Research activities
Research interests: Quantum many-body physics / Quantum optics, measurement, information / AMO physics / Machine learning
Shedding quantum light on quantum many-body systems
Background：Recently, a possibility of controlling many-body states through their coupling to external degrees of freedom has attracted great attention. In particular, the so-called Floquet engineering, i.e., controlling many-body states via periodically driven classical light, has been well explored in this decade. Meanwhile, it has largely remained an open question how and even whether quantum nature of light can be used to control the phase of matter. In view of recent developments, this question is now within experimental reach.
Research：We have proposed a possibility of controlling quantum phase of matter via its coupling to fluctuating quantum light without external driving. Specifically, we have shown the existence of the cavity-induced superradiant transition in an umbiguious manner for the first time, without relying on common assumptions that will be ill-justified in ultrastrong light-matter coupling regimes. Moreover, devising a new unitary transformation, we have proposed a general nonperturbative approach to analyze quantum light-matter interaction at arbitrary coupling strengths.
[Quantum light-induced phase transition] PRX 2020, PRL 2022, arXiv 2022, arXiv 2023, arXiv 2023
[Nonperturbative approach to quantum electrodynamics] PRL 2021, PRR 2022, arXiv 2022
Physics of open systems
Background：A quantum system coupled to an external environment dramatically alters its dynamics due to non-unitary perturbations. While such a behavior has been extensively studied for quantum systems with small degrees of freedom, recent experimental advances to measure and manipulate many-body systems at the single quantum level provide a radically new possibility to study quantum many-body physics open to an external world.
Research：We have studied the fundamental aspects of many-body physics in quantum systems subject to the backaction from an environment, with particular focus on their quantum criticality, out-of-equilibrium dynamics, and quantum thermalization. The interactions with an external observer or environment are shown to fundamentally alter the underlying physics and give rise to new types of many-body phenomena beyond the conventional paradigm, such as the violations of c-theorem and Lieb-Robinson bound. Studying non-Hermitian systems as a specific class of open quantum systems, we have also revealed their unconventional topological aspects and information flows.
[Non-unitary many-body physics] Nat. Commun. 2017, PRA 2016, PRL 2018, PRA 2017, PRL 2018, PRB 2020, PRB 2020, PRL 2020, PTEP 2020, PRR 2020
[Non-Hermitian systems] PRX 2018, PRB 2018, Nat. Commun. 2019, PRL 2018, PRL 2019, Nat. Commun. 2020, PRR 2022, PRB 2022, arXiv 2023, arXiv 2023
[Review article] Adv. Phys. 2021
Quantum impurity problems in- and out-of-equilibrium
Background：The ability to manipulate many-body systems at the single quantum level allows one to study many-body physics in quantum systems that are strongly correlated with an external environment in a highly controlled manner. In such situations, the strong system-bath correlations invalidate the Born-Markov approximation. We thus need to explicitly take into account the degrees of freedom of the environment rather than eliminating them as done in the master-equation approach. A quantum impurity problem is the most fundamental paradigm of such strongly correlated open quantum systems.
ResearchWe have developed a versatile and efficient theoretical approach to solving generic quantum spin-impurity problems in and out of equilibrium and then applied it to reveal previously unexplored nonequilibrium dynamics. In spite of the rich theoretical toolbox, analysis of the long-time dynamics has remained very challenging due to the large entanglement in the time-evolved state. To overcome the challenge, we have introduced a new canonical transformation that can completely decouple the impurity and the environmental degrees of freedom. We have benchmarked our approach by the MPS calculations and revealed new types of nonequilibrium dynamics that are difficult to explore in the previous approaches. We have also unveiled novel out-of-equilibrium dynamics of Rydberg gases and magnetic polarons in strongly coupled regimes.
[Non-perturbative approach to quantum impurity problems] PRL 2018, PRB 2018, PRB 2018
[Out-of-equilibrium dynamics of magnetic polarons] PRB 2018
[Kondo-Central spin problems in Rydberg gases] PRL 2019, PRA 2019
Machine learning of nonequilibrium open systems
Background：When a system is open to two heat baths with different temperatures, nonzero current can be generated in a steady-state regime. Thermoelectric nanomachines convert such heat flows into useful electrical work and can work as nanoscale heat engines. Without many-body interactions, a set of the best nanoscale thermoelectric has been identified by the Landauer theory. Yet, the question about to what extent thermodynamic performance can be improved with interaction has remained open due to the combinatorially large search space.
Research：We have applied global search algorithm known as the differential evolution to tackle the problem of finding the best interacting thermoelectric and pointed out that the figure of merit and power factor can be improved by orders of magnitudes for the optimal interaction. We also constructed the framework of reinforcement learning applicable to a wide class of nanoscale thermal machines. We have also applied deep reinforcement learning to feedback control of quantum systems under observation.
[Differential evolution, Gaussian regression] Commun. Phys. 2021, Sci. Rep. 2022
[Deep reinforcement learning] PRL 2020, PRApp 2022
Super-resolved imaging: from quantum gases to biomolecules
Background：Two objects with a distance less than a wavelength of light cannot be resolved. This limit known as the diffraction limit has long imposed severe challenges on wide areas of science ranging from physics to life science. The state-of-the-art experimental techniques in microscopy are not exceptions. Two prime examples are quantum gas microscopes in the field of ultracold atoms, and super-resolved fluorescent microscopy in life science; due to the diffraction limit, measurements inevitably become destructive in quantum gas microscopes, and the slow temporal resolution of super-resolved fluorescent microscopy severely restricts the utility to the study of live-cell phenomena.
Research：We have developed a versatile method to surpass the diffraction limit and overcome the difficulties mentioned above. In particular, we have shown that, by tracking the collapse of the many-body wavefunction, the atom-number distribution can be measured with a high fidelity beyond the diffraction limit. In such a situation, we have pointed out the possibility of realizing a "nondestructive" quantum gas microscope. The approach can be generalized to classical systems such as fluorescent molecules. We have constructed a robust numerical method to locate positions of multiple light emitters. The method will allow us to achieve the theoretical-limit time resolution of super-resolution microscopy.
[Diffraction-unlimited nondestructive measurement of ultracold atoms in optical lattice] PRL 2015
[Precise multi-emitter localization method for classical objects in continuous spac] Opt. Lett. 2016
Nonequilibrium statistical mechanics (classical and open quantum systems)
BackgroundWhile the equilibrium statistical mechanics provides a powerful framework to describe equilibrium properties of a macroscopic system, it has been scarcely understood how we can understand systems far from equilibrium in a unified manner. Recently, there has been remarkable progress in this direction. In particular, the important relations called Jarzynski equality and fluctuation theorems have been found.
Research：We have given a general achievable upper bound for extractable work under feedback control and generalized the fluctuation theorems so as to be applicable to error-free measurements. We have also constructed a theoretical framework to describe nonequilibrium statistical mechanics of open quantum systems and obtained the generalized Jarzynski equality.
[Classical systems] PRE 2014, PRE 2021
[Open quantum systems] PRA 2016