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ABSTRACT: Not all quantum protocols require entanglement to outperform their classical
alternatives. The nonclassical correlations that lead to this quantum advantage
are conjectured to be captured by quantum discord. Here we demonstrate that
discord can be explicitly used as a resource: certifying untrusted entangling
gates without generating entanglement at any stage. We implement our protocol
in the single-photon regime, and show its success in the presence of high
levels of noise and imperfect gate operations. Our technique offers a practical
method for benchmarking entangling gates in physical architectures in which
only highly-mixed states are available.
01/2013;
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Xiao-Song Ma,
Johannes Kofler,
Angie Qarry,
Nuray Tetik,
Thomas Scheidl,
Rupert Ursin,
Sven Ramelow,
Thomas Herbst,
Lothar Ratschbacher, Alessandro Fedrizzi,
Thomas Jennewein,
Anton Zeilinger
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ABSTRACT: The counterintuitive features of quantum physics challenge many common-sense assumptions. In an interferometric quantum eraser experiment, one can actively choose whether or not to erase which-path information (a particle feature) of one quantum system and thus observe its wave feature via interference or not by performing a suitable measurement on a distant quantum system entangled with it. In all experiments performed to date, this choice took place either in the past or, in some delayed-choice arrangements, in the future of the interference. Thus, in principle, physical communications between choice and interference were not excluded. Here, we report a quantum eraser experiment in which, by enforcing Einstein locality, no such communication is possible. This is achieved by independent active choices, which are space-like separated from the interference. Our setup employs hybrid path-polarization entangled photon pairs, which are distributed over an optical fiber link of 55 m in one experiment, or over a free-space link of 144 km in another. No naive realistic picture is compatible with our results because whether a quantum could be seen as showing particle- or wave-like behavior would depend on a causally disconnected choice. It is therefore suggestive to abandon such pictures altogether.
Proceedings of the National Academy of Sciences 01/2013; · 9.68 Impact Factor
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ABSTRACT: Quantum computers are unnecessary for exponentially efficient computation or simulation if the Extended Church-Turing thesis is correct. The thesis would be strongly contradicted by physical devices that efficiently perform tasks believed to be intractable for classical computers. Such a task is boson sampling: sampling the output distributions of n bosons scattered by some linear-optical unitary process. Here, we test the central premise of boson sampling, experimentally verifying that 3-photon scattering amplitudes are given by the permanents of submatrices generated from a unitary describing a 6-mode integrated optical circuit. We find the protocol to be robust, working even with the unavoidable effects of photon loss, non-ideal sources, and imperfect detection. Scaling this to large numbers of photons will be a much simpler task than building a universal quantum computer.
Science 12/2012; · 31.20 Impact Factor
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ABSTRACT: We introduce an efficient method for characterizing any multi-mode linear
photonic network. Our method employs a standard laser source and intensity
measurements to directly determine all moduli and non-trivial phases of the
matrix describing the network. We experimentally demonstrate our method by
characterizing a $6{\times}6$ fibre-optic network and independently verify the
results via nonclassical two-photon interference.
10/2012;
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Devin H Smith,
Geoff Gillett,
Marcelo P de Almeida,
Cyril Branciard, Alessandro Fedrizzi,
Till J Weinhold,
Adriana Lita,
Brice Calkins,
Thomas Gerrits,
Howard M Wiseman,
Sae Woo Nam,
Andrew G White
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ABSTRACT: Quantum steering allows two parties to verify shared entanglement even if one measurement device is untrusted. A conclusive demonstration of steering through the violation of a steering inequality is of considerable fundamental interest and opens up applications in quantum communication. To date, all experimental tests with single-photon states have relied on post selection, allowing untrusted devices to cheat by hiding unfavourable events in losses. Here we close this 'detection loophole' by combining a highly efficient source of entangled photon pairs with superconducting transition-edge sensors. We achieve an unprecedented ∼62% conditional detection efficiency of entangled photons and violate a steering inequality with the minimal number of measurement settings by 48 s.d.s. Our results provide a clear path to practical applications of steering and to a photonic loophole-free Bell test.
Nature Communications 01/2012; 3:625. · 7.40 Impact Factor
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ABSTRACT: Topological phases exhibit some of the most striking phenomena in modern physics. Much of the rich behaviour of quantum Hall systems, topological insulators, and topological superconductors can be traced to the existence of robust bound states at interfaces between different topological phases. This robustness has applications in metrology and holds promise for future uses in quantum computing. Engineered quantum systems--notably in photonics, where wavefunctions can be observed directly--provide versatile platforms for creating and probing a variety of topological phases. Here we use photonic quantum walks to observe bound states between systems with different bulk topological properties and demonstrate their robustness to perturbations--a signature of topological protection. Although such bound states are usually discussed for static (time-independent) systems, here we demonstrate their existence in an explicitly time-dependent situation. Moreover, we discover a new phenomenon: a topologically protected pair of bound states unique to periodically driven systems.
Nature Communications 01/2012; 3:882. · 7.40 Impact Factor
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ABSTRACT: We present a simple technique to reduce the emission rate of higher-order
photon events from pulsed spontaneous parametric down-conversion. The technique
uses extra-cavity control over a mode locked ultrafast laser to simultaneously
increase repetition rate and reduce the energy of each pulse from the pump
beam. We apply our scheme to a photonic quantum gate, showing improvements in
the non-classical interference visibility for 2-photon and 4-photon
experiments, and in the quantum-gate fidelity and entangled state production in
the 2-photon case.
10/2011;
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ABSTRACT: Entangled photons play a pivotal role in the distribution of quantum
information in quantum networks. However, the frequency bands for optimal
transmission and storage of photons are not necessarily the same. Here we
experimentally demonstrate the coherent frequency conversion of photons
entangled in their polarization, a widely used degree of freedom in photonic
quantum information processing. We verify the successful entanglement
conversion by violating a Clauser-Horne-Shimony-Holt (CHSH) Bell inequality and
fully confirm that our characterised fidelity of entanglement transfer is close
to unity using both state and process tomography. Our implementation is robust
and flexible, making it a practical building block for future quantum networks.
06/2011;
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ABSTRACT: One of the most striking features of quantum mechanics is the appearance of
phases of matter with topological origins. These phases result in remarkably
robust macroscopic phenomena such as the edge modes in integer quantum Hall
systems, the gapless surface states of topological insulators, and elementary
excitations with non-abelian statistics in fractional quantum Hall systems and
topological superconductors. Many of these states hold promise in the
applications to quantum memories and quantum computation. Artificial quantum
systems, with their precise controllability, provide a versatile platform for
creating and probing a wide variety of topological phases. Here we investigate
topological phenomena in one dimension, using photonic quantum walks. The
photon evolution simulates the dynamics of topological phases which have been
predicted to arise in, for example, polyacetylene. We experimentally confirm
the long-standing prediction of topologically protected localized states
associated with these phases by directly imaging their wavefunctions. Moreover,
we reveal an entirely new topological phenomenon: the existence of a
topologically protected pair of bound states which is unique to periodically
driven systems. Our experiment demonstrates a powerful new approach for
controlling topological properties of quantum systems through periodic driving.
05/2011;
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ABSTRACT: Tests such as Bell's inequality and Hardy's paradox show that joint probabilities and correlations between distant particles in quantum mechanics are inconsistent with local realistic theories. Here we experimentally demonstrate these concepts in the time domain, using a photonic entangling gate to perform nondestructive measurements on a single photon at different times. We show that Hardy's paradox is much stronger in time and demonstrate the violation of a temporal Bell inequality independent of the quantum state, including for fully mixed states.
Physical Review Letters 05/2011; 106(20):200402. · 7.37 Impact Factor
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ABSTRACT: We study the entanglement dynamics of discrete time quantum walks acting on
bounded finite sized graphs. We demonstrate that, depending on system
parameters, the dynamics may be monotonic, oscillatory but highly regular, or
quasi-periodic. While the dynamics of the system are not chaotic since the
system comprises linear evolution, the dynamics often exhibit some features
similar to chaos such as high sensitivity to the system's parameters,
irregularity and infinite periodicity. Our observations are of interest for
entanglement generation, which is one primary use for the quantum walk
formalism. Furthermore, we show that the systems we model can easily be mapped
to optical beamsplitter networks, rendering experimental observation of
quasi-periodic dynamics within reach.
02/2011;
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ABSTRACT: Many applications in optical quantum information processing benefit from careful spectral shaping of single-photon wave-packets. In this paper we tailor the joint spectral wave-function of photons created in parametric downconversion by engineering the nonlinearity profile of a poled crystal. We designed a crystal with an approximately Gaussian nonlinearity profile and confirmed successful wave-packet shaping by two-photon interference experiments. We numerically show how our method can be applied for attaining one of the currently most important goals of single-photon quantum optics, the creation of pure single photons without spectral correlations.
Optics Express 01/2011; 19(1):55-65. · 3.59 Impact Factor
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Thomas Scheidl,
Rupert Ursin,
Johannes Kofler,
Sven Ramelow,
Xiao-Song Ma,
Thomas Herbst,
Lothar Ratschbacher, Alessandro Fedrizzi,
Nathan K Langford,
Thomas Jennewein,
Anton Zeilinger
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ABSTRACT: Bell's theorem shows that local realistic theories place strong restrictions on observable correlations between different systems, giving rise to Bell's inequality which can be violated in experiments using entangled quantum states. Bell's theorem is based on the assumptions of realism, locality, and the freedom to choose between measurement settings. In experimental tests, "loopholes" arise which allow observed violations to still be explained by local realistic theories. Violating Bell's inequality while simultaneously closing all such loopholes is one of the most significant still open challenges in fundamental physics today. In this paper, we present an experiment that violates Bell's inequality while simultaneously closing the locality loophole and addressing the freedom-of-choice loophole, also closing the latter within a reasonable set of assumptions. We also explain that the locality and freedom-of-choice loopholes can be closed only within nondeterminism, i.e., in the context of stochastic local realism.
Proceedings of the National Academy of Sciences 11/2010; 107(46):19708-13. · 9.68 Impact Factor
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ABSTRACT: Non-classical states of light, such as entangled photon pairs and number states, are essential for fundamental tests of quantum mechanics and optical quantum technologies. The most widespread technique for creating these quantum resources is spontaneous parametric down-conversion of laser light into photon pairs. Conservation of energy and momentum in this process, known as phase-matching, gives rise to strong correlations that are used to produce two-photon entanglement in various degrees of freedom. It has been a longstanding goal in quantum optics to realize a source that can produce analogous correlations in photon triplets, but of the many approaches considered, none has been technically feasible. Here we report the observation of photon triplets generated by cascaded down-conversion. Each triplet originates from a single pump photon, and therefore quantum correlations will extend over all three photons in a way not achievable with independently created photon pairs. Our photon-triplet source will allow experimental interrogation of novel quantum correlations, the generation of tripartite entanglement without post-selection and the generation of heralded entangled photon pairs suitable for linear optical quantum computing. Two of the triplet photons have a wavelength matched for optimal transmission in optical fibres, suitable for three-party quantum communication. Furthermore, our results open interesting regimes of non-linear optics, as we observe spontaneous down-conversion pumped by single photons, an interaction also highly relevant to optical quantum computing.
Nature 07/2010; 466(7306):601-3. · 36.28 Impact Factor
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Thomas Scheidl,
Rupert Ursin, Alessandro Fedrizzi,
Sven Ramelow,
Xiao-song Ma,
Thomas Herbst,
Robert Prevedel,
Lothar Ratschbacher,
Johannes Kofler,
Thomas Jennewein,
Anton Zeilinger
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ABSTRACT: A significant limitation of practical quantum key distribution (QKD) setups is currently their limited operational range. It has recently been emphasized (X. Ma, C.-H. F. Fung, and H.-K. Lo., Phys. Rev. A, 76:012307, 2007) that entanglement-based QKD systems can tolerate higher channel losses than systems based on weak coherent laser pulses (WCP), in particular when the source is located symmetrically between the two communicating parties, Alice and Bob. In the work presented here, we experimentally study this important advantage by implementing different entanglement-based QKD setups on a 144~km free-space link between the two Canary Islands of La Palma and Tenerife. We established three different configurations where the entangled photon source was placed at Alice's location, asymmetrically between Alice and Bob and symmetrically in the middle between Alice and Bob, respectively. The resulting quantum channel attenuations of 35~dB, 58~dB and 71~dB, respectively, significantly exceed the limit for WCP systems. This confirms that QKD over distances of 300~km and even more is feasible with entangled state sources placed in the middle between Alice and Bob. Comment: 14 pages, 5 figures, 1 table
07/2010;
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ABSTRACT: Single-photon pairs created in the nonlinear process of spontaneous
parametric downconversion form the backbone of fundamental and applied
experimental quantum information science. Many applications benefit from
careful spectral shaping of the single-photon wave-packets. In this paper we
tailor the joint spectral wave-function of downconverted photons by modulating
the nonlinearity of a poled crystal without affecting the phase-matching
conditions. We designed a crystal with a Gaussian nonlinearity profile and
confirmed successful wave-packet shaping by two-photon interference
experiments. We numerically show how our method can be applied for attaining
one of the currently most important goals of single-photon quantum optics, the
creation of pure single photons without spectral correlations.
05/2010;
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ABSTRACT: We demonstrate that the concept of information offers a more complete
description of complementarity than the traditional approach based on
observables. We present the first experimental test of information
complementarity for two-qubit pure states, achieving close agreement with
theory; We also explore the distribution of information in a comprehensive
range of mixed states. Our results highlight the strange and subtle properties
of even the simplest quantum systems: for example, entanglement can be
increased by reducing correlations between two subsystems.
02/2010;
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ABSTRACT: Quantum entanglement enables tasks not possible in classical physics. Many quantum communication protocols require the distribution of entangled states between distant parties. Here we experimentally demonstrate the successful transmission of an entangled photon pair over a 144 km free-space link. The received entangled states have excellent, noise-limited fidelity, even though they are exposed to extreme attenuation dominated by turbulent atmospheric effects. The total channel loss of 64 dB corresponds to the estimated attenuation regime for a two-photon satellite quantum communication scenario. We confirm that the received two-photon states are still highly entangled by violating the CHSH inequality by more than 5 standard deviations. From a fundamental point of view, our results show that the photons are virtually not subject to decoherence during their 0.5 ms long flight through air, which is encouraging for future world-wide quantum communication scenarios. Comment: 5 pages, 3 figures, replaced paper with published version, added journal reference
02/2009;
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Thomas Scheidl,
Rupert Ursin,
Johannes Kofler,
Sven Ramelow,
Xiao-song Ma,
Thomas Herbst,
Lothar Ratschbacher, Alessandro Fedrizzi,
Nathan Langford,
Thomas Jennewein,
Anton Zeilinger
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ABSTRACT: The predictions of quantum mechanics can be in striking contradiction with local realism if entanglement exists between distant systems. Bell's theorem shows that local realistic theories, such as classical physics, place a strong restriction on observable correlations between different systems in experiments, giving rise to Bell's inequality [1]. This allows an experimental test of whether nature itself agrees with local realism or quantum mechanics. To derive his inequality, Bell made three assumptions: realism (objects possess definite properties prior to and independent of observation), locality (space-like separated events cannot causally influence each other), and freedom of choice (the choice of measurement settings is free or random). In experimental tests of Bell's inequality, there may be "loopholes" which allow observed violations to still be explained by local realistic theories. Many Bell tests have been performed which violate Bell's inequality [2-13], some which have closed individual loopholes, specifically the locality loophole [11] and the fair-sampling loophole [12]. Another crucial loophole, which has been discussed theoretically in Ref. [14] but not yet addressed experimentally, is related to Bell's freedom-of-choice assumption. Here we report an experiment using entangled photons, which for the first time closes this loophole by randomly switching measurement settings and space-like separating the setting choice from the photon pair emission. There has previously been much experimental and theoretical progress towards a complete loophole-free Bell test (e.g., Refs [14-18]). However, our experiment, which simultaneously closes the locality and the freedom-of-choice loopholes, is the first to close more than one of the three crucial loopholes at the same time.
12/2008;
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ABSTRACT: Two-photon anti-bunching at a beamsplitter is only possible if the photons are entangled in a specific state, anti-symmetric in the spatial modes. Thus, observation of anti-bunching is an indication of entanglement in a degree of freedom which might not be easily accessible in an experiment. We experimentally demonstrate this concept in the case of the interference of two frequency entangled photons with continuous frequency detunings. The principle of anti-symmetrisation of the spatial part of a wavefunction and subsequent detection of hidden entanglement via anti-bunching at a beamsplitter may facilitate the observation of entanglement in other systems, like atomic ensembles or Bose-Einstein condensates. The analogue for fermionic systems would be to observe bunching. Comment: Published version, 10 pages, 3 figures
07/2008;
