Between Light and Mechanics, A Quantum Connection

In a new microscopic system, light can be converted into a mechanical oscillation and then back to light in an interaction so strong that it allows control of the oscillator motion at the level where quantum mechanics governs its behavior.

Physicists at the EPFL’s Laboratory of Photonics and Quantum Measurement have shown that it is possible to control the motion of an object that is large enough to be seen with the naked eye at the level where quantum mechanics dominates. They achieved this by illuminating the object with laser light.

The structure, a carefully crafted 30-micron-diameter glass donut on a microchip, can vibrate at a well-defined frequency, acting as a racetrack for light, which can travel the circumference of the donut. In turning the bend, the light exerts a little force on the glass surface, an effect called “radiation pressure.”

Electron microscopy image of the glass donut, which is smaller than the diameter of a hair. It is connected to the supporting chip by four spokes to ensure that the structure can vibrate for a long time, like a good tuning fork. Light can circulate around the circumference of the donut up to a million times. As the light bounces against the walls of the structure, it exerts a small force on the glass, which can influence the vibrations of the structure. (Image: © K-Lab/EPFL)

Although this force is very small, in these structures it can become appreciable, since light circles the structure up to a million times before being lost. The radiation pressure force can make the ring move, causing it to vibrate like a finger running along the rim of a wineglass. But it can in fact also dampen the vibrations and thus diminish the oscillatory motion.

Cooling is crucial to reach the regime of quantum mechanical motion, as this is normally overshadowed by random thermal fluctuations. For this reason, the structure is brought to a temperature of less than 1 degree above absolute zero. Then, radiation pressure damping by laser light launched into the donut cools the motion down by an extra factor of 100. The oscillator is cooled so much that it spends a large fraction of the time in its quantum ground state.

But even more importantly: The interaction between light and the movement of the oscillator can be made so strong that the two form an intimate connection. A small excitation in the form of a light pulse can fully transform into a small vibration and back again.

For the first time, this transformation between light and motion occurs within a time period short enough that the quantum properties of the original light pulse are not lost in the process through decoherence. By outpacing decoherence, the current results provide a powerful way to control the quantum properties of the oscillator motion, and to see the peculiar predictions of quantum mechanics at play in manmade objects.

The results are published in this week’s issue of Nature.

For more information, visit: www.epfl.ch