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# Quantum defect

The term quantum defect is highly ambiguous. Various meanings are discussed below.

## Quantum defect in laser science

In laser science, the term quantum defect refers to the fact that the energy of a pump photon is generally higher than that of an signal photon (photon of the output radiation). The difference of energies goes to the heat; this heat may carry away the excess of entropy delivered with the multimode uncoherent pump.

The quantum defect of a laser can be defined as part of the energy of the pumping photon, which is lost (not turned into photons at the lasing wavelength) in the gain medium at the lasing.[1] At given frequency $~\omega_{\rm p}~$ of pump and given frequency $~\omega_{\rm s}~$ of lasing, the quantum defect $~q=\hbar\omega_{\rm p}-\hbar\omega_{\rm s}~$. Such quantum defect has dimension of energy; for the efficient operation, the temperature of the gain medium (meadured in units of energy) should be small compared to the quantum defect.

At a fixed pump frequency, the higher the quantum defect, the lower is the upper bound for the power efficiency.

## Quantum defect in Rydberg atoms

The quantum defect of a Rydberg atom refers to a correction applied to the equations governing Rydberg atom behavior to take into account the fact that the inner electrons do not entirely screen their associated charge in the nucleus[2].

## References

1. ^ T.Y.Fan (1993). "Heat generation in Nd:YAG and Yb:YAG". IEEE Journal of Quantum Electronics 29: 1457-1459.
2. ^ http://www.phy.davidson.edu/StuHome/joesten/IntLab/final/rydberg.htm , Rydberg Atoms and the Quantum Defect at the site of Davidson College, Physics department