Optical and Gamma Superradiance

А.В. Андреев

А.В. Андреев

L. I. Men’shikov

Wei 威 Qin 覃, Dong-Chen Zheng, Jia-Ying 佳颖 Lin 林 +2 more

Abstract We consider a Bose–Einstein condensate loaded inside an optical cavity and exposed to two crossed coherent pump fields with same imbalance parameter γ . We identify different effects between pure standing wave fields ( γ = 1) and the pump beams combining standing wave and running wave ( γ ≠ 1). In particular, for γ = 1, the system only hosts a normal phase and a superradiant phase. In contrast, for γ ≠ 1, the system features three distinctive phases: the normal phase (NP), superradiant phase 1 (SR 1 ), and superradiant phase 2 (SR 2 ). Importantly, the superradiance is subdivided into different types characterized by the photon phase. Furthermore, we determine perturbatively the phase boundary separating the normal phase and the superradiant phases, and find that there exists a competitive relationship of energy minimum on the overlapping region between SR 1 and SR 2 . Interestingly, the transition between the normal phase to SR 1 or SR 2 is identified to be a second-order phase transition, while the transition between SR 1 and SR 2 is a first-order transition. When the first-order phase transition occurs, the phase of the photons changes abruptly from 0 to π /2.

A. V. Andreev

Ralf Röhlsberger

The electromagnetic transitions of Mössbauer nuclei provide ideal two-level systems to study the cooperative effects of X-rays with matter. Recently, we have observed the collective Lamb shift (CLS) in nuclear γ-ray superradiance [Röhlsberger, R.; Schlage, K.; Sahoo, B.; Couet, S.; Rüffer, R. Science 2010, 328, 1248–1251]. An ensemble of resonant 57Fe nuclei was embedded in the center of a planar cavity and was collectively excited by synchrotron radiation into a purely superradiant state. The decay of the superradiant state can be almost two orders of magnitude faster than the single-atom nuclear decay and can be accompanied by a CLS of more than 10 times the natural linewidth. Here we review the basic features of cooperative effects that occur in nuclear γ-ray superradiance in comparison with similar effects observed in the optical regime of the electromagnetic spectrum. We show that nuclear resonant scattering of synchrotron radiation opens new ways for systematic studies of cooperative effects in the interaction of X-rays with matter.