Heralded amplification for precision measurements with spin ensembles

We propose a simple heralded amplification scheme for small rotations of the collective spin of an ensemble of particles. Our protocol makes use of two basic primitives for quantum memories, namely, partial mapping of light onto an ensemble, and conversion of a collective spin excitation into light. The proposed scheme should be realizable with current technology, with potential applications to atomic clocks and magnetometry.

Figure 1

Proposed implementation of heralded amplification with an ensemble of three-level systems. (a) Level structure and transitions for the write and read processes. Thin arrows correspond to the write and read photons, whereas thick arrows signify the write and read laser beams. (b) Schematic setup. The atomic ensemble (AE) is originally in the small-amplitude coherent state $|\alpha \rangle$. An auxiliary photon produced by the single-photon source (SPS) is partially stored in the ensemble with the help of the write beam, where $t$ is the amplitude for the storage to occur. The state of the ensemble is subsequently read out with the help of the read beam. If a read photon is detected in the single-photon detector (SPD), then the transmitted light is in the amplified coherent state $|\alpha /t\rangle$, which can be measured by homodyne detection (HD).