Swinburne University of Technology - Melbourne Australia

Atom chip to open frontiers unknown

Story by Graeme O'Neil

In an ultrahigh-vacuum chamber at Swinburne University of Technology, a million ultracold rubidium-87 atoms hover just beneath the surface of a silicon chip coated with a thin magneto-optical film.

Tailored to create a shaped, perpendicular magnetic field, the magnetic film confines and shepherds the rubidium atoms on the chip, in much the same way as electrons are guided along conducting wires on an electronic microchip.

Cooled to a temperature of a few billionths of a degree Kelvin, just above absolute zero (minus 273˚C), and confined by a magnetic microtrap on the chip, the ultracold atoms fall into the lowest energy state of the trap and no longer jostle for room - they exhibit almost no random thermal motion.

The atoms condense to a state where they behave as a single super-atom of rubidium-87 and exhibit coherent, wave-like properties - rather like the coherent light from a laser. For several seconds, the chip holds the atoms in an exotic, fifth state of matter called a Bose-Einstein condensate.

If all this sounds 'sci-fi' it's because in many ways it is. Bose-Einstein condensate is a new frontier whose boundaries have yet to be measured, but are more than likely to take humankind to new realms of technological and industrial capability.

Just as the first lasers mystified scientists as to their possible applications, so too now with Bose-Einstein condensate. In theory, they could at the very least be the basis for quantum computing - that is, computers able to use atoms to store data and complete in seconds computations that would take today's most powerful supercomputers years.

The first Bose-Einstein condensate was demonstrated just 12 years ago by US physicists Eric Cornell and Carl Wieman, at the Joint Institute for Laboratory Astrophysics in Boulder - 70 years after the prediction by Albert Einstein. Cornell and Wieman shared the Nobel Prize for physics in 2001 with Wolfgang Ketterle of the Massachusetts Institute of Technology, another pioneer in the field.

Since then others have joined the quest to develop Bose-Einstein condensates into a stable, applicable technology. In Australia, Professor Peter Hannaford's research centre at Swinburne University of Technology, a partner with the Australian National University and the University of Queensland in the Australian Research Council Centre of Excellence for Quantum-Atom Optics (ACQAO), has now designed a miniaturised atom chip for producing and manipulating Bose-Einstein condensates.

The advantage of miniaturising the apparatus onto an atom chip, Professor Hannaford says, is that it creates very steep gradients in the magnetic field that tightly confine the rubidium atoms in deep magnetic 'wells'. This allows Bose-Einstein condensates to be produced in just a few seconds, and also allows very precise control over the atoms at the quantum level.

A unique feature of the atom chip developed by the Swinburne group of Professor Andrei Sidorov, Dr Brenton Hall and Professor Hannaford, compared with atom chips developed by other international research groups, is its use of permanent magnetic films instead of current-carrying wires to produce the precisely shaped magnetic potentials.

"With permanent magnetic devices, we should be able to minimise the technical 'noise' relative to those based on current-carrying wires. And the absence of any ohmic heating allows lower operating temperature and more complex magnetic patterns to be integrated into a smaller area," Professor Hannaford says.

The Swinburne atom chip is made by depositing a thin film of a magnetic alloy of terbium, gadolinium, iron and cobalt on a glass or silicon substrate.

The atoms are chilled by a technique known as laser cooling. When hit by a laser beam tuned slightly above the wavelength of an absorption line, the moving atoms preferentially absorb photons in a direction opposite to that of the atoms' velocity. As thermal motion slows to a crawl, their temperature drops to a few millionths of a degree Kelvin, allowing them to be trapped by magnetic fields which hold them against gravity in a vacuum for up to several minutes.

A technique called evaporative cooling can then lower their temperature even further - a radio-frequency field is applied which selectively ejects any atoms with higher than average kinetic energy, leaving the remainder to equilibrate at a lower kinetic energy and temperature, rather like the cooling of a cup of coffee.

The atoms are now tightly confined within a magnetic microtrap at just a few billionths of a degree above absolute zero. Professor Sidorov says the Bose-Einstein condensate state is detected by releasing the ultracold atoms from the trap and measuring the expansion of the atom cloud with time.

Sharp, central peaks in the atomic density indicate that the majority of the atoms move very slowly within a very tiny volume a few tens of micrometres below the chip's surface.

The Bose-Einstein condensate persists only a few seconds, but the Swinburne atom chip's technology allows it to be re-formed again within a few seconds, compared with a minute for the laboratory-scale apparatus used to demonstrate the first Bose-Einstein condensate more than a decade ago.

Professor Sidorov says current microfabrication technology will allow the integration of individual atom optical devices into atomic circuits, serving as highly sensitive detectors, atomic waveguides or interferometers in which the ultracold atoms are split into two paths and then recombined.

Atom chips will make it easier for physicists to investigate the remarkable properties of Bose-Einstein condensates, but what will be their practical applications?

Professor Sidorov says: "It's too early to say what impact atom chips and Bose-Einstein condensates will have. But lasers revolutionised the world when, at the time of their invention, few people thought they would be useful for anything."

The laser is now the basis of a multi-billion dollar industry - and nobody in 1960 could have anticipated that it would one day play a central role in the chill-and-still process that creates Bose-Einstein condensates.

Suspended within the Bose-Einstein condensate, the atoms are extremely sensitive to disturbance by external fields, including variations in the Earth's gravitational field.

Dr Hall says this suggests the possibility of using the atom chip to build a super-sensitive quantum sensor to map the faint gravitational signatures of mineral deposits hidden beneath the landscape. But he says that, at its current state of development, the Swinburne atom chip cannot yet match the sensitivity of existing gravity sensors.

Dr Hall says the atom chip would also allow physicists to make much more accurate measurements of a quantum effect known as the Casimir-Polder force. This is a physical force that arises between uncharged particles due to fluctuations in energy fields that arise in the quantum vacuum in the space between them. The force can be accurately measured when the atoms in a Bose-Einstein condensate are brought very close to the surface of the chip.

"We can say atom chips are already looking very promising," Dr Hall says, "but their usefulness may be isolated to certain industries, and they're not likely to become as ubiquitous as the laser."

One of those 'practical devices' could possibly be a quantum computer: a computer that would use individual atoms located in a regular lattice-like pattern to store data.

"The first practical devices involving atom chips and Bose-Einstein condensates could be a decade away," Dr Hall says. "But we're hoping to do a little better than that."