Scientists use lasers to cool membrane to record-low temperature

Researchers at the University of Basel have achieved a remarkable feat by employing a novel technique to cool a small membrane to temperatures nearing absolute zero. This groundbreaking advancement utilizing laser light holds significant potential for the development of highly sensitive sensors.

The concept of solar sails, initially proposed by the German astronomer Johannes Kepler four centuries ago, revolves around the notion that light exerts a force when it interacts with an object. This force explains why the tails of comets align away from the sun.

In contemporary times, scientists harness the force of light to decelerate and cool atoms and particles, typically relying on intricate setups. However, a team of researchers, led by Prof. Dr. Philipp Treutlein and Prof. Dr. Patrick Potts at the University of Basel, has achieved a major breakthrough. They have successfully cooled an ultrathin membrane to temperatures approaching absolute zero (-273.15 degrees Celsius) using laser light alone. Their remarkable findings have been recently published in the esteemed journal Physical Review X.

Feedback without measurement

Physicist Maryse Ernzer, a Ph.D. student and the primary author of the research paper, emphasizes the uniqueness of their approach by stating, “Our method stands out because it accomplishes cooling without requiring any form of measurement.” Traditional feedback loops necessitate measurements, which, according to the principles of quantum mechanics, alter the quantum state and introduce disturbances.

To circumvent this issue, the scientists from Basel devised a coherent feedback loop that leverages laser light as both a sensor and a damper. Through this method, they successfully dampened and cooled the thermal vibrations of a half-millimeter-sized membrane composed of silicon nitrate.

In their experimental setup, a laser beam was directed onto the membrane, and the light reflected by the membrane was captured in a fiber optic cable. As the membrane vibrated, it induced small changes in the oscillation phase of the reflected light. This oscillation phase contained information about the real-time motion state of the membrane, which was then utilized, with a slight time delay, to apply precise force on the membrane at the appropriate moment using the same laser light.

Ernzer compares this process to slowing down a swing by briefly touching the ground with one’s feet at the right moment. To achieve an optimal delay of approximately 100 nanoseconds, the researchers employed a 30-meter-long fiber optic cable.

Close to absolute zero

Dr. Manel Bosch Aguilera, a postdoctoral researcher who contributed to the study, highlights the involvement of Professor Potts and his collaborators in developing a theoretical framework for the new technique. Through their calculations, they determined the optimal settings that would lead to the lowest achievable temperatures. These predictions were subsequently confirmed by the experimental results.

The team successfully cooled the membrane to an impressive temperature of 480 micro-Kelvin, which is less than one-thousandth of a degree above absolute zero.

Moving forward, the researchers have set their sights on further refining their experiment to reach the membrane’s quantum mechanical ground state—the lowest temperature achievable for its oscillations. Additionally, they aim to generate squeezed states of the membrane, which hold great potential for constructing highly accurate sensors. These sensors could find applications in various fields, including atomic force microscopy, enabling the scanning of surfaces with nanometer-level precision.

Source: University of Basel

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