Alessandro Fedrizzi

University of Queensland, Brisbane, Queensland, Australia

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Publications (66)305.16 Total impact

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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.
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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.
    Nature Physics 12/2014; DOI:10.1038/nphys3233 · 20.60 Impact Factor
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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.
    Physical Review Letters 10/2014; 114(9). DOI:10.1103/PhysRevLett.114.090402 · 7.73 Impact Factor
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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.
    Scientific Reports 06/2014; 4. DOI:10.1038/srep06955 · 5.08 Impact Factor
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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
    Physics 02/2014; 7. DOI:10.1103/Physics.7.25
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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. DOI:10.1103/PhysRevLett.112.020401 · 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. DOI:10.1103/PhysRevLett.111.230504 · 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). DOI:10.1038/nphoton.2013.47 · 29.96 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. DOI:10.1364/OE.21.013450 · 3.53 Impact Factor
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    ABSTRACT: form only given. 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].Here we report an experimental verification of one key assumption of BosonSampling: that multi-photon interference amplitudes are given by the permanents of submatrices of a larger unitary describing the photonic circuit. We built a tunable photonic circuit consisting of a central 3x3 fiber beamsplitter (Fig. 1) and exploited orthogonal polarization modes to extend the network to 6x6 modes. We developed a direct characterization method [2] to obtain the unitary description of this network and compared theoretical interference patterns predicted from this unitary with an experimental signature obtained via non-classical interference of three single photons [3].Our results show good agreement with theory, and we can rule out an explanation of the observed interference via classical means. We conclude that small-scale BosonSampling can be performed in the presence of unavoidable optical loss, imperfect photon sources, and inefficient detection [3].To reach a regime of 20 to 30 photons, where BosonSampling experiments are expected to start outperforming modern computers, we need to precisely quantify the contributions of realistic noise created in current photon sources. To this end, we modeled the detrimental effects of spectral and photon-number impurity [4] of independently generated photon pairs on the expected multi-photon interference patterns. We tested our model by fully - apping out three-photon interference as a function of individual temporal delays. While a full-scale demonstration is still out of reach, our results promise that a scaling-up of BosonSampling to single-photon numbers reached in state-of-the-art quantum optics experiments is feasible.
    International Quantum Electronics Conference; 05/2013
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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.
    2013 Conference on Lasers & Electro-Optics Europe & International Quantum Electronics Conference CLEO EUROPE/IQEC; 05/2013
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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.
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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.
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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.
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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.
    Physical Review A 01/2013; 89(4). DOI:10.1103/PhysRevA.89.042323 · 2.99 Impact Factor
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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; 110(4). DOI:10.1073/pnas.1213201110 · 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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    [Show abstract] [Hide abstract]
    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; 339(6121). DOI:10.1126/science.1231440 · 31.48 Impact Factor

Publication Stats

1k Citations
305.16 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