A Cavity Quantum Electrodynamics QC

An alternative cavity QED approach to realize a QC is based on the concept of trapped qubits similar to the ion trap proposal (Sect. 6.4) was proposed recently (Pellizzari et al. 1995). The qubits are represented by atoms or ions and are fixed in space by appropriate trapping techniques. There is an obvious advantage of fixed qubits over flying qubits. Namely, we can perform quantum gates between any pair of qubits without having to make sure that the involved qubits are in the right place at the right time. This can be difficult for more complicated networks. In contrast 

6 Quantum Computers First Steps Towards a Realization 

The proposed solution to this problem is based on dark states , an effect which can only be understood quantum mechanically. A detailed explanation of dark states and their use in quantum computing would exceed the scope of this chapter. Instead, a short outline of the basic idea may suffice. Let us consider an atom with three levels, two energetically lower-lying stable states pi) and p2) 2is well as a (decaying) excited state e). If a laser is coupled to one of the transitions pi) k) or p2) — je) Rabi oscillations as discussed in Sect. 6.2.2 will take place. Every time the atom is in the excited state it can decay by spontaneous emission of a photon. However, if both transitions are driven by lasers a very interesting effect can take place the Rabi oscillations will cease and the atom remains in the stable ground states. No excitation will take place even though two lasers 

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