Quantum versus classical polarization states: when multipoles count

01.01.2014, 00:00

Newsletter 7

The standard notion of polarization comes from treating light as a beam. This suggests the existence of a well-defined direction of propagation and a specific transverse plane within which the tip of the electric field describes an ellipse. This polarization ellipse can be elegantly visualized by using the Poincaré sphere and is determined by the Stokes parameters. In the quantum domain, this classical setting can be immediately mimicked in terms of the Stokes operators, which can be obtained from the Stokes parameters by quantizing the field amplitudes. However, the appearance of hurdles such as, e.g., the presence of hidden polarization, shows that the resulting theory is insufficient. The root of these difficulties can be traced to the fact that classical polarization is chiefly built on first-order moments of the Stokes variables, whereas higher-order moments can play a major role for quantum fields. In this work we have developed a multipole expansion of the density matrix that naturally sorts successive moments of the Stokes variables. The dipole term, being just the first-order moment, can be identified with the classical picture, while the other multipoles (the figure shows one example) account for higher-order fluctuations.

Contact: lsanchez(at)ucm(dot)es
Group: Leuchs Division
Reference:  L L Sánchez-Soto et al., J. Phys. B: At. Mol. Opt. Phys. 46 104011 (2013).

The paper was chosen for IOPselect.