4 September 2008

Verging on absolute zero

By
Cosmos Online
We've gone to space, split the atom, and created devices small enough to travel through our blood. But it seems that in science, as in nature, there are some places we still can't reach.
Thermometer

Credit: iStockphoto

Absolute zero is a physical limit that will almost certainly remain beyond our grasp, although lately, physicists have been getting within a hair’s breadth of that goal. What they’ve found is that in the ultimate cold, matter becomes truly bizarre.

Absolute zero: zero degrees on the Kelvin scale, or -273.15 °C is as cold as it gets. Nothing in nature gets this cold; the coldest known region of space, the Boomerang nebula in the constellation Centaurus, is a balmy one degree Kelvin.

It’s a physical limit, theoretically impossible to reach. Attaining absolute zero would break the law of thermodynamics, which says that getting there would require an infinite amount of work. At absolute zero, there is no heat. All atomic motion ceases — atoms no longer move or vibrate; they have no thermal energy whatsoever.

Even getting close to the limit is problematic. Objects cooled to absolute zero must be kept as still as possible. Anything touching an object being cooled to absolute zero would pass on its thermal energy, so instead, physicists use lasers to trap the atoms and damp their movement to slow them down to near as near motionless as possible.

The technique – known as laser cooling – has taken atoms to within a few billionths of a degree of 273.15°C.

Nobel science

At this extreme cold, the quantum world takes over and some very strange and useful behaviour is observed.

For instance, certain materials may become superconductors, materials with zero electrical resistance and the ability to exclude their inner magnetic field. Superconducting magnets are used in particle accelerators and nuclear magnetic resonance (NMR) machines to image the brain, as well as in studies of the intrinsic properties of matter.

A number of institutions have recently formed with the common goal of taking atoms down to temperatures even closer to zero. And with good reason too: ultra-cold atoms have been the topic of two recent Nobel prizes in physics (in 1997 for the development of methods to cool and trap atoms with laser light; and in 2001 for studies of the properties of Bose-Eintsein condensates).

Scientists studying cold atoms are lining up to take the next prize, says quantum physicist Maciej Lewenstein, who leads the quantum optics theory group at the Institute of Photonic Sciences in Barcelona, Spain. “Concerning Nobel Prizes in this area, it’s only a question of who’s next,” he adds.

In July, Lewenstein presented his theories on cold atom research along with some Europe’s pre-eminent cold atom scientists at the European Science Foundation’s Euroscience Open Forum (ESOF) in Barcelona.

Bose-Einstein condensate

The first matter known to have quantum properties, Bose-Einstein condensate (BEC), was predicted by Satyendra Nath Bose in 1925.

Bose later worked with Albert Einstein, who modified and generalised Bose’s initial theory, in publishing his research on the topic. It wasn’t until in 70 years later, though, in 1995, that the first BEC was produced at the U.S. National Institute of Science and Technology and the University of Colorado in Boulder.

Bose-Einstein condensate is a strange state of matter composed of identical atoms existing at temperatures close to absolute zero. In the extreme cold, the atoms’ energies drop to the lowest possible state and they start to act something like a shoal of fish. Locked together at the same energy, they behave as if they are no longer a number of individuals, but instead act as one giant super atom.

BEC can only be made with certain particles called bosons that have a particular atomic spin. The extreme cold causes the atoms to behave like waves rather than particles and ‘overlap’ one another. It’s a difficult thing to imagine, but it seems that’s characteristic of things at the extreme cold.

Quantum physicist Christopher Foot, at the University of Oxford in the U.K., also presented his work on cold atoms at the July meeting in Barcelona. He explains that this strangeness doesn’t stop with atoms overlapping – atoms at temperatures close to absolute zero also become ‘entangled’ too.

Spooky actions

“Small particles such as atoms and electrons behave in strange ways that often seem very weird when compared to our everyday experience of large ‘ordinary’ objects such as a tennis ball or football,” he says.

“A single quantum object can exist in two places at once, but this is not really as strange as it first appears when considered in terms of waves. However, there is a second property of quantum systems of two or more particles that is truly difficult to understand,” says Foot. “Indeed Einstein pointed out a consequence of [entanglement] which is so bizarre that he thought there must be something wrong.”

Atoms possess certain properties, such as their weight, charge, and the direction of spin of their electrons. At close to absolute zero, though, the direction of spin is like the Duke of York’s men: neither up nor down. “It is in a state of indecision,” says Foot.

A pair of atoms in this undecided state has what Einstein called a “spooky” influence on each other, even at a distance. These entangled atoms can communicate to their partners without the information following any path as we traditionally understand it. It’s as if the information is teleported from one atom to another.

“By understanding [entanglement] we can do new things such as build quantum computers that, in the future, could store and process far more information than ordinary computers and may outperform them in certain applications, e.g. cracking the encryption commonly used to transmit information
electronically,” says Foot.

Ultra-precise clocks

Another practical applications of atoms at very cold temperatures has already been realised; the development of ultra precise clocks that lose less than one second in 100 million years.

Atomic clocks operate using the natural frequency of the atom. Every atom of a particular type – say rubidium – has electrons at certain energy levels. A photon hitting a rubidium atom will bounce one of its electrons into a higher energy level, creating a measurable frequency – this is the tick of the atomic clock.

At very cold temperatures, though, it is much easier to measure the natural frequency of the atoms (the clock’s ‘tick’), meaning cold atoms make more accurate clocks. Cold atom clocks are created by laser cooling, in tandem with the use of microwaves to form a gravitational trap – a so-called ‘atomic fountain’.

Atomic clocks can accurately measure extremely subtle variations in physical parameters, such as changes in the Earth’s gravitational field.

“In a few years clocks will be able to monitor local changes of the Earth gravitational potential by using relativity, which might help us forecast tsunamis, earthquakes, or global climate warming,” said quantum physicist Christophe Salomon, director of the cold Fermi gas group at the Superior Normal School, in Paris, France and principal investigator for the ACES (Atomic Clock Ensemble in Space) space clock mission.

Cold beams of matter

Also in development by cold atom scientists are atomic lasers. Atomic lasers are created from a super cold cloud of atoms. Slowly releasing the atoms in a steady stream creates lasers made from focused beams of matter rather than light.

Atomic lasers could be used to make extremely accurate sensors for magnetic, electric and gravitational fields or to make precise measurements of rotation and accelerations, for instance on satellites or spacecraft that are out of reach of radio signals.

Recently, scientists from the Australian National University in Canberra published the details of the world’s first refillable atom laser in journal Nature Physics. Built by the same team that produced the first BEC in Australia, in 2001, the ANU team created an atom laser using a cloud of BEC that taps into a neighbouring cloud to refuel; much like a water bucket is refilled by a tap.

Although currently expensive and confined to research labs, devices based on atom lasers will find uses in the real world, says John Close, head of the ANU’s Atom Laser Group. These uses might missiles that secretly track their own path, or spacecraft that can navigate without assistance from Earth. But we’ll probably have to wait at least another two years before the first practical devices that contain atoms lasers are built, he adds.

In the meantime, as scientists push closer to the ultimate cold, their path is a road strewn with discovery.

Heather Catchpole is a science writer based in Sydney and a regular contributor to COSMOS.
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