Spectral characterization of non-Gaussian quantum noise: Keldysh approach and application to photon shot noise

Yu-Xin Wang,A A Clerk

doi:10.1103/physrevresearch.2.033196

Abstract

Having accurate tools to describe non-classical, non-Gaussian environmental fluctuations is crucial for designing effective quantum control protocols and understanding the physics of underlying quantum dissipative environments. We show how the Keldysh approach to quantum noise characterization can be usefully employed to characterize frequency-dependent noise, focusing on the quantum bispectrum (i.e., frequency-resolved third cumulant). Using the paradigmatic example of photon shot noise fluctuations in a driven bosonic mode, we show that the quantum bispectrum can be a powerful tool for revealing distinctive non-classical noise properties, including an effective breaking of detailed balance by quantum fluctuations. The Keldysh-ordered quantum bispectrum can be directly accessed using existing noise spectroscopy protocols.

Highlights

An accurate description of environmental fluctuations is crucial for quantum information processing and quantum control
We show that the quantum bispectrum” (QBS) reveals important new physics and distinct quantum signatures: at low temperatures, qualitatively new features emerge that would never be present in a classical model with only thermal fluctuations
We show that the QBS is a generic tool for revealing the breaking of detailed balance and violation of Onsager-like symmetry relations

Summary

INTRODUCTION

An accurate description of environmental fluctuations is crucial for quantum information processing and quantum control. Ability distribution to describe the noise, and to assess whether the noise can be faithfully mimicked by completely classical noise processes [10,12] It has a direct operational meaning: the “quantum polyspectra” we introduce are exactly the quantities that contribute to the dephasing of a coupled qubit at each order in the coupling. These quantities can be measured using the same non-Gaussian noise spectroscopy techniques designed for classical noise sources [3,4,14]; one does not have to decide in advance whether the noise is classical or quantum to perform the characterization. We find that the photon shot noise QBS violates detailed balance at low temperatures

KELDYSH ORDERING AND QUANTUM POLYSPECTRA

QUANTUM NOISE MODEL

Drive-independent fluctuations

Driven fluctuations

IMAGINARY BISPECTRUM AND VIOLATIONS

CONCLUSIONS