Quantum computing is entering a new era of remotely-accessible quantum machines and, given their recent development, computation is more than likely accompanied by errors. One such error—quantum leakage—is an often-overlooked imperfection that amounts to quantum information escaping from the desired computational space and whose presence is rarely identified by a remote user. In work published in Physical Review A (DOI:https://doi.org/10.1103/PhysRevA.99.032328) Armands, Animesh, and George adapt one of dimension witness protocols designed for the purpose of a remote discovery of leakage and equip it with statistically robust, user-defined confidence levels before applying to a remotely accessed quantum processor. They find a circuit component "transmon" acting in a higher computational space than advertised.

Their study constitutes the first, model-independent experimental discovery of leakage in a remotely-accessed quantum computer. They have achieved this by a substantive theoretical development of the method of delays, originally adopted from classical chaos theory and proposed for quantum systems in a path-breaking paper almost a decade ago.

Such finding confirms the imperfection of current quantum computers, but at the same, guides the engineers to small step improvements that would eventually lead to the ultimate goal of fault-tolerant quantum computation.