Light Slows at the Nanoscale

It sounds like a magic trick: slowing down light, often to a standstill. Yet physicists have done it many times in a variety of media, from gases to diamonds. Now a U.S. group has achieved the feat on nanoscale silicon chips—an advance that could be a step toward building "quantum" networks that are fundamentally secure.

Some of the most effective ways to slow light came in the late 1990s. Researchers were learning how to take an opaque medium—typically a gas of atoms—and turn it transparent by zapping it with a laser tuned to a certain frequency. The laser would jolt the gas's atoms into a new energy state in which they could no longer absorb light and had to let it pass—the definition of transparency.

Such "electromagnetically induced transparency," (EIT) was key to slowing down light. In 1999, researchers in the United States discovered that, in a cooled gas of sodium atoms, EIT could slow down a pulse of light to just 17 meters per second—some 20 million times slower than its speed in a vacuum. Two years later, researchers went a step further and switched off the control laser during the EIT process, bringing the pulse to a standstill. For a fraction of a second, the pulse effectively froze in an atomic gas, to be released only when the laser was switched back on.

Strictly speaking, what slows down in EIT is not the photons themselves, but the "group velocity" of light: the average speed of the wave packets that transmit the light's energy. Even this limited sort of slow or stopped light could find uses in signal processing, however—particularly in the so-called quantum memories scientists will need to develop quantum networks.

Quantum networks exploit the inherent fuzziness of quantum mechanics to transmit information that is fundamentally secure from eavesdroppers. Unlike normal information, quantum information becomes corrupt as soon as it is measured, and for this reason memories need to store it without actually doing any recording. Because light is often the carrier of choice for quantum information, slow light could be an ideal quantum memory, but the atomic gases used for EIT are hard to integrate with normal electronics.

Physicist Oskar Painter and colleagues at the California Institute of Technology in Pasadena have now demonstrated slow light in a medium that should be far easier to integrate: a nanoscale silicon chip. The researchers etched tiny holes onto the chip and then cooled it down to just 9° above absolute zero. When they shined a laser onto the chip, it deformed the holes in a way that made their structure intimately linked to the light.

To perform EIT in this system, Painter's group shined another laser onto the chip. The presence of both lasers caused a "beating" in their beams, which made the nanoscale holes vibrate like a tuning fork. The oscillations stored the second laser's light, just as an atomic gas would do. The researchers say the light was effectively slowed to about 40 metres per second, or 8 million times slower than its speed in a vacuum.

"In my opinion, this is a very exciting development," says Hugues de Riedmatten, an expert in EIT at the Institute of Photonic Sciences in Barcelona, Spain. De Riedmatten calls the experiment an "important step" toward viable quantum memory but adds that it doesn't slow light as much as some other techniques do.

Indeed, Painter's group isn't the first to demonstrate EIT in an "optomechanical" system that blends light with mechanics. But it is probably the first optomechanical system to use EIT to slow light substantially, a development that could be crucial for quantum networks.