Quantum entanglement can be too much of a good thing

An overdose of the spooky connection can break down quantum computing systems, researchers find

Physicists have long thought that quantum entanglement, a mysterious link between separated particles that Einstein called “spooky action at a distance,” would allow quantum computers to solve certain hard math problems much faster than ordinary computers. But now it seems that entanglement can also be a nuisance.

Instead of speeding up the process, too much entanglement can break down the entire system, researchers report in a paper to appear in Physical Review Letters.

Entanglement occurs when quantum particles of energy or matter — such as atoms, ions or photons — interact, becoming intimately connected regardless of how far apart they later become. When two objects are entangled, their properties are mutually related.

“Consider two atoms,” says David Gross of Technical University of Braunschweig in Germany, lead author of the new paper. “We can think of a particle like an atom with tags identifying its properties saying, ‘I’m here,’ ‘I’m going in that direction at this speed.’ Now, if I have two or more atoms that have interacted with one another, then it is no longer true that an individual atom has any properties of its own. Only the unified system has well-defined properties I can measure. When I measure one of the particles, the other one produces a result that is related — as if they had communicated.”

Folk wisdom among computing scientists holds that a quantum system of many particles — atoms in a molecule, photons, or ions trapped in an electromagnetic field — with enough entanglement can produce very powerful computational capabilities.

“When you manipulate one of the particles in the system, that does something to the other one, allowing you to perform computational operations you can’t do on a classical computer,” explains Peter Shor, a theoretical computer scientist at MIT.

But not just any entanglement will do, Gross and collaborators say. “What we are showing is that, for entanglement to be useful, it must come in the right amount,” Gross says. “If you have too little of it, quantum computing doesn’t take place. But if you have too much, then that particular quantum system will not have quantum computational properties at all.”

Conventional computers, regardless of efficiency or hardware, all work the same way, manipulating strings of 0s and 1s. Quantum computing changes the rules. Instead of binary arithmetic, it manipulates quantum states. “The only quantum computers developed so far have around 10 or fewer quantum states, so they can only solve toy problems,” says Shor.

People designing quantum computers would like to have more states and more systems available to use. “Our results clarify the direction researchers must look to find useful entangled states, because some states that might have been thought as potential resources are known to be useless,” says Jens Eisert of the University of Potsdam in Germany and one of the paper’s authors.

“The study is definitely not a blow to the existing quantum computing paradigms,” Eisert asserts, “because the results do not discard useful known quantum systems.”

But the authors of the study do believe that their results challenge current views about the relationship between computational power and entanglement. “There are certain states of the system which are known to help with sophisticated, difficult computations, but those are difficult to create at the lab,” Gross says. Those challenges may limit the freedom that physicists have in building quantum computers.

“They are saying that most quantum states are in such complicated forms of entanglement that it’s not easy to make them do what you want them to do,” comments Shor. “I don’t think anybody had proved this rigorously before…. It’s an interesting theoretical result.”

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