Scalable noisy quantum circuits for biased-noise qubits

In this work, we consider biased-noise qubits affected only by bit-flip errors, which is motivated by existing systems of stabilized cat qubits. This property allows us to design a class of noisy Hadamard tests involving entangling and certain non-Clifford gates, which can be conducted reliably with only a polynomial overhead in algorithm repetitions. On the flip side, we also found classical algorithms able to efficiently simulate both the noisy and noiseless versions of our specific variants of the Hadamard test. We propose to use these algorithms as a benchmark of the biasness of the noise at the scale of large circuits. The bias being checked on a full computational task makes our benchmark sensitive to crosstalk or time-correlated errors, which are usually invisible from individual gate tomography. For realistic noise models, phase-flip will not be negligible, but in the Pauli-Twirling approximation, we show that our benchmark could check the correctness of circuits containing up to 10⁶ gates, several orders of magnitude larger than circuits not exploiting a noise-bias. Our benchmark is applicable for an arbitrary noise-bias, beyond Pauli models.