Quantum computers promise superfast calculations that precisely simulate the natural world, 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.
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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.
quantum_peaksAfter 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.
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