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Superradiance in quantum optics occurs when a collection of atoms exhibits a cooperative, spontaneous emission of photons. We understand this in the following way:
A single atom is approximated as a two-level system. When the atom is found in its excited state, there is a well-defined probability per unit time, quantified by Fermi’s golden rule, that the atom will emit a photon and return to its ground state. If the same atom is continuously excited, e.g. by an external laser, there will be an assigned emission rate. If two such atoms are placed close to each other, at a distance comparable to the emission wavelength, their joint emission rate will not be twice that of a single atom. This is due to quantum interference. Specifically, the wave functions that are used to calculate the emission probabilities are interfering with each other, and so the emission probability and rate, will be enhanced or diminished based on weather this interference is constructive or destructive. As a result, two atoms of the same kind that form a non-separable state, will have an emission rate that oscillates sinusoidally as a function of their distance, above (superradiance) and below (subradiance) their individual rates.
Within EnHydro, we have shown that superradiance and subradiance may also be observed in a hydrodynamic setting. This was first shown theoretically, using a model of bipartite tunnelling in the hydrodynamic pilot-wave system. This demonstration satisfied all three criteria encountered in quantum optics as follows. First, we considered a classical two-level system in which one of the two states may be treated as the lower-energy state, in the sense that it is more likely to arise. Second, we produced a bipartite system, consisting of two such two-level systems, coupled in such a way that their collective behavior cannot be specified in terms of a linear combination of its individual subsystems. Third, we demonstrated that the probability of transition from state to state in each subsystem varies in a sinusoidal fashion with distance between the two subsystems.
More recently, we have also observed superradiance in an experimental hydrodynamic setting. In this setting, the effect was created by droplet emission resulting from the interfacial fracture of two parametrically excited hydrodynamic cavities.
Ref: K Papatryfonos, M Ruelle, C Bourdiol, A Nachbin, JWM Bush, M Labousse, Hydrodynamic superradiance in wave-mediated cooperative tunneling, Communications Physics 5 (1), 1-7 (2022) [pdf]