Uncertainty Regions for the Vacuum, Coherent, and Squeezed States
Schematic diagram of the uncertainty areas in the generalized coordinate and momentum (X, P) phase space of a photon squeezed state (ellipse) and a coherent state (circle). In quantum optics, X and P are the two quadratures of the electric field. Notice that the coherent state has the same uncertainty area as the vacuum state (circle at the origin), and that its area is circular, while the squeezed state has an elliptical uncertainty area. Therefore, in the direction parallel to the q/2 line, the squeezed state has a smaller noise than the coherent state.
Image Source: X. Hu, Quantum Fluctuations In Condensed Matter Systems, UM Ph.D. Thesis 1997, Page 5.
Information on squeezed states in quantum optics can be found in:
D. Walls, Nature 306, 141, 1983.
M.C. Teich and B.E.A. Saleh, 1988 in Progress in Optics 26, 1, ed. E. Wolf.