Observation of entanglement propagation in a quantum many-body system (1401.5387v3)

Abstract: The key to explaining a wide range of quantum phenomena is understanding how entanglement propagates around many-body systems. Furthermore, the controlled distribution of entanglement is of fundamental importance for quantum communication and computation. In many situations, quasiparticles are the carriers of information around a quantum system and are expected to distribute entanglement in a fashion determined by the system interactions. Here we report on the observation of magnon quasiparticle dynamics in a one-dimensional many-body quantum system of trapped ions representing an Ising spin model. Using the ability to tune the effective interaction range, and to prepare and measure the quantum state at the individual particle level, we observe new quasiparticle phenomena. For the first time, we reveal the entanglement distributed by quasiparticles around a many-body system. Second, for long-range interactions we observe the divergence of quasiparticle velocity and breakdown of the light-cone picture that is valid for short-range interactions. Our results will allow experimental studies of a wide range of phenomena, such as quantum transport, thermalisation, localisation and entanglement growth, and represent a first step towards a new quantum-optical regime with on-demand quasiparticles with tunable non-linear interactions.

• The paper experimentally demonstrates how varying the interaction range enables detailed observation of magnon quasiparticle-mediated entanglement propagation in trapped-ion systems.
• The paper employs a one-dimensional Ising spin model with tunable power-law interactions to probe the breakdown of traditional light-cone bounds in long-range regimes.
• The paper validates its findings using quantum state tomography and numerical models, offering new insights for quantum simulation and information processing.

Overview of Entanglement Propagation in Quantum Many-Body Systems

This paper presents a significant experimental investigation into the dynamics of entanglement propagation within a quantum many-body system modeled by trapped ions. The paper is rooted in understanding how quasiparticles facilitate entanglement distribution in such systems, with a particular focus on how the interaction range influences quasiparticle behavior and the entanglement dynamics.

The authors employ an experimental setup using trapped ions to simulate a one-dimensional Ising spin model. This setup allows the controlled paper of quasiparticles, specifically magnons, being engineered and analyzed within these systems. The ability to manipulate the interaction range between spins is a pivotal aspect of this research, enabling the authors to explore regimes from short-range to long-range interactions corresponding to different exponents of a power-law decay ($\alpha$) in the interaction strength. Such flexibility sheds light on phenomena that remain challenging to observe directly in other quantum systems.

The paper reports observing the dynamics of magnon quasiparticles and their role in distributing entanglement across a spin chain of trapped ions. The dynamics following both global and local quantum quenches are studied, with compelling insights into how entanglement is propagated and how the interaction range affects this propagation. Specifically, the breakdown of the light-cone picture, generally applicable in short-range systems as dictated by Lieb-Robinson bounds, is demonstrated for systems with long-range interactions. This signifies that, unlike nearest-neighbor interaction systems, larger bounds on information propagation velocity should be conceived when long-range interactions are considered.

The authors' work exhibits strong numerical verification of theoretical models, coupled with experimental fidelity. The observations of entanglement propagation from the central spin in a one-dimensional chain are substantiated through quantum state tomography. The entanglement growth was measured through the spin-spin correlations and the single-spin von Neumann entropy, which neatly captures the distillation of entanglement as the magnon wavefront moves through the system. This methodology affords validation of theoretical predictions in this experimentally accessible regime.

Implications and Future Directions

The implications of this paper are multifaceted, potentially influencing quantum information processing, the paper of quantum phase transitions, and improving the understanding of many-body physics. The detailed observation and manipulation of quasiparticle dynamics pave the way for experimental investigations into quantum transport phenomena and systems that exhibit thermalization or localization under various interaction conditions.

From a practical perspective, this research demonstrates a pathway to simulate and explore quantum dynamics in systems that would otherwise remain theoretically bounded or inaccessible due to the complexity of the underlying equations. The tunable constraints within their experimental framework suggest prospects in both quantum simulation and modeling astrophysical phenomena potentially governed by similar physical laws.

The immediate future direction would logically extend these studies to longer ion chains, exploring higher dimensions, and examining the influence of system imperfections on entanglement propagation dynamics. Additionally, incorporating disorder or simulating higher temperature conditions could provide a richer context to emulate real-world quantum systems, presenting further avenues for advancing the understanding of quantum many-body entanglement dynamics.

As the interest in exploring quantum systems with long-range interactions grows, spurred by both theoretical and experimental advances, studies like this one provide a rigorous, experimentally grounded framework to question and redefine some core tenets in quantum mechanics, especially concerning quantum speed limits and entanglement distribution.

This paper is an exemplary case of how experimental advancements can bridge the gap with theoretical expectations, providing new insights into the realms of quantum information science and quantum thermodynamics.