Principles and brief history of Cavity QED 1st S.Haroche, specialized Solvay Lecture, June 4 2010 From the Bohr-Einstein photon box thought experiment… …to the super-high Q cavities of today’s real experiments…. …exploring the quantum dynamics of atoms and photons in a confined space has progressed a lot… Bohr’s draft of a box storing and releasing a photon to test quantum laws… …and its «realization» by Gamov… 1 Early History of Cavity QED: controlling spontaneous emission Early History: Tailoring spontaneous emission in a confined space Spontaneous processes are random. Only their rate can be predicted and, in the case of photon emission, estimated by classical arguments based on Maxwell’s equations The spontaneous emission rate of an excited state depends on the atom’s state, but also on the structure of the surrounding vacuum, which determines the density of modes into which photons are emitted: an atom within boundaries does not radiate as in free space. Similar effects in beta decay: a neutron lives longer in a nucleus than in free space! Spontaneous emission enhancement predicted by E.Purcell in 1945… …and the possibility to inhibit spontaneous emission in atoms suggested by D.Kleppner in 1981 Antennae radiating near reflecting surfaces Dipole \\ to mirror and image cancell Dipole # to mirror and image add up In close-spaced gap, field modes with polarization \\ to mirrors are suppressed When mirrors are curved, focusing effects enhance the resonances, leading to huge emission enhancement factors Emission inhibited Emission enhanced "c/" "c/" 100-1000 or larger Mode density in cavity (peak increases with Q) Dispersive effects: cavity Lamb shifts, Casimir effect Freespace mode density 2l/! When gap is increased to l = l/2, mode density jumps and undergoes resonances for larger l values l/! First demonstration of Purcell effect on atoms Ionization of 23S Ionization of 22P Ionization signal in a ramped electric field applied to the atoms after they leave the cavity. The 22P state ionizes in a larger field, thus at a later time than 23S. Signals corresponding to an average of N atoms crossing together the cavity: N = 3.5, 2 and 1.3 for traces a,b,c respectively. Cavity on resonance (solid line) or offresonance (dashed line). Rydberg atoms prepared in state 23S in a cavity (V=70mm3) resonant with transition 23S22P (!=340 GHz). Enhancement factor: ! = " C / " (23S #22 P ) = 530 at P.Goy, J-M.Raimond, M.Gross et S.Haroche, PRL 50, 1903 (1983) Inhibiting the spontaneous emission of circular Rydberg atoms microwave transition Atom prepared in circular Rydberg state n=22 (orbit parallel to metal plates) Ionisation detector ! / 2L Inhibited transition n=22 # n’=21 at " = 0.45mm R.G.Hulet, E.S.Hilfer et D.Kleppner, PRL 55, 2137 (1985). Atomic transmission versus "/2L: " is swept by Stark effect, L being kept constant. The sharp signal increase for "/2L=1 demonstrates the inhibition of s.e. of the Rydberg atom which survives longer in its initial state. Many enhancement and inhibition experiments in microwave, infrared and optical part of spectrum realized since these pionneering studies… Collective emission in cavity: from Purcell to Dicke Atoms located at equivalent nodal positions in cavity are symetrically coupled to field: they evolve during emission in a subspace invariant by atomic permutation. There is no way to know which atom has emitted when a photon is lost… Two atoms: e, e ! "S = 1 ( e, g + ge 2 ) ! g, g Strong correlations with entanglement spontaneously build up between atoms, making collective dipole larger than when atoms radiate independently Due to this correlation, the spontaneous emission occurs faster than for single atom: this is Dicke superradiance Superradiance rate proportional to number N of atoms A double enhancement effect: ! C (N ) = " N! 0 Purcell factor: ~ number of images in cavity wall collectively emitting with one atom Number of atoms radiating collectively together Observation of Dicke superradiance in a cavity Sample of N=3200 Sodium atoms prepared in Rydberg state 29S, emitting collectively in a cavity resonant with 29S-28P transition at != 162 GHz. The single atom spontaneous emission rate in free space on this transition is $0 =43s-1. Purcell factor: % = $atC / $0 ~ 70. The atom-cavity coupling is switched-off after variable time by applying an electric field in cavity (Stark effect). For each interaction time t , we measure the number of atoms in states 29S and 28P after cavity exit. From an ensemble of 900 realizations of experiment, we reconstruct the histograms of the number Ne of excited atoms as a function of t (in units of tD ~ %N/$0 = 460 ns). Agreement between experimental histograms and theory (solid lines in black) J-M Raimond, P.Goy, M.Gross, C.Fabre et S.Haroche, Phys.Rev.Lett. 49, 1924 (1982) 2. The strong coupling regime of CQED in time-domain: Rydberg-atom microwave experiments From Purcell to Rabi: the strong coupling regime of Cavity QED $c ( &c 2 " !c =
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