Condensates create Josephson arrays
Aug 2, 2001
The rate of discovery in Bose condensate research continues apace with new breakthroughs occurring every month. In the latest advance Italian physicists have created an array of Josephson junctions in a condensate (F S Cataliotti et al. 2001 Science 293 843). The achievement could have implications for researchers working in fields as diverse as optical fibres and biological molecules.
Bose condensation happens in a gas of atoms when the de Broglie wavelength of the atoms becomes comparable with their average separation. All the atoms in the gas then collapse into the same quantum ground state, which gives the condensate many unusual properties. Gases must be cooled to within a fraction of a Kelvin above absolute zero for condensation to occur.
Massimo Inguscio and co-workers at the European Laboratory for Nonlinear Spectroscopy (LENS) and the Istituto Nationale per la Fisica della Materia, both in Florence, the universities of Florence and Padova, and the SISSA institute in Trieste, produced a condensate of rubidium-87 using a magnetic trap and a laser standing wave. The standing wave essentially splits the trap into an array of disk-shaped wells, each about 400 nanometres apart. There are about 200 wells, each containing 1000 or so atoms, and the height of the energy barrier between the wells can be changed by adjusting the laser intensity. Quantum theory allows the atoms to tunnel through the barriers.
In a standard Josephson junction two superconductors or two superfluids are separated by a potential barrier. However, the phase difference between the two sides causes a Josephson current to flow through the barrier. Inguscio and co-workers have observed a similar current of atoms in their device, and they have confirmed that the behaviour of the system is described by a nonlinear Schrodinger equation.
A large number of experiments have already been performed with two- and three-dimensional arrays of Josephson junctions. However, it is difficult to create one-dimensional arrays in these systems. Bose condensates therefore offer the opportunity to observe the large number of new effects that have been predicted to exist in these systems. New effects are expected in quantum phase transitions, nonlinear dynamics, optical fibres, biological molecules and other areas of research.