If Nature is surprising, Quantum Physics is mindbogling. It can be non-intuitive, it can lead to results difficult to accept, but it does it job when it comes to describe Nature at the elementary particles level. Although that is not its only realm. In a recent publication (Nature Communications 7, Article number: 12172, July 2016), scientists from the University of Vienna (Austria) and the Rockefeller University (USA) go one step beyond and answer affirmatively to the question: Can a person see a single photon?
We physicists are used to doing experiments shielding our setups from any external perturbation, just to be sure that what we are measuring is what we want to study. When we do experiments in quantum optics this goal is not easy. Thermal fluctuations, electronic noise, straight light or even quantum fluctuations can blur our measurements. This differentiation from the micro-world to the macro-world makes us think that quantum effects can not be detected at the macroscopic level, that we are not going to be able to create Schrödinger´s cat with an actual cat (besides having all youtube followers attacking us). But this is not true. Otherwise we would not have transistors, computers, mobile phones, magnetic resonance imaging, and many more.
There is one question that has been in the air and in the conferences for many years: What is the limit of perception of the human eye? In the 1980s and 1990s some researchers tried to answer it, but the equipment of the moment could only conclude that the human eye could detect pulses of five photons.
By that time, researchers could not create single photons in a controllable way. They used light attenuated from a laser. But this type of light is called coherent, and when it is pulsed and attenuated, each pulse has not a well defined number of photons. There is a statistics, called Poissonian, that tells us the probability of having zero, one two, three, ... photons in each pulse. We can talk about mean photon number but we cannot know exactly how many photons will have each one of the pulses.
Now, by using spontaneous parametric downconversion (SPDC), these physicists have created correlated pairs of photons. From each pair, one was sent to a detector, to herald the creation of the pair, and the other was sent to the eye of a person. This way, the researchers knew when a photon was crated and could wait for the response of the person.
Of course, the room was completely dark and had been dark for a while, so the patient could adapt her vision to darkness and have the eye more sensitive to light.
The research conducted followed a statistical analysis of the detections made by three different people, trying to quantify the goodness of the detection and to distinguish all the detections from a purely random process.
It seems that our human eye is sensitive enough to a single photon. Even though this does not guarantee that we can detect every single photon sent to our eye. Also, the research was carried out only on three individuals. A more robust statistics should have been more conclusive, though.
In any case, this surprising result closes one question that remained open for a few decades and opens the door do quantum experiments were human beings can be part of the detection side. Maybe we could read soon on research on quantum entanglement resolved by a person herself.
Physicist, working in quantum optics and nonlinear dynamics in optical systems. Loves to communicate science.