Spectral Compression of Narrowband Single Photons with a Resonant Cavity

We experimentally demonstrate a spectral compression scheme for heralded single photons with narrow spectral bandwidth around 795 nm, generated through four-wave mixing in a cloud of cold ${}^{87}\mathrm{Rb}$ atoms. The scheme is based on an asymmetric cavity as a dispersion medium and a simple binary phase modulator, and can be, in principle, without any optical losses. We observe a compression from 20.6 MHz to less than 8 MHz, almost matching the corresponding atomic transition.

Figure 1

Concept of spectral compression. The top row shows temporal intensity profiles $|\psi (t){|}^2$ in various stages of the spectral compression, the bottom row the corresponding power spectra $|\mathrm{\Psi }(\omega ){|}^2$. The initial pulse is dispersed by a cavity, leading to a new temporal shape, but an unchanged spectrum. An electro-optical modulator (EOM) manipulates the phase ${\varphi }^{\prime }(t)$ of the pulse which leads to a narrower spectrum.

Figure 2

Schematic setup for generation and spectral compression of heralded single photons. $D_{S,I}$: single-photon detectors; EOM: electro-optical modulator; PBS: polarizing beam splitter; QWP: quarter-wave plate; IF: interference filter. Inset: energy level scheme for four-wave mixing in ${}^{87}\mathrm{Rb}$.

Figure 3

Detection time distribution for the heralded photon before (open blue circles) and after (filled red dots) the dispersion cavity. We fit an exponential decay Eq. (6) to the initial time correlation (blue solid line), from which we infer the photon bandwidth ${\mathrm{\Gamma }}_p$. The simulated temporal profile after the photon passed through the dispersion cavity [red line, calculated from Eq. (2)], matches our experimental data (filled dots) well.

Figure 4

Spectral profile of heralded photons before (blue) and after (red) spectral compression, obtained by measuring the photon transmission rate through the Fabry-Perot cavity at different cavity detunings. The solid lines are calculated from Eq. (5), with ${\mathrm{\Gamma }}_p$ inferred from the temporal envelope measurement of the photons from the source, and ${\mathrm{\Gamma }}_c$ by experimentally characterizing the cavity bandwidth. Shaded areas cover 50% of the total power for each spectrum.