Classical and quantum operation modes of the reversible Josephson-junction logic circuits

One of the most important features of the quantum computing circuits that distinguishes them from the traditional computational models is their full, physical and logical, reversibility at all stages of the calculations. This makes the large-scale architecture of potential quantum and classical reversible computers quite similar. The goal of this work is to analyze this similarity in the case of reversible superconducting logic circuits based on the recently suggested elementary gate, the nSQUID. Specifically, we present the results of numerical simulations and some limited measurements of a reversible 8-bit circular shift register fabricated at HYPRES, Inc., and working in the classical mode. The measured energy dissipation per bit operation per nSQUID is still about 20 times higher than the thermodynamic reversibility threshold of k(B)T In 2 at T approximate to 4 K. We also discuss qualitatively operation of the reversible circuits of galvanically-coupled nSQUIDs in the quantum mode which provides the basis for implementation of "flying" superconducting qubits. Such qubits have several attractive features for Josephson-junction quantum logic devices beyond the single- or two-qubit circuits.