QUANTUM IS IN THE EYE OF THE BEHOLDER FOCUS
QUANTUM LIGHT AND THE EYE Studies of the intensity threshold for human vision date back to the late 1800s, but fully reliable results were obtained much later, in the 1940s, through the pioneering work of Hecht et al. [ 4 ]. This research accounted for what the authors defined as“ the best physical and physiological conditions.” By using filtered light, both in wavelength and intensity, and paying meticulous attention to the calibration of absorptions in the eye media and rhodopsin, the authors established that“ to see, it is necessary for 1 quantum of light to be absorbed by 5 to 14 retinal rods” [ 4 ]. The technology available at that time did not allow for control over the photon number. Subsequent investigations demonstrated the capability of retinal cells to detect single photons, thanks to a combination of four factors highlighted in [ 5 ]: 1. high efficiency, 2. low noise, 3. significant amplification, and 4. reproducibility of the signal waveform.
The advent of quantum photonics now allows for the production of single photons with an excellent degree of approximation, and this has led to experiments conducted with superior control. In particular, Phan and coworkers reported in 2014 [ 6 ] that ex-vivo retinal cells from Xenopus toads could produce a detectable signal when illuminated with single photons at 532 nm – the reported value of the second-order correlation g( 2)( 0)= 0.08 corresponds to a multiphoton suppression by a factor 12 with respect to classical light of equivalent intensity. The signal from individual cells, collected by means of an electrode, confirmed the presence of those remarkable detection properties.
The mere presence of such a physiological capability does not imply that this is then actually used in the visual function. In fact, a complex arrangement of neural cells processes the signal emerging from the receptors before this is transmitted over the optical nerve, and then there is the active interpretation of the stimulus by the brain. The confirmation that humans can actually see single photons directly – i. e. that they can be perceived by the visual system – has been published in 2016 by Tinsley et al. [ 7 ]. The authors have asked subjects to give a yes / no answer upon the stimulation from a single-photon source and to rate their confidence in their response. The probability of seeing a photon was found to exceed the random guessing baseline( p see = 0.516 ± 0.010), reaching even higher significance for high-confidence events( p see = 0.60 ± 0.03).
Using sub-Poissonian statistics for investigating vision is an intriguing exploration, partly connected
QUANTUM CRYPTOGRAPHY AND QUANTUM METROLOGY to the question of whether and how quantumness is preserved in energy transduction in biosystems [ 8 ]. This however barely strokes the surface of the potential quantum photonics may hold for these investigations. There exists a much richer toolbox that can be put to good use. The eye can be considered as a quantum detector, thus its description can be elaborated in terms of detector tomography. This idea was first presented by van der Reep et al., considering the domain of the intensity response [ 9 ]. They showed that, even taking in due consideration the limitations of subjective responses from psychophysical tests and the
All quantum objects must satisfy Heisenberg’ s relation ∆x∆p ≥ ħ – 2 accounting for how the uncertainty on the quantity x limits the one on the conjugated quantity p. Photons can thus be prepared in quantum states with enhanced precision for measurements with respect to classical light, by suppressing the intrinsic fluctuations in one quantity by increasing those of another: this applies to polarization, to orbital angular momentum and to phase φ and photon number N, using the proper units. This implies, for instance, that a beam with suppressed phase fluctuations would show increased fluctuations of its intensity, and vice versa. In addition, an eavesdropper attempting to read the value of x would be caught, since it necessarily produces an increase in ∆p. By exploiting the opportunities granted by Heisenberg’ s relation and turning its limitations to our advantage, it could be possible to combine precision and security in a single cryptographically protected scheme.
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