The 1998 Wolf Prize has been awarded to Yakir Aharonov and Sir Michael Berry for the discoveries that bear their names – the Aharonov- Bohm effect and the Berry phase. Both effects – subtle and unexpected consequences of quantum theory – have subsequently been observed in many different types of experiment.
The official citation states that the awards are made “for the discovery of quantum topological and geometrical phases, specifically the Aharonov-Bohm effect, the Berry phase, and their incorporation into many fields of physics”. These fields include optics, nuclear physics, fluid physics, chemistry, molecular physics, string theory, gravitational physics, cosmology, solid-state physics, the foundations of quantum mechanics and quantum computing.
In 1959 Aharonov and the late David Bohm proposed a thought experiment in which a charged particle is directed towards a tube of magnetic flux. The particle is split quantum mechanically so that one component passes to one side, say the left, of the tube, while the other passes to the right. Although the magnetic field is zero outside the tube, the vector potential associated with the field is not zero. This led Aharonov and Bohm to predict that the two components of the electron would acquire different phases due to their interaction with the vector potential – even though the field itself was zero – and that the difference between these phases could be detected via interference. The effect was soon observed in experiments.
In 1984 Berry showed that, under certain conditions, quantum systems could acquire what is known as a geometric phase. Berry was studying quantum systems in which the Hamiltonian describing the system is slowly changed so that it eventually returns to its initial form. Berry showed that the adiabatic theorem widely used to describe such systems was incomplete. In particular he found that the system acquired a phase factor that depended on the path followed, but not on the rate at which the Hamiltonian was changed. This geometric phase factor, now known as the Berry phase, was later shown to be a generalization of the Aharonov-Bohm effect.
Aharonov currently holds joint appointments at Tel Aviv University in Israel and the University of South Carolina in the US. Berry is Royal Society Research Professor at the University of Bristol in the UK. The Prize, worth $100, 000, will be presented by the president of Israel, Ezer Weizman, in Jerusalem in May.
