発表者名 
荻原 琢磨 
指導教員名 
求 幸年 教授 
発表題目（英語） 
Efficient realfrequency solver for dynamical meanfield theory 
要旨（英語） 
Ab initio electronic structure calculations based on density functional theory (DFT) have been important theoretical and numerical tools in modern materials science. However, despite of their great success, the DFTbased methods have often failed to predict properties of correlated electron materials such as transition metal oxides. For example, standard DFTbased methods, local density approximation (LDA) and generalized gradient approximation (GGA), fail to reproduce experimentally observed crystal structures of strongly correlated electron materials. As a typical example, LDA and GGA have been known to underestimate the volume of FeO. To improve the accuracy of the DFTbased methods, a combination of DFT and dynamical meanfield theory (DMFT) [1] called DFT+DMFT has been proposed [2]. Indeed, the DFT+DMFT reproduces the crystal structure of FeO [3, 4].
The DMFT maps a correlated lattice model onto a quantum impurity model in a selfconsistent fashion so that the local oneparticle Green functions of the lattice and impurity models become identical. The impurity model consisting of a correlated impurity site and noninteracting bath sites is much easier to solve than the original lattice model. Since the introduction of the DMFT, there has been an enormous development on how to solve the quantum impurity models.
In our previous presentation, we reviewed an important previous work [5]. In this work, an exact diagonalization (ED) technique was used to solve the impurity model, and the DMFT loop was implemented by using real frequency to avoid analytical continuation. Na?ve implementation of the ED method has a problem that the Hilbert space dimension grows exponentially as the number of the bath sites increases. In Ref. [5], the Hilbert space dimension is effectively reduced by selecting basis functions with natural orbitals. Then they succeeded in solving a single impurity model with three hundred bath sites. It is highly desirable to extend the previous method to multiorbital systems and to realize an efficient impurity solver for the DFT+DMFT. Towards this goal, first, we have been working to reproduce the result of the previous work. In the presentation, we will report our progress.
[1] A. Georges, G. Kotliar, W. Krauth, and M. J. Rozenberg, Rev. Mod. Phys. 68, 13 (1996).
[2] G. Kotliar et al., Rev. Mod. Phys. 78, 865 (2006).
[3] K. Haule and T. Birol, Phys. Rev. Lett.115, 256402 (2015).
[4] A. Paul and T. Birol, Annu. Rev. Mater. Res. 49, 31 (2019).
[5] Y. Lu, M. Hoppner, O. Gunnarsson, and M. W. Haverkort, Phys. Rev. B 90, 085102 (2014). We have constructed a midinfrared spectroscopy system using a midinfrared comb and an extremelyhighorder dispersion element and a twodimensional array type detector. Our laboratory has constructed highly sensitive, highresolution and realtime midinfrared spectroscopic system in the 35 um band. The 35 um band has sharp absorption lines of light molecules, and a highresolution spectroscopic system is required. On the other hand, the 812 um band has wide and complex absorption lines of heavy molecules such as aromatic compounds. So wider spectral system is required in the 812 um band.
In this presentation, I will introduce the setup of midinfrared spectroscopy system in the 812 um band . 
発表言語 
日本語 

発表者名 
加藤 啓輔 
指導教員名 
中村 泰信 教授 
発表題目（英語） 
Magnon based quantum tranducer 
要旨（英語） 
Recently, quantum computation has been attracting much attention by virtue of its potential to dramatically improve computational power for certain tasks in both academic and industrial areas. Amongst physical platforms for quantum computation, the superconducting qubit(quantum bit) is the most widely researched system around the world. The superconducting qubit is basically electric circuits made of superconducting material. It behaves as a quantum twolevel system whose transition frequency is in the microwave
regime and its properties like the transition frequency and the magnitude of interaction with the external environment are tunable by changing circuit parameters. Although its tunability and some other points give an advantage over other physical systems that behave as a qubit, its microwave energy scale forces it to work in a dilution refrigerator because quantum states can easily decay in room temperature.
Now I shall cast an eye on the quantum network. Quantum network is the network that connects many quantum devices in a quantum manner and allows more complicated computation, distribution of quantum entanglement, and so on. Each device is to be connected using optical fibre because of its low transmission loss. However, since a superconducting qubit works in the microwave regime, we need to convert quantum states from superconducting qubits to optical photons in the reversible manner. The physical system to do so is called quantum transducer.
It is not an easy task to convert quantum states from superconducting qubits to optical photons because energy magnitude of each systems are quite different. There are several candidates for quantum transducer. In the midst of them, I shall explain one using magnon, the quanta of ferromagnetic resonance. First, I confirmed that the NMR spectral shift (Knight shift), which measures the magnitude of the local magnetic field created by the electron spins, is scaled to the spin susceptibility and found that the spectrum is widened with decreasing temperature, suggesting an increasing inhomogeneity of the local spin susceptibility upon cooling.
Second, the nuclear spinlattice relaxation rate 1/T1, which measures the intensity of electron spin fluctuations, showed a temperature dependence similar to in the spinliquid compound κ(ET)2Cu2(CN)3. A previous study found both materials to behave similar in spin susceptibility; thus, κ(ET)4Hg2.89Br8 is regarded as a doped spin liquid hosting both mobile carriers and a quantum spin liquid.
Furthermore, a decreases in the Knight shift and a cubic temperature dependence of 1/T1 below 10 K suggest that superconductivity emerging in this material has spinsinglet dwave symmetry with highly enhanced fluctuations above Tc. 
発表言語 
日本語 

発表者名 
上島 卓也 
指導教員名 
沙川 貴大 教授 
発表題目（英語） 
Powerefficiency tradeoff of heat engines in the nonlinear 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 limit, the Carnot efficiency can be achieved, where, however, the heat engine cannot produce nonzero power due to its longtime operation. When it comes to operating heat engines in a finite time, power and efficiency cannot be optimized simultaneously in general. This tradeoff relation is wellestablished 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 those parameters can no longer characterize the tradeoff relation beyond the linear response regime, thus it is a difficult task to explore the tradeoff relation there.
Pareto front and efficiency bound are two promising approaches to investigate the tradeoff relation in the nonlinear regime. Focusing on small timeindependent systems like quantum dots, we calculated the Pareto front numerically and confirm that both power and efficiency are enhanced by the nonlinearity. Furthermore, we developed a new efficiency bound taking into account higherorder fluctuations of power. According to our calculation in the quantum dots, our new bound turned out to be tighter than the conventional efficiency bound. We plan further investigate larger and complicated systems and extract universal properties of the tradeoff relation from them.
In this presentation, I will show the aforementioned results on power and efficiency and discuss the tradeoff relation in the nonlinear regime. 
発表言語 
日本語 

発表者名 
川﨑 彬斗 
指導教員名 
古澤 明 教授 
発表題目（英語） 
Temporal mode function engineering of NonGaussian state 
要旨（英語） 
In continuous variable quantum information processing with light, highly nonclassical states called nonGaussian states are essential to realize faulttolerant and universal quantum computation. The standard method for generating nonGaussian states is called "heralding state preparation". In this method, nonGaussian state can be generated on one side of the two entangled modes by projective measurement of the other side.
On the other hand, for quantum computation in real experimental systems, it is important to properly define the temporal mode function that describes the shape of the wave packet of light that contains the quantum state. For example, a wave packet defined in finite time with no DC component has been proposed to be useful for largescale highspeed quantum computation [1].There have been previous studies [2, 3] on the generation of nonGaussian states defined by temporal mode function of simple shapes in heralding scheme, but there has been no method to freely shape an arbitrary temporal mode function. We propose that arbitrary temporal mode functions can be engineered by combining a broadband quantum entanglement source and appropriate frequency filtering in measurement system of the heralding scheme.
[1] J. Yoshikawa et al., APL Photonics 1, 060801 (2016).
[2] H. Ogawa et al., Phys. Rev. Lett. 116. 233602 (2016).
[3] S. Takeda et al., Phys. Rev. A 87, 043803 (2013). 
発表言語 
英語 
