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the repetitive frequency of the driver laser, resulting in higher total intensity. is method does not involve dete-rioration of correlation as the pulse intensity remains the same, but past research invariably reported that the upper most frequency was around 76 MHz or below. e authors have succeeded in attaining a higher operation rate without involving more noise by combining a new driver laser capable of generating 2.5 GHz repetition frequency with a high-purity quantum entanglement source. Higher fre-quency was enabled by using a frequency comb source developed independently in NICT[4]. e driver pulse laser must satisfy such requirements as: variable wavelength, variable pulse width, high frequency rate, and operational stability. e frequency comb source developed in NICT can provide all these features. Figure 1 shows the setup overview and the result of the experiment. e experimen-tal result indicates clarity of interference produced by the entangled photon pairs as plotted against the intensity of driver laser output. e higher the interference visibility, the higher the purity of quantum entanglement. e plot clearly indicates the use of the 2.5 GHz frequency comb source has the eect of maintaining high visibility even under higher intensity operation (red line) as compared with the case of a conventional driver laser (blue line). In summary, we achieved a much greater system operation rate, as high as more than 30 times that of conventional systems, by designing an entirely new system based on the frequency comb source.3Quantum communication protocol using a quantum entanglement source: a new phenomenonAmong the communication protocols based on a quan-tum entanglement source, the one called entanglement swapping provides the networking basis for quantum cryptography and quantum computing. Figure 2 shows the two stages that constitute entanglement swapping. First, point A and B, and point B and C respectively share dif-ferent pairs of entangled photons. At this point of time, the two photon pairs—one shared between A and B, and the other between B and C—have no correlation whatsoever. Next, Bell measurement (a special technique to project two photons onto the quantum entanglement basis) is taken at point B to detect the arrival of the photons. e measure-ment is guratively described as trying to capture a photon blindfolded, but it can cause the formation of new quantum entanglement between A and C by intentionally letting the incoming direction (A or C) of photons remain obscured. As illustrated in Fig.3, the experimental setup includes optical elements (e.g. a mirror that exhibits high reectance only at specic wavelengths), a NICT developed a super-conducting single-photon detector, as well as the quantum entanglement source developed in NICT (see description in previous section). Application of these proprietary de-vices enabled the experiment to generate high-purity en-tanglement swapping at a much higher rate (the success count of entanglement swapping observation was 1,000 times or higher than reported by previous studies)[5]. e lower right plot of Fig.3 shows the results of correlation measurements on photon polarization arriving at point A and C, indicating good visibility much larger than 33% (the generally accepted threshold value to guarantee the exis-tence of quantum entanglement). ese results are signi-cant in paving the way for a new domain of entanglement swapping experiments on optical ber networks that have been quite impractical due to slow speed.e high-speed, high-purity quantum entanglement source and detection method also enables the observation of new phenomena in quantum optics. An example is Holland-Burnett interference, a photon-photon quantum coherence well known in quantum optics. Although the observation was limited only to a two-photon system up to the present, availability of a much faster source and detector enabled the observation of multi-photon systems. We conducted interference measurements involving up to 6 photons, and revealed the occurrence of a variety of quantum interference patterns depending on the number of photons[6]. Further, in collaboration with the research-ers of U.S. National Institute of Standards and Technology (NIST), we developed a frequency resolving measurement method applied to the photons aer the interference. is technique was successfully applied to the frequency resolv-ing measurement of Hong-Ou-Mandel (HOM) interfer-ence, a well-known phenomenon as one of the most basic quantum interference phenomena[7]. e HOM interfer-ence fringes have been hitherto considered to disappear in certain parameter regions, but the experiment revealed for the rst time that strong quantum interference remains to exist even in the regions among the frequency resolved photons. Although these results are not directly linked to information telecommunication, we consider them to be signicant scientic contributions in expanding quantum optics—a basic science from which a variety of achieve-ments may be derived. e frequency resolving technology and the novel knowledge concerning quantum coherence 514-1 Optical Quantum Control Technologies