It was with great pleasure when on Tuesday morning I read that Serge Haroche and Dave Wineland had been awarded the 2012 Nobel Prize in Physics "for ground-breaking experimental methods that enable measuring and manipulation of individual quantum systems."
Quantum mechanics is the branch of physics that describes how matter and energy behave at the most fundamental scale. Everything is, at its most basic, quantum-mechanical. Atoms obey the laws of quantum mechanics, and so do soccer balls. So why wasn't famed Argentine soccer player Lionel Messi also part of the citation? After all, one of the key features of quantum mechanics is the ability of a single atom to be in several places at once, and Messi can manipulate a soccer ball to similar effect. That's because atoms behave in far stranger ways than soccer balls. Over the last few decades, Haroche and Wineland have measured and manipulated individual atoms in ways that are stranger and more stirring than any World Cup soccer match.
At the beginning of the twentieth century, founders of the then new discipline named it quantum theory, after the Latin word meaning, "how much". They used the word quantum to signify phenomena that one normally thinks of as continuous, such as waves of light or of sound, that are fundamentally discrete. Light, for example, is made up of elementary excitations called photons--particles of light. Similarly, sound is composed of particles called phonons. The motions of electrons in atoms are also quantized, resulting in a discrete set of possible energy states for atoms. When an atom moves from one such energy state to another, it absorbs and emits photons and phonons. In quantum mechanics, continuous things turn out to be discrete. Conversely, things that we think of as being discrete and chunky like atoms or, for that matter, soccer balls have a continuous, wave-like quality to them. This interplay between continuous and discrete natures is known as wave particle duality.
Quantum mechanics is famously weird--its predictions are strange and counter-intuitive. A single atom can be in two places at once. A measurement made on one particle can apparently instantaneously affect another particle light years away, an effect that Schrödinger called entanglement, but that Einstein termed "spooky action at a distance." The detailed dynamics of the interaction of atoms with light and sound is a complex quantum dance replete with all sorts of quantum weirdness. As atoms absorb and emit light, they are in two different energy states at once, and the emitted photons and phonons are entangled with the atoms and with each other. Haroche and Wineland are masters at controlling and directing this herd of elementary particles.
The atom wranglers
To see exactly how deep an impression Haroche and Wineland made on our field, let's take a trip to the American West, where I was living in the summer of 1994. There, a wrangler is a cowboy whose job it is to herd horses and cattle. Cattle on the range are ornery, and it requires lots of wrangling to convince a herd to go one way or another. That summer in Santa Fe, I wasn't wrangling cattle but herds of atoms to make them exchange photons and phonons with each other. I wanted them to perform a particular task. I wanted to make the atoms compute.
Computers operate by flipping bits, the smallest possible chunks of information. A bit represents the distinction between two possible states--yes or no, true or false, zero or one. Their quantum nature makes atoms, photons, and phonons digital; they naturally represent bits. For example, an atom in its lowest energy state or ground state can be taken to represent zero, while the same atom in its first excited energy state represents one. A polarized photon whose electric field wiggles back and forth horizontally can be taken to represent zero, while the same photon wiggling back and forth vertically represents one. The presence of a phonon can be taken to represent one, while its absence represents zero. A herd of atoms exchanging photons and phonons is basically a bunch of bits flipping back and forth. If you could wrangle that herd so that it flipped its bits in the right way, I reasoned, those flipping bits would constitute a computation.
In the 1980s, David Deutsch and Richard Feynman had come up with applications for such quantum computers, if they could only be built. In 1993, I had come up with a plan to make a herd of atoms compute. If you took that herd and zapped them with a carefully crafted string of pulses of laser light--a technique called electromagnetic resonance--then the natural interactions between the atoms would allow them to flip their bits in the form of a computation. In fact, their quantum nature would make them compute in a particularly weird way. As an atom emitted or absorbed a photon, it could be in both the ground state and the first excited state at the same time. The atoms quantum bit, or "qubit," would represent zero and one simultaneously. Interactions between atoms and light could create entanglement and spooky action at a distance.
My plan looked good on paper, as I'm sure did many a plan to take a herd of Texas cattle from Red River to Kansas along the Chisholm trail. But to put it into action, I needed a real atom wrangler, an experimentalist who could actually coax atoms and photons into doing what she or he wanted. Folks told me that the best atom wrangler around was Jeff Kimble of Caltech. So I made the trip from Santa Fe to Pasadena to meet with Jeff and his atoms.
Jeff Kimble is a tall Texan, well-known for having squeezed light harder than it had ever been squeezed before. After we shook hands, my fingers knew how that light felt. Along with Serge Haroche, Jeff is the world master at dripping atoms through optical cavities and having each atom interact strongly with an individual particle of light. Jeff looked at my plan, laughed because he knew it would be difficult to implement, and then got us down to the real business of making atoms and photons perform quantum logic. He also suggested that I head up to Colorado to talk with Dave Wineland.
For years, Dave Wineland and his group at the National Institute of Standards and Technology in Boulder had been building ion traps in order to make more accurate atomic clocks. But when he saw a proposal in 1995 by two brilliant theorists at the University of Innsbruck, Ignacio Cirac and Peter Zoller, he could make ions compute. Cirac and Zoller had come up with an alternative plan for building a quantum computer. Like my proposal, their method involved zapping atoms with lasers. In their case, however, the atoms were ions trapped by electromagnetic fields and interacting with each other by the exchange of phonons--quantized vibrations induced by the electromagnetic repulsion between the ions. Chris Monroe, a young scientist running Dave's ion trap, immediately set to work. Within just a few months, Chris and Dave had the first ion trap quantum computer up and running, performing simple quantum logic operations. When I visited them that year, they had that ion trap twisting and jumping with quanta and had lassoed ions and photons to perform all kinds of quantum weirdness.
The quantum cat's meow
Perhaps the funkiest quantum state that Wineland and his group produced was a so-called "Schrödinger's cat" state. Erwin Schrödinger, in his work of 1935 where he coined the notion of entanglement, suggested the following quantum thought experiment: Imagine an apparatus where the presence of an atom in one place triggers a sequence of events that kills a cat, while the presence of the atom in another leaves the cat alive. Now have the atom be here and there at the same time--completely allowed by the laws of quantum mechanics. The result is a cat that is simultaneously dead and alive.
In the Wild West, a sheriff tracking a bandit would put posters that read, "Wanted: dead or alive." If Schrödinger's cat had been the bandit, the bounty could have been double--it was both dead and alive at the same time. If there was any scientist who could make good on that bounty, it was Dave Wineland. In fact, with his wiry build and full moustache, Dave bears a striking resemblance to the legendary lawman Wyatt Earp. More peaceful than Earp, and kinder to animals than Schrödinger, Dave and his colleagues trained their lasers on an ion in their trap to construct a kind of "Schrödinger's kitten," a quantum of a state in which a whole herd of sound particles (phonons) is both over here (dead) and over there (alive) at the same time.
Meanwhile, in the decidedly non-wild western confines of the École normale supériure, the brilliant French scientist Serge Haroche was persuading herds of photons to be both here and there at the same time. Born in Morocco, Haroche's career was marked by a meteoric rise through France's grandes écoles. He was a true quantum wrangler--ever sensitive to the nuances and moods of the atoms--using his atom-optical Schrödinger's kittens to probe atoms' delicate relationships with their environment. The process by which a quantum system such as an atom flips between being here and there simultaneously to being here or there is called decoherence. The interaction between an atom and other atoms and photons in its surroundings causes that environment effectively to measure the position of the atom. Haroche noted that the highly non-classical quantum state of the Schrödinger's kitten caused it to be highly sensitive to decoherence.
Weird entangled states are often highly sensitive to decoherence--they lose their ability to be in two places simultaneously. This sensitivity makes quantum computation more challenging than ordinary digital computation, and that made Haroche skeptical about the possibilities of large-scale quantum computing. Interestingly, the same physics that makes some entangled states more sensitive to decoherence makes others completely insensitve. These insensitive states are the basis for quantum error correcting codes that allow quantum bits to be flipped in a way that is protected from environmental influences.
I first met Haroche at a conference in Santa Barbara in 1997, where he was declaring that sensitivity to the environment would prevent quantum computers from ever being constructed. I teased Haroche for being too much like Einstein, who had made great contributions to quantum mechanics without ever fully believing in it, a ribbing which today remains true: Haroche has made heroic contributions to the field of quantum computing without believing such computers can be built.
The quantum sensitivity Haroche identified certainly makes quantum computers hard to build, but it's also that very sensitivity that makes funky quantum phenomena such as Schrödinger's cat states the basis for hyper-sensitive detectors and measurement devices. Recently, Wineland's group and others around the world have used quantum interactions between light and atoms to produce optical-frequency atomic clocks that have the potential to be many orders of magnitude more precise than existing atomic clocks. What's bad for quantum computation is good for precision measurement--if life deals you quantum lemons, make quantum lemonade.
Back in the quantum saddle
By wrangling atoms and light, Wineland, Haroche, Kimble, and many others--Nobel laureates and not--have attained an intimacy with the quantum world as never before. These 'quantum whisperers' know how to coax atoms and photons down their weird paths because they know just how weird those paths are. You don't convince people to do things by quarreling with them. Yes, they can be moved by force, but in the end people, horses, soccer balls, and atoms attain great things only when treated with sensitivity and respect.