Realization of Measurement and the Standard Quantum Limit
What measurement is there? It is a difficult question but the importance of this question has increased much in connection with the effort to detect gravitational radiation. For monitoring the position of a free mass such as the gravitational-wave interferometer , it is usually supposed that the sensitivity is limited by the so called standard quantum limit (SQL) [2,3]. In the recent controversy –, started with Yuen’s proposal  of a measurement which beats the SQL, the meaning of the SQL has been much clarified. In order to settle this controversy, rigorous treatment of the question on what measurement there is seems the key point.
KeywordsMeasurement Probability Probe System Contractive State Density Operator Position Measurement
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