In this chapter, we present a proposal for the realization of an entangling gate for photons. The protocol works deterministically and is experimentally feasible under realistic conditions. We start outlining the general problem of quantum information processing with photons and describe the current status in this field. Then we introduce the proposed scheme and present its main features. This part is followed by a review of the concepts and procedures employed in our proposal. Finally, we explain the quantum gate, in detail, in the last section of this chapter, where we also briefly discuss the performance of the gate in the presence of imperfections.
26.1.1 Quantum information processing with light
Photons play a key role in applications of quantum information because they are ideally suited to transmit quantum states between distant sites. This feature makes them indispensable for the realization of quantum communication protocols and the construction of quantum networks.
While being a good flying carrier of information, photons are naturally less adequate for storage than the long-lived matter degrees of freedom. For this reason, matter systems are employed as quantum memories. In order to combine both elements in a quantum network (i.e., photons as flying qubits and atoms as memory devices), light-matter interface schemes have been developed to transfer quantum states of light onto an atomic system. Some of these schemes are based on quantum-nondemolition interactions, electromagnetically induced transparency, and Raman processes. Moreover, Raman processes have been used to entangle two distant atomic ensembles, which is an important step to realize quantum repeaters and, thus to solve the problem of losses and decoherence that exists in a photonic channel.
Apart from the transmission and storage of information, in a quantum network it is necessary to process quantum states. However, manipulation of quantum states of light is still challenging, since it requires the ability to create entanglement between photons. This task is difficult because photons are noninteracting particles, in principle. One possibility to entangle photons is to employ materials that possess optical nonlinearities, but so far, there are no materials available whose nonlinearities are strong enough to allow for short gate times. An alternative approach was put forward by Knill et al., which requires only linear optical operations and measurements. However, this scheme is probabilistic and not very efficient in practice.
In this chapter, we present a scheme for the realization of a deterministic entangling gate for photons by using an atom lattice. An atom lattice consists of an ensemble of cold atoms loaded in the periodic optical potential created by a standing wave. In order to use the lattice as a quantum register for quantum computation, it is necessary to prepare it in a Mott insulating phase, in which the number of atoms in each site of the potential (i.e., in each of its minima) is approximately constant, and set equal to 1.
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