Tadamasa Kimura, Yoshihiro Nambu, Takaaki Hatanaka, Akihisa Tomita, Hideo Kosaka, Kazuo Nakamura
We have demonstrated single-photon interference over 150 km using time-division interferometers for quantum cryptography, which were composed of two integrated-optic asymmetric Mach-Zehnder interferometers, and balanced gated-mode photon detectors. The observed fringe visibility was more than 80% after 150-km transmission.
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Cryptographic scheme proposed by Bennett, Brassard, and Mermin [Phys. Rev. Lett. {\bf 68}, 557 (1992)] is reformulated in a version involving two polarizing Mach-Zehnder interferometers. Such a form, although physically equivalent to the original one, makes its security explicit, suggestive and easy to explain to non-experts.
We experimentally demonstrate that a long distance actively stabilized Mach-Zehnder fibre optical interferometer can be used to reliably implement the GV95 quantum cryptography protocol employing orthogonal states. A proof-of-principle experiment using an interferometer composed of 1 km of spooled optical fibres connecting Alice and Bob is performed. The active stabilisation against phase drifts is done with a classical channel wavelength-multiplexed with the quantum channel and a high stable visibility implying a QBER of 2.2 % is shown.
Quantum Cryptography over 23km of installed Telecom fiber using a novel interferometer with Faraday mirrors is presented. The interferometer needs no alignment nor polarization control and features 99.8% fringe visibility. A secret key of 20kbit length with a error rate of 1.35% for 0.1 photon per pulse was produced.
We constructed an optical interferometer for a Bennett-Brassard quantum key distribution system using integrated optics based on planar lightwave circuit technology, and tested its operation and stability. Experimental results show that this interferometer is useful in implementing a practical quantum key distribution system.
We present a time-multiplexed interferometer based on Faraday mirrors, and apply it to quantum key distribution. The interfering pulses follow exactly the same spatial path, ensuring very high stability and self balancing. Use of Faraday mirrors compensates automatically any birefringence effects and polarization dependent losses in the transmitting fiber. First experimental results show a fringe visibility of 0.9984 for a 23km-long interferometer, based on installed telecom fibers.
Amplitude squeezed pulsed light has been produced using a microstructured silica fibre. By spectrally filtering after the nonlinear propagation in the fibre a squeezing value of -1.7dB has been measured. A quantum key distribution scheme based on squeezed light from such microstructured fibres is proposed.
We present a new protocol for practical quantum cryptography, tailored for an implementation with weak coherent pulses. The key is obtained by a very simple time-of-arrival measurement on the data line; an interferometer is built on an additional monitoring line, allowing to monitor the presence of a spy (who would break coherence by her intervention). Against zero-error attacks (the analog of photon-number-splitting attacks), this protocol performs as well as standard protocols with strong reference pulses: the key rate decreases only as the transmission $t$ of the quantum channel. We present…
A hierarchical metropolitan quantum cryptography network upon the inner-city commercial telecom fiber cables is reported in this paper. The seven-user network contains a four-node backbone net with one node acting as the subnet gateway, a two-user subnet and a single-fiber access link, which is realized by the Faraday-Michelson Interferometer set-ups. The techniques of the quantum router, optical switch and trusted relay are assembled here to guarantee the feasibility and expandability of the quantum cryptography network. Five nodes of the network are located in the government departments and …
The degree of freedom of orbital angular momentum (OAM) is an important resource in high-dimensional quantum information processing, as the quantum number of OAM can be infinite. The Dove prism (DP) is a most common tool to manipulate the OAM light, such as in interferometers. However, the Dove prism does not preserve the polarization of the photon states and decreases the sorting fidelity of the interferometer. In this work, we analyze the polarization-dependent effect of the DP on single-path Sagnac interferometers. The results are instructive to quantum information processing with OAM light…
Based on principle of quantum mechanics, quantum cryptography provides an intriguing way to establish secret keys between remote parties, generally relying on actual transmission of signal particles. Surprisingly, an even more striking method is recently proposed by Noh named as `counterfactual quantum cryptography' enabling key distribution, in which particles carrying secret information are seemly not being transmitted through quantum channel. We experimentally give here a faithful implementation by following the scheme with an on-table realization. Furthermore, we report an illustration on …
We report the first demonstration of quantum key distribution over a standard telecom fiber exceeding 100 km in length. Through careful optimisation of the interferometer and single photon detector, we achieve a quantum bit error ratio of 8.9% for a 122km link, allowing a secure shared key to be formed after error correction and privacy amplification. Key formation rates of up to 1.9 kbit/sec are achieved depending upon fiber length. We discuss the factors limiting the maximum fiber length in quantum cryptography.
Noise and imperfection of realistic devices are major obstacles for implementing quantum cryptography. In particular birefringence in optical fibers leads to decoherence of qubits encoded in polarization of photon. We show how to overcome this problem by doing single qubit quantum communication without a shared spatial reference frame and precise timing. Quantum information will be encoded in pair of photons using ``tag'' operations which corresponds to the time delay of one of the polarization modes. This method is robust against the phase instability of the interferometers despite the use of…
We report a fully guided-wave source of polarisation entangled photons based on a periodically poled lithium niobate waveguide mounted in a Sagnac interferometer. We demonstrate the source's quality by converting polarisation entanglement to postselection-free energy-time entanglement for which we obtain a near-optimal $S$-parameter of $2.75 \pm 0.02$, i.e. a violation of the Bell inequality by more than 35 standard deviations. The exclusive use of guided-wave components makes our source compact and stable which is a prerequisite for increasingly complex quantum applications. Additionally, our…
Cavity-enhanced induced coherence without induced emission
本研究开展理论探索，将腔增强自发参量下转换与ZWM干涉测量结合，利用单共振光学参量振荡器产生窄带单光子，窄化光子带宽并提升相干性与光子对产生效率，填补相关理论空白，推动量子密码等领域应用发展。
We demonstrate photonic quantum circuits that operate at the stringent levels that will be required for future quantum information science and technology. These circuits are fabricated from silica-on-silicon waveguides forming directional couplers and interferometers. While our focus is on the operation of quantum circuits, to test this operation required construction of a spectrally tuned photon source to produce near-identical pairs of photons. We show non-classical interference with two photons and a two-photon entangling logic gate that operate with near-unit fidelity. These results are a …
Fabrication of single nickel-nitrogen (NE8) defect centers in diamond by chemical vapor deposition is demonstrated. Under continuous-wave 745 nm laser excitation single defects were induced to emit single photon pulses at 797 nm with a linewidth of 1.5 nm at room temperature. Photon antibunching of single centers was demonstrated using a Hanbury-Brown and Twiss interferometer. Confocal images revealed approximately 10^6 optically active sites/cm^2 in the synthesized films. The fabrication of an NE8 based single photon source in synthetic diamond is important for fiber based quantum cryptograph…
Since the invention of Bennett-Brassard 1984 (BB84) protocol, many quantum key distribution (QKD) protocols have been proposed and some protocols are operated even in field environments. One of the striking features of QKD is that QKD protocols are provably secure unlike cryptography based on computational complexity assumptions. It has been believed that, to guarantee the security of QKD, Alice and Bob have to monitor the statistics of the measurement outcomes which are used to determine the amount of the privacy amplification to generate a key. Recently a new type of QKD protocol, called rou…
The Franson interferometer, proposed in 1989 [J. D. Franson, Phys. Rev. Lett. 62:2205-2208 (1989)], beautifully shows the counter-intuitive nature of light. The quantum description predicts sinusoidal interference for specific outcomes of the experiment, and these predictions can be verified in experiment. In the spirit of Einstein, Podolsky, and Rosen it is possible to ask if the quantum-mechanical description (of this setup) can be considered complete. This question will be answered in detail in this paper, by delineating the quite complicated relation between energy-time entanglement experi…
Integrated photonic circuits are one of the most promising platforms for large-scale photonic quantum information systems due to their small physical size and stable interferometers with near-perfect lateral-mode overlaps. Since many quantum information protocols are based on qubits defined by the polarization of photons, we must develop integrated building blocks to generate, manipulate, and measure the polarization-encoded quantum state on a chip. The generation unit is particularly important. Here we show the first integrated polarization-entangled photon pair source on a chip. We have impl…
Programmable Bell State Generation in an Integrated Thin Film Lithium Niobate Circuit
本研究在集成薄膜锂铌酸盐电路中实现了可编程贝尔态生成，利用可编程干涉仪和高亮度光子对源，展示了高保真度的纠缠态制备。
Photons from distant astronomical sources can be used as a classical source of randomness to improve fundamental tests of quantum nonlocality, wave-particle duality, and local realism through Bell's inequality and delayed-choice quantum eraser tests inspired by Wheeler's cosmic-scale Mach-Zehnder interferometer gedankenexperiment. Such sources of random numbers may also be useful for information-theoretic applications such as key distribution for quantum cryptography. Building on the design of an "astronomical random-number generator" developed for the recent "cosmic Bell" experiment [Handstei…
Entangled two-photon absorption in transmission-based experiments: deleterious effects from linear optical losses
本研究利用Hong-Ou-Mandel干涉图作为传感设备，揭示线性光学损耗对基于透射的纠缠双光子吸收实验的有害影响，并证明在标准条件下ETPA未被明确检测到。
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