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Entangling Appliance: Solid-State Memories Pave the Way to Practical Quantum Communication

Two groups of physicists have managed to shift the quantum entanglement between two photons onto an entangled state between one photon and a quantum memory
Quantum memory in a crystal



Wolfgang Tittel/University of Calgary

Full-scale quantum computers, with all the number crunching, code cracking and jaw-dropping processing power researchers expect them to deliver, remain a mere twinkle in the eye of physicists and computer scientists. It is a twinkle supported by promising experimental and theoretical work, but a twinkle nonetheless—to date only rudimentary quantum processors have been built.

Such computers would harness the physical properties of quantum bits, or qubits, to expand the reach of computation. Whereas ordinary bits can be 0 or 1, a qubit can be in a superposition of 0 and 1 simultaneously—and qubits can be mutually entangled, meaning that their properties are linked.

Along the road to building a full-fledged quantum computer lies a critical experimental milestone that would be a significant achievement in its own right: the construction of a simpler device called a quantum repeater, essentially a relay station for qubits that could enable quantum communication systems, an analogue to the fiber-optic telecommunications already in wide use. These systems would harness the same properties as quantum computers to transmit information, encoded on photon qubits, over long distances. With a slew of advances in the past year or so, the latest of which are reported in a pair of complementary papers published online January 12 in Nature, researchers now say that quantum repeaters could be within reach within five to 10 years. (Scientific American is part of Nature Publishing Group.)

Two groups of physicists have devised quantum memories from crystals laced with rare earth elements that are capable of storing an entangled photon and then releasing it a short time later. That kind of memory would be useful for long-distance quantum cryptography, for instance, in which individual photons from an entangled pair are sent to two parties who wish to share a unique and secure link. The photons would act as a kind of shared cryptographic key, but the problem is that the individual photons have a maximum range of about 100 kilometers in an optical fiber.

"The solution to extend the range is repeaters," explains Harvard University physicist Mikhail Lukin, who did not contribute to the new studies, "which are essentially small quantum computers that you insert along this channel, and they help to clean up and purify the entangled states along the way." To do that, researchers need a material memory that is capable of taking in a photonic qubit, holding on to it for a quick rejuvenation, and then reemitting it with its quantum properties intact. With a repeater every 10 kilometers, say, each photon could be repeatedly refreshed along its journey before it had a chance to degrade.

Although the two studies differ in specifics, both are notable for shifting an entangled state between two photons into an entangled state between one of those photons and a group of atoms in a solid. Such quantum cross-pollination has been achieved before, but it usually involves highly technical setups such as single trapped atoms cooled by lasers to a fraction of a kelvin (0 kelvin is absolute zero). The crystal memories, on the other hand, need only be cooled to a few K by more conventional means.

"Laser-cooled atoms, I think it's great science, but I have the feeling that it's a little too complicated to move into real applications," says Wolfgang Tittel of the University of Calgary in Alberta, one of the studies' co-authors, who says that the closed-cycle coolers required for crystal memories are much closer to an off-the-shelf technology. "Once you buy it, you have it, and ours was running for an entire year," he says. Mikael Afzelius of the University of Geneva in Switzerland, a co-author of the other study, sounds a similar tone. "Our solid-state approach could prove to be more scalable," he says, "which would be important for future quantum networks."

Tittel's group employed a crystal doped with thulium, a rare earth metal, that can hold on to a photon for seven nanoseconds before releasing it again, quantum state intact. The photon, once absorbed into the crystal memory, creates an excitation in a collection of atoms within the memory that then reproduces the photon a short time later in a sort of optical echo. The group demonstrated that the echo photon retained its entanglement with the original photon's mate.

The other group achieved a similar feat with a crystal doped with neodymium, another rare earth metal. Afzelius and his colleagues achieved longer storage times of up to 200 nanoseconds, but their experiment did not allow them to verify entanglement between the two photons once one of them was stored in and retrieved from the memory.

"I think this is a significant advance," Lukin says, praising in particular the broadband nature of the crystals, which would allow them to receive very rapid pulses of light. He notes that several improvements in the crystal-memory technology would have to be made before a practical quantum memory for a repeater could be implemented: longer storage times, higher efficiencies and the ability to recall a photon on demand rather than after a predetermined time.

When asked independently, Lukin, Tittel and Afzelius all give roughly the same answer for how far away a prototype quantum repeater might be: five years, maybe 10. "If you had asked me three or four years ago, I would have said a very long time," Tittel says. But with a handful of major quantum-information science papers appearing just in 2010, things are looking up. "Everything is pretty much there, but everything has to be improved," he says. "From a fundamental point of view, I can see which way to go."

Adds Afzelius: "I think we are looking at five to 10 years before we will see laboratory experiments that have progressed enough to attempt a first practical quantum repeater link between, let's say, two cities. From that point it will be an engineering challenge to make our experimental systems more compact and affordable, but I believe that is within reach with current technologies."

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