Quantum electrodynamics in a semiconductor vacuum

An isolated atom is, in many regards, the quintessential quantum system, but it interacts only weakly with its electromagnetic environment. As described by cavity quantum electrodynamics (QED), however, atom–photon interactions can be manipulated by placing an atom in a highly reflective optical cavity (see the article by Serge Haroche and Daniel Kleppner, Physics Today, January 1989, page 24 ). With modern nanofabrication techniques, the coupling of an artificial atom, such as a quantum dot or superconducting qubit, to a nearby transmission-line resonator or other microwave circuit can analogously be engineered, in what’s been dubbed circuit QED (see Physics Today, November 2004, page 25 , and the article by J. Q. You and Franco Nori, November 2005, page 42 ). Although many properties of artificial atoms can be readily tuned, the quantum states typically have short coherence times. Now Andrea Morello and colleagues at the University of New South Wales in Sydney, Australia, report a promising new scheme: a lone phosphorus-31 atom embedded in an isotopically purified substrate of silicon-28 and magnetically coupled to a superconducting niobium resonator (the figure shows the cross section). With no net nuclear or electron spin, the ²⁸Si substrate behaves like a magnetic vacuum, isolating the ³¹P atom’s unpaired electron and enabling exceptionally long coherence times. The researchers show that for experimentally realizable configurations, the coupling between the spin and the microwave circuit should be strong enough to allow coherent control and nondemolition measurement of the spin state. (G. Tosi et al., AIP Advances 4, 087122, 2014, doi:10.1063/1.4893242 .)