Flying Qubits Atoms

6 Quantum Computers First Steps Towards a Realization 

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Photons in quantum optical cavities also constitute excellent qubit candidates [52]. Resonant coupling of atoms with a single mode of the radiation field was experimentally achieved 25 years ago [53], and eventually the coherent coupling of quantum optical cavities with atoms or (simple) molecules was suggested as a means to achieve stable quantum memories in a hybrid quantum processor [54]. There might be a role to play for molecular spin qubits in this kind of hybrid quantum devices that combine solid-state with flying qubits. 

An experimental approach to realize quantum logic gates based atomic Ramsey interferometry is presented in Sect. 6.5. The quantum bits are represented by atoms. A beam of atoms is sent through a microwave resonator. The resonator provides the necessary interaction between the atoms to perform quantum logic gates. This is an example for the concept of flying qubits. Experiments along these lines are carried out at the Ecole Normale Superiore in Paris. 

An alternative realization of the flying qubit concept is briefly discussed in Sect. 6.7. The quantum bits here are stored in polarizations of single photons and the coupling between the photons is provided by atoms. This is the only implementation of quantum logic discussed in this chapter that is not based on atoms as the carrier of quantum bits. 

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... 

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