Biomedical subjects

Philip Walther

Publications and source records attributed to Philip Walther.

8 recordsLinked to original sources

PubMed2007

High-speed linear optics quantum computing using active feed-forward.

As information carriers in quantum computing, photonic qubits have the advantage of undergoing negligible decoherence. However, the absence of any significant photon-photon interaction is problematic for the realization of non-trivial two-qubit gates. One solution is to introduce an effective nonlinearity by measurements resulting in probabilistic gate operations. In one-way quantum computation, the random quantum measurement error can be overcome by applying a feed-forward technique, such that the future measurement basis depends on earlier measurement results. This technique is crucial for achieving deterministic quantum computation once a cluster state (the highly entangled multiparticle state on which one-way quantum computation is based) is prepared. Here we realize a concatenated scheme of measurement and active feed-forward in a one-way quantum computing experiment. We demonstrate that, for a perfect cluster state and no photon loss, our quantum computation scheme would operate with good fidelity and that our feed-forward components function with very high speed and low error for detected photons. With present technology, the individual computational step (in our case the individual feed-forward cycle) can be operated in less than 150 ns using electro-optical modulators. This is an important result for the future development of one-way quantum computers, whose large-scale implementation will depend on advances in the production and detection of the required highly entangled cluster states.

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PubMed2006

Comparative financial analysis of minimally invasive surgery to open surgery for small renal tumours < or =3.5 cm: a single institutional experience.

OBJECTIVE: We analysed total hospital costs by comparing minimally invasive surgery (MIS) procedures, such as laparoscopic cryoablation (LCA), laparoscopic partial nephrectomy (LPN), and hand-assisted laparoscopic nephrectomy (HALN), with conventional surgery. METHODS: Between March 2000 and July 2005, 184 consecutive patients underwent surgery for a small, organ-confined renal tumour < or =3.5 cm in diameter. The distribution of patients among the surgical procedures was: HALN (n=53); LPN (n=20); open radical nephrectomy (ORN; n=20); open partial nephrectomy (OPN; n=71); and LCA (n=20). Total hospital costs were analysed for each procedure. RESULTS: Patients undergoing OPN at a mean age of 58+/-13 yr were significantly younger those undergoing HALN, ORN, and LCA. The mean hospital length of stay in the LCA group (2.0+/-1.2 d) was shorter than all other groups (p<0.05). Higher surgical costs occurred with LCA, LPN, and HALN compared (p<0.05) with ORN and OPN. However, total financial costs were lower for LCA and HALN with more obvious differences between LCA and the other four groups. CONCLUSIONS: The costs of MIS remain competitive with traditional surgery. Although the surgical costs were higher, LCA had the lowest total hospital costs for the renal tumour < or =3.5 cm at our institution. Long-term oncologic efficacy studies will be needed to fully appreciate the cost-efficacy ratio of MIS.

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PubMed2006

Experimental entangled entanglement.

All previous tests of local realism have studied correlations between single-particle measurements. In the present experiment, we have performed a Bell experiment on three particles in which one of the measurements corresponds to a projection onto a maximally entangled state. We show theoretically and experimentally that correlations between these entangled measurements and single-particle measurements are too strong for any local-realistic theory and are experimentally exploited to violate a Clauser-Horne-Shimony-Holt-Bell inequality by more than 5 standard deviations. We refer to this possibility as "entangled entanglement."

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PubMed2005

Experimental violation of a cluster state bell inequality.

Cluster states are a new type of multiqubit entangled states with entanglement properties exceptionally well suited for quantum computation. In the present work, we experimentally demonstrate that correlations in a four-qubit linear cluster state cannot be described by local realism. This exploration is based on a recently derived Bell-type inequality [V. Scarani et al., Phys. Rev. A 71, 042325 (2005)] which is tailored, by using a combination of three- and four-particle correlations, to be maximally violated by cluster states but not violated at all by GHZ states. We observe a cluster-state Bell parameter of 2.59+/-0.08, which is more than 7sigma larger than the threshold of 2 imposed by local realism.

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PubMed2004

Communications: quantum teleportation across the Danube.

Efficient long-distance quantum teleportation is crucial for quantum communication and quantum networking schemes. Here we describe the high-fidelity teleportation of photons over a distance of 600 metres across the River Danube in Vienna, with the optimal efficiency that can be achieved using linear optics. Our result is a step towards the implementation of a quantum repeater, which will enable pure entanglement to be shared between distant parties in a public environment and eventually on a worldwide scale.

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PubMed2004

Realization of a photonic controlled-NOT gate sufficient for quantum computation.

We report the first experimental demonstration of a quantum controlled-NOT gate for different photons, which is classically feed forwardable. In the experiment, we achieved this goal with only the use of linear optics, an entangled ancillary pair of photons, and postselection. The techniques developed in our experiment are of significant importance for quantum information processing with linear optics.

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PubMed2004

De Broglie wavelength of a non-local four-photon state.

Superposition is one of the most distinctive features of quantum theory and has been demonstrated in numerous single-particle interference experiments. Quantum entanglement, the coherent superposition of states in multi-particle systems, yields more complex phenomena. One important type of multi-particle experiment uses path-entangled number states, which exhibit pure higher-order interference and the potential for applications in metrology and imaging; these include quantum interferometry and spectroscopy with phase sensitivity at the Heisenberg limit, or quantum lithography beyond the classical diffraction limit. It has been generally understood that in optical implementations of such schemes, lower-order interference effects always decrease the overall performance at higher particle numbers. Such experiments have therefore been limited to two photons. Here we overcome this limitation, demonstrating a four-photon interferometer based on linear optics. We observe interference fringes with a periodicity of one-quarter of the single-photon wavelength, confirming the presence of a four-particle mode-entangled state. We anticipate that this scheme should be extendable to arbitrary photon numbers, holding promise for realizable applications with entanglement-enhanced performance.

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PubMed2003

Long-distance free-space distribution of quantum entanglement.

We demonstrate the distribution of quantum entanglement via optical free-space links to independent receivers separated by 600 m, with no line of sight between each other. A Bell inequality between those receivers is violated by more than four standard deviations, confirming the quality of the entanglement. This outdoor experiment represents a step toward satellite-based distributed quantum entanglement.

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