Quantum movements of small glass sphere controlled
A football is not a quantum particle. There are crucial differences between the things we know from everyday life and tiny quantum objects. Quantum phenomena are usually very fragile. To study them, one normally uses only a small number of particles, well shielded from the environment, at the lowest possible temperatures.
Through a collaboration between the University of Vienna, the Austrian Academy of Sciences and TU Wien, however, it has now been possible to measure a hot glass sphere consisting of about one billion atoms with unprecedented precision and to control it at the quantum level. Its movement was deliberately slowed down until it assumed the ground state of lowest possible energy. The measurement method almost reached the limit set by Heisenberg’s uncertainty principle — physics just does not allow for any more precision than that. This was made possible by applying special methods from control engineering to quantum systems. The results have now been published in the scientific journal Nature.
Perfect precision is impossible
The measurement influences the measured object — this is one of the most basic principles of quantum theory. “Werner Heisenberg came up with a famous thought experiment — the so-called Heisenberg microscope” explains physicist Lorenzo Magrini, the first author of the study from the University of Vienna. “If you want to measure the position of an object very precisely under a microscope, you have to use light with the shortest possible wavelength. But short wavelength means higher energy, so the movement of the particle is disturbed more strongly.” You just cannot accurately measure the location and the state of motion of a particle at the same time. The product of their uncertainties is always limited by Planck’s constant — this is the so-called Heisenberg uncertainty principle. However, it is possible to find out how close one can get to this limit set by nature.
Prof. Markus Aspelmeyer’s team at the University of Vienna is investigating this using a glass sphere with a diameter of less than 200 nanometres, consisting of about one billion particles — very small by our everyday standards, but still very large compared to objects usually studied in quantum physics.
The glass sphere can be kept in place with a laser beam. The atoms of the sphere are heated up by the laser, and the internal temperature of the sphere rises to several hundred degrees Celsius. This means that the atoms of the glass sphere are wobbling around violently. In the experiment, however, it was not the wobbling movements of the individual atoms that were studied, but the collective motion of the sphere in the laser trap. “These are two completely different things, just as the movement of a pendulum in a pendulum clock is something different from the movement of the individual atoms inside the pendulum,” says Markus Aspelmeyer. More