of the driven cavity system [@Kamenev2011book; @Nazarov2003], with a relevance that goes beyond the analysis of just a single measurement setup. Further, this is the ordering that is “chosen" by our qubit: if one simply interprets the qubit dephasing as arising from classical noise, then the Keldysh ordered bispectrum (with its imaginary part) plays the role of the bispectrum of this effective classical noise.

[*Conclusions–* ]{} We have shown how the Keldysh approach to quantum noise provides a meaningful way to define the polyspectra of non-classical, non-Gaussian noise. In the experimentally-relevant case of photon shot noise fluctuations in a driven-damped resonator, the quantum bispectrum reveals distinct quantum features and a surprising quantum-induced breaking of detailed balance. We stress that our approach amounts to interpreting the dephasing of a qubit by quantum noise as arising from an effective classical noise process. As such, the same noise spectroscopy techniques that have been used successfully to measure classical bispectra with qubits [@Viola2016; @Oliver2019] can be directly used (without modification) to measure our quantum bispectra.

This work was supported as part of the Center for Novel Pathways to Quantum Coherence in Materials, an Energy Frontier Research Center funded by the U.S. Department of Energy, 