Soft Decoding of a Qubit Readout Apparatus

TL;DR

This paper demonstrates that incorporating soft information from analog signals in qubit readout significantly improves quantum state detection and estimation, even with realistic noise and small qubit numbers.

Contribution

It introduces a method to utilize soft information in qubit readout, leading to notable improvements in quantum state detection and estimation.

Findings

Enhanced state detection with quantum repetition code

Improved quantum state and parameter estimation

Advantages persist despite non-Gaussian noise and small qubit numbers

Abstract

Qubit readout is commonly performed by thresholding a collection of analog detector signals to obtain a sequence of single-shot bit values. The intrinsic irreversibility of the mapping from analog to digital signals discards soft information associated with an \emph{a posteriori} confidence that can be assigned to each bit value when a detector is well characterized. Accounting for soft information, we show significant improvements in enhanced state detection with the quantum repetition code as well as quantum state or parameter estimation. These advantages persist in spite of non-Gaussian features of realistic readout models, experimentally relevant small numbers of qubits, and finite encoding errors. These results show useful and achievable advantages for a wide range of current experiments on quantum state tomography, parameter estimation, and qubit readout.