座長：佐藤 達郎、島崎 紘太
|氏名： 鎌谷 拓実
指導教員名： 森本 高裕 准教授
発表題目（英語）： Higgs modes in noncentrosymmetric superconductors
要旨（英語）： Superconductors are one of the most important topics in studies of condensed matter physics. Recently, collective excitations in superconductors are attracting much attention. One typical example is Higgs modes, which are amplitude modes of superconducting gap function and are the condensed matter counterpart of the Higgs particle that is an origin of mass in particle physics. Higgs modes in superconductors are experimentally observed by Raman spectroscopy and optical measurements of third harmonic generation .
In multiband superconductors such as MgB2 and iron based superconductors, the gap function contains multiple band degrees of freedom and becomes a matrix generally. Consequently, there appears another collective mode that corresponds to the oscillation of the relative phase between different bands, which is called Leggett modes. So far, Leggett modes are observed in Raman experiments using MgB2, while there is no optical measurement of Leggett modes. From the view point of detailed research and operability, it is highly desired to observe Leggett modes via optical responses.
Motivated by these, we study optical responses of Higgs and Leggett modes in multi-band superconductors. To this end, we use Green function methods, where collective excitations are described within random phase approximation (RPA). We find that Higgs and Leggett modes appear in linear responses through optical absorption, in the case of noncentrosymmetric superconductors. This is in contrast to the fact that the excitation of Higgs modes does not appear in linear responses and only appear in third order responses (third harmonic generation) in centrosymmetric superconductors. In particular, this suggests that the Leggett modes can be observed by linear optical responses in noncentrosymmetric multiband superconductors, which facilitates its optical detection. Since Leggett modes can appear below the superconducting gap (in contrast to the Higgs modes that appear right at the superconducting gap), Leggett modes can be detected as a single peak in absorption spectra.
 R. Matsunaga et al., Science 345, 6201 (2014)
 G. Blumber et al., Physics Review Letters 99, 227002 (2007)
|氏名： 古賀 淳平
指導教員名： 石坂 香子 教授
発表題目（英語）： Ultrafast phase transition dynamics in photoinduced TaTe2
要旨（英語）： Ultrafast electron diffraction (UED) is time resolved measurement for development of electron diffraction images after photoexcitation by pump-probe method. This method reveals ultrafast optical response and lattice dynamics by short pulse laser. And photoexcitation process by pump pulse leaser can generate non-equilibrium states, and meta-stable states. These states attracting attention because physical properties and structures which cannot be observed in equilibrium state can appear.
Transition metal dichalcogenides (TMDs) indicate various crystal structures corresponding to their composition and temperature. Some TMDs are known to cause photoinduced structural phase transition at picosecond or sub-picosecond time scale. Probing and controlling the state in them is one of important quests in physics, and various experiments have been done.
In this presentation, I will introduce lattice dynamics of TMDs, and our experiment data of TaTe2, a kind of TMDs, measured by UED.
|氏名： 上島 卓也
指導教員名： 沙川 貴大 教授
発表題目（英語）： Investigating Power-Efficiency Tradeoff beyond Linear Response Regime
要旨（英語）： According to thermodynamics, the efficiency of heat engines generally cannot surpass the universal efficiency, called Carnot efficiency. In the reversible process, the Carnot efficiency can be achieved but the heat engine cannot produce nonzero power due to its long-time operation. When it comes to operating heat engines in a finite time, power and efficiency cannot be optimized simultaneously in general. This trade-off relation is well established in the linear response regime, where the temperatures of the reservoirs are slightly different, and the tradeoff relation is fully characterized only by two parameters. However, it is known that these parameters lose its significance beyond the linear response regime, thus it is a difficult task to explore the trade-off relation there. Taking advantage of nonlinearity to optimize heat engines will offer a significant improvement in the practical application such as energy supply and reuse of waste heat.
To investigate tradeoff relations in the nonlinear regime we focus on analyzing small systems on the basis of stochastic thermodynamics, while we also plan to extend the setup to larger and complicated systems and extract universal properties from them. In this presentation, I will discuss my theoretical and numerical calculations of power and efficiency for quantum dots and show the trade-off relation in the nonlinear regime.
|氏名： 小林 拓豊
指導教員名： 吉岡 孝高 准教授
発表題目（英語）： Development of Laser Systems for Cooling and Measuring Temperature of Positronium
要旨（英語）： Positronium (Ps), a bound state of an electron and its anti-particle (a positron), was discovered in 1951 and has since been extensively investigated to develop and test the bound-state quantum electrodynamics (QED). Since Ps is a purely leptonic system, it is free of hadronic corrections, and its properties can be calculated perturbatively with very small uncertainty and compared with precise measurements. One of the most useful targets is measurement of the Ps 1S-2S transition frequency, which can be precisely determined thanks to its narrow linewidth of the resonance. Improvement of the precision from the current 2.6 ppb to 0.1 ppb will lead not only to more rigorous test on the bound-state QED, but also to an observation of a tiny gravitational redshift on the transition frequency, which will be the first precise measurement of gravitational effect on antimatter.
The improved precision of the transition frequency requires cooling of Ps because a very wide Doppler broadening (~300 GHz at room temperature), which results from the light mass of Ps, causes seriously large uncertainties. One of the experimental challenges for cooling Ps is that Ps annihilates into photons with a lifetime of 142 ns. Laser cooling is a promising method which is efficient enough to cool Ps in the short duration. We have been developing a special light source which has a wide bandwidth and a long pulse duration to overcome difficulties arising from the large Doppler broadening and the short annihilation lifetime.
In order to confirm cooling of Ps, we need to measure the temperature of Ps. We also have been developing another laser to measure velocity distributions via the Doppler broadening of the linewidth of the Ps 1S-2P transition.
In this presentation, I will introduce the designs and features of these two lasers, and show the current status of the development.
|氏名： 川﨑 彬斗
指導教員名： 古澤 明 教授
発表題目（英語）： The development of broadband quantum memory
要旨（英語）： In quantum information processing with light,a large-scale computational platform, called cluster states, was realized last year .By externally injecting non-classical states, called non-Gaussian states, into these cluster states, fault tolerance and universality can be realized.
Some non-gaussian states like GKP qubits are generated by interference of multiple probabilistically generated states. The interference rate can be increased exponentially by synchronizing the interference timing with quantum memories .
In this research, we aim to realize high generation rates of non-Gaussian states using a quantum memory with a concatenated optical cavity. A quantum memory with concatenated optical cavity consists of a memory cavity which store quantum states and a shutter cavity which control the timing of storing and reading out quantum states.It is known that the product of the storage time τ and the bandwidth of quantum memory δ needs to be increased in order to increase the rate of state generation . However, the current quantum memory with concatenated optical cavity  has a trade-off relationship between storage bandwidth and storage time. Thus a mechanism for increasing the bandwidth while preserving the storage time needs to be developed. In this research we try to solve this problem by introducing a new method into a shutter cavity.
In the presentation, I will give an overview of the research and its current progress.
W.Asavanant et al., Science 366.6463(2019)
H. M. Vasconcelos et al., Optics letters 35.19 (2010)
J. Nunn et al., Physical review letters 110.13 (2013)
J.Yoshikawa et al., Physical Review X 3.4(2013)
|氏名： 小林 弘和
指導教員名： 渡辺 悠樹 准教授
発表題目（英語）： Modern theory of polarization and orbital magnetization
要旨（英語）： Polarization and magnetization are fundamental and familiar quantities, but their classical definition is not well-defined under periodic boundary conditions (PBCs) for the ambiguity of the position. This difficulty was solved by the new definition called modern theory . These definitions are based on the Bloch wave function and well-defined under PBCs. The modern theory is also interesting in that it shows us the connection to Berry phase and Bulk-Boundary correspondence.
In this presentation, I will introduce the modern theory and present the problem which I am working on. My current goal is to extend the theory to higher orders by constructing a unified derivation of the modern theory.
 R. Resta, Rev. Mod. Phys. 66, 3, 899-915 (1994)
 T. Thonhauser, Davide Ceresoli, David Vanderbilt, and R. Resta Phys. Rev. Lett. 95, 137205 (2005)
|氏名： 熊崎 紘介
指導教員名：沙川 貴大 教授
発表題目（英語）： Implementing time-dependent Hamiltonians by autonomous quantum machines
要旨（英語）： To implement unitary operations on a quantum system by interaction with a time-dependent external field, the strength of the interaction is needed to be precisely controlled. In the full quantum setup, the external quantum system switching the interaction is called a clock, though it is theoretically nontrivial how a classical field emerges there. Here, we consider an autonomous quantum machine where the total Hamiltonian of the system and the clock is time-independent and focus on how the time-dependent Hamiltonian of the system is effectively implemented via such an autonomous and fully quantum machine.
In this presentation, I will show that the time evolution and the energy change in the system with a time-dependent Hamiltonian can be perfectly implemented with an ideal clock of infinite dimension, and introduce a finite clock model that approximately reproduces the ideal clock. I discuss how the time-dependent Hamiltonian is implemented with the clock and show some numerical results that demonstrate that the performance of the finite clock model asymptotically approaches to the ideal clock.