Squeezed noise due to two-level system defects in superconducting resonator circuits

TL;DR

This paper presents a quantum theoretical model explaining noise and squeezing phenomena in superconducting resonators caused by two-level system defects, aligning with recent experimental observations.

Contribution

It introduces a fully quantum model incorporating TLS fluctuations to explain noise squeezing and power-dependent phase noise in superconducting resonators.

Findings

Predicts noise squeezing in amplitude quadrature

Shows noise enhancement in phase direction away from resonance

Suggests TLS fluctuations are essential for accurate modeling

Abstract

Motivated by recent surprising experimental results for the noise output of superconducting microfabricated resonators used in quantum computing applications and astronomy, we develop a fully quantum theoretical model to describe quantum dynamics of these circuits. Building on theoretical techniques from quantum optics, we calculate the noise in the output voltage due to two-level system (TLS) defects. The theory predicts squeezing for the noise in the amplitude quadrature with respect to the input noise, which qualitatively reproduces the noise ellipse observed in experiment. We show that noise enhancement along the phase direction persists for pump frequencies away from resonance. Our results also suggest that intrinsic TLS fluctuations must be incorporated in the model in order to describe the experimentally observed dependence of the phase noise on input power.