Enhanced Quantum Control Beats Previous Squeezing Limits

Optimising a single collective transverse field now enhances spin squeezing in a two-dimensional system with dipolar interactions. Achieving substantial squeezing in finite-range interacting systems proved challenging until now, but this strategy surpasses the two-axis-twisting benchmark. Ang Li of the Tsinghua University and colleagues used rotor-spin-wave theory with a power-law interaction value of α = 3 to achieve this breakthrough in squeezing enhancement.

A new method enhances ‘spin squeezing’, a key resource for building more sensitive quantum technologies. The technique overcomes existing limitations when controlling interactions within quantum systems, enabling stronger and more dependable squeezing than previously attainable. The advance uses rotor-spin-wave theory to efficiently manage complex calculations for larger systems, demonstrating that optimising a single control field is sufficient to sharply improve squeezing.

Spin squeezing gently compresses a cloud of quantum particles to reduce uncertainty in measurements, much like focusing a blurry image, and enhancing it allows for more precise readings. This advance addresses a longstanding challenge in controlling interactions within quantum systems, particularly those with finite-range interactions where particles do not influence each other equally. Ang Li of the University of Science and Technology of China and colleagues employed rotor-spin-wave theory, a simplified map for complex quantum interactions, to efficiently calculate optimal control strategies for larger systems. Remarkably, optimising just one control field surpasses the performance of the two-axis-twisting benchmark, a standard measure of squeezing quality.

Optimised transverse fields enable enhanced spin squeezing beyond two-axis twisting

Spin squeezing, a key resource for quantum technologies, now surpasses the two-axis-twisting (TAT) benchmark, achieving a squeezing parameter exceeding that of traditional methods by a substantial margin. A single collective transverse field was optimised using rotor-spin-wave theory, dynamically suppressing unwanted interactions and confining the system’s evolution within the most beneficial quantum state. This approach demonstrates a favourable linear scaling with system size, indicated by a crossover time increasing by only 0.01 units per additional spin.

The breakthrough allows for more precise quantum measurements and enhanced sensitivity in quantum devices, previously unattainable due to computational limitations in modelling finite-range interactions. Detailed analysis of the total spin squared operator revealed that the optimised field actively suppresses transitions between quantum states, confining the system’s evolution and preventing the decay observed in uncontrolled scenarios. The control field synchronises squeezing and rotation, mitigating over-twisting and maintaining high levels of squeezing, as confirmed by visualisation using the Husimi Q function. Currently, these results focus on two-dimensional systems and do not yet demonstrate scalability to the three-dimensional architectures required for practical quantum computing. Importantly, this control strategy maintains effectiveness even when incorporating dephasing noise, a realistic imperfection in quantum systems.

Collective excitation modelling of long-range interacting quantum spin systems

Rotor-spin-wave theory serves as the mathematical framework central to this work, simplifying complex quantum interactions. The technique recasts the behaviour of many interacting spins into the language of collective excitations, effectively reducing the computational burden of simulating larger systems. By focusing on these collective modes, control strategies previously inaccessible due to computational bottlenecks could be explored, allowing efficient calculation of how to best manipulate the system.

The investigation centred on a two-dimensional XX spin model, focusing on systems with finite-range interactions where the coupling strength diminishes with distance between particles. Specifically, the researchers examined the case where interactions follow a power law with an exponent of 3, mirroring those found in Rydberg atom arrays and dipolar quantum gases. This approach enabled the optimisation of control parameters even with periodic boundary conditions and the inclusion of dephasing noise, demonstrating its suitability for experimental implementation in quantum devices. Exact diagonalization or matrix product states are restricted to smaller or one-dimensional systems, highlighting the advantages of this method.

Spin squeezing enhancement versus theoretical limitations in extended rotor-spin-wave models

This work establishes a new benchmark for spin squeezing, surpassing the performance of two-axis twisting techniques, but the reliance on rotor-spin-wave theory introduces a trade-off. Comparisons with more computationally intensive methods like matrix product states confirm that the theory’s accuracy demonstrably diminishes under open boundary conditions and with the inclusion of dephasing noise. A key consideration for scaling towards practical quantum devices is understanding how much error accumulates as system size increases and noise becomes more prevalent.

Acknowledging limitations in the theory’s precision with open boundaries and noise is important, as comparisons with more complex matrix product states reveal discrepancies. Nevertheless, this work delivers a strong advance by demonstrating substantial spin squeezing enhancement, exceeding established benchmarks, through a surprisingly simple control mechanism. This optimised protocol’s durability, even under realistic conditions, offers a valuable and scalable framework for building more effective quantum technologies and exploring quantum entanglement.

Scientists at theTsinghua University and the Hainan University have established a new, strong method for enhancing spin squeezing, a technique used to reduce measurement uncertainty in quantum systems. By applying an optimised control field to a two-dimensional model, they surpassed the performance of standard ‘two-axis twisting’ techniques, despite the complexities of modelling interactions between particles. Extending the theory to include realistic imperfections like boundary conditions and noise proved vital for scalability.

Scientists demonstrated substantial enhancement of spin squeezing in a two-dimensional model with dipolar interactions, exceeding the performance of two-axis twisting. This achievement matters because spin squeezing is a key resource for improving the precision of quantum measurements and witnessing quantum entanglement. The researchers utilised rotor-spin-wave theory to explore control strategies at larger scales and also extended this theory to account for realistic conditions such as open boundaries and dephasing noise, providing a scalable framework for future work.