Abstract
Spontaneous emission from nuclear spins is measured for the first time, we believe. This is possible because of the use of a low-noise SQUID amplifier with noise temperature (~0.2 K) less than that of the 1.5 K helium bath. The spontaneous emission power is ~5-10% of the background circuit Nyquist noise, which corresponds to the ratio of the effective spin resistance Rs (proportional to the nuclear susceptibility) and circuit resistance R. Although only one spin flip occurs per 108 yr due to spontaneous omission, the effect is magnified, because all 1022 spins in the sample contribute independent of the temperature. This technique implies that spectra of spins can be measured without the requirement of a net polarization and might be useful for samples where spin lattice relaxation times are impracticaily long. Radiation damping terms which are added to the Bloch equations allow for spin noise analysis of power flow between the spin system and cavity. The equivalence is obtained of predictions of Nyquist's theorem at all spin temperatures Ts and bath temperatures T in conformity with the predictions of the Einstein detailed balance equation for spontaneous emission. The model of a macroscopic spin pendulum applies where fluctuation through a mean square angle 〈θ2〉 from magnetization alignment along the polarizing field accounts for coupling of the fluctuating spins to the single-cavity mode. Several aspects of the model for spin noise analysis are applicable to inverted two-level systems.
© 1987 Optical Society of America
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