TLS characterization, which has previously been limited to TLSs within (thin) Josephson tunneling barriers made from aluminum oxide. He...

Using a calibrated uniform dc electric field, we modify the energy potential of randomly occurring two-level systems (TLSs) in an insulating film and probe them using a superconducting microwave resonator at millikelvin temperatures. The excitation energy of individual TLSs depends on the electric field and the $z$-component of their electric dipol...

The need to increase transistor packing density beyond Moore's Law and the need for expanding functionality, real-estate management and faster connections has pushed the industry to develop complex 3D package technology which includes System-in-Package (SiP), wafer-level packaging, through-silicon-vias (TSV), stacked-die and flex packages. These st...

The need to increase transistor packing density beyond Moore's Law and the need for expanding functionality, realestate management and faster connections has pushed the industry to develop complex 3D package technology which includes System-in-Package (SiP), wafer-level packaging, through-silicon-vias (TSV), stacked-die and flex packages. These sta...

Amorphous solids contain nanoscale two-level systems (TLSs) which are of interest in quantum computing because they are a source of decoherence and potential qubits. We report on individual TLSs studied using cavity quantum electrodynamics (CQED). A superconducting resonator forms the cavity and an insulating film containing the TLSs is in the cent...

We report a reproducible technique for the fabrication of sharp superconducting Nb tips for scanning tunneling microscopy (STM) and scanning tunneling spectroscopy. Sections of Nb wire with 250 $\mu$m diameter are dry etched in an SF$_6$ plasma in a Reactive Ion Etcher. The gas pressure, etching time and applied power are chosen to produce a self-s...

We describe the design, construction, and performance of an ultra-low temperature, high-field scanning tunneling microscope (STM) with two independent tips. The STM is mounted on a dilution refrigerator and operates at a base temperature of 30 mK with magnetic fields of up to 13.5 T. We focus on the design of the two-tip STM head, as well as the sa...

In superconducting devices used for quantum computing and astronomy photon detectors, two-level systems (TLS) in insulators act as defects which couple to microwave fields and create deleterious phenomena. As a result, they have been the subject of fundamental science and device optimization studies. According to a recent theory by Burin et al., th...

Photon number splitting is observed in a transmon coupled to a superconducting quasi-lumped-element resonator in the strong dispersive limit. A thermal population of 5.474 GHz photons at an effective resonator temperature of T = 120mK results in a weak n = 1 photon peak along with the n = 0 photon peak in the qubit spectrum in the absence of a cohe...

We describe the design, construction and operation of a scanning tunneling microscope (STM) with two tips that can independently acquire simultaneous scans of a sample. The STM is mounted on a dilution refrigerator and the setup includes vibration isolation, rf-filtered wiring, an ultra high vacuum (UHV) sample preparation chamber and sample transf...

While transistor gate lengths may continue to shrink for some time, the semiconductor industry faces increasing difficulties to satisfy Moore's Law. One solution to satisfying Moore's Law in the future is to stack transistors in a 3-dimensional (3D) formation. In addition, the need for expanding functionality, real-estate management and faster conn...

Hybrid quantum systems can be formed that combine the strengths of multiple platforms while avoiding the weaknesses. Here we report on progress toward a hybrid quantum system of neutral atom spins coupled to superconducting qubits. We trap laser-cooled rubidium atoms in the evanescent field of an ultrathin optical fiber, which will be suspended a f...

We have investigated the decoherence of quantum states in two Al/AlOx/Al Cooper-pair boxes coupled to lumped element superconducting LC resonators. At 25 mK, the first qubit had an energy relaxation time T1 that varied from 30 us to 200 us between 4 and 8 GHz and displayed an inverse correlation between T1 and the coupling to the microwave drive li...

We have observed the Autler-Townes doublet in a superconducting Al/AlOx/Al transmon qubit that acts as an artificial atom embedded in a three-dimensional Cu microwave cavity at a temperature of 22 mK. Using pulsed microwave spectroscopy, the three lowest transmon levels are isolated, eliminating unwanted effects of higher qubit modes and cavity mod...

We have fabricated and tested an Al/AlOx/Al dc SQUID phase qubit on a sapphire substrate with a qubit junction area 0.17 μm2 and critical current I0,J1 = 1.4 μA. The qubit is shunted by an interdigitated capacitor and isolated from the bias leads by an inductive isolation network using a larger Josephson junction with area 2 μm2 and critical curren...

We examine a dc SQUID phase qubit with an on-chip low-pass resonant LC filter that transforms the line impedance, improving the qubit lifetime. Unusual features in the spectroscopy suggest dynamics more complicated than a simple two-level system. To model this behavior, we consider a lumped-element circuit model of the SQUID that includes the filte...

We report on the quadrupling of the transition spectrum of an Al/AlOx/Al Cooper-pair box (CPB) charge qubit in the 4.0-7.3 GHz frequency range. The qubit was coupled to a quasi-lumped element Al superconducting resonator and measured at a temperature of 25 mK. We obtained good matches between the observed spectrum and the spectra calculated from a...

We have measured the switching current versus applied magnetic flux of a highly asymmetric, hysteretic dc superconducting quantum interference device (SQUID) from 30 mK to 400 mK. The SQUID loop had an inductance of 1 nH, and the capacitances of the two Al-Al2O3-Al Josephson junctions comprising the SQUID were 100 fF and 5 fF. As expected, the swit...

The spectral density SΦ(f)=A2/(f/1 Hz)α of magnetic flux noise in ten dc superconducting quantum interference devices (SQUIDs) with systematically varied geometries shows that α increases as the temperature is lowered; in so doing, each spectrum pivots about a nearly constant frequency. The mean-square flux noise, inferred by integrating the power...

We have designed a biased bridge resonator (BBR), which allows us to probe amorphous dielectric films by simultaneously applying a quasi-static electric bias field in addition to a microwave electric field. The BBR is made with a bridge arrangement of capacitors using superconducting aluminum electrodes and operated at millikelvin temperatures. Mea...

To create a hybrid quantum system, we plan to trap neutral atoms in the evanescent optical field from an optical nanofiber and move them to within a few microns above a SQUID in a dilution refrigerator that operates at 10 mK. A key component in this experiment is a long section (10 cm) of optical fiber with a uniform diameter of about 500 nm, suffi...

We present progress toward a hybrid quantum system in which microwave quanta stored in a superconducting flux qubit are coupled through a magnetic dipole interaction to laser-trapped atoms. In initial experiments, our goal will be to couple a microfabricated superconducting LC resonator to the 6.835 GHz hyperfine splitting in an ensemble of ^87Rb a...

We have developed a tunable ``lumped-element" thin-film superconducting Al microwave resonator [1] and used it for measuring two level systems. The device is intended for coupling to the hyperfine splitting of trapped ^87Rb atoms at 6.83 GHz. By moving a superconducting Al pin towards the inductor of the resonator using a piezo stage, we can tune t...

We are developing a dual-tipped scanning tunneling microscope (STM) that operates at milliKelvin temperatures. The two tips can be connected and brought into tunneling with a superconducting sample to form a SQUID loop. Our scheme involves holding one of the tips fixed while the other is scanned to image spatial variations in the gauge invariant ph...

Spectroscopy of an Al/AlOx/Al dc SQUID phase qubit revealed peaks suggestive of dispersive photon shifts in a Jaynes-Cummings model, where the role of the resonator is played by an on-chip rf LC filter. A lumped element analysis of the filter-qubit system reveals qubit and resonator modes at the expected frequencies (330 MHz and 8.7 GHz) but an iso...

We have fabricated and tested an Al/AlOx /Al dc SQUID phase qubit on a sapphire substrate. The qubit is shunted by an interdigitated capacitor and isolated from the bias leads by an inductive isolation network using a larger Josephson junction. Additional high frequency filtering is provided by an on-chip LC filter which consists of square spiral i...

We have developed a design for a tunable transmon qubit with an on-chip flux bias. The transmon is fabricated with two sub-micron Al/AlOx/Al tunnel junctions and coupled to a superconducting planar lumped-element resonator. A coplanar transmission line provides flux coupling and tuning of the qubit's transition energies. We will discuss the design...

In the dressed atom picture, a three-level system can interact with two photons via the Autler-Townes (AT) effect, where the system exhibits two peaks separated by the generalized Rabi frequency of the coupling photon. The system can also exhibit electromagnetically induced transparency (EIT), where the first excited state is made transparent to th...

We have designed, fabricated, and measured a transmon made from a single Al/AlOx/Al Josephson-Junction on a sapphire substrate with f01˜ 5 GHz. The transmon was mounted in a 3D microwave cavity (OFHC copper, fc˜ 7.5GHz), similar to other recent experimentsfootnotetextPaik, H. et al. Phys. Rev. Lett. 107, 240501.^,footnotetextRigetti, C. et al. Phys...

For many types of superconducting qubits, magnetic flux noise is a source of pure dephasing. Measurements on a representative dc superconducting quantum interference device (SQUID) over a range of temperatures show that $S_\Phi(f) = A^2/(f/1 \hbox{Hz})^\alpha$, where $S_\Phi$ is the flux noise spectral density, $A$ is of the order of 1 $\mu\Phi_0 \...

Nanofibers are a promising tool for hybrid systems of atomtronics and quantum information. We present the construction and characterization protocol that allows us to reliably produce nanofibers with a waist up to 10 cm in length and down to 500 nm in diameter operating around 780 nm (Rb D2 line). By controlling the angle in the tapered region at c...

We have constructed a robust system for studying atom-light interactions in atomtronics and hybrid quantum information. We require the loading of atomic dipole traps formed on tapered optical nanofibers and other photonic structures from magneto-optical traps. A commercially available UHV manipulator allows for controlled translation of the structu...

We have developed a frequency tuning system for a ``lumped-element'' thin-film superconducting Al microwave resonator [1] on sapphire intended for coupling to hyperfine ground states of cold trapped ^87Rb atoms, which are separated by about fRb=6.83 GHz. At T=12 mK and on resonance at 6.81 GHz, the loaded quality factor was 120,000. By moving a car...

We have investigated the loss mechanisms of the TE101 mode (resonant frequency f0= 8 GHz) of a superconducting Al microwave cavity. The internal quality factor Qint of the cavity has been measured for a range of temperatures from 23 mK to 360 mK in a low photon number regime and from 360 mK to Tc˜ 1.1 K in a high photon number regime, both with and...

Measurements of low frequency magnetic flux noise in dc SQUIDs demonstrate a spectral density Sφ(f)=A^2/f^α in which the magnitude A scales only weakly with the washer geometry and typically 0.5<α<1. An analytical model assuming non-interacting spins localized to the surface of the SQUID loop predicts that A^2R/W for R/W1. Here, R and W are the out...

We report on the design, fabrication and measurement of an Al/AlOx/Al transmon qubit coupled to a quasi-lumped element superconducting resonator. Our resonator, which has a resonant frequency of 5.4,GHz, and a loaded quality factor Ql 30,000 is, in turn, coupled to a transmission line. The qubit is designed to have EJ/Ec> 30 to significantly decrea...

We report loss in a thin-film dc electric-field tunable LC resonator built with superconducting aluminum and silicon nitride dielectric. To measure the loss we continually apply microwave power on resonance and monitor the transmitted power. At milli-Kelvin temperatures, loss is limited by two-level systems in the dielectric which are saturated wit...

Recently, Kim et al. have reported that the interaction of a Cooper-pair box (CPB) with discrete charge fluctuators can decrease the relaxation time (T1) of the first excited state of a CPB when operating the CPB near the transition frequency of a charged two-level system (TLS).ootnotetextZ. Kim et al., Physical Review B, 78 144506 (2008). Using a...

It is known that amorphous dielectrics are a major source of decoherence in superconducting qubits due to energy absorption by two-level systems coupled to the electric fields. Linear resonators have been applied extensively to study loss in different dielectrics used in qubit circuits due to their versatility and relative simplicity in design, fab...

Superconducting quantum information circuits use various amorphous dielectrics for capacitors, and alumina is the ubiquitous barrier material for Josephson junctions within these devices. The exposure of the devices to air allows water molecules to penetrate the dielectric films along grain boundaries, and become adsorbed onto internal surfaces. In...

We describe a thin-film superconducting Nb microwave resonator, tunable to within 0.3 ppm of the hyperfine splitting of $^{87}$Rb at $f_{Rb}=6.834683$ GHz. We coarsely tuned the resonator using electron-beam lithography, decreasing the resonance frequency from 6.8637 GHz to 6.8278 GHz. For \emph{in situ} fine tuning at 15 mK, the resonator inductan...

We present a scheme to couple trapped $^{87}$Rb atoms to a superconducting flux qubit through a magnetic dipole transition. We plan to trap atoms on the evanescent wave outside an ultrathin fiber to bring the atoms to less than 10 $\mu$m above the surface of the superconductor. This hybrid setup lends itself to probing sources of decoherence in sup...

We examine the transmission through nonideal microwave resonant circuits. The general analytical resonance line shape is derived for both inductive and capacitive coupling with mismatched input and output transmission impedances, and it is found that for certain non-ideal conditions the line shape is asymmetric. We describe an analysis method for e...

One important application of scanning SQUID microscopes is fault detection in integrated circuits and multi-chip modules. However, the present generation of computer processors operate at over 1 GHz, well above the bandwidth of the present generation of SQUID microscopes. Towards this end, we present results on a cryo-cooled 4.2 K scanning SQUID mi...

We have observed anomalous switching curves (s-curves) in a dc SQUID phase qubit. The SQUID has two Al/AlO_x/Al Josephson junctions in a 1.5 nH loop. One junction as a phase qubit and the other acts as a detector junction. The qubit junction area is 2 μm², the critical current is 0.08 μA and the junction has an added low-loss S...

The loss in superconducting microwave resonators at low-photon numbers and low temperatures is not well understood but has implications for achievable coherence times in superconducting qubits. We have fabricated single-layer resonators with a high quality factor by patterning a superconducting aluminum film on a sapphire substrate. Four resonator...

We present an experimental investigation of lumped-element superconducting LC resonators designed to provide different types of coupling to a transmission line. We have designed four resonator geometries including dipole and quadrupole configured inductors connected in parallel with low-loss SiN_x dielectric parallel-plate capacitors. The...

We examine the behavior of low frequency flux noise measured in ten Nb-NbO_x-Pbln dc SQUIDs in the temperature range 1 K to 20 mK. As previously reported, the flux noise power spectrum S_Φ(f) typically scales with frequency as A/f^α. Remark ably, the excess noise power A at 1 Hz increases as the temperature is decrease...

We fabricated a dc SQUID phase qubit with a sub- μm Al/AlO_x/Al qubit junction and an interdigitated shunting capacitor on a sapphire substrate. The qubit junction had a critical current of 135 nA, and the isolation junction had a critical current of 8.3 μA. The shunting capacitance was about 1.5 pF. To reduce the unwanted effects of two-...

Recent proposals in quantum computing center on the creation of hybrid quantum processors. Here we report progress on an experiment to couple an ensemble of ^87Rb atoms to a superconducting, thin-film LC resonator at 20 mK through a magnetic dipole transition. We have demonstrated tuning of the LC resonator to within 2 kHz of the 6.8GHz hyperfine s...

We present a scheme and our advances to magnetically couple 87Rb atoms to a superconducting flux qubit by trapping the atoms in the evanescent wave outside an tapered optical fiber.

Low-temperature properties of PECVD SiNx dielectric films are measured within the capacitor of superconducting LC resonators. Experiments are made at temperatures from 30 to 300 mK, and at storage energies from 1 to 10^6 photons in a resonant cavity. While the power and temperature dependence of the loss agrees with two-level system (TLS) theory ab...

Our system consists of an Al/AlOx/Al Cooper-pair box (CPB) charge qubit coupled to a lumped element resonator, which in turn is coupled to a transmission line. From the measured Rabi frequency, for a given microwave frequency f and amplitude in the transmission line, we can extract the coupling of qubit to the transmission line. We observe an order...

We present data on the dephasing properties of our Al/AlOx/Al Cooper-pair box (CPB) qubit. The CPB had a charging energy EC/h = 6.25 GHz and a maximum EJ/h = 19 GHz which was decreased by an external magnetic field to an effective EJ/h of 6.1 GHz. The qubit was capacitively coupled to a lumped element microwave resonator (f0= 5.446 GHz, QL= 1.8x10^...