Profile of David Wineland and Serge Haroche, 2012 Nobel laureates in physics.

As a postdoctoral student at the Massachusetts Institute of Technology in 1978, I trolled the literature, seeking inspiration for my career as an experimental physicist. One article that caught my attention described the first laser-cooling experiment. David Wineland et al. at the National Bureau of Standards (NBS, now the National Institute of Standards and Technology) in Boulder, CO, had trapped a cloud of hot (about 700 K) ions and cooled them to near absolute zero (T < 40 K) by shining light on them (1). (See also ref. 2, which reports an independent and nearly simultaneous demonstration of laser cooling of ions.) Later that year, I was hired by NBS in Gaithersburg, MD and, inspired by Wineland’s landmark experiment, began to work on laser-cooling of neutral atoms. Earlier, in 1975 with Hans Dehmelt, Wineland had first proposed laser cooling (3), simultaneously with Ted Hansch and Art Schawlow (4). I often talk about laser-cooling, showing a slide of the idea’s origins and remarking that among Wineland, Dehmelt, Hansch, and Schwalow, three had received Nobel prizes in physics. On October 9, 2012, the Royal Swedish Academy of Sciences completed the foursome by awarding the 2012 Nobel Prize in Physics to David Wineland. The award is shared with Serge Haroche of the Collège de France, Paris “for ground-breaking experimental methods that enable measuring and manipulation of individual quantum systems.” Wineland and Haroche realized a longstanding dream of quantum physics: studying the behavior of single quantum objects. The founders of quantum mechanics believed that studying a single quantum system, like a single atom or a single photon, was beyond the realm of experimental possibility. Many believed that it did not even make sense to talk about a single atom; only the behavior of an ensemble could be meaningful. In fact, Schrödinger asserted: “. . .we never experiment with just one electron or atom ... In thought experiments, we sometimes assume that we do; this invariably entails ridiculous consequences. . .” (5). The groups of Haroche and Wineland turned this idea on its head; not only did they use individual atoms and photons to elucidate some of the strangest aspects of quantum mechanics, they have even used them to make practical devices. A key advance of Wineland’s group was to go from trapping a cloud of many “hot” ions (a few degrees above absolute zero) to holding individual ions in the trap “ground state.” Quantum mechanics shows that the energy of center-of-mass motion of any confined particle takes on only specific (quantized) values. Ordinary particles at ordinary temperatures are distributed among millions and millions of such energy levels. However, at the temperatures in Wineland’s experiment, microdegrees above absolute zero, the single ion is—with near 100% probability—in one particular energy level, the lowest, ground state. Starting with such a pure and well-defined quantum state, Wineland and his colleagues experimentally demonstrated some of the classic thoughtexperiments of modern physics. Among these experiments were the observation of quantum jumps as a single atom changed its state after absorbing or emitting a photon and the creation of “Schrödinger cat” states: having an ion in two macroscopically separated places at the same time. Haroche did much the same thing, but trapping photons, particles of light (or microwaves) in a cavity, where the photons bounce back and forth between two mirrors a billion times before escaping. Cooling the mirrors to less than a kelvin nearly eliminates the thermal photons in the cavity so Haroche’s group could study one or a few photons that they deliberately put into the cavity. By passing single atoms through the cavity and analyzing the state of the atoms as they emerged, Haroche’s group determined exactly how many photons the cavity held. In a dramatic demonstration of a classic thought-experiment, Haroche and colleagues put an indeterminate number of photons into the cavity, then began to measure the number, watching the cavity progressively “collapse” to a specific (but unpredictable) number of photons. Then, as photons slowly leaked out of the cavity, they could see the “death” of each individual photon. Haroche et al. also created “cat” states where the cavity, before measurement, was in a superposition containing National Institute of Standards and Technology physicist David Wineland adjusts an ultraviolet laser beam used to manipulate ions in a high-vacuum apparatus containing an “ion trap.” Copyright Geoffrey Wheeler.