Absolute zero is really cold – −273.15 °C to be exact. At temperatures close to absolute zero matter begins to act strangely at an atomic scale.
In nature when matter undergoes a transition from one form to another, such as a liquid into a gas, it is known as a phase transition. In physics this is well understood and is present in many of the things we encounter in nature, not to mention the formation of the universe more generally. But at the quantum level phase transitions are even more involved and fascinating.
For starters it’s challenging to view them experimentally, although it is possible to catch glimpses of what is taking place, capturing the whole picture of what is happening during a phase transition is another matter. Very different systems show the same phenomena, a concept known as ‘universality’ in physics.
Extremely cold states of matter are tested in Bose-Einstein condensates in which atoms ‘gather’ together and behave as if they were a single atom, thereby allowing quantum effects to be visible on a much larger scale.
Condensates are incredibly exciting in physics, and are relevant across a broad range of conditions — from sub-atomic particles to the early Universe itself – leading to bizarre phenomena such as superfluidity and superconductivity, which are useful to technologies.
While the Bose-Einstein condensate was first predicted in 1924 it wasn’t directly observed experimentally until 1995. Now for the first time it is possible to visualise the whole process of how matter undergoes such a phase transition into the ‘quantum world’ for a large particle number.
In this system modeled by Professor Nick Proukakis and colleagues, a thermal gas of tens of millions of atoms is rapidly cooled down (quenched) to less than a millionth of a degree above absolute zero under extremely controlled conditions.
It is a close simulation of what experimenters actually work with: a gas of ultra-cold atoms suspended inside a vacuum by lasers and magnets within a ‘magnetic trap’. Importantly what affects this transition is the way in which you quench the system or how fast you change it from warm to cold.
So how does this phase transition actually take place? It is generally understood that order forms locally in a random manner, and the competition of such ordered patches of matter leads to the final system. This is very similar to the way in which a collection of randomly-oriented mini-magnets arranges itself into a well-formed large-scale magnet, as temperature is lowered.
“The prevailing model describing this process dynamically, originally proposed to understand early Universe physics, provides a direct prediction for the average number of defects forming, and relates this to the speed with which the system is cooled”, says Prof Proukakis
The numerical model used by Prof Proukakis and colleagues allows scientists to see this in action as the atoms undergo a phase transition from disorder/noise to coherence. These simulations provide a deeper understanding of how the atoms in the condensate actually obtain a collective identity, and how ‘defects’ such as vortices play an important role in the quantum evolution of the system.
As the system is quenched at the start of the animation above it causes a number of defects (vortices of different shapes and sizes) to appear. A vortex amounts to a hole in the density of the atomic condensate around which the rest of the fluid flows, a kind of quantum ‘whirlpool’, or ‘mini-tornado’. Such a structure appears randomly, as a result of fluctuations, which explains its unusual (non-symmetric) appearance. In this simulation the vortices collide with each other, sometimes violently, before they gradually decay.
When the defects die out, the model shows a growth of ‘green regions’, corresponding to higher-density areas of coherence in which the atoms are working together, not unlike “a random collection of rowers synchronising under an excellent cox”, says Prof Proukakis. These regions of coherence eventually join up bringing the system into equilibrium. The randomness of the storm eventually gives way to a state of tranquility.
Continuous imaging of a phase transition is very hard to do experimentally and currently can’t be done in real-time during all stages as precisely as it can in the simulation.
“The key thing is that we have access to the whole region – once we have the numerically generated field of the system we can do any kind of analysis that we want”.
Timing is important. The duration of the quench plays a key role in the details of establishing the quantum system, something that is observed with great accuracy through the visualisations. As many envisaged practical devices rely on a rapid crossing from classical to quantum behaviour, understanding the intricate details and controlling this process has importance for quantum technologies, including precision measurement.
Ultra-cold atoms are extremely sensitive to changes in electromagnetic fields and because they are nearly stationary, they are also useful for detecting changes in gravitation. In the long-term, this could, for example, mean better search capabilities for underwater vessels like submarines, or advanced guidance for spacecraft.
The European-wide research QuantERA Consortium involves Newcastle and other partners exploring the frontiers of quantum dynamics and boosting quantum research and technologies throughout Europe.
This research was done in collaboration with colleagues in the Bose-Einstein Condensation Centre at University of Trento, a QuantERA partner, and the National Changhua University of Education in Taiwan.
For further information about this work contact Professor Nick Proukakis, School of Mathematics, Statistics and Physics, Newcastle University: nikolaos.proukakis@ncl.ac.uk .
Read the Research Paper (open access):
I.-K. Liu, S. Donadello, G. Lamporesi, G. Ferrari, S.-C. Gou, F. Dalfovo & N. P. Proukakis Dynamical equilibration across a quenched phase transition in a trapped quantum gas. Communications Physics volume 1, Article number: 24 https://doi.org/10.1038/s42005-018-0023-6
brett.cherry@newcastle.ac.uk