Bright spot for quantum research

HU research team and partners have directly measured the particle exchange phase of photons for the first time

Figure 1: Conceptual sketch of the interferometer setup: an entangled pair of photons (red beam) is directed into the interferometer, producing two distinct substituents in the central polarization beam splitter (PBS), as shown in b : Either the photon in path 1 is transmitted and the photon in path 2 is reflected or vice versa. The quantum superposition of these scenarios leads to interference between states that are physically reversed versions of each other and reveal the particle exchange phase _x. The blue beam is generated by an attenuated laser and serves as a reference signal to determine the effective optical path length difference, _1 and _2. Fig.: HU Berlin

This experiment provides direct evidence of a surprising quantum phenomenon that is observed only in quantum objects of the same type. Thus quantum research is taking an important step forward.

The particles the research team is tracking are difficult to understand. Physicists study the quantum particles of electromagnetic waves, also called photons, that make up light. Photons can only be differentiated if they have different wavelengths, vibrate in different directions, or are at different points in space and time.

“When two photons that are indistinguishable in terms of wavelength and direction of oscillation meet and separate again, so to speak, they have lost their identity,” explains Kurt Bush. “Imagine we send two twins through two doors into a common room. When you walk out again, we can’t tell if you used the same door for each one,” says Oliver Benson. Nevertheless, something happens in quantum mechanics. According to the so-called symmetry postulate, there are two categories of elementary particles: bosons and fermions. These types of particles are different when you swap them with each other.

In the example this would mean when each twin exits the room through the other door. Nothing changes with a boson – with a fermion, the quantum mechanical wave function that describes particles acquires a phase shift, also known as the exchange phase. “In the twin example, you can imagine it this way: If we send two twins into a room in lockstep and they come out through different doors, they will remain in lockstep. As a boson, The twins first exit space with the same foot they first stepped into space. However, as fermions, they both need one more step and now move with the other foot as they leave the room, “So far, it has been possible to show that photons are bosonic through indirect measurements and mathematical calculations,” says Benson. “In our most recent experiment, we directly measured the particle exchange phase of photons for the first time and thus provided direct evidence of his bosonic character.”

To directly demonstrate the exchange symmetry of a state for two identical particles, the team set up an optical instrument with an interferometer. The heart of the structure – the size of a small table – are two beam splitters. Two photons are then sent to the interferometer and passed through the beam splitter in two different ways. Photons are exchanged with each other along one of the two paths, while they remain unchanged on the other. At the exit of the interferometer, both photons were rerouted to the second beam splitter. “Depending on whether the photons are bosonic or fermonic, the two photons are in lockstep and reinforce each other or they are out of step and quench each other,” explain the physicists.

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Press release from Humboldt University of Berlin on June 2, 2021