Fig. 4: Measuring OTOC(2) in the classically challenging regime.
From: Observation of constructive interference at the edge of quantum ergodicity
a, \({{\mathcal{C}}}_{\text{off-diag}}^{(4)}\) measured on a set of 65-qubit circuits each having t = 23 cycles. Inset, Qubit geometry. B acts simultaneously on three different qubits. b, Experimental SNRs for circuits measured with system sizes ranging from 18 to 40 qubits. Error bars correspond to the 95% confidence interval of an empirical error model (Supplementary Information sections II.F.3 and II.F.4). Error bars in a are based on the same empirical error model. c, Estimated time to compute \({{\mathcal{C}}}_{\text{off-diag}}^{(4)}\) of a single circuit in a on the Frontier supercomputer using tensor-network contraction. The estimate was obtained by running a specially designed optimization algorithm46,47,48,49 on 20 Google Cloud virtual machines (totalling 1,200 CPUs) up to a period of 24 h (x axis). Estimates using a publicly available library cotengra47 lead to costs that are ten times higher after the same optimization time. TNCO, tensor-network contraction.
