PETERBOROUGH, N.H. It sounds wildly impractical: define magnetic waveguides on silicon chips so as to route clumps of supercold atoms into cavities, where their wave functions can interfere with one another. The result? In effect, it's circuits in which atoms process information.
Impractical or not, the technique is being perfected at a half-dozen research labs around the world in an attempt to build an atomic analogue of laser optoelectronics. Scientists are already doing useful things with these chips and say it might be just a few years before commercial versions appear. Indeed, proponents predict that the technology not only will be practical but could represent as fundamental a development as the laser itself.
The interest in building single-chip Bose-Einstein condensate (BEC) processors, which use cold atoms instead of electrons to process information, lies in the unique advantages that atoms bring to the mature science of coherent radiation. BECs share the property that makes laser light so useful: coherence. A group of atoms at close to absolute zero all have the same wave function and therefore represent one large superparticle.
Development is just at the point of creating some basic components, such as interferometers. But at least one researcher Dana Anderson at the University of Colorado believes atom tunneling will soon be demonstrated. Just as electron tunneling is a key property of electrons that makes semiconductor devices work, atom tunneling would do the same for BEC chips, making it possible to build analogues of transistors and other devices.
"In the manner that ultracold atoms interact with each other, there are excellent analogues that can be made between electronic devices and atomic ones," said Anderson, who heads an optical laboratory at the university. His group has already built a Michaelson interferometer that uses a standing light wave to split and recombine a beam of coherent atoms.
If the nascent discipline the Defense Advanced Research Projects Agency calls "atomtronics" turns out to be practical, what kinds of applications could atom chips address? "Ultracold-atom science is still sufficiently new and evolving so rapidly that identifying applications should be considered target practice with closed eyes," Anderson said. Nevertheless, he ticked off a number of applications, some near-term and others more speculative.
In the near-term category, Anderson cited inertial sensing, slow-light and nonlinear optical phenomena, and quantum computing. Further down the road, he sees the technology moving out of physics labs and into mainstream electronics. A single chip with its own source of coherent atoms would enable atom versions of conventional components such as an ultralow-noise microwave amplifier.
Atom chip technology "can be placed into the hands of potential users who can then invent, design and create useful systems without having the enormous expertise currently required to produce ultracold atoms," Anderson said.
Photonic gyroscopes sense motion, but because photons have almost no mass, they are relatively insensitive. In contrast, BECs have mass and the same ability as light to register shifts via coherent interference. Theoretical estimates put BEC gyroscopes at about 11 orders of magnitude higher in sensitivity than optical versions.
"Cold atoms have enormous potential in this application domain, which includes accelerometers, gyroscopes and timing devices," Anderson said. "Already, cold-atom systems have achieved the most accurate measurements, by far, of the acceleration, g, due to the Earth's gravity." In addition, atomic circuits could support on-chip clocks that would be as accurate as atomic clocks.
And where photons are weak when it comes to the kinds of interactions required by logic devices, cold atoms remedy that by generating strong nonlinear action between photons. The hope is that they could be combined with integrated-laser optical circuits to make a superior information-processing capability.
"It is rather clear that one can integrate cold-atom technology and optical technology on a single chip," Anderson said. He pointed to "signs of that" emerging in work at MIT, in Germany "and in our own work at the University of Colorado."
In quantum computing, meanwhile, no capability has emerged that's comparable to the integrated circuit, in which large numbers of gates can be easily replicated. Anderson pointed to atom chips as "a reasonable candidate" for the job. In fact, he said, "if it is possible to do atom-chip-based logic gates, then it is indeed probably the way to do it, since we can benefit from the substantially well-developed electronics industry infrastructure."
So far Anderson has reduced a benchtop BEC apparatus a vacuum chamber used to cool atoms to the point where they enter the BEC state to a unit that can be held in one hand. The bulkiest part of the contraption is an ion pump that's needed to load the cold atoms onto a microelectromechanical-system chip. The ultimate goal is self-contained chips that use magnetic fields and laser diodes to cool the atoms so that the system has a ready supply of cold matter.
The breakthrough that made atom chips a practical proposition was the discovery that groups of atoms within a few nano-Kelvins of absolute zero could exist in a suspended state above a surface at room temperature. That obviated the need to cool the entire chip to ultralow temperatures.
"The BEC couples only very weakly to the environment and in a way that we understand," said Ed Hinds, a researcher at the Imperial College of London's Centre for Cold Matter. "Even though the BEC has a temperature of nano-Kelvins and the chip is at room temperature, the quantum relaxation times can be many seconds, or even hundreds of seconds in the best cases."
That, Hinds said, is "very similar" to electronics, where electrons and holes have a finite time for scattering or recombining. "But this time can be controlled, and in devices where it needs to be long, this can often be achieved at room temperature."
Hinds and his colleagues are looking into building quantum processing arrays using atom chips. While photons must travel at the speed of light simply to exist, atoms particularly ultracold ones can be very slow-moving. Hinds is attempting to line them up in coherent arrays.
"We are aiming to make a string of 100 atoms," he said. "Then we will find out what level of control is really possible. At the moment, this is a hard question to answer, and it will still be some years before anyone can do so."
On the other hand, Hinds is confident that improved laser cavities will enable totally integrated atom chips with their own BEC sources on board. "I would say it is bound to happen," he said, "and probably quite soon."