A single excitation in a quantum world doesn't have to make a decision whether to stay or go. Daniel Whiting and colleagues from the Joint Quantum Centre in Durham, United Kingdom have demonstrated that an excitation can be simultaneously stored in two groups of atoms in a thermal atomic vapour, allowing a single photon to both ‘stay’ in stationary atoms, and to ‘go’ with a moving group of atoms.
Atomic vapours usually have a complex, multi-level atomic structure. Instead of trying to prepare a well-defined initial state of such a system, the researchers realized a new coherent control scheme by isolating atoms in selected states. A strong magnetic field and strong laser dressing enables only two atomic velocity groups to be excited, with well-defined relative phase. Spontaneous emission then heralds the storage of a single collective excitation of the two velocity groups; in the lab frame, one group is stationary while the other is moving.
The coherent nature of the storage is demonstrated by observing Doppler-beating of the single photon simultaneously emitted from the two atomic-velocity groups. The collective nature of the excitation allows readout of the photons in a well-defined direction. The demonstrated interferometric measurement scheme offers possibilities for coherent state manipulation at the single-photon level in atomic vapour cells, and a way of generating tuneable bi-chromatic photons.
The research is published in Physical Review Letters 118, 253601 (2017).