  Realistic quantum sensors face a trade-off between the number of sensors
measured in parallel and the control and readout fidelity ($F$) across the
ensemble. We investigate how the number of sensors and fidelity affect
sensitivity to continuous and intermittent signals. For continuous signals, we
find that increasing the number of sensors by $1/F^2$ for $F<1$ always recovers
the sensitivity achieved when $F=1$. However, when the signal is intermittent,
more sensors are needed to recover the sensitivity achievable with one perfect
quantum sensor. We also demonstrate the importance of near-unity control
fidelity and readout at the quantum projection noise limit by estimating the
frequency components of a stochastic, intermittent signal with a single trapped
ion sensor. Quantum sensing has historically focused on large ensembles of
sensors operated far from the standard quantum limit. The results presented in
this manuscript show that this is insufficient for quantum sensing of
intermittent signals and re-emphasizes the importance of the unique scaling of
quantum projection noise near an eigenstate.
