Peter B. R. Nisbet-Jones, Jerome Dilley, Annemarie Holleczek, Oliver Barter, Axel Kuhn
Quantum information processing (QIP) has become an attractive interdisciplinary research topic. With single quantum systems controlled to encode elementary 'quantum' bits (qubits) of information, a fundamental enhancement of computing and information security is now in reach. Particular attention is paid to QIP in linear-optics quantum circuits (LOQC), which are in principle scalable to larger networks if it were not for the spontaneous nature of parametric down conversion (PDC) photon sources. Here, we demonstrate that single photons deterministically emitted from a single atom into an optical cavity can be equally used for LOQC, thus levying these restrictions. With a 500 ns coherence time, also a sub-dividision of photons into several time bins of arbitrary amplitudes and phases is possible. In particular, in place of storing a simple qubit in one photon (being present or absent), the subdivision into d time bins is now used to encode arbitrary qudits in one photon. We verify the fidelity of the encoding with a series of quantum-homodyne measurements. These are performed by sending the photons of interest together with single reference photons (acting as local oscillator) into a small quantum network consisting of one single beamsplitter, and monitoring the photon-photon correlations between its output ports in a time resolved manner.
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http://arxiv.org/abs/1203.5614
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