This is the Standard Quantum Limit for such a 2-pulse procedure. In principle, if we limit our measurement to two pulses only and do not care about perturbing mirror position afterwards, the second pulse measurement uncertainty, Δ Δ x m e a s ( t j + 1 ) , can, in theory, be reduced to 0 (it will yield, of course, Δ Δ p b . a . ( t j + 1 ) → → ∞ ∞ ) and the limit of displacement measurement error will reduce to: