The existence of permanent currents is surprising, even for experts, since they occur in ordinary, non-superconducting metals where, due to electrical resistance, current can only flow with applied voltage. The measured steady currents are based on an effect of quantum physics influencing the movement of electrons in metals. Ultimately, they can be thought of as an expression of the same movement that allows the electrons in the atom to constantly circle around the atomic nucleus.
An experimental demonstration of continuous currents is difficult. They cannot be directly measured with a conventional flow meter because they flow only in metal rings with a diameter of about one micrometer, or micron. A micron is about one hundredth of the diameter of a human hair and is comparable to the size of the wires in a silicon computer chip. In previous experiments, attempts were made to demonstrate the constant flows using the magnetic field caused by them. (A magnetic field is always generated when a current flows through a wire.) For that, highly sensitive magnetic field probes were used, so-called SQUIDs (Superconducting Quantum Interference Device). The results of these experiments, however, were inconsistent and sometimes differed widely from the theoretical predictions.
The experiments now conducted at Yale University under the direction of Jack Harris were based on a different and novel strategy. The metal rings were applied to the tip of a nanocantilever – a kind of swinging miniature diving board. The current flowing in the rings led to a magnetic force on the cantilever and could thus be demonstrated by means of changes in vibrations of the “springboard.” After many years of optimization, using this method it has now been possible to demonstrate the continuous currents much more accurately than ever before and to measure them.
The most comprehensive, theoretical predictions for these experiments go back to the 15-year-old doctoral thesis by Felix von Oppen, who is currently a researcher at Dahlem Center for Complex Quantum Systems at Freie Universität Berlin. Working jointly with American scientists von Oppen’s results were expanded based on facts established by the new experimental method. In particular, it was necessary to take effects of the theory of relativity (the so-called spin-orbit coupling) into account in the theoretical calculations. Measurements taken after these adjustments were made, confirm the theoretical predictions with great accuracy.
The greatly improved method of measurement has not only resolved an old enigma, but also thrown open the door to numerous experiments that scientists anticipate will provide new insights into the behavior of electrons in metals. New results could, for example, identify metals that could potentially serve as superconductors or elucidate the behavior of qubits, the building blocks of a future quantum computer.