発表者名 
熊崎 紘介 
指導教員名 
沙川 貴大 教授 
発表題目（英語） 
Implementing noncommutative timedependent Hamiltonians by a quantum clock 
要旨（英語） 
When considering manipulating a quantum system with external control, there are two theoretical frameworks to treat the time dependence of the Hamiltonian. One is a nonautonomous framework in which the classical external field is considered and the Hamiltonian of the system is timedependent, and the other is an autonomous framework in which the external quantum system interacting with the system is explicitly considered and the Hamiltonian of the whole system is timeindependent. In this study, we investigate the relationship between these frameworks.
We focus on the fact that the timedependent Hamiltonian can be implemented approximately to the system by using an ancilla system called a clock [1]. In particular, we investigate how the error between the state evolution with a finitedimensional clock as an ancilla system and the evolution of the timedependent Hamiltonian scales with respect to the dimension of the clock. In a previous study, the scaling of the exponential decay at the time after one cycle of the clock has been theoretically shown, but the analysis of the implementation error of the state evolution was restricted to the case where the timedependent Hamiltonian commutes with itself at different times [2]. In our study, we perform numerical calculations for the case where the Hamiltonian does not commute with itself at different times and observe the scaling of the exponential decay of the implementation error at the time after one cycle as in the commutative case. The scaling of the error is also analyzed for times other than after one cycle, and in contrast, a powerlaw decay is observed. Furthermore, we have theoretically derived such a powerlaw decay. This result suggests that high accuracy implementation with the clock can only be achieved at specific times.
[1] A. S. L. Malabarba et al., New J. Phys. *17*, 045027 (2015).
[2] M. P. Woods, R. Silva and J. Oppenheim, Ann. Henri Poincare *20, *125218(2019). Recently, a new criterion of the second law of thermodynamics is proposed, which is called concavity [1]. An energy eigenstate that meets the criterion indeed obeys the principle of maximum work for adiabatic processes, which is an expression of the second law. The concavity also can be used to judge whether a mixed state obeys the second law or not and would be useful to study quantum heat engines.
In quantum heat engines [2], there are several examples that show higher efficiency than the Carnot efficiency [3]. However, the reason why these quantum engines show high efficiency has not been clarified yet.
In this presentation, I will introduce the concept of concavity for a single energy eigenstate and a mixed state, and show a numerical estimate of the efficiency of a single qubit heat engine.
[1] C. Itoi and M. Amano, J. Phys. Soc. Jpn. 89, 104001 (2020)
[2] J. Goold et al., J. Phys. A: Math. Theor. 49 143001 (2016)
[3] J. Roßnagel et al., PRL 112, 030602 (2014) 
発表言語 
日本語 

発表者名 
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 and construction of ultrafast electron diffraction 
要旨（英語） 
Optical manipulating the structure and function of materials remains one of the ultimate challenges of science. MTe2(M=V, Nb, Ta) is known to form MM bonding, leading to the formation of a double zigzag chain in room temperature. It is also reported that in VTe2, this VV bonding is related to topological surface states. However, the ultrafast response of MTe2 to optical driving remains unknown.
Ultrafast electron diffraction (UED) is time resolved measurement for time dependence of electron diffraction images after photoexcitation by pumpprobe method with short pulse laser. This method reveals ultrafast optical response and lattice dynamics. In this study, we have used UED for elucidating photoinduced dynamics of TaTe2. We observed ultrafast intensity increase in the Bragg intensity (< 0.5 ps), that implies structural phase transition.
In this presentation, I will discuss the origin of photoinduced structural phase transition from the analysis of the electron diffraction intensity and introduce reconstruction of UED equipment to improve time resolution. Here, we focus on pyrochlore oxides (A2B2O7) which are theoretically predicted to be another candidate of the host of flat band structure. Among them, Sn2Nb2O7 and Sn2Ta2O7 are specifically expected to be promising compounds according to some first principle calculations. We aim to synthesize highquality single crystalline thin films of them by pulsed laser deposition method in order to materialize the flat band structure.
In this presentation, I will explain why ferromagnetism emerges in the flat band system, and why pyrochlore oxides can be candidates of it.
Besides, I will introduce my experimental results of synthesizing and evaluating thin films of these pyrochlore oxides and discuss difficulties in inducing ferromagnetism in them. 
発表言語 
日本語 

発表者名 
小林 拓豊 
指導教員名 
吉岡 孝高 准教授 
発表題目（英語） 
A Laser Cooling Experiment of Positronium 
要旨（英語） 
Positronium (Ps) is a hydrogenlike atom consisting of an electron and its antiparticle (a positron). Ps is an important research target for precise verification of fundamental physics because it is a simple system consisting of two leptons. If Ps can be cooled down to below 10 K, it is expected to lead to important applications such as the verification of the Standard Model of elementary particles by precise measurement of its atomic properties, the search for matterantimatter asymmetry, and the realization of BoseEinstein condensation for the first time in a system containing antimatter. We are working on the realization of laser cooling of Ps.
In order to realize effective cooling of Ps, a special light source is required to adapt to the characteristics of Ps, which has the short lifetime as 142 ns and the wide Doppler width around 500 GHz at room temperature. We have developed a prototype laser based on our original design. The laser was transported and installed at an accelerator facility where Ps can be produced. In addition, we have constructed a laser system to evaluate the velocity distribution of Ps by measuring the Doppler spectrum of the 1S2P transition.
In this presentation, I will introduce the design of the laser cooling experiment and show the results so far. 
発表言語 
日本語 

発表者名 
阪本 天志 
指導教員名 
吉岡 孝高 准教授 
発表題目（英語） 
Exploration of laser cooling method for carbon atoms 
要旨（英語） 
Laser cooling and trapping of atoms at very low temperatures are techniques of great value in a wide range of applications, such as precision spectroscopy, observation of quantum degenerate states, and
chemical reaction studies.
However, these techniques can be applied only to limited atomic species, such as alkali and alkaliearth metals which have convenient energy levels suited to current laser technology.
In particular, many atomic species of chemical and biological interest, including carbon, are currently out of reach due to the lack of sufficiently powerful lasers in deep ultraviolet which are required for cooling.
In this presentation, I will introduce our research plan and current progress of light source development toward the realization of laser cooling of carbon atoms with the background of a proposal for a new laser cooling method[1].
[1] A. M. Jayich et al. Physical Review X6, 041004 (2016) 
発表言語 
日本語 
