Near Absolute Zero: Cooling of a Small Membrane through Laser Techniques

In an experiment conducted in Basel, a membrane is targeted with a laser beam (central square). The light reflected by the laser is delayed via an optical fiber (in violet), enabling the cooling of the membrane to less than one-thousandth of a degree above absolute zero. The experiment was executed by the Physics Department at the University of Basel.

A team of scientists from the University of Basel has devised a novel method that has successfully cooled a tiny membrane to temperatures approaching absolute zero using laser beams. Such membranes, once cooled to these extreme levels, may be applied in the development of exceptionally sensitive sensors.

Around four centuries ago, the renowned German astronomer Johannes Kepler conceived the idea of solar sails. Kepler’s belief was that these sails could be used to propel vessels across the universe. He theorized that a force is created when light is reflected off an object, and this concept explained why comet tails are always oriented away from the sun.

In contemporary times, scientists utilize light force to decelerate and cool atoms and other particles, often requiring complex equipment. A research group at the University of Basel, guided by Prof. Dr. Philipp Treutlein and Prof. Dr. Patrick Potts, has managed to cool a slender membrane to a temperature almost reaching absolute zero, -273.15 degrees Celsius, solely with the use of laser light. The findings were recently shared in the scientific publication Physical Review X.

Unique Cooling Without Observation

“Our method stands out as we effectuate this cooling without any form of measurement,” asserts physicist Maryse Ernzer, the lead author of the study. According to quantum mechanics, measurements—commonly essential in feedback loops—result in alterations of the quantum state, thereby causing disruptions. The Basel team innovated a coherent feedback loop where the laser light serves as both a sensor and a stabilizer. This method allowed them to reduce and cool the thermal vibrations of a silicon nitrate membrane of about half a millimeter.

During the experiment, the scientists focused a laser beam onto the membrane, leading the reflected light into a fiber optic cable. The membrane’s vibrations slightly altered the phase of the reflected light’s oscillations. This alteration in phase, embodying information on the membrane’s immediate motion, was then utilized with a time lag to exert precise force on the membrane at the proper moment using the same laser light.

Ernzer likens this to slowing down a swing by momentarily touching one’s feet to the ground at the right instant. To accomplish the best delay of approximately 100 nanoseconds, a 30-meter-long fiber optic cable was employed.

Achieving Temperatures Almost Absolute Zero

“Professor Potts and his team created a theoretical model of the technique and determined the settings that would likely reach the lowest temperatures; this was subsequently confirmed experimentally,” explains Dr. Manel Bosch Aguilera, a postdoctoral contributor to the study. The researchers managed to cool the membrane down to 480 micro-Kelvin, a temperature less than one-thousandth of a degree above absolute zero.

Next, the scientists aim to refine their experiment to reach the membrane’s lowest feasible temperature—its quantum mechanical ground state. Following this, it could be feasible to create specific squeezed states of the membrane, which are intriguing for creating sensors that provide improved measurement accuracy. Such sensors might be utilized in applications like atomic force microscopes, which enable surface scanning with nanometer precision.

Reference: “Optical Coherent Feedback Control of a Mechanical Oscillator” by Maryse Ernzer, Manel Bosch Aguilera, Matteo Brunelli, Gian-Luca Schmid, Thomas M. Karg, Christoph Bruder, Patrick P. Potts, and Philipp Treutlein, published on 15 May 2023 in Physical Review X. DOI: 10.1103/PhysRevX.13.021023

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• Physical Review X
• University of Basel, Department of Physics