Quantum entanglement聽can link two objects even when they are separated by extremely large distances. But a new study has found a limit at which such quantum correlations stop 鈥� and surprisingly, something even stronger may begin.
鈥淗onestly, we are at the edge of science here,鈥� says at Paris-Saclay University in France.
To verify that two quantum objects are entangled, physicists use what are called Bell tests: they repeatedly measure the system to find out all possible states it might be in, then create a 鈥減robability distribution鈥� to show how likely the system is to be in any of these states.
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Bancal and , also at Paris-Saclay University, have now calculated exactly which probability distributions are allowed by quantum theory. If quantum objects have a probability distribution that doesn鈥檛 match any of these, it suggests they actually belong to some more exotic, post-quantum theory.
Physicists first began studying this idea in the 1980s, and since then, several research teams have inched towards discovering a boundary between quantum and post-quantum behaviour. For their part, Barizien and Bancal focused on a quantum system that could only be in one of two states. An example of this is a quantum bit or qubit 鈥� the building block of quantum computers and quantum communication devices.
Normally, physicists would calculate the probability distribution of these states based on the physical details of the object. But the pair devised a way to invert this method: instead, they started with the many possible probability distributions and determined which could be matched to their physical quantum system.
Previously, researchers had mapped out some quantum probability distributions by studying cases where entanglement between objects was as strong as possible. Bancal and Barizien managed to complete the map by expanding from these more extreme examples to all cases of entanglement. Connections stronger than the entanglements included in the map would be evidence of a post-quantum effect.
Barizien says that it was like collecting pieces of a complex puzzle from past works, mathematical literature and their own insights, until they all came together remarkably successfully. 鈥淭his is a great technical achievement,鈥� says at the National University of Singapore.
at University Grenoble-Alpes in France says that fully characterising the set of all possible quantum correlations becomes exceedingly difficult as objects become more complex 鈥� because there are more potential states for the system. So it is notable and important that the new study could be exact. 艩upi膰 says that while it is fairly easy to see where classical correlations end and quantum ones begin, it is more problematic to work out when something quantum crosses into something that is possibly post-quantum.
at the Slovak Academy of Sciences says that, because the new work applies to qubits, it may also offer mathematical tools for making quantum communication and computing protocols more secure.
This is because the finding allows researchers to learn about a quantum device just from making measurements of its properties and determining how they are correlated, instead of having to know the details of its hardware 鈥� something that is impossible for traditional computing devices. 鈥淲e don鈥檛 have to trust the manufacturer of the device. We can just test the device on 鈥榃hat is it doing?鈥� And results like this make the tests more rigorous,鈥� says Plesch.
But there may also be consequences for how we think about quantum theory, says Scarani. Some probability distributions that lie beyond the post-quantum border break laws of physics that would make them impossible to find in nature. But others don鈥檛. This opens a big question: is our world entirely inside of the quantum border 鈥� or not?
The question remains tantalisingly open, especially since there aren鈥檛 any rigorous and agreed upon post-quantum theories. So far, no experiment has managed to cross the edge of quantumness. But if that ever did happen, it would be the results of this study that would help us realise we had gone post-quantum, says Barizien.
Nature Physics
