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The studies of insulator to super-fluid transitions in many body systems and their realization in optical lattices have opened great possibilities for simulating many body systems. It is thus interesting to explore which other systems permit such phases and simulations, especially if the problem of accessibility of the individual sites is not present. Particularly arresting will be to find such phases in a system of photons which, by being non-interacting, are unlikely candidates for the studies of many-body phenomena.

D. G. Angelakis, M. Santos and S. Bose have managed to show (quantum-ph/0606159), that a Mott phase can arise in an array of coupled high Q electromagnetic cavities between which photons can hop, when each cavity is coupled to a single two level system. The latter could be an atom or a quantum dot or even a Cooper pair box. In this phase each atom-cavity system has the same integral number of polaritonic (atomic plus photonic) excitations. It occurs for resonant photonic and atomic frequencies when photon blockade provides an effective repulsion between the excitations in each atom-cavity system. Detuning the atomic and photonic frequencies suppresses this repulsion and induces a transition from the Mott phase to a photonic superfluid.(See also M.J. Hartmann et al., for a related proposal using clouds of four level atoms, quantum-ph/0606097 and Nature Physics). Angelakis et al., also showed that for zero detuning, the system can simulate the dynamics of a spin chain with arbitrary number of excitations. This could be used for various quantum information processing tasks.

 The most promising technologies for implementing these ideas are shown below.

In the first, an array of coupled defects in a photonic band gap material is shown. These could be doped with single atoms, or quantum dots already existing in the substrate. The second one is showing a series of coupled toroidal microresonators. Here single atoms could be trapped near the surface and coupled with the resonator’s circulating light modes. The third one describes a circuit quantum electrodynamics architecture. There two superconducting charged qubits-Cooper pair boxes-are coupled strongly to a coplanar transmission line resonator (All three experimental images shown below are for illustrations purposes only and with permission from the corresponding groups).

This work will feature in a focus article in New Scientist later on this month and is currently under review for publication in Physical Review Letters.