Added Disorder Firmly Traps Ultracold Atoms in a Synthetic Lattice

A new method creates a Generalised Aubry-André chain with added hopping disorder within a Momentum Space Lattice using a Bose-Einstein Condensate of rubidium-87 atoms. Joel M. Sunil and colleagues at the Indian Institute of Science Education and Research demonstrate precise control over disorder configurations and spatial correlations, a feat difficult with traditional methods. The findings reveal that uncorrelated disorder strengthens localisation, while spatially correlated disorder can partially delocalise states. This platform validates as a set of tools to study a wider range of disordered quantum systems.

Creating tailored disorder in atomic systems via momentum space lattices

The Momentum Space Lattice (MSL) technique functioned as a complex 3D maze for ultracold rubidium-87 atoms, creating a lattice within the atoms’ momentum space rather than physically constructing one with light. Conventional optical lattice simulators rely on interfering laser beams to create a periodic potential in real space, confining atoms at discrete lattice sites. However, this approach struggles to introduce true disorder, as the potential is inherently ordered. The MSL circumvents this limitation by employing a series of ‘Bragg transitions’, where atoms absorb and re-emit light, changing their momentum and effectively ‘hopping’ between locations within the lattice. These transitions are driven by multiple laser beams, carefully configured to create the desired lattice structure in momentum space. The MSL allows independent control of the hopping strength and energy at each site, simulating a Generalised Aubry-André (GAA) chain with precisely tailored disorder, a key advancement over traditional methods. A Bose-Einstein condensate of approximately 4×10⁴ rubidium-87 atoms, cooled to 50 nanokelvins, was utilised to create a 21-site Generalised Aubry-André (GAA) chain within the MSL, employing 1064nm laser beams to induce Bragg transitions and control hopping strength and onsite energy. The choice of 1064nm wavelength is crucial for efficient manipulation of rubidium-87 atoms. Rabi frequency was limited to 1.4kHz to avoid off-resonant driving, ensuring that the atoms primarily respond to the intended Bragg transitions and minimising unwanted excitations. This precise control over the laser parameters is fundamental to the success of the MSL technique.

Momentum space lattices reveal disorder’s impact on many-body localisation transitions

Unprecedented control enabled the observation of a reduction in localization transition sharpness, changing from a distinct step to a smoothed crossover, thanks to the MSL platform. In many-body quantum systems, the transition between localized and delocalized phases is often sharp and well-defined. However, the introduction of disorder can significantly alter this transition. The MSL platform allowed researchers to meticulously control the type and strength of disorder, revealing a smoothing of the localization transition. Experiments reveal that uncorrelated hopping disorder consistently enhances localization across all phases of the GAA model, while spatially correlated disorder induces partial delocalization within localized states near strong hopping bonds. Uncorrelated disorder introduces randomness in the hopping amplitude between adjacent sites, effectively hindering the propagation of quantum information and strengthening localization. Conversely, spatially correlated disorder introduces patterns in the hopping amplitudes, creating pathways for partial delocalization. Further numerical analysis confirmed that spatially correlated hopping disorder induces partial delocalization specifically within localized states near areas of strong atomic bonding. This suggests that the correlations in the disorder can create resonant pathways, allowing atoms to tunnel through the lattice more easily. Over a range of disorder strengths and correlation lengths, experimental results quantitatively matched simulations of Bose-Einstein Condensate dynamics within the MSL, demonstrating the platform’s precision. However, these findings currently focus on one-dimensional systems and do not yet demonstrate how these principles translate to the more complex, and practically relevant, three-dimensional materials. Extending these studies to higher dimensions remains a significant challenge, requiring more complex MSL configurations and increased computational resources.

Controlling quantum disorder via a momentum space lattice realisation

Understanding how disorder impacts quantum systems is an increasingly important focus for scientists, a challenge with implications for materials science and beyond. Disorder is ubiquitous in real materials, arising from imperfections in the crystal structure, impurities, and thermal fluctuations. These imperfections can significantly alter the electronic and optical properties of materials, impacting their performance in various applications. While creating precisely controlled, quasi-periodic systems with ultracold atoms has become routine, replicating true disorder, randomness without predictable patterns, has proven elusive. Quasi-periodic systems exhibit order, but not the perfect periodicity of crystals. This work successfully demonstrates a Generalised Aubry-André model incorporating hopping disorder, acknowledging the platform’s present limitations regarding system size and coherence times, as these factors currently restrict observation of truly long-range correlations. The number of sites in the GAA chain (21) and the duration of the experiment are limited by the coherence time of the Bose-Einstein condensate. Longer coherence times would allow for the creation of larger lattices and the observation of long-range correlations.

A new method for simulating disordered quantum systems using a Bose-Einstein condensate of rubidium-87 atoms has been established by this experiment. Manipulating the atoms’ momentum created a lattice to simulate a Generalised Aubry-André chain, differing from conventional approaches by offering site-specific control over atomic behaviour. Random variations in how easily atoms move between locations strengthen the tendency for quantum particles to become localized, effectively smoothing transitions between quantum states. This approach offers unique advantages, enabling precise control over disorder configurations and spatial correlations, and will likely enable exploration of complex quantum behaviours and begin a major era in materials science. The ability to tailor disorder at the single-site level opens up new avenues for investigating fundamental questions in condensed matter physics, such as the nature of the many-body localization transition and the emergence of topological phases in disordered systems. Furthermore, this platform could be used to design and simulate novel materials with tailored properties, potentially leading to breakthroughs in areas such as superconductivity and quantum computing.

This research successfully demonstrated a Generalised Aubry-André model with added hopping disorder using a Bose-Einstein condensate of 87Rb atoms. The method creates a lattice via momentum manipulation, allowing for precise control over disorder and spatial correlations, unlike previous simulations. Introducing random variations in atomic movement strengthened the tendency for quantum particles to become localised, altering transitions between quantum states. The platform, comprising a 21-site chain, provides a means to study disordered quantum systems and represents a step towards understanding complex quantum behaviours.