Description
Quantum nonlocality describes a stronger form of quantum correlation than that of entanglement [1]. It refutes Einstein's belief of local realism and is among the most distinctive and enigmatic features of quantum mechanics. It is a crucial resource for achieving quantum advantages in a variety of practical applications, ranging from cryptography and certified random number generation via self-testing to machine learning.
While the few-body case is well-explored, the detection of nonlocality especially in quantum many-body systems, is notoriously challenging.
In this talk, I first motivate the usage of energy as a detector for many-body Bell nonlocality [2,3] and the certification of Bell correlation depth [4]. If an appropriate Hamiltonian is chosen, it can be used as a Bell correlation witness and by variationally decreasing the energy of a many-body system across a hierarchy of thresholds, we certify an increasing Bell correlation depth from experimental data.
In a recent work [5], we show that this theoretical concept is experimentally viable. We report an experimental certification of genuine multipartite Bell correlations, which signal nonlocality in quantum many-body systems, up to 24 qubits with a fully programmable superconducting quantum processor.
As an illustrating example, we variationally prepare the low-energy state of a two-dimensional honeycomb model with 73 qubits and certify its Bell correlations by measuring an energy that surpasses the corresponding classical bound with up to 48 standard deviations.
Our results establish a viable approach for preparing and certifying multipartite Bell correlations, which provide not only a finer benchmark beyond entanglement for quantum devices, but also a valuable guide towards exploiting multipartite Bell correlation in a wide spectrum of practical applications.
References
[1] N. Brunner, D. Cavalcanti, S. Pironio, V. Scarani, and S. Wehner, Bell Nonlocality, Rev. Mod. Phys. 86, 419 (2014).
[2] Tura, J., De las Cuevas, G., Augusiak, R., Lewenstein, M., Acín, A., & Cirac, J. I. Physical Review X, 7(2), 021005. (2017).
[3] P. Emonts, M. Hu, A. Aloy, and J. Tura, Effects of Topological Boundary Conditions on Bell Nonlocality, arXiv:2405.14587.
[4] G. Svetlichny, Distinguishing Three-Body from Two-Body Nonseparability by a Bell-Type Inequality, Phys. Rev. D 35, 3066 (1987).
[5] K. Wang et al., Probing Many-Body Bell Correlation Depth with Superconducting Qubits, Phys. Rev. X 15, 021024 (2025).
