= Ramsey separated oscillatory fields experiment
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A moving <chemical atom> or <molecule> can experience the two pulses of <Ramsey interferometry> by flying through two short oscillatory-field regions. Their common <oscillator> supplies the reference <quantum phase>; the flight between them supplies the interrogation time. The long interval need not contain a driving field. This turns a spatial apparatus into a coherent pulse sequence and makes narrow <spectral lines> possible without filling the entire flight path with a uniform oscillatory field.
<Norman Foster Ramsey, Jr.>'s 1950 paper calculates the two-state transition <probability> and averages it over molecular <velocities>. It is a theoretical investigation, rather than a report of an <atomic clock>'s measured accuracy.<A Molecular Beam Resonance Method with Separated Oscillating Fields by Norman Foster Ramsey, Jr.>[Physical Review 78 (1950), pp. 695–699, abstract: separated interaction regions, transition probability and molecular-velocity average]{ref}
His 1989 lecture contrasts an electric-resonance apparatus photograph with a magnetic-resonance schematic. Coherent drive regions bracket a static-field flight region; state-dependent deflection converts transitions into detector changes, while differing flight times suppress side fringes.<Experiments with Separated Oscillatory Fields and Hydrogen Masers by Norman Foster Ramsey, Jr.>[1989 lecture, pp. 553–558, Figs. 1–2, 4–6]{ref}
The readout infers a change of internal <quantum state> through state-dependent transmission, as in the <Rabi molecular beam magnetic resonance experiment>. A useful control is to block one drive region: a narrow fringe requiring both interactions must disappear. Changing the second region's <oscillation phase> should move the fringe, whereas an incoherent background need not move. These are consequences of the model below, not additional historical measurements. A detector dip alone neither proves perfect preparation nor determines all apparatus-induced <frequency> shifts; <cesium-beam clock procedure> adds the controls needed for a practical <atomic clock>.
The publication sequence begins with <Norman Foster Ramsey, Jr.>'s 1949 article https://doi.org/10.1103/PhysRev.76.996[A New Molecular Beam Resonance Method], Physical Review 76, p. 996, published 1 October; the fuller theoretical paper followed in 1950.https://journals.aps.org/pr/abstract/10.1103/PhysRev.76.996[Byline and publication record]{ref}
NIST-7 supplied a practical realization: <optical pumping> moved <caesium> populations into the $F=3$ manifold, without isolating $m_F=0$. Two separated <microwave> interactions probed the clock transition; <fluorescence> detected transferred $F=4$ atoms. A synthesized reference, tied to a <hydrogen maser>, sampled opposite sides of resonance for <feedback control>. Beam reversal tested cavity <quantum phase> bias; independent <velocity> determinations checked motion corrections. The observed central fringe resolved the transition, but power-dependent velocity weighting and light shifts still required evaluation.<Primary Atomic Frequency Standards at NIST by D. B. Sullivan and others>[pp. 50–52, §§2.2–2.2.4, Fig. 2]{ref}
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