Quasiparticle Decoherence in d-wave Superconducting Qubits

TL;DR

This paper challenges the belief that gapless quasiparticles in d-wave superconductors inevitably cause severe decoherence in qubits, showing that phase-dependent conductance and midgap states can allow quantum effects to be observed.

Contribution

It introduces a self-consistent linear response formalism demonstrating that quasiparticle conductance in d-wave qubits can be phase-dependent, enabling quantum coherence under certain conditions.

Findings

Quasiparticle conductance is strongly phase dependent.

Midgap states and nodal quasiparticles influence conductance and decoherence.

Quantum behavior may be observable with appropriate d-wave junction parameters.

Abstract

It is usually argued that the presence of gapless quasiparticle excitations at the nodes of the d-wave superconducting gap should strongly decohere the quantum states of a d-wave qubit, making quantum effects practically unobservable. Using a self-consistent linear response non-equilibrium quasiclassical formalism, we show that this is not necessarily true. We find quasiparticle conductance of a d-wave grain boundary junction to be strongly phase dependent. Midgap states as well as nodal quasiparticles contribute to the conductance and therefore decoherence. Quantum behavior is estimated to be detectable in a qubit containing a d-wave junction with appropriate parameters.