Photons pass the famous Bell test but without entanglement, shows latest study
"Our new work may provide a new perspective to people's understanding of non-local correlations," said Xiao-Song Ma, one of the authors of the study and a professor at Nanjing University.
How to entangle without entanglement
The foundation of this breakthrough lies in a 60-year-old idea. In 1964, physicist John Bell designed a test, now known as the Bell test, to check whether nature follows the rules of quantum mechanics or obeys more traditional, local theories, where distant objects can't instantly influence each other.All strong violations of Bell's inequality so far have relied on entangled particles. That's because entanglement was believed to be the only way to produce the non-local correlations needed to beat Bell's test. However, in this new study, scientists built a setup that seemed to defy this rule. Instead of using entangled particles, they created photons using four special crystals.When illuminated with lasers, each crystal emitted a pair of photons with measurable properties like polarization (the direction the light wave oscillates) and phase (how its wave wiggles in space and time).
The photons then traveled through a carefully designed maze of optical devices, crystals, lenses, and beam splitters before reaching two separate detectors, labeled Alice and Bob. Normally, in a Bell test, Alice and Bob each measure one half of an entangled pair.What's interesting here is that the experiment was built in a way that explicitly avoided creating entanglement. The researchers even included extra components to block any accidental entanglement between properties like frequency or speed.
Yet, when they crunched the numbers using Bell's inequality, the photons seemed to talk to each other in a non-local way, just like entangled ones—but how was this even possible?
The answer may lie in a lesser-known quantum property, called indistinguishability by path identity. "We report the violation of the Bell inequality that cannot be described by quantum entanglement in the system but arises from quantum indistinguishability by path identity," the study authors note.
Due to this property, it became impossible to tell which photon came from which crystal, and the paths taken by photons overlapped and blended perfectly. The particles became fundamentally indistinct and led to non-local correlations that entanglement usually provides.
The significance of indistinguishability
The experiment raises exciting but controversial possibilities. If indistinguishability can mimic or even replace entanglement in some cases, it might open up new routes for building quantum devices, especially ones that are simpler to engineer.
However, there are also important caveats. Some physicists point out that the team used a method called post-selection, where only certain photon detection events are counted. They argue this might artificially boost the appearance of quantum correlations.
Moreover, it is also possible that there might still be entanglement involved, just not between photons, but at the level of the quantum fields that create them.
The study authors acknowledge these concerns and are already working on improvements. They aim to eliminate post-selection by increasing the number of photons their crystals can produce. If successful, it could mark a major milestone in quantum foundations.
The study is published in the journal Science Advances.
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