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In 1990, Wayne M. Itano, Daniel J. Heinzen, J. J. Bollinger and David J. Wineland reported inhibited transitions in about 5,000 ${}^9{\mathrm{Be}}^{+}$ ions held in a cylindrical Penning trap. Trapped magnesium ions, illuminated at 280 nm, kept the beryllium ions cold. A 320.7 MHz radio frequency field coupled two ground-state hyperfine structure sublevels during a 256 ms pulse adjusted for complete transfer. Up to 64 intervening 313 nm pulses addressed an optical transition from level 1: scattering distinguished it from level 2. Afterward, calibrated fluorescence counts over 100 ms measured the final quantum state population. This ensemble readout did not select ions that remained in the initial quantum state at every check; departure followed by return also contributes. The apparatus made inhibition by repeated quantum measurements experimentally accessible.^[ref]

David J. Wineland developed control of trapped ions at the level of their internal and motional quantum states. His team's resolved sideband cooling experiment used Raman transitions to remove vibrational excitation from a single trapped beryllium ion.

Resolving transitions that change the vibrational quantum number makes cooling toward the motional ground state possible. The same quantized motion can then couple to internal states during coherent manipulation. Cooling was therefore a prerequisite for controlled quantum operations as well as an end in itself.^[ref]

Electron shelving distinguishes an internal quantum state outside a cycling optical transition from one that scatters many photons. In the mercury-ion example in David J. Wineland's lecture, $S\leftrightarrow P$ excitation produces $194\mathrm{nm}$ fluorescence; transfer to a metastable atomic level $D$ suppresses it. A photomultiplier records bright/dark intervals. Coherent $282\mathrm{nm}$ interrogation precedes detection to avoid perturbation by the detection beam.^[ref]

“Dark” means weak scattering under a particular detection setting. The motional ground state is dark to an ideal red motional sideband for a different reason: there is no lower number state. It can still fluoresce strongly on a cycling detection transition. Shelving's population readout also need not preserve a quantum superposition's quantum coherence; scattered photons carry information about the internal state, and their recoil can disturb motion.

Awarded in equal shares to Serge Haroche and David J. Wineland for experimental methods enabling measurement and manipulation of individual quantum systems.^[ref]

Wayne M. Itano, Daniel J. Heinzen, J. J. Bollinger and David J. Wineland, Physical Review A 41(5), 2295–2300, published 1 March 1990.