Quantum nondemolition detection and four-mode squeezing
Typically, when a laser beam is measured with a photodetector, the light is absorbed. Thus information about the amplitude of the beam is obtained at the cost of its destruction. A portion of the beam can be diverted with a beam-splitter and measured, leaving the remainder of the beam available for further measurements. At the quantum level, this also destroys any correlation between the amplitude of the detected beam and the transmitted beam. Both of these measurements are termed "quantum demolition" measurements because they do not provide information about the quantum state of the light subsequent to the measurement event. A method of measuring light amplitude which did not alter that amplitude would be valuable to experimentalists. Such a method has now been realized using nonlinear optics.
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