Entangled photons make better messengers

Quantum communication gets a boost from a six-pack of light

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

SPOOKY LIGHT Using a laser (shown), physicists at Stockholm University in Sweden have produced six photons bound in a quantum state that could enable safe transmission of information over long distances. Courtesy of Magnus Rådmark/Stockholm University

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.

“This is an exciting landmark in experimental capabilities,” comments physicist Aephraim Steinberg of the University of Toronto, who was not involved in the work. Creating the six-photon entanglement is an impressive technical achievement, he says. “This is the first demonstration of such large entangled states” with high quality.

Quantum communication offers an absolutely secure way to send secret messages, such as encoded military secrets or financial transactions. But quantum information is fragile, quickly destroyed by even slight interactions with the environment.

While a conventional bit of information can have only one value, 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, whether a stray atom or a scientist trying to measure its properties, 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.

Fortunately, theorists have shown that some quantum-mechanical systems are immune to certain interactions. One of these resilient systems is a set of four or more photons that are intimately bound, or entangled, a property of quantum systems that links particles’ fates even when they are separated by large distances.

Delicate quantum bits find safety in numbers. The more photons are entangled, the more data can be encoded and transmitted reliably. Four photons can encode one robust qubit of information, and six photons can encode two, theorists have calculated.

Now, a team of physicists led by Magnus Rådmark of Stockholm University has experimentally demonstrated a set of six entangled photons that can fly down flawed, noisy fiber-optic cables and emerge unscathed.

“You’ll get exactly the same state out as you sent in, even if the fiber is being stressed and the temperature is changing, and all of the environmental factors that would normally make it a no-go,” Steinberg says.

The key to preserving the state is to make sure all six photons are altered in exactly the same way. 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 close 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.”

As a bonus, this property means that the sender and receiver don’t need to agree on which way is up. Changing the reference frame is just another rotation, the same kind of noise the photons ignored in the fiber.

The photon sextet could also be useful in quantum computing, which could in principle manipulate entangled qubits to solve certain problems that are impossibly difficult for conventional computers.

Lisa Grossman is the astronomy writer. She has a degree in astronomy from Cornell University and a graduate certificate in science writing from University of California, Santa Cruz. She lives near Boston.

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