Based on programmable quantum simulator, HKU discovers highly entangled quantum spin liquid

Recently, a joint team of scholars from the University of Hong Kong and Harvard University explored the subtle interactions between different phase transitions and discovered highly entangled quantum spin liquids by performing a quantum Monte Carlo analysis of the Riedberg atomic model. Their results were published in the journal Nature Communications under the title "A triangular lattice quantum dimer model with variable dimer density" [1].

01The most promising system to show quantum dominance: the Reedeberg atomic array

Quantum innovation using Reedeburg atoms

Among the many quantum computing and simulation platforms, the Riedberg atom array is considered to be the most promising system among the many programmable quantum simulator platforms that have shown quantum advantages in recent years due to its largest number of quantum bits and highest experimental accuracy. This optical lattice consists of individual neutral alkaline earth atoms with significant dipole moments: they are trapped in an array of microscopic dipole traps that can be moved optically at will to form the desired lattice geometry. In this, each atom can be excited to its Riedberg state, and pairs of excited states interact through their dipole moments via long-range interactions.

Many consider such an array of Riedberg atoms to be the system with the highest experimental accuracy and number of quantum bits of any programmable quantum simulator platform in recent years. The observation of quantum phase transitions and the characterization of topological order from Riedberg atom arrays has been reported at a tremendous rate.

However, these long-range interactions in optical lattices and the Reedeburg blocking mechanism have both advantages and disadvantages. On the one hand, as mentioned above, they yield high precision for experimental quantum control; but on the other hand, they also enforce constraints on the system to be modeled.

Such quantum-constrained many-body systems are among the most difficult to study from a theoretical and numerical point of view. Without a precise theoretical understanding of the complete phase diagram and the new quantum phases, future experiments will have no guidance to continue.

02Mapping the expected phase diagram of the Riedberg array

The experiment was carried out by Zheng Yan, Assistant Professor, and Ziyang Meng, Associate Professor, Department of Physics, The University of Hong Kong, together with Professor Subir Sachdev, a renowned physicist at Harvard University (a member of the American Academy of Arts and Sciences), and Dr. Yan-Cheng Wang, a researcher at the Hangzhou Institute of Innovation, Beijing University of Aeronautics and Astronautics (Yuhang) [2].

They designed a new triangular lattice quantum dimer model with soft constraints (soft constraints) to be as close as possible to the experimental conditions, and developed a scanning clustering algorithm for quantum Monte Carlo simulations that can effectively solve such soft-constrained quantum many-body systems.

Their simulations and theoretical analysis have successfully mapped the expected phase diagram (phase diagram) of the Riedberg array on the Kagome lattice: not only the expected and conventional nematic and interlaced solid phases are found, but also exotic and highly entangled Z2 quantum spin liquids (QSLs) with large parameter regimes in the phase diagram.

The phase diagrams obtained in this work. Within different phases, even Z2 quantum spin liquids (QSLs) and odd Z2 QSLs are topologically ordered novel states of matter that are expected to exist in the Reedeburg Atomic Array experiment in the Kagome lattice. Source: The University of Hong Kong

They identified these novel phases by designing string operators and non-local measurements of other physical observables in quantum Monte Carlo simulations. the distinction between QSL and mundane paramagnetic phases was successfully distinguished. Most excitingly, a pathway connecting the odd Z2 QSL, the plain paramagnetic (PM) phase, and the even QSL and solid phases is revealed, which is useful for guiding the Reedeberg array experiments.

phase transition between the QSL and PM phases.

In addition, they investigate the dynamics and interactions of fractional quasiparticles (visons) in the Z2 QSL to give more experimentally possible evidence.

These results highlight the richness of the constrained model they derive for the Riedberg array system and exploit the various new phases induced by the long-range interactions and the Riedberg blocking mechanism.

Reference links：

[1]https://www.nature.com/articles/s41467-022-33431-5

[2]https://phys.org/news/2022-10-physicists-entangled-states-programmable-quantum.html