Slowing down single photons

Lene Vestergaard Hau

Kali Wilson, Bethany Little, Genevieve Gariepy +3 more

Slow-light media are of interest in the context of quantum computing and enhanced measurement of quantum effects, with particular emphasis on using slow light with single photons. We use light-in-flight imaging with a single-photon avalanche diode camera array to image in situ pulse propagation through a slow-light medium consisting of heated rubidium vapor. Light-in-flight imaging of slow-light propagation enables direct visualization of a series of physical effects, including simultaneous observation of spatial pulse compression and temporal pulse dispersion. Additionally, the single-photon nature of the camera allows for observation of the group velocity of single photons with measured single-photon fractional delays greater than 1 over 1 cm of propagation.

Shengwang Du

Kali Wilson, Bethany Little, Genevieve Gariepy +3 more

Summary form only given. Slow light has been explored for building quantum networks, with particular interest in slowing the group velocity of single photons [1], and more recently exploited to enhance the measurement of small phase shifts. Generally, slow-light effects have been characterized as the net effect of a pulse propagating through the slow-light medium, i.e., as a pulse delay time Δt measured with a fast photodiode at the output of the medium [2]. In this work, we use a single-photon imaging camera to observe slow light in situ, and thus provide a direct measurement of spatial pulse compression and temporal dispersion as the pulse travels through the slow light medium, in this case a hyperfine absorption doublet in hot Rb vapor. Our method combines light-in-flight imaging techniques with a camera comprised of an array of single-photon avalanche diodes (SPAD camera) [3] to image the photons scattered by the Rb vapor in the direction of the camera as shown in Fig. 1(a). In addition, the single photon nature of the SPAD detector allows us to obtain a measurement of the single photon group velocity. As shown in Fig. 1(c) and (d), we observe a significant delay, on the order of nanoseconds, in the detection of the photons scattered when the pulse first enters the slow-light medium. This lag in scattered-photon arrival time is a direct visualization of the slowing down of the single-photon group velocity. The pulses used here had a temporal full width at half maximum (FWHM) of τ ~1 ns, with measured group velocities as low as vg~ 0.006c. At these low group velocities we observe a full fractional pulse delay of up to F D = Δt/τ ~ 40 over 7 cm of propagation, and F D ~ 5 for the scattered single photons, which propagate through ~ 1 cm of Rb vapor prior to exiting the cell en-route to the camera.

Zhifan Zhou, Zhongzhong Qin, Yami Fang +4 more

We demonstrate photodetection with single-photon level sensitivity using slow light through a four-wave mixing process with high dispersion and high gain in hot rubidium vapor. The delay times are dependent on the few-photon input states.

Axel Kuhn