Interferometric autocorrelation in the ultraviolet utilizing spontaneous parametric down-conversion inside an enhancement cavity.

Autocorrelation is a common method to estimate the duration of ultrashort laser pulses. In the ultraviolet (UV) regime it is challenging to employ the process of second-harmonic generation, most prominently due to absorption in nonlinear crystals at very short wavelengths. Here we show how to utilize spontaneous parametric down-conversion (SPDC) to generate an autocorrelation signal in the infrared (IR) for UV pulses. Our method utilizes the nth-order emission of the SPDC process, which occurs for low pumping powers proportional to the nth power of the UV intensity. Thus, counting 2n down-converted photons directly yields the nth-order autocorrelation. The method, now with detection of near-IR photons, is applied to the first direct measurement of ultrashort UV pulses circulating inside a UV enhancement cavity.

Permutationally invariant quantum tomography.

We present a scalable method for the tomography of large multiqubit quantum registers. It acquires information about the permutationally invariant part of the density operator, which is a good approximation to the true state in many relevant cases. Our method gives the best measurement strategy to minimize the experimental effort as well as the uncertainties of the reconstructed density matrix. We apply our method to the experimental tomography of a photonic four-qubit symmetric Dicke state.

Highly efficient state-selective submicrosecond photoionization detection of single atoms.

We experimentally demonstrate a detection scheme suitable for state analysis of single optically trapped atoms in less than 1 µs with an overall detection efficiency η exceeding 98%. The method is based on hyperfine-state-selective photoionization and subsequent registration of the correlated photoion-electron pairs by coincidence counting via two opposing channel electron multipliers. The scheme enables the calibration of absolute detection efficiencies and might be a key ingredient for future quantum information applications or precision spectroscopy of ultracold atoms.

Towards long-distance atom-photon entanglement.

We report the observation of entanglement between a single trapped atom and a single photon at remote locations. The degree of coherence of the entangled atom-photon pair is verified via appropriate local correlation measurements, after communicating the photon via an optical fiber link of 300 m length to a receiver 3.5 m apart. In addition, we measured the temporal evolution of the atomic density matrix after projecting the atom via a state measurement of the photon onto several well-defined spin states. We find that the state of the single atom dephases on a time scale of 150 micros, which represents an important step towards long-distance quantum networking with individual neutral atoms.

Experimental observation of four-photon entangled Dicke state with high fidelity.

We present the experimental observation of the symmetric four-photon entangled Dicke state with two excitations |D_{4};{(2)}. A simple experimental setup allowed quantum state tomography yielding a fidelity as high as 0.844+/-0.008. We study the entanglement persistency of the state using novel witness operators and focus on the demonstration of a remarkable property: depending on the orientation of a measurement on one photon, the remaining three photons are projected into both inequivalent classes of genuine tripartite entanglement, the Greenberger-Horne-Zeilinger and W class. Furthermore, we discuss possible applications of |D_{4};{(2)} in quantum communication.

Experimental demonstration of four-party quantum secret sharing.

Secret sharing is a multiparty cryptographic task in which some secret information is split into several pieces which are distributed among the participants such that only an authorized set of participants can reconstruct the original secret. Similar to quantum key distribution, in quantum secret sharing, the secrecy of the shared information relies not on computational assumptions, but on laws of quantum physics. Here, we present an experimental demonstration of four-party quantum secret sharing via the resource of four-photon entanglement.

Experimental one-way quantum computing.

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.

Distributing entanglement and single photons through an intra-city, free-space quantum channel.

We have distributed entangled photons directly through the atmosphere to a receiver station 7.8 km away over the city of Vienna, Austria at night. Detection of one photon from our entangled pairs constitutes a triggered single photon source from the sender. With no direct time-stable connection, the two stations found coincidence counts in the detection events by calculating the cross-correlation of locally-recorded time stamps shared over a public internet channel. For this experiment, our quantum channel was maintained for a total of 40 minutes during which time a coincidence lock found approximately 60000 coincident detection events. The polarization correlations in those events yielded a Bell parameter, S=2.27+/-0.019, which violates the CHSH-Bell inequality by 14 standard deviations. This result is promising for entanglement-based freespace quantum communication in high-density urban areas. It is also encouraging for optical quantum communication between ground stations and satellites since the length of our free-space link exceeds the atmospheric equivalent.

Compact source of polarization-entangled photon pairs.

We present a compact source of polarization-entangled photon pairs at a wavelength of 805 nm using a violet single-mode laser diode as the pump source of type-II spontaneous parametric down-conversion. The source exhibits entanglement and pair-rate comparable to conventional systems utilizing large frame ion lasers thus significantly increases the practicality of novel quantum information or quantum metrology applications.

Practical quantum key distribution with polarization entangled photons.

We present an entangled-state quantum cryptography system that operated for the first time in a real-world application scenario. The full key generation protocol was performed in real-time between two distributed embedded hardware devices, which were connected by 1.45 km of optical fiber, installed for this experiment in the Vienna sewage system. The generated quantum key was immediately handed over and used by a secure communication application.

A step towards global key distribution.

Large random bit-strings known as 'keys' are used to encode and decode sensitive data, and the secure distribution of these keys is essential to secure communications across the globe. Absolutely secure key exchange between two sites has now been demonstrated over fibre and free-space optical links. Here we describe the secure exchange of keys over a free-space path of 23.4 kilometres between two mountains. This marks a step towards accomplishing key exchange with a near-Earth orbiting satellite and hence a global key-distribution system.

Feasible "Kochen-Specker" experiment with single particles

We present a simple experimental scheme which can be used to demonstrate an all-or-nothing-type contradiction between noncontextual hidden variables and quantum mechanics. The scheme, which is inspired by recent ideas by Cabello and Garcia-Alcaine, shows that even for a single particle, path and spin information cannot be predetermined in a noncontextual way.

Quantum cryptography with entangled photons

By realizing a quantum cryptography system based on polarization entangled photon pairs we establish highly secure keys, because a single photon source is approximated and the inherent randomness of quantum measurements is exploited. We implement a novel key distribution scheme using Wigner's inequality to test the security of the quantum channel, and, alternatively, realize a variant of the BB84 protocol. Our system has two completely independent users separated by 360 m, and generates raw keys at rates of 400-800 bits/s with bit error rates around 3%.

Experiments towards falsification of noncontextual hidden variable theories

We present two experiments testing the hypothesis of noncontextual hidden variables. The first one is based on observation of two-photon pseudo-Greenberger-Horne-Zeilinger correlations, with two of the originally three particles mimicked by the polarization degree of freedom and the spatial degree of freedom of a single photon. The second one, a single-photon experiment, utilizes the same trick to emulate two particle correlations, and is an "event ready" test of a Bell-like inequality, derived from the noncontextuality assumption. Modulo fair sampling, the data falsify noncontextual hidden variables.

Stable solid-state source of single photons

Fluorescence light observed from a single nitrogen-vacancy center in diamond exhibits strong photon antibunching: The measured pair correlation function g((2))(0) shows that only one photon is emitted at a time. Nitrogen-vacancy centers are well localized, stable against photobleaching even at room temperature, and can be addressed in simple experimental configurations.

Experimental test of quantum nonlocality in three-photon Greenberger-Horne-Zeilinger entanglement

Bell's theorem states that certain statistical correlations predicted by quantum physics for measurements on two-particle systems cannot be understood within a realistic picture based on local properties of each individual particle-even if the two particles are separated by large distances. Einstein, Podolsky and Rosen first recognized the fundamental significance of these quantum correlations (termed 'entanglement' by Schrodinger) and the two-particle quantum predictions have found ever-increasing experimental support. A more striking conflict between quantum mechanical and local realistic predictions (for perfect correlations) has been discovered; but experimental verification has been difficult, as it requires entanglement between at least three particles. Here we report experimental confirmation of this conflict, using our recently developed method to observe three-photon entanglement, or 'Greenberger-Horne-Zeilinger' (GHZ) states. The results of three specific experiments, involving measurements of polarization correlations between three photons, lead to predictions for a fourth experiment; quantum physical predictions are mutually contradictory with expectations based on local realism. We find the results of the fourth experiment to be in agreement with the quantum prediction and in striking conflict with local realism.

All-fiber three-path Mach-Zehnder interferometer.

We report the realization of a three-path Mach-Zehnder interferometer using single-mode fibers and two integrated 3 x 3 fiber couplers. We observed enhanced phase sensitivity, as compared with two-path interferometers, with a visibility of the interference pattern of more than 97%. This interferometer has an analog in two-photon interferometry, and we believe it to be the first nontrivial example of N x N multiport interferometers.