発表者名 
馬場 正太郎 
指導教員名 
中村 泰信 教授 
発表題目（英語） 
Design of broadband Josephson parametric amplifiers based on a coupledresonator model 
要旨（英語） 
Superconducting qubits are an outstanding candidate for quantum computing. However, to develop a faulttolerant quantum computer with many superconducting qubits, one big problem is how to control many qubits in the dilution refrigerator and decrease the number of room temperature control instruments. Frequencymultiplexed qubit readout is one solution to tackle this problem and broadband JPAs are necessary for frequencymultiplexed qubit readout.
A standard way to measure qubit states is a reflection measurement of a resonator coupled to a qubit. This protocol is widely used because of small measurement back action and protection of qubit decoherence by readout resonators. However, due to the low signaltonoise ratio(SNR), we need low noise, high gain amplifiers for qubit measurement by one microwave pulse and Josephson parametric amplifiers(JPA) are necessary for this purpose. We have to amplify some microwave frequencies simultaneously by one amplifier for frequencymultiplexed qubit readout, so we need broadband JPAs.
There are two approaches to construct broadband JPAs. One is to achieve impedance matching between JPA and the outside world coupled to JPA[1,2]. The other is to use a nonlinear transmission line with many Josephson junctions[3].
In this presentation, I will explain the operation and the impedance matching of broadband JPAs and talk about methods to design broadband JPAs based on a coupledresonator model by focusing on the similarity between JPAs with impedance matching and bandpass filters. I also show JPAs designed by a coupledresonator model can achieve wide bandwidth and high gain compared to JPAs with impedance matching.
[1]T.Roy et al., Appl. Phys. Lett. 107, 262601 (2015).
[2]J.Y.Mutus et al., Appl. Phys. Lett. 104, 263513 (2014).
[3]C.Macklin et al., Science 350, 307 (2015). 
発表言語 
日本語 

発表者名 
西澤 葉 
指導教員名 
賀川 史敬 准教授 
発表題目（英語） 
MFM measurement of multiferroics (FexZn1x)2Mo3O8 (The topic could be changed) 
要旨（英語） 
The multiferroic material Fe2Mo3O8 shows anomalous hysteresis broadening with doping of Zn in Fe sites. I'm trying to discover what process leads this broadening. I use the MFM (Magnetic Force Microscopy) method to measure real space images. It is implied that anomalously broadening areas showed different phase change dynamics with not anomalously broadening areas. 
発表言語 
日本語 

発表者名 
半田 光 
指導教員名 
高橋 陽太郎 准教授 
発表題目（英語） 
Terahertz spectroscopy of soft phonon in (Pb,Sn)Te thin film 
要旨（英語） 
In terahertz region, dynamics of elementary excitation and electron, for example Drude response, phonon and antiferromagnetic resonance of spin, can be observed, so terahertz region recently attracts a great deal of attention.
It has been reported that in SrTiO3, by using intense terahertz pulses, soft mode resonance frequency hardens at high field, so nonlinearity is observed [1]. And it has also been reported that by using intense terahertz pulses, ferroelectric soft mode in SrTiO3 drives strongly and, displacement of ferroelectric soft mode is comparable to ferroelectric phase induced by pressure [2]. These studies suggest that transient ferroelectric phase is induced by intense terahertz pulses.
(Pb, Sn)Te is good candidate for large nonlinearity by intense terahertz pulses.
(Pb, Sn)Te is narrow gap semiconductor, and PbTe is paraelectric and SnTe is ferroelectric.
In this presentation, I will talk about composition ratio dependence of soft mode in (Pb, Sn)Te and nonlinearity induced by intense terahertz pulses. I will explain the nonlinearity by using anharmonic potential model.
[1] I. Katayama *et al*., PRL(2012)
[2] X. Li *et al*., Science (2019). 
発表言語 
日本語 

発表者名 
福田 光 
指導教員名 
十倉 好紀 卓越教授 
発表題目（英語） 
Topological transport properties of noncoplanar magnets 
要旨（英語） 
In recent years, there has been a lot of research that has brought a topological perspective to physics, including the quantum Hall effect.
Topological materials are considered to be candidates for the next generation of electronic devices because of their nondissipative nature and robustness to fluctuations. According to the Berry phase theory, it is the Berry curvature that characterizes these topological materials, which acts as a giant effective magnetic field. This shows that controlling Berry curvature is equivalent to controlling giant effective magnetic field and nondispersive currents. Wavenumber space, like topological insulators, is the most typical example where Berry curvature appears, but the Berry curvature of the electronic band is difficult to control by the external field because its energy scale is much larger than that of the external field. On the other hand, since Berry curvature occurs in arbitrary parameter space, the policy of using Berry curvature in spin space has also been proposed. For example, in a noncoplanar magnetic structure, the itinerant electrons are subjected to a huge effective magnetic field.
Therefore, in noncoplanar magnetic materials, the Berry curvature can be easily controlled by a magnetic field through the manipulation of spins. In this presentation, I will talk about the noncoplanar magnets *R*2Mo2O7 (*R* : rareearth elements) and their exotic transport properties. 
発表言語 
日本語 
