Quantum leakage detection using a model-independent dimension witness
Armands Strikis, Animesh Datta, George C. Knee · Physical Review A · 2019
Users of quantum computers must be able to confirm they are functioning as intended, even when they are remotely accessed. In particular, if the Hilbert space dimension of the quantum computer components is not as advertised---for instance, if they suffer leakage---errors can ensue and protocols may be rendered insecure. We refine the method of delayed vectors, adapted from classical chaos theory to quantum systems, and apply it remotely on the IBMQ platform---a quantum computer composed of transmon qubits. The method witnesses, in a model-independent fashion, dynamical signatures of higher-dimensional processes. We present evidence, under mild assumptions, that the IBMQ transmons suffer state leakage, with a $p$ value no larger than $5\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}$ under a single-qubit operation. We also estimate the number of shots necessary for revealing leakage in a two-qubit system.