Best quantum messages travel in packs of six: experiment improves ability to send entangled photons.

To send a quantum message, it helps to have a photon six-pack.

When bound together by a process called quantum entanglement, a set of six photons can withstand the hard knocks that ordinarily would erase quantum information, researchers have shown.

Papers describing the new experiment appear in the Oct. 9 Physical Review Letters and the October Physical Review A. A team led by Magnus Radmark of Stockholm University has experimentally demonstrated that a set of six entangled photons can fly down noisy fiber-optic cables and emerge unscathed.

Quantum communication offers an absolutely secure way to send secret messages, such as encoded military secrets or financial transactions. While a conventional bit of information can have a value of only 0 or 1, a quantum bit, or qubit, exists as a combination of 0 and 1 simultaneously. A qubit stays in this undecided state until something interacts with it, forcing it into a single state. This collapse of possibilities, known as quantum decoherence, can be detected farther down the line to catch eavesdroppers. But it can also keep qubits from reaching their destination intact.

[ILLUSTRATION OMITTED]

A set of four or more photons that are entangled--a property of quantum systems that links particles' fates even when they are separated by large distances--would be immune to certain interactions. Temperature changes around the fiber-optic cable can alter the way it bends light, which in turn can rotate photons unpredictably. But if the photons travel in a tight pack, they will all feel the same twists and bends.

"If I take all six photons and rotate them in the same way, I will get exactly the same state I started with," says Mohamed Bourennane of Stockholm University, a coauthor on the papers. "It's like nothing has happened."