The quantum coherence of a polariton condensate has been seen to oscillate as the condensate decays. The discovery was made by researchers in Russia, the UK, and Iceland who were led by Alexis Askitopoulos at the Skolkovo Institute of Science and Technology. The oscillations are magnetic in nature and the team suggests that the phenomenon could be used to develop new instruments for measuring magnetic fields.
An exciton–polariton (often referred to as a polariton) is a quasiparticle that occurs in semiconductors. It comprises a photon of light that is coupled to an exciton, which itself comprises an electron and a hole. Polaritons can be produced by sending a light pulse into a semiconductor-based microcavity.
Polaritons are bosons. This means that a dense ensemble of the quasiparticles can form a Bose-Einstein condensate, in which a large number of the polaritons are in the same quantum state. Such a condensate has macroscopic properties that are defined by its quantum nature. These properties can be determined by studying the photons emitted by the condensate as it decays.
Waning coherence
In recent research, Askitopoulos’ team prepared a polariton condensate by firing a 20 µs long light pulse into a microcavity. They then watched as the condensate decayed over time, measuring a coherence function that is related to the overall quantum nature of the condensate.
Polaritonic condensate reveals universal law in an out-of-equilibrium system
Instead of decaying smoothly as seen in other condensates, they found that the coherence function periodically rose and fell as it decayed, with a remarkably uniform frequency. By examining these oscillations, they identified Larmor precession as a likely cause of this behaviour. This involves the rotation of the condensate’s magnetic moments about a magnetic field, which is created by polariton interactions.
In total, Askitopoulos and colleagues observed some 100,000 full precessions within a single optical pulse. They found that both the decay speed and the frequency of Larmor precession was directly influenced by the density of polaritons within the condensate. By periodically boosting the system’s coherence, this precession persisted millions of times longer than the lifetime of an individual polariton.
Askitopoulos’ team suggests that this precession could be controlled using optical methods, which could lead to better techniques to study polariton condensates. One possible application of the discovery is the development of new types of magnetometers. These are devices that measure the strength, direction, and relative change of a local magnetic field.
The research is described in Physical Review Letters.
