RESEARCH INTERESTS

Nonequilibrium Physics

• Stochastic thermodynamics and information thermodynamics
  • We found thermodynamics uncertainty relations for non-steady states in both continuous- and discrete-time Markov processes and applied it to measurement and feedback control (Maxwell’s demon).[Phys. Rev. Lett. 125, 140602 (2020)]
  • We found universal kinetic bound on the static response of a generic nonequilibrium observable to external perturbations in terms of the dynamical activity (or traffic) that quantifies the frequency of stochastic state transitions of a Markov process, named as response kinetic uncertain relation (R-KUR).[arXiv:2410.20800]
• Deep learning theory
  • We derived analytical formulae of the noise and model fluctuations of the stochastic gradient descent (SGD) algorithm in deep learning with a finite learning rate.[ICML 2021]
  • We analyzed analytically the fundamental properties of the minibatch noise in discrete-time SGD.[ICLR 2022 Spotlight]
  • We derived the stationary distribution and found a power-law escape rate from a local minimum for minibatch SGD.[ICML 2022 Spotlight]
• Quantum thermodynamics and information thermodynamics
  • We constructed a new type of quantum information engine that can store useful work cumulatively and transport a quantum particle unidirectionally by harnessing purely quantum fluctuations with the aid of Maxwell’s demon, whose maximum power and maximum transport velocity are well-defined and the optimal operation time is specified. We proposed an improved definition of the efficiency by including all possible energy flow involved in the engine cycle. We discussed possible experimental implementations with existing techniques, especially those for cold atom systems and optical lattices.[arXiv:2303.08326]
  • We experimentally realized Maxwell’s demon using a 62-qubit superconducting quantum processor, being the first experiment using an isolated quantum many-body system. [Phys. Rev. A 109, 062614 (2024)]
  • We generalize the response kinetic uncertainty relation (R-KUR) to the steady state of Markovian open quantum systems obeying Lindblad master equations, named the Quantum Response Kinetic Uncertainty Relation (QR-KUR). The response precision of a measured observable is upper bounded by two terms. One is the conventional quantum dynamical activity which quantifies the frequency of quantum jumps in the steady state. The other contribution is genuinely quantum because it characterizes the perturbation-induced quantum transitions between the steady-state subspace and its complementary subspaces and it vanishes in the classical limit. The direct physical meaning of the latter contribution was elusive until our explanation. We also provide quite general sufficient conditions such that either of the two terms vanishes, indicating their equal importance. We illustrate our bound and the sufficient conditions in a two-level atom which can be solved exactly.[arXiv:2501.04895]