From the very first Michelson Interferometer invented over 100 years ago to today’s kilometre-scale gravitational wave detectors the sensitivity of interferometric length measurements has been improved by about 10 orders of magnitude and is now limited by the so-called Standard Quantum Limit (SQL), a manifestation of Heisenberg’s Uncertainty Principle. The SQL is comprised of the inevitable combination of sensing noise (photon shot noise) and back action noise (photon radiation pressure noise) when repeatedly measuring the position of a test mass. However, by measuring a different variable, i.e. the test mass velocity (speedmeter) instead of its position (position-meter), it is possible to evade back action noise. The momentum of a free test mass can be measured continuously to arbitrary accuracy without being limited by the SQL. Since a Sagnac interferometer is sensitive only to the time-dependent part of the arm-length difference it is automatically a speed meter and therefore brings measurements beyond the SQL into our reach.Theoretical analyses have shown that the speedmeter approach is the most promising track towards wide-band sub-SQL measurements. An experimental test of this technique is urgently required! Therefore, my three main objectives of this proposal are: 1) Realisation of an ultra-low noise, quantum radiation pressure dominated speedmeter test bed. 2) Experimental demonstration of back action noise supression in a Sagnac speedmeter. 3) Development of speedmeter based sub-SQL interferometery for future gravitational wave detectors such as the Einstein Telescope.By the end of this project I will have demonstrated the sub-SQL potential of the Sagnac speedmeter configuration. A positive outcome of this project is expected to lead to the Sagnac speedmeter superseding the Michelson interferometer as state-of-the-art instrument for ultra-high sensitivity lengths measurements.