In the Basel experiment, a laser beam is directed at a membrane (a square in the middle). Using reflected laser light, delayed by a (violet) fiber optic cable, the membrane is then cooled to less than a thousand degrees above absolute zero. Credit: University of Basel, Department of Physics
Researchers from the University of Basel have developed a new technique that has successfully cooled a small membrane to temperatures close to that absolute zero using only laser light. Such highly cryogenic membranes could, for example, find applications in highly sensitive sensors.
Centuries ago, about 400 years to be exact, the famous German astronomer Johannes Kepler conceived of the idea of solar sails. It was believed that these sails could propel ships across the universe. Kepler assumed that light, when reflected by an object, produces a force. This idea also provided an explanation for the phenomenon of a comet’s tail always pointing away from the sun.
Nowadays, scientists use the power of light to, among other things, slow down and cool atoms and other particles. Normally, one would need a complex device to do so. A team of researchers at the University of Basel led by Prof. Dr. Philipp Trotlin and Prof. Dr. Patrick Potts has succeeded in cooling a thin film to a temperature close to absolute zero of minus 273.15 degrees. Celsius using nothing but laser light. They recently published their findings in the scientific journal X physical review.
Unmeasured feedback
“What makes our method special is that we achieve this cooling effect without making any kind of measurement,” says physicist Marise Ernzer, PhD. Student and first author of the research paper. According to the laws of quantum mechanics, a measurement, as is usually required in a feedback loop, leads to a change in the quantum state and thus to perturbations. To avoid this, the Basel scientists developed a so-called coherent feedback loop in which laser light acts as both a sensor and a damper. In this way, they cooled and cooled the thermal vibrations of a silicon nitrate membrane about half a millimeter in size.
In their experiment, the researchers directed a laser beam at the membrane and beamed the light reflected from the membrane onto a fiber-optic cable. In the process, the vibrations of the membrane caused subtle changes in the oscillatory phase of the reflected light. Information about the instantaneous kinetic state of the membrane in that oscillation phase was then used, with a time delay, to apply the appropriate amount of force to the membrane at the appropriate moment with the same laser light.
“This is a bit like slowing down a swing by touching the ground briefly with one’s foot at the right time,” explains Ernzer. To achieve an optimal delay of about 100 nanoseconds, the researchers used a 30-meter-long fiber-optic cable.
close to absolute zero
“Professor Potts and his collaborators developed a theoretical description of the new technology and calculated the settings at which we could expect to achieve lower temperatures; this was then confirmed by experiment,” says Dr Manel Bosch Aguilera, who contributed to the study as a postdoctoral researcher. He and his colleagues managed to cool the membrane to 480 microkelvins — less than a thousandth of a degree above the temperature of absolute zero.
In the next step, the researchers want to improve their experiment so that the membrane reaches the lowest possible temperature—the quantum mechanical ground state of the membrane’s oscillations, that is. Subsequently, it should also be possible to create the so-called compressed states of the membrane. These cases are particularly interesting for building sensors because they allow for higher scaling Accuracy. Possible applications for such sensors include atomic force microscopes, which are used to scan surfaces with nanometer resolution.
Reference: “Optical Coherent Feedback Control of a Mechanical Oscillator” By Marise Ehrenser, Manel Bosch-Aguilera, Matteo Brunelli, Gian-Luca Schmid, Thomas M. Karge, Christoph Broder, Patrick B. Potts, and Philip Treutlin May 15, 2023, Available Here. X physical review.
DOI: 10.1103/PhysRevX.13.021023