Dicke Superradiance and Hanbury Brown and Twiss Intensity Interference: Two Sides of the Same Coin

Superradiance is one of the outstanding problems in quantum optics since Dicke introduced the concept of enhanced directional spontaneous emission by an ensemble of identical two-level atoms, situated in a collective highly entangled Dicke state [1]. However, the production of Dicke states with higher number of excitations remains a challenge. One option is the repeated measurements of photons at particular positions starting from the fully excited system. This amounts to measuring the m-th order photon correlation function for N > m emitters. In this case, if the detection is unable to identify the individual photon source, the collective system cascades down the ladder of symmetric Dicke states each time a photon is recorded, even for widely separated emitters. This is another example of measurement induced entanglement among parties which do not directly interact with each other [2–9]. Following this approach we show that an enhanced directional emission of spontaneous radiation can be produced via measurement of higher order correlation functions starting from initially uncorrelated systems [10,11]. The approach is applicable to a wide variety of quantum emitters like trapped atoms, ions, quantum dots or NV-centers. Surprisingly, the method is also valid for initially uncorrelated incoherent classical emitters. This is experimentally confirmed with up to eight statistically independent thermal light sources [11]. In both cases, i.e., for classical and quantum sources, the arrangement to measure the higher order correlation functions corresponds to a generalized Hanbury Brown and Twiss setup. This shows that the two phenomena, superradiance and the Hanbury Brown and Twiss effect, stem from the same interference phenomenon and are two sides of the same coin.