Single-shot qubit readout typically combines high readout contrast with
long-lived readout signals, leading to large signal-to-noise ratios and high
readout fidelities. In recent years, it has been demonstrated that both readout
contrast and readout signal lifetime, and thus the signal-to-noise ratio, can
be enhanced by forcing the qubit state to transition through intermediate
states. In this work, we demonstrate that the sub-Poissonian relaxation
statistics introduced by intermediate states can reduce the single-shot readout
error rate by orders of magnitude even when there is no increase in
signal-to-noise ratio. These results hold for moderate values of the
signal-to-noise ratio (S≲100) and a small number of
intermediate states (N≲10). The ideas presented here could have
important implications for readout schemes relying on the detection of
transient charge states, such as spin-to-charge conversion schemes for
semiconductor spin qubits and parity-to-charge conversion schemes for
topologically protected Majorana qubits.Comment: 10 pages, 6 figures. Two appendices have been added. This version is
close to the final published versio