Quantum communication and computing with atomic ensembles using a light-shift-imbalance-induced blockade

Recently, we have shown that for conditions under which the so-called light-shift imbalance induced blockade occurs, the collective excitation of an ensemble of a multilevel atom can be treated as a closed two-level system. In this paper, we describe how such a system can be used as a quantum bit (qubit) for quantum communication and quantum computing. Specifically, we show how to realize a controlled-NOT gate using the collective qubit and an easily accessible ring cavity, via an extension of the so-called Pellizzari scheme. We also describe how multiple, small-scale quantum computers realized using these qubits can be linked effectively for implementing a quantum internet. We describe the details of the energy levels and transitions in an Rb87 atom that could be used for implementing these schemes.