Partial-wave interference

In quantum scattering. Two research groups, one in New Zealand, the other in the Netherlands, have banged quantum gases together at velocities of up to 20 cm/s. Both groups cooled clouds of rubidium-87 atoms to the nanokelvin regime. Each group then manipulated magnetic fields to create a double-well potential with roughly half of the original cloud on each side. The physicists then quickly changed the potential back to a single well; the two clouds then accelerated to the potential minimum whereupon their constituent atoms scattered off each other. Absorption images revealed the resulting halo distributions of atoms. At collision energies less than about 100 µK, the distribution was spherically symmetric, due to pure s-wave scattering. At energies above about 300 µK, the distribution was dumbbell-shaped, from pure d-wave scattering. At intermediate energies, quantum interference effects were apparent as shown here in an absorption image taken at a collision energy of 192 µK by the New Zealand physicists. The Dutch group took similar images and also used their data to directly determine the scattering length for the cold bosonic gas: 102 Bohr radii, which agrees with the accepted value of 99 a ₀. (N. R. Thomas et al., Phys. Rev. Lett. 93, 173201, 2004;http://dx.doi.org/10.1103/PhysRevLett.93.173201 C. Buggle et al., Phys. Rev. Lett. 93, 173202, 2004 http://dx.doi.org/10.1103/PhysRevLett.93.173202 .)