Focus Week on Non-equilibrium Quantum Dynamics

30 Sept 2024, 09:00 → 4 Oct 2024, 22:20 Asia/Tokyo

Dates: September 30 -- October 4, 2024

Venue:  Kavli IPMU Lecture Hall, Kashiwa campus, UTokyo (co-hosted by Kavli IPMU and ISSP)

Overview:

The focus of the workshop is the non-equilibrium dynamics of quantum systems. While the study of dynamics of quantum systems has often been regarded as difficult, we have seen impressive progress in recent years, both in theoretical and experimental aspects, e.g. non-Hermitian systems; breaking of ETH; applications of generalized concepts of symmetries; applications of holographic techniques to quantum dynamics; technological breakthroughs in quantum devices. In this workshop, we will try to engage with different communities inside Japan, in addition to inviting worldwide experts from abroad.

Invited speakers: 

Masayuki Hashisaka (ISSP)
Yunfeng Jiang (Yau Center, Southeast U.)
Gyu-Boong Jo (HKUST)
Kohei Kawabata (ISSP)
Jong Yeon Lee (UIUC)
Chihiro Matsui (UTokyo)
Pratik Nandy (RIKEN and YITP)
Takashi Oka (ISSP)
Silvia Pappalardi (U. Köln)
Pedram Roushan (Google)
Kareljan Schoutens (U. Amsterdam)
Krishnendu Sengupta (IACS)
Hal Tasaki (Gakushuin U.)
Takafumi Tomita (IMS)
Frank Verstraete (Cambridge and Ghent)
Derek S. Wang (IBM)

Organizing Committee Members :

Hosho Katsura (UTokyo physics)
Yuan Miao (IPMU)
Masaki Oshikawa (ISSP)
Masahito Yamazaki (IPMU)

Note: We plan not to use zoom platform for the talks, and all the talks will be in person. 

Deadlines: If you would like to present a poster (and/or get the travel support), the deadline for the registration of the poster session is **August 20, 2024**. The deadline for the registration of the conference is **September 20, 2024**.

Address:
Kavli Institute for the Physics and Mathematics of the Universe (Kavli IPMU),
the University of Tokyo, 5-1-5 Kashiwa-no-ha, Kashiwa City, Chiba 277-8583, Japan

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Funding: This conference is supported by Kavli IPMU, ISSP and JST PRESTO Grant No. JPMJPR225A. 

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Workshop_booklet_0927.pdf

Monday 30 September

Registration

Opening remarks

1 A detailed portrait of many-body dynamics on a quantum computer

Interacting many-body quantum systems and their dynamics, while fundamental to modern science and technology, are formidable to simulate and understand. However, by discovering their symmetries, conservation laws, and integrability one can unravel their intricacies. Here, using up to 124 qubits of a fully programmable quantum computer, we uncover local conservation laws and integrability in one- and two-dimensional periodically-driven spin lattices in a regime previously inaccessible to such detailed analysis. We focus on the paradigmatic example of disorder-induced ergodicity breaking, where we first benchmark the system crossover into a localized regime through anomalies in the one-particle-density-matrix spectrum and other hallmark signatures. We then demonstrate that this regime stems from hidden local integrals of motion by faithfully reconstructing their quantum operators, thus providing a detailed portrait of the system's integrable dynamics. Our results demonstrate a versatile strategy for extracting hidden dynamical structure from noisy experiments on large-scale quantum computers.

Speaker: Derek S. Wang (IBM Quantum)

10:30 Coffee break

2 What is thermal equilibrium and how do we get there?

We present a simple, rigorous example (which is essentially a free fermion chain) of an isolated macroscopic (but finite) quantum system that exhibits thermalization (in a phenomenological sense) from any pure initial state in the microcanonical energy shell. The essential ingredients of the proof are the large-deviation type strong ETH (energy eigenstate thermalization hypothesis) bound and the absence of degeneracy in the energy spectrum. As far as we know, this is the first concrete realization of the philosophy on the foundation of equilibrium statistical mechanics proposed by von Neumann in 1929.

Speaker: Hal Tasaki (Gakushuin University)

12:00 Lunch

3 Fermionic Bricks in the Wall

We study brick wall quantum circuits enjoying a global fermionic symmetry. The constituent 2-qubit gate, and its fermionic symmetry, derive from a 2- particle scattering matrix in integrable, supersymmetric quantum field theory in 1+1 dimensions. Our 2-qubit gate, as a function of three free parameters, is of so-called free fermionic or matchgate form, allowing us to derive the spectral structure of both the brick wall unitary $U_F$ and its, non-trivial, hamiltonian limit $H_\gamma$ in closed form. We find that the fermionic symmetry pins $H_\gamma$ to a surface of critical points, whereas breaking that symmetry leads to non-trivial topological phases. We explore quench dynamics for this class of circuits.

Speaker: Kareljan Schoutens (University of Amsterdam)

IPMU2024_Schoutens.pdf

15:00 Tea time

4 Aspect of Floquet physics in closed quantum systems

In this talk, we shall discuss two aspects of a periodically driven closed quantum system. First, we shall discuss the existence of signatures of Hilbert space fragmentation (HSF) in a driven interacting fermi chain in its prethermal regime at special drive frequencies. We provide analytical expression for these drive frequencies, show that such prethermal regime can be exponentially long in the large drive amplitude regime, and discuss feasibility of realization of such systems in experiments. Second, we shall discuss a class of two-rate periodic drive protocol for closed quantum systems where the drive frequencies have integer ratio. We show that for protocols obeying certain conditions, there is a large class of non-integrable models where one obtains an exact Floquet flat band. Near these flat bands, heating in these driven systems is significantly reduced which may be useful, for example, in application to qubit manipulation and quantum state preparation.

Speaker: Krishnendu Sengupta (Indian Association for the Cultivation of Science)

kstalk_ipmu.pdf

Tuesday 1 October

5 Controlling ultrafast Rydberg dynamics with ultracold atoms in optical tweezers

Rydberg atoms, with their giant electronic orbitals, exhibit dipole-dipole interaction reaching the GHz range at a distance of a micron, making them a prominent contender for realizing ultrafast quantum operations. However, such strong interactions have never been harnessed so far because of the stringent requirements on the atom position fluctuation and the necessary excitation strength. Here, we introduce novel techniques to enter this regime and explore it with two strongly-interacting single atoms. This interaction is the key to the realization of an ultrafast two-qubit gate for cold-atom quantum computers. The techniques demonstrated here open the path for ultrafast quantum simulation and computation

Speaker: Takafumi Tomita (Institute for Molecular Science)

10:30 Coffee break

6 Exploring Non-Hermitian Phenomena in Ultracold Fermions

Ultracold fermions, traditionally prepared in well-isolated environments with minimal dissipation, have recently emerged as a novel platform for investigating non-Hermitian physics and broader open quantum systems. This exploration is facilitated
through precise control of dissipation mechanisms. In this presentation, I will elucidate the methods for realizing such non-Hermitian systems using atomic ensembles, and discuss our recent experimental observations. These include the
demonstration of the non-Hermitian skin effect in two dimensions and chiral spin transfer near exceptional points. Furthermore, I will outline our ongoing research into novel quantum dynamics that extend beyond conventional non-Hermitian frameworks within these open quantum systems.

Speaker: Gyu-Boong Jo (The Hong Kong University of Science and Technology (HKUST),)

12:00 Group Photo

12:10 Lunch

7 Anyons and Edge Transport in Quantum Hall Systems

Recent experiments verified abelian anyonic statistics of quasiparticles in Laughlin's fractional quantum Hall state of Landau level filling $\nu = 1/3$, about 40 years after the theoretical proposal in the 1980s. The experiments were achieved due to the progress in the long-standing theoretical and experimental research on edge transport in quantum Hall systems. In this talk, I will introduce some of our experiments and discuss recent developments in anyons and edge-transport research.

Speaker: Masayuki Hashisaka (Institute for Solid State Physics (ISSP), University of Tokyo)

15:00 Tea time

8 Gong Show

9 Poster Session 1

Wednesday 2 October

10 Exact correlation functions of quantum integrable circuits from algebraic geometry

In this talk, I will discuss some exact results on correlation functions of strings of spin operators for an integrable quantum circuits. These observables can be used for error calibration and mitigation of quantum simulation platforms. In the first part, I will discuss the method for the computation which includes algebraic Bethe ansatz and computational algebraic geometry. In the second part, I will present the exact results in both real space and Fourier space and discuss their physical implications.

Speaker: Yunfeng Jiang (Shing-Tung Yau Center and School of Physics, Southeast University)

10:30 Coffee break

11 Novel quantum dynamics with superconducting qubits

In recent years, superconducting qubits have emerged as one of the leading platforms for quantum computation and simulation. We utilize these Noisy Intermediate Scale Quantum (NISQ) processors to study quantum dynamics. I will present some of our recent works in studying robustness of bound states of photons [1], universality classes of dynamics in the 1D Heisenberg chain [2], and braiding of non-Abelian excitations [3]. These works point to the subtleties of non-equilibrium dynamics of highly entangled states in many-body systems; they provide evidence that in the absence of full-fledged quantum processors, the NISQ processors have challenged and guided our conventional wisdom.

[1] Morvan et al., Nature 612, 240–245 (2022)
[2] Rosenberg et al., Science 384, 48-53 (2024)
[3] Andersen et al., Nature 618, 264–269 (2023)

Speaker: Pedram Roushan (Google Quantum AI)

Tokyo_Sep2024_MIP_LGT.pdf

12:00 Lunch

12 Entanglement and tensor networks in the generalised Landau paradigm

Speaker: Frank Verstraete

15:00 Tea time

Free afternoon

Thursday 3 October

10:00 Coffee break

13 Understanding Phases and Transitions in Mixed States

With the rapid development of quantum simulator platforms, understanding the stability of quantum phases in the presence of environmental coupling has become increasingly important. As a pure quantum state evolves into a mixed state due to decoherence, traditional notions of quantum phases need to be reconsidered.

In this talk, I will present recent progress on mixed-state phases from an information-theoretic perspective, which contrasts sharply with conventional descriptions that rely on order parameters and fail to capture mixed-state phases. The presentation will be divided into three parts: (i) spontaneous symmetry breaking in mixed states, (ii) symmetry-protected topological states under decoherence and the extraction of long-range entangled states, and (iii) decohered topological orders and intrinsic error thresholds. If time permits, I will discuss how fault tolerance can be incorporated into this framework using a newly developed spacetime SPT (symmetry-protected topological) picture, highlighting the deep connection between fault tolerance and phase stability in open quantum systems.

Speaker: Jong Yeon Lee (University of Illinois at Urbana-Champaign)

12:00 Lunch

14 A variety of partially solvable models: From closed spin chains to open spin chains

Non-thermal energy eigenstates of thermalizing isolated quantum systems have been intensively studied these days, as counter examples for the strong eigenstate thermalization hypothesis. These states are called “quantum many-body scars (QMBS)”, which exhibit specific properties such as low entanglement entropies and persistent oscillations. Surprisingly, many of QMBS are exactly solvable states of non-integrable Hamiltonians. This fact strongly motivates us to study partially solvable quantum systems. We show that partial solvability of a quantum many-body system can be maintained even when the system is coupled to boundary dissipators under certain conditions. We propose two mechanisms that support partially solvable structures in boundary dissipative systems: The first one is based on the restricted spectrum generating algebra, while the second one is based on the Hilbert space fragmentation. From these structures, we derive exact eigenmodes of the Gorini-Kossakowski-Sudarshan-Lindblad equation for a family of quantum spin chain models with boundary dissipators, where we find various intriguing phenomena arising from the partial solvability of the open quantum systems, including persistent oscillations (quantum synchronization) and the existence of the matrix product operator symmetry. We discuss how the presence of solvable eigenmodes affects long-time behaviors of observables in boundary dissipative spin chains based on numerical simulations using the quantum trajectory method.

This talk is based on the collaboration work arXiv:2409.03208.

Speaker: Chihiro Matsui (University of Tokyo)

2024Sep30.pdf

15:00 Tea time

15 Non-Hermitian Topology in Hermitian Topological Matter: Wannier Localizability and Detachable Topological Boundary States

Non-Hermiticity gives rise to distinctive topological phenomena absent in Hermitian systems. However, connection between such intrinsic non-Hermitian topology and Hermitian topology has remained largely elusive. Here, considering the bulk and boundary as an environment and system, we demonstrate that anomalous boundary states in Hermitian topological insulators exhibit non-Hermitian topology [1, 2]. We study the self-energy capturing the particle exchange between the bulk and boundary, and show that it detects Hermitian topology in the bulk and induces non-Hermitian topology at the boundary. As an illustrative example, we reveal the non-Hermitian topology and concomitant skin effect inherently embedded within chiral edge states of Chern insulators. We also identify the emergence of hinge states within effective non-Hermitian Hamiltonians at surfaces of three-dimensional topological insulators. Furthermore, we comprehensively classify our correspondence across all the tenfold symmetry classes of topological insulators and superconductors.

Based on this correspondence and K-theory, we complete the tenfold classification of Wannier localizability and detachable topological boundary states [3, 4]. While topology can impose obstructions to exponentially localized Wannier functions, certain topological insulators are exempt from such Wannier obstructions. The absence of the Wannier obstructions can further accompany topological boundary states that are detachable from the bulk bands. Here, we elucidate a close connection between these detachable topological boundary states and non-Hermitian topology. Identifying topological boundary states as non-Hermitian topology, we demonstrate that intrinsic non- Hermitian topology leads to the inevitable spectral flow. By contrast, we show that extrinsic non-Hermitian topology underlies the detachment of topological boundary states and clarify anti-Hermitian topology of the detached boundary states.

References:
[1] F. Schindler, K. Gu, B. Lian, and K. Kawabata, PRX Quantum \textbf{4}, 030315 (2023).
[2] S. Hamanaka, T. Yoshida, and K. Kawabata, arXiv:2405.10015.
[3] D. Nakamura, K. Shiozaki, K. Shimomura, M. Sato, and K. Kawabata, arXiv:2407.09458.
[4] K. Shiozaki, D. Nakamura, K. Shimomura, M. Sato, and K. Kawabata, arXiv:2407.18273.

Speaker: Kohei Kawabata (Institute for Solid State Physics (ISSP), University of Tokyo)

16 Poster Session 2

Friday 4 October

17 Driven Dirac systems and novel quantum anomalous states

Floquet engineering, which involves controlling systems through time-periodic driving, provides a powerful method for coherently manipulating quantum materials and realizing dynamical states with novel functionalities. This presentation reports our recent experimental and theoretical findings on the dynamical states realized in Dirac electrons under various fields.

Circularly Polarized Laser [1]: Nonlinear optical response experiments suggest the generation of chiral gauge fields and the creation of emergent Weyl points.

AC-Magnetic Fields [2]: We demonstrate the emergence of π-Landau levels and the chiral anomaly-induced homodyne effect when Dirac electrons are subjected to a time-oscillating magnetic field B cos(Ωt).

Propagating Fields [3]: Dirac electrons in a propagating wave V cos(Qx−Ωt) exhibit sensitivity to the field speed v = Ω/Q. We classify these states and explore the associated topological phase transitions.

References:
[1] N. Yoshikawa et al., arXiv:2209.11932; Y. Hirai et al., arXiv:2301.06072
[2] S. Kitamura and T. Oka, arXiv:2407.08115
[3] T. Oka, arXiv:2407.21458

Speaker: Takashi Oka (Institute for Solid State Physics (ISSP), University of Tokyo)

Oka_iPMUs.pdf

10:30 Coffee break

18 Physics of tridiagonal matrices

Eigenvalues of Hermitian matrices encapsulate the core statistical and dynamical characteristics of quantum systems. Typically, this involves diagonalizing the Hamiltonian, where the diagonal elements represent the eigenvalues of the Hamiltonian. In this talk, I introduce an alternative approach: tridiagonalizing the Hamiltonian. I will discuss the properties of the tridiagonal matrix elements, highlighting that both chaotic and integrable systems share certain features, yet exhibit distinct statistical behaviors in these elements. If time permits, I will also show how these properties can be extended to non-Hermitian systems through singular value decomposition.

Speaker: Pratik Nandy (Riken & Kyoto University)

K-complexity IPMU_new.pdf

12:00 Closing Remarks