VASILI SEMENOV
Research Professor
Physics and Astronomy
vasili.semenov@stonybrook.edu | (631)-632-8931, Physics B-108
Research Group Website

Biography
I graduated from Moscow State University. At the beginning of my career, I was affiliated with Moscow State University and with two industrial institutions. The last three decades I have been affiliated with Stony Brook University.

Research Statement
I am working in superconductor electronics. This broad area is attracting attention due to its various unique features. In particular, circuits of RSFQ logic/memory family keep world record for clock/sampling frequency of digital circuits. Circuits of nSQUID logic family demonstrated the lowest specific energy dissipation about k B T per logic operation. The third unique feature of superconductor electronics is presentation and processing information by single flux quanta. The motion of these quanta can be described as propagation of very short voltage pulses. Such pulses are exotic for conventional electronics but they are inherent for neuromorphic circuits emulating behavior of neuron networks. This is very exiting fact especially if we note that the pulses propagate along the circuits with speeds close to the speed of light. Our inputs to these areas of superconductor electronics are illustrated by three attached paper abstracts.

For a full list of my publications, see here.

Highlighted Publications

RSFQ logic/memory family: A new Josephson-junction technology for sub-terahertz-clock-frequency digital systems, Konstantin K Likharev, Vasili K Semenov.

Recent developments of the rapid single-flux-quantum (RSFQ) circuit family are reviewed. Elementary cells of the family can generate, pass, memorize, and reproduce picosecond voltage pulses V (t) with nominally quantized area Jv (t) dt=~ 0, corresponding to transfer of a single magnetic flux quantum~ o= h/2e across a Josephson junction. Functionally, each cell can be viewed as a combination of a logic gate and an output latch (register) controlled by clock pulses, which are physically similar to the signal pulses. Hand-shaking style of local exchange by the clock pulses enables one to increase complexity of the LSI RSFQ systems without loss of operating speed. The simplest components of the RSFQ circuitry have been experimentally tested at clock frequencies exceeding 100 GHz, and an increase of the speed beyond 300 GHz is expected as a result of using an up-to-date fabrication technology. The review includes a discussion of possible future developments and applications of this novel, ultrafast digital technology.

Progress with physically and logically reversible superconducting digital circuits, Jie Ren, Vasili K Semenov.

We continue to develop a new Superconductor Flux Logic (SFL) family based on nSQUID gates with fundamentally low energy dissipation and the ability to operate in irreversible and reversible modes. Prospective computers utilizing the new gates can keep conventional logically irreversible architectures. In this case the energy dissipation is limited by fundamental thermodynamic laws and could be as low as a few kBT s per logic operation. Highly exotic and less practical logically and physically reversible circuit architectures are more attractive for us because they enable a reduction of the specific energy dissipation well below the thermodynamic threshold kBT ln2. The reversible option is of interest to us because we can then experimentally demonstrate that all technical mechanisms of the energy dissipation could be cut below the fundamental thermodynamic limit. In other words, we like to set the energy dissipation record for all conventional digital technologies that (if measured in kBT ) is about one million times below the best figures achieved in commercially available semiconductor circuits. Besides, we believe that diving below the thermodynamic threshold would have impressive scientific and philosophical im pacts. In the paper we introduce a new timing belt clocking scheme and present new circuits. While we still work with test circuits, some of them contain two 8-stage shift registers, one with direct and the other with inverted outputs. The energy dissipation per nSQUID gate per bit measured at 4 K temperature is already below the thermodynamic threshold. We are confident that we passed through the critical phase of the project and we simply need more time to make more sophisticated circuits. The extremely low energy dissipation converts our circuits into a natural candidate to support circuitry for any sensors operating at milli-Kelvin temperatures.

BioSFQ Circuit Family for Neuromorphic Computing: Bridging Digital and Analog Domains of Superconductor Technologies, Vasili K Semenov, Evan B Golden, Sergey K Tolpygo.

Superconductor single flux quantum (SFQ) technology is attractive for neuromorphic computing due to low energy dissipation and high, potentially up to 100 GHz, clock rates. We have recently suggested a new family of bioSFQ circuits (V.K. Semenov et al., IEEE TAS, vol. 32, no. 4, 1400105, 2022) where information is stored as a value of current in a superconducting loop and transferred as a rate of SFQ pulses propagating between the loops. This approach, in the simplest case dealing with positive numbers, requires single-line transfer channels. In the more general case of bipolar numbers, it requires dual-rail transfer channels. To address this need, we have developed a new comparator with a dual-rail output. This comparator is an essential part of a bipolar multiplier performing an X⋅Y operation on two analog currents X and Y. The multiplier has been designed, fabricated, and tested. We also present bioSFQ circuits for implementing an analog bipolar divide operation Y/X and a square root operation √X . We discuss strategic advantages of the suggested bioSFQ approach, e.g., an inherently asynchronous character of bioSFQ cells which do not require explicit clock signals. As a result, bioSFQ circuits are free of racing errors and tolerant to occasional collision of propagating SFQ pulses. This tolerance is due to the stochastic nature of data signals generated by comparators operating within their gray zone. The circuits were fabricated in the eight-niobium-layer fabrication process SFQ5ee developed for superconductor electronics at MIT Lincoln Laboratory.