Aug 5 2019
When a tray of water is placed in the freezer, ice cubes are formed. Currently, scientists from CU Boulder and the University of Toronto have realized a similar transition with the help of ultracold atom clouds.
In research recently reported in the journal Science Advances, the team learned that it could push these quantum materials to experience transitions between “dynamical phases”—fundamentally, jumping between two states wherein the atoms act in totally different ways.
This happens abruptly, and it resembles the phase transitions we see in systems like water becoming ice. But unlike that tray of ice cubes in the freezer, these phases don’t exist in equilibrium. Instead, atoms are constantly shifting and evolving over time.
Ana Maria Rey, Study Co-Author and Fellow, JILA
The findings, she continued, offer a new perspective into materials that are challenging to explore in the laboratory.
“If you want to, for example, design a quantum communications system to send signals from one place to another, everything will be out of equilibrium,” said Rey, a fellow at JILA, a joint institute between CU Boulder and the National Institute of Standards and Technology (NIST). “Such dynamics will be the key problem to understand if we want to apply what we know to quantum technologies.”
Antisocial atoms
Scientists have noticed parallel transitions before in ultracold atoms, but exclusively among a few dozen charged atoms, or ions.
Rey and her co-workers, in contrast, looked at clouds composed of numerous uncharged, or neutral, fermionic atoms. She defined fermionic atoms as the introverts of the periodic table of elements. They do not want to share their space with other atoms, which can render them harder to regulate in cold atom laboratories.
We were really wandering in a new territory not knowing what we would find.
Joseph Thywissen, Study Co-Author and Professor of Physics, University of Toronto
To steer that new territory, the scientists exploited the weak interactions that happen between neutral atoms—but only when those atoms collide into each other in a narrow space.
Initially, Thywissen and his team in Canada cooled a gas composed of neutral potassium atoms to merely a fraction of a degree below absolute zero. Secondly, they tweaked the atoms so that their “spins” all pointed in one direction.
Such spins are a natural characteristic of all atoms, Thywissen explained, a little like a compass reacting to a magnetic field.
Once the atoms were all standing in order, the team then modified them to alter how strongly they interacted with each other. This was when the fun started.
We ran the experiment using one kind of magnetic field, and the atoms danced in one way. Later, we ran the experiment again with a different magnetic field, and the atoms danced in a completely different way.
Joseph Thywissen, Study Co-Author and Professor of Physics, University of Toronto
Two dances
In the first dance—or when the atoms hardly interacted at all—these particles ended up in chaos. The atomic spins started to rotate at their own rates and swiftly all pointed in random directions.
Imagine it like standing in a room full of numerous clocks with second hands all ticking at various tempos.
But that was just part of the tale. When the team boosted the strength of the interactions between atoms, they stopped behaving like chaotic individuals and more like a collective group. Their spins still ticked, in other words, but they ticked in harmony.
In this synchronous stage, “the atoms are no longer independent,” said Peiru He, a graduate student in physics at CU Boulder and one of the lead authors of the new paper. “They feel each other, and the interactions will drive them to align with each other.”
With the right adjustments, the team also found that it could achieve something else: turn back time, triggering both the synchronized and disordered phases to return to their former state.
Finally, the scientists were only able to preserve those two varied dynamical phases of matter for about 0.2 seconds. If they can expand that time, He said, they may be capable of making even more stimulating observations.
“In order to see richer physics, we probably have to wait longer,” He said.
Other co-authors on the study include Scott Smale, Ben Olsen, Kenneth Jackson, Haille Sharum and Stefan Trotzky from the University of Toronto, and Jamir Marino from JILA.
Source: https://www.colorado.edu