Trapped-Ion State Detection through Coherent Motion
Till P. Rosenband, David Hume, Chin-Wen Chou, David R. Leibrandt, Michael J. Thorpe, David J. Wineland
Coherent control of trapped atomic ions has led to several recent advances in quantum information processing1 and precision spectroscopy2. A basic requirement for these and other quantum-metrology experiments is the ability to detect the state of an individual ion. Current detection techniques36 are applicable only to relatively simple atomic systems, which precludes precision measurement and control of most atomic and molecular species. Here, we explore a detection strategy based on state-dependent excitation of coherent motion that can be applied to a larger class of ion systems. We couple a spectroscopy ion to a control ion in the same trap via mutual Coulomb repulsion. For state detection, off-resonant laser excitation induces harmonic motion of the two ions with an amplitude that depends on the state of the spectroscopy ion. We implement two methods of measuring this motion with the control ion. In the first, the ion velocity modulates fluorescence from the control ion, which is observed via time-resolved photon counting7, 8. We demonstrate this technique by detecting the clock states in 27Al+ (1S0 or 3P0) using 25Mg+ as a control ion. In the second method, coherent motion near the ground state switches the atomic state of the control ion via resolved sideband transitions4, 9. This approach provides higher sensitivity to motion at small amplitudes and is demonstrated by performing projective measurements of several Zeeman sublevels in the 27Al+1S0 ground state. As opposed to previous methods5, 6, state detection can be performed without spontaneous emission, thereby avoiding optical pumping on the spectroscopy ion. Applications ranging from simplified detection in portable optical clocks to precision molecular spectroscopy10 could benefit from these techniques.