Getting atoms to do what they want isn’t easy – but it’s at the heart of so much ground-breaking research in physics that creating and controlling new forms of matter is of particular interest and an active field of research.
The new study, published in the journal Physical Review Letters, reveals an entirely new way to sculpt ultra-cold atoms into different shapes using laser light.
The ultra-cold atoms, cooled to temperatures close to absolute zero (-273 degrees Celsius), are of great interest to researchers because they allow them to see and explore physical phenomena that would otherwise be impossible.
At these temperatures, which are colder than outer space, the clusters of atoms form a new state of matter (not solid, liquid or gas) known as Bose-Einstein condensates (BEC). In 2001, physicists were awarded the Nobel Prize for generating such a capacitor.
The defining characteristic of BEC is that its atoms behave quite differently from what we normally expect. Instead of acting as independent particles, they all have the same (very low) energy and are coordinated with each other.
This is similar to the difference between photons (particles of light) coming from the Sun, which may have different wavelengths (energies) and oscillate independently, and those in lasers, which have the same wavelength and oscillate together.
And in this new state of matter, the atoms act more like a single wave-like structure than a collection of individual particles.
The researchers were able to display wave-like interference patterns between two different BECs and even produce moving “BEC droplets”. The latter can be considered as the atomic equivalent of a laser beam.
And in the latest study, conducted with colleagues Gordon Rupp and Gian Luca Ubo, it looked at how specially designed lasers could be used to manipulate ultra-cold atoms in BEC.
And the idea of using light to move things is not new: when light falls on an object, it can exert a (very small) force. This radiative pressure is the principle behind the idea of solar sails, in which the force that sunlight exerts on large mirrors can be used to propel a spacecraft through space.
However, in this study, the researchers used a specific type of light that can not only “push” atoms, but also rotate them around, like an “optical wrench”.
These lasers appear as bright rings (or cakes) rather than dots and have a twisted (helical) wave front.
And under the right conditions, when such a twisted light is shone on a moving BEC, the atoms in it are first attracted towards the bright ring before orbiting around it.
As the atoms rotate, both the light and the atoms begin to form droplets that rotate around the original direction of the laser beam before being ejected outward and away from the ring.
And the number of drops is twice the number of light turns. By changing the number or direction of twists in the initial laser beam, we have complete control over the number of droplets formed, and the speed and direction of their subsequent rotation.
The twisted light shines onto a moving BEC, sculpting it into a ring before breaking it into a number of BEC droplets that orbit around the direction of the light before releasing and twisting away.
We can even prevent the atomic droplets from escaping the loop so that they continue to spin longer, producing a form of an extremely cold atomic current.
This approach of shining twisted light through ultra-cold atoms opens up a new and simple way to manipulate matter and sculpt it into other unconventional and complex shapes.
One of the most exciting potential applications of BECs is in the generation of “atomic electronic circuits,” in which matter waves from ultracold atoms are directed and manipulated by optical and/or magnetic fields to form advanced equivalents of electronic circuits and devices such as transistors and diodes.
The ability to reliably manipulate the shape of the BEC will eventually help in the creation of atomtronic circuits.
And our ultra-cold atoms, which here act like an “atomic superconducting quantum interference device,” have the potential to provide much higher hardware than conventional electronic devices.
This is because neutral atoms lose less information than the electrons that normally make up the current. We also have the ability to change device features more easily.
Even more exciting, however, is the fact that our method allows us to produce complex electronic circuits that would be impossible to design using ordinary materials.
This could help design highly controllable and easily reconfigurable quantum sensors capable of measuring small magnetic fields that would otherwise be unmeasurable.
These sensors could be useful in fields ranging from basic physics research to discovering new materials or measuring signals from the brain.