Extracting Coherence Information From Random Circuits Using “Speckle Purity Benchmarking”

Budgeting the contributions of coherent and incoherent noise sources is an important component of benchmarking quantum gates. Typically, methods such as Cross Entropy Benchmarking (XEB) or Randomized Benchmarking are used to measure an error-per-gate that includes noise and control errors. These sequences can be extended to quantify the decay of a quantum state due to noise only by measuring the state purity with tomography as described in previous publications. Here, we introduce a method that allows us to extract the same information with exponentially fewer sequences from raw XEB data. We introduce “Speckle Purity Benchmarking” which quantifies the purity via the contrast (or “speckliness!”) of output bitstring probabilities. Pure quantum states generated by the XEB procedure will have high contrast, while incoherent mixtures will have low contrast. Compared to conventional XEB, this procedure can be done with zero information about the actual quantum process. Additionally, this can be scaled to a handful of qubits.