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Physical Review Letters

Observation of Quantum Jumps

Thilo Sauter, Werner Neuhauser, R. Blatt, P. E. Toschek

Published Oct 6, 1986• 649 citations
Publisher

Abstract

We have recorded the laser-excited resonance fluorescence of one to three ${\mathrm{Ba}}^{+}$ ions and observed Bohr's "quantum jumps" when an ion decayed to the metastable $^{2}D_{\frac{5}{2}}$ state, suddenly quenching its fluorescence. Most of the jumps mark Raman-Stokes scattering from the $^{2}S_{\frac{1}{2}}$ and $^{2}D_{\frac{3}{2}}$ levels.

DOI
10.1103/physrevlett.57.1696

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We have recorded the laser-excited resonance fluorescence of one to three Ba+{\mathrm{Ba}}^{+}Ba+ ions and observed Bohr's "quantum jumps" when an ion decayed to the metastable 2D52^{2}D_{\frac{5}{2}}2D25​​ state, suddenly quenching its fluorescence. Most of the jumps mark Raman-Stokes scattering from the 2S12^{2}S_{\frac{1}{2}}2S21​​ and 2D32^{2}D_{\frac{3}{2}}2D23​​ levels.
We detect the radiatively driven electric quadrupole transition to the metastable 2D52^{2}D_{\frac{5}{2}}2D25​​ state in a single, laser-cooled Hg II ion by monitoring the abrupt cessation of the fluorescence signal from the laser-excited $^{2}S_{\frac{1}{2}}\ensuremath{\rightarrow}^{2}P_{\frac{1}{2}}$ first resonance line. When the ion "jumps" back from the metastable DDD state to the ground SSS state, the $S\ensuremath{\rightarrow}P$ resonance fluorescence signal immediately returns. The statistical properties of the quantum jumps are investigated; for example, photon antibunching in the emission from the DDD state is observed with 100% efficiency.
One of the experiments made possible by the trapping and cooling of a single atom is to observe the quantum jumps to a metastable state in the atom by monitoring the resonance fluorescence of a strong transition in which at least one of the states is coupled to the metastable state. When the atomic electron moves to the metastable state, the fluorescence from the strongly driven transition disappears. When the electron drops back into the strongly driven transition, the fluorescence abruptly returns. Statistical analysis of these random interruptions to the fluorescence signal implies photon antibunching in the emission of the weak photon.
Quantum jumps due solely to spontaneous Raman scattering between the Zeeman sublevels of a single 24Mg+^{24}\mathrm{Mg}^{+}24Mg+ ion have been observed in the fluorescence emitted by the ion. A theory of quantum jumps for this system predicts that coherences between excited levels cause the ratio of the mean duration of the "fluorescence-on periods" to the mean duration of the "fluorescence-off periods" to be independent of laser intensity. The measured value agrees with the predicted one to within the measurement precision of 2%. The distribution of the durations of the off periods also agrees with theory.
Recent experiments have found abrupt changes in the intensity of laser light scattered off single ions and atoms. These abrupt changes have been interpreted as resulting from Bohr's ``quantum jumps'' of the ion or atom from one state to another. In Bohr's description, such quantum jumps are not predicted by the Schr\"odinger equation and are introduced by additional postulates into quantum theory. We consider here an alternative explanation of these experiments using only properties of solutions of the Schr\"odinger equation and avoiding Bohr's added postulate.