Imagine two people on opposite sides of the world want to share confidential information with complete privacy. One promising option is to use quantum entanglement, a strange property of physics in which two particles become so interconnected that they act as a single system, even when separated by vast distances. Changes to one instantly affect the other, no matter how far apart they are.
Entanglement is the backbone of quantum cryptography, a technology that taps into the eerie correlations between particles — like photons and electrons — to share information in ways no classical system can. The resulting messages are theoretically safe from eavesdropping: any attempt to intercept a message disturbs its quantum state, instantly tipping off both sender and receiver.
However, there’s a catch: To entangle two particles, you first have to bring them together in the same physical location so they can interact locally, making the process expensive and logistically difficult. It’s as if you had to meet in person to exchange money, rather than sending it through the bank.
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Borrowing from The Entanglement Reservoir
But what if new users of quantum cryptography didn’t need to generate their own entangled particle pairs? What if each party could simply borrow entanglement from a local “reservoir,” and save themselves the hassle?
In a recent paper in the journal Physical Review A, an international team of researchers reported that this sort of workaround is indeed possible.
“Entanglement can be transferred from the reservoir, like from a bank to the customers,” says co-author Chirag Srivastava, a postdoc at the University of Gdańsk in Poland. “They need not come together.”
When formulating quantum thought experiments, physicists often refer to fictional characters named Alice and Bob. In this study, to explain their novel method, the authors introduce two new characters: Charu and Debu.
Maximal Entanglement
Alice and Bob share an entangled state, and thus can communicate securely. Charu and Debu, two other distant parties, want their own shared entanglement. But rather than build one from scratch, Charu interacts locally with Alice, while Debu interacts locally with Bob. Through quantum operations known as “unitaries”— essentially careful manipulations of the particles’ states — a portion of Alice and Bob’s entanglement is transferred to Charu and Debu.
In principle, the researchers show, this process can be repeated indefinitely, with each new Charu-Debu pair receiving a slice of quantum connection. But the party can’t go on forever. Entanglement is a finite resource — the more people you share it with, the smaller each portion becomes. Even the largest bank can’t loan endless sums of money.
That said, even modest amounts of entanglement can enable certain tasks. In quantum teleportation, for example, you “teleport” a quantum state from one entangled particle to the other; so long as you don’t need perfect fidelity, partial entanglement will do the trick. And given a large enough reservoir, you could borrow more for larger tasks that require maximal entanglement.
Solving The Quantum Communication Puzzle
The researchers envision a future where a few such entanglement reservoirs serve as hubs across the globe, allowing countless users to borrow entanglement without generating it themselves. That could save lots of resources and help scale up an international network of secure quantum communication.
Still, other challenges remain. For one, maintaining entangled states after they’ve been established is notoriously difficult. As entangled particles interact with their separate environments, they tend to “decohere,” gradually losing their fragile quantum connection to each other.
Entangled particles are especially prone to decoherence during transportation, and this study eliminates the need to move them long distances. Granted, it’s also difficult to preserve entanglement even in a controlled laboratory setting. Srivastava says this problem must still be addressed. But he and his colleagues have solved one big piece of the quantum communication puzzle: how to efficiently share entanglement around the world.
“The transferring part,” Srivastava says, “we think that’s doable.”
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