The Quantum World Can Be A Surprising Place
September 7, 2012

Ultracold Atoms Reveal Surprising New Quantum Effects

April Flowers for - Your Universe Online

Thermalization is the process of particles reaching thermal equilibrium through mutual interaction. We see this process around us every day. Take for example, ice cubes dropped into a pot of hot water. The cubes melt and cannot remain stable. Well-ordered ice crystals turn into a disordered liquid as the molecules of the ice and the molecules of the water reach thermal equilibrium, ending up at the same temperature.

Research at the Vienna Center for Quantum Science and Technology (VCQ) at the Vienna University of Technology has shown that in the quantum world the transition to thermal equilibrium is more interesting and more complicated that previously assumed.

A "quasi-stationary intermediate state" can emerge between an ordered initial state and the statistically mixed final state. This intermediate state already exhibits some equilibrium-like properties, but some of the distinct order of the initial state remains visible for a remarkably long time.

This mid-way transition state is called "pre-thermalization," and is predicted to play a major role in many different non-equilibrium processes in quantum physics. For example, it could help us to understand the state of the early universe.

"In our experiments we start with a one-dimensional quantum gas of ultracold atoms, a so-called Bose-Einstein condensate, which is then rapidly split into two using an atomchip", Professor Jörg Schmiedmayer explains. When the two parts of the condensate are immediately rejoined, they create an ordered matter-wave interference pattern.

"The shape of this interference pattern shows us that the two clouds have not yet forgotten that they originally came from the same atom cloud", says Schmiedmayer.

The split atom is expected to tend towards thermal equilibrium after some time. After more time during which the order in the interference pattern decays, the two halves of the system are rejoined.

"The astonishing thing about this is that the order does not directly reach a minimum. First, it decays rapidly, but then it remains in an intermediate state — the so-called pre-thermalized state", says Michael Grin.

Schmiedmayer and Grin's research group has been working on these experiments for several years.

"At first, it was not clear how to interpret this phenomenon. The experiments had to be improved and the corresponding theory needed further development", says Schmiedmayer. In close cooperation with Professor Eugene Demler's theory group at Harvard University the surprising results could now be explained. "The observed disorder in the intermediate state does not depend on the temperature of the initial state. It is introduced into the system by the laws of quantum physics when the atom cloud is split into two", Schmiedmayer says.

The transition to thermal equilibrium is important in many fields of quantum physics because a quantum experiment can never be done at exactly zero temperature. Therefore, scientists always have to deal with temperature effects.

Carrying out calculations or storing data in a quantum computer inevitably creates non-equilibrium states, which tends towards a thermal equilibrium, destroying the quantum state.

This novel intermediate state has implications for the physics of quark-gluon plasma. All of the matter in the universe was in a non-equilibrium state of quark-gluon plasma just fractions of a second after the Big Bang. Today, quark-gluon plasma is created in large particle colliders. These experiments showed that certain aspects of the plasma tend towards a thermal equilibrium much faster than one would have assumed.

"Pre-thermalization" was postulated in a theoretical framework developed at Heidelburg University to explain this. Scientists speculate that this could be linked to an intermediate state, similar to the one discovered in the ultracold atom clouds.

The processes associated with the decay of a quantum system to thermal equilibrium could also tell us more about the relationship between quantum physics and the classical macroscopic world.

"Our atom clouds offer us the possibility to study the fascinating crossover from non-equilibrium states towards thermal equilibrium in detail", says Jörg Schmiedmayer. "That way, we hope to achieve a deeper understanding of non-equilibrium processes, which are omnipresent in nature."

For the experiment, a special kind of atom chip was created at The Center for Micro- and Nanostructures (ZMNS) at the Vienna University of Technology.