This is a seminar jointly organized by the three groups working on quantum physics and technology at Department of Physics, Kindai University, namely Condensed-Matter Theory (CMT), Quantum Control (QC), and QMB Laboratories.

### Scheduled talks

Fiscal year 2021

Time and Date: 10:45-, July 28, 2021

Room: Rm. 31-808, 8th Floor, 31st Bldg. + Webcast via Zoom

Speaker: Suwa, Mizuki (General Relativity and Cosmology Lab., Kindai Univ.)

Title: Only in Japanese

Abstract: Only in Japanese

### Talks in the past

Fisical year 2021

Time and Date: 10:45-, April 14, 2021

Room: Rm. 31-808, 8th Floor, 31st Bldg. + Webcast via Zoom

Speaker: Kokubo, Haruya (CMT)

Title: Wave pattern formation of quantum Kelvin-Helmholtz instability in binary superfluids

Abstract: Kelvin-Helmholtz instability (KHI) is an instability of an interface in phase separated two component fluids with relative velocity. Sinusoidal growth appears at the interface over time, forming a large spiral structure by this instability [1]. A Weber number is a dimensionless number defined by the ratio of the inertial force of the fluid to the surface tension. This number can characterize the dynamics of interface growth driven by the KHI [2].

An interface in a superfluid causes a phenomenon similar to KHI, and in fact, it has been studied experimentally in a superfluid He [3]. In a Bose-Einstein condensate (BEC) of ultra cold atoms, the dissipation mechanism can be approximately ignored, and thus an instability phenomenon similar to the dynamic instability in the ideal classical fluid can be seen. The KHI in BECs has been studied theoretically [4].

In this talk, we will show that the pattern forming dynamics caused by the KHI can be classified by the Weber number defined for two component superfluids with a relative velocity. This Weber number(We) can be written the ratio an interface thickness to the wave length of the most unstable mode of the KHI in a superfluid. On We << 1, an interface forms finger pattern. On We >> 1, an interface forms either sealskin pattern or zipper pattern depending on the interface thickness.

[1]Hydrokinetic solutions and observations. Kelvin, Load (William Thomson), Phil. Mag. (4), vol.42, 362-377.

[2]Simulation Of Viscous Stabilization Of Kelvin- Helmholtz Instability. AT Dinh, et al., Advances in Fluid Mechanics III

[3]Shear Flow and Kelvin-Helmholtz Instability in Superfluids R. Blaauwgeers, et al. PRL 89, 155301

[4]Quantum Kelvin-Helmholtz instability in phase-separated two-component Bose-Einstein condensates Hiromitsu Takeuchi, et al. PRB 81, 094517

Time and Date: 9:00-, April 21, 2021

Room: Webcast via Zoom

Speaker: Iigaya, Kiyohito (California Institute of Technology)

Title: Neural principles of subjective value construction

Abstract: It is an open question how humans construct the subjective value of complex objects (stimuli), such as artistic paintings or photographs. While great progress has been made toward understanding how the brain *adjusts* the value of objects through reinforcement-learning, little is known about how the value *arises* in the brain in the first place. Here, we propose and provide evidence that the brain constructs the value of a novel stimulus by extracting and assembling common features. Notably, because those features are shared across a broad range of stimuli, we show that simple linear regression in the feature space can work as a single neural mechanism to construct the value across stimulus domains. In large-scale behavioral experiments with human participants, we show that a simple model of feature abstraction and linear summation can predict the subjective value of paintings, photographs, as well as shopping items whose values change according to different goals. The model shows a remarkable generalization across stimulus types and participants, e.g. when trained on liking ratings for photographs, the model successfully predicts a completely different set of art painting ratings. Also, we show that these general features emerge through image recognition training in a deep convolutional neural network, without explicit training on the features, suggesting that features relevant for value computation arise through natural experience. Furthermore, using fMRI, we found evidence that the brain actually performs value computation hierarchically by transforming low-level visual features into high-level abstract features which in turn are transformed into valuation. We conclude the feature-based value computation is a general neural principle enabling us to make flexible and reliable value computations for a wide range of complex stimuli.

Time and Date: 10:45-, May 12, 2021

Room: Rm. 31-808, 8th Floor, 31st Bldg. + Webcast via Zoom

Speaker: Goto, Shimpei (QMB)

Title: How to make typical(-like) states from product states

Abstract: Recent improvements of imaginary time evolution algorithm in quantum circuits have triggered the development of an algorithm for simulating quantum many-body systems at finite temperatures [1], which has been considered very difficult even in quantum computations. Such an algorithm uses the random sampling of initial product states to evaluate the trace of operators.

In this talk, we show that the trace evaluation based on the random sampling of initial product states could lead to severe inefficiency in 100-qubit scale systems [2]. In order to resolve the sampling inefficiency, we propose two methods: One is effective in classical computers, and the other is designed for fault-tolerant quantum computers.

[1] Shi-Ning Sun *et al*., PRX Quantum 2, 010317 (2021).

[2] Shimpei Goto, Ryui Kaneko, and Ippei Danshita, arXiv:2103.04515.

Time and Date: 10:45-, May 19, 2021

Room: Rm. 31-808, 8th Floor, 31st Bldg. + Webcast via Zoom

Speaker: Kukita, Shingo (QC)

Title: Heisenberg-limited quantum metrology by collective dephasing

Abstract: The goal of quantum metrology is the precise estimation of physical parameters using quantum properties such as entanglement. This estimation usually consists of three steps: initial state preparation, time evolution during which information of the parameters is imprinted in the state, and readout of the state. Decoherence during the time evolution typically degrades the performance of quantum metrology and is considered to be one of the major obstacles to realizing entanglement-enhanced sensing. We show, however, that under suitable conditions, this decoherence can be exploited to improve the sensitivity [1]. In this talk, I will introduce a sensing scheme utilizing Markovian collective dephasing. Assume that we have two axes, and our aim is to estimate the relative angle between them. Our results reveal that the use of Markvoian collective dephasing to estimate the relative angle between the two directions affords Heisenberg-limited sensitivity. Moreover, our scheme is robust against environmental noise: it is possible to achieve the Heisenberg limit even under the effect of independent dephasing.

[1]: arXiv:2103.11612 [quant-ph]

Time and Date: 10:45-, May 26, 2021

Room: Rm. 31-808, 8th Floor, 31st Bldg. + Webcast via Zoom

Speaker: Kondo, Yasushi (QC)

Title: Quantum Zeno-effect realized in NMR

Abstract: A quantum Zeno-effect is a very counter-intuitive phenomenon and illustrates a difference between classical and quantum measurements: The measurements can be performed without disturbing a system of interest in classical mechanics, while it is not the case in quantum mechanics. We discuss a quantum Zeno-effect experiment realized with a standard high precision NMR spectrometer at Kindai University [1].

[1] Y. Kondo, Y. Matsuzaki, K. Matsushima, and J. G. Filgueiras; New J. Phys. 18, 013033 (2016).

Time and Date: 10:45-, June 2, 2021

Room: Rm. 31-808, 8th Floor, 31st Bldg. + Webcast via Zoom

Speaker: Kaneko, Ryui (QMB)

Title: Tensor-network study of quench dynamics of antiferromagnetic correlations in a two-dimensional quantum Ising model

Abstract: Quantum simulators using Rydberg atom arrays have attracted growing interest owing to rapid technological advances [1]. The Rydberg atom arrays can realize the quantum Ising model, which is a fundamental model in statistical physics. Quench dynamics of antiferromagnetic correlations have been observed in the two-dimensional Ising systems [2,3]. Very recently, the number of controllable atoms has exceeded 200 [4,5]. On the other hand, it is also necessary to confirm the validity of the experimental results by numerical simulations on classical computers. To this end, we have applied the tensor-network method [6,7] using projected entangled pair states (PEPS) [8,9], which can handle infinite two-dimensional systems. We have calculated the real-time evolution of antiferromagnetic correlations in the quantum Ising model when the system is quenched from a disordered state. We have found that the estimated phase velocity is maximized locally near the transition point of the ground state phase diagram.

[1] A. Browaeys and T. Lahaye, Nat. Phys. 16, 132 (2020).

[2] E. Guardado-Sanche et al., Phys. Rev. X 8, 021069 (2018).

[3] V. Lienhard et al., Phys. Rev. X 8, 021070 (2018).

[4] S. Ebadi et al., arXiv:2012.12281.

[5] D. Bluvstein et al., Science 371, 1355 (2021).

[6] C. Hubig and J. I. Cirac, SciPost Phys. 6, 031 (2019).

[7] P. Czarnik et al., Phys. Rev. B 99, 035115 (2019).

[8] T. Nishino et al., Prog. Theor. Phys. 105, 409 (2001).

[9] F. Verstraete, J. I. Cirac, arXiv:cond-mat/0407066.

Time and Date: 10:45-, June 9, 2021

Room: Rm. 31-808, 8th Floor, 31st Bldg. + Webcast via Zoom

Speaker: Mikkelsen, Mathias (QMB)

Title: Connecting scrambling and work statistics in the interacting harmonic oscillator

Abstract: The non-equilibrium excitations created by sudden changes in the physical parameters of a quantum system (quenches) are well-described by the work probability distribution which establishes a connection to thermodynamics quantities such as the irreversible work [**1**]. The work probability distribution is closely related to the delocalisation of the initial state in the eigenspace of the final Hamiltonian. A different measure of delocalisation, namely the dynamic delocalisation of operators in Hilbert space, known as scrambling, has seen a lot of interest for interacting systems in the last 5 years [**2**]. The scrambling can be quantified by so-called out of-time order correlators (OTOCs). Some specific OTOCs associated with an Ising spin-chain have been measured in an ion setup [**3**]. Such measurements involve a time-reversal of the Hamiltonian, however, and are therefore very difficult for general continuum systems due to the kinetic energy term.In our work [**4**] we investigate interacting particles in a harmonic oscillator after a quench of the trapping frequency, utilizing numeric solutions for up to 5 particles and fully analytic solutions for 2 particles. We show that the scrambling of the single-particle canonical operators is closely related to the work probability distribution, particularly that the infinite-time average of the scrambling is proportional to the work fluctuations. Furthermore we show that our results can be extrapolated to N particles. This further elucidates the role of irreversibility, which is closely related to scrambling, in the quench and links the scrambling to an experimentally accessible quantity, namely the work statistics [**5**] for an important continuum system.

[**1**] M. Á. García-March, T. Fogarty, S. Campbell, T. Busch, and M. Paternostro, *New J. Phys.* **18**, 103035 (2016)

[**2**] B. Swingle, *Nat. Phys. ***14**, 988–990 (2018).

[**3**] M. Gärttner, J. G. Bohnet, A. Safavi-Naini, M. L. Wall, J. J. Bollinger, and A. M.Rey, *Nat. Phys. ***13**, 781–786 (2017).

[**4**] M. Mikkelsen, T. Fogarty and Th. Busch, arXiv:2009.14478 (2020)

*note that this currently only has the 2-particle solution, will be updated soon.

[**5**] M. Cetina, M. Jag, R. S. Lous, I. Fritsche, J. T. M. Walraven, R. Grimm, J. Levinsen, M. M. Parish, R. Schmidt, M. Knap, and E. Demler, *Science*** 354**, 96 (2016)

Time and Date: 10:45-, June 16, 2021

Room: Rm. 31-808, 8th Floor, 31st Bldg. + Webcast via Zoom

Speaker: Danshita, Ippei (QMB)

Title: Superfluidity of two-orbital Bose gases in optical lattices

Abstract: Recent experiments have used two-component bosonic atoms in state-dependent optical lattices in order to realize two-orbital Bose gases [1,2]. In this kind of systems, one can induce orbital hybridization by making Rabi coupling between the two internal states of the atom through microwaves [1] or lasers [2]. In this study, we study superfluidity of the two-orbital Bose gases in a situation that one component is delocalized all over the system while the other is localized by a deep optical lattice. We analyze the two-orbital Bose-Hubbard model within the Gross-Pitaevskii mean-field theory and calculate the nonlinear band structure of the energy by using the pseudo-arclength method [3]. We find three different cases with respect to the breakdown of a supercurrent. In one of the cases, when the strength of the orbital hybridization increases, a transition from a superfluid to another superfluid occurs. We discuss how to realize this transition in future experiments.

[1] L. Krinner, M. Stewart, A. Pazmino, J. Kwon, and D. Schneble, Nature **559**, 589 (2018).

[2] L. Riegger, Ph.D. Thesis (2019).

[3] M. Kunimi and Y. Kato, Phys. Rev. A **91**, 053608 (2015).

Time and Date: 10:45-, June 23, 2021

Room: Rm. 31-808, 8th Floor, 31st Bldg. + Webcast via Zoom

Speaker: Kagamihara, Daichi (QMB)

Title: BCS-BEC crossover of an ultracold Fermi gas in state-dependent optical lattices

Abstract: Recent experimental developments in ultracold atomic physics enable us to simulate various interesting many-body systems. State-dependent optical lattices make it possible to realize the Hubbard model whose hopping amplitudes depend on atomic internal states [1,2]. Furthermore, one can induce orbital hybridization via Rabi coupling between two internal states by microwaves or lasers [1,2].

In this work, we investigate the superfluid properties of a Fermi gas with attractive interaction in the state-dependent lattices. We consider the so-called Bardeen-Cooper-Schrieffer(BCS)-Bose-Einstein Condensate(BEC) crossover phenomena at absolute zero temperature. We discuss how differences in hopping amplitudes and hybridization affect superfluid properties. We also discuss possibilities of realization of breached-pair (Sarma) phase which is suggested to realize in this system [3].

[1] L. Krinner, M. Stewart, A. Pazmino, J. Kwon, and D. Schneble, Nature **559**, 589 (2018).

[2] L. Riegger, Ph.D. Thesis (2019).

[3] W. V. Liu, F. Wilczek, and P. Zoller, Phys. Rev. A **70**, 033603 (2004).

Time and Date: 10:45-, June 30, 2021

Room: Rm. 31-808, 8th Floor, 31st Bldg. + Webcast via Zoom

Speaker: Kasamatsu, Kenichi (CMT)

Title: On truncated Wigner methods for compact U(1) and SU(2) variables

Abstract: The purpose of this study is to explore real time dynamics of quantum systems described by dynamical variables in compact U(1) or SU(2) groups. We employ the truncated Wigner approximation (TWA), which includes quantum correction to the classical mean-field trajectory, demonstrating it by using a few spin-1/2 system. We also show the formulation of TWA with (winding)number-phase representation, which is useful in our problem.

Time and Date: 10:45-, July 7, 2021

Room: Webcast via Zoom

Speaker: Mizuno, Ryota (Kyoto University)

Title: Development of efficient approximation methods in dynamical mean field theory for multi-degree-of-freedom systems

Abstract: Although several impurity solvers in the dynamical mean field theory (DMFT) have been proposed, especially in the multi-degree-of-freedom systems, there are practical difficulties arising from a trade-off between costs and applicability. At least in principle, exact methods, such as the continuous quantum Monte Carlo method (CT-QMC)[2] and the exact diagonalization method (ED) [3], have a broad scope of application. However, especially in multi-degree-of-freedom systems, it is not uncommon that we cannot carry out the calculation practically due to its very high numerical costs. On the contrary, the iterative perturbation theory (IPT) [4-6] has a very low numerical cost, although its scope of application is quite limited.

Given the above, in this study, we provide a new interpretation for IPT from the perspective of the frequency dependence of the two-particle vertices and extended the method such that it can be applied to multi-degree-of-freedom systems [7]. We validated this method by applying it to several models, such as the single-orbital square lattice, the two-orbital square lattice, and the bilayer model, and by comparing it with the numerically exact CT-QMC method. We confirm that our method shows good agreements with CT-QMC. We also propose a simplification of the local two-particle full vertex inspired by the new interpretation of IPT. By using this simplified form of the full vertex, we also develop two low-cost methods to take into account the non-local correlation to DMFT. We apply these methods to the models mentioned above and confirm that our methods can capture important behaviors such as the pseudo-gap. In this talk, we explain the details of the methods and the results.

**Reference**

[1] A. Georges, G. Kotliar, W. Krauth, and M. J. Rozenberg: Rev. Mod. Phys. **68**, 13 (1996).

[2] A. N. Rubtsov, V. V. Savkin, and A. I. Lichtenstein: Phys. Rev. B **72**, 035122 (2005).

[3] M. Caffarel and W. Krauth: Phys. Rev. Lett. **72**, 1545 (1994).

[4] H. Kajueter and G. Kotliar: Phys. Rev. Lett. **77, **131 (1996).

[5] M. S. Laad *et al.*: Phys. Rev. B. **73**, 045109 (2006).

[6] N. Dasari *et al.*:* *The European Physical Journal B. **89**, 202 (2016).

[7] R, Mizuno, M. Ochi, K. Kuroki: arXiv:2101.04466

Time and Date: 10:45-, July 14, 2021

Room: Rm. 31-808, 8th Floor, 31st Bldg. + Webcast via Zoom

Speaker: Inoue, Takumi (Cosmology Lab., Kindai Univ.)

Title: Only in Japanese

Abstract: Only in Japanese

Time and Date: 9:00-, July 21, 2021

Room: Webcast via Zoom

Speaker: Güngördü, Utkan (Kyoto University)

Title: Robust implementation of quantum gates despite always-on exchange coupling in silicon quantum dots

Abstract: Single spin qubits in SiMOS quantum dots provide a promising platform for scalable quantum computing, owing to well-developed fabrication techniques and suppressed Overhauser effect due to isotropic enrichment, with gate fidelities ultimately limited by charge noise. Although qubit frequencies and exchange coupling strengths are electrically controllable, there can be severe constraints on the range of tunability and bandwidth, leading to always-on couplings and crosstalk. We consider a double quantum dot device working in this regime with always-on exchange coupling [1], and describe how a controlled-Z (CZ) gate and arbitrary one-qubit gates which are robust against charge noise can be implemented by smoothly pulsing the microwave source while eliminating the crosstalk [2]. We find that the most significant deviations from the rotating wave approximation, which are analogous to the Bloch-Siegert shift in a two-level system, can be compensated using local virtual gates. These results can be extended to a linear chain to three quantum dots [3].

[1] W. Huang, C. H. Yang, K. W. Chan, T. Tanttu, B. Hensen, R. C. C. Leon, M. A. Fogarty, J. C. C. Hwang, F. E. Hudson, K. M. Itoh, A. Morello, A. Laucht, and A. S. Dzurak; Nature (London) 569, 532 (2019)

[2] U. Güngördü, J. P. Kestner; Phys. Rev. B 101, 155301 (2020)

[3] D. W. Kanaar, S. Wolin, U. Güngördü, J. P. Kestner; arXiv:2101.08840 (2021)