Frog eyes show prowess as quantum detectors

The human eye, once acclimated to darkness, can detect a burst of light containing as few as a hundred photons. That strongly suggests that rod cells—the retinal photoreceptors that specialize in night vision—can detect single quanta of light; even after accounting for focusing, it’s unlikely that any two photons from so faint a flash would arrive at the same one of the roughly 120 million rods that line the inner eye. Now Leonid Krivitsky and coworkers at the Agency for Science, Technology and Research in Singapore have tested that proposition using rods plucked from the eyes of Xenopus frogs. The experimental setup is pictured here: Photons are delivered via an optical fiber to a rod held in a pipette, where electrodes are used to record spikes in the cell’s transmembrane potential. To generate pulses containing just a single photon, Krivitsky and company exploit a phenomenon known as spontaneous parametric down conversion. They shine UV light into a nonlinear crystal, which occasionally spits out a pair of visible photons. One of those photons is steered to an avalanche photodiode, where the photon’s detection triggers the opening of a shutter that allows the second photon to pass to the waiting rod. The team’s results indicate that an impinging photon has a roughly 30% chance of eliciting a spike—an efficiency comparable with that of commercially available single-photon detectors. (N. M. Phan et al., Phys. Rev. Lett., in press.)