ABSENCE OF REENTRANCE IN THE TWO-DIMENSIONAL XY-MODEL WITH RANDOM PHASE SHIFT

We show, that the 2D XY-model with random phase shifts exhibits for low temperature and small disorder a phase with quasi-long-range order, and that the transition to the disordered phase is {\it not} reentrant. These results are obtained by heuristic arguments, an analytical renormalization group calculation, and a numerical Migdal-Kadanoff renormalization group treatment. Previous predictions of reentrance are found to fail due to an overestimation of the vortex pair density as a consequence of independent dipole approximations. At positions, where vortex pairs are energetically favored by disorder, their statistics becomes effectively fermionic. The results may have implications for a large number of related models.Comment: 5 pages, latex, with 2 figures, one author added, minor text changes, to be published in J. de Physique

Entanglement-enhanced optical gyroscope

Fiber optic gyroscopes (FOG) based on the Sagnac effect are a valuable tool in sensing and navigation and enable accurate measurements in applications ranging from spacecraft and aircraft to self-driving vehicles such as autonomous cars. As with any classical optical sensors, the ultimate performance of these devices is bounded by the standard quantum limit (SQL). Quantum-enhanced interferometry allows us to overcome this limit using non-classical states of light. Here, we report on an entangled-photon gyroscope that uses path-entangled NOON-states (N=2) to provide phase supersensitivity beyond the standard-quantum-limit

A fundamental test and an application of quantum entanglement

Diese Arbeit beschreibt zwei Experimente, die auf Korrelationsmessungen zwischen verschränkten Photonen basieren. Die Photonen werden dabei zwischen den kanarischen Inseln La Palma und Teneriffa 144 km räumlich voneinander getrennt. Das erste Experiment trägt zur Diskussion darüber bei, ob quantenmechanische Vorhersagen innerhalb eines lokal-realistischen Rahmens beschrieben werden können. Diese Frage spielt seit der Veröffentlichung des berühmten Einstein-Podolsky-Rosen “Paradoxons” [1] eine fundamentale Rolle in der Begründung der Quantenmechanik. Das beschriebene Experiment ist ein Test der CHSH Form [2] der Bell’schen Ungleichung [3] und schließt gleichzeitig zwei der drei “Schlupflöcher” für lokalen Realismus, die in einem experimentellen Test der Bell’schen Ungleichung auftreten können. Es sind dies das Locality und das Freedom-of-choice Schlupfloch. Letzteres wurde bis heute experimentell nicht adressiert und zu allererst in unserem Experiment durch raumzeitliche Trennung der Wahl der Analysatorstellung und der Photonemission geschlossen. Das dritte Schlupfloch, das Fair-sampling Schlupfloch [4], konnte wegen zu niedriger Detektionseffizienz leider nicht geschlossen werden. Da unser Experiment jedoch das Erste ist, das mehr als ein Schlupfloch gleichzeitig schließt und die CHSH Ungleichung mit mehr als 16 Standardabweichungen durch Sexp = 2.37 ± 0.023 verletzt, repräsentieren unsere Resultate die bis heute schlüssigste Verletzung des lokalen Realismus. Gleichzeitig stellt unser Experiment einen wichtigen Schritt in Richtung eines vollkommen schlupflochfreien Bell Tests dar, eine der bedeutensten ungelösten Herausvorderungen der fundamentalen Physik. Im zweiten Experiment werden die faszinierenden Eigenschaften verschränkter Photonen ausgenutzt, um die verschränkungsbasierte “Verteilung” quantenkryptographischer Schlüssel (quantum key distribution, QKD) zu demonstrieren. Diese technische Anwendung quantenmechanischer Eigenschaften ist wohl eine der ausgereiftesten im Bereich der Quanteninformation und Quantenkommunikation. Für QKD Experimente bei denen man hohen Abschwächungen im Quantenkanal ausgesetzt ist, wie etwa in zukünftigen satellitenbasierten Netzwerken oder Glasfasernetzwerken, ist es wichtig die effizientesten Systeme zu verwenden. Es wurde kürzlich gezeigt [5], dass QKD mit verschränkten Photonen höhere Abschwächungen tolerieren kann als Systeme die auf schwachen Laserpulsen basieren. Das ist vorallem der Fall, wenn die Quelle verschränkter Photonen symmetrisch zwischen den Empfängerstationen, Alice und Bob, liegt. In unserem Experiment untersuchen wir diesen wichtigen Vorteil eines symmetrischen Systems und implementieren drei unterschiedliche experimentelle Aufbauten. Diese benutzen einen 144 km langen optischen Kanal zwischen den kanarischen Inseln La Palma und Teneriffa und weisen Photonpaar Abschwächungen von 35 dB, 58 dB beziehungsweise 71 dB auf. Dabei wurde die Quelle der verschränkten Photonenpaare entweder direkt bei Alice, asymetrisch zwischen Alice und Bob oder symmetrisch in der mitte zwischen Alice und Bob platziert. Wir zeigen, dass unsere experimentellen Resultate sehr gut mit dem theoretischen Modell übereinstimmen, welches auf eine aktuelle Arbeit [5] bezogen ist, jedoch an unsere experimentellen Parameter angepasst wurde. Verglichen mit dem zu erwartenden Photonenverlust bei der Übertragung von einem Satelliten im “low-earth-orbit” (LEO) zur Erde [6] geben unsere Resultate Grund zur Annahme, dass verschränkungsbasierte QKD Systeme geeignet sind, solche Übertragungen sowohl in einem Einzel- als auch Doppellink Szenario [7] durchzuführen.This work describes two experiments that are based on correlation measurements between entangled photons, spatially separated by 144 km between the Canary Islands, La Palma and Tenerife. The first of which contributes to the debate of whether or not quantum mechanical predictions can be described within a local realistic frame, a question that plays a fundamental role in the foundation of quantum mechanics ever since the famous Einstein-Podolsky-Rosen (EPR) “paradox” [1]. The experiment presented is a test of the CHSH form [2] of Bell’s inequality [3], simultaneously closing two out of three possible “loopholes” for local realism that can arise in an experimental Bell test. These two loopholes are the locality loophole and the freedom-of-choice loophole. The latter has not been addressed experimentally so far and was closed for the first time in our experiment by space-like separating the setting choice from the photon pair emission. Unfortunately, the third crucial loophole, i.e., the fair-sampling loophole [4], could not be closed due to inefficient photon detection. However, our experiment is the first to close more than one loophole at a time. By violating the CHSH inequality by more than 16 standard deviations with Sexp = 2.37 ± 0.023, this is the most conclusive violation of local realism to date and represents an important step towards a completely loophole-free Bell test, which is one of the most significant still-unresolved challenges in fundamental physics. Within the second experiment described, the intriguing properties of photonic entanglement are exploited for demonstrating entanglement based quantum key distribution (QKD), probably one of the most mature applications in the field of quantum information and quantum communication. In high loss situations, such as in the case of future satellite based or optical fiber based quantum communication networks, it is important to implement the most efficient experimental QKD scheme. It has recently been emphasized [5] that entanglement based quantum key distribution systems can tolerate higher channel losses compared to systems based on weak coherent laser pulses. This is in particular the case when the entangled photon source is located symmetrically between the two receiver stations, called Alice and Bob. We experimentally studied this important advantage by implementing different entanglement based QKD setups on a 144 km free-space link between the two Canary Islands, La Palma and Tenerife. We studied three different configurations that operated at two-photon attenuations of 35 dB, 58 dB and 71 dB, respectively. In these experiments, the entangled photon source was placed either at Alice’s location, asymmetrically between Alice and Bob or symmetrically in the middle between Alice and Bob. In addition, we show that our experimental results agree well with the theoretical model devised in [5], which we applied to our experimental parameters. Compared to the expected link attenuations in a low-earth-orbit (LEO) satellite to ground scenario [6], as it might be implemented in a future network, we expect from our results that entanglement based QKD systems are suitable to be used within either a single-downlink configuration or a configuration with two simultaneous downlinks [7]

Experimental quantum teleportation over a high-loss free-space channel

We present a high-fidelity quantum teleportation experiment over a high-loss free-space channel between two laboratories. We teleported six states of three mutually unbiased bases and obtained an average state fidelity of 0.82(1), well beyond the classical limit of 2/3. With the obtained data, we tomographically reconstructed the process matrices of quantum teleportation. The free-space channel attenuation of 31 dB corresponds to the estimated attenuation regime for a down-link from a low-earth-orbit satellite to a ground station. We also discussed various important technical issues for future experiments, including the dark counts of single-photon detectors, coincidence-window width etc. Our experiment tested the limit of performing quantum teleportation with state-of-the-art resources. It is an important step towards future satellite-based quantum teleportation and paves the way for establishing a worldwide quantum communication network

Quantum erasure with causally disconnected choice

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

Experimental test of photonic entanglement in accelerated reference frames

The quantization of the electromagnetic field has successfully paved the way for the development of the Standard Model of Particle Physics and has established the basis for quantum technologies. Gravity, however, continues to hold out against physicists' efforts of including it into the framework of quantum theory. Experimental techniques in quantum optics have only recently reached the precision and maturity required for the investigation of quantum systems under the influence of gravitational fields. Here, we report on experiments in which a genuine quantum state of an entangled photon pair was exposed to a series of different accelerations. We measure an entanglement witness for $g$ values ranging from 30 mg to up to 30 g - under free-fall as well on a spinning centrifuge - and have thus derived an upper bound on the effects of uniform acceleration on photonic entanglement. Our work represents the first quantum optics experiment in which entanglement is systematically tested in geodesic motion as well as in accelerated reference frames with acceleration a>>g = 9.81 m/s^2.Comment: 7 pages, 5 figure