Ultracold Atomic Systems

-Kaviranjana Antony

What do superconductors, neutron stars and quantum computers have in common? Before we get to that, let me introduce you to one of the most peculiar phenomena in physics: ultracold atomic systems. Put simply, when you cool atoms down in the range of several microkelvins (µK), thus emphasising their quantum mechanical properties, you end up with an ultracold atomic system.

The infamous Bose-Einstein Condensate is an ultracold system, wherein individual atoms have no free energy to move around because the temperature is dangerously close to absolute zero. As a result, the entire collection of atoms behaves as a single atom i.e, they occupy the same quantum state, becoming indistinguishable from one another. This clearly violates the Pauli Exclusion Principle (which states that fermions cannot have identical quantum states) and are hence described by a completely new set of principles: the Bose-Einstein statistics.

Bizarre properties like superfluidity begin to emerge at these cryogenic temperatures, wherein fluids flow with no loss of kinetic energy. Such fluids will just continue rotating indefinitely once they are stirred and can flow through narrow channels forever without seeming to experience any friction. Physicists have created exactly such kind of vortices using ultracold fermionic gases using isotopes of elements such as lithium.

Superfluidity is central to the theory of superconductivity.

Superconductors are fascinating materials and form an extremely promising area of research. Superconductors exhibit zero electrical resistance and have no magnetic fields in them; which occurs below a particular critical temperature. Superconductivity is observed at very low temperatures i.e such materials have critical temperatures around 30K.

Electrons in superconductors are bound to each other in pairs at incredibly low temperatures called Cooper pairs. This mutual attraction is brought about by the interaction between the electrons themselves and the crystal lattice. More accurately, it is the quantised unit of a vibrating crystal lattice, “the phonon”, analogous to how a photon is a quantised light wave. Likewise, a phonon can be considered as a quantised sound wave.

Now, an electron-phonon interaction in the material at very low temperatures gives rise to a Cooper pair. Several Cooper pairs may occupy the same quantum state. This sounds hugely similar to what happens in ultracold systems! To massively oversimplify things: This tendency for all the Cooper pairs to “condense” into an identical quantum state results in the phenomenon of superconductivity.

A superconductor can be considered a superfluid, except the constituent particles, are electrically charged whereas in superfluids the atoms that make them up are electrically neutral. This adds up because we know in a superconductor that a current can flow in a superconductor almost perpetually once initiated, just as superfluids can flow through narrow tubes endlessly!

What if neutron stars, some of the densest stellar objects ever discovered, could do the same? That is, form analogues to Cooper pairs? But instead, replace the electrons with neutrons and the reason for the attraction to be a long-range attractive nuclear force. This is exactly what many theories that try to explain what is inside a neutron star say. In that case, you’d expect superfluidity and superconductivity inside the neutron star, and as unexpected as that sounds, several theories do suggest that a neutron star looks like an “onion with layers of superconductors and superfluids”.

Because ultracold atoms are excellent at highlighting the quantum mechanical properties of the system, they come in handy in quantum simulations. To study various condensed matter systems experimentally and how they behave under certain conditions, you would require a great degree of control yet need to maintain the intrinsic properties of the system. This is where ultracold atomic systems come in, acting as an analogue to the actual system being studied, hence serving as a crucial tool for experimentalists.

For this very reason, ultracold systems have been proposed as platforms for quantum computing. In a 2017 MIT approach, researchers showed that collections of ultracold molecules can retain the information stored in them, for hundreds of times longer than previously achieved in these materials. Such configurations could provide the framework for qubits (analogous to bits in classical computing) essential for quantum computing.

It is truly beyond belief that something as elementary as exceptionally low temperatures can span such a wide range of exotic manner and fascinating properties, and more so that these properties insert themselves into such varying scales of the universe. When you think about it, being able to characterise the same set of properties at the electronic level as well as the cosmological level is mind-blowing indeed and captures the true essence of physics in a way like no other.


Cover image credits: https://trivediresearch.org.ohio-state.edu/wp-content/uploads/2013/05/cold-atoms-1.jpg









References: Image reference: Image courtesy of Symmetry magazine, a joint Fermilab/SLAC publication. Artwork by Sandbox Studio, Chicago. Image reference Title: Galaxy rotation under the influence of dark matter.ogv Author: Ingo Berg