In this example, light pulse phase ϕ ϕ ^ ^ j serves as the readout observable O of the Meter. Then we suppose that the phase ϕ ϕ ^ ^ j r e f l measurement error introduced by the detector is much smaller than the initial uncertainty of the phases Δ Δ ϕ ϕ . In this case, the initial uncertainty will be the only source of the position measurement error: