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

Improvement in secure transmission of information is an urgent need for governments, corporations and individuals. Quantum key distribution (QKD) promises security based on the laws of physics and has rapidly grown from proof-of-concept to robust demonstrations and deployment of commercial systems. Despite these advances, QKD has not been widely adopted, and large-scale deployment will likely require chip-based devices for improved performance, miniaturization and enhanced functionality. Here we report low error rate, GHz clocked QKD operation of an indium phosphide transmitter chip and a silicon oxynitride receiver chip—monolithically integrated devices using components and manufacturing processes from the telecommunications industry. We use the reconfigurability of these devices to demonstrate three prominent QKD protocols—BB84, Coherent One Way and Differential Phase Shift—with performance comparable to state-of-the-art. These devices, when combined with integrated single photon detectors, pave the way for successfully integrating QKD into future telecommunications networks.

We present high performance fiber-coupled niobium titanium nitride superconducting nanowire single photon detectors fabricated on thermally oxidized silicon substrates. The best device showed a system detection efficiency (DE) of 74%, dark count rate of 100 c/s, and full width at half maximum timing jitter of 68 ps under a bias current of 18.0 µA with a practical Gifford-McMahon cryocooler system. We also introduced six detectors into the cryocooler and confirmed that the system DE of all detectors was higher than 67% at the dark count rate of 100 c/s.

Hong-Ou-Mandel (HOM) interference[1] unveils a distinct behavior of identical particles which cannot be distinguished from each other. Especially for bosons, two separated identical particles passing through a beamsplitter always go together into one of the output ports, but that is not the case with other particles including fermions or classical ones. So far many elemental properties of quantum physics and information [2] have been discovered through the concatenated HOM effects, which has been demonstrated in photons [1,3,4,5,6,7,8] and recently in plasmons [9,10], atoms [11] and phonons [12]. However, all demonstrations in optical region employed two particles in different spatial modes. Here we first report the HOM interference between two photons in a single spatial mode with different frequencies (energies) by using a partial frequency conversion. The demonstrated frequency-domain interferometer allows us to replace spatial optical paths by optical frequency multiplexing, which opens up a distinct architecture of the quantum interferometry.In the past three decades since the HOM interference has been proposed and demonstrated with two photons from spontaneous parametric down-conversion (SPDC) process [1], a huge varieties of experiments based on the HOM interference revealed fundamental properties in quantum physics, especially in quantum optics [2], and its applications are widely spreading over quantum information processing, such as quantum computation [13,14,15,16], quantum key distribution [17,18], quantum repeater [19,20,21] and quantumoptical coherence tomography [22]. HOM interference has been observed with photons generated not only from nonlinear optical phenomenon but also from quantum dots [3,4], trapped neutral atoms[5], trapped ions[6], NV centers[7] and SiV centers[8] in diamond. Furthermore not only photons but also other bosonic particles, e.g., surface plasmons [9,10], Helium 4 atoms[11] and phonons [12] show the HOM interference. In spite of such demonstrations using various kinds of physical systems, to the best of our knowledge, all of them essentially used the spatial or polarization degree of freedom for the HOM interference, including the use of polarization modes of photons that are easily converted to and from spatial modes. The demonstrations use the beamsplitter (BS) which mixes the two particles in different spatial/polarization modes.In this letter, we report the first observation of the HOM interference between two photons with different frequencies in optical region. In contrast to the spatial interferometer, the frequency-domain HOM interferometer is implemented in a single spatial mode with a nonlinear optical frequency conversion [23,24,25]. In the experiment, we input a 780 nm photon and a 1522 nm photon to the frequency converter that partially converts the wavelengths of the photons between 780 nm and 1522 nm as shown in Fig. 1a. We measured coincidence counts between the output photons at 780 nm and those at 1522 nm from the frequency converter. The observed HOM inter...

Integrated photonics has enabled much progress towards quantum technologies. Many applications, e.g., quantum communication, sensing, and distributed cloud quantum computing, require coherent photonic interconnection between separate on--chip subsystems. Large--scale quantum computing architectures and systems may ultimately require quantum interconnects to enable scaling beyond the limits of a single wafer, and towards multi--chip systems. However, coherently connecting separate chips remains a challenge, due to the fragility of entangled quantum states. The distribution and manipulation of entanglement between multiple integrated devices is one of the strictest requirements of these systems. Here, we report the first quantum photonic interconnect, demonstrating high--fidelity entanglement distribution and manipulation between two separate photonic chips, implemented using state--of--the--art silicon photonics. Path--entangled states are generated on one chip, and distributed to another chip by interconverting between path and polarization degrees of freedom, via a two--dimensional grating coupler on each chip. This path--to--polarization conversion allows entangled quantum states to be coherently distributed. We use integrated state analyzers to confirm a Bell--type violation of S=2.638±0.039 between the two chips. With further improvements in loss, this quantum photonic interconnect will provide new levels of flexibility in quantum systems and architectures.