Ghostly particles detected in condensates of light and matter | 1/26/2020 | Staff
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Bose-condensed quantum fluids are not forever. Such states include superfluids and Bose-Einstein condensates (BECs).

There is a beautiful purity in such exotic states, in which every particle is in the same quantum state, allowing quantum effects to be seen at a macroscopic level visible on a simple microscope.

Reality - Particles - Condensate - Absolute - Particles

In reality though, not all particles stay in the condensate even at absolute zero where, classically, particles are expected to stand still. Instead, interaction-induced quantum fluctuations make the particles collide, unavoidably expelling some particles out of the condensate, a phenomenon called "quantum depletion."

This effect is incredibly strong in superfluid helium-4, the first known superfluid, such that 90% of the particles are expelled out of the condensate. However, in extremely dilute, ultracold atomic gases, which form the typical Bose-Einstein condensates (BEC) we know, the effect is much weaker, almost negligible.

Quantum - Depletion - Theory - Nikolay - Bogoliubov

Although quantum depletion has been well described theoretically (by the 70-year-old theory developed by Nikolay Bogoliubov), it has historically known to be difficult to measure in an atomic BEC for a number of reasons.

Instead of atomic particles, physicists at the Australian National University (ANU) use exciton-polaritons, hybrid particles with both light and matter character, which allows detection of momentum without any distortion.

ANU - Team - Prof - Elena - Ostrovskaya

The ANU team, led by Prof Elena Ostrovskaya, successfully detected the expelled particles by blocking the light, using a razor edge, emitted by the incredibly bright condensate. "It is like recreating a solar eclipse," says the study's lead author Dr. Maciej Pieczarka. "The moon blocks the bright sun (the condensate) and exposes its glorious corona (the excitations)."

The study represents the first direct observation of quantum depletion in a non-equilibrium Bose-Einstein condensate (BEC).

Result - Study - Challenge - Physics - Nonequilibrium

A surprising result of the study offers a new challenge for the physics of nonequilibrium quantum fluids. Exciton-polariton condensates can be tuned from more matter-like (excitonic) to more light-like (photonic), allowing for comparison with...
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