A team of researchers led by GUO Guangcan, member of the Chinese Academy of Sciences, experimentally reveal that nonlocality can be produced from single-particle contextuality by using two-particle correlations which do not violate any Bell inequality by themselves. This experiment also verifies the free will theorem and shows that the results of the measurements on the photons are not determined by their past.

The findings are expected to contribute to the design of a quantum system integrating quantum computation and communication together.

Quantum nonlocality is at the heart of quantum physics. A fundamental problem is the origin of quantum nonlocality. The relationship between quantum nonlocality and other fundamental quantum phenomena, such as quantum contextuality, is of special interest. It has been shown that quantum nonlocality is essential for device-independent secure communication and quantum contextuality supplies the power for fault-tolerant universal quantum computation. It raises the question of whether single-particle contextuality and two-party nonlocality can coexist, so the same quantum system can provide both resources simultaneously. 

Prof. LI Chuanfeng, Prof. LIU Biheng, Prof. HAN Yongjian from CAS Key Laboratory of Quantum Information, University of Science and Technology of China (USTC) and their collaborators designed a delicate beam splitter, with which they generated a two-photon four qubit hyper-entangled state with high fidelity. They sent the photons to Alice and Bob, who carried out a series of quantum measurements to check quantum nonlocality and quantum contextuality. Scientists then tested the quantum contextuality.

The results violate the Peres-Mermin inequality, which is valid for nontextuality hidden variable theories, by 165 standard deviations. Scientists showed that the Alice-Bob quantum correlations, by themselves, do not reveal nonlocality. At last, when taking the quantum contextuality at Alice's side into consideration, they observed the violation of a Bell-like inequality by 66 standard deviations, which reveals Bell nonlocality. This means single-particle contextuality and two-party nonlocality can coexist.

What’s more, this inequality can be deduced from free will theorem. The violation of this inequality implies that the results of the measurements on the photons are not determined by their past.

Then what determines the results of the measurements? Is it the free will of Alice and Bob? We still have no answer.

This work is published in Physical Review Letters on November 25^th titled as “Nonlocality from Local Contexuality”. This work is supported from Ministry of Science and Technology of China, National Natural Science Foundation of China, The Chinese Academy of Sciences and Synergetic Innovation Center of Quantum Information and Quantum Physics.