Superradiance, Superabsorption and a Photonic Quantum Engine

Kyungwon An
Seoul National U (Korea)
ICAP 2022
Tuesday, Jul 19, 9:20 AM

Superradiance, Superabsorption and a Photonic Quantum Engine

A superradiant state is a special superposition state of atoms capable of undergoing superradiance immediately without the usual delay involved with Dicke states. We can prepare a superradiant state in a cavity by preparing N atoms in the same superposition state of the ground and excited states. By sending atoms through a nanohole array aperture with a period equal to the atomic transition wavelength, we can impose a common phase to all individual superposition states and localize the atoms exactly at antinodes of a cavity while they traverse the cavity. Surprisingly, these correlated atoms generate superradiance in the cavity even when the mean number of intracavity atoms is much less than unity. It turns out that these time-separated atoms are correlated via the long-lived cavity field so that a single intracavity atom can undergo superradiance as if the preceding atoms outside the cavity were also in the cavity. Another interesting feature of this superradiance is that the emission is amplified like a laser but without exhibiting a threshold. Through second-order-correlation measurements, we observed that the cavity field is coherent even when the mean number of photons is less than unity, in contrast to the thermal light emitted from the usual thresholdless lasers under the same condition.

The superradiant state can also be used to realize the long-sought superabsorption, the opposite of superradiance. It is well known that the bright state responsible for superradiance also has an absorption rate the same as the superradiant emission rate. This notion motivated theorists to suggest various ideas for realizing superabsorption, but none has worked so far. From theoretical considerations, we found that a superradiant state can absorb light instead of emitting light if its phase is chosen properly depending on the phase of the input field. Based on this idea, we have recently realized superabsorption by reversing the superradiance process in time. The maximum number of photons absorbed was proportional to the square of the number of atoms, proving the cooperative nature of superabsorption. In addition, the superradiant state can be used to realize a superradiant photonic quantum engine. Here, the atoms entering the cavity are our fuel and their nonzero excited state population can be associated with an effective temperature by the Boltzmann factor. Our engine operates between a thermal state and a superradiant state of reservoir at the same reservoir temperature. Photons emitted by superradiance exert radiation pressure to the cavity mirrors to perform a work. The observed efficiency of the engine was 98%. Our quantum engine can serve as a testbed for quantum thermodynamics in nanoscale systems.