Is there a connection more extreme than quantum entanglement?
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Quantum entanglement allows two objects to be inextricably linked even when they’re separated by extremely large distances. But a new study has found a limit where such quantum correlations stop – and surprisingly, something even stronger may begin.
“Honestly, we are at the edge of science here,” says Jean-Daniel Bancal at the Paris-Saclay University in France.
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To verify that two quantum objects are entangled, physicists use so-called Bell tests: they repeatedly measure the system to find out all possible states it might be in, then create a “probability distribution” to show how likely the system is to be in any of these states.
Bancal and Victor Barizien, also at the Paris-Saclay University, have now calculated exactly which probability distributions are allowed by quantum theory. If quantum objects have a probability distribution that does not match any quantum systems, 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 the boundary between quantum and post-quantum behaviour. Barizien and Bancal focused on a quantum system that could only be in one of two states. One example of this type of system is a quantum bit or qubit – the building block of quantum computers and quantum communication devices – which end up in only one of two states, 0 or 1, when measured.
Normally, physicists would calculate the probability of these states based on the physical details of the object. But the pair devised a way to invert this method: instead, they determined which of the many possible probability distributions could be matched to a given physical quantum system.
Bancal and Barizien found a mathematical relationship between cases where objects are partially entangled and where they are as entangled as possible. This helped them leverage past calculations that focused on the most entangled case possible – and greatly generalise them. 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.
“This is a great technical achievement,” says Valerio Scarani at the National University of Singapore.
Ivan Šupić at University Grenoble-Alpes in France says that fully characterising the set of all possible quantum correlations becomes exceedingly complicated as objects become more complex – as there are more potential states for the system. So it is notable and important that the new study could be exact. Šupić says it’s actually fairly easy to see where classical correlations end and quantum ones begin, but the problem is to know when quantum crosses into something that is possibly post-quantum.
Martin Plesch 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 – something that is impossible for traditional computing devices. “We don’t have to trust the manufacturer of the device. We can just test the device on “What is it doing?” And results like this make the tests more rigorous,” Plesch says.
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 do not. This opens a big question: is our world entirely inside of the quantum border – or not?
The question remains tantalisingly open, as there aren’t 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 new study that would help us realize that we had gone post-quantum, says Barizien.
Topics:
quantum physics