The spectral density of the Nyquist noise current in a tuned circuit coupled to a sample of nuclear spins has been measured at ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ temperatures with a dc SQUID used as a rf amplifier. When the sample is in thermal equilibrium, a dip is observed in the spectral density at the Larmor frequency. For zero spin polarization, on the other hand, a bump in the spectral density is observed. This bump is due to temperature-independent fluctuations in the transverse component of magnetization, and represents spontaneous emission from the spins into the circuit.

• Received 12 August 1985

DOI:https://doi.org/10.1103/PhysRevLett.55.1742