Brazilian scientists participate in international research team that cools a nano-object to the lowest possible energy state, paving the way for development of ultrasensitive detectors and quantum experiments (Nature)

Luminous cooling
2011-11-09

Brazilian scientists participate in international research team that cools a nano-object to the lowest possible energy state, paving the way for development of ultrasensitive detectors and quantum experiments.

Luminous cooling

Brazilian scientists participate in international research team that cools a nano-object to the lowest possible energy state, paving the way for development of ultrasensitive detectors and quantum experiments.

2011-11-09

Brazilian scientists participate in international research team that cools a nano-object to the lowest possible energy state, paving the way for development of ultrasensitive detectors and quantum experiments (Nature)

 

By Fábio de Castro

Agência FAPESP
– An international group of researchers which includes Brazilian scientists used a laser to cool a mechanical nano-object to its lowest possible energy state—the so-called zero point energy—for the first time.

According to the authors, in using light to put a solid mechanical system into zero-point energy state—where it behaves accord to the laws  of quantum physics—the study opens pathways to the development of extremely sensitive mass and force detectors as well as opening perspectives for the realization of quantum experiments in macroscopic systems.

The study, published in the October 6 edition of Nature magazine, was carried out by researchers at the California Institute of Technology (Caltech) in the United States together with a team from the University of Vienna, in Austria. Due to its importance, the article received a commentary in the magazine itself.

One of the authors is Brazil’s Thiago Alegre, currently professor in the Applied Physics Department at the Gleb Wataghin Physics Institute at Universidade Estadual de Campinas (Unicamp). Over the last three years, Alegre has been working on his post-doctoral studies at Caltech, after finishing his doctorate at Unicamp with FAPESP funding.

With the use of a laser, the group cooled the mechanical mode of an optical micro cavity to its zero-point energy state. The feat had only been performed before in optical trap systems containing few atoms. 

“We used an optical resource, the laser, to cool a solid macroscopic mechanical system to its lowest possible energy state—which has been a dream for scientists for nearly a decade. It had only been done with a few atoms or ions before, but we managed to do it with a system composed of billions of atoms. The study clears the path for quantum experiments in macroscopic systems like, for example, the quantum entanglement between light and mechanical movement,” said Alegre to Agência FAPESP.

For the experiment, the scientists projected and fabricated a nanometric optical cavity composed of a small silicon beam in which holes measuring some 200 nanometers were carefully positioned. The structure is about 560 nanometers wide and 15 microns long. The micron and nanometer are, respectively, one millionth and one billionth of a meter.

 “This geometry forms an optical cavity where only one frequency—or color—of a laser beam can be confined. The system has the capacity to be a mechanical oscillator, able to also trap phonons—the particles associated with mechanical oscillations, just as photons are associated with electromagnetic oscillations, or light,” he explained.

In confining photons and phonons in one place, the tiny structure intensifies the interactions between mechanical vibrations and light. “It’s an optomechanical system. The light that crosses the cavity, carrying information about the amplitude and oscillation of the system, or the number of phonons, can be associated to the temperature of the oscillation mode,” said Alegre.

In carefully choosing the frequency of the stimulation laser, the researchers can extract mechanical energy through the light that leaves the cavity, cooling the system says the scientist. This way, they create an efficient interface between an optical system and a mechanical system where information can flow from one to the other.

Establishing a “dialogue” between the mechanical and optical worlds has important scientific implications, according to Alegre. In another study published in Nature at the beginning of 2011, the same group showed the effects of the mechanical system on light, an interaction that theoretically makes the creation of optical memories possible.  

“And in the study we just published, we showed the effect of the optical part on the mechanical part. In showing the interaction from both sides, we opened up possibilities for greater control,” affirmed Alegre.

Zero kelvin

One of the resources used by to study on quantum effects in macroscopic scale has been the experiments that use Bose-Einstein condensation—a phase of material formed by atoms at near absolute-zero temperatures. But according to Alegre, to work this way, the first step is to bring the system to its fundamental state, lowering its global temperature to a few dozen millikelvins. 

“Arriving at the fundamental state means working at temperatures near to zero kelvin, which is quite complex and expensive. In our experiment, we didn’t lower the system’s global temperature. We worked with a temperature of around 20 kelvin. Instead of lowering the temperature of the whole system, we created an optical path so that only the vibrational mode would reach near zero kelvin,” he explained.

According to Alegre, the scientists created an escape for the phonons via light. “When it is trapped in the optic cavity, light, which has less energy than the cavity, tends to gain energy, or in other words, to change color. This is only possible if it absorbs mechanical energy from the system, which is then cooled,” he said.

The article Laser cooling of a nanomechanical oscillator into its quantum ground state (doi:10.1038/nature10499), by Jasper Chan, Thiago Alegre and others can be read by Nature subscribers at www.nature.com.
 

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