Schematic diagram of the uncertainty areas in the phase difference and charge number (f, n) phase space of:
(a) the vacuum state,
(b) a "number" state,
(c) a coherent state, and
(d) a squeezed state.
In these diagrams, the phases are measured in units of (EC/EJ)^0.25, while the charge numbers in units of (EJ/EC)^0.25. In addition, the "number" state here is not an eigenstate of the charge number operator n. It is actually and eigenstate of the harmonic part of the Hamiltonians. Notice that the coherent state has the same uncertainty area as the vacuum state, and that both areas are circular, while the squeezed state has an elliptical uncertainty areas. Therefore, in the director parallel to the q/2 line, the squeezed state has a smaller noise than both the vacuum and coherent states.
Image Source: X. Hu, Quantum Fluctuations In Condensed Matter Systems,
UM Ph.D. thesis 1997, Page 103.
X. Hu and F. Nori, UM Preprint, 1995;.