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

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: TBA

Abstract: TBA

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: 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)

### Talks in the past

Fisical year 2021