Matter waves on a microchip

A few years ago, the first atom lasers were built, devices that produce a beam of atoms with many of the properties of a laser beam. Now physicists at the Max Planck Institute for Quantum Optics and the Ludwig-Maximilians-Universitaet in Munich have demonstrated that atom lasers can be integrated on a microchip (Nature, 4 Oct. 2001). This result dramatically simplifies the production of laser-like matter waves, and can be compared to the step from single transistors to integrated microelectronics. The new devices are expected to become key components in future technologies ranging from ultraprecise measurements to quantum information processing.

A standard lithographic process from microelectronics is used to produce the chip for the magnetic microtrap.

Quantum mechanics has taught us that atoms behave like waves, spreading out in space and being able to interfere much like light waves do. The tiny size of these matter waves and their fast and irregular movement play together to hide them from even a microscope's view under normal conditions. This changed dramatically when the first "atom lasers" were built a few years ago. By realizing a phenomenon known as "Bose-Einstein condensation", physicists were able to put thousands of atoms into the same quantum state, and to take real-life images of this amplified atomic matter wave with an ordinary video camera. Many scientists believe that Bose-Einstein condensates and atom lasers will lead to revolutionary new technologies, just as laser has revolutionized fields as diverse as eye surgery, consumer audio electronics and high-speed telecommunication. However, much like the early lasers, the first atom lasers were bulky machines that filled entire research laboratories. Indeed, to suspend ("trap") the cloud of atoms, they used large, water-cooled electromagnets that consumed as much electrical power as a few dozen 'hair dryers'. Moreover, the most advanced vacuum technologies were needed to isolate the atoms from their environment while they were being cooled down to the near-absolute zero temperature which is required for Bose-Einstein condensation.

Now, Wolfgang Hänsel, Peter Hommelhoff, Theodor W. Hänsch and Jakob Reichel of the Max Planck Institute for Quantum Optics and the University of Munich have dramatically simplified atom laser construction by using a thumbnail-sized microchip to achieve Bose-Einstein condensation. In their experiments, the cloud of condensed atoms hovers just above the micron-sized gold wires which are inscribed on the chip. The microchip not only replaces the coils and cuts the power consumption to a small fraction of what it used to be. In addition, it drastically shortens the time required to produce the condensate, from about minute to a few seconds. This in turn relaxes the stringent vacuum requirements of the older experiments.

The chip is mounted upside down in an evacuated glass cell. A silver coating on the chip reflects laser beams, which are necessary to capture the Rubidium atoms from the background vapour.

But the new technique has another advantage, which may well turn out to be the most important one. Just as in microelectronics, many "atom-optical" components may be integrated on a single chip. In these first experiments already, the Munich researchers have successfully transported the fragile matter waves along the chip surface, using a special arrangement of microwires to create a "magnetic conveyor belt". This demonstrates the versatility of the new method, and will likely become a standard tool in future devices which employ Bose-Einstein condensates. As a next step, the team proposes an integrated atom interferometer, which could be used as an ultra-sensitive magnetic field probe. Other devices will follow. The new chip technique is so simple that many laboratories will adopt it and use it in new applications.

Chip substrate (original size: 2.3 cm x 1.9 cm): the Rubidium atoms are trapped in a distance of 50 to 200 micrometers from the substrate. Alternating currents in the undulating wires produce a magnetic conveyor belt, which transports the Bose-Einstein condensate along the chip surface.

Artists' view of the experimental apparatus. The chip is mounted upside down in an evacuated glass cell. Below, a colour-coded image shows actual experimental data of a Bose-Einstein condensate that is transported in the conveyor belt (horizontal), and then released into free fall. The shape of the expanding cloud contains information of the formerly trapped atoms and shows that the atoms remain condensed throughout the whole transport (the asymmetric red feature in the center of the expanded clouds is a key signature for Bose-Einstein condensation).