1. Context

Light from a laser is fundamentally characterized by four main parameters: wavelength, direction of propagation, energy and polarization state. Current techniques make it easy to control the first three parameters. The wavelength is generally fixed by the laser and, in linear propagation, undergoes little or no variation during propagation. The direction of propagation can be easily controlled using mirrors or waveguides, such as optical fibers. Likewise, the energy (or number of photons) of the laser beam can be easily reduced using attenuators, or increased using amplifiers (such as an erbium-doped fiber amplifier). The evolution of these three parameters is generally perfectly predictable, and systems for controlling them independently of each other are now well established and commercially available.

Polarization, on the other hand, is a trickier parameter to...