A team led by Prof. LI Chuanfeng and Prof. XU Jinshi from the University of Science and Technology of China (USTC) collaborating with Prof. CHEN Jingling from Nankai University and Prof. Adán Cabello from the University of Seville, studied the single-system version of multipartite Bell nonlocality and observed the highest degree of quantum contextuality in single system. Their work was published in Physical Review Letters on June 13th.
Quantum contextuality refers to the phenomenon that the measurements of quantum observables cannot be simply considered as revealing preexisting properties. It is a distinctive feature in quantum mechanics and a crucial resource for quantum computation. Contextuality defies noncontextuality hidden-variable theories and is closely linked to quantum nonlocality. In multipartite systems, quantum nonlocality arises as the result of the contradiction between quantum contextuality and noncontextuality hidden-variable theories. The extent of nonlocality can be measured by the violation of Bell inequality and previous researches showed that the violation increases exponentially with the number of quantum bits involved. However, while single-particle high-dimensional system offers more possibilities for measurements compared to multipartite systems, the quest to enhance contextual correlation’s robustness remains an ongoing challenge.
The schematic diagram for extracting contextuality from three-party nonlocality. (Image by LIU Zhenghao et al.)
To construct and observe more robust quantum contextuality in single-particle system, the team adopted a graph-theoretic approach to quantum correlations. They associated the commutation relations between measurements used in nonlocality correlations with a graph of exclusivity, and then looked for another set of measurements in the single high-dimensional system that have commutation relation isomorphic to the graph. This approach fully quantify the nonclassical properties of quantum correlations using graph parameters.
The team found that after transforming the Mermin-Ardehali-Belinskii-Klyshko (MABK) Bell inequality into noncontextuality inequality using the above approach, the maximum violation is the same but the required Hilbert space dimension is smaller compared to the dimension of the original Bell inequality. Further research indicated that this phenomenon of contextuality concentration, wherein contextuality transitions from nonlocality correlations to single-particle high-dimensional correlations, is widely observed within a class of nonlocality correlations previously discovered by the team. In the experiment, the team developed a spatial light modulation technique to achieve high-fidelity quantum state preparation and measurement in a seven-dimensional quantum system based on photon spatial mode encoding.
By ensuring minimal disturbance between the initial and subsequent measurements, the team observed a violation exceeding 68 standard deviations in the noncontextuality inequality derived from the three-party MABK inequality. The ratio between the quantum violation value and the classical limit reached 0.274, setting a new record for the highest ratio in single-particle contextuality experiments.
The discovery of quantum contextuality concentration not only lays the foundation for observing more quantum correlations but also holds the potential to advance the realization of quantum computation in various physical systems.
Paper link: https://doi.org/10.1103/PhysRevLett.130.240202
(Written by MA Xuange, edited by ZHANG Yihang, USTC News Center)