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Making Waves with Atoms

Writer's picture: Mishkat BhattacharyaMishkat Bhattacharya

Background


One of the revelations of quantum mechanics is that the same objects can behave as particles as well as waves. Light was the first object to be verified to show this behavior. Young, Maxwell, Hertz and others showed that light behaved like a wave; Planck, Einstein and others showed light behaved like it was made of particles.


Later, de Broglie, Schrodinger and others showed that material particles like electrons and atoms also displayed wave like behavior. Einstein, following work by Bose, predicted in 1924 a situation where a collection of atoms behaved like a giant (matter) wave: a Bose-Einstein condensate (BEC). A BEC was realized in the laboratory in 1995 - the year I joined graduate school - leading to a Nobel prize in 2001. It is coming up to 30 years since BEC was made; in this post I will describe three exciting areas of research that use BECs as a platform.


Interferometry


One of the fundamental characteristics of waves is that they interfere. This can be seen for water waves in a pond by dropping stones in nearby locations. Likewise, matter waves also interfere. This was famously originally demonstrated at an experiment at MIT, where two BECs were combined to yield bright and dark fringes in the location of the atoms.


Matter wave interferometry is a technique developed mostly in the 1990s, where one starts with a BEC, divides it into two pieces, and then recombines them. Before the recombination, one of the two pieces is made to interact with some force, which we desire to measure. The presence of the force shifts the interference pattern from when it is absent, and this change can be used to obtain information about the force.


Atoms move much more slowly than photons, and hence are more sensitive to forces that vary slowly. Thus, using matter wave interferometry, we can measure the acceleration of a vehicle, and also important constants of physics such as the gravitational constant and the fine structure constant. Recent matter wave interferometers have opened up exciting prospects for using entanglement to increase resolution.


Atomtronics


Electronics - relying on the flow of electrons through wires - has revolutionized society. Notwithstanding its crucial presence in computers, cell phones and various other devices, electronics technology has now started approaching its limits.


One problem facing the field stems from the fact that currents flowing in metallic wires face resistance, which leads to heating. The speed and power of modern day computers are practically limited by how many circuit components can be packed into a certain volume of space without overheating the device.


One way out of this problem is to look for high-temperature superconductors - materials which do not offer resistance to the flow of electric current. This is an exciting and ongoing, but still unsuccessful, pursuit. The challenges are that superconductivity shows up only at low temperatures or high pressures and not under the ambient conditions where machines typically operate.


Another way out is to use the fact that BECs display superfluidity (though BEC-formation is neither necessary nor sufficient for superfluidity!) - wherein neutral atoms can flow without friction. This has led to the burgeoning field of 'atomtronics', where matter waves flow without generating heat, in various kinds of circuits. Analogous to regular electronics, elements like transistors and diodes can also be set up using neutral atoms.


Although the atoms themselves are at very low temperatures (nanoKelvins!), atomtronics works at room temperature, as the atoms are well isolated from their surroundings. The challenge is that these circuits require high vacuum for the isolation, and as of yet these vacuum setups can hardly match the miniaturization, complexity or portability of standard electronic chips. Still, this is an exciting area of research, one that I have recently entered.


Turbulence


One of the great fundamental unsolved problems in classical fluid mechanics is that of turbulence. It is also a topic of practical importance, e.g. in aircraft design and weather forecasting. Turbulence consists of flow whose properties (pressure, velocity) are irregular (chaotic). It can be seen in a variety of systems, such as when we drag something (our finger) through water. If we move the finger slowly, the liquid flows past it smoothly. This is called laminar flow. If we move the finger fast enough, irregular flow, bubbles and froth result. This is turbulent flow. A cool video demonstration is available here.


A full understanding of classical turbulence, from the underlying Navier-Stokes equation, still eludes us. For example, we cannot always predict the speed at which laminar flow becomes turbulent.


Enter BEC into this picture. Dragging an obstacle through a BEC results in quantum turbulence, since the BEC represents the quantum mechanical manifestation of the wave nature of matter (in other words it's a fluid). Similar studies have been carried out earlier in superfluid Helium.


In contrast to classical turbulence, quantum turbulence i) occurs in a superfluid, so there is no damping to convert kinetic energy to heat and ii) implies that the circulation of structures that are shed, such as vortices, is quantized (so the quantum wavefunction describing the superfluid is single-valued) and hence not arbitrary. Typically energy considerations lead the circulation to take on its lowest possible value, i.e. a single quantum.


These interesting differences, which can sometimes simplify the phenomena, have contributed to making quantum turbulence an interesting frontier of contemporary physics research.



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