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arXiv:quant-ph/0608052 · Physical Review Letters

Entanglement Generation by Fock-State Filtration

Kevin J. Resch, Jeremy L. O’Brien, Till J. Weinhold, Kaoru Sanaka, B. P. Lanyon, Nathan K. Langford, Andrew G. White

Published May 16, 2007• 22 citations
arXivHTMLPDF

Abstract

We demonstrate a Fock-state filter which is capable of preferentially blocking single photons over photon pairs. The large conditional nonlinearities are based on higher-order quantum interference, using linear optics, an ancilla photon, and measurement. We demonstrate that the filter acts coherently by using it to convert unentangled photon pairs to a path-entangled state. We quantify the degree of entanglement by transforming the path information to polarization information; applying quantum state tomography we measure a tangle of T=(20+/-9)%.

DOI
10.1103/physrevlett.98.203602

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We demonstrate a Fock-state filter which is capable of preferentially blocking single photons over photon pairs. The large conditional nonlinearities are based on higher-order quantum interference, using linear optics, an ancilla photon, and measurement. We demonstrate that the filter acts coherently by using it to convert unentangled photon pairs to a path-entangled state. We quantify the degree of entanglement by transforming the path information to polarization information; applying quantum state tomography we measure a tangle of T=(20+/-9)%.
Unprecedented optical nonlinearities can be generated probabilistically in simple linear-optical networks conditioned on specific measurement outcomes. We describe a highly controllable quantum filter for photon number states, which takes advantage of such a measurement-induced amplitude nonlinearity. The basis for this filter is multiphoton nonclassical interference which we demonstrate for one- and two-photon states over a wide range of beam splitter reflectivities. Specifically, we show that the transmission probability, conditional on a specific measurement outcome, can be larger for a two-photon state than a one-photon state; this is not possible with linear optics alone.
An optical-quantum-circuit entanglement filter can pass a pair of pho-tons with the desired polarization correlations. Photons are an important information-conveyance medium for enhanced computers and communications based on quantum mechanics. Filtering multiple photon states is a key capabili-ty in quantum-information science and technology, where one-quantum binary-digit (one-qubit) filters, or polarizers, have found wide application. Filtering based on the correlation or en-tanglement of photons requires multi-qubit filters with qubit-qubit interactions. We demonstrate an optical-entanglement filter that passes a pair of photons if they have the desired polarization correlations.1 The entangling capability of the filter was verified, distinguishing it from classical ones. Because our entanglement filter acts on photonic qubits, it is promising for quantum tech-
We propose a scheme to implement a single-mode quantum filter, which selectively eliminates the one-photon state in a quantum state $\ensuremath{\alpha}|0〉+\ensuremath{\beta}|1〉+\ensuremath{\gamma}|2〉.$ The vacuum state and the two-photon state are transmitted without any change. This scheme requires single-photon sources, linear optical elements, and photon detectors. Furthermore, we demonstrate how this filter can be used to realize a two-qubit projective measurement and to generate multiphoton polarization entangled states.
A macrostate consisting of N approximately 3.5x10{4} photons in a quantum superposition and entangled with a far apart single-photon state (microstate) is generated. Precisely, an entangled photon pair is created by a nonlinear optical process; then one photon of the pair is injected into an optical parametric amplifier operating for any input polarization state, i.e., into a phase-covariant cloning machine. Such transformation establishes a connection between the single photon and the multiparticle fields. We then demonstrate the nonseparability of the bipartite system by adopting a local filtering technique within a positive operator valued measurement.