Publications (30)11.21 Total impact
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Article: Unshunted QOS Comparator for Qubit Readout
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ABSTRACT: State measurements of flux-based superconducting quantum bits (qubits) require the sensitive discrimination of small differences of magnetic flux, with measurement times short compared to the timescales of coherent quantum mechanical behaviour. The single-flux-quantum (SFQ) superconducting comparator may be the best device for this purpose. An SFQ comparator design using a quasi-one-junction SQUID (QOS) acting as the comparator junction has two advantages over earlier designs—increased sensitivity and the possibility to eliminate shunt resistors. Here we report the first measurements of the sensitivity of an unshunted QOS comparator. The gray zone of this device at 4.2K is 6 mΦo.Journal of Physics Conference Series 07/2006; 43(1):1413. -
Article: A simple circuit to supply constant flux biases for superconducting quantum computing
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ABSTRACT: We present and demonstrate experimentally a persistent-current bias loop that is intended to provide an on-chip stable dc magnetic field to flux-bias the flux qubits in a superconducting quantum computer. The bias loop is initialized to approximately the desired persistent current and then the external source is disengaged; this procedure prevents the fluctuations in the bias sources from causing decoherence in the qubit.Journal of Physics Conference Series 07/2006; 43(1):1397. -
Article: Scalable fault-tolerant quantum computation in DFS blocks
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ABSTRACT: We investigate how to concatenate different decoherence-free subspaces (DFSs) to realize scalable universal fault-tolerant quantum computation. Based on tunable $XXZ$ interactions, we present an architecture for scalable quantum computers which can fault-tolerantly perform universal quantum computation by manipulating only single type of parameter. By using the concept of interaction-free subspaces we eliminate the need to tune the couplings between logical qubits, which further reduces the technical difficulties for implementing quantum computation. Comment: 4 papges, 2 figures07/2004; -
Article: Dispersive Manipulation of Paired Superconducting Qubits
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ABSTRACT: We combine the ideas of qubit encoding and dispersive dynamics to enable robust and easy quantum information processing (QIP) on paired superconducting charge boxes sharing a common bias lead. We establish a decoherence free subspace on these and introduce universal gates by dispersive interaction with a LC resonator and inductive couplings between the encoded qubits. These gates preserve the code space and only require the established local symmetry and the control of the voltage bias. Comment: 5 pages, incl. 1 figure07/2003; -
Article: Superconducting Quantum Computing without Switches
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ABSTRACT: This paper presents a very simple architecture for a large-scale superconducting quantum computer. All of the SQUID qubits are fixed-coupled to a single large superconducting loop.12/2002; -
Article: Quantum computation with untunable couplings.
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ABSTRACT: Most quantum computer realizations require the ability to apply local fields and tune the couplings between qubits, in order to realize single bit and two bit gates which are necessary for universal quantum computation. We present a scheme to remove the necessity of switching the couplings between qubits for two bit gates, which are more costly in many cases. Our strategy is to compute with encoded qubits in and out of carefully designed interaction free subspaces analogous to decoherence free subspaces. We give two examples to show how universal quantum computation is realized in our scheme with local manipulations to physical qubits only, for both diagonal and off diagonal interactions.Physical Review Letters 12/2002; 89(19):197903. · 7.37 Impact Factor -
Article: Quantum computation with un-tunable couplings
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ABSTRACT: Most quantum computer realizations require the ability to apply local fields and tune the couplings between qubits, in order to realize single bit and two bit gates which are necessary for universal quantum computation. We present a scheme to remove the necessity of switching the couplings between qubits for two bit gates, which are more costly in many cases. Our strategy is to compute in and out of carefully designed interaction free subspaces analogous to decoherence free subspaces, which allows us to effectively turn off and turn on the interactions between the encoded qubits. We give two examples to show how universal quantum computation is realized in our scheme with local manipulations to physical qubits only, for both diagonal and off diagonal interactions. Comment: 5 pages, 2 figures09/2002; -
Article: A general framework of nondistortion quantum interrogation
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ABSTRACT: We present a general framework to study nondistortion quantum interrogation which preserves the internal state of the quantum object being detected. We obtain the necessary and sufficient condition for successful performing nondistortion interrogation for unknown quantum object when the interaction between the probe system and the detected system takes place only once. When the probe system and interrogation process have been limited we develop a mathematical frame to determine whether it is possible to realize NQI processes only relying on the choice of the original probe state. We also consider NQI process in iterative cases. A sufficient criterion for NQI is obtained.04/2002; -
Article: Nondistortion quantum interrogation using Einstein-Podolsky-Rosen entangled photons
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ABSTRACT: We propose a scheme for nondistortion quantum interrogation defined as an interaction-free measurement that preserves the internal state of the object being detected. In our scheme, two Einstein-Podolsky-Rosen entangled photons are used as the probe and polarization-sensitive measurements are performed at the four ports of the Mach-Zehnder interferometer. In comparison with the previous single-photon scheme, it is shown that the two-photon approach has a higher probability of initial state preserving interrogation of an atom prepared in a quantum superposition. In the case that the presence of the atom is not successfully detected, the experiment can be repeated since the initial state of the atom is unperturbed.Phys. Rev. A. 09/2001; 64(4). -
Article: High efficiency Nondistortion Quantum Interrogation of atoms in quantum superpositions
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ABSTRACT: We consider the nondistortion quantum interrogation (NQI) of an atom prepared in a quantum superposition. By manipulating the polarization of the probe photon and making connections to interaction free measurements of opaque objects, we show that nondistortion interrogation of an atom in a quantum superposition can be done with efficiency approaching unity. However, if any component of the atom's superposition is completely transparent to the probe wave function, a nondistortion interrogation of the atom is impossible.06/2001; -
Article: Design of an RSFQ Control Circuit to Observe MQC on an rf-SQUID
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ABSTRACT: We believe that the best chance to observe macroscopic quantum coherence (MQC) in a rf-SQUID qubit is to use on-chip RSFQ digital circuits for preparing, evolving and reading out the qubit's quantum state. This approach allows experiments to be conducted on a very short time scale (sub-nanosecond) without the use of large bandwidth control lines that would couple environmental degrees of freedom to the qubit thus contributing to its decoherence. In this paper we present our design of a RSFQ digital control circuit for demonstrating MQC in a rf-SQUID. We assess some of the key practical issues in the circuit design including the achievement of the necessary flux bias stability. We present an "active" isolation structure to be used to increase coherence times. The structure decouples the SQUID from external degrees of freedom, and then couples it to the output measurement circuitry when required, all under the active control of RSFQ circuits. Research supported in part by ARO grant # DAAG55-98-1-0367.03/2001; -
Article: A "Tipping Pulse" Scheme for rf-SQUID Qubits
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ABSTRACT: We introduce a technique to control the macroscopic quantum state of an rf SQUID qubit. We propose to employ a stream of single flux quantum (SFQ) pulses magnetically coupled to the qubit junction to momentarily suppresses its critical current. This effectively lowers the barrier in the double-well rf-SQUID potential thereby increasing the tunneling oscillation frequency between the wells. By carefully choosing the time interval between SFQ pulses one may accelerate the interwell tunneling rate. Thus it is possible to place the qubit into a chosen superposition of flux states and then effectively to freeze the qubit state. We present both numerical simulations and analytical time-dependent perturbation theory calculations that demonstrate the technique. Using this strategy one may control the quantum state of the rf-SQUID in a way analogous to the \pi pulses in other qubit schemes. Research supported in part by ARO grant # DAAG55-98-1-0367.03/2001; -
Article: Conditions for Nondistortion Interrogation of Quantum System
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ABSTRACT: Under some physical considerations, we present a universal formulation to study the possibility of localizing a quantum object in a given region without disturbing its unknown internal state. When the interaction between the object and probe wave function takes place only once, we prove the necessary and sufficient condition that the object's presence can be detected in an initial state preserving way. Meanwhile, a conditioned optimal interrogation probability is obtained. Comment: 5 pages, Revtex, 1 figures, Presentation improved, corollary 1 added. To appear in Europhysics Letters03/2001; -
Article: Nondistortion Quantum Interrogation using EPR entangled Photons
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ABSTRACT: We propose a novel scheme for nondistortion quantum interrogation (NQI), defined as an interaction-free measurement which preserves the internal state of the object being detected. In our scheme, two EPR entangled photons are used as the probe and polarization sensitive measurements are performed at the four ports of the Mach-Zehnder interferometer. In comparison with the previous single photon scheme, it is shown that the two photon approach has a higher probability of initial state preserving interrogation of an atom prepared in a quantum superposition. In the case that the presence of the atom is not successfully detected, the experiment can be repeated since the initial state of the atom is unperturbed. Comment: 4 pages, 2figures03/2001; -
Article: Toward a Systematic Design Methodology for Large Multigigahertz Rapid Single Flux Quantum Circuits
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ABSTRACT: Rapid single flux quantum (RSFQ) digital circuits have reached the level of medium- to large-scale of integration. At this level, existing design methodologies, developed specifically for RSFQ circuits, have become computationally inefficient. Applying mature semiconductor methodologies to the design of RSFQ circuits, one encounters substantial difficulties originating from the differences between both technologies. In this paper, a new design methodology aimed at large-scale RSFQ circuits is proposed. This methodology is based on a semiconductor semicustom design approach. An established design methodology for small-scale RSFQ digital circuits, based on circuit (junctionlevel) simulation and device parameter optimization, is used for the design of basic RSFQ cells. A library composed of about 20 basic RSFQ cells has been developed based on this approach. A novel design methodology for large-scale circuits, presented in this paper, is based on logic (gate-level) simulation and timing optimization. This methodology has been implemented around the Cadence integrated design environment and used successfully at the University of Rochester for the design of two large-scale digital circuits.01/2001; -
Article: Tools for the Computer-Aided Design
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ABSTRACT: The realization of large integrated circuits depends upon the application of computer-aided design (CAD) tools. This paper summarizes the results of a survey of CAD tools targeting superconducting digital electronics. Five categories of tools: circuit simulators, circuit optimizers, layout tools, inductance estimators, and logic simulators are discussed in detail. Within each category, a comparison of several currently available CAD tools is presented, and a tool which has been adapted for use or developed at the University of Rochester is discussed in greater detail. In addition, tools for timing analysis as well as integrated design environments that permit the effective data interchange among various tools and support libraries of design models are discussed. Future tools for timing optimization, automated logic synthesis, and automated layout synthesis are shown to be necessary for the design of superconducting circuits at the very large scale of integration (VLSI) level of integration. Trends regarding changes in the requirements for effective CAD tools are discussed, and expected improvements to existing tools and features of new tools currently under development are presented. Index Terms--- CAD, inductance extraction, layout, optimization, RSFQ, simulation, superconducting electronics.01/2001; -
Article: Experimental Investigation of Local Timing Parameter Variations in RSFQ Circuits
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ABSTRACT: Circuit parameter variations resulting from the fabrication process affect the timing parameters of Rapid Single Flux Quantum (RSFQ) digital circuits. This determines the maximum clock rate and the yield of the circuit. It is generally believed that the global parameter variations (target-to-wafer) are much more significant in this regard than the local parameter variations (on-chip), but there has been little experimental evidence for this. This experiment measures the distribution of local parameter variations of the timing parameter of RSFQ circuits. The experiment consists of a 10 by 10 matrix of nominally identical RSFQ "clock rings" covering an integrated circuit area, a total of 3500 Josephson junctions. Each ring is activated individually, and its frequency is measured with accuracy better than 1%. I. Introduction Fabrication-induced parameter variations are by far the limiting factor which determines the maximum clock rate of complex RSFQ circuits. The parameter variation pr...01/1999; -
Article: Timing Jitter and Bit Errors in a 64-Bit Circular Shift Register
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ABSTRACT: The bit-error rate of a 64-bit single-flux-quantum circular shift register, operating at a clock frequency of 10-16 GHz was measured. Error incidence depends on the values of the clock and data bias currents and on the clock frequency. Timing violation arising from thermal jitter is the dominant error mechanism. The jitter per JTL stage is estimated to be 340 fs based on the error rate data. This corresponds to a noise temperature of 10 K. I. INTRODUCTION D IGITAL ELECTRONICS based on the single flux quantum (SFQ) offers an appealing alternative to conventional transistor- based circuits. A feature of this technology is that the clock must be delivered to each basic logic element. These circuits typically have a regular systolic array architecture, in which the data flow through the circuit without feedback (see, for example, [2]). Circuits with recurrent data paths, in which the data is required to circulate around closed loops, are subject to more stringent timing constraints and...10/1998; -
Article: A Clock Distribution Scheme for Large RSFQ Circuits
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ABSTRACT: ãA primary issue in maximizing the performance of large scale synchronous digital systems is the clock distribution scheme. We present a novel clocking scheme, developed specifically for RSFQ logic, which is based on the concurrent flow of the clock and data signals. The scheme permits the circuit throughput to be independent of inter-cell connection delays and significantly reduces the dependence of the throughput on the clock-to-output delay of the cells. Concurrent flow clocking is particularly well suited for structured architectures. The simulated maximum clock frequency of an RSFQ decimation digital filter currently under development at the University of Rochester can be as much as seven times higher using concurrent-flow clocking rather than conventional (counterflow) clocking. This advantage, however, is reduced to a factor of two due to fabrication process parameter variations in present day superconductive technologies. I. INTRODUCTION Full exploitation of the speed of Rapid ...03/1998; -
Article: Choice Of The Optimum Timing Scheme For Rsfq Digital Circuits
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ABSTRACT: Rapid Single Flux Quantum (RSFQ) logic is a superconducting digital circuit technology that has emerged as a possible alternative to advanced semiconductor technologies for large scale ultra-high speed, very low power digital applications. Timing of RSFQ circuits at frequencies of tens to hundreds of gigahertz is a challenging and still unresolved problem. In many circuit topologies of current medium to large scale RSFQ circuits synchronous clocking outperforms asynchronous schemes in speed, device/area overhead, and simplicity of the design procedure. Circuit yield and performance of large synchronous circuits is limited by process parameter variations. As a result, alternative synchronization techniques, including but not limited to asynchronous timing, should be considered for certain circuit topologies. We discuss our methodology for the optimal choice of the timing scheme for medium to large RSFQ circuits. INTRODUCTION The recent achievements of superconductive circuits using Ra...03/1998;
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1996–2006
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University of Rochester
- Department of Electrical and Computer Engineering
Rochester, NY, USA
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