Quantum cooling in two dimensions

Since the early days 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 (Zurich), Markus Aspelmeyer (Vienna), Oriol Romero-Isart (Innsbruck), and Romain Quidant (Zurich) is attempting to answer precisely this question within the ERC-Synergy project Q-Xtreme.

Control over all dimensions of movement

To explore the limits of the quantum world, the project is employing nanoscopic glass particles levitated in high vacuum by a strongly focused laser beam, cooling their motion near the absolute limit, i.e. into the quantum ground state.

Several experiments in Zurich and Vienna, supported by theoretical calculations by Dr. Gonzalez-Ballestero and Prof. Romero-Isart at university of Innsbruck, have led to the first demonstrations of such ground-state cooling along one of the three directions of particle motion, leaving the motion along the other two directions "hot".

"Achieving ground-state cooling along more than one direction is key for exploring novel quantum physics," emphasizes Dr. Gonzalez-Ballestero of the Institute of Quantum Optics and Quantum Information at the Austrian Academy of Sciences and the Department of Theoretical Physics at the University of Innsbruck. “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” The so-called “Dark State Effect” prevented cooling to the full ground state.

With different frequencies towards the goal

Now, the research at the Photonics Laboratory of ETH Zurich has succeeded for the first time in cavity-based, two dimensional ground-state cooling of a mechanical oscillator. Based on theoretical predictions from the Innsbruck team, the Swiss physicists were able to circumvent the dark-state effect. "To do so, we designed the frequencies at which the particle oscillates in the two directions differently and carefully adjusted the polarization of the laser light," says Lukas Novotny of ETH Zurich.

The work, published in Nature Physics, 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 was financially supported by the European Research Council ERC and the European Union, among others.