dephasing–* ]{} In what follows, we focus on the physics of the frequency-dependent third cumulant, the so-called quantum bispectrum (QBS) $S_2[\omega_1,\omega_2]$; we drop the subscript $2$ hereafter. The quantum non-Gaussian noise of interest will be the energy fluctuations of a driven-damped bosonic mode. As we will see, the frequency-dependent QBS reveals a host of physics that is not apparent if one only considers the low-frequency behaviour of fluctuations (as has been studied earlier [@Harris2010; @Clerk2011; @Clerk2016]).

Our “bath" here is a driven damped cavity mode $c$ (frequency $\omega_c$, Markovian energy decay rate $\gamma$). Coupling the photon number of this mode to an ancilla qubit, working in a rotating frame at the drive frequency, and letting $\hat{\rho}$ denote the qubit-cavity reduced density matrix, the system dynamics follows the master equation $$\dot {\hat \rho}  =  
    - i [ \hat H_0 + \hat H_{\mathrm{int}} (t) ,\hat \rho ] + \gamma ( {\bar n}_\mathrm{th} + 1 ) \mathcal{D} [ \hat c ]\hat \rho  + \gamma {\bar n}_\mathrm{th} \mathcal{D} [\hat c^\dag ]\hat \rho.
\label{Eq:meq}$$ Here $ \mathcal{D} [ \hat A  ]\hat \rho  = \hat A\rho {\hat A^\dag } -  ( {{\hat A^\dag }\hat A\hat \rho  + \hat \rho {\hat A^\dag }\hat A}  )/2$ is the 