If two identical quantum particles collide and rebound, the very act of observing one of them interferes with the behaviour of the second. The interference pattern depends on whether the particles are bosons or fermions. In the latest issue of Nature Yoshihisa Yamamoto of Stanford University, California, and colleagues report that they have measured this effect in fermion collisions for the first time.
Bosons (for example, photons) cause constructive interference patterns. This means that the probability that both particles are detected in the same direction is enhanced by the intereference.
On the other hand fermions (such as electrons), are predicted to cause destructive interference patterns. This suggests that, compared to classical physics, the probability of finding the particles moving in the same direction after a collision should decrease. Such quantum interference is responsible for the Pauli exclusion principle, which states that two electrons can never occupy the same state.
Yamamoto and colleagues at Stanford and NTT Basic Research Laboratories in Japan have designed an experiment to observe this behaviour. The apparatus depended on a mesoscopic electron beam splitter etched onto a gallium arsenide electron gas system. To reduce current noise, the apparatus was cooled to 1.6 Kelvin. According to their calculations, the fermion collision noise level should be 52% that of the classical collision noise level if no quantum effects occur. However, the researchers measured a suppression of collision noise level which averages to 56% of the classical value, thus indicating the presence of quantum interference.
