Spaced-out electrons

摘要

Like the gentle patter of raindrops, we expect photons, the quanta of sunlight, to arrive at Earth at random intervals, their arrival times distributed in just the same, natural way that customers arrive at a box office to buy tickets for a play. A histogram of the number of people or photons arriving per unit time follows what is known as the Poisson distribution. But in 1909, in the first clear evidence for wave-particle duality, Einstein pointed out that the width of this distribution for sunlight contains both the Poisson contribution of random arrival times, and a second 'wave-noise' contribution, which causes the photons to arrive in bunches. An experiment performed by Kiesel et al., reported on page 392 of this issue, shows that the same principle applies to a beam of coherent electrons―but with the opposite effect. In contrast to the bunching of photons, this experiment confirms the theoretical prediction that a stream of coherent electrons will 'anti-bunch', tending to become more equally spaced than the classical Poisson prediction (Fig. 1, overleaf). In the quantum regime, electrons have an innate tendency to avoid each other, thereby demonstrating a fundamental difference in the way light and electrons interfere with themselves.