The scientists of the Faculty of Physics of Tomsk State University Peter Kazinsky and Vladislav Ryakin and TSU graduate Polina Shevchenko studied photon emission caused by the measurement of the state of Dirac particles, such as electrons. They found that measuring the state of a particle does not just change its quantum state, but can also lead to photon emission. The results are published in the Physical Review D (Q1).
— As far as we know, this is a new type of radiation connected with purely quantum process of collapse of a particle state during measurement. In particular, such radiation exists when we measure the state of a freely moving particle. The study of this radiation opens new horizons for the study of quantum mechanics and its applications, says Professor of the Department of Quantum Field Theory at the TSU Faculty of Physics Peter Kazinsky.
The study is focused on a theory that explains how projective measurement (a type of quantum measurement that leads to the collapse of a particle wave function) can lead to photon emission. This radiation has properties similar to edge or transient radiation, which occurs when charged particles pass through a boundary between two media.
— To better understand the phenomenon, we can draw an analogy with classical electrodynamics. Imagine that you abruptly stop a charged ball. As a result of this abrupt change of motion, the ball emits electromagnetic waves. In the quantum case, the abrupt change occurs not due to a change in its trajectory, but due to a change in the wave function, which leads to a similar radiation effect, says Vladislav Ryakin, co-author of the work, a graduate student of the Department of Quantum Field Theory of the TSU Faculty of Physics.
The radiation during measurement can be used to study the dynamics of the collapse of a particle wave function. It can also serve as a source of high-energy photons, although the measurement time imposes a limit on the maximum energy of the emitted photons.
Major fields of the study:
When we measure the state of a quantum particle, its wave function collapses to one of the possible states. This process may lead to the emission of photons.
Forced emission occurs when photons are already present in the system. The wavefunction acts as a kind of charged fluid and the radiation amplitudes add up coherently.
Spontaneous emission occurs when there were no photons in the initial state, but they appear as a result of the measurement. In this case, the radiation is incoherent.
When the spin projection of a particle is measured, the radiation is determined by the magnetic moment of a particle. It is noteworthy that if the spin of a particle does not change as a result of the measurement (so-called quantum non-destructive measurement), there is no radiation.
When we measure a particle momentum, the emission resembles the edge emission from a particle beam with a certain momentum. In the case of a coordinate measurement, the radiation arises from the abrupt change in the wave function of a particle in space.
If two particles are entangled in spin, measuring the spin of one of them can result in radiation that is created only in the region wh ere the particle being measured is located. The second particle, although entangled, emits nothing.
The full paper “Radiation of Dirac fermions induced by projective measurement” is published in Physical Review D (Q1), see also on arXiv.
Text and photos are taken from the website of TSU Faculty of Physics