On-chip squeezed light could improve sensors

A microchip-based way to create squeezed light could assist a range of precision measurements and provide a viable route toward real-world on-chip sensor applications and technology.

Monitoring a mechanical object’s motion, even with a touch as gentle as that of light, fundamentally alters its dynamics. Squeezed light, with its quantum fluctuations below that of the vacuum field, was proposed nearly three decades ago as a way of overcoming the standard quantum limits in precision force measurements.

Squeezed light was recently generated in a system of ultracold gas-phase atoms, engineered at the California Institute of Technology (Caltech); the system is a solid-state, optomechanical system fabricated from a silicon microchip and composed of a micromechanical resonator coupled to a nanophotonic cavity.

“We work with a material that’s very plain in terms of its optical properties,” said graduate student Amir Safavi-Naeini. “We make it special by engineering or punching holes into it, making these mechanical structures that respond to light in a very novel way.”

(a) Scanning electron microscope image of the silicon micromechanical resonator used to generate squeezed light. Light is coupled into the device using a narrow waveguide and reflects off a back mirror formed by a linear array of etched holes. Upon reflection, the light interacts with a pair of double nanobeams (micromechanical resonator/optical cavity), which are deflected in a way that tends to cancel fluctuations in the light. (b) Numerical model of the differential in-plane motion of the nanobeams. Photo courtesy of Caltech/Amir Safavi-Naeini, Simon Groeblacher and Jeff Hill

A waveguide feeds laser light into a cavity created by two tiny silicon beams in the new system. Once there, the light bounces back and forth because of the engineered holes, which in effect turn the beams into mirrors that vibrate when photons strike them. The particulate nature of the light introduces quantum fluctuations that affect those vibrations.

Typically, such fluctuations mean that, to get a good signal reading, you would have to increase the power of the light to overcome the noise. But increasing power introduces other problems, such as excess heat. In the new system, the light and beams interact strongly with each other – so strongly that the beams impart the quantum fluctuations they experience back on the light.

In the experiment, a detector measuring the noise in the light as a function of frequency showed that in a range centered around 28 MHz, the system produces light with less noise than what is present in a vacuum – the standard quantum limit.

“But one of the interesting things,” Safavi-Naeini said, “is that by carefully designing our structures, we can actually choose the frequency at which we go below the vacuum.”

“This system should enable a new set of precision microsensors capable of beating standard limits set by quantum mechanics,” said applied physics professor Oskar Painter, senior author of a paper on the work. “Our experiment brings together, in a tiny microchip package, many aspects of work that has been done in quantum optics and precision measurement over the last 40 years.”

The work appears in Nature (doi: 10.1038/nature12307). The researchers expect that, with further improvements, the technology could be used to make integrated microscale devices for precision metrology applications.