Development of Quantum-Non-Demolition laser interferometer using displacement noise cancelling technique.

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High-sensitivity displacement sensor had been developed to beat Standard Quantum Limit (SQL) using a technique of Displacement-noise Free Interferometer (DFI). Extracting the idea and results of prototype DFI, a package of basic techniques needed to observe and then overcome the SQL was demonstrated. After a series of upgrades of instruments including vacuum enclosure and intensive light source, we have tried to improve the sensitivity of the instrument as a displacement sensor. Employing the monolithic interferometer that fix the optics rigidly on the single base plate for a Mach-Zhender interferometer, the sensitivity of the interferometer was limited by the shot noise (quantum noise) down to several Hz, which corresponds to the level less sensitive by only factor of 10 to reach to the SQL.