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Standard Quantum Limit

Acronym: SQL

Definition: a limit for noise levels set by quantum mechanics

German: Standard-Quantenlimit

Categories: quantum optics, fluctuations and noise

How to cite the article; suggest additional literature


The standard quantum limit for the noise of some optical measurement scheme usually refers to the minimum level of quantum noise which can be obtained without the use of squeezed states of light. Some examples are:

  • The standard quantum limit for high-frequency intensity noise of a laser is the shot noise limit, and a lower intensity noise level can be obtained only with amplitude-squeezed light.
  • There is also a standard quantum limit for phase noise, which is reached by a laser only at high noise frequencies.
  • The sensitivity of various optical measurements, e.g. of length measurements with interferometers [2], has a standard quantum limit.
  • The same applies to certain correlations e.g. of intensity noise between two optical beams.

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[1]C. M. Caves, “Quantum-mechanical radiation-pressure fluctuations in an interferometer”, Phys. Rev. Lett. 45 (2), 75 (1980), doi:10.1103/PhysRevLett.45.75
[2]M. T. Jaekel and S. Reynaud, “Quantum limits in interferometric measurements”, Europhys. Lett. 13, 301 (1990), doi:10.1209/0295-5075/13/4/003
[3]H. M. Wiseman, “Light amplification without stimulated emission: beyond the standard quantum limit to the laser linewidth”, Phys. Rev. A 60 (5), 4083 (1999), doi:10.1103/PhysRevA.60.4083
[4]V. Giovannetti et al., “Quantum-enhanced measurements: beating the standard quantum limit”, Science 306 (5700), 1330 (2004), doi:10.1126/science.1104149

(Suggest additional literature!)

See also: quantum noise, squeezed states of light, shot noise
and other articles in the categories quantum optics, fluctuations and noise


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