Atoms that are cooled below a millikelvin can also be held in space with either static magnetic fields, laser fields, or a combination of a weak magnetic field and circularly polarized light. Atoms traps can be used to accumulate a large number of laser-cooled atoms; confine them for further cooling and use them for studies of atomic collisions at very low temperatures.
A magnetic trap exerts forces on atom via the atom’s magnetic moment. The potential energy of a magnetic dipole in a magnetic field B is given by –μ.B, where μ is the magnetic moment, normally on the order of 1 μB.
An illustration of atoms that are magnetically trapped is shown below:
The spherical quadrupole trap above shows some of the magnetic field lines and the direction of the currents. At the center of the trapping coils, the B-field is 0 and increases linearly as you move radially outward. An atom with its magnetic dipole aligned antiparallel to the B-field minimizes its energy by seeking regions where the field magnitude is smallest i.e. the atom experiences a force that drives it to the center of the trap. If the atom moves slowly in the trap, it can remain aligned antiparallel to the magnetic field, even if the field changes direction. Typically, if the magnetic moment precesses or spins rapidly around a slowly changing magnetic field, it continues to spin around a changing field axis. In quantum mechanics, we say that, the state of the atom “adiabatically” follows external field conditions that define the quantum state. We can also have atoms held with a laser beam by using intense light tuned far from an atomic resonance to polarize the atom. The induced dipole moment p on the atom points in the same direction as the driving electric field as long as the frequency of the light is below the atom resonance frequency. Therefore, the potential energy of the atom in the light field, -p.ε, causes the atom to seek regions of highest laser intensity.
Also read: The Equivalence Principle & Black Holes
