Lynden K. Shalm

Strong Loophole-Free Test of Local Realism

We performed an loophole-free test of Bells inequalities. The probability that local realism is compatible with our results is less than 5.9×10^-9.

Significant-Loophole-Free Test of Bell's Theorem with Entangled Photons.

Local realism is the worldview in which physical properties of objects exist independently of measurement and where physical influences cannot travel faster than the speed of light. Bells theorem states that this worldview is incompatible with the predictions of quantum mechanics, as is expressed in Bells inequalities. Previous experiments convincingly supported the quantum predictions. Yet, every experiment requires assumptions that provide loopholes for a local realist explanation. Here, we report a Bell test that closes the most significant of these loopholes simultaneously. Using a well-optimized source of entangled photons, rapid setting generation, and highly efficient superconducting detectors, we observe a violation of a Bell inequality with high statistical significance. The purely statistical probability of our results to occur under local realism does not exceed 3.74×10^{-31}, corresponding to an 11.5 standard deviation effect.

Three-photon energy–time entanglement

Many-particle entangled states and entanglement between continuous properties are valuable resources for quantum information, but are notoriously difficult to generate. An experiment now entangles the energy and emission times of three photons, creating generalized Einstein–Podolsky–Rosen correlations.

Direct generation of three-photon polarization entanglement

A three-photon entangled Greenberger–Horne–Zeilinger state is directly produced by cascading two entangled down-conversion processes. Experimentally, 11.1 triplets per minute are detected on average. The three-photon entangled state is used for state tomography and as a test of local realism by violating the Mermin and Svetlichny inequalities.

Squeezing and over-squeezing of triphotons.

Quantum mechanics places a fundamental limit on the accuracy of measurements. In most circumstances, the measurement uncertainty is distributed equally between pairs of complementary properties; this leads to the ‘standard quantum limit’ for measurement resolution. Using a technique known as ‘squeezing’, it is possible to reduce the uncertainty of one desired property below the standard quantum limit at the expense of increasing that of the complementary one. Squeezing is already being used to enhance the sensitivity of gravity-wave detectors and may play a critical role in other high precision applications, such as atomic clocks and optical communications. Spin squeezing (the squeezing of angular momentum variables) is a powerful tool, particularly in the context of quantum light–matter interfaces. Although impressive gains in squeezing have been made, optical spin-squeezed systems are still many orders of magnitude away from the maximum possible squeezing, known as the Heisenberg uncertainty limit. Here we demonstrate how an optical system can be squeezed essentially all the way to this fundamental bound. We construct spin-squeezed states by overlapping three indistinguishable photons in an optical fibre and manipulating their polarization (spin), resulting in the formation of a squeezed composite particle known as a ‘triphoton’. The symmetry properties of polarization imply that the measured triphoton states can be most naturally represented by quasi-probability distributions on the surface of a sphere. In this work we show that the spherical topology of polarization imposes a limit on how much squeezing can occur, leading to the quasi-probability distributions wrapping around the sphere—a phenomenon we term ‘over-squeezing’. Our observations of spin-squeezing in the few-photon regime could lead to new quantum resources for enhanced measurement, lithography and information processing that can be precisely engineered photon-by-photon.

Experimental characterization of qutrits using symmetric informationally complete positive operator-valued measurements

Z. E. D. Medendorp1,†, F. A. Torres-Ruiz, L. K. Shalm, G. N. M. Tabia, C. A. Fuchs, and A. M. Steinberg Centre for Quantum Information & Quantum Control and Institute for Optical Sciences, Department of Physics, University of Toronto, 60 St George Street, Toronto, Ontario, Canada M5S 1A7 Center for Optics and Photonics, Universidad de Concepción, Casilla 160-C, Concepción, Chile Departamento de Ciencias F́ısicas, Universidad de La Frontera, Temuco, Casilla 54-D, Chile. Perimeter Institute for Theoretical Physics, Waterloo, Ontario N2L 2Y5, Canada †zmedendo@lakeheadu.ca

Experimental three-photon quantum nonlocality under strict locality conditions

Violation of the classical bound of the three-particle Mermin inequality by nine standard deviations is experimentally demonstrated by closing both the locality and freedom-of-choice loopholes; only the fair-sampling assumption is required. To achieve this, a light source for producing entangled multiphoton states and measurement technologies for precise timing and efficient detection were developed.

Multiparticle state tomography: hidden differences.

We address the problem of completely characterizing multiparticle states including loss of information to unobserved degrees of freedom. In systems where nonclassical interference plays a role, such as linear-optics quantum gates, such information can degrade interference in two ways, by decoherence and by distinguishing the particles. Distinguishing information, often the limiting factor for quantum optical devices, is not correctly described by previous state-reconstruction techniques, which account only for decoherence. We extend these techniques and find that a single modified density matrix can completely describe partially coherent, partially distinguishable states. We use this observation to experimentally characterize two-photon polarization states in single-mode optical fiber.

Spectral correlation measurements at the Hong-Ou-Mandel interference dip

We present an efficient tool capable of measuring the spectral correlations between photons emerging from a Hong-Ou-Mandel interferometer. We show that for our spectrally factorizable spontaneous downconversion source the Hong-Ou-Mandel interference visibility decreases as the photons frequency spread is increased to a maximum of 165 nm. Unfiltered, we obtained a visibility of

Generating polarization-entangled photon pairs using cross-spliced birefringent fibers

92.0 \pm 0.2 \%