Superstrong coupling regime of cavity quantum electrodynamics

Abstract

We describe a qualitatively new regime of cavity quantum electrodynamics, the super-strong coupling regime. This regime is characterized by atom-field coupling strengths of the order of the free spectral range of the cavity, resulting in a significant change in the spatial mode functions of the light field. It can be reached in practice for cold atoms trapped in an optical dipole potential inside the resonator. We present a nonperturbative scheme that allows us to calculate the frequencies and linewidths of the modified field modes, thereby providing a good starting point for a quantization of the theory.PACS numbers: 42.50. Fx,42.50.Pq, A striking characteristic of cavity quantum electrodynamics (CQED) is the conceptual simplicity of the systems involved. Typically, photons in a single cavity mode interact with atoms with a very small relevant number of internal quantum states [1]. On the experimental side this simplicity leads to the precise control of most system parameters and to the laboratory realization of many idealized theoretical models and Gedankenexperiments. For example, strongly nonclassical states of the light field such as e.g. number states [2,3] can be created, the entanglement between light and atoms can be studied, and important questions related to the quantum measurement process can be addressed. Over the last two decades experimentalists further expanded the scope of CQED by achieving increasing control over the translational degrees of freedom of the atoms via laser cooling and other cooling schemes, and CQED also plays an important role in quantum information research.In the strong coupling regime of CQED the coherent interaction between a single atom and the light field, characterized by the Rabi frequency g, dominates over the decoherence processes induced by the coupling to the environment, and characterized by the spontaneous decay rate γ and the cavity damping rate κ, g > γ, κ.(1)In contrast to these three characteristic frequencies, whose relative role in CQED has been explored in great detail in the past, the role of the free spectral range ω FSR of the resonator has largely been ignored so far. However, if one could achieve experimental conditions such thatthe coupled atoms-cavity system would enter a qualitatively new regime. In this regime the coupling between atoms and light is strong already during one round trip in the resonator, which is in contrast to the conventional strong coupling regime where sufficiently strong coupling is achieved through recycling of the light by means of a high Q cavity. Because the spatial mode pattern inside the resonator is established during one round trip it is easy to see that in the super strong coupling limit the atoms can affect the spatial mode structure itself, and not just the occupation of the modes as is typically the case in conventional CQED. The reason why that regime has not been clearly identified in the past is that ω FSR = c/2L, where L is the resonator length, is under most circumstances much too large to lead to significant e...

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