Xiao-song Ma

Quantum teleportation over 143 kilometres using active feed-forward

The quantum internet is predicted to be the next-generation information processing platform, promising secure communication and an exponential speed-up in distributed computation. The distribution of single qubits over large distances via quantum teleportation is a key ingredient for realizing such a global platform. By using quantum teleportation, unknown quantum states can be transferred over arbitrary distances to a party whose location is unknown. Since the first experimental demonstrations of quantum teleportation of independent external qubits, an internal qubit and squeezed states, researchers have progressively extended the communication distance. Usually this occurs without active feed-forward of the classical Bell-state measurement result, which is an essential ingredient in future applications such as communication between quantum computers. The benchmark for a global quantum internet is quantum teleportation of independent qubits over a free-space link whose attenuation corresponds to the path between a satellite and a ground station. Here we report such an experiment, using active feed-forward in real time. The experiment uses two free-space optical links, quantum and classical, over 143 kilometres between the two Canary Islands of La Palma and Tenerife. To achieve this, we combine advanced techniques involving a frequency-uncorrelated polarization-entangled photon pair source, ultra-low-noise single-photon detectors and entanglement-assisted clock synchronization. The average teleported state fidelity is well beyond the classical limit of two-thirds. Furthermore, we confirm the quality of the quantum teleportation procedure without feed-forward by complete quantum process tomography. Our experiment verifies the maturity and applicability of such technologies in real-world scenarios, in particular for future satellite-based quantum teleportation.Xiao-song Ma, 2 Thomas Herbst, Thomas Scheidl, Daqing Wang, Sebastian Kropatschek, William Naylor, Alexandra Mech, 1 Bernhard Wittmann, 1 Johannes Kofler, 1 Elena Anisimova, 6 Vadim Makarov, 6 Thomas Jennewein, Rupert Ursin, and Anton Zeilinger 2, 3 Institute for Quantum Optics and Quantum Information (IQOQI), Austrian Academy of Sciences, Boltzmanngasse 3, A-1090 Vienna, Austria Vienna Center for Quantum Science and Technology, Faculty of Physics, University of Vienna, Boltzmanngasse 5, A-1090 Vienna, Austria Faculty of Physics, University of Vienna, Boltzmanngasse 5, A-1090 Vienna, Austria Max Planck Institute of Quantum Optics, Hans-Kopfermann-Str. 1, 85748 Garching/Munich, Germany Institute for Quantum Computing and Department of Physics and Astronomy, University of Waterloo, 200 University Avenue West, Waterloo, ON, N2L 3G1, Canada Department of Electronics and Telecommunications, Norwegian University of Science and Technology, NO-7491 Trondheim, Norway (Dated: May 1, 2014)

Quantum discord as resource for remote state preparation

Quantum discord is the total non-classical correlation between two systems. This includes, but is not limited to, entanglement. Photonic experiments now demonstrate that separable states with non-zero quantum discord are a useful resource for quantum information processing and can even outperform entangled states.

Experimental demonstration of fractional orbital angular momentum entanglement of two photons.

The singular nature of a noninteger spiral phase plate allows easy manipulation of spatial degrees of freedom of photon states. Using two such devices, we have observed very high-dimensional spatial entanglement of twin photons generated by spontaneous parametric down-conversion.

Experimental delayed-choice entanglement swapping

In 2000, Asher Peres put forward the paradoxical idea that entanglement could be produced after the entangled particles have been measured, even if they no longer exist. Researchers now experimentally demonstrate this idea using four photons.

Quantum simulation of the wavefunction to probe frustrated Heisenberg spin systems

Quantum simulations, where one quantum system is used to emulate another, are starting to become experimentally feasible. Here, four-photon states are used to simulate spin tetramers, which are important in the description of certain solid-state systems. Emerging frustration within the tetramer is observed, as well as evolution of the ground state from a localized to a resonating-valence-bond state.

Feasibility of 300 km quantum key distribution with entangled states

A significant limitation of practical quantum key distribution (QKD) setups is currently their limited operational range. It has recently been emphasized (Ma et al 2007 Phys. Rev. A 76 012307) that entanglement- based QKD systems can tolerate higher channel losses than systems based on weak coherent laser pulses (WCP), in particular, when the source is located symmetrically between the two communicating parties, Alice and Bob. In the work presented here, we experimentally study this important advantage by implementing different entanglement-based QKD setups on a 144km free-space link between the two Canary Islands of La Palma and Tenerife. We established three different configurations where the entangled photon source was placed at Alices location, asymmetrically between Alice and Bob and symmetrically in the middle between Alice and Bob, respectively. The resulting quantum channel attenuations of 35, 58 and 71dB, respectively, significantly exceed the limit for WCP systems (Ma et al 2007 Phys. Rev. A 76 012307). This confirms that QKD over distances of 300km and even more is feasible with entangled state sources placed in the middle between Alice and Bob.

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.

Delayed-choice gedanken experiments and their realizations

The wave-particle duality dates back to Einsteins explanation of the photoelectric effect through quanta of light and de Broglies hypothesis of matter waves. Quantum mechanics uses an abstract description for the behavior of physical systems such as photons, electrons, or atoms. Whether quantum predictions for single systems in an interferometric experiment allow an intuitive understanding in terms of the particle or wave picture, depends on the specific configuration which is being used. In principle, this leaves open the possibility that quantum systems always either behave definitely as a particle or definitely as a wave in every experimental run by a priori adapting to the specific experimental situation. This is precisely what is tried to be excluded by delayed-choice experiments, in which the observer chooses to reveal the particle or wave character -- or even a continuous transformation between the two -- of a quantum system at a late stage of the experiment. We review the history of delayed-choice gedanken experiments, which can be traced back to the early days of quantum mechanics. Then we discuss their experimental realizations, in particular Wheelers delayed choice in interferometric setups as well as delayed-choice quantum erasure and entanglement swapping. The latter is particularly interesting, because it elevates the wave-particle duality of a single quantum system to an entanglement-separability duality of multiple systems.

Experimental violation of a Bell inequality with two different degrees of freedom of entangled particle pairs

We demonstrate hybrid entanglement of photon pairs via the experimental violation of a Bell inequality with two different degrees of freedom (DOF), namely, the path (linear momentum) of one photon and the polarization of the other photon. Hybrid entangled photon pairs are created by spontaneous parametric down conversion and coherent polarization to path conversion for one photon. For that photon, path superposition is analyzed, and polarization superposition for its twin photon. The correlations between these two measurements give an

Quantum interference in heterogeneous superconducting-photonic circuits on a silicon chip

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