Interfacing Collective Atomic Excitations and Single Photons

We study the performance and limitations of a coherent interface between collective atomic states and single photons. A quantized spin-wave excitation of an atomic sample inside an optical resonator is prepared probabilistically, stored, and adiabatically converted on demand into a sub-Poissonian photonic excitation of the resonator mode. The measured peak single-quantum conversion efficiency of $\chi =0.84(11)$ and its dependence on various parameters are well described by a simple model of the mode geometry and multilevel atomic structure, pointing the way towards implementing high-performance stationary single-photon sources.

Figure 1

(a) Setup for the conditional generation of single photons using a sample of laser-cooled Cs atoms inside an optical resonator. (b) Level scheme for the system with hyperfine and magnetic sublevels $|F,m_F⟩$. The atomic sample is initially prepared in $|g⟩$ by optical pumping.

Figure 2

Conditional ($R_c$, solid circles) and unconditional ($R_u$, open squares) retrieval with model predictions versus intracavity write photon number $n_w$ at a write-read delay of 80 ns. The single-quantum conversion efficiency $\chi$ can also be obtained as the $y$-axis intercept of the linear fit to $R_c$ (solid line). Inset: Nonclassical write-read correlation $g_{wr}>2$ with model (solid line) and theoretical limit $g_{wr}\lesssim 1/n_w$ (dashed line).

Figure 3

Magnon-photon conversion efficiency $\chi$ versus read optical depth $N\eta$ at a write-read delay of 120 ns. The optical depth is extracted from the write scattering rate and known intensities and detunings. The dashed line shows the predicted conversion ${\chi }_0$ for a three-level system; the solid line is the prediction from a model including dephasing from additional excited states.

Figure 4

Conditional single-photon conversion efficiency $\chi$ versus the delay time between write and read pulses $\tau$. The two time scales, as apparent in the inset, are due to the superposition of a short- and a long-wavelength magnon in the standing-wave resonator.