Reversible Fluxon Logic With Optimized CNOT Gate Components

Kevin D. Osborn and Waltraut Wustmann

For the paper "Reversible Fluxon Logic With Optimized CNOT
Gate Components"

IEEE Transactions on Applied Superconductivity ( Volume: 31, Issue: 2, March 2021, Paper: 1300213)

Reversible logic gates were previously implemented
in superconducting circuits as adiabatic-reversible gates, which are
powered with a sufficiently slow clock. In contrast, we are studying
ballistic-reversible gates, where fluxons serve to both encode the
information and power the gates. No power is applied to the gate
apart from the energy of the input fluxons, and the two possible
flux polarities represent the bit states. Undamped long Josephson
junctions (LJJs), where fluxons move at practically constant speed
from inertia, form the input and output channels of the gates.
LJJs are connected in the gates by circuit interfaces, which are
designed to allowthe ballistic scattering frominput to output fluxon
states, using the temporary excitation of a localized mode. The
duration of the resonant scattering determines the operation time
of the gate, approximately a few Josephson plasma periods. Due to
the coherent conversions between fluxon and localized modes, the
ballistic gates can be very efficient: In our simulations, only a few
percent of the fluxon’s energy are dissipated in the gate operation.
Ballistic-reversible gates can be combined with other nonballistic
gate circuits to extend the range of gate functionalities. Here, we
describe how the CNOT can be built as a structure that includes the
Identity-else-Same-gives-NOT (IDSN) and store-and-launch (SNL)
gates. The IDSN is a 2-b ballistic gate, which we describe and
analyze in terms of equivalent 1-b circuits. The SNL is a clocking
gate, which allows the storage of a bit and the clocked launch of a
fluxon on a bit-state-dependent output path. In the CNOT, the SNL
gates provide the necessary routing and fluxon synchronization for
the input to the IDSN gate.

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