Researchers at the University of Basel have developed a new technique that successfully cools a small membrane to temperatures close to absolute zero, using only laser light. Such highly cooled membranes may find application in highly sensitive sensors, for example.

A century ago, or more precisely, about 400 years ago, the famous German astronomer Johannes Kepler dreamed up the concept of solar sails. He believed that these sails could propel ships through space. Kepler proposed that light reflected from an object generates a force. This idea also provided an explanation for the fact that the tails of comets always point away from the sun.

Today, scientists use the power of light to slow and cool atoms and other particles, among other things. As a rule, this requires a complex apparatus. Professor at the University of Basel, Dr. Philip Troitlein and Professor Dr. A group of researchers led by Patrick Potts has managed to cool a very thin membrane to near absolute zero—minus 273.15 degrees Celsius—without using anything. but laser light. They recently published their findings in a scientific journal. Physical Examination X.

unmeasured feedback

“The peculiarity of our method is that we get the cooling effect without any measurements,” says physicist Maryse Ernzer. student and first author of a scientific paper. According to the laws of quantum mechanics, measurement normally required in a feedback loop leads to a change in the quantum state and thus to a perturbation. To avoid this, the Basel scientists developed a so-called coherent feedback loop, in which the laser light acts both as a sensor and a damper. In this way, they damped and cooled the thermal fluctuations of the silicon nitrate membrane, which is about half a millimeter in size.

In their experiment, the researchers aimed a laser beam at the membrane and fed the light reflected from the membrane into a fiber optic cable. In this process, the vibrations of the membrane caused minor changes in the vibrational phase of the reflected light. Information about the instantaneous state of membrane motion present in this phase of oscillation was then used with a time delay to apply the right amount of force to the membrane at the right time with the same laser light.

“It’s like slowing down a swing by briefly tapping your feet on the ground at the right moment,” Ernzer explains. The researchers used a 30-meter fiber optic cable to achieve the optimum delay of about 100 nanoseconds.

close to absolute zero

“Professor Potts and colleagues developed a theoretical description of the new technique and calculated the parameters we can expect to reach the lowest temperatures, which was later confirmed by experiment,” said Dr. Manel Bosch Aguilera. He and his colleagues were able to cool the membrane down to 480 microKelvin, which is less than a thousandth of absolute zero.

In the next step, the researchers carried out their experiments to the lowest possible temperature of the membrane – the quantum mechanical ground state of the membrane’s oscillations, ie. After that, it should also be possible to create so-called compressed states of the membrane. Such cases are of particular interest for the creation of sensors, as they allow higher measurement accuracy. Possible applications of such sensors include atomic force microscopes used to scan surfaces at nanometer resolution. Source