Estimating the coherence of noise in mid-scale quantum systems
While the power of quantum computers is commonly acknowledged to rise
exponentially, it is often overlooked that the complexity of quantum noise
mechanisms generally grows much faster. In particular, quantifying whether the
instructions on a quantum processor are close to being unitary has important
consequences concerning error rates, e.g., for the confidence in their
estimation, the ability to mitigate them efficiently, or their relation to
fault-tolerance thresholds in error correction. However, the complexity of
estimating the coherence, or unitarity, of noise generally scales exponentially
in system size. Here, we obtain an upper bound on the average unitarity of
Pauli noise and develop a protocol allowing us to estimate the average
unitarity of operations in a digital quantum device efficiently and feasibly
for mid-size quantum systems. We demonstrate our results through both
experimental execution on IQM Spark (TM), a 5-qubit superconducting quantum
computer, and in simulation with up to 10 qubits, discussing the prospects for
extending our technique to arbitrary scales.
Pedro Figueroa-Romero Miha Papič Adrian Auer Inés de Vega