In an attempt to study solid objects in the quantum realm, a team of scientists have used lasers to levitate and cool a nanoparticle to 0.00001°C above absolute zero, the lowest temperature possible. By dramatically chilling the solid object to its ground state, which is the lowest energy level that the particle can occupy, quantum mechanics can begin to dominate its motion. This will allow scientists to observe quantum effects on a “macroscopic” level.
When at the quantum scale (i.e. very small), the heat and energy of a particle become interchangeable. So in order for the particle to cool down, its motion needs to be inhibited. To do so with lasers has been achieved before with large clouds of hundreds of millions of atoms, but quantum control of solids, whose density is a billion times higher than atomic clouds, has not been as forthcoming.
However, a team of researchers from the University of Vienna, the Austrian Academy of Sciences, and the Massachusetts Institute of Technology (MIT) have reported in Science the success of their laser-cooling method to make a 140-nanometer-wide glass bead enter the quantum regime.
The particle, which is 1,000 times smaller than a grain of sand, was optically trapped by a tightly focused laser beam with quantized frequencies in a vacuum. Levitation is achieved because of the interactions between the light and the particle itself. The team credit this trick to Nobel Laureate Arthur Ashkin, who originally introduced the idea in the 1980s.
In the optical cavity that the oscillating particle is trapped in, the aim is to reduce the particle’s motion. To do so requires control of the scattering processes between the photons and the particle that govern changes in its kinetic energy.
As the laser beam is reflected off the mirrors in the cavity, they interfere and produce a standing wave. If the particle is maneuvered to an intensity minimum of the wave (node), the photons of light cannot elastically scatter off the object, leaving only inelastic scattering to worry about.
When the right trapping frequency is selected, inelastic scattering processes that would increase the kinetic energy of the particle are suppressed, while those that would reduce the kinetic energy are enhanced. This results in the continual removal of energy, and therefore cooling of the particle, until it reaches its ground state.
Called “cavity cooling by coherent scattering”, the method was proposed by Austrian physicist Helmut Ritsch at the University of Innsbruck and, independently, by study co-author Vladan Vuletic and Nobel laureate Steven Chu.
Previous systems to cool solids to a quantum regime involved clamped oscillators, where the environment of the equipment itself proved a challenge. However, by levitating the solid, the cooling can be achieved in a room-temperature environment.
"Optical levitation brings in much more freedom: by changing the optical trap – or even switching it off – we can manipulate the nanoparticle motion in completely new ways", said Nikolai Kiesel, co-author and assistant professor at the University of Vienna, in a statement. "A decade ago we started this experiment motivated by the prospect of a new category of quantum experiments. We finally have opened the door to this regime."
Being able to study relatively large solid objects in the quantum realm is a huge deal. Observing how quantum superposition, the principle that a physical system may be in one of many configurations, impacts compact solid objects, rather than diffuse gases, could advance scientists' understanding of quantum gravity. More generally, the team hope their research will further the study of quantum phenomena involving large masses.
[H/T: New Scientist]