座長：ﾁﾝ ｾｲﾃｲ、吉沢 徹
| 氏名： 熊澤
指導教員名： 小芦 雅斗 教授
発表題目（英語）： Secure decoy-state quantum key distribution by calibration of light sources
要旨（英語）： The goal of quantum key distribution (QKD) is to allow two distant users, the sender (Alice) and the receiver (Bob), to communicate in absolute security in the presence of an eavesdropper, Eve. The security of the decoy-state BB84 QKD, one of the most frequently demonstrated protocols, has been proven in a number of papers. These proofs assume that the probability distribution of the photon number from a light source is Poissonian distribution. However, to ensure this assumption, we need to make sure that infinite number of inequalities about the probability distribution of photon number are satisfied. Unfortunately, there is no way to calibrate experimentally all of these inequalities. In this presentation, we address these issues by analyzing the security of QKD only assuming what we can check in an experiment. We suggest how to calibrate the photon-number statistics of the light sources and estimate how the calibration affects the performance of the secure key generation rate.
| 氏名： 阪口
指導教員名： 古澤 明 教授
発表題目（英語）： Generation of non-Gaussian state for realization of non-Gaussian gate
要旨（英語）： One of the ultimate goals of quantum optics is to realize the arbitrary unitary operation on quantum state of light. For the realization of universal operation on light, it is known that we need to realize at least one "non-Gaussian gate" which corresponds to third or higher order nonlinear optical effect. Non-Gaussian operations are difficult to be implemented by ordinary nonlinear crystal because the nonlinear effect is too weak for single-photon power level. Instead, there is a proposal of indirect measurement-based method to implement such a gate with non-Gaussian ancillary state, quantum measurement and feed-forward.
For experimental reasons, however, still there is a problem to implement the measurement-based method. We have to use delay lines to have the feed-forwarded state wait for the measurement and feed-forward process since the state flies in light speed. The problem is that the longer the delay lines become, the more unstable the optical system becomes in general. We must conduct the measurement and feed-forward process as soon as possible.
In this presentation, I will explain the method how to generate the fast-measurable non-Gaussian ancillary state, and show you the result of single-photon generating experiment with this method.
| 氏名： 越川
指導教員名： 岩佐 義宏 教授
発表題目（英語）： Spintronics and nonreciprocal electric transport in a noncentrosymmetric semiconductor
要旨（英語）： Polar conductors are recently attracting a growing interest as a potential material stage. The nonreciprocal response of particles in systems with a broken inversion symmetry is not only an important technological functionality but also the salient physical property which offers the deep implication about the exotic excitation or interaction in solids. Semiconductor diode with p-n junction is the most famous and representative example of nonreciprocal electric transport where the I-V characteristic differs between the right and left directions. Usually the external magnetic field combined with the relativistic spin-orbit interaction is needed for the rectification effect of electrons in solids. So far, nonreciprocal electric responses are studied in polar interfaces or twisted chiral structures but only one example of such a rectification effect in three-dimensional crystal with chirality has been reported and the microscopic mechanism is not well understood. Here I demonstrate both experimentally and theoretically that this effect is dramatically enhanced in three-dimensional bulk polar systems such as BiTeBr when the electron density is low, in which the rectification of abundant current can be manipulated by the external magnetic field.
| 氏名： 佐野
指導教員名： 求 幸年 教授
発表題目（英語）： Beyond Kitaev quantum spin liquids: Two extensions of the Kitaev model
要旨（英語）： The quantum spin liquid (QSL) is an exotic phase of quantum spin systems which does not show any conventional magnetic order even at zero temperature. A particular class of QSLs exhibits long-range quantum entanglement and exotic excitations with emergent statistics. The Kitaev model is a quantum spin model which possesses such exotic features in the exact QSL ground states. The model has received growing attention because of the possibility of its realization in real materials, such as d5 transition metal oxides with strong spin-orbit coupling.
In this study, we investigate two extensions of the Kitaev model. One is to explore the Kitaev-type interaction for the d7 electron configuration in the high-spin state. We find an effective model for the spin-orbital entangled pseudo-spins, which includes ferromagnetic Kitaev interactions in addition to the isotropic Heisenberg interactions. I will present the derivation of the model and discuss the result in comparison with that for the case with d5 electron configuration in the low-spin state.
The second extension is to introduce the coupling to itinerant electrons. Considering a layered system combining the Kitaev magnet and a metal, we obtain an extended Kondo lattice model with an unusual coupling between itinerant electrons and localized Kitaev pseudo-spins. I will present the derivation of the model and discuss the anticipated physics in this model.
| 氏名： 金堂
指導教員名： 三尾 典克 特任教授
発表題目（英語）： Construction of the time-correlated single photon counting system for the observation of enhancement of spontaneous emission
要旨（英語）： Optical near-field is in the vicinity of the surface or an interface of materials. If the material is metal and its size is smaller than the wavelength of light, the magnitude of the electric field |E| of the near field is much greater than that of the illuminating (or exciting) light. Under such a strong field, many interesting phenomena have been observed such as surface-enhanced Raman scattering (SERS) and enhancement of spontaneous emission of excited quantum systems.
One of our collaborative researchers has a sample system which has especially strong near field. In this system, we expect that much greater enhancement of spontaneous emission would be observed. To measure the spontaneous emission rate of the quantum system in a near field, I am constructing a time-correlated single photon counting (TCSPC) apparatus, which has Hanbury Brown-Twiss interferometer set-up. Before measuring the spontaneous emission rate, the characteristics of the TCSPC apparatus is investigated. By using a beat signal of light from the laser stabilized by two-mode method, its time axis is calibrated. By using a crosstalk signal of single photon detectors, its time resolution is estimated.
鈴木 啓 史
指導教員名： 中村 泰信 教授
発表題目（英語）： On-chip circulator for quantum information processing with propagating microwave photon
要旨（英語）： With the advent of quantum information processing with superconducting qubits, the ability to connect spatially separated qubits has been attracting high attention. Propagating microwave photon is one of the candidates for communication medium for short-range communications, and it requires to route single photon without loss or added noise.
Circulator, which is a non-reciprocal three- or four-port device in which a microwave photon entering any port is transmitted to the next port in rotation only, is one of the most important element for propagating wave manipulation. Circulators with ferromagnetic crystals which is widely used now have large losses and not suitable for integration since they cannot coexist with superconducting circuits because of its strong magnetic field.
We have newly devised a circulator which can control propagating direction with low loss. In this presentation, I will talk about how to realize our new circulator and some results.