Boundary Effects Overcome in Promising Quantum Computing Arrays

Researchers at Brown University, Yash M. Lokare and Matthew J. Coley-O’Rourke, have developed a new strategy to stabilise quantum order within finite Rydberg atom arrays. Lokare and colleagues address a key challenge in quantum simulation, where boundary effects often disrupt the observation of predicted bulk quantum phases of matter in these ultracold atom systems. Their protocol uses the disordered phase inherent in Rydberg systems, effectively mitigating boundary interference and enabling smaller arrays to exhibit bulk-like quantum behaviour in both one- and two-dimensional configurations. This advancement is a vital step towards realising the full potential of Rydberg atom arrays as a platform for exploring complex quantum phenomena, with implications for materials science and fundamental physics.

Stabilising quantum order in Rydberg arrays via edge manipulation and the disordered phase

Over 200 atoms now reliably exhibit bulk-like quantum order, a significant increase from previous limitations of approximately 50 atoms. Boundary effects had previously prevented stable observation at larger scales, hindering the ability to accurately model many-body quantum systems. These boundary effects arise because atoms located at the edges of the array experience fewer nearest neighbours compared to those in the interior, leading to altered interactions and disrupting the formation of the desired bulk quantum phase. This advance allows for accurate modelling of complex quantum phenomena in finite Rydberg atom arrays, resolving a longstanding challenge in quantum simulation. The technique centres on manipulating the edges of these arrays, driving them into unbiased configurations dependent on bulk physics and effectively eliminating disruptive boundary interference. This is achieved by leveraging the inherent properties of Rydberg atoms, which exhibit strong, long-range interactions when excited to high principal quantum numbers.

The disordered phase, a naturally occurring state within Rydberg systems characterised by a lack of long-range order, stabilised quantum order without requiring larger array sizes or altered atomic interactions, opening possibilities for more precise and scalable quantum modelling in both one- and two-dimensional systems. This disordered phase acts as a ‘buffer’ at the edges, effectively screening the boundary atoms from the bulk and reducing the influence of the reduced coordination. Density matrix renormalization group simulations, a powerful computational method used to study strongly correlated quantum systems, demonstrated the technique’s efficacy by retaining all long-range atomic interactions without simplification. These simulations are crucial for verifying the theoretical predictions and understanding the underlying physics. In one-dimensional systems, crystalline phases typically transition between distinct states separated by a ‘floating phase’ characterised by continuous variation in atomic density, but experiments on lattices of fewer than 200 atoms previously showed discrete, quantized density fluctuations caused by edge pinning. This pinning occurs because the boundary atoms are constrained and cannot freely adjust their positions to minimise the system’s energy. Two-dimensional simulations predicted a stable ‘star phase’ at specific interaction strengths, a configuration where atoms arrange themselves in a star-like pattern. Yet finite arrays exhibited a competing ‘square phase’ dominating the ground state because atoms at the edges favoured denser packing than those within the array. The simulations explored how this edge behaviour influences the observed phases. The ‘square phase’ is energetically favourable for edge atoms due to the reduced number of neighbours, but it is not the true ground state of the bulk system.

Mitigating boundary effects enhances fidelity of Rydberg atom array quantum simulations

Stabilising quantum order in these carefully controlled Rydberg atom arrays promises a leap forward in modelling complex materials and phenomena, including high-temperature superconductivity, magnetism, and topological phases of matter. Rydberg atom arrays offer a unique platform for quantum simulation because the strong interactions between Rydberg atoms can be precisely controlled using external laser fields, allowing researchers to engineer artificial quantum systems with desired properties. However, this work relies entirely on numerical simulations, representing an important step but not definitive proof. Translating these calculations into a working laboratory demonstration remains a substantial hurdle, given the inherent challenges of manipulating ultracold atoms with sufficient precision. Maintaining the necessary level of control over the atomic positions and interactions requires sophisticated experimental techniques, including optical tweezers and high-resolution imaging.

Acknowledging the need for laboratory validation does not diminish the importance of this research. A practical computational strategy to overcome limitations inherent in current quantum simulation platforms has been identified, specifically highly controlled systems of ultracold atoms used to model other materials. By addressing boundary effects in these small-scale arrays, scientists can more accurately replicate the behaviour of larger, more complex quantum systems, accelerating materials discovery and fundamental physics research. The ability to simulate larger systems with fewer atoms is particularly valuable, as it reduces the experimental complexity and cost. This approach effectively renders the boundaries irrelevant to the quantum simulation, allowing for more reliable results. Furthermore, the technique is not limited to specific array geometries or atomic interactions, making it a versatile tool for exploring a wide range of quantum phenomena. Future research will focus on implementing this strategy in experimental Rydberg atom arrays and validating the simulation results, paving the way for more accurate and efficient quantum simulations.

By developing a strategy to minimise boundary effects, researchers successfully demonstrated a method for improving the accuracy of quantum simulations using ultracold Rydberg atom arrays. This is important because boundaries in small experimental systems have previously hindered accurate replication of the behaviour of larger quantum materials like those exhibiting superconductivity or magnetism. Numerical simulations in one- and two-dimensional systems confirmed the technique’s effectiveness in both ordered and critical phases. The authors intend to validate these findings with physical experiments using Rydberg atom arrays, potentially leading to more reliable quantum simulations.