FIG. 2. (Color online) Conditional probability of revealing the difference of two squeezed vacuum states |ψ1〉 = S(r)|0〉 and |ψ2〉 = S(s)|0〉 in the ideal case (η = 1) as a function of δ− = |r − s|. Solid lines, from top to bottom, correspond to the optimal squeezed-states comparator (blue) and the universal comparator (red line). The dashed line is the upper bound on the probability in the case of only two possible squeezings. See Sec. IV C for details.
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