Photons are easily generated spontaneously from energy

19.07.2016 17:00

Humans can see a single photon

Dr. Heidemarie Hurtl IMP Communications
IMP - Research Institute for Molecular Pathology GmbH

Researchers at the Vienna Institute for Molecular Pathology (IMP) and at Rockefeller University in New York have shown for the first time that humans can perceive a single photon. For their experiments, they used a quantum light source and combined it with a sophisticated psycho-physical approach. The scientific journal “Nature Communications” publishes the results in its current issue.

Despite numerous studies that have been carried out on this topic for over seven years, the absolute lower limit of human vision has not yet been determined with certainty. It is known from older studies that test subjects who have adapted to the dark can perceive flashes of light that consist of five to seven photons. For a long time, however, it was not clear whether a single photon is visible.

An interdisciplinary team headed by the quantum physicist Alipasha Vaziri has now been able to answer this question positively in an extensive series of tests. Vaziri is Associate Professor and Head of the Laboratory of Neurotechnology & Biophysics at Rockefeller University and also heads a working group at the IMP in Vienna, where the experiments were carried out with volunteers.

Amazing precision despite adverse conditions

“We were able to show for the first time that the human eye is actually able to recognize a single photon,” explains Alipasha Vaziri. "This is really remarkable and shows the astonishing efficiency to which evolution can advance the sensitivity of the sense organs, in this case to the unity of physical quantity itself."

What particularly fascinates the physicist: “Here a photon, the smallest unit of light, meets a biological system consisting of billions of cells. The extremely weak signal goes through several steps of biological signal processing up to conscious perception and is not lost despite all possible sources of noise. To make matters worse, the environment is warm and humid - usually a real nightmare for measurements at the quantum level. Any human-made detector would have to be heavily cooled and carefully shielded to produce such results. "

Quantum light source creates entangled photons

Earlier attempts had failed because neither the mature technology nor the appropriate psycho-physical approaches were available. Vaziri: “It is not easy to generate light that consists of exactly one or a defined number of photons. In light from classical sources, the number of photons is statistically distributed. By dimming you can only reduce the average number of photons of a light pulse, the exact number cannot be determined. "

The lack of suitable light sources was therefore a major challenge in developing the experimental approach. The researchers solved the problem by constructing a light source that was previously only used in the field of quantum optics and quantum information. The principle is based on so-called spontaneous parametric fluorescence, in which a high-energy photon in an optical crystal spontaneously decays into two entangled photons with lower energy, the sum of the energies of the two photons corresponding to that of the original. In the experiment, one of the photons was directed to the subject's eye, while the other hit a detector at the same time.

“The setup of this camera was a tough nut to crack,” says co-lead author Jonathan Tinsley, who carried out some of the experiments as a master’s student. “In addition, we had to build special darkrooms for the experiments, which perfectly shielded light and noise.” A total of around nine months was worked on the experimental setup.

First indication of the perception of individual photons

To evaluate the experiments, the researchers chose a protocol that was used for the first time in this context. They used the method of forced choice (two-alternative forced-choice, 2AFC), in which the subjects have to choose from two alternatives for each round. Specifically, the test subjects had to choose between two time intervals, only one of which a photon flashed. More than 30,000 such passes were finally evaluated and showed with statistical significance that individual photons can be perceived by the human eye.

In addition to this finding, the experiments delivered another unexpected result: the chance of perceiving a photon increased if a photon had already hit the eye shortly before. In subsequent experiments, the researchers want to clarify how this phenomenon comes about. In addition, numerous other questions arise: How can biological systems develop such sensitivity and precision? How are the weak signals filtered out of the background noise? Are the observed phenomena limited to the sense of sight or do they provide general information on signal processing in living beings? Alipasha Vaziri and his team will get to the bottom of these questions in the years to come.

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Original publication
Jonathan N. Tinsley, Maxim I. Molodtsov, Robert Prevedel, David Wartmann, Jofre Espigulé-Pons, Mattias Lauwers and Alipasha Vaziri: Direct Detection of a Single Photon by Humans. Nature Communications, July 19, 2016. DOI: 10.1038 / ncomms12172.

Press contact IMP
Dr. Heidemarie Hurtl
IMP Communications
Research Institute of Molecular Pathology
+43 (0)1 79730 3625
[email protected]

Press contact Rockefeller University
Zach Veilleux
Communications and Public Affairs
The Rockefeller University
+ 1-212-327-8982 or similar
+ 1-347-978-4723 m
[email protected]

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