Marc J. Feldman

Prospects for quantum coherent computation using superconducting electronics

We discuss the prospects and challenges for implementing a quantum computer using superconducting electronics. It appears that Josephson junction devices operating at milli-Kelvin temperatures can achieve a quantum dephasing time of milliseconds, allowing quantum coherent computations of 10/sup 10/ or more steps. This figure of merit is comparable to that of atomic systems currently being studied for quantum computation.

Multiparameter optimization of RSFQ circuits using the method of inscribed hyperspheres

Rapid Single Flux Quantum (RSFQ) circuits must be designed with large operating margins because of variations in the fabrication process. We describe a routine to optimize the yield, rather than the critical margin, of RSFQ circuits, based on the method of inscribed hyperspheres. The operating region is mapped out with a minimum of circuit simulations taking into account the relative size of the process variations. The method is effective for as many as nine parameters. We show that the standard optimization of critical margin will often give a seriously wrong result for multidimensional parameter space.<<ETX>>

Timing of Multi-Gigahertz Rapid Single Flux Quantum Digital Circuits

Rapid Single Flux Quantum (RSFQ) logic is a digital circuit technology based on superconductors 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. Despite the many fundamental differences between RSFQ and semi- conductor logic at the device and at the circuit level, timing of large scale digital circuits in both technologies is principally governed by the same rules and constraints. Therefore, RSFQ offers a new perspective on the timing of ultra-high speed digital circuits.This paper is intended as a comprehensive review of RSFQ timing, from the viewpoint of the principles, concepts, and language developed for semiconductor VLSI. It includes RSFQ clocking schemes, both synchronous and asynchronous, which have been adapted from semiconductor design methodologies as well as those developed specifically for RSFQ logic. The primary features of these synchronization schemes, including timing equations, are presented and compared.In many circuit topologies of current medium to large scale RSFQ circuits, single-phase synchronous clocking outperforms asynchronous schemes in speed, device/area overhead, and simplicity of the design procedure. Synchronous clocking of RSFQ circuits at multigigahertz frequencies requires the application of non-standard design techniques such as pipelined clocking and intentional non-zero clock skew. Even with these techniques, there exist difficulties which arise from the deleterious effects of process variations on circuit yield and performance. As a result, alternative synchronization techniques, including but not limited to asynchronous timing, should be considered for certain circuit topologies. A synchronous two-phase clocking scheme for RSFQ circuits of arbitrary complexity is introduced, which for critical circuit topologies offers advantages over previous synchronous and asynchronous schemes.

Tools for the computer-aided design of multigigahertz superconducting digital circuits

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.

A technique for noise measurements of SIS receivers

We present a simple new technique to determine the noise temperature of the RF input section of a superconducting quasiparticle heterodyne receiver. This quantity is difficult to measure by existing methods. The new technique uses standard hot/cold-load measurements, and the precision should be as good as the hot/cold-load determination of receiver noise temperature. For most receivers, correction terms will be much smaller than the quantum temperature /spl planck//spl omega//k. >

Quantum computation with untunable couplings.

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.

Nonlinear characteristics of T-branch junctions: Transition from ballistic to diffusive regime

Nonlinear electrical characteristics of nanostructured T-branch junctions (TBJs) made of two-dimensional electron gas in an InGaAs∕InAlAs heterostructure were studied by a systematic variation of both the device size and the operating temperature. We have found that two distinct mechanisms are responsible for the electronic transport in TBJs and their resulting nonlinear characteristics, namely, the nonlinear ballistic effect at low applied voltages and the intervalley transfer at high voltages. Detailed experimental analysis for each mechanism and their contributions with respect to the TBJ’s nanochannel length and operating temperature are discussed.

Error rate of a superconducting circuit

The bit‐error rate of the Josephson junction single‐flux‐quantum comparator was measured as a function of bias current offset, at clock rates up to 10 GHz. The bit‐error rate versus offset is a smooth curve, measured over 16 decades of incidence, which linearly extrapolates to 10−49 for optimal bias. The lowest rate actually measured was 5×10−17, corresponding to 4 errors counted in 130 h.

A clock distribution scheme for large RSFQ circuits

A primary issue in maximizing the performance of large scale synchronous digital systems is the clock distribution scheme. We present 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 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 parameter variations in present day superconductive technologies.<<ETX>>

Optoelectronic generation and detection of single‐flux‐quantum pulses

We report on the direct observation of a single‐flux‐quantum (SFQ) pulse. The response of a metal–semiconductor–metal photodiode to a femtosecond laser pulse was used to switch Josephson junctions and to generate an SFQ voltage pulse on a superconducting microstrip line. The detailed shape of the pulse was measured optoelectronically, using a cryogenic electro‐optic sampling system. The measured SFQ pulse had a width of 3.2 ps, an amplitude of 0.67 mV, and a total pulse content of 2.1±0.2 mV×ps, corresponding to the quantum of magnetic flux h/2e. With larger excitation, multiple SFQ pulses were observed. Numerical simulations are shown to be qualitatively similar to our experimental results.