First observation of quantum many-body scar state using superconducting processor at Zhejiang University

In a recent paper, "Many-body Hilbert space scars on superconducting processors," published Oct. 13 in Nature Physics [1], scientists at Zhejiang University and collaborators demonstrated for the first time quantum many-body scar (QMBS) states: a powerful mechanism for maintaining coherence between interacting quantum bits, offering the possibility of multiparty entanglement for various high processing speeds and low-power applications in quantum information science and technology.

Corresponding author of this article: Haohua Wang (left) and Lei Ying (right), Zhejiang University and Alibaba-Zhejiang University Joint Research Center for Frontier Technologies

01Implications for quantum many-body systems: delaying thermalization and maintaining coherence

"QMBS states have the intrinsic and universal capability of multiparty entanglement, which makes them extremely attractive for applications such as quantum sensing and metrology." Lei Ying explained.

Classical or binary computing relies on transistors, which can only represent 1 or 0 at the same time. in quantum computing, quantum bits can represent both 0 and 1, which can exponentially speed up certain computational processes.

"In quantum information science and technology, it is often necessary to assemble a large number of basic information processing units - quantum bits - together." Yingcheng Lai, a professor at Arizona State University and one of the corresponding authors of this paper, explains [2], "For applications like quantum computing, it is crucial to maintain a high degree of coherence or quantum entanglement between quantum bits. However, the inevitable interactions between quantum bits and environmental noise can destroy coherence in a very short time: within about 10 nanoseconds, due to the fact that many interacting quantum bits form a many-body system."

Ying-Cheng Lai (Lai)

Chinese theoretical physicist/electrical engineer working on chaos theory and complex dynamical systems; he is one of the pioneers in the field of relativistic quantum chaos. Currently, he is a Regents' Professor at Arizona State University; he also holds a Chair in Electrical Engineering at ISS.

Key to this research is a deep understanding of delayed thermalization to maintain coherence, which is also considered a key research goal for quantum computing.

"From fundamental physics, we know that in a system of many interacting particles, for example, molecules in a closed volume, thermalization processes will occur." Yingcheng Lai said, "The contention between many quantum bits will invariably lead to quantum thermalization - the process described by the so-called 'eigenstate thermalization hypothesis' - which will destroy the coherence between quantum bits. "

Yingcheng Lai said these findings will help advance quantum computing and will have applications in cryptography, secure communications and cybersecurity, as well as other technologies.

02Superconducting system achieves quantum many-body scarring beyond the limits of classical simulations

In this article, scientists report the experimental observation of a new class of quantum many-body scar (QMBS) states on a superconducting (SC) processor.

Unlike previous implementations in kinetically constrained Riedberg atom arrays, in this experiment the scientists designed QMBS by weakly decoupling a part of the Hilbert space on a computational basis.This approach was inspired by the topology of the Su-Schrieffer-Heeger model of polyacetylene: it was used to create a nearly decoupled subspace with a hypercubic graph structure. This subspace gives rise to emerging QMBS phenomena, including the resurrection of many-bodies from a special initial state residing in the hypercube, and the "scarred" band of characteristic scar states.

At the same time, the whole system thermalizes as the cross-coupling between neighboring quantum bits becomes weaker. One of the advantages of the experimental SC platform is the tunable XY coupling between quantum bits on a 6*6 square lattice configuration, which allows scientists to simulate many-body systems with one-dimensional (1D) and quasi-one-dimensional systems with a comb shape.

Ultimately, circuits with up to 30 quantum bits and 29 couplers were experimentally studied, with Hilbert space dimensions well beyond the limits of classical simulations. Measurements of entanglement entropy and quantum fidelity by population dynamics and quantum state laminar scans demonstrate the emergence of powerful QMBS states, and by directly comparing their slow dynamics with conventional thermalized states, scientists demonstrate their realization of a new QMBS paradigm on a solid-state SC platform.

This paves the way for the systematic exploration of scarring, and other forms of "breaking" in highly tunable, interacting systems.

experimental setup and identification of QMBS states by quantum state laminar scans. Where, (a) the figure shows the experimental superconducting circuit with quantum bits and couplers in a square geometry.

Experimentally observed dynamics of quantum bits. Where (a)-(b) are contour plots of the experimental quantum bit population as a function of interaction time for QMBS and fast thermalization states, respectively. (c)-(d):The generalized imbalance I(t) extracted from the (a)-(b) plots as a function of the interaction time. (e)-(f):Fourier transform amplitude of the imbalance in (c)-(d), which characterizes the squared overlap between the initial state and the energy eigenstate.

QMBS states in a comb tensor system. (a) Comb tensor topology in a superconducting processor with L = 20 quanta. (b) Initial product state with Fourier squared amplitude. (c,d) Dynamics of imbalance (top), four-qubit fidelity (middle), and four-qubit entanglement entropy (bottom).

03First observation of quantum scars in a solid-state device opens the door to the study of many-body phenomena

In this experiment, the team implemented QMBS states in a superconducting circuit and effectively simulated quantum many-body systems with one- and quasi-one-dimensional geometries described by the unconstrained spin-1/2XY model: this represents the first experimental observation of QMBS states in a solid-state device. Furthermore, this study provides the first in-depth characterization of QMBS on large subsystems using quantum state laminar scans.

This opens the door for studying QMBS states and other many-body phenomena with large Hilbert spaces in a feasible way. For example, there are inevitable imperfections in the observation and characterization of long-lived quantum states in strongly interacting solid-state systems in classical programmable computers: cross-coupling between quantum bits, random disorder, environment-induced decoherence and decoherence, etc. The robustness of QMBS states in solid-state systems can greatly extend the coherence time for specific quantum information operations.

Also, this work points out the need to further investigate non-thermal states in experimental platforms to generate QMBS states against quantum thermalization for quantum information and quantum metrology applications.

Reference links：

[1]https://www.nature.com/articles/s41567-022-01784-9

[2]https://scitechdaily.com/quantum-computing-breakthrough-qubits-for-a-programmable-solid-state-superconducting-processor/