For quantum computer, add a dash of disorder

Flawed crystals could help couple light to matter

Embracing chaos just might help physicists build a quantum brain. A new study shows that disorder can enhance the coupling between light and matter in quantum systems, a find that could eventually lead to fast, easy-to-build quantum computers.

FROM DISORDER, ARRANGEMENT Light bouncing around a disordered crystal spontaneously arranged itself in bright spots, represented by the tall spikes in this figure. L. Sapienza

Quantum computers promise superfast calculations that precisely simulate the natural world (SN Online: 1/22/10), but physicists have struggled to design the brains of such machines. Some researchers have focused on designing precisely engineered materials that can trap light to harness its quantum properties. To work, scientists have thought, the crystalline structure of these materials must be flawlessly ordered — a nearly impossible task.

The new study, published in the March 12 Science, suggests that anxious physicists should just relax. A group of researchers at the Technical University of Denmark in Lyngby have shown that randomly arranged materials can trap light just as well as ordered ones.

“We took a very interesting, different approach: relaxing all these ordered structures and using disorder” as a resource, says study coauthor Peter Lodahl. “Let it play with you instead of playing against you.”

One approach to quantum computing relies on entangling photons and atoms, or binding their quantum states so tightly that they can influence each other even across great distances. Once entangled, a photon can carry any information stored in the atom’s quantum state to other parts of the computer. To get that entangled state, physicists pin light in tiny cavities to increase the likelihood of quantum interaction with neighboring atoms.

Lodahl and his colleagues didn’t set out to trap light. They wanted to build a waveguide, a structure designed to send light in a particular direction, by drilling carefully spaced holes in a gallium arsenide crystal. Because the crystal bends light much more strongly than air does, light should have bounced off the holes and traveled down a channel that had been left clear of holes.

But in some cases, the light refused to move. It kept getting stuck inside the crystal.

“At first we were scratching our heads,” Lodahl says. “Then we realized it was related to imperfections in our structures.” If imperfect materials could trap light, Lodahl thought, then physicists could couple light and matter with much less frustration.

To see if disorder could help materials trap light, Lodahl and colleagues built a new waveguide, this time deliberately placing the holes at random intervals. They also embedded quantum dots, tiny semiconductors that can emit a single photon at a time, in the waveguide as a proxy for atoms that could become entangled with the photons.

After zapping the quantum dots with a laser to make them emit photons, the researchers found that 94 percent of the photons stayed close to their emitters, creating spots of trapped light in the crystal. That’s about as good as previous results using more precisely ordered materials. Intuitively, physicists expect light to scatter in the face of disorder, but in this case colliding light waves built each other up and collected in the material.

The quantum dots also emitted photons 15 times faster after a light spot formed around them.

“This is the essence of our discovery: We used localized modes not just to trap light but to enhance interaction between light and matter,” Lodahl says.

That’s the first mile marker on the road to entanglement, notes Diederik Wiersma, a physicist at the European Laboratory for Non-linear Spectroscopy in Florence, Italy. “It has not been achieved as quantum entanglement yet, but it’s the important step that everyone has to make to get there.”

The system produced several separate light traps at once. If the light traps can be entangled with each other, the system could someday lead to a quantum network in a randomly organized crystal.

Wiersma thinks of the potential product as a “quantum brain.” Like a human brain, a quantum brain is not a perfectly ordered structure, he says. “Nature doesn’t need a symmetric structure. It just needs your brain to be working.”

Lisa Grossman

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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