# Research News

## Uprooting defects to enable high-performance III–V optoelectronic devices on silicon

A new defect elimination strategy in highly mismatched heteroepitaxy is demonstrated to achieve ultra-low dislocation density using a highly scalable process.

## Checking the outputs of quantum computers

As the first generation of quantum computers are now reaching the point where they can answer such otherwise impossible questions, it is necessary to consider how the answers of these early devices can be confirmed correct. The full power of quantum computing includes a wide range of problems for which this is not possible, instead demanding new techniques to test quantum computers. A new test has now been proposed by Samuele, Theodoros, and Animesh (Quantum Information Science) which can be used to make sure the quantum computer is working correctly without using excessive additional resources while still testing the entire quantum computer. Published in the New Journal of Physics, this protocol uses circuits which have the same form as the desired circuit but are formulated to give known outcomes. Based on the accuracy of these circuits they are able to place a statistical bound on how close the distribution the quantum computer gives is to the correct distribution.

## Efficiently evaluating the fundamental limit to quantum sensing

New work from the Quantum Information Science group, now published in Physical Review Letters, demonstrates that the Holevo Cramér-Rao bound, the fundamental limit to how precise any sensor can be, can be evaluated by numerically efficient methods. Computation of the Holevo Cramér-Rao bound requires the solving of a non-linear optimisation problem. In this publication Francesco, Jamie, and Animesh demonstrate that the necessary optimisation can be expressed as a convex optimisation problem. This realisation allows efficient numerical evaluation of the Holevo Cramér-Rao bound, opening up the possibility of practically applying it in quantum sensing problems.

## Spin-selective Aharonov-Casher caging in a topological quantum network

Quantum systems can often be found to exhibit wave-like properties. As such, matter waves are known to interfere just as water waves, leading to regions of destructive interference - an absence of matter - while in regions of constructive interference, matter can become "localized". However, this behavior is not only true for matter properties of quanta, but also for their other intrinsic properties such a spin: certain spin projections can be shown to vanish while others remain measurable: this is the basis of so-called spin filters. In a recent paper, we show that this behaviour can also be found in systems with higher spins such as spin-1, spin-3/2 and so on. In Phys. Rev. B 100, 161108(R), we demonstrate how this allows the construction of spin filters and, indeed, spin "cages" in which certain spin projections remain "imprisoned" and can no longer contribute to transport. Such studies show the many surprises one is expecting for so-called spintronics devices.

## Knot your regular crystal of atoms

Topology has played a central role in modern physics. New phases of matter and phase transitions (*1*, *2*), as well as electronic band theory (*3*), are understood in terms of topological concepts. On page 1449 of this issue, Tai and Smalyukh (*4*) harness topology to create a fundamentally new type of crystal, built with knots tied in a chiral fluid (see the figure). They used a liquid crystal doped with a chiral molecule, which caused all of the molecules to rotate like a corkscrew along a preferred direction, the helical axis. Using electric fields, they created vortex lines in the helical axis and tied them into knots that act like “atoms” but on the micrometer scale. Different knotted particles were created by careful illumination with laser tweezers, and their interactions were tuned so that they spontaneously assembled into two- or three-dimensional lattices.

Gareth Alexander's paper appears in *Science, *Vol. 365, Issue 6460, pp. 1377

DOI: 10.1126/science.aaz0479

## Understanding CP violation in B+ → π+π+π− decays

The LHCb collaboration has, this week, published long-awaited results on matter-antimatter asymmetries in B+ → π+π+π− decays, explaining the curious variation of the asymmetry across the phase space.