Comparison of the Hanbury Brown - Twiss effect for bosons and fermionsDutch version (FOM persbericht)
Fifty years ago Robert Hanbury Brown and Richard Twiss (HBT) discovered photon bunching in light emitted by a chaotic source and
used this effect to determine stellar diameters. The quantum interpretation of bunching relies on the constructive
interference between amplitudes involving two indistinguishable photons, and its additive character is linked to the Bose
nature of photons.
The bunching effect can also be observed with bosonic atoms, as demonstrated by the group of Chris Westbrook and Alain Aspect in Orsay
for metastable Helium-4. In Amsterdam we have, in a collaboration with the Orsay group, made a comparison of the HBT effect
for bosonic Helium-4 and fermionic Helium-3. Our experimental setup allows cooling and trapping of both helium isotopes to
temperatures around one microKelvin above absolute zero.
The experimental setup is shown in the figure below.
A cold cloud of metastable helium atoms is released at the switch-off of a magnetic trap. The cloud falls under the influence of gravity onto a time-resolved and position-sensitive detector and detects single atoms. The inset shows conceptually the two 2-particle amplitudes (in black or grey) that interfere to give either bunching or antibunching.
To observe this effect helium atoms have to be cooled to temperatures close to absolute zero. Only then can the atoms be confined to a small enough volume in space with velocities that are low enough to see the effect. How this is done can be found here. In the plot below the bunching effect for helium-4 (in red) and the antibunching effect for helium-3 (in blue) are shown in a plot of the normalized correlation function, that describes the number of pairs that are observed at one detector point for different time differences. These time differences are translated to vertical pair separations. Each point represents the normalized number of pairs observed in a certain time difference bin.
It is the limited detector resoltution that prevents the observation of the full correlation function. We do not see that it reaches 2 for the bosons and 0 for the fermions. The effect of the resolution has been circumvented by using a diverging atom lens. This causes a smaller effective source size and therefore a larger antibunching effect for the fermions, as illustrated by the measurement below. Shown is the normalized correlation function for Helium-3 with (dark blue squares) and without (light blue circles) a diverging atomic lens in the x-y plane.