Dipole and lattice traps

There are many ways of manipulating and confining atoms using electric and magnetic fields. Doppler cooling and magneto-optical traps (MOTs) use the radiation force of photons resonant with transitions in the atoms we wish to cool and/or confine. In an optical lattice trap, the trapping mechanism is the interaction between the radiation-induced dipole in the atom and the intensity gradient of the radiation.

When an atom interacts with a light field, this light causes a shift in the energy levels of the atom. It is the gradient of this energy shift that causes the dipole force. If the frequency of the light is lower than the resonance in the atom, the atom is attracted to the region of highest intensity, which gives the lowest energy state. This type of trap is a conservative trap, which means it does not reduce the temperature or velocity of the atoms it confines in the way that a MOT does. The atoms must therefore be cooled before they can be loaded into such a trap, which is often done by pre-cooling in a MOT. One can think of the dipole trap like a bowl with atoms rolling around inside it.  The hotter the atoms, the higher up the bowl they will roll. However, if the atoms are too hot, they will roll up and over the bowl edge and be lost to the trap. The intensity of the laser beams determines the temperature of the atoms the trap will be able to hold, with greater intensity corresponding to a deeper trap, able to hold hotter atoms.

A dipole lattice trap is formed by crossing two laser beams or retro-reflecting a single laser beam back on itself. This causes an interference pattern to form, creating a linear “lattice” pattern of high-intensity regions, each of which can trap atoms in the same way as a dipole trap does. These trapping regions are separated by half the laser wavelength.

Why is an optical lattice trap good for metrology?

An optical lattice can be used to hold an ensemble of atoms during an atomic frequency measurement. This set-up is then called a 'lattice clock'. In other types of neutral atom clocks, including Rb and Cs fountain clocks, the clock atoms have to be released from their trap during a frequency measurement because the incident trapping laser light causes a shift of the atomic energy levels. This causes the atoms to ballistically expand at the point in the experiment when one wants then to be the most still. Although not too significant for microwave standards, this expansion creates velocity-related frequency shifts that can be quite large for frequency standards based on optical transitions.  However, as first proposed by Katori, et al^[1], a careful choice of lattice trap laser frequency will shift both the ground and excited states of the clock transition equally. This enables the atoms to be held during the clock measurement, significantly reducing these velocity-related systematic shifts and uncertainties of the clock system. In this way an optical clock made of neutral atoms gains many of the positive features of the single ion optical clock systems, but retains the excellent signal-to-noise ratio and therefore statistics of the neutral atom optical standards. In addition to the confinement that the lattice creates, there are also ways to further cool the atoms once they are being held in a lattice trap, and these lead to further reductions in the systematic uncertainties of the clock system.

1. H. Katori et al., Phys. Rev. Lett. 91, 173005 (2003).