Coherent Control of Single-Photon Absorption and Reemission in a Two-Level Atomic Ensemble

We demonstrate coherent control of single-photon absorption and reemission in a two-level cold atomic ensemble. This is achieved by interfering the incident single-photon wave packet with the emission (or scattering) wave. For a photon with an exponential growth waveform with a time constant equal to the excited-state lifetime, we observe that the single-photon emission probability during the absorption can be suppressed due to the perfect destructive interference. After the incident photon waveform is switched off, the absorbed photon is then reemitted to the same spatial mode as that of the incident photon with an efficiency of 20%. For a photon with an exponential decay waveform with the same time constant, both the absorption and reemission occur within the waveform duration. Our experimental results suggest that the absorption and emission of a single photon in a two-level atomic ensemble may possibly be manipulated by shaping its waveform in the time domain.

Figure 1

A single photon propagates through a two-level atomic ensemble. ${\psi }_{\mathrm{in}}$ and ${\psi }_s$ denote the incident and elastically scattered single-photon wave packets.

Figure 2

(a) Experimental setup. The ${}^{85}\mathrm{Rb}$ energy levels are $|1⟩=|5S_{1/2},F=2⟩$, $|2⟩=|5S_{1/2},F=3⟩$, $|3⟩=|5P_{1/2},F=3⟩$, and $|4⟩=|5P_{3/2},F=3⟩$. (b) Two-photon coincidence counts collected for 900 s with 1 ns bin width between the detectors $D_1$ and $D_2$, with no atoms in MOT2. $\tau$ is the relative time between the anti-Stokes photon detection time $t_2$ at $D_2$ and the Stokes trigger time $t_1$ at $D_1$. Plot (1) is measured without the EOM modulation. Plots (2) and (3) are the exponential growth and decay waveforms shaped by the EOM, respectively.

Figure 3

Photon coincidence counts (between the detectors $D_1$ and $D_2$) after the heralded anti-Stokes photons with an exponential growth waveform pass through the two-level atomic ensemble at (a) $\mathrm{OD}=0$ (vacuum), (b) $\mathrm{OD}=3$, and (c) $\mathrm{OD}=8$. The exponential growth waveform in (a) has a time constant of 26.5 ns. The insets (d) and (e) show the measured conditional autocorrelation $g_c^{(2)}$ as functions of coincidence window width.

Figure 4

Photon coincidence counts (between the detectors $D_1$ and $D_2$) after the heralded anti-Stokes photons with an exponential decay waveform pass through the two-level atomic ensemble at (a) $\mathrm{OD}=0$ (vacuum), (b) $\mathrm{OD}=3$, and (c) $\mathrm{OD}=8$. The exponential decay waveform in (a) has a time constant of 26.5 ns. The insets (d) and (e) show the measured conditional autocorrelation $g_c^{(2)}$ as functions of coincidence window width.