Alessandro Fedrizzi

University of Queensland, Brisbane, Queensland, Australia

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Publications (62)261.57 Total impact

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    ABSTRACT: Quantum mechanics allows for correlations that are stronger than anything that can be achieved in the classical world. There are, however, theories compatible with relativity which allow for even stronger correlations, while sharing many characteristics with quantum mechanics. The principle of information causality offers a possible explanation for why the world is quantum---and not described by one of these other models. Generalizing the no-signaling condition it suggests that the amount of accessible information must not be larger than the amount of transmitted information. Here we study this principle experimentally in the classical, quantum and post-quantum regimes. We simulate correlations that are stronger than allowed by quantum mechanics by exploiting the effect of polarization-dependent loss in a photonic Bell-test experiment.
    06/2014;
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    ABSTRACT: We optically demonstrate violation of the CHSH-Bell inequality and Tsirelson’s bound via loss and postselection. This enables us to more easily distinguish between entangled and unentangled states, and violates information causality with the postselected data.
    CLEO: QELS_Fundamental Science; 06/2014
  • Alessandro Fedrizzi
    02/2014;
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    ABSTRACT: Quantum physics constrains the accuracy of joint measurements of incompatible observables. Here we test tight measurement-uncertainty relations using single photons. We implement two independent, idealized uncertainty-estimation methods, the three-state method and the weak-measurement method, and adapt them to realistic experimental conditions. Exceptional quantum state fidelities of up to 0.999 98(6) allow us to verge upon the fundamental limits of measurement uncertainty.
    Physical Review Letters 01/2014; 112(2):020401. · 7.73 Impact Factor
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    ABSTRACT: The key requirement for quantum networking is the distribution of entanglement between nodes. Surprisingly, entanglement can be generated across a network without direct transfer-or communication-of entanglement. In contrast to information gain, which cannot exceed the communicated information, the entanglement gain is bounded by the communicated quantum discord, a more general measure of quantum correlation that includes but is not limited to entanglement. Here, we experimentally entangle two communicating parties sharing three initially separable photonic qubits by exchange of a carrier photon that is unentangled with either party at all times. We show that distributing entanglement with separable carriers is resilient to noise and in some cases becomes the only way of distributing entanglement through noisy environments.
    Physical Review Letters 12/2013; 111(23):230504. · 7.73 Impact Factor
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    ABSTRACT: Photons are critical to quantum technologies since they can be used for virtually all quantum information tasks: in quantum metrology, as the information carrier in photonic quantum computation, as a mediator in hybrid systems, and to establish long distance networks. The physical characteristics of photons in these applications differ drastically; spectral bandwidths span 12 orders of magnitude from 50 THz for quantum-optical coherence tomography to 50 Hz for certain quantum memories. Combining these technologies requires coherent interfaces that reversibly map centre frequencies and bandwidths of photons to avoid excessive loss. Here we demonstrate bandwidth compression of single photons by a factor 40 and tunability over a range 70 times that bandwidth via sum-frequency generation with chirped laser pulses. This constitutes a time-to-frequency interface for light capable of converting time-bin to colour entanglement and enables ultrafast timing measurements. It is a step toward arbitrary waveform generation for single and entangled photons.
    Nature Photonics 07/2013; 7(5). · 27.25 Impact Factor
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    ABSTRACT: We report an experimental demonstration of BosonSampling: an intermediate-model of quantum-computing. We verify that the scattering probabilities for three-photon interference are given by permanents of sub-matrices of a larger unitary matrix describing the optical network
    CLEO: QELS_Fundamental Science; 06/2013
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    ABSTRACT: We introduce an efficient method for fully characterizing multimode linear-optical networks. Our approach requires only a standard laser source and intensity measurements to directly and uniquely determine all moduli and non-trivial phases of the matrix describing a network. We experimentally demonstrate the characterization of a 6×6 fiber-optic network and independently verify the results via nonclassical two-photon interference.
    Optics Express 06/2013; 21(11):13450-8. · 3.55 Impact Factor
  • International Quantum Electronics Conference; 05/2013
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    ABSTRACT: Information gain in communication is bounded by the information encoded in the physical systems exchanged between sender and receiver. Surprisingly, this does not hold for quantum entanglement, which can increase even though the communicated system carries no entanglement at all. Here we demonstrate this phenomenon in a four-photon experiment where two parties sharing initially separable (unentangled) state get entangled by exchanging a photon that is {\it at all times} not entangled with either of them. Our result validates a long-standing assert in quantum information and has important practical implications in quantum networking, where entanglement must be reliably distributed across many nodes at low resource-cost.
    03/2013;
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    ABSTRACT: Quantum computers are unnecessary for exponentially-efficient computation or simulation if the Extended Church-Turing thesis---a foundational tenet of computer science---is correct. The thesis would be directly contradicted by a physical device that efficiently performs a task believed to be intractable for classical computers. Such a task is BosonSampling: obtaining a distribution of n bosons scattered by some linear-optical unitary process. Here we test the central premise of BosonSampling, experimentally verifying that the amplitudes of 3-photon scattering processes 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. Strong evidence against the Extended-Church-Turing thesis will come from scaling to large numbers of photons, which is a much simpler task than building a universal quantum computer.
    03/2013;
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    ABSTRACT: Linear photonic devices comprised of simple beamsplitters and phase shifters can implement any unitary operator for quantum information processing. The significant practical challenge is to characterize such an interferometric device once it is built. Performing quantum process tomography requires the full suite of quantum tools such as N-mode quantum state preparation and measurement, and is, despite progress on more efficient methods, slow and impractical for large interferometric devices. Here we introduce a simple technique to characterize the unitary matrix of a linear photonic device using standard laser sources and photodetectors, without the requirement for active locking or single-photon sources. Our method is precise and efficient, requiring only 2N-1 measurement configurations for a N-path network. We use it experimentally to characterise an integrated 3x3 fused-fibre coupler and highlight its precision by comparing measured quantum interference patterns with those predicted using the classically-estimated unitary. We observe excellent agreement between the two experimental methods.
    03/2013;
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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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    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; · 9.81 Impact Factor
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    ABSTRACT: The extended Church-Turing thesis posits that any computable function can be calculated efficiently by a probabilistic Turing machine. If this thesis held true, the global effort to build quantum computers might ultimately be unnecessary. The thesis would however be strongly contradicted by a physical device that efficiently performs a task believed to be intractable for classical computers. BosonSampling-the sampling from a distribution of n photons undergoing some linear-optical process-is a recently developed, and experimentally accessible example of such a task.
    Photonics Society Summer Topical Meeting Series, 2013 IEEE; 01/2013
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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 demonstrate spectral compression of single photons via sum-frequency generation with bright spectrally chirped laser pulses. The converted photon maintains the strong temporal correlation with an idler photon and its center frequency is tunable.
    Frontiers in Optics; 10/2012
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    ABSTRACT: Quantum steering allows the verification of shared entanglement even with an untrusted measurement device. We show the first photonic "detection loophole free" violation of a steering inequality by 48 standard deviations.
    Quantum Information and Measurement; 03/2012
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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 often not 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 characterize our near-perfect entanglement transfer using both state and process tomography. Our implementation is robust and flexible, making it a practical building block for future quantum networks.
    Physical Review A 01/2012; 85(1). · 3.04 Impact Factor
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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. · 10.02 Impact Factor

Publication Stats

793 Citations
261.57 Total Impact Points

Institutions

  • 2009–2014
    • University of Queensland
      • School of Mathematics and Physics
      Brisbane, Queensland, Australia
  • 2007–2013
    • Austrian Academy of Sciences
      • Institut für Quantenoptik und Quanteninformation - IQOQI Innsbruck
      Vienna, Vienna, Austria
  • 2012
    • Harvard University
      • Department of Physics
      Cambridge, MA, United States
  • 2011
    • Princeton University
      • Department of Chemistry
      Princeton, NJ, United States
  • 2006
    • University of Vienna
      • Basic Experimental Physics Training and Didactics Group
      Vienna, Vienna, Austria