Collective Photon Emission of Correlated Atoms in Free Space

Superradiance is one of the enigmatic problems in quantum optics since Dicke introduced the concept of enhanced spontaneous emission by an ensemble of identical two-level atoms, situated in collective highly entangled Dicke states [1-5]. As denoted by the title of Dicke’s paper "Coherence in Spontaneous Radiation Processes", the puzzle lies in the combination of spontaneous emission, i.e., the absence of coherent radiation, and the appearance of an interference pattern scaling at its maximum with the square of the number of atoms.

While single excited Dicke states have been investigated since long to study superradiance, the analysis of Dicke states with higher number of excitations has been scarce. We study these states from the perspective of producing constructive interference [6]. Moreover, we demonstrate how these states can be generated via successive measurement of photons at particular positions starting from the fully excited system [7-10]. 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 [11]. We applied this scheme to demonstrate directional super- and subradiance with two trapped ions [12].

Recently, it has been shown using a master equation approach, that the density matrix of the atomic ensemble in the small sample limit remains separable throughout its time evolution, demonstrating that entanglement is not required for superradiant light emission [13-16]. On the other hand, it turns out that quantum correlations as quantified by the quantum discord are present throughout the emission process playing a key role in the cooperative behavior [17].