In nuclear magnetic resonance (NMR), oscillating magnetic fields drive changes in atoms’ nuclear spins. However, NMR isn’t localized enough to manipulate only a single nuclear spin in a tightly spaced array of other, identical spins; such specific manipulation is an important goal for future quantum computers and electromagnetic sensors. Now Andrea Morello and his team at the University of New South Wales in Australia have developed a technique for manipulating the spin of a single atom using an electric field produced at the tip of a nanometer-sized electrode.
The suggestion that electric fields could precisely control nuclear spins dates to the 1960s. Nuclear electric resonance experiments since then have achieved transitions of large numbers of nuclear spins in a bulk crystal. Morello and his colleagues started by investigating the possibility of single-spin control using NMR. They used a magnetic antenna to probe a single antimony atom embedded in silicon. But during the experiment, they realized that high power had damaged the microwave antenna and transformed it into an open circuit. As a result, the antenna was focusing a strong electric field directly on the Sb atom. Having realized that they were manipulating the nucleus electrically, the researchers applied an oscillating voltage to a metal electrode positioned directly above the Sb atom to further improve the performance. Simulations revealed that the field distorted the atomic bonds around the Sb nucleus and caused it to reorient itself.
Because electric fields decay rapidly with distance from the electrode and can be screened easily, Morello’s technique allows control that is localized enough to manipulate different nuclei individually. In quantum computing applications, that level of control could offer the precision to drive one qubit without affecting the other in a two-qubit gate, which is the fundamental building block of a scalable quantum computer. (S. Asaad et al., Nature 579, 205, 2020.)
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