Quantum controlled solids. Probing the boundary between quantum and classical physics.

Researchers are pushing the boundary between classical and quantum physics — levitating and cooling glass nanobeads into a quantum state.

Laser-cooling a minuscule glass nanoparticle — thousands of times smaller than a grain of sand — bringing it into a quantum state has allowed researchers the probe the fuzzy boundary that exists between quantum and classical physics. Whilst by everyday standards the particle the team were able to manipulate is tiny, it has the density of a solid object and is millions of times larger and more complex than the atomic-objects like single atoms, ion and molecules that have traditionally used to investigate ‘quantum motion.’ The team consisted of physicists from institutes including the University of Vienna and MIT, their research is published in the journal Science.

“Classically, we expect a solid to have a well-defined position, momentum, etc. while in quantum mechanics this is not the case,” explains Dr Markus Aspelmeyer, co-author of the paper and professor of quantum optics at the University of Vienna. “These uncertainties in the observable properties of a quantum system are one of the key features, along with quantum superpositions, that give rise to all of the exotic phenomena that is so characteristic of quantum mechanics.”

In such a superposition, Aspelmeyer says objects can behave as if they were in two states at the same time. He points to the example of the Schrodinger’s cat thought experiment, in which the cat is simultaneously alive and dead. “This defies our understanding from everyday life,” he adds. “In a more relevant example, objects can be placed in a superposition of multiple positions, interacting with the world around it at each location simultaneously.”

Physicists have been aware for some time that the quantum properties of individual atoms can be manipulated by lasers. This level of quantum control has even previously been demonstrated with clouds containing hundreds of millions of atoms, influencing them to enter into macroscopic quantum states of matter such as quantum gases and Bose-Einstein condensates — a state of matter in which separate atoms cooled to near absolute zero behave as a single quantum entity.

The team took this existing research further by extending it to a solid with a density billions of times greater than gas, in which the atoms are bound together moving in unison across the object’s centre of mass.

“In our case, the quantum ground state, all thermal energy has been removed from the system,” Aspelmeyer explains. “A classical object would then stand perfectly still, while quantum theory predicts a finite amount of energy — the vacuum energy — that can never be removed and that results in a fundamental uncertainty of position and momentum — the ground state uncertainty.”

“Look, no wires!” Levitating solids with lasers

The researcher goes on to explain the key difference between the team’s experiment and work carried out in the past with quantum solids. Whereas previous experiments have used an anchored mechanical oscillator — usually something like a very small diving board that bends up and down — their solid is not firmly anchored to an environment but is, instead, levitated in an optical trap.

“We can, in principle, quickly change the optical trap to provide a changing potential landscape for the particle to move in,” Aspelmeyer says. “This flexibility allows for strong nonlinear features that cannot be generated by the previously used clamped oscillators.”

The method was originally introduced by Nobel laureate Arthur Ashkin already many decades ago becoming a well-established tool used for isolating atoms from their environment.

The hope is that such experiments could open up the possibility of macroscopic quantum states that involve large mass. Just as is the case with Bose-Einstein condensates, these quantum states could become incredibly useful in the development of quantum technology and quantum computers.

“This will be interesting in the future when we want to increase the mass of our objects even more and study quantum effects of systems with a large mass,” says Aspelmeyer. “Also, in a solid, all atoms are bound to move together. This means that a solid forces all of its atoms to be in one place.

“Again, this is interesting in the future because it allows us to generate quantum states in which all atoms are in one place or another together, which is otherwise extraordinarily difficult to achieve with atomic gases.”

Achieving quantum control of such large macroscopic particles is, as you may imagine, no mean feat. In fact, it represents a major challenge.

Quantum control may be cool, but it isn’t easy

For the object they intended to experiment with the team selected a glass nanoparticle-containing roughly 10⁸ atoms. To put that into perspective, a single grain of sand contains around 4.33 x 10 ¹⁹ atoms!

In order to achieve quantum control of this macroscopic object the team first has to isolate the said object from any environmental influence, removing almost all of its thermal energy, thus cooling it down to near absolute zero — the theoretical point at which physicists estimate that all atomic movement will cease. At this temperature — 0 kelvin or -273⁰C —the rules of quantum mechanics become the dominating factor in the particle’s motion. As detailed above, the team achieved this level of isolation by optically trapping the particle within a tightly focused laser beam in a vacuum, but this still left them with the considerable challenge of cooling the particle to near -273⁰C.

Scientists from Vienna, Kahan Dare (left) and Manuel Reisenbauer (right) working on the experiment that cooled a levitated nanoparticle to its motional quantum ground state. (© Lorenzo Magrini, Yuriy Coroli/University of Vienna)

“The real challenge is for us to cool the particle motion into its quantum ground state,” says Uros Delic, lead author of the paper from the University of Vienna. “Laser cooling via atomic transitions is well established and a natural choice for atoms, but it does not work for solids.”

In order to combat this drawback, the team have been developing a laser-cooling method referred to as ‘cavity cooling by coherent scattering’ — first proposed by Helmut Ritsch at the University of Innsbruck and in a study by Vladan Vuletic and Nobel laureate Steven Chu. By improving upon this method by withdrawing more and more gas from the cavity, thus creating a purer vacuum, and sending in more photons, the team have been able to cool the motion of the glass nanoparticle straight into the quantum regime. Delic points out a certain irony with this procedure: “The surface of our glass bead is extremely hot — around 300°C — because the laser heats up the electrons in the material. But the motion of the centre of mass of the particle is ultra-cold, around 0.00001°C away from absolute zero.

“Thus, we can show that the hot particle moves in a quantum way.”

“With the addition of cavity cooling by coherent scattering to our toolbox of techniques we saw rapid progress but this came after almost a decade of research,” adds Aspelmeyer. “We subsequently improved on our realization of this technique and managed to cool to the ground state. A strong feature of this system is the additional capability to modify the optical trapping potential allowing us to modify the particle’s quantum state in a very versatile way.

“This is a powerful tool which we hope to leverage in the future to explore other quantum phenomena.”

The researchers are excited about future prospects for building upon their study. “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”, says Nikolai Kiesel, co-author and Assistant Professor at the University of Vienna.

Several such schemes have been proposed, and may now become possible. For example, in combination with the newly achieved motional ground state the authors expect that this opens new opportunities for unprecedented sensing performance, the study of fundamental processes of heat engines in the quantum regime, as well as the study of quantum phenomena involving large masses.

Dr Markus Aspelmeyer believes that studies developing from this research could eventually help physicists tackle the biggest hurdle standing in the way of a union between classical, relativistic and quantum physics; the question of quantum gravity.

“One strong motivation is to push these experiments much further into the regime of large masses,” he concludes. “In that way, we may one day be able to experimentally address the question ‘how does a quantum system gravitate?’”