Tiny quantum storage device fits on a chip

A newfangled data storage device, which takes up less than a millionth the amount of space of its predecessors, could be a key component of futuristic communication systems.

Scientists fashioned a tiny crystal that stores snippets of quantum information — which unlike computer data “bits” that come only in 0s and 1s, can also exist as both 0 and 1 simultaneously. This crystal is the first quantum memory device of its kind that could fit on a chip alongside nano-sized instruments for detecting and sending signals written in quantum bits. This work, reported online August 31 in Science, improves prospects for establishing a widespread, ultrasecure network of quantum communications.

Crystal quantum memory devices hoard data by absorbing photons, each of which carry one quantum bit, or qubit, of data. Generally, the bigger the chunk of crystal, the greater the chance that one of its atoms will absorb a photon streaking through the material. For a typical, cube-shaped crystal to snare photons as well as the new storage device, it would have to be about a millimeter across, says study coauthor Tian Zhong, a molecular engineer at the University of Chicago. But the crystal that Zhong and colleagues built is only about 10 micrometers long and 0.7 micrometers wide — about as wide as a bacterial cell.

The secret to this storage device’s tiny size is its shape, which resembles a Toblerone chocolate bar: a triangular prism with notches etched along the top. “The grooves towards the ends of the [crystal] collectively behave as two mirrors, one on each side,” Zhong says.

Photons that enter the crystal at one end bounce back and forth between these “mirrors” a few thousand times before they can escape, which increases their likelihood of getting absorbed by an atom along the way. So the crystal’s serrations helped it capture a decent number of photons despite its small number of atoms.

Zhong’s team tested their crystal’s quantum storage capabilities by using an optical fiber to inject it with one bunch of photons, and then another. The quantum bit of information carried on each photon was 0 if it belonged to the earlier pulse, 1 if it belonged to the later pulse, or 0 and 1 simultaneously if (thanks to the bizarre rules of quantum mechanics) it belonged to both the earlier and later pulse.

Some of the photons that whizzed through the crystal got absorbed by neodymium atoms, each of which could hold onto a photon for about 75 nanoseconds. Once these photons escaped, they zipped back out of the crystal, through the optical fibers whence they came. So the researchers saw one blip of light come out of the crystal, followed by another.

When Zhong and colleagues compared the two original bursts of light to the echoes that emerged from the crystal, they found that, for the most part, each photon had preserved its original identity as a 0, 1, or both. “This is what demonstrates that the device can perform quantum memory,” says study coauthor Andrei Faraon, a physicist at Caltech. “It doesn’t destroy that information.”

This kind of supersmall memory device could help expand the reach of future networks of quantum computers, says Jevon Longdell, a physicist at the University of Otago in New Zealand who wasn’t involved in the work.

Photons carrying quantum data through fiber-optic cables have a tendency to get lost along the way, so messages encoded in quantum bits can’t make nonstop trips longer than about 100 kilometers, Longdell says. But small chips packing photon detectors, generators and these memory devices could serve as a kind of photon pit stop between optical fibers, to help quantum messages go the distance.