= Haroche cavity photon nondemolition experiment
{c}
In the 2007 <cavity quantum electrodynamics> experiment of <Christine Guerlin> and collaborators, including <Sébastien Gleyzes>, <Michel Brune>, <Jean-Michel Raimond> and <Serge Haroche>, an injected <coherent state> occupied a 51.1 GHz <microwave> cavity with <niobium>-coated mirrors exhibiting <superconductivity>. Velocity-selected <rubidium> <chemical atoms> were prepared as circular <Rydberg atoms> in levels with <principal quantum numbers> 50 and 51. Repeated atomic readouts progressively narrowed the inferred <photon number> to a <Fock state>; later records revealed jumps. The cavity damping time was $T_c=0.130\,\mathrm{s}$. Empty-cavity <Ramsey interferometry> scans fixed the analysis-phase origin; weak injected fields refined the response calibration. Finite fringe contrast required calibrated <likelihoods>; photon losses could interrupt convergence.<Progressive field-state collapse and quantum non-demolition photon counting by Christine Guerlin and others>[Progressive field-state collapse and quantum non-demolition photon counting, PDF pp. 2–6, Eqs. (1)–(2), Figs. 2–4 and Methods]{ref}
The functional sequence is preparation, first <Ramsey interferometry> zone $R_1$, dispersive cavity $C$, second zone $R_2$, and state-selective detection. The first $\pi/2$ <microwave> pulse prepares an atomic <quantum superposition>; <dispersive atom-cavity coupling> adds a <photon number>-dependent relative <quantum phase> while suppressing energy exchange. The second pulse converts that <quantum phase> into atomic populations; <state-selective field ionization> reads $e$ or $g$. Successive <chemical atoms> carry the meter records away while the same field remains available.<Controlling Photons in a Box and Exploring the Quantum to Classical Boundary by Serge Haroche>[Controlling Photons in a Box and Exploring the Quantum to Classical Boundary, discussion of Fig. 10 (apparatus) and Fig. 11 (repeated atomic detection)]{ref}
\Image[/haroche-ramsey-cavity-photon-readout.png]
{title=Atoms carry photon-number information from the cavity to a detector}
{description=Original functional schematic, not a scaled apparatus drawing. The atom is the meter; the final detector reads the atom.}
This arrangement makes <quantum nondemolition measurement> useful for following a field repeatedly and preparing a <Fock state> by conditioning. The <photon number> is inferred from a sequence of physical records; it is not a directly visible label attached to a photon. Each atomic probe makes a weak, partial <quantum measurement> of <photon number>, and the accumulated records can approach a resolved count. A single binary meter need not identify one among many possible <Fock states>, as <perfect discrimination of pure states requires orthogonality>.
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