Either autonomous feedback controls floating motion of nanoparticles through a cavity or measurement-based feedback by external forces. Recent experiments on measurement-based cooling of a single nanoparticle in the ground state employ linear velocity feedback (also known as cold damping) and call for the application of electrostatic forces to charged particles via external electrodes.

At ETH’s Department of Information Technology and Electrical Engineering, a group of researchers led by Professor Lukas Novotny has developed a method to capture and cool multiple nanoparticles to a few millikelvins independently of their electrical charge.

Jayadev Vijayan, a postdoc in Novotny’s lab and lead author of the paper, said: “In our research group, we have perfected the cooling of a few electrically charged nanoparticles over the past ten years. With the new method, which also works for electrically neutral objects, we can now capture several particles at the same time for the first time, which opens up completely new perspectives for research.”

In their tests, the researchers used optical tweezers, commonly known as a sharply focused laser beam, to capture a tiny glass sphere less than 200 nanometers in size in a vacuum device. Due to its kinetic energy, the sphere oscillates back and forth in the optical tweezers.

The amplitude of the oscillation increases with the particle’s kinetic energy, which increases with the particle’s temperature. A light detector, which receives the laser light scattered by the sphere, can determine how strongly and in which direction the sphere oscillates at a given moment in the optical tweezers.

Using this information, scientists slowed down and cooled the nanoparticle. This is achieved by using an electronically controlled deflector to slightly change the direction of the laser beam, which in turn changes the location of the optical tweezers by shaking it in the exact opposite direction of the sphere’s oscillation.

To counteract the movement of the sphere as it moves to the left, the tweezers are quickly switched to the right; when it moves to the right, the deflector shifts the tweezers to the left. By doing so, the amplitude of the oscillation and, consequently, the effective temperature gradually decreases to several thousandths of a degree above absolute zero (-273.15 degrees Celsius).

The scientists used an innovative technique to cool two nanoparticles at the same time. They tweak the optical tweezers they use to capture the spheres so that the oscillation frequencies of the individual particles vary slightly. In this method, the movements of the two spheres can be differentiated using the same light detector and the two tweezers can each be given a different cooling strategy.

Vijayan explains, “The simultaneous cooling can easily be scaled up to a few nanoparticles. Since we have full control over the positions of the particles, we can arbitrarily tune the interactions between them; that way we can study quantum effects of multiple particles, such as entanglement, in the future.”

The study could lead to the study of quantum phenomena of such particles or to build sensitive sensors.

Magazine reference:

  1. Vijayan, J., Zhang, Z., Piotrowski, J. et al. Scalable all-optical cold damping of suspended nanoparticles. Wet. Nanotechnology. 18, 49-54 (2023). DOI: 10.1038/s41565-022-01254-6