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In the everyday world that humans experience, objects behave in a predictable way, explained by classical physics. One of the important aspects of classical physics is that nothing, not even information, can travel faster than the speed of light. However, in the 1930s, scientists discovered that very small particles abide by some very different rules. One of the most mind-boggling behaviors exhibited by these particles is quantum entanglement鈥攚hich Albert Einstein famously called "spooky action at a distance."

In quantum entanglement, two particles can become linked so that their properties are correlated, even when separated by large distances. If you measure a property of one particle鈥攕uch as its orientation鈥攜ou instantly know the corresponding property of the other, no matter how far apart they are. Although this correlation appears to happen instantaneously, it cannot be used to send information faster than light. Instead, it reveals a deep and puzzling connection that defies classical explanation, while still respecting the fundamental speed limit set by relativity. This phenomenon is known as "nonlocality"鈥攖he appearance of effects at a distance that would be impossible under classical physics.

Up until recently, it was thought that only entangled particles could exhibit this nonlocality. But a new study, in Science Advances, has used Bell's inequality to test whether nonlocal quantum correlations can arise from other non-entanglement quantum features.

The experiment used photons generated by laser light hitting a particular type of crystal in such a way that it is impossible to determine their source. The setup ensures that the photons cannot become entangled before their detection at two separate detectors. The researchers used Bell's inequality to determine if the experiment resulted in violations of local realism.

According to their calculations, the experiment resulted in a violation of the Bell inequality, exceeding the threshold by more than four standard deviations. This kind of violation using unentangled photons had not been seen before. The researchers say these violations of Bell's inequality arise from a property called quantum indistinguishability by path identity, instead of entanglement.

"Our work establishes a connection between quantum correlation and quantum indistinguishability, providing insights into the fundamental origin of the counterintuitive characteristics observed in quantum physics," the study authors write.

While this work might be groundbreaking, there are still some possible issues that need to be ironed out in future studies. For example, the experiment relies on post selection鈥攚here only certain photons are detected, possibly giving misleading results.

Another possible issue comes from a locality loophole due to the phase settings of the detectors not being separated properly. However, the study authors are aware of this study's limitations and are eager to find fixes to these issues and try again.

They end by saying, "We not only expect that tailored loopholes and local hidden variable to the work reported here can be identified, but also expect that they will be consistently excluded by hardware improvements of high-quality quantum photonic devices and experiments, as we witnessed in the 90-year endeavor in the violations of local realism with entangled particles.

"Moreover, our work could very well lead to other interesting experiments, such as in the development of the Bell experiment. In analogy to the Bell experiment with two particles, we expect that quantum mechanics will lastly prevail."

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