Andrea Morello and colleagues report a demonstration of Bell’s inequality violation for a pair of qubits embedded in silicon. Bell’s inequality was introduced by physicist John Stewart Bell during the 1960s. A violation of the inequality by a pair of quantum objects placed far apart from one another is a demonstration of the nonlocal nature of quantum entanglement and of the impossibility of describing such entanglement by classical mechanics. Bell’s inequality violation has been demonstrated before, so in this sense the work by Morello and co-workers is not conceptually new. What matters, however, is the degree to which the violation occurs. The amount by which the two qubits (represented by the electron spin and the nuclear spin associated with a single phosphorus donor in silicon) violate Bell’s inequality is close to the theoretical limit. The results are, therefore, a direct demonstration that quantum entanglement can be created reliably in silicon solid-state devices, which are the basic components of existing technology.
Bell’s theorem sets a boundary between the classical and quantum realms, by providing a strict proof of the existence of entangled quantum states with no classical counterpart. An experimental violation of Bell’s inequality demands simultaneously high fidelities in the preparation, manipulation and measurement of multipartite quantum entangled states. For this reason the Bell signal has been tagged as a single-number benchmark for the performance of quantum computing devices. Here we demonstrate deterministic, on-demand generation of two-qubit entangled states of the electron and the nuclear spin of a single phosphorus atom embedded in a silicon nanoelectronic device. By sequentially reading the electron and the nucleus, we show that these entangled states violate the Bell/CHSH6 inequality with a Bell signal of 2.50(10). An even higher value of 2.70(9) is obtained by mapping the parity of the two-qubit state onto the nuclear spin, which allows for high-fidelity quantum nondemolition measurement (QND) of the parity. Furthermore, we complement the Bell inequality entanglement witness with full two-qubit state tomography exploiting QND measurement, which reveals that our prepared states match the target maximally entangled Bell states with over 96% fidelity. These experiments demonstrate complete control of the two-qubit Hilbert space of a phosphorus atom, and show that this system is able to maintain its simultaneously high initialization, manipulation and measurement fidelities past the single-qubit regime.
Another advance in quantum technology is improved magnetic sensing properties in nitrogen–vacancy center in diamond
Ronald Hanson and colleagues report the improved magnetic sensing properties of the spin of an electron associated with a nitrogen–vacancy (NV) center in diamond. The electron spin around an NV centre is protected from environmental magnetic noise and can be used to monitor external magnetic fields. But the sensitivity of these measurements depends on the state of the spin before the measurements take place. Hanson and colleagues applied an adaptive protocol, which uses the results of subsequent measurements to initialize the state of the spin in an iterative way. Once again, the sensing properties of NV centres are known, and so are adaptive protocols. But the results show to what extent the sensitivity can be improved and demonstrate the ability of NV centres to measure fast varying magnetic fields, both important technological achievements.