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Optics InfoBase Conference Papers

Einstein-Podolsky-Rosen Steering with No Detection Loophole over 1 km of Optical Fibre

Howard M. Wiseman, D. A. Evans, Cyril Branciard, Eric G. Cavalcanti, Adam Bennet, D. J. Saunders, Geoff J. Pryde

Published Jan 1, 2011• 0 citations
Publisher

Abstract

Whether for quantum cryptography or for testing fundamental properties of the universe, it will be essential to demonstrate nonclassical effects over longer and longer distances. The barrier to doing so is loss of photons during propagation, because considering only those cases where a photon is detected opens a “detection loophole” in the security of any quantum information task in which parties or devices are untrusted. EPR-steering is a nonclassical effect which allows one party to verify that he shares entanglement with another party, even though he doesn't trust her. Using new, arbitrarily loss-tolerant tests, we perform the first detection-loophole-free demonstration of EPR-steering with entangled photon pairs. The parties are separated by a coiled 1 km optical fibre, giving total losses of 8.9 dB (87%).

DOI
10.1364/iqec.2011.i1183

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Whether for quantum cryptography or for testing fundamental properties of the universe, it will be essential to demonstrate nonclassical effects over longer and longer distances. The barrier to doing so is loss of photons during propagation, because considering only those cases where a photon is detected opens a “detection loophole” in the security of any quantum information task in which parties or devices are untrusted. EPR-steering is a nonclassical effect which allows one party to verify that he shares entanglement with another party, even though he doesn't trust her. Using new, arbitrarily loss-tolerant tests, we perform the first detection-loophole-free demonstration of EPR-steering with entangled photon pairs. The parties are separated by a coiled 1 km optical fibre, giving total losses of 8.9 dB (87%).
Demonstrating nonclassical effects over longer and longer distances is essential for both quantum technology and fundamental science.The main challenge is the loss of photons during propagation, because considering only those cases where photons are detected opens a ''detection loophole'' in security whenever parties or devices are untrusted.Einstein-Podolsky-Rosen steering is equivalent to an entanglement-verification task in which one party (device) is untrusted.We derive arbitrarily loss-tolerant tests, enabling us to perform a detection-loophole-free demonstration of Einstein-Podolsky-Rosen steering with parties separated by a coiled 1-km-long optical fiber, with a total loss of 8.9 dB (87%).
Tests of the predictions of quantum mechanics for entangled systems have provided increasing evidence against local realistic theories. However, there remains the crucial challenge of simultaneously closing all major loopholes—the locality, freedom-of-choice and detection loopholes—in a single experiment. An important sub-class of local realistic theories can be tested with the concept of 'steering'. The term 'steering' was introduced by Schrödinger in 1935 for the fact that entanglement would seem to allow an experimenter to remotely steer the state of a distant system as in the Einstein–Podolsky–Rosen (EPR) argument. Einstein called this 'spooky action at a distance'. EPR-steering has recently been rigorously formulated as a quantum information task opening it up to new experimental tests. Here, we present the first loophole-free demonstration of EPR-steering by violating three-setting quadratic steering inequality, tested with polarization-entangled photons shared between two distant laboratories. Our experiment demonstrates this effect while simultaneously closing all loopholes: both the locality loophole and a specific form of the freedom-of-choice loophole are closed by having a large separation of the parties and using fast quantum random number generators, and the fair-sampling loophole is closed by having high overall detection efficiency. Thereby, we exclude—for the first time loophole-free—an important class of local realistic theories considered by EPR. Besides its foundational importance, loophole-free steering also allows the distribution of quantum entanglement secure event in the presence of an untrusted party.
Loophole-free quantum nonlocality often demands experiments with high complexity (defined by all parties' settings and outcomes) and multiple efficient detectors. Here, we identify the fundamental efficiency and complexity thresholds for quantum steering using two-qubit entangled states. Remarkably, it requires only one photon detector on the untrusted side, with efficiency ε>1/Xε> 1/Xε>1/X, where X≥2X \geq 2X≥2 is the number of settings on that side. This threshold applies to all pure entangled states, in contrast to analogous Bell-nonlocality tests, which require almost unentangled states to be loss-tolerant. We confirm these predictions in a minimal-complexity (X=2X = 2X=2 for the untrusted party and a single three-outcome measurement for the trusted party), detection-loophole-free photonic experiment with $ε= (51.6 \pm 0.4)\% $.
Shared quantum correlations find application in a wide range of modern quantum information protocols. This thesis presents several optical experiments towards the improved certification of shared quantum correlations, using photonic qubits. The first experiment demonstrates the Einstein–Podolsky–Rosen (EPR) steering effect with no detection loophole. Polarisation–entangled photon pairs generated in a Sagnac interferometer were used to test new loss–tolerant EPR steering inequalities. To test the robustness of the loss–tolerant EPR steering nequalities, a transmission loss of −4.3dB was introduced using a 1km optical fibre loop, finding inequality violations for n = 10 and n = 16 measurement settings with no detection loophole (equating to a total loss of 87%). The findings demonstrate how the new loss–tolerant protocols can be used to certify the EPR steering effect for arbitrarily high losses, and may find application in one–sided device–independent quantum key distribution protocols. The second experiment introduces EPR steering in the context of an EPR steering game. The rules of the game allow for certification of shared quantum correlations via EPR steering inequalites without requiring trust in devices or parties external to a quantum referee. In particular, the experiment successfully demonstrates EPR steering in a measurement–device independent framework; a framework which has typically been accessible only through Bell inequality violations. The findings suggest the possibility for quantum–refereed one–sided device–independent quantum key distribution protocols.
Entanglement is the key resource for many long-range quantum information tasks, including secure communication and fundamental tests of quantum physics. These tasks require robust verification of shared entanglement, but performing it over long distances is presently technologically intractable because the loss through an optical fiber or free-space channel opens up a detection loophole. We design and experimentally demonstrate a scheme that verifies entanglement in the presence of at least 14.8 ± 0.1 dB of added loss, equivalent to approximately 80 km of telecommunication fiber. Our protocol relies on entanglement swapping to herald the presence of a photon after the lossy channel, enabling event-ready implementation of quantum steering. This result overcomes the key barrier in device-independent communication under realistic high-loss scenarios and in the realization of a quantum repeater.