Coupled Superconductors

J. Aarts, A. A. Golubov, V. A. Oboznov +3 more

John Lawrence Stevens

Martin Leijnse, Karsten Flensberg

We show how superconductors can be used to couple, initialize, and read out spatially separated spin qubits. When two single-electron quantum dots are tunnel coupled to the same superconductor, the singlet component of the two-electron state partially leaks into the superconductor via crossed Andreev reflection. This induces a gate-controlled singlet-triplet splitting which, with an appropriate superconductor geometry, remains large for dot separations within the superconducting coherence length. Furthermore, we show that when two double-dot singlet-triplet qubits are tunnel coupled to a superconductor with finite charging energy, crossed Andreev reflection enables a strong two-qubit coupling over distances much larger than the coherence length.

Roberto D. Merlin

Marcin Mierzejewski, J. Zieliński, P. Entel

Naoki Takeuchi, Kota Arai, Nobuyuki Yoshikawa

Abstract Adiabatic superconductor logic (ASL) families are energy-efficient because they can operate with a switching energy much less than the I cΦ0 product, where I c is the critical current for Josephson junctions, Φ0 is the flux quantum, and I cΦ0 is generally on the order of 10–19 J. A switching energy of 0.03I cΦ0 has been demonstrated by adiabatic quantum-flux-parametron (AQFP) logic, which indicates the possibility of extremely energy-efficient digital circuits using ASL. ASL uses signal transformers to propagate and invert the signal current; otherwise, it is difficult to design inverters. However, signal transformers are generally difficult to miniaturize. Therefore, to realize high-density and energy-efficient superconductor circuits, ASL that does not use signal transformers is required. In this paper, we propose ASL without signal transformers, which we call directly coupled quantum-flux-parametron (DQFP). DQFP logic is based on AQFP, but it can invert signal current without using signal transformers. We conducted numerical simulation of a DQFP inverter chain to show that DQFP logic can invert signal current without signal transformers, and that DQFP logic can operate adiabatically. Then, we designed and fabricated basic DQFP circuits, such as an inverter chain and a full adder. All the circuits were found to have wide operating margins. Our results indicate that DQFP logic is suitable for high-density and energy-efficient superconductor circuits.