Abstract

The temporal evolution of a quantum system is retarded by repeated measurements of the observable involved in the evolution. This impediment, the ”Quantum Zeno Effect” [1], has been proven indistinguishable from the effect of dephasing the system’s wave function, except in an individual quantum system [2]. Whereas a measurement on an ensmble results in an expectation value, with an individual system it yields an eigenvalue, and the system becomes reduced to an eigenstate of the observable. As long as the evolution of the quantum system between two subsequent reduction is coherent, there is no base for invoking dephasing. –We have demonstrated the retardation of the light-driven quantum evolution on the line S_1/2 – D_5/2 of a single cooled ¹⁷²Yb⁺ ion placed in the node of the electric field of a 2-mm Paul trap. Irradiation by laser light whose spectral width did not exceed 500 Hz (over 1s) coherently drove this transition, and was alternated with interrogation by light scattering on the S_1/2 – P_1/2 resonance line (Fig. 1). In the resulting trajectories made up of ”on” and ”off” detections, ”on”-”off” pairs indicate acts of the ion’s excitation on the E2 line. The normalized number of these acts yields the absorption probability. With stepwise detuning of the drive light, absorption spectra have been recorded ( Fig.2, top). The modulation is caused by Rabi nutation, i.e. it proves coherent evolution to have happend, that has been stroboscopically sampled (bottom). The trajectories of measurements are made up of sequences of 1…q…equal results whose numbers decrease upon their length q increasing. This statistics demonstrates state reduction to happen with each individual measurement and identifies the degree to which the observations have impeded the quantum evolution of the ion.

© 2000 IEEE