What are the limits of quantum physics? This is a question that has been researched around the world for decades. If we want to make the properties of the quantum world technically usable, we need to understand whether objects that are significantly larger than atoms and molecules can also exhibit quantum phenomena.
For example, small glass spheres with a diameter of one hundred nanometres can be examined – still over a thousand times smaller than a grain of sand, but huge by quantum standards. For years, attempts have been made to show the extent to which such spheres still exhibit quantum properties. A research group at ETH Zurich, with theoretical support from TU Wien (Vienna), has now achieved a breakthrough: they were able to show that the rotational vibrations of such particles behave in accordance with quantum physics, not only when they are cooled to near absolute zero using complex cooling methods, but even at room temperature.
Vibration quanta: only certain wobbles are allowed
"A microscopic particle will always wobble a little," says Carlos Gonzalez-Ballestero from the Institute of Theoretical Physics at TU Wien. "This oscillation depends on the temperature and on how the particle is influenced by its environment."
In everyday life, we assume that any kind of oscillation is possible. The pendulum of a clock, for example, can be swung to any angle, and it can be set into oscillation a little more strongly or a little more weakly – just as you like. In the quantum world, however, things are different: if you look at oscillations with very low energy, you find that there are very specific "oscillation quanta".
There is a minimum vibration, known as the "ground state", a slightly higher vibration that carries a little more energy (the "first excited state"), and so on. There is no state in between, but the particle can exist in a quantum physical combination of different vibration states – this is one of the central concepts of quantum physics.
"It is very difficult to put a nanoparticle into a state where its quantum properties become apparent," says Carlos Gonzalez-Ballestero. "You have to let the particle float in order to isolate it from any interference as much as possible. And normally you also have to ensure extremely low temperatures, close to absolute zero, which is minus 273.15 degrees Celsius."
The rotation freezes, the particle remains hot
ETH Zurich and TU Wien have now developed a technique that allows a very specific aspect of the nanoparticle to be brought into a quantum physical state, even though the particle itself is in a hot, disordered state.
'We use a nanoparticle that is not perfectly round, but slightly elliptical,' explains Carlos Gonzalez-Ballestero. "When you hold such a particle in an electromagnetic field, it starts to rotate. Our question was: Can we see the quantum properties of this rotational vibration? Can we extract energy from this rotational movement until it is mainly in the quantum ground state?'
Laser beams and mirror systems were used for this purpose. 'The laser can either supply energy to the nanoparticle or take energy away from it," explains Carlos Gonzalez-Ballestero. 'By adjusting the mirrors in a suitable way, you can ensure that energy is extracted with a high probability and only added with a low probability. The energy of the rotational movement thus decreases until we approach the quantum ground state.'
To achieve this, however, a number of difficult theoretical problems had to be solved – the quantum noise of the lasers had to be correctly understood and controlled.
Record-breaking quantum purity
Finally, it was actually possible to demonstrate that the rotation can be brought into a state that corresponds almost exclusively to the quantum mechanical ground state. The amazing thing about this is that the nanoparticle has not cooled down – on the contrary, it is actually several hundred degrees hot.
"You have to consider different degrees of freedom separately," explains Carlos Gonzalez-Ballestero. "This allows the energy of the rotational movement to be reduced very effectively without having to reduce the internal thermal energy of the nanoparticle at the same time. Amazingly, the rotation can freeze, so to speak, even though the particle itself has a high temperature."
This made it possible to create a state that is significantly 'purer' in terms of quantum physics than was previously possible with similar particles – even though cooling was not required. "This is a technically astonishingly practical way of pushing the boundaries of quantum physics," says Carlos Gonzalez-Ballestero. "We can now study the quantum properties of objects in a stable and reliable way, which was previously hardly possible."
