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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


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The standard quantum limit for the noise of some optical measurement scheme usually refers to the minimum level of quantum noise which can generally 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. 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. (See the article on the Schawlow–Townes linewidth.)
  • 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.


[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

See also: quantum noise, squeezed states of light, shot noise

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