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arXiv:quant-ph/9909075 · Physical Review A

Optical qubit by conditional interferometry

Matteo G. A. Paris

Published Aug 15, 2000• 43 citations
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Abstract

We suggest a method to prepare any chosen superposition ${a}_{0}|0〉{+a}_{1}|1〉$ of the vacuum and one-photon states. The method is based on a conditional double interferometer fed by a one-photon state and a coherent state. The scheme involves only linear optical elements and avalanche photodetectors, and therefore it should be realizable with current technology. A realistic description of the triggering photodetectors is employed, i.e., we assume that they can only check, with a certain efficiency, whether or not any photon is present. We discuss two working regimes, and show that output states with fidelity arbitrarily close to unit may be obtained, with nonvanishing conditional probability, also for low quantum efficiency at the photodetectors.

DOI
10.1103/physreva.62.033813

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We propose an optical scheme to generate a superposition of coherent states with enhanced size adopting an interferometric setting at the single-photon level currently available in the laboratory. Our scheme employs a nondegenerate optical parametric amplifier together with two beam splitters so that the detection of single photons at the output conditionally implements the desired superposition of second-order photonic operations. We analyze our proposed scheme by considering realistic on-off photodetectors with nonideal efficiency in heralding the success of conditional events. A high-quality performance of our scheme is demonstrated in view of various criteria such as quantum fidelity, mean output energy, and measure of quantum interference.
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We propose a scheme to generate entanglement between a single-photon qubit in the polarization basis and a coherent state of light. The required resources are a superposition of coherent states, a polarization entangled photon pair, beam splitters, the displacement operation, and four photodetectors. Even when realistic detectors with a limited efficiency are used, an arbitrarily high fidelity can be obtained by adjusting a beam-splitter ratio and the displacement amplitude at the price of reducing the success probability. Our analysis shows that high fidelities may be obtained using on-off detectors with low efficiencies and available resource states under current technology.
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 NNN-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.