We describe a scheme that allows for the generation of any desired N-photon state on demand. Under ideal conditions, this requires only N single photon sources, laser pulses and linear optics elements. First, the sources should be initialized with the help of single-qubit rotations and repeat-until-success two-qubit quantum gates [Y.L. Lim, A. Beige and L.C. Kwek, Phys. Rev. Lett. 95 030505 (2005).]. Afterwards, the state of the sources can be mapped onto the state of N newly generated photons whenever needed.
We describe a scheme that allows for the generation of any desired N-photon state on demand. Under ideal conditions, this requires only N single photon sources, laser pulses and linear optics elements. First, the sources should be initialized with the help of single-qubit rotations and repeat-until-success two-qubit quantum gates [Y.L. Lim, A. Beige and L.C. Kwek, Phys. Rev. Lett. 95 030505 (2005).]. Afterwards, the state of the sources can be mapped onto the state of N newly generated photons whenever needed.
Photon entanglement is an essential ingredient for linear optics quantum computing schemes, quantum cryptographic protocols and fundamental tests of quantum mechanics. Here we describe a setup that allows for the generation of polarisation-entangled N-photon states on demand. The photons are obtained by mapping the entangled state of N atoms, each of them trapped inside an optical cavity, onto the free radiation field. The required initial state can be prepared by performing postselective measurements on the collective emission from the cavities through a multiport beamsplitter.
We present a scheme to generate the polarization-entangled two-photon state 1/2(∣H〉∣V〉+∣V〉∣H〉), which is of much interest in the field of quantum information processing. Furthermore, we demonstrate the capability of this concept in respect of a generalization to entangle N-photon states for interferometry and lithography. This scheme requires single-photon sources, linear optical elements, and a multifold coincidence detection.
The problem of on-demand generation of entanglement between single-atom qubits via a common photonic channel is examined within the framework of optical interferometry. As expected, for a Mach-Zehnder interferometer with coherent laser beam as input, a high-finesse optical cavity is required to overcome sensitivity to spontaneous emission. We show, however, that with a twin-Fock input, useful entanglement can in principle be created without cavity enhancement. Both approaches require single-photon resolving detectors, and best results would be obtained by combining both cavity feedback and twin-Fock inputs. Such an approach may allow a fidelity of 0.99 using a two-photon input and currently available mirror and detector technology. In addition, we study interferometers based on NOON states, i.e., maximally entangled N-particle states, and show that they perform similarly to the twin-Fock states, yet without the need for high-precision photodetectors. The present interferometrical approach can serve as a universal, scalable circuit element for quantum information processing, from which fast quantum gates, deterministic teleportation, entanglement swapping, etc., can be realized with the aid of single-qubit operations.
We introduce an architecture for robust and scalable quantum computation using both stationary qubits (e.g., single photon sources made out of trapped atoms, molecules, ions, quantum dots, or defect centers in solids) and flying qubits (e.g., photons). Our scheme solves some of the most pressing problems in existing nonhybrid proposals, which include the difficulty of scaling conventional stationary qubit approaches, and the lack of practical means for storing single photons in linear optics setups. We combine elements of two previous proposals for distributed quantum computing, namely the efficient photon-loss tolerant build up of cluster states by Barrett and Kok [Phys. Rev. A 71, 060310(R) (2005)] with the idea of repeat-until-success (RUS) quantum computing by Lim et al. [Phys. Rev. Lett. 95, 030505 (2005)]. This idea can be used to perform eventually deterministic two qubit logic gates on spatially separated stationary qubits via photon pair measurements. Under nonideal conditions, where photon loss is a possibility, the resulting gates can still be used to build graph states for one-way quantum computing. In this paper, we describe the RUS method, present possible experimental realizations, and analyze the generation of graph states.
We generalize in higher dimensions the so-called ``flying-qubit scheme'' that was described in the paper by Lim, Beige, and Kwek [Phys. Rev. Lett. 95, 030505 (2005)]. In that paper, the authors proposed a scheme according to which distant atoms get entangled during a measurement, in the Bell basis, of photons (flying qubits) emitted by them. We show that although in principle a generalization of this scheme to arbitrary dimensions is possible, this theoretical proposal is not presently feasible in all dimensions because only qubit Bell states have been successively measured until now. Nevertheless we show that a many-qubits generalization of the flying-qubit scheme factorizes and reduces to the realization, in parallel, of many individual single-qubit schemes, for which it is known that they are realizable experimentally with the techniques that are available today. In other words, our approach shows that when d is an even prime power (d=2m), the flying-qudit scheme reduces to m flying-qubit schemes. For d=2m and arbitrary m the implementation of a generalized, maximally entangling, conditional qudit phase gate ``with insurance'' or ``repeat-until-success'' is thus shown to be feasible in practice by coupling m (pairs of) two-level atoms to m (pairs of) two-level (polarized) photons. Moreover, due to the parallelism of the task, the time necessary for completing successfully the task scales logarithmically in a function of m while at the same time the dimension of the Hilbert space scales exponentially which presents promising perspectives regarding quantum informational realizations such as the quantum computer.