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Qudit entangled states have proven to offer significant advantages with respect to qubit states regarding the implementation of quantum cryptography or computation schemes. Here we propose and experimentally implement a scalable scheme for preparing and analyzing these states in the time-energy degree of freedom of two-photon pairs. Using the scheme, the entanglement of (2×4)-dimensional states is demonstrated. [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]

Standard quantum computation is based on sequences of unitary quantum logic gates that process qubits. The one-way quantum computer proposed by Raussendorf and Briegel is entirely different. It has changed our understanding of the requirements for quantum computation and more generally how we think about quantum physics. This new model requires qubits to be initialized in a highly entangled cluster state. From this point, the quantum computation proceeds by a sequence of single-qubit measurements with classical feedforward of their outcomes. Because of the essential role of measurement, a one-way quantum computer is irreversible. In the one-way quantum computer, the order and choices of measurements determine the algorithm computed. We have experimentally realized four-qubit cluster states encoded into the polarization state of four photons. We characterize the quantum state fully by implementing experimental four-qubit quantum state tomography. Using this cluster state, we demonstrate the feasibility of one-way quantum computing through a universal set of one- and two-qubit operations. Finally, our implementation of Grover's search algorithm demonstrates that one-way quantum computation is ideally suited for such tasks. [ABSTRACT FROM AUTHOR]

We demonstrate the principle of complementarity in quantum mechanics in a single-photon interference experiment. Single photons are provided by isolated, optically pumped nitrogen-vacancy centers in diamond, which can be easily addressed by confocal microscopy. In order to observe the particle-like behavior of photons, we perform an elementary Welcher-Weg measurement, detecting photons behind a beam splitter. In contrast, if we dispense with this Welcher-Weg information, we observe interference fringes with a visibility of about 96%, revealing the wave nature of the photon. [ABSTRACT FROM AUTHOR]

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]

Abstract. Quantum teleportation enables one to transfer any arbitrary quantum state from one particle to another. In the experiments presented here we use parametric down-conversion to produce entangled photon pairs and two-photon interferometry to perform the necessary projection onto entangled states, the so-called Bell state analysis. This allows us to teleport the polarization state from one photon to the other and to entangle photons that have never interacted with one another in the past. [ABSTRACT FROM AUTHOR]