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It is demonstrated that frequency pulling by Δm=±1 hyperfine σ transitions, here called Ramsey pulling, is a real calculable effect in atomic beam frequency standards. An analytic expression for the effects of Ramsey pulling is derived using perturbative techniques for the driving σ transitions, while treating the primary π transitions exactly. It is shown that these Ramsey pulling effects are intrinsically different from Rabi pulling in origin, manifestation, and elimination. These predictions are compared to measurements performed on a cesium beam atomic frequency standard, giving good quantitative verification of the theory and a clear demonstration of the existence of these effects.

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We calculate the signal-to-noise ratio in a dc SQUID systenz as a function of source impedance, taking into account the effects of current and voltage noise sources in the SQUID. The optimization of both tuned and untuned voltmeters and magnetometers is discussed and typical sensitivities are predicted using calculated noise spectra. The calculations are based on an ideal symmetric dc SQUID with [3 = 2LIo/CPo = 1 and moderate noise rounding (F=2rrkBT/Io~o=O.05), where ~o is the flux quantum, T is the temperature, L is the SQUID inductance, and Io is the critical current of each ]unction. The optimum noise temperatures of tuned and untuned voltmeters are found to be 2.8 (~o L/R)Tand 8(wL/R)T(1 + 1.Sa 2 + O. 7a 4) 1/e/a 2respectively ' where w/2rr is the signal frequency, assumed to be much less than the Josephson frequency, and ~ is the coupling coefficient between the SQUID and its input coil. It is found that tuned and untuned magnetometers can be characterized by optimum effective signal energies given by (16kBTLB/a 2R)[I + (1 + 1.5a 2 + O. 7a 2) 1/2 + O. 75a 2] and 2ksTiRiB/oo 2Lp respectively, where B is the bandwidth, R~ is the resistance representing the losses in the tuned circuit at temperature Ti, and Lp is the inductance of the pickup coil.

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