The mapping of photonic states to collective excitations of atomic ensembles is a powerful tool which finds a useful application in the realization of quantum memories and quantum repeaters. In this work we show that cold atoms in optical lattices can be used to perform an entangling unitary operation on the transferred atomic excitations. After the release of the quantum atomic state, our protocol results in a deterministic two qubit gate for photons. The proposed scheme is feasible with current experimental techniques and robust against the dominant sources of noise.
The mapping of photonic states to collective excitations of atomic ensembles is a powerful tool which finds a useful application in the realization of quantum memories and quantum repeaters. In this work we show that cold atoms in optical lattices can be used to perform an entangling unitary operation on the transferred atomic excitations. After the release of the quantum atomic state, our protocol results in a deterministic two qubit gate for photons. The proposed scheme is feasible with current experimental techniques and robust against the dominant sources of noise.
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.
In a quantum network, transmission of quantum states is performed by photonic channels, which distribute them between remote locations. However, the storage and manipulation of the photons is more problematic. Recently, several schemes have been proposed to store photons efficiently by interfacing the photonic channel with an atomic system, i.e. an atomic ensemble. In those schemes, the photonic input state is transferred to a collective atomic state. But the manipulation of photonic states requires the ability to perform entangling operations. We show how to do this manipulation and perform a deterministic entangling gate between photons. To this order, we interface a system of cold neutral atoms in an optical lattice with the photonic channel, such that now the atoms are not only used to store information, but also to process it by means of cold controlled collisions in the lattice.
Quantum optics provides tools for the accurate control of light using atoms and also for manipulating atomic states using light. These techniques are being used for many applications, including quantum information processing and the generation of exotic quantum states. This thesis contains proposals for implementing a photonic quantum memory and a photon-photon gate, which are essential elements of photonic quantum information processing. Furthermore it proposes a scheme for the creation of many-body entangled states. First a proposal for a new quantum memory protocol, called the atomic frequency sweep quantum memory, is presented. A two-level polariton model is derived that explains the coherent storage and retrieval of light through the manipulation of the atomic resonance frequency. This is followed by a scheme for a deterministic photonic controlled-PHASE gate based on the strong interaction between two stationary collective Rydberg excitations in an atomic ensemble. Distortion effects caused by nonuniform interaction are quantified and compensation techniques for these effects are proposed. Finally a proposal is presented that uses Rydberg dressing for the generation of energy cat states in an atomic medium, i.e. superposition states of all the atoms being in the ground or excited state, where these two states are connected by an optical transition and thus have a significant difference in energy. Considering the fragility of the state, the effects of many different imperfections and decoherence sources are quantified. The resulting cat state would allow testing of energy decoherence models with greatly improved sensitivity.
Quantum information science and atom optics are among the most active fields in modern physics. One of our main activities is to use atomic qubits as quantum memory and exploit photonic qubits for information transfer and processing to achieve efficient linear optics quantum information processing (QIP). On the one hand, utilizing the interaction between laser pulses and ultra-cold atomic ensembles we experimentally investigate the potentials of atomic ensembles in the gas phase to build quantum repeaters for long-distance quantum communication, that is, to develop a new technological solution for quantum repeaters making use of the effective qubit-type entanglement of two cold atomic ensembles by a projective measurement of individual photons by spontaneous Raman processes. On the other hand, building on our long experience in research on multi-photon entanglement, The techniques that are being developed will lay the basis for future large-scale realizations of linear optical QIP with atoms and photons.
Large scale quantum information processing requires stable and long-lived quantum memories. Here, using atom-photon entanglement, we propose an experimentally feasible scheme to realize decoherence-free quantum memory with atomic ensembles, and show one of its applications, remote transfer of unknown quantum state, based on laser manipulation of atomic ensembles, photonic state operation through optical elements, and single-photon detection with moderate efficiency. The scheme, with inherent fault-tolerance to the practical noise and imperfections, allows one to retrieve the information in the memory for further quantum information processing within the reach of current technology.
Quantum information has been witnessing great science value and latent application since 1980's. The research work presented here consist of mainly two important parts: manipulations of multiphoton entanglement and atomic ensembles based quantum memory. In first part, the experimental technique multi-photon entanglement is further developed to study fundamental issues in quantum mechanics, remarkable applications to quantum communication and quantum computation. Specifically, we have demonstrated a bit-flip error-free transfer of quantum information, violation of Bell's inequality beyond Tsirelson's bound, teleportation of a two-qubit composite system, as well as the one-way computing by two-photon-four-qubit cluster state. To overcome un-scalability problem due to probabilistic feature in linear optical quantum information processing, we investigated in the second part the physics of atomic ensembles based quantum memory. We show that theoretically, entanglement between distant locations can be deterministically generated. The experimental work has thoroughly developed the necessary techniques and we have achieved deterministic single photon source, and interference of the photons from independent atomic ensembles, teleportation between photonic and atomic qubits, a novel way to create a robust entanglement between an atomic and a photonic qubit, and memory based entanglement swapping. We believe, the developed techniques here would dramatically facilitate progresses in many fields including global quantum communication, linear optical quantum computation and the foundations of quantum mechanics etc.