The human eye can see a single photon

A person’s eye is still much more sensitive to light than has always been thought. A human eye can see even a single photon. This is the smallest physical unit of light. An experiment with a special quantum light source has proved this fact. This was reported by researchers in the journal "Nature Communications". It is not yet clear how our visual system can perceive this weak photon signal.

Vienna (Austria). Normally, we humans are not really considered super eyes. Many animals can see much better than humans, especially at night. But where is the limit in the perception of light?? The vision of the eyes has been studied for about 70 years. To this day, it is still unclear where the limits lie.

How well our eyes see?

So far, studies have not shown comprehensive results on whether people consciously respond to a signal when the rods, or our visual sensory cells, are already reacting to a small photon. One of the reasons for this is that there is simply a lack of technology with which to shoot the single photons at a subject’s eye.

Study leader Alipasha Vaziri of the University of Vienna explains that it is not very easy to produce a light that consists of a defined number of photons. The number of photons is very statistically distributed in a light from classical sources. The average photon number of a light can be reduced by dimming it. Thus, the exact number can not be determined.

With light or without

Researchers solved this problem and constructed a light source previously known only from quantum information and quantum optics. Through an optical crystal, a high-energy photon is used to decay into two entangled photons of low energy. While one photon was directed to the eye of a test subject, the other photon hit a detector.

The test subjects were young males with optimal visual acuity. They sat in a light-isolated room and after a short familiarization phase had the task to indicate in which temporal intervals they could detect a light in the absolutely dark room. In addition, they had to indicate whether and how sure they were. The researchers repeated this experiment with a wide variety of participants for more than 30.000 times.

Here one can speak of no more coincidence

The test subjects were correct in more than half of all cases. The hit rate was thus much higher than if it were pure chance. If the researchers were to evaluate only those experiments in which the test subjects felt safe, the figure would rise to as much as 60 percent.

This means that the human eye is actually able to see a small photon. This led the researchers to conclude that evolution has improved the sensitivity of human sensory organs. In this case even up to the unit of a physical quantity.

What fascinates the physicists is the fact that a small photon, which is the smallest unit of light, can be detected by billions of cells. The faint light first passes through several steps of biological signal processing until it is consciously perceived. Despite the different steps, the noise is not lost.

The puzzling priming effect

The sensitive human retina reacts very differently to light. This does not only depend on individual differences. Another experiment showed that the hit rate of test subjects increased when researchers sent two photons to the eyes five seconds apart.

The scientists explained that the detection of single photons can increase the sensitivity of the visual system to such a test in a low-light condition. This effect is called the priming effect. It trains the eye to respond even to weak signals. It is still unclear which neurophysiological mechanisms are responsible for this, as the researchers report in the scientific journal Nature.

It is also unclear how the human eye manages to detect a small photon from the background noise. In the coming year, Alipasha Vaziri and his team will investigate this question in more detail.

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