Particles that appear unentangled achieved significant results in the renowned Entanglement test. This experiment offers fresh insights into the peculiarities of the quantum realm.

Nearly sixty years ago, physicist John Stewart Bell devised a method to determine whether our universe can be better explained through quantum mechanics or traditional theories. The pivotal distinction lies in quantum theory’s incorporation of “abbiotics,” or effects that can persist across vast distances. Remarkably, every experimental implementation of Bell’s tests to date supports the idea that our physical reality is non-local, indicating that we reside in a quantum world.

However, these experiments primarily focused on particles that are closely associated via quantum entanglement. Now, Xiao-Song Ma from Nanjing University in China, along with his team, claims they conducted the Bell Test without relying on entanglement. “Our research may offer a novel viewpoint on non-local correlations,” he states.

The experiment commenced with four specialized crystals, each generating two light particles, or photons, when exposed to a laser. These photons possess various properties measurable by researchers, such as polarization and phase, which describe their behavior as electromagnetic waves. The researchers guided the photons through an intricate arrangement of optical devices, including crystals and lenses, prior to detection.

A standard Bell test experiment involves two fictional experimenters, Alice and Bob, evaluating the properties of correlated particles. By correlating their observations with the “inequality” equation, Alice and Bob can ascertain whether the particles are linked in a non-local manner.

In the new experiment, Alice and Bob were represented by sets of optical devices and detectors instead of interlinked photons. In fact, the researchers incorporated devices in the setup to prevent the intertwining of particle frequencies and velocities. Nonetheless, when Alice and Bob’s measurements were analyzed using the inequality equation, the results indicated a stronger correlation among photons than what could be explained by local effects alone.

Mario Clen from the Max Planck Institute for the Light of Light in Germany suggests that this might be linked to another peculiar property of photons. They indicate it is impossible to identify which photons were “born” within the crystal and what paths they took, making them indistinguishable. Previously, Clen, along with colleagues, utilized this property, termed “distinguishability by path identity,” to entangle photons. However, in this scenario, they confirmed that only one type of quantum peculiarity remains indistinguishable.

The team has yet to formulate a definitive theory explaining how entanglement outcomes can manifest in the Bell test without entanglement actually being employed, but Ma proposes that several underlying quantum phenomena could be indistinguishable as a condition. Thus, even strategies that lack entanglements might serve as the fundamental components necessary to create non-local correlations.

Krenn and Ma express hope that fellow physicists will propose new alternative theories and identify experimental gaps within the Bell test. This mirrors the historical development surrounding the standard Bell test, where nearly five decades elapsed between the initial experiment and the establishment of quantum theory, successfully ruling out all alternative explanations.

One contentious aspect may be the “post-selection” technique utilized by the team. Stefano Paesani at the University of Copenhagen in Denmark argues that this raises questions about whether unentangled photons can be convincingly recognized as non-local within Bell’s tests. After the selection process, he contends that the experiments resemble more traditional scenarios where entanglement exists.

Jeff Randeen from the University of Ottawa, Canada, asserts that while the Bell test can create experiments to examine light, this “holds no profound significance concerning the nature of the universe or reality.”

In such circumstances, there exists the potential for Alice and Bob to act as identical observers or to generate correlations that researchers might misinterpret as non-local effects. Lundeen maintains that the new experiment doesn’t completely eliminate the possibility that Alice and Bob were colluding. “Thus, this experiment doesn’t quite carry the same weight as the renowned violation of Bell’s inequality,” he states.

“This represents one of the elegant extensions of a landmark finding from the ‘Glorious Age’ of the 1990s,” notes Aephraim Steinberg at the University of Toronto, Canada. Nevertheless, in his assessment, traces of entanglement remain in the new experiment—not at the photon level, but rather within the quantum field.

Looking forward, the team aims to enhance the apparatus to address some of these criticisms. For instance, by generating more photons from each crystal, researchers could avoid relying on selection thereafter. “Our collaborative group has already pinpointed several critical potential shortcomings, which we are eager to tackle in the future,” states Ma.