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abstract: 'Having accurate tools to describe non-classical, non-Gaussian environmental fluctuations is crucial for designing effective quantum control protocols. 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.'
author:
- 'Yu-Xin Wang'
- 'A. A. Clerk'
bibliography:
- 'refFCS.bib'
title: 'Spectral characterization of non-Gaussian quantum noise: Keldysh approach and application to photon shot noise'
---

[*Introduction–* ]{}An accurate description of environmental fluctuations is crucial for quantum information processing and quantum control. While it is common to assume noise that is both classical and Gaussian, there are many physically-relevant situations where these assumptions fail [@Cywinski2014; @Viola2014; @Viola2016; @Oliver2019; @Ramon2019]. Understanding how to usefully characterize non-Gaussian, non-classical noise in a frequency-resolved manner could enable the design of more optimal dynamical decoupling protocols, enhancing qubit coherence. It could also provide fundamental 