Evanescent-wave Johnson Noise from Superconductors
Abstract
We compute the evanescent-wave Johnson noise (EWJN) in the vacuum half-space above a superconductor, and the resulting relaxation time (T1) of spin and charge qubits placed at nanometer distances from the surface. The electromagnetic response is described by a single microscopic transverse current-response kernel $Q(q, ω)$ for a BCS superconductor. This is computed for varying densities of both non-magnetic impurities and magnetic impurities, for arbitrary frequency and temperature and for wave vectors q ≪ kF (the Fermi wavevector). When combined with the fluctuation-dissipation theorem and the nonlocal surface impedances of the half-space, this yields the magnetic and electric field noise at any distance z ≫ kF−1 from the surface, from which we obtain T1. Just below Tc the magnetic noise is enhanced relative to the normal state by the coherence (Hebel-Slichter-type) peak of the dissipative conductivity and drops exponentially at lower temperatures; the electric noise shows no coherence peak. The theory predicts that there is a zero-temperature noise floor induced by magnetic impurities. In the gapless regime produced by pair breaking, the finite subgap density of states ν(0) yields a temperature-independent noise spectral density and a relaxation rate bounded by T1−1(T)≤[ν(0)/νF]2 T1,N−1(T) for T≪ Tc, with equality in the extreme nonlocal regime. Here ν(0) and νF are the superconducting and normal-state densities of states at the Fermi energy, and T1,N(T) is the relaxation time the same electrode would produce in its normal state at the same temperature.
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BibTeX
@misc{mallubhotla2026evanescentwave,
title = {{Evanescent-wave Johnson Noise from Superconductors}},
author = {Hruday Mallubhotla and Gustav Romare and Ilya Esterlis and Maxim Vavilov and Robert Joynt and Alex Levchenko},
howpublished = {arXiv},
year = {2026},
month = sep,
doi = {10.48550/arxiv.2609.36497},
eprint = {2609.36497},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2609.36497},
}
RIS
TY - UNPB
TI - Evanescent-wave Johnson Noise from Superconductors
AU - Hruday Mallubhotla
AU - Gustav Romare
AU - Ilya Esterlis
AU - Maxim Vavilov
AU - Robert Joynt
AU - Alex Levchenko
PY - 2026
DA - 2026/09/29
PB - arXiv
DO - 10.48550/arxiv.2609.36497
UR - https://arxiv.org/abs/2609.36497
AB - We compute the evanescent-wave Johnson noise (EWJN) in the vacuum half-space above a superconductor, and the resulting relaxation time (T_1) of spin and charge qubits placed at nanometer distances from the surface. The electromagnetic response is described by a single microscopic transverse current-response kernel $Q(q, ω)$ for a BCS superconductor. This is computed for varying densities of both non-magnetic impurities and magnetic impurities, for arbitrary frequency and temperature and for wave vectors q ≪ k_F (the Fermi wavevector). When combined with the fluctuation-dissipation theorem and the nonlocal surface impedances of the half-space, this yields the magnetic and electric field noise at any distance z ≫ k_F^−1 from the surface, from which we obtain T_1. Just below T_c the magnetic noise is enhanced relative to the normal state by the coherence (Hebel-Slichter-type) peak of the dissipative conductivity and drops exponentially at lower temperatures; the electric noise shows no coherence peak. The theory predicts that there is a zero-temperature noise floor induced by magnetic impurities. In the gapless regime produced by pair breaking, the finite subgap density of states ν(0) yields a temperature-independent noise spectral density and a relaxation rate bounded by T_1^−1(T)≤[ν(0)/ν_F]^2 T_1,N^−1(T) for T≪ T_c, with equality in the extreme nonlocal regime. Here ν(0) and ν_F are the superconducting and normal-state densities of states at the Fermi energy, and T_1,N(T) is the relaxation time the same electrode would produce in its normal state at the same temperature.
ER -