Recently, non-Hermitian (NH) quantum phenomena have attracted a great deal of attention. We here address the following three issues in this context.

1. Non-Hermitian Kondo Effect [1]:

We first investigate the Kondo effect in an open quantum system motivated by recent experiments with ultracold alkaline-earth atoms. Due to inelastic collisions and the associated atom losses, this system is described by a NH extension of the Kondo problem. We show that the non-Hermiticity induces anomalous reversion of renormalization- group, leading to a unique quantum phase transition. Furthermore, by exactly solving the non-Hermitian Kondo Hamiltonian, we obtain the critical line consistent with the renormalization-group flow.

2. Non-Hermitian Tomonaga-Luttinger Liquids [2]:

We next demonstrate the universal properties of dissipative Tomonaga-Luttinger (TL) liquids by calculating correlation functions and performing finite-size scaling analysis of a non-Hermitian XXZ spin chain as a prototypical model in one-dimensional open quantum many-body systems. Our analytic calculation is based on effective field theory with bosonization, finite-size scaling approach in conformal field theory, and the Bethe-ansatz solution. We uncover that the model in the massless regime with weak dissipation belongs to the universality class characterized by the complex-valued TL parameter, which is related to a complex generalization of c=1 conformal field theory.

3. Non-Hermitian Fermionic Superfluidity [3]:

Finally, we address a NH BCS superfluidity with a complex-valued interaction arising from inelastic scattering between fermions. We find unconventional phase transitions unique to NH systems: the superfluidity shows reentrant behavior with increasing dissipation, as a consequence of non-diagonalizable exceptional points in the quasiparticle Hamiltonian for weak attractive interactions. For strong attractive interactions, the superfluid gap is enhanced by dissipation due to an interplay between the BCS-BEC crossover and the quantum Zeno effect.

[1] M. Nakagawa, N. Kawakami and M. Ueda, Phys. Rev. Lett. 121, 203001 (2018)

[2] K. Yamamoto, M. Nakagawa, M. Tezuka, M. Ueda, and N.Kawakami, Phys. Rev. B 105, 205125 (2022); K. Yamamoto and N. Kawakami, arXiv:2207.04395

[3] K. Yamamoto, M. Nakagawa, K. Adachi, K. Takasan, M. Ueda and N. Kawakami, Phys. Rev. Lett. 123, 123601 (2019); K. Yamamoto, M. Nakagawa, N. Tsuji, M. Ueda, and N. Kawakami, Phys. Rev. Lett. 127, 055301 (2021?

Host: Miguel A. Cazalilla

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