A gated quantum dot strongly coupled to an optical microcavity.Nature 2019Nat
The strong-coupling regime of cavity quantum electrodynamics (cavity QED) represents the light-matter interaction at the fully quantum level. Adding a single photon shifts the resonance frequencies, a profound nonlinearity. Cavity QED is a test bed of quantum optics1-3 and the basis of photon-photon and atom-atom entangling gates4,5. At microwave frequencies, success in cavity QED has had a transformative effect6. At optical frequencies, the gates are potentially much faster; the photons can propagate over long distances and can be detected easily. Following pioneering work on single atoms1-3,7, solid-state implementations using semiconductor quantum dots are emerging8-15. We present here a gated, ultralow-loss, frequency-tunable microcavity device. The gates allow both the quantum-dot charge and its resonance frequency to be controlled electrically; crucially, they allow cavity feeding10,11,13-17 to be eliminated. Even in the microcavity, the quantum dot has a linewidth close to the radiative limit. In addition to a very pronounced avoided-crossing in the spectral domain, we observe a clear coherent exchange of a single energy quantum between the 'atom' and cavity in the time domain (vacuum Rabi oscillations). Decoherence arises predominantly via the atom and photon loss channels. The coherence is exploited to probe the transitions between the singly and doubly excited photon-atom system via photon-statistics spectroscopy18.