Circuit-enhanced spin flips

The spontaneous emission of a photon by an excited-state atom or molecule may seem like a process inherent to the material system. But it depends just as much on the vacuum into which the photon is emitted. Specifically, the transition probability is proportional to the density of final photon states of the right energy. Since the 1980s, researchers have exercised control over atomic spontaneous emission by placing the atoms in an optical cavity: Transitions near a cavity resonance are enhanced, whereas others are suppressed (see the article by Serge Haroche and Daniel Kleppner, Physics Today, January 1989, page 24 ). Now Patrice Bertet , Audrey Bienfait (both at the Atomic Energy Commission in Saclay, France), and their collaborators have demonstrated the same effect in a different system: the electron spins in bismuth-doped silicon. Rather than a closed cavity, the researchers used a micron-scale LC circuit on the sample’s surface, which alters the density of photon states in a similar way. At low temperatures, each Bi nucleus binds an unpaired electron and splits its spin states by 7.4 GHz, near the circuit’s resonant frequency; applying a weak magnetic field tunes the transition energy into and out of resonance. Without the circuit, the spins have a spontaneous emission lifetime of 10 000 years—although they can relax in half an hour via nonradiative mechanisms, such as coupling to phonons. With the circuit, the researchers can reduce the relaxation time to as little as a third of a second. Although specific applications may be a long way off, enhanced spontaneous emission may find use in initializing a register of spins used as qubits. (A. Bienfait et al., Nature, in press, doi:10.1038/nature16944 .)