Quantum nondemolition measurement of a light field component by a feedback compensated beam splitter

In conventional quantum nondemolition measurements, the interaction between signal and probe preserves the measured variable. Alternatively, it is possible to restore the original value of the variable by feedback. In this paper, we describe a quantum nondemolition measurement of a quadrature component of the light field using a feedback compensated beam splitter. The noise induced by the vacuum port of the beam splitter is compensated by a linear feedback resulting in an effective amplification of the observed variable. This amplification is then be reversed by optical parametric amplification to restore the original value of the field component. The measurement backaction required by the uncertainty principle is a fundamental feature of quantum mechanics. In particular, this principle can be applied in quantum communications to prevent or detect eavesdropping [1]. However, most measurements in quantum optics are performed by irreversibly absorbing the measured light. It is therefore a special challenge to devise experimental realizations of back-action evasion measurements that allow the non-destructive observation of light field properties [2]. Usually, quantum nondemolition measurements of the light field are realized by a non-linear coupling between the signal field and a meter field [3–7]. In the following, a simple beam splitter will be used to couple the signal and the meter fields. While this method initially adds noise to the observed property of the signal field, it has been shown that this noise can be compensated by a measurement dependent feedback [8]. The result of this compensation is an amplification of the observed field component. By reversing this amplification in an optical parametric amplifier, the original value of the observable is restored. It is then possible to realize a quantum nondemolition measurement of a quadrature component using only one single mode parametric amplification step performed after the original beam splitter measurement. This approach thus represents a considerable simplification compared to the previous realizations of quadrature quantum nondemolition measurements which used two mode parametric amplification in order to couple the meter mode to the signal mode [5–7].