Alessandro Fedrizzi

University of Queensland, Brisbane, Queensland, Australia

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Publications (68)306.11 Total impact

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    ABSTRACT: Transport phenomena on a quantum scale appear in a variety of systems, ranging from photosynthetic complexes to engineered quantum devices. It has been predicted that the efficiency of quantum transport can be enhanced through dynamic interaction between the system and a noisy environment. We report the first experimental demonstration of such environment-assisted quantum transport, using an engineered network of laser-written waveguides, with relative energies and inter-waveguide couplings tailored to yield the desired Hamiltonian. Controllable decoherence is simulated via broadening the bandwidth of the input illumination, yielding a significant increase in transport efficiency relative to the narrowband case. We show integrated optics to be suitable for simulating specific target Hamiltonians as well as open quantum systems with controllable loss and decoherence.
    No preview · Article · Apr 2015
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    ABSTRACT: Scaling up linear-optics quantum computing will require multi-photon gates which are compact, phase-stable, exhibit excellent quantum interference, and have success heralded by the detection of ancillary photons. We investigate implementation of the optimal known gate design which meets these requirements: the Knill controlled-Z gate, implemented in integrated laser-written waveguide arrays. We show that device performance is more sensitive to the small deviations in the coupler reflectivity, arising due to the tolerance values of the fabrication method, than phase variations in the circuit. The mode fidelity was also shown to be less sensitive to reflectivity and phase errors than process fidelity. Our best device achieves a fidelity of 0.931±0.001 with the ideal 4×4 unitary circuit and a process fidelity of 0.680±0.005 with the ideal computational-basis process.
    Full-text · Article · Feb 2015
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    ABSTRACT: Quantum mechanics is an outstandingly successful description of nature, underpinning fields from biology through chemistry to physics. At its heart is the quantum wavefunction, the central tool for describing quantum systems. Yet it is still unclear what the wavefunction actually is: does it merely represent our limited knowledge of a system, or is it an element of reality? Recent no-go theorems argued that if there was any underlying reality to start with, the wavefunction must be real. However, that conclusion relied on debatable assumptions, without which a partial knowledge interpretation can be maintained to some extent. A different approach is to impose bounds on the degree to which knowledge interpretations can explain quantum phenomena, such as why we cannot perfectly distinguish non-orthogonal quantum states. Here we experimentally test this approach with single photons. We find that no knowledge interpretation can fully explain the indistinguishability of non-orthogonal quantum states in three and four dimensions. Assuming that some underlying reality exists, our results strengthen the view that the entire wavefunction should be real. The only alternative is to adopt more unorthodox concepts such as backwards-in-time causation, or to completely abandon any notion of objective reality.
    Full-text · Article · Dec 2014 · Nature Physics
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    ABSTRACT: We fully characterize the reduced dynamics of an open quantum system initially correlated with its environment. Using a photonic qubit coupled to a simulated environment we tomographically reconstruct a superchannel---a generalised channel that treats preparation procedures as inputs---from measurement of the system alone, despite its coupling to the environment. We introduce novel quantitative measures for determining the strength of initial correlations, and to allow an experiment to be optimised in regards to its environment.
    Full-text · Article · Oct 2014 · Physical Review Letters
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    Martin Ringbauer · Alessandro Fedrizzi · Dominic W. Berry · Andrew G. White
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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.
    Full-text · Article · Jun 2014 · Scientific Reports
  • Martin Ringbauer · Alessandro Fedrizzi · Dominic W. Berry · Andrew G. White
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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.
    No preview · Conference Paper · Jun 2014
  • Alessandro Fedrizzi

    No preview · Article · Feb 2014 · Physics
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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.
    Full-text · Article · Jan 2014 · Physical Review Letters
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    ABSTRACT: In this chapter we review the process of parametric down-conversion (PDC) and discuss the different methods to use PDC as a heralded single-photon source.PDC is a non-linear optical process, where an incoming pump photon decays, under energy and momentum conservation, into a photon-pair. The creation of photons in pairs allows for the implementation of a single-photon source by detecting one photon (trigger) to herald the presence of its partner (signal). The engineering possibilities of PDC enable the generation of single-photons with high rates in a wide range of frequencies.This chapter is structured as follows: Section 11.2 describes the principles of PDC in non-linear media. We derive the quantum state of the generated photon-pairs, investigate the spectral purity and photon-number purity of the heralded signal photon and discuss the achievable single-photon generation rates. In section 11.3 we turn towards experimental realizations and introduce bulk crystal PDC. Section 11.4 elaborates on the use of periodic poling to engineer the PDC process. Finally, section 11.5 gives an overview over PDC in waveguides.A comparison of experimental data from various heralded singe-photon sources based on PDC is presented in section 11.6 with an overview of nonlinear materials suited for PDC given in section 11.7.
    No preview · Article · Dec 2013 · Experimental Methods in the Physical Sciences
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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.
    Full-text · Article · Dec 2013 · Physical Review Letters
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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.
    Full-text · Article · Jul 2013 · Nature Photonics
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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.
    No preview · Conference Paper · Jul 2013
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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
    No preview · Conference Paper · Jun 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.
    Full-text · Article · Jun 2013 · Optics Express
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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, experimentally accessible example of such a task [1].
    No preview · Conference Paper · May 2013
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    ABSTRACT: Optical quantum computing (QC) increasingly uses integrated optics based experiments which permit circuit compactness and phase stability. However, despite the rapid adaptations of integrated waveguide devices for quantum photonics, initial gate demonstrations operate in post-selection, thus not allowing scaling of a quantum circuit beyond the depth of a single gate. Recently, a number of quantum circuits have been demonstrated using the femtosecond laser direct write (FLDW) technique. This technique induces refractive index change in glass substrates which can form three-dimensional waveguide devices. Here we demonstrate a potentially scalable waveguide gate for QC, a controlled-phase gate or Knill gate, produced using the FLDW technique. This gate produces a phase shift on a target qubit conditional on the state of a control qubit, as shown in Fig. 1(a). It requires four photons for operation, two of which act as the target and control path-encoded qubits and two ancillas which herald the successful operation of the circuit.
    No preview · Conference Paper · May 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.
    No preview · Article · Mar 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.
    No preview · Article · Mar 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.
    No preview · Article · Mar 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.
    Preview · Article · Jan 2013 · Physical Review A

Publication Stats

2k Citations
306.11 Total Impact Points

Institutions

  • 2009-2014
    • University of Queensland
      • • School of Mathematics and Physics
      • • ARC Centre of Excellence for Quantum Computation and Communication Technology
      Brisbane, Queensland, Australia
  • 2005-2013
    • Austrian Academy of Sciences
      • Institute for Quantum Optics and Quantum Information - IQOQI Innsbruck
      Wien, Vienna, Austria
  • 2011
    • University of Waterloo
      • Institute for Quantum Computing
      Ватерлоо, Ontario, Canada
  • 2004
    • University of Vienna
      Wien, Vienna, Austria