Spaced-out electrons.

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). 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. In the 1950s, the astronomers Robert Hanbury Brown and Richard Twiss devised the famous experimental arrangement, known as intensity interferometry, to study these effects for photons in the form of light from distant stars. They split the light into two beams using a half-silvered mirror, then compared the arrival times of photons at two separate detectors. Due to bunching, they saw that coincident photon arrivals were more likely than expected by chance. Hanbury Brown and Twiss realized they could use this bunching to infer the angular size of distant stars. The similar experiment performed by Kiesel et al. with electrons finds coincident arrivals less probable, which indicates antibunching. In fact, although an anti-bunching effect was reported three years ago for electrons in solids, Kiesel and colleagues’ experiment is the first to mimic the original of Hanbury Brown and Twiss for a beam of free particles. It is because the effect is so much weaker with electrons that it has taken nearly fifty years to detect it. Kiesel et al. used a field-emission point source of electrons to illuminate two detecQuantum physics