Yuto Ashida

Parity-time-symmetric quantum critical phenomena

Synthetic non-conservative systems with parity-time (PT) symmetric gain–loss structures can exhibit unusual spontaneous symmetry breaking that accompanies spectral singularity. Recent studies on PT symmetry in optics and weakly interacting open quantum systems have revealed intriguing physical properties, yet many-body correlations still play no role. Here by extending the idea of PT symmetry to strongly correlated many-body systems, we report that a combination of spectral singularity and quantum criticality yields an exotic universality class which has no counterpart in known critical phenomena. Moreover, we find unconventional low-dimensional quantum criticality, where superfluid correlation is anomalously enhanced owing to non-monotonic renormalization group flows in a PT-symmetry-broken quantum critical phase, in stark contrast to the Berezinskii–Kosterlitz–Thouless paradigm. Our findings can be experimentally tested in ultracold atoms and predict critical phenomena beyond the Hermitian paradigm of quantum many-body physics.

Diffraction-Unlimited Position Measurement of Ultracold Atoms in an Optical Lattice.

We consider a method of high-fidelity, spatially resolved position measurement of ultracold atoms in an optical lattice. We show that the atom-number distribution can be nondestructively determined at a spatial resolution beyond the diffraction limit by tracking the progressive evolution of the many-body wave function collapse into a Fock state. We predict that the Pauli exclusion principle accelerates the rate of wave function collapse of fermions in comparison with bosons. A possible application of our principle of surpassing the diffraction limit to other imaging systems is discussed.

Quantum critical behavior influenced by measurement backaction in ultracold gases

Recent realizations of quantum gas microscope offer the possibility of continuous monitoring of the dynamics of a quantum many-body system at the single-particle level. By analyzing effective non-Hermitian Hamiltonians of interacting bosons in an optical lattice and continuum, we demonstrate that the backaction of quantum measurement shifts the quantum critical point and gives rise to a unique critical phase beyond the terrain of the standard universality class. We perform mean-field and strong-coupling-expansion analyses and show that non-Hermitian contributions shift the superfluid--to-Mott-insulator transition point. Using a low-energy effective field theory, we discuss critical behavior of the one-dimensional interacting Bose gas subject to the measurement backaction. We derive an exact ground state of the effective non-Hermitian Hamiltonian and find a unique critical behavior beyond the Tomonaga-Luttinger liquid universality class. We propose experimental implementations of post-selections using quantum gas microscopes to simulate the non-Hermitian dynamics and argue that our results can be investigated with current experimental techniques in ultracold atoms.

Parity-time-symmetric topological superconductor

Nonequilibrium open systems effectively described by non-Hermitian Hamiltonians with parity-time (

Information Retrieval and Criticality in Parity-Time-Symmetric Systems

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Quantum-trajectory thermodynamics with discrete feedback control

) symmetry have recently attracted considerable attention due to their properties with no Hermitian counterparts. In particular, there exists a growing interest in non-Hermitian topological phases of matter. Here, the authors show that a

Exploring the anisotropic Kondo model in and out of equilibrium with alkaline-earth atoms

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Multiparticle quantum dynamics under real-time observation

-symmetric topological superconducting wire possesses two distinct types of unconventional edge modes, those with complex energies and with nonorthogonal Majorana zero modes. The latter induce anomalous particle transport with nonlocal currents present only at the edges. This work explores a unique nonequilibrium topological phenomenon that results from the interplay between

Precise multi-emitter localization method for fast super-resolution imaging

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General achievable bound of extractable work under feedback control

symmetry and topological superconductivity.