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Quantum correlations, often observed as violations of Bell inequalities, are critical to our understanding of the quantum world, with far-reaching technological and 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 states to independent observers. We measured a violation of Mermin's inequality with 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. [ABSTRACT FROM AUTHOR]

Optical quantum communication utilizing satellite platforms has the potential to extend the reach of quantum key distribution (QKD) from terrestrial limits of ~200 km to global scales. We have developed a thorough ~ numerical simulation using realistic simulated orbits and incorporating the effects of pointing error, diffraction, atmosphere and telescope design, to obtain estimates of the loss and background noise which a satellite-based system would experience. Combining with quantum optics simulations of sources and detection, we determine the length of secure key for QKD, as well as entanglement visibility and achievable distances for fundamental experiments. We analyse the performance of a low Earth orbit satellite for downlink and uplink scenarios of the quantum optical signals. We argue that the advantages of locating the quantum source on the ground justify a greater scientific interest in an uplink as compared to a downlink. An uplink with a ground transmitter of at least 25 cm diameter and a 30 cm receiver telescope on the satellite could be used to successfully perform QKD multiple times per week with either an entangled photon source or with a weak coherent pulse source, as well as perform long-distance Bell tests and quantum teleportation. Our model helps to resolve important design considerations such as operating wavelength, type and specifications of sources and detectors, telescope designs, specific orbits and ground station locations, in view of anticipated overall system performance. [ABSTRACT FROM AUTHOR]

Entangled quantum particles have correlations stronger than those allowed by classical physics. These correlations are at the heart of deep foundational questions in quantum mechanics, and form the basis of many emerging quantum technologies. Although the discrete variables of up to 14 ions and the continuous variables between three intense optical beams have been entangled, it has remained an open challenge to entangle the continuous properties of three or more individual particles. Here we experimentally demonstrate genuine tripartite continuous-variable entanglement between three separated particles. In our set-up the three particles are photons created directly from a single input photon; the creation process leads to quantum correlations between the energies and emission times of the photons. The entanglement between our photons is the three-party generalization of the Einstein-Podolsky-Rosen correlations for continuous variables, and could serve as a valuable resource in a wide variety of quantum information tasks. [ABSTRACT FROM AUTHOR]

The statistical fluctuations in free-space links in the turbulent atmosphere are important for the distribution of quantum signals. To that end, we first study statistics generated by the turbulent atmosphere in an entanglement-based free-space quantum key distribution (QKD) system. Using the insights gained from this analysis, we study the effect of link fluctuations on the security and key generation rate of decoy state QKD concluding that it has minimal effect in the typical operating regimes. We then investigate the novel idea of using these turbulent fluctuations to our advantage in QKD experiments. We implement a signal-to-noise ratio filter (SNRF) in our QKD system which rejects measurements during periods of low transmission efficiency, where the measured quantum bit error rate is temporarily elevated. Using this, we increase the total secret key generated by the system from 78 009 bits to 97 678 bits, representing an increase of 25.2% in the final secure key rate, generated from the same raw signals. Lastly, we present simulations of a QKD exchange with an orbiting low earth orbit satellite and show that an SNRF will be extremely useful in such a situation, allowing many more passes to extract a secret key than would otherwise be possible. [ABSTRACT FROM AUTHOR]

In general, quantum computer architectures which are based on the dynamical evolution of quantum states, also require the processing of classical information, obtained by measurements of the actual qubits that make up the computer. This classical processing involves fast, active adaptation of subsequent measurements and real-time error correction (feed-forward), so that quantum gates and algorithms can be executed in a deterministic and hence error-free fashion. This is also true in the linear optical regime, where the quantum information is stored in the polarization state of photons. The adaptation of the photon’s polarization can be achieved in a very fast manner by employing electro-optical modulators, which change the polarization of a trespassing photon upon appliance of a high voltage. In this paper we discuss techniques for implementing fast, active feed-forward at the single photon level and we present their application in the context of photonic quantum computing. This includes the working principles and the characterization of the EOMs as well as a description of the switching logics, both of which allow quantum computation at an unprecedented speed. [ABSTRACT FROM AUTHOR]

Quantum entanglement is the main resource to endow the field of quantum information processing with powers that exceed those of classical communication and computation. In view of applications such as quantum cryptography or quantum teleportation, extension of quantum-entanglement-based protocols to global distances is of considerable practical interest. Here we experimentally demonstrate entanglement-based quantum key distribution over 144 km. One photon is measured locally at the Canary Island of La Palma, whereas the other is sent over an optical free-space link to Tenerife, where the Optical Ground Station of the European Space Agency acts as the receiver. This exceeds previous free-space experiments by more than an order of magnitude in distance, and is an essential step towards future satellite-based quantum communication and experimental tests on quantum physics in space. [ABSTRACT FROM AUTHOR]

The use of satellites to distribute entangled photon pairs (and single photons) provides a unique solution for long-distance quantum communication networks. This overcomes the principle limitations of Earth-bound technology, i.e., the range of the order of 100 km afforded by both optical fiber and by terrestrial free-space links. [ABSTRACT FROM PUBLISHER]

Abstract. Quantum cryptography bases the security of quantum key exchange on the laws of quantum physics and is likely to become the first application employing quantum effects for communication. Here we present performance tests of a new design based on polarization encoding of attenuated, coherent light pulses. Our measurements show that this compact setup can achieve an effective key-bit rate in the kHz range with low alignment requirements and thus offers the tools for last and user-friendly quantum key exchange. [ABSTRACT FROM AUTHOR]

We demonstrate the distribution of polarization entangled photons of wavelength 810 nm through standard telecom fibers. This technique allows quantum communication protocols to be performed over established fiber infrastructure, and makes use of the smaller and better performing setups available around 800 nm, as compared to those which use telecom wavelengths around 1550 nm. We examine the excitation and subsequent quenching of higher-order spatial modes in telecom fibers up to 6 km in length, and perform a distribution of high quality entanglement (visibility 95.6%). Finally, we demonstrate quantum key distribution using entangled 810 nm photons over a 4.4 km long installed telecom fiber link. [ABSTRACT FROM AUTHOR]

The full text of this article is available in the PDF provided. [ABSTRACT FROM AUTHOR]