Quantum non-demolition measurements play an important role in quantum theory and many of its applications.In theory they are the most fundamental quantum measurements, but in practice their realization can be chal-lenging due to realistic constraints. In optics for example, most measurements are destructive since photons getabsorbed by the detector. While some simple single particle non-demolition measurements are routinely done inoptical setups by using a second degree of freedom to encode the results at an intermediate stage, measurementsof degenerate non-local observables involving multiple photons remain challenging, especially when these aredone at intermediate measurement strengths. Here we present an optical setup for performing variable strengthnon-demolition measurements of non-local observables in a pre and postselected setting. At the heart of thesetup is an apparatus that can be used to turn a strong (projective) measurement into an arbitrary strengthmeasurement by using a quantum eraser. We present our initial calibration results for this apparatus.
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