Swiss and Austrian scientists have managed to cool nanoparticles to quantum ground-state in two motional dimensions

Glass nanoparticles trapped by lasers in extreme vacuum are considered a promising platform for exploring the limits of the quantum world.

Since the advent of quantum theory, the question at which sizes an object starts being described by the laws of quantum physics rather than the rules of classical physics has remained unanswered.

A team formed by Lukas Novotny (ETH Zurich), Markus Aspelmeyer (Institute for Quantum Optics and Quantum Information (IQOQI) at the Austrian Academy of Sciences), Oriol Romero-Isart (University of Innsbruck), and Romain Quidant (ETH Zurich) is attempting to answer precisely this question within the ERC-Synergy project Q-Xtreme.

A crucial step on the way to this goal is to reduce the energy stored in the motion of the nanoparticle as much as possible, i.e. to cool the particle down to the so-called quantum ground-state.

The Q-Xtreme team has been working together on ground-state cooling of nanoparticles for a long time.

Several experiments in Zurich and Vienna, supported by theoretical calculations by Dr Gonzalez-Ballestero (IQOQI and University of Innsbruck) and Professor Romero-Isart, have led to the first demonstrations of such ground-state cooling of a nanoparticle, either by dampening the particle motion using electronic control (active feedback) or by placing the particle between two mirrors (cavity-based cooling).

So far in experiments, the ground state has been achieved only along one of the three directions of particle motion, leaving the motion along the other two directions ‘hot’.

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Novel quantum physics


Gonzalez-Ballestero said: “Achieving ground-state cooling along more than one direction is key for exploring novel quantum physics, but so far this achievement remained elusive as it was challenging to make the mirrors between which the particle is positioned interact efficiently with the motion along some of the three directions.”

This is the so-called ‘Dark Mode Effect’ prevented cooling to the full ground state.

Now, the research at the Photonics Laboratory of ETH Zurich has succeeded for the first time in ground-state cooling of a nanoparticle along two directions of motion.

A glass sphere, about a thousand times smaller than a grain of sand, is completely isolated from its environment in a high vacuum and held by a strongly focused laser beam while simultaneously being cooled to near absolute zero.

Based on theoretical predictions from the Innsbruck team, the Swiss physicists were able to circumvent the dark-state problem.

Novotny said: “To do so, we designed the frequencies at which the particle oscillates in the two directions differently and carefully adjusted the polarisation of the laser light.”

The work demonstrates that it is possible to reach the minimum energy state for the three motional directions. It also allows to create fragile quantum states in two directions, which could be used to create ultrasensitive gyroscopes and sensors.

The research is published in Nature Physics.

Image: The vacuum chamber with the experimental setup to levitate a particle inside of a cavity. The cavity consists of two mirrors coated to be extremely reflective for infrared light. The cylindrical part in the centre holds a lens at its tip to focus the infrared laser down to a point at which the particle is trapped. © Johannes Piotrowski.