Abstract:A secure communication network with quantum key distribution in a metropolitan area is reported. Six different QKD systems are integrated into a mesh-type network. GHz-clocked QKD links enable us to demonstrate the world-first secure TV conferencing over a distance of 45km. The network includes a commercial QKD product for long-term stable operation, and application interface to secure mobile phones. Detection of an eavesdropper, rerouting into a secure path, and key relay via trusted nodes are demonstrated in this network. ©2011 Optical Society of AmericaOCIS codes: (270.5568) Quantum cryptography; (060.5565) Quantum communications. References and links1. N. Gisin, G. Ribordy, W. Tittel, and H. Zbinden, "Quantum cryptography," Rev. Mod. Phys. 74(1), 145-195 (2002). 2. V. Scarani, H. Bechmann-Pasquinucci, N. J. Cerf, M. Dušek, N. N. Lütkenhaus, and M. Peev, "The security of practical quantum key distribution," Rev. Mod. Phys. 81(3), 1301-1350 (2009

We present generation of photon-subtracted squeezed states at 860 nm, from nearly pure, continuous-wave squeezed vacua generated with a periodically-poled KTiOPO(4) crystal as a nonlinear medium of a subthreshold optical parametric oscillator. We observe various kinds of photon-subtracted squeezed states, including non-Gaussian states similar to the single-photon state and superposition states of coherent states, simply by changing the pump power. Nonclassicality of the generated states clearly shows up as its negative region around the origin of the phase-space distributions, i.e., the Wigner functions. We obtain the value, -0.083 at the origin of the Wigner function, which is largest ever observed without any correction for experimental imperfections.

A continuous-variable tripartite entangled state is experimentally generated by combining three independent squeezed vacuum states and the variances of its relative positions and total momentum are measured. We show that the measured values violate the separability criteria based on the sum of these quantities and prove the full inseparability of the generated state.PACS numbers: 03.65. Ud, 03.67.Mn, 42.50.Dv The remarkable proposal of quantum teleportation [1] demonstrates that the quantum correlations of a shared entangled state enable two parties to reliably exchange quantum information. So far, several experiments on quantum communication with discrete-variable states have been carried out. In the domain of continuous variables (CVs), the unconditional quantum teleportation of arbitrary coherent states [2,3,4] and quantum dense coding [5] have been demonstrated. These successful experiments show the advantage of CV bipartite entanglement for the implementation of quantum protocols; that is, the simplicity of its generation and manipulation and the applicability of efficient homodyne techniques to its detection.CV entanglement may also be applicable to quantum protocols involving more than two parties. For example, tripartite entanglement (the entanglement shared by three parties) enables one to construct a quantum teleportation network [6], to build an optimal one to two telecloner [7], or to perform controlled dense coding [8]. CV tripartite entanglement can be generated in a similar way as in the case of CV bipartite entanglement. It only requires combining three modes using linear optics, where at least one of these modes is in a squeezed state [6]. In fact, as pointed out in Ref. [9], CV tripartite entanglement has already been generated in the CV quantum teleportation experiment of Ref.[2], although no further investigation was made there. On the other hand, the separability properties of tripartite states are more complicated than in the bipartite case; three-mode Gaussian states are classified into five different classes [9]. In order to exploit the tripartite entanglement for three-party quantum protocols such as that from Ref.[6], the state involved has to be fully inseparable (class 1 in Ref. [9]). Although the output state that emerges from the beam splitters with one or more squeezed input states * Electronic address: takao@ap.t.u-tokyo.ac.jp is in principle fully inseparable for any nonzero squeezing [6], inevitable losses in the real experiment may destroy the genuine tripartite entanglement and convert the state into a partially or fully separable one. This would make a true tripartite quantum protocol fail. In other words, the success of a true tripartite quantum protocol (e.g., a coherent-state quantum teleportation network with fidelities better than one half) is a sufficient criterion for the full inseparability of the state involved [6]. It should be noted here that the success of a tripartite quantum protocol between two parties with the help of the third party (e.g., via a momentum d...

We propose and demonstrate a novel method to generate a large-amplitude coherent-state superposition (CSS) via ancilla-assisted photon subtraction. The ancillary mode induces quantum interference of indistinguishable processes in an extended space, widening the controllability of quantum superposition at the conditional output. We demonstrate this by a simple time-separated two-photon subtraction from continuous wave squeezed light. We observe the largest CSS of traveling light ever reported without correcting any imperfections, which will enable various quantum information applications with CSS states.

In a new branch of quantum computing, information is encoded into coherent states, the primary carriers of optical communication. To exploit it, quantum bits of these coherent states are needed, but it is notoriously hard to make superpositions of such continuous-variable states. We have realized the complete engineering and characterization of a qubit of two optical continuous-variable states. Using squeezed vacuum as a resource and a special photon-subtraction technique, we could with high precision prepare an arbitrary superposition of squeezed vacuum and a squeezed single photon. This could lead the way to demonstrations of coherent state quantum computing.

Abstract:We theoretically and experimentally investigate the spectral tunability and purity of photon pairs generated from spontaneous parametric down conversion in periodically poled KTiOPO 4 crystal with group-velocity matching condition. The numerical simulation predicts that the purity of joint spectral intensity (P JSI ) and the purity of joint spectral amplitude (P JSA ) can be kept higher than 0.98 and 0.81, respectively, when the wavelength is tuned from 1460 nm to 1675 nm, which covers the S-, C-, L-, and U-band in telecommunication wavelengths. We also directly measured the joint spectral intensity at 1565 nm, 1584 nm and 1565 nm, yielding P JSI of 0.989, 0.983 and 0.958, respectively. Such a photon source is useful for quantum information and communication systems.

We demonstrate pulsed polarization-entangled photons generated from a periodically poled KTiOPO(4) (PPKTP) crystal in a Sagnac interferometer configuration at telecom wavelength. Since the group-velocity-matching (GVM) condition is satisfied, the intrinsic spectral purity of the photons is much higher than in the previous scheme at around 800 nm wavelength. The combination of a Sagnac interferometer and the GVM-PPKTP crystal makes our entangled source compact, stable, highly entangled, spectrally pure and ultra-bright. The photons were detected by two superconducting nanowire single photon detectors (SNSPDs) with detection efficiencies of 70% and 68% at dark counts of less than 1 kcps. We achieved fidelities of 0.981 ± 0.0002 for |ψ(-)〉 and 0.980 ± 0.001 for |ψ(+)〉 respectively. This GVM-PPKTP-Sagnac scheme is directly applicable to quantum communication experiments at telecom wavelength, especially in free space.

We have developed a high-performance single-photon source (SPS) operating at 1.5 µm wavelength. The source is an InAs/InP quantum dot with a horn-shaped nanostructure. A resonant excitation to the p-shell state helps achieve a single-photon efficiency of 5.8% after coupling into a single-mode fiber with a second-order correlation value of g(2)(0)∼0.055. The performance of the source has been assessed by integrating it into a conventional quantum key distribution system. We have successfully transmitted secure keys over a 50 km commercial fiber, exceeding the previously reported range for an SPS operating below 1.3 µm.