Leakage in IBM's quantum computer
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 (who began this work as a part of his MPhys project), 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.
A study of chiral liquid-crystal droplets shows how defects lead to geometric arrangements similar to atoms in a molecule, and provides a new framework for analysing novel chiral materials.
We investigate a simple tight-binding Hamiltonian to understand the stability of spin-polarized transport of states with an arbitrary spin content in the presence of disorder. The general spin state is made to pass through a linear chain of magnetic atoms, and the localization lengths are computed. Depending on the value of spin, the chain of magnetic atoms unravels a hidden transverse dimensionality that can be exploited to engineer energy regimes where only a selected spin state is allowed to retain large localization lengths. Our results show that the spin filtering effect is robust against weak disorder and hence the proposed system should be a good candidate model for experimental realizations of spin-selective transport devices.
Room-temperature spin transport has now been shown in both pivotal semiconductor materials, Ge and Si, providing new opportunities for the future of semiconductor spintronics.