Thermal Noise in ir Quantum Counters

Because an ion's strength of interaction with the lattice vibrations is not appreciably greater than with the radiation field, for energy gaps $>{\displaystyle {lim}^{\sim }}{10}^3\hspace{0.17em}{\mathrm{cm}}^{\text{−1}}$ , the 300°K background radiation field will play an important role in determining the equilibrium population of the near‐infrared electronic levels. As a consequence, the Infrared Quantum Counter fluorescence produced by thermal population of the device's infrared level (a generalized form of spontaneous emission noise) cannot be reduced indefinitely by choosing arbitrarily cold cryogenic baths. It is shown that ``soft'' lattices with weak ion‐lattice interactions produce the most noise of this type, yet yield the best detector sensitivity. Qualitative experimental confirmation is provided, and the IRQC is shown to be potentially useful, if laser pumping can be achieved, despite the noise limitation imposed by the thermal background.