Illustrated Physics
physics, drawn
Concept

Optical lattice — where it appears

A periodic pattern of light intensity made by crossing laser beams, in which cold atoms feel a potential and settle one to a site. Its geometry is set by the beams' directions, its depth by their power, and it holds the atoms of the most accurate clocks.

Named by 2 essays across 2 fields — each of them below, with the objects they name alongside it.

A hill that is always ahead of the atom. The light-shift potentials of the two ground sublevels of a spin-½ atom in two counter-propagating beams with crossed linear polarisations, over one and a half wavelengths, in units of the well depth. The polarisation of the light turns from σ+ to linear to σ− every quarter wavelength, and the two sublevels see sinusoidal potentials a quarter wavelength out of step. Optical pumping transfers the atom from one sublevel to the other fastest exactly where its own potential is highest — the fastest pumping and the hilltop coincide, which the figure checks — and that is the bottom of the other potential. The heavy line is an atom that starts with 3.3 well depths of kinetic energy: each time it reaches a hilltop it is pumped down by exactly one well depth, climbs the next hill, and is pumped down again, 3 times, until it no longer has the energy to reach a hilltop and is left oscillating in one well. The energy is carried off by the pumping photon, which leaves bluer than the light that drove it by the depth of the well.

The limit that belonged to a simpler atom

The theory of laser cooling predicted a floor, and the first careful measurement came in six times below it. Nothing was wrong with the measurement or the arithmetic. The floor belonged to an atom with one ground state, and real atoms have several — which lets the light build a hill in front of every atom, move it to the bottom before it can roll back, and repeat the trick until the atom is a few microkelvin from rest.

astrophysics · Radiation pressure 3 beams crossing: change the phases and the pattern only slides. The intensity where 3 plane waves of one wavelength cross in a plane, their directions spread evenly round the circle, for three different sets of relative phases, each panel spanning 2.2 wavelengths. Darkest shading is brightest, in five steps of a fifth of the peak. The three patterns are the same lattice of bright spots moved sideways: for each of the altered sets there is a shift that reproduces the first pattern to 2.0 per cent of the peak. With 3 beams in two dimensions there are 2 relative phases and 2 directions to slide in, so every change of phase is a slide.

The lattice no phase can bend

Cross a few laser beams of one colour and they paint a crystal of light in the space where they overlap. The beams' phases drift whenever a mirror trembles, and it would seem the pattern must tremble with them. Whether it does depends on a count. Three beams in a plane, or four in space, give a lattice whose shape no change of phase can alter — the phases can only slide it. One beam more and the phases decide the shape, and must be held still.

waves · Superposition

Named alongside it

The objects these essays reach for when they reach for this one.

Laser coolingDoppler limitInterferenceInterferometryLight shiftOptical molassesOptical pumpingPhasePlane wavePolarisationRadiation pressureReciprocal lattice

All concepts