Scientists have received direct experimental confirmation that the destruction of a photon in a part of the light beam does not change the shape of the beam profile (ie, "does not cast a shadow"), but it can change its brightness. Previously, this effect was demonstrated only in a simplified regime, when the beam was split into two channels, and the removal of photons in one channel led to changes in the other.
The article by researchers Direct test of the “quantum vampire's” shadow absence with use of thermal light, prepared by a group of physicists from the Center for Quantum Technologies of the Physics Faculty of Moscow State University, was published in the Optics Letters journal.
To confirm the "quantum vampire" effect, CCT physicists have created a device in which one photon is removed from the vampire-shaped part of the heat beam. For comparison, the situation was also considered when classical absorption of light occurred in the same region, leading to the fact that, on average, one photon was lost. If in the classical case the beam profile changed and “a shadow was visible”, then in the quantum case, when one photon was destroyed, there was no shadow.
Recall that a "quantum vampire" is an effect that, under certain conditions, a body that is in the path of the light "does not cast a shadow." If in everyday life we are accustomed to the fact that any object that gets in the way of a part of the light stream causes a shadow (a dip in illumination), then in the quantum world, if an object is designed in such a way that it absorbs exactly one photon, instead of "forming a shadow" behind the obstacle there is a subsidence or an increase in illumination (depending on the properties of the radiation source) over the entire area of the light beam.
The effect allows for a better understanding - on an intuitive level - of how the photon annihilation operator works, which is the basis of quantum mechanics and is practically used in a wide variety of applications and technologies. For example, it can be used to physically simulate a quantum heat engine or Maxwell's photonic demon. The splitting off of a photon makes it possible to increase the sensitivity of thermal field interferometers, expand the possibilities of optical quantum computing, and increase the efficiency of quantum key distribution systems.
For the first time the effect of the "quantum vampire" was experimentally discovered by the group of Alexander Lvovsky. Scientists conducted a test experiment in which one or two photons were split into two channels by a beam splitter, then conditional destruction of one photon was realized in one of the channels, and this led to the fact that the photon was destroyed simultaneously in both beams.
Later, the CCT employees in their work in 2018 proved that this effect will be fulfilled not only for quantum states of light with a given number of photons, but also for classical light from a heat source, that is, it does not have a truly quantum nature.
