Quantum many-body scars in Rydberg atom arrays

Short answer: Quantum many-body scars are special eigenstates that stay nonthermal even though the system as a whole thermalizes. Theory papers in 2019 and 2020 found exact scar states in Rydberg-blockaded atom chains and 2D arrays, and a Science paper reports an experiment with up to 200 Rydberg atoms in which periodic driving stabilised coherent revivals and a subharmonic response. These are laboratory physics results.

What are quantum many-body scars?

Quantum many-body scars are specific eigenstates within a many-body system that exhibit nonthermal behavior. In the context of Rydberg atom chains, these states represent a violation of the strong eigenstate thermalization hypothesis. They are distinct from typical thermal states because they retain structural order despite being at an effectively infinite temperature. Theoretical work has identified these states as potentially responsible for atypical dynamics observed in experiments, such as unusual oscillatory quench dynamics.

The existence of these scars implies that the system does not fully thermalize into a chaotic spread throughout Hilbert space. Instead, coherent revivals associated with these scars can be stabilized. This behavior is linked to discrete time-crystalline order, where the system exhibits a robust subharmonic response to periodic driving. The authors describe scars as offering new ways to steer many-body dynamics.

How do Rydberg atom arrays simulate these phenomena?

The Science paper describes Rydberg atom arrays as a powerful platform for simulating emergent phenomena in quantum many-body systems. These systems consist of neutral atoms, specifically 87Rb, trapped in optical tweezers and arranged into arbitrary patterns. A spatial light modulator generates these patterns, allowing for the exploration of quantum dynamics in systems ranging from one-dimensional chains to two-dimensional arrays. The atoms are held in a programmable optical lattice, enabling the study of strongly interacting qubits in various spatial dimensions.

The physical implementation relies on the Rydberg blockade regime, where interactions between excited atoms prevent neighboring excitations. This creates an effective model, often referred to as the PXP model, which captures the essential physics of the system. By subjecting these arrays to periodic excitation or rapid quenches, researchers can investigate nonequilibrium dynamics. The programmable nature of the setup allows for the mapping of Hilbert space dynamics, geometry dependence, and phase diagrams of the emergent scar phenomena.

What does theory say about scars?

Theoretical investigations have uncovered exact quantum many-body scar states in both one-dimensional and two-dimensional models. In the one-dimensional Rydberg-blockaded atom chain, researchers discovered eigenstates at infinite temperature that can be represented exactly as matrix product states with a finite bond dimension. These states exist for both periodic and open boundary conditions, explicitly demonstrating the violation of the strong eigenstate thermalization hypothesis. They show signatures of translational symmetry breaking with a period-2 bond-centered pattern.

In two-dimensional PXP models, which serve as effective models for Rydberg atom arrays, exponentially many exact scar states have been identified. These states are remarkably simple valence bond solids, even though they reside at an effectively infinite temperature. For particular boundary conditions, these eigenstates possess integer-valued energies. The level-spacing statistics of these states are well described by Wigner-Dyson statistics, further distinguishing them from typical thermal states. Certain charge-density-wave initial states give rise to strong oscillations in Rydberg excitation density after a quantum quench, with towerlike structures appearing in their overlaps with eigenstates.

How are scars stabilized and controlled?

Stabilizing coherent revivals associated with quantum many-body scars is achieved through periodic driving. This driving generates a robust subharmonic response, akin to discrete time-crystalline order. Under such driven excitation, the array of atoms can stabilize, freezing periodically into configurations that resemble time crystals. This control over nonequilibrium quantum dynamics is challenging because interactions typically lead to thermalization and chaotic spreading. However, the presence of scars allows for new ways to steer complex dynamics in many-body systems.

The authors of the Science paper describe understanding and controlling such dynamics as central to contemporary condensed matter physics. The work is basic research on programmable quantum simulators, not an application.

FAQ

Are these scars experimentally observed?

Researchers have observed unusual oscillatory quench dynamics in Rydberg atom chains, which theoretical works link to scar states. A Science paper reports experiments with up to 200 atoms in which periodic driving stabilised the array, freezing periodically into what looked like time crystals.

Do scars violate thermalization?

Yes, scar states strongly violate the eigenstate thermalization hypothesis. They remain ordered at effectively infinite temperature, preventing the chaotic spreading of information typically seen in thermalizing systems.

What is the role of periodic driving?

Periodic driving stabilizes coherent revivals associated with scars. It generates a robust subharmonic response, creating a discrete time-crystalline order that prevents the system from fully thermalizing into noise.

Are these states exact in theory?

Theoretical models identify exact scar states in one-dimensional and two-dimensional PXP models. These states are represented as matrix product states or valence bond solids with specific symmetry properties.

What atoms are used in these experiments?

Experiments utilize neutral 87Rb atoms trapped in optical tweezers. These atoms are arranged into programmable patterns using spatial light modulators to simulate many-body dynamics.