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arXiv:1912.02979 · Physical Review A

Observation of nonlocality sharing via not-so-weak measurements

Tianfeng Feng, Changliang Ren, Yuling Tian, Maolin Luo, Haofei Shi, Jing‐Ling Chen, Xiaoqi Zhou

Published Sep 28, 2020• 43 citations
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Abstract

Nonlocality plays a fundamental role in quantum information science. Recently, it has been theoretically predicted and experimentally demonstrated that the nonlocality of an entangled pair may be shared among multiple observers using weak measurements with moderate strength. Here we devise an optimal protocol of nonlocality sharing among three observers and show experimentally that nonlocality sharing may be also achieved using weak measurements with near-maximum strength. Our result sheds light on the interplay between nonlocality and quantum measurements and may find applications in quantum steering, unbounded randomness certification, and quantum communication network.

DOI
10.1103/physreva.102.032220

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Nonlocality plays a fundamental role in quantum information science. Recently, it has been theoretically predicted and experimentally demonstrated that the nonlocality of an entangled pair may be shared among multiple observers using weak measurements with moderate strength. Here we devise an optimal protocol of nonlocality sharing among three observers and show experimentally that nonlocality sharing may be also achieved using weak measurements with near-maximum strength. Our result sheds light on the interplay between nonlocality and quantum measurements and may find applications in quantum steering, unbounded randomness certification, and quantum communication network.
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Bell nonlocality plays a fundamental role in quantum theory. Numerous tests of the Bell inequality have been reported since the ground-breaking discovery of the Bell theorem.Up to now, however, most discussions of the Bell scenario have focused on a single pair of entangled particles distributed to only two separated observers. Recently, it has been shown surprisingly that multiple observers can share the nonlocality present in a single particle from an entangled pair using the method of weak measurements [Phys. Rev. Lett. {\bf 114}, 250401 (2015)]. Here we report an observation of double CHSH-Bell inequality violations for a single pair of entangled photons with strength continuous-tunable optimal weak measurements in photonic system for the first time. Our results not only shed new light on the interplay between nonlocality and quantum measurements but may also be significant for important applications such as unbounded randomness certification and quantum steering.
Quantum correlation is a fundamental resource for various quantum information tasks. It is thus of importance to share the correlation to utilize it for many parties, but sharing quantum correlation among multiple parties is strictly restricted by the well-known monogamy relations. Nonetheless, this restriction can be relaxed when weak measurements are employed. Here, we experimentally demonstrate multiple-observer quantum steering by exploiting sequential weak measurements. Specifically, we observe simultaneous triple violations of the quantum steering inequality among four observers for a bipartite entangled system. Our results not only provide fundamental insights into the relation between quantum steering and measurement disturbance, but also suggest that quantum steering might be repeatably exploited to find applications to, for example, unbounded randomness certification and sharing secret keys among multiple parties simultaneously.
A well-known property of quantum nonlocality is monogamy. However, recent research by Silva \emph{et al.} shows that multiple observers can share the nonlocality by using weak measurements [Phys. Rev. Lett. 114, 250401 (2015)]. There is an open question left in their result: whether the nonlocality of a single particle from an entangled pair can be shared among more than two observers that act sequentially and independently of each other? In this work, we analytical and numerical shows that it is possible to observe a triple violation of CHSH inequality between Alice and three Bobs when the measurements of each of the several observers at one side are unbiased with respect to the previous observers. This result overturns the conclusions and proofs of previous related work on this issue which has been widely shared in the scientific community before.
We investigate the trade-off between information gain and disturbance for von Neumann measurements on spin-1/2 particles, and derive the measurement pointer state that saturates this trade-off, which turns out to be highly unusual. We apply this result to the question of whether the nonlocality of a single particle from an entangled pair can be shared among multiple observers that act sequentially and independently of each other, and show that an arbitrarily long sequence of such observers can all violate the Clauser-Horne-Shimony-Holt-Bell inequality.