Scientists at ANU have designed a back-to-front filter which uses lasers to process clouds of atoms.

The device is back-to-front because it creates mirrors made of laser beams that reflect atoms. It is a reversed version of the well-known Fabry-Perot Interferometer, which uses mirrors to reflect lasers back and forth.

The theoretical design for the device is published in the Nature journal Scientific Reports.

Although similar to the conventional Fabry-Perot Interferometer, lead author Dr Manju Perumbil from the Department of Quantum Science said the atom-optics version had distinct differences.

“Atom-optics displays many counter-intuitive quantum phenomena, such as wave-particle duality and quantum tunnelling,” she said.

Because atoms have mass and are sensitive to gravity, the atom-optics Fabry-Perot interferometer could offer new sensing capabilities.

“It has a wide range of application on earth as well as in space!” Dr Perumbil said.

“An atomic Fabry-Perot interferometer is a promising candidate for space-based interferometry for acceleration and gravity sensing, due to its simple design and smaller size.”

Dr Perumbil completed her PhD in the Department of Quantum Sciences in 2020 and is now based in Kerala, India.

Conventional Fabry-Perot interferometers use interference of light bouncing between two mirrors to separate light frequencies with high precision. Examples include narrowing the bandwidth of lasers and separating out particular light frequencies, such as the hydrogen alpha line, a crucial frequency in astronomy that is emitted from hydrogen in stars.

Dr Perumbil’s atom-optics design for the device uses an atom laser in a pulsed mode – clouds of rubidium atoms that have been cooled to billionths of a degree kelvin, and formed a state known as a Bose-Einstein condensate, in which the atoms are highly coherent and acting collectively.

The clouds are then propelled towards a pair of laser beams travelling at right angles to the moving atom cloud.

The laser beams are able to reflect the atoms because their frequency is slightly larger than a resonant transition in the rubidium.

The interactions between the rubidium states and the laser photons normally results in most of the atoms being repulsed, as laser photons would be from a mirror. However, a small fraction of the atoms quantum tunnel through the first laser beam, and find themselves in a cavity, trapped between the two beams. They bounce back and forth between the beams with a small fraction of them quantum tunneling out again.

The probability of the particles tunneling through the cavity is dependent on the momentum of the cloud of atoms - a quantum analogue to the wavelength of laser light in the traditional Fabry-Perot. Atoms with some energies preferentially escape (called resonant transmission), while others are strongly reflected. This effectively forms an energy filter for atoms.

 “The interesting difference here is that atoms interact – they bounce off each other – where photons do not interact,” said Professor John Close, who leads the research team.

“This meant we found there were clear regimes where you could see Fabry-Perot effect was more pronounced, and others in which the resonances were washed out.”

The theoretical study was designed to be as directly applicable to laboratory parameters as possible, to enable the most effective streamlined development of the device, said Ms Perumbil’s supervisor, Dr Stuart Szigeti.

“We have only recently begun developing the capabilities needed to implement a viable experimental demonstration.

“I think the next steps are the most exciting: experimentally demonstrating atomic Fabry-Perot resonances and calculating the fundamental sensitivity limits of an atomic Fabry-Perot accelerometer,” Dr Szigeti said.

Atom-optics systems are a new realm of control, to add to lasers (built from organised photons) and electronics (built from electrons arrayed in doped semiconductors), said Professor Close.

“Every time we get control of a new particle we see huge gains. What is the third going to give?

“I think free atoms are going to make superb chip-based systems, such as sensors, because they respond to gravity, acceleration and rotation – they’ll be on the front line of the Internet Of Things,” Professor Close said.

Contact

Dr Manju Perumbil

Kerala, India

E: u5692850@alumni.anu.edu.au

Dr Stuart Szigeti

ANU Research School of Physics

E: stuart.szigeti@anu.edu.au

T:  (02) 612 58896