In this note we want to discuss different quantum non-demolition techniques, often also called quantum noise reduction methods, for the design study towards a European third-generation earth-bound laser interferometer gravitational-wave detector, the Einstein Telescope (ET). We will on the one hand analyze the theoretically possible improvement in sensitivity produced by a certain QND technique for a detector of the envisaged scales and on the other hand we shall also comment on problems and disadvantages in the implementation of such techniques and their feasibility for the gravitational-wave detection.

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