  Non-locality and quantum measurement are two fundamental topics in quantum
theory and theirinterplay attracts intensive focus since the discovery of Bell
theorem. Non-locality sharing amongmultiple observers is predicted and
experimentally observed. However, only one-sided sequentialcase, i.e., one
Alice and multiple Bobs is widely discussed and little is known about two-sided
case.Here, we theoretically and experimentally explore the non-locality sharing
in two-sided sequentialmeasurements case in which one entangled pair is
distributed to multiple Alices and Bobs. Weexperimentally observed double EPR
steering among four observers in the photonic system for thefirst time. In the
case that all observers adopt the same measurement strength, it is observedthat
double EPR steering can be demonstrated simultaneously while double Bell-CHSH
inequalityviolations are shown to be impossible. The exact formula relating
Bell quantity and sequential weakmeasurements for arbitrary many Alices and
Bobs is also derived, showing that no more doubleBell-CHSH inequality
violations is possible under unbiased input condition. The results not
onlydeepen our understanding of relation between sequential measurements and
non-locality but alsomay find important applications in quantum information
tasks.
