Fragility of entanglement no bar to quantum secrets

When it comes to quantum entanglement, we can have our cake and eat it. A quantum cryptography scheme works even after the delicate entanglement behind it has been destroyed, suggesting that this spooky property is more practical than we thought.

"Most people think entanglement would be useless if it is destroyed," says Zheshen Zhang at the Massachusetts Institute of Technology. "The most surprising fact is, although the entanglement itself is completely destroyed, the benefit still survives."

Entanglement, a quantum link between disparate objects, can enable ultra-secure cryptography, but even a slight jostle can destroy it, so systems that rely on it are not always useful in practice. Or so it seemed.

A few years ago Jeffrey Shapiro, also at MIT, devised a cryptography scheme in which one person, Alice, entangles two beams of light. She sends one to Bob, and retains the other as a key. Bob changes phase of the beam he receives to mean "yes", or leaves it alone to mean "no", amplifies it and sends it back.

Only Alice, who holds the entangled partner, can work out whether Bob made the change, making the system super secure. Crucially, she should be able do this even if the entanglement itself is gone.

Relic correlations

Now Shapiro's team, including Zhang, has recreated the set-up in the lab. The entanglement was destroyed because of the amplification and transport of the light. However, Alice was still able to use her beam to decode Bob's message, because of relic correlations between the two beams.

Although no longer entangled, photons in the beams are still more likely than chance to share the same state. The experiment revealed that for every million bits Bob sends to Alice, only one will be misinterpreted. "The quantum state really helps even though we end up with no quantumness at all," says Zhang.

Tim Ralph of the University of Queensland in Brisbane, Australia, says: "This is quite counterintuitive and hence interesting and surprising. Environmental noise usually cannot be avoided under practical conditions, so having protocols that work in the presence of environmental noise could be useful."

However, he notes that to combine the beams to reveal the correlations, you need to know precisely how far they have travelled, which could be challenging.

The result might have implications for quantum computing too, where delicate entanglement enables "qubits" to carry out multiple computations in parallel. "This particular example is secure communication, but it implies that entanglement can be used in a more practical way," says Zhang.

Journal reference: Physical Review Letters, DOI: 10.1103/PhysRevLett.111.010501

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