Quantum Optics and Quantum Information

Our article, A quantum advantage for inferring causal structure, by Katja Reid, Megan Agnew, Lydia Vermeyden, Dominik Janzing, Rob Spekkens, and Kevin Resch just appeared in Nature Physics.  Giulio Chirabella wrote a News and Views piece on it entitled, Quantum information: Good causes.

Abstract: The problem of inferring causal relations from observed correlations is relevant to a wide variety of scientific disciplines. Yet given the correlations between just two classical variables, it is impossible to determine whether they arose from a causal influence of one on the other or a common cause influencing both. Only a randomized trial can settle the issue. Here we consider the problem of causal inference for quantum variables. We show that the analogue of a randomized trial, causal tomography, yields a complete solution. We also show that, in contrast to the classical case, one can sometimes infer the causal structure from observations alone. We implement a quantum-optical experiment wherein we control the causal relation between two optical modes, and two measurement schemes—with and without randomization—that extract this relation from the observed correlations. Our results show that entanglement and quantum coherence provide an advantage for causal inference.

Stories on IQC website here and Perimeter Institute website here.

Our new article Storage and Retrieval of THz-Bandwidth Single Photons Using a Room-Temperature Diamond Quantum Memory by D.G. England, K.A.G. Fisher, J-P.W. MacLean, P.J. Bustard,
R. Lausten, K.J. Resch, and B.J. Sussman was published in Physical Review Letters.  The work is the result of a great collaboration between our group and Ben Sussman’s Quantum Technology group at the National Research Council of Canada.  This work was selected for a synopsis in Physics and as an Editor’s suggestion.

Abstract: We report the storage and retrieval of single photons, via a quantum memory, in the optical phonons of a room-temperature bulk diamond. The THz-bandwidth heralded photons are generated by spontaneous parametric down-conversion and mapped to phonons via a Raman transition, stored for a variable delay, and released on demand. The second-order correlation of the memory output is g(2) = 0.65 ± 0.07, demonstrating a preservation of nonclassical photon statistics throughout storage and retrieval. The memory is low noise, high speed and broadly tunable; it therefore promises to be a versatile light-matter interface for local quantum processing applications.

Update March 8, 2015: Our work was covered in an article by S.M. Dambrot on Phys.org.

Check out our new paper in Nature Photonics entitled, Direct generation of three-photon polarizaton entanglement, by Deny R. Hamel, Lynden K. Shalm, Hannes Hübel, Aaron J. Miller, Francesco Marsili, Varun B. Verma, Richard P. Mirin, Sae Woo Nam, Kevin J. Resch, and Thomas Jennewein.

Abstract: Non-classical states of light are of fundamental importance for emerging quantum technologies. All optics experiments producing multi-qubit entangled states have until now relied on outcome post-selection, a procedure where only the measurement results corresponding to the desired state are considered. This method severely limits the usefulness of the resulting entangled states. Here, we show the direct production of polarization-entangled photon triplets by cascading two entangled downconversion processes. Detecting the triplets with high-efficiency superconducting nanowire single-photon detectors allows us to fully characterize them through quantum state tomography. We use our three-photon entangled state to demonstrate the ability to herald Bell states, a task that was not possible with previous three-photon states, and test local realism by violating the Mermin and Svetlichny inequalities. These results represent a significant breakthrough for entangled multi-photon state production by eliminating the constraints of outcome post-selection, providing a novel resource for optical quantum information processing.

Update Oct 18, 2014: Our article was discussed on physorg.com, sciencenews.org, photonics.com, and the Popular Science website

We have a new paper out (online) in Nature Photonics today entitled, Experimental three-photon quantum nonlocality under strict locality conditions, by Chris Erven, Evan Mayer-Scott, Kent Fisher, Jonathan Lavoie, Brendon Higgins, Zhizhong Yan, Chris Pugh, Jean-Phillipe Bourgoin, Robert Prevedel, Krister Shalm, Laura Richards, Nick Gigov, Raymond Laflamme, Gregor Weihs, Thomas Jennewein, and Kevin Resch.   This paper is the result of a great collaboration between three IQC groups and a former IQC faculty member, now at University of Innsbruck.

Abstract: Quantum correlations, often observed as violations of Bell inequalities, are critical to our understanding of the quantum world, with far-reaching technologicaland fundamental impact. Many tests of Bell inequalities have studied pairs of correlated particles. However, interest in multi-particle quantum correlations is driving the experimental frontier to test larger systems. All violations to date require supplementary assumptions that open results to loopholes, the closing of which is one of the most important challenges in quantum science. Seminal experiments have closed some loopholes, but no experiment has closed locality loopholes with three or more particles. Here, we close both the locality and freedom-of-choice loopholes by distributing three-photon Greenberger–Horne–Zeilinger entangled statesto independent observers. We measured a violation of Mermin’s inequalitywith parameter 2.77 ± 0.08, violating its classical bound by nine standard deviations. These results are a milestone in multi-party quantum communication and a significant advancement of the foundations of quantum mechanics.

Update April 16, 2014: Geoff Pryde discussed our work in a Nature Photonics News and Views article Entanglement à trois

Our new paper, Quantum Computing on encrypted data, by Kent Fisher, Anne Broadbent, Krister Shalm, Zhizhong Yan, Jonathan Lavoie, Robert Prevedel, Thomas Jennewein, and Kevin Resch was published in Nature Communications.

Abstract: The ability to perform computations on encrypted data is a powerful tool for protecting privacy. Recently, protocols to achieve this on classical computing systems have been found. Here, we present an efficient solution to the quantum analogue of this problem that enables arbitrary quantum computations to be carried out on encrypted quantum data. We prove that an untrusted server can implement a universal set of quantum gates on encrypted quantum bits (qubits) without learning any information about the inputs, while the client, knowing the decryption key, can easily decrypt the results of the computation. We experimentally demonstrate, using single photons and linear optics, the encryption and decryption scheme on a set of gates sufficient for arbitrary quantum computations. As our protocol requires few extra resources compared with other schemes it can be easily incorporated into the design of future quantum servers. These results will play a key role in enabling the development of secure distributed quantum systems.