A new method of cooling membrane with lasers

Using a new technique, researchers at the University of Basel have managed to cool a small particle to near-zero temperatures using only laser light. Such supercooled membranes could, for example, find use in highly sensitive sensors.

400 years ago, German astronomer Johannes Kepler came up with the idea of ​​solar sails that could be used by ships to navigate the universe. He suspected that light emits energy when something is visible. This theory also helped him explain why the tails of comets point away from the sun.

Today, scientists use the energy of light, among other things, to slow and cool atoms and particles. Usually, one needs complex tools to do this. A group of researchers at the University of Basel led by Prof. Dr. Philipp Treutlein and Prof. Dr. Patrick Potts has now managed to cool a thin film to a near-zero temperature of 273.15 degrees Celsius without using anything. but laser light. They recently published their results in a scientific journal Physical Review X.

Comment without measurement

“What makes our method unique is that we achieve this cooling without making any kind of measurement,” says physicist Maryse Ernzer, PhD student and first author of the research paper. According to the laws of quantum mechanics, measurement, as required for long-term transitions, leads to changes in the quantum state and causes disturbances. To avoid this, the Basel scientists have developed a so-called coherent feedback loop in which the laser light acts as both a sensor and a damper. In this way they relax and cool the thermal vibrations of the membrane made of silicon nitrate around half a millimeter in size.

In their experiment, the researchers directed a laser beam onto a membrane and fed the light reflected by the membrane into a fiber optic cable. In doing so, the vibration of the membrane caused a slight change in the oscillation phase of the visible light. Information about the instantaneous movement of the membrane in the oscillation phase was used, with a time delay, to apply the right force on the membrane at the right time with the same laser light. “It’s like slowing down the vibration by gently touching the ground with one’s feet at the right time,” explains Ernzer. To achieve the ideal delay of about 100 nanoseconds, the researchers used a 30 meter long fiber optic cable.

Close to zero

“Professor Potts and his colleagues made a theoretical description of this new method and calculated the settings that we can expect to achieve the highest temperature; which was confirmed by the experiment”, says Dr. Manel Bosch Aguilera, who supported the study as a postdoc. He and his colleagues were able to cool the membrane down to 480 micro-Kelvin – less than a thousand degrees above zero temperature.

In the next step, the researchers want to control their experiment until the membrane reaches a very low temperature – the limit of the mechanical rotation of the membrane, that is. After that, it should be possible to make what are called compressed membranes. Such states are very interesting for building sensors because they allow their measurement to be very precise. Possible applications of such sensors are atomic force microscopes, which are used to analyze nanometer-sized surfaces.

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