Room: Rm. 31-502, 5th Floor, 31 Bldg. + Webcast via Zoom

Speaker: Nagao, Kazuma (RIKEN)

Title: Squeezed coherent state path integral methods for fermions

Abstract: Spontaneous symmetry broken phases, such as Bose-Einstein condensates of dilute bosonic atoms and superconductors of electrons, are ubiquitous and fundamental quantum phases in condensed matter physics, AMO physics, and high-energy physics. In recent years, the Higgs mode of fermionic superconductor/superfluid systems, which is an emergent amplitude excitation mode of the order parameter field, has been extensively explored in many experiments including pump-probe experiments for BCS-type superconductors [1] and a radiofrequency spectroscopy experiment for ultracold two-component Fermi gases [2].

In this work, towards establishing a versatile framework to describe those experimental systems, we develop a new path integral representation of fermionic superfluid systems that gives a unified description of the fermionic quasiparticle excitations and the bosonic collective excitations of the order parameter fluctuations. Our approach gives a direct fermionic generalization of the squeezed-coherent-state path integral method for bosons [3]. In this talk, we apply this formalism to BCS superconductors and discuss that a generalized Lagrangian defined for a squeezed fermionic coherent state shows a gapped excitation branch corresponding to the Higgs excitation mode and reproduces the well-known BCS quasiparticle gapped spectrum in a single framework [4]. Additionally, I will also talk about an ongoing project on an application of the method to contact interacting Fermi gas systems [5]. In this work, we analyze the generalized Lagrangian in terms of the momentum-shell renormalization group method and present a new finding on the ground state BCS- BEC crossover phenomenon of the dilute Fermi gas systems.

[1] R. Shimano and N. Tsuji, Annu. Rev. Condens. Matter Phys. 11, 103 (2020).

[2] A. Behrle et al., Nat. Phys. 14, 781 (2018).

[3] I. M. H. Seifie, V. P. Singh, and L. Mathey, Phys. Rev. A 100, 013602 (2019).

[4] K. Nagao, D. Li, and L. Mathey, arXiv:2102.03113.

[5] K. Nagao and L. Mathey, in preparation.

Time and Date: 10:45-, August 2, 2022

Room: Simulation and Experiment Room, 3rd Floor, 31 East Bldg. + Webcast via Zoom

Speaker: Andreas Thomasen (QunaSys)

Title: Contemporary approaches to variational quantum algorithms for quantum chemistry

Abstract: Variational quantum algorithms were initially proposed as enabling the first use-cases of quantum computers, namely determination of ground state properties of molecules, with the variational quantum eigensolver being the most familiar and well-known among them. There are signs that such algorithms have inherent short-comings that in the short term will render them unable to produce useful results on noisy intermediate-scale quantum (NISQ) devices due to noise and even for high-fidelity architectures may not converge due to the occurrence of barren plateaus in the parameter search space. In this seminar we give an overview of these problems and discuss recent approaches to mitigating and potentially overcoming them.

In this presentation, we report our proposal to measure optical spin conductivity of ultracold atomic gases [4]. In this proposal, an alternating spin current is induced by using a time-dependent gradient of magnetic field or the optical Stern-Gerlach effect. The generated current is measured by observing dynamics of spin density profile.

To demonstrate what information the frequency dependence of the spin conductivity can capture, we computed the conductivity of several cold-atomic systems; a spin-1/2 s-wave Fermi superfluid, spin-1 Bose Einstein condensate, Tomonaga-Luttinger liquid, and topological Fermi superfluid [4,5]. In these systems, nontrivial spin excitations result in the frequency dependence of the spin conductivity quite different from that of the conventional Drude conductivity.

References:

[1] C. C. Homes *et al.,* Phys. Rev. Lett. **71**, 1645 (1993).

[2] Y. S. Lee *et al.,* Phys. Rev. B **66**, 041104(R) (2002).

[3] R. R. Nair *et al,* Science **320**, 1308 (2008); K. F. Mak *et al.,* Phys. Rev. Lett. **101**, 196405 (2008).

[4] Y. Sekino, H. Tajima, and S. Uchino, arXiv:2103.02418.

[5] H. Tajima, Y. Sekino, and S. Uchino, Phys. Rev. B **105**, 064508 (2022).