J. Phys. II France
Volume 2, Numéro 4, April 1992
Page(s) 573 - 577
DOI: 10.1051/jp2:1992152
J. Phys. II France 2 (1992) 573-577

Quantum mechanics and the science of measurements

Norman F. Ramsey

Lyman Laboratory of Physics, Harvard University, Cambridge, MA 02138, U.S.A.

(Received 29 July 1991, accepted 7 January 1991)

The accuracies of measurements of almost all fundamental physical constants have increased by factors of about 10,000 during the past 60 years. Although some of the improvements are due to greater care, most are due to new techniques based on quantum mechanics. In popular accounts of quantum mechanics, such great emphases is placed on the Heisenberg Uncertainty Principle that it often appears that the primary effect of quantum mechanics should be to diminish measurement accuracy whereas in most cases it is the validity of quantum mechanics that makes possible the vastly improved measurement accuracies. Seven quantum features that have a profound influence on the science of measurements are: (1) Existence of discrete quantum states of energy $W_{\rm i}$. (2) Energy conservation in transitions between two states. (3) Electromagnetic radiation of frequency $\nu$ is quantized with energy $h\nu$ per quantum. (4) The identity principle. (5) The Heisenberg Uncertainty Principle. (6) Addition of probability amplitudes (not probabilities) so $P=\vert\psi_1 +\psi_2 \vert^2 \neq \vert\psi_1\vert^2 + \vert\psi_2 \vert^2$. (7) Wave and coherent phase phenomena. Of these seven quantum features, only the Heisenberg Uncertainty Principle limits the accuracy of measurements, and its affect is often negligibly small. The other six features make possible much more accurate measurements of quantum systems than with almost all classical systems and the identity principle provides meaning and significance to highly precise measurements with quantized systems. These effects are discussed and illustrated.

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