# Evanescent-wave Johnson Noise in small devices

## TL;DR

This paper develops a theoretical framework for evanescent-wave Johnson noise affecting qubits near metallic structures, providing analytical results for various geometries and discussing implications for quantum decoherence.

## Contribution

It introduces a simplified classical electrodynamics approach to calculate local noise spectra for different geometries, enhancing understanding of qubit decoherence mechanisms.

## Key findings

- Analytical expressions for noise spectral density near cylindrical conductors
- Results for dipole-like noise sources and geometries
- Comparison with experimental decoherence times

## Abstract

In many quantum computer architectures, the qubits are in close proximity to metallic device elements. The fluctuating currents in the metal give rise to noisy electromagnetic fields that leak out into the surrounding region. These fields are known as evanescent-wave Johnson noise. The noise can decohere the qubits. We present the general theory of this effect for charge qubits subject to electric noise and for spin and magnetic qubits subject to magnetic noise. A mapping of the quantum-mechanical problem onto a problem in classical electrodynamics simplifies the calculations. The focus is on relatively simple geometries in which analytical calculations can be done. New results are presented for the local noise spectral density in the vicinity of cylindrical conductors such as small antennae, noise from objects that can be treated as dipoles, and noise correlation functions for several geometries. We summarize the current state of the comparison of theory with experimental results on decoherence times of qubits. Emphasis is placed on qualitative understanding of the basic concepts and phenomena.