Mesoscopic behaviour of the neutral Fermi gas 3He confined in quantum wires

The behaviour of electron gases in restricted geometries provides a means to explore the fundamental quantum-mechanical properties of fermion gases at mesoscopic length scales1. But the existence of Coulomb repulsion between electrons unavoidably complicates the physics. Quantum gases of neutral fermions — such as 3He quasiparticles in a dilute solution of 3He in 4He, cooled to millikelvin temperatures2 — therefore offer a means of probing regimes completely inaccessible to electronic systems. Here we demonstrate the quantum exclusion of a 3He fermion gas from a network of narrow channels, connected to a reservoir of 3He/4He solution. The effect is expected from simple quantum-mechanical arguments, which predict that the 3He atoms cannot enter the channels when their wavelength exceeds √2 times the channel width. By adjusting the temperature of the solution, the energy of the particles and hence their average wavelength can becontrolled. In this way, we observe temperature-dependent changes in the penetration of the 3He quasiparticles into the channels. Our results demonstrate the macroscopic response of an atomic gas to basic quantum-mechanical restrictions at the mesoscopic level.