K. Dobek, M. Karpiński, R. Demkowicz-Dobrzański, K. Banaszek, and P. Horodecki
Phys. Rev. Lett. 106, 030501 (2011)
We report experimental generation of a noisy entangled four-photon state that exhibits a separation between the secure key contents and distillable entanglement, a hallmark feature of the recently established quantum theory of private states. The privacy analysis, based on the full tomographic reconstruction of the prepared state, is utilized in a proof-of-principle key generation. The inferiority of distillation-based strategies to extract the key is exposed by an implementation of an entanglement distillation protocol for the produced state.
Noé Curtz, Rob Thew, Christoph Simon, Nicolas Gisin, and Hugo Zbinden
Optics Express, Vol. 18, Issue 21, pp. 22099-22104 (2010)
We report a coherence-preserving photon frequency down-conversion experiment based on difference-frequency generation in a periodically poled Lithium niobate waveguide, at the single-photon level. The coherence of the process has been demonstrated by measuring the phase coherence of pseudo single-photon time-bin qubits after frequency conversion with an interference visibility of >96 %. This interface could be of interest for quantum repeater based hybrid networks.
Filippo Caruso, Nicolò Spagnolo, Chiara Vitelli, Fabio Sciarrino, and Martin B. Plenio
Phys. Rev. A 83, 013811 (2011)
M. Cramer, M. B. Plenio, and H. Wunderlich
Phys. Rev. Lett. 106, 020401 (2011)
Nat. Commun. 1, 149 (2010)
Quantum state tomography—deducing quantum states from measured data—is the gold standard for verification and benchmarking of quantum devices. It has been realized in systems with few components, but for larger systems it becomes unfeasible because the number of measurements and the amount of computation required to process them grows exponentially in the system size. Here, we present two tomography schemes that scale much more favourably than direct tomography with system size.
New J. Phys. 12 083034 (2010)
Phys. Rev. Lett. 105, 200501 (2010)
Nat. Commun. 2 , 184 (2011)
The results of local measurements on some composite quantum systems cannot be reproduced classically. This impossibility, known as quantum nonlocality, represents a milestone in the foundations of quantum theory. Quantum nonlocality is also a valuable resource for information-processing tasks, for example, quantum communication, quantum key distribution, quantum state estimation or randomness extraction. Still, deciding whether a quantum state is nonlocal remains a challenging problem.
Phys. Rev. Lett. 105, 160501 (2010)
We propose a hybrid (continuous-discrete variable) quantum repeater protocol for long-distance entanglement distribution. Starting from states created by single-photon detection, we show how entangled coherent state superpositions can be generated by means of homodyne detection. We show that near-deterministic entanglement swapping with such states is possible using only linear optics and homodyne detectors, and we evaluate the performance of our protocol combining these elements.
Appl. Phys. Lett. 97, 031104 (2010)