Slow Light Beam Splitter

We demonstrate a slow light beam splitter using rapid coherence transport in a wall-coated atomic vapor cell. We show that particles undergoing random and undirected classical motion can mediate coherent interactions between two or more optical modes. Coherence, written into atoms via electromagnetically induced transparency using an input optical signal at one transverse position, spreads out via ballistic atomic motion, is preserved by an antirelaxation wall coating, and is then retrieved in outgoing slow light signals in both the input channel and a spatially-separated second channel. The splitting ratio between the two output channels can be tuned by adjusting the laser power. The slow light beam splitter may improve quantum repeater performance and be useful as an all-optical dynamically reconfigurable router.

Figure 1

Schematic of slow light beam splitter operation. An optical signal pulse enters an atomic EIT medium in Channel 1 together with a strong optical control field. Rapid atomic motion in a wall-coated cell with no buffer gas distributes atomic coherence throughout the medium. Output optical signal pulses leave the medium in both Channel 1 and a spatially-separated Channel 2 defined by the position of a second control field. The propagation direction of the two control fields, and hence the two output signal pulses, need not be the same.

Figure 2

Schematic of the apparatus used in the coated cell slow light beam splitter experiments. (See text for details.)

Figure 3

Example measurements of EIT spectra in Chs. 1 and 2, demonstrating steady-state beam splitter operation. (a) Both the broad pedestal and narrow peak of the coated-cell EIT line shape are visible in the Ch. 1 output with large Ch. 1 input control power of $600\mu \mathrm{W}$. Only the narrow EIT structure is observed in Ch. 2 with the Ch. 2 control field on because coherence associated with the broad pedestal dephases before it is transferred from Ch. 1 to Ch. 2. Ch. 1 input signal power of $9\mu \mathrm{W}$; Ch. 2 input control power of $150\mu \mathrm{W}$. (b) Only the narrow EIT line shape structure is visible with weak Ch. 1 control power of $270\mu \mathrm{W}$ and other parameters as in (a). Note: frequency range $\simeq 100$-times smaller than in (a). (For both plots: linear scales for power; Ch. 2 background from control field leakage due to finite extinction ratio of PBS.)

Figure 4

Pulsed slow light beam splitter. (a) Signal pulses detected from each channel. With Ch. 2 control field on, a slow light pulse is read out in both channels. Input powers: Ch. 1 control $270\mu \mathrm{W}$, peak signal $9\mu \mathrm{W}$, Ch. 2 control $150\mu \mathrm{W}$. (b) Tunable slow light beam splitter. Transferred pulse percentage (area of Ch. 2 pulse normalized to area of Ch. 1 pulse without Ch. 2 light) can be optimized as a function of Ch. 2 power for fixed Ch. 1 powers. Dashed lines added to guide the eye.

Figure 5

Demonstration of phase coherent signal transport in the slow light beam splitter. For each channel, a small part of the control field is interfered with the signal field at the photodetectors such that the output signal amplitude is a measure of the phase difference between the signal and control fields. Near identical variation is observed in the two channel outputs as the phase difference between the input signal and control fields in Ch. 1 is scanned by nearly $2\pi$. Vertical offset in Ch. 1 and Ch. 2 output signals is caused by slightly different control field admixtures. Ch. 1 and Ch. 2 control fields $160\mu \mathrm{W}$. Input Ch. 1 probe field $13\mu \mathrm{W}$.