Researchers uncover hysteresis in isolated spin systems with twelve particles and thousands of atoms

The behaviour of complex magnetic materials presents a long-standing challenge in physics, and now researchers are exploring these properties in entirely isolated quantum systems. Moritz Hornung, Eduard J. Braun, and Sebastian Geier, all from the Physikalisches Institut at the Universität Heidelberg, alongside their colleagues, demonstrate a form of magnetic memory in a system of interacting quantum spins, even when shielded from external influences. The team observes a phenomenon akin to hysteresis, where a system’s state depends on its history, by carefully controlling the energy of these isolated spins, achieved using ultracold Rydberg atoms. This observation, mirroring behaviour seen in traditional magnetic materials but occurring in a fundamentally different setting, reveals a novel metastable regime and opens exciting possibilities for understanding the low-energy structure of isolated quantum systems, potentially paving the way for new explorations of quantum magnetism.

Rydberg Atoms Explore Collective Quantum Interactions

Researchers are investigating the collective behavior of a large number of Rydberg atoms trapped using laser light. Rydberg atoms, with their exaggerated properties, provide an ideal platform for studying strong interactions and quantum phenomena. The goal is to understand how these atoms interact, how their behavior can be manipulated, and how this collective behavior might be harnessed for quantum information processing or simulating complex quantum systems. The team focuses on understanding the magnetic properties of these interacting atoms and how they respond to external fields. Rydberg atoms become strongly interactive when excited to high energy levels, extending their influence over relatively large distances.

By manipulating and measuring the magnetization of the atomic ensemble, researchers gain insights into these interactions. They employ zero-field and field annealing protocols, using microwave pulses to prepare and measure the magnetization. A crucial parameter in this research is the C6 coefficient, which quantifies the strength of the van der Waals interaction between Rydberg atoms. The team traps atoms and excites them to a specific Rydberg state, initially polarizing them in one direction. They then use microwave pulses to manipulate the atoms and apply annealing protocols, allowing the atoms to evolve under the influence of their interactions.

By measuring how the magnetization components change over time, researchers can track the dynamics of the interacting atoms and compare the results to simulations. These simulations model the system’s behavior, validating the experimental findings. Experiments reveal that the interactions between Rydberg atoms are direction-dependent in the presence of a magnetic field. Oscillations observed in the magnetization components during the annealing process are attributed to imperfections in the experimental setup. However, the experimental results generally align with the simulations, confirming the understanding of the system.

The magnetization components eventually relax to zero, indicating the system is reaching equilibrium. The interaction strength, quantified by the C6 coefficient, also changes in the presence of a magnetic field, reflecting the anisotropic interactions. This research provides valuable insights into the complex interactions between Rydberg atoms and how these interactions can be controlled. This knowledge is crucial for developing quantum computers, simulating complex quantum systems, and furthering our understanding of many-body physics. Future research will focus on improving experimental control, exploring different Rydberg states, and designing new quantum protocols based on Rydberg atom interactions.

Isolated Spins Exhibit Energy-Dependent Hysteresis

Researchers have discovered energy-dependent hysteresis in an isolated system of interacting spins, a phenomenon previously observed only in conventional spin glasses that interact with a thermal environment. This breakthrough demonstrates that complex magnetic behavior can emerge even when a system is entirely isolated, opening new avenues for exploring fundamental physics. The team devised innovative annealing protocols, akin to controlled cooling, to manipulate the energy within this isolated spin system and observe its response. Experiments reveal that the susceptibility of the spin system bifurcates at a specific energy level, indicating the presence of distinct magnetic regimes.

This behavior was confirmed through both detailed numerical simulations involving twelve interacting spins and experiments utilizing thousands of Rydberg atoms, which act as dipolar interacting spins. By varying the speed of the annealing process, researchers were able to initialize the system at different mean energies, allowing them to map out the energy-dependent hysteresis. The simulations demonstrate that fast annealing ramps leave the system in a high-energy state, while slower ramps allow it to settle into a lower-energy ground state. Notably, the team observed a metastable, non-thermal regime, suggesting the possibility of novel states of isolated spin systems not previously understood. Measurements of the linear magnetic susceptibility show distinct responses depending on whether a probe field is applied during the annealing process or afterwards, providing further evidence of the system’s complex energy landscape. These findings not only deepen our understanding of isolated quantum systems but also offer a new platform for investigating complex magnetic phenomena and potentially developing novel quantum technologies.

Isolated Spins Exhibit Hysteresis and Magnetic Regimes

Researchers have demonstrated energy-dependent hysteresis in an isolated system of interacting spins, a phenomenon previously observed only in conventional spin glasses that interact with a thermal environment. By carefully controlling the energy within this isolated system, using a method analogous to cooling or heating magnetic materials, the team observed a bifurcation in the system’s susceptibility, indicating the presence of distinct magnetic regimes. This control is achieved through specifically designed annealing protocols that manipulate the energy levels of the spins. The findings reveal that the isolated spin system exhibits behaviors reminiscent of both paramagnetic and spin glass states, offering insights into the emergence of different magnetic orderings.

Importantly, the researchers identified a metastable, non-thermal regime, suggesting the possibility of novel states of matter in isolated quantum systems. While acknowledging that the precise nature of the second identified regime requires further investigation, the team proposes that future studies could explore potential phase transitions using quench protocols and time reversal symmetry measurements. These protocols, applicable to various spin models, also offer a framework for assessing the performance of quantum computing platforms based on annealing. This research expands our understanding of isolated quantum systems and provides a new avenue for exploring complex magnetic phenomena and potentially developing novel quantum technologies.