Title: Multi-time correlators in continuous measurement of qubit observables,
Abstract: We consider multi-time correlators for output signals from linear detectors,
continuously measuring several qubit observables at the same time. Using the
quantum Bayesian formalism, we show that for unital (symmetric) evolution in
the absence of phase backaction, an $N$-time correlator can be expressed as a
product of two-time correlators when $N$ is even. For odd $N$, there is a
similar factorization, which also includes a single-time average. Theoretical
predictions agree well with experimental results for two detectors, which
simultaneously measure non-commuting qubit observables.