The Josephson effect is a macroscopic manifestation of quantum mechanics usually seen in superconductors. Observation of this effect in a gas of ultracold atoms demonstrates the underlying unity of solid and gaseous systems.
On page 579 of this issue1, Levy et al. report the observation of the Josephson effect in a cold atomic gas. Brian Josephson discovered the original version of this effect in 1962, when he was a young graduate student at the University of Cambridge2, and it earned him a share of the 1973 Nobel Prize in Physics.
Josephson considered what happens when two superconducting plates are placed next to each other with an insulating layer between them. In a world determined by classical physics, this makes a simple capacitor that stores up static electric charges. But quantum mechanics warns us that it is hard to pin a particle down in any one place. Because of this, if the Josephson junction is formed with a sufficiently thin insulator, electrons from one plate can tunnel through the barrier to the other plate, resulting in a flow of current.
For plates made of normally conducting metal, this tunnelling is haphazard, and the effect is equivalent to that of a resistor shorting the capacitor. But in a superconductor, where current can flow without resistance, the quantum state of the electrons is highly correlated, and the tunnelling becomes coherent. Josephson's breakthrough was to realize that this meant interference could be observed, because the tunnelling wavefunction from one electrode combines with that from the other in a way that depends on their relative phase. This interference gives rise to two main effects. First, a steady current can flow through the junction even when no voltage is applied. Second, when a steady voltage is applied, an oscillating current results. These are known respectively as the d.c. and a.c. Josephson effects. They are at the heart of many important technologies, particularly in the measurement of electric and magnetic fields.
Levy et al.1 do not use superconductors, but start with a gas of rubidium atoms in the form of a Bose–Einstein condensate. Condensates are a kind of superfluid, in which the atoms share a quantum wavefunction just as electrons do in a superconductor. The atoms are at a temperature just a few billionths of a degree above absolute zero, and are held in vacuum using a magnetic trap (a box to keep them from drifting away). Levy et al. divided this trap in two by sending a tightly focused laser beam through its centre. The beam formed a barrier for the atoms that was analogous to the insulating layer in the original Josephson effect; it was similarly narrow enough that atoms on one side had a non-zero probability of tunnelling through to the other.
Following a suggestion made by Giovanazzi et al.3, the authors observed the a.c. Josephson effect by moving the beam from the centre of the box towards one side (Fig. 1a). Atoms in a condensate normally repel one another, so the energy of the atoms on the side that was compressed increased as they were crowded against their neighbours. This interaction energy, also known as the chemical potential, served as the equivalent of the voltage in an electrical circuit: it made the compressed atoms want to flow across the barrier to the other side. But when the authors monitored the number of atoms on the compressed side, they saw that it did not simply decrease; instead, it oscillated in time. This is because the tunnelling rate depends on the interference of the two quantum waves representing the two separated condensates, and the sign of this interference oscillates in time when the condensates have different energies. Previous experiments had exposed the same sort of effect in an array of many condensates4, but this is the first time that a simple, two-state system directly analogous to a superconducting Josephson junction has been observed.
Levy and colleagues' atomic junction1 also exhibited the d.c. Josephson effect. In this case, the laser was again displaced to one side, but much more slowly (Fig. 1b). As it moved, the authors found that atoms were flowing through the barrier, but that the densities of the atoms, and therefore the chemical potentials of the two condensates, remained precisely equal. This is the analogue of a supercurrent — a flow without a voltage to drive it. They observed this behaviour as long as the beam was moved more slowly than a critical velocity of about 40 µm s−1. A corresponding critical value of the current occurs in the superconducting case.
Even compared with superconductors, Bose–Einstein condensates are delicate and difficult to produce. So it is unlikely that the atomic Josephson effect will find applications as widespread as the electrical version. But the authors do point out the possibility of using the atomic effect to create an exquisitely sensitive rotation detector that might find use in 'inertial guidance' systems for rockets and aircraft.
Above all, however, this work represents a milestone in the effort to map the physics of solid matter onto atomic systems of ultracold gases. The advantage of this mapping is that atomic systems are much more easily controlled. For instance, it was a simple matter for Levy et al. to vary the height and width of their tunnelling barrier by adjusting the optics of their laser beam. In a superconducting system, such tests would require the fabrication of entirely new samples. More generally, laser beams can be used to create defect-free 'atomic crystals' of arbitrary symmetry and adjustable binding strength5. Even the interactions between atoms — analogous to the charge for electrons — can be tuned6.
The hope is that a 'proving ground' might be created, where theories could be tested under various well-controlled conditions and where the complicated phenomena of real material systems could be sorted out. Levy et al.1 have strengthened this link by showing that atomic condensates can mimic superconductors in one of their hallmark effects.
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New Journal of Physics (2008)