First published on Cosmos Magazine site, 25/9/18
An experiment with a cloud of ultracold atoms squashed into a quantum pancake has revealed never-before seen quantum effects that could lead to more efficient electronics, including high temperature superconductors.
A team at Swinburne University in Australia observed a quantum anomaly in lithium-6 gas cooled to a few billionths of a degree above absolute zero and squashed between two laser beams.
“We’re seeing quantum mechanics that’s visible on a macroscopic scale – a large collection of tens of thousands of atoms all behaving quantum mechanically,” says project leader Chris Vale, a researcher at Swinburne’s Centre of Excellence in Future Low-Energy Electronics Technologies.
The Australian team used laser beams focused into a flat plane to create a pancake of lithium-6 gas with a radius of 200 microns and the thickness of a single atom – around 500 nanometers. Then the researchers compressed the gas slightly with a magnetic field, to set it vibrating. Shining another laser from below, they measured the vibration frequency of the gas cloud by watching the shadow of the cloud on a camera.
The frequency of this radial vibration, known as a breathing mode, gave the telltale sign of a quantum anomaly: it vibrated 2.5% faster than the classical model predicted.
Symmetry in the classical model dictates that gas properties such as pressure and density should scale in a straightforward way as the size of the gas cloud oscillates. But the full quantum analysis predicts a higher frequency: the classical theory breaks down because of strong interactions between the gas particles.
To further test the model, the scientists jammed more atoms between the laser beams, and turned the pancake from a two-dimensional crepe into a fatter flapjack. Its three-dimensional nature conformed to the classical model.
Vale says the interactions between the gas particles in the quantum crepe was mid-way between two well-known states of matter: Bose-Einstein condensates, in which atoms in a gas interact strongly and exhibit uniform quantum behaviour, and low temperature superconductivity, in which electrons in a material form weakly-bound pairs known as Cooper pairs that can carry electricity through a material with no energy loss.
“There’s a continuous crossover between these two limits, what happens in the middle is not well understood,” he says
“There’s a lot of interesting physics there. For example, this is where we find the highest superfluid transition temperatures, in the intermediate zone where the binding energy of the fermion pairs is similar to the natural energy scale for the system, the Fermi energy.”
By studying a two-dimensional system, Vale hopes to spark developments of new materials for the electronics industry, for example topological insulators or high-temperature superconductors.
In the current highest-temperature superconductors, ceramics based on copper oxide, the superconducting current is carried in two-dimensional layers within the material – although exactly how the electrons pair up is not fully understood.
The power of the experiments at Swinburne is their simplicity, says Vale – the microscopic properties of the lithium atoms and their interactions are precisely known, which is not always possible in more complex materials such as solids.
“In a sense, our cold atoms are acting as a quantum simulator, where we can test models of many-body physics with precisely known inputs that can be difficult to pin down in other materials,” he says.