Single Photon Vision: A Quantum Optics Lens on the Human Eye.

Can we see a single photon? Rod photoreceptor cells are known to respond to this infinitesimal stimulus but whether it persists through neural circuits to elicit conscious perception is a question yet unanswered. While previous studies have been firmly rooted in the classical realm and plagued by observer bias, we harness a true heralded single-photon source to probe the visual system. The heralded photon is generated with Type II spontaneous parametric down-conversion (SPDC) at 505 nm, the peak wavelength of the rod’s spectral sensitivity, and is channeled toward a trained observer in our two-alternative forced-choice experimental design. This procedure minimizes false positive results in both observer and detector by arbitrarily choosing the direction of emanation and including trials without a photon entirely. The subject answers where they ‘saw’ the photon each time and statistically significant correct identification of the photon’s location suggests single-photon sensitivity in the observer. If we can consciously perceive one photon, cascading questions ask the precise location of wavefunction-collapse in the visual system, potentially tackled by sending superposition states of light to the eye, and the possibility of human observers acting in novel tests of nonlocality, reconciling quantum mechanics and human perception.