Quantum Horse Races and Crystals of Light
SINGAPORE—I'd heard of quantum dice, quantum poker, quantum roulette, and even quantum Russian roulette, but a quantum horse race? I learned about this surreal game of chance last December during a symposium at the Centre for Quantum Technologies in Singapore. Start with a row of rubidium atoms, place your bets, let ʼem go, and measure their positions some time later. The atoms don't gallop so much as ooze: their quantum wavefunctions begin as sharp peaks and spread laterally. In fact, they ooze out both sides of the starting gate. "Since these are quantum horses, they can run in two directions simultaneously," said German experimental physicist Immanuel Bloch. (Watch his animation or whole talk.)
It wasn't the quantum weirdness that wowed me—we should all be used to that by now—but the fact we're talking about individual atoms here. Atoms. In olden times (before 1990), physicists worried that quantum uncertainty might foil attempts to manipulate matter on this scale. Even Richard Feynman's famous nanotech lecture in 1959 envisioned atomic artisanship only "ultimately—in the great future." Today experimentalists see atoms, poke atoms, prod atoms. Atoms' quantumness has not been a hindrance but a benefit. Bloch's horse race has the serious purpose of demonstrating that single atoms could store and process data in a quantum computer.
The tough part isn't so much that atoms are small, but that they don't stay put and that they bunch up. Bloch's team and others bring them to heel by cooling them to a temperature of nanokelvins and pouring them into an optical lattice, which, depending on your poetic frame of mind, you might call an optical egg crate or a crystal of light. It consists of laser beams that overlap and interfere, forming a stationary grid of bright and dark spots. The light exerts electromagnetic forces that wrest atoms into these spots (typically the bright ones) and pin them there. The atoms are spaced perhaps 400 nanometers apart, so they reach a density of about 100 trillion atoms per cubic centimeter—which is a lot of atoms per cubic centimeter, but still only about a hundred-thousandth the density of hydrogen gas at room temperature and pressure. So these systems let physicists explore a domain they seldom otherwise enter, a frigid, sparse realm where quantum is king.
In particular, experimentalists are exploring the central problem in the physics of materials (what physicists call condensed matter): how matter makes the transition between different phases. The lattice-imprisoned aggregate of atoms can melt, like an ice cube, except that the transition arises from a rebalancing of energy rather than a change in temperature or pressure.
The atoms' energy is both kinetic (they jump from one site to another through the process of quantum tunneling) and interactive (they repel one another). When the laser intensity is low, the electromagnetic forces trapping atoms are weak, so kinetic energy dominates and atoms are footloose. They no longer call one site home, but ooze all over the place, acting in unison as one ginormous quantum wave—a phase of matter called a superfluid. When the laser intensity is strong, the interaction energy dominates. Atoms hold one another at arm's length and stay confined to specific spots on the grid—a phase of matter called a Mott insulator. A decade ago, Bloch and his colleagues gradually dialed up the intensity of their laser to watch the transition from superfluid to insulator.
This ability to fine-tune the system also lets physicists take pictures of individual atoms. First, crank up the interaction energy so that atoms space themselves out. Then, illuminate them with light that they absorb and re-emit. Through an optical microscope, you can see the atoms fluoresce. If their overall density is low enough, each spot is a single atom.
A similar procedure can control atoms one by one. First, shine a laser beam on the atom you'd like to muck with; this shifts its energy levels and makes it vulnerable to microwave radiation. Then, beam in a microwave pulse to flip the atom's spin. Bloch's team has applied this technique to create the world's smallest pixel display (see picture). The atoms stay in place, so this approach differs from the nanosculpture that IBM scientists famously used two decades ago to create the world's smallest corporate logo. Like the horse race, nanoartistry is practice for building a quantum computer.
There are all sorts of other fun experiments you can do. Last year, Bloch's team tracked the insulator-superfluid transition and showed that the system goes through a "hidden" phase of matter—a subtly patterned arrangement that conventional theory doesn’t capture. Theorists have drawn on ideas from string theory, of all places, to help explain these transitions. Lately, the team has played with the Higgs mechanism. To most physics aficionados, the name "Higgs" conjures up the boson that particle physicists may finally be on the verge of discovering. But the concept actually originates in condensed matter: it explains why electromagnetic forces do not propagate freely in a superconductor. (In fact, the particle-physics Higgs is a sort of superconductor in which weak nuclear forces do not propagate freely.) Bloch's team switched on the Higgs mechanism in its simplified system. The "bosons" were collective vibrations of the atoms.
Yet another experiment touches on the fundamental question of what determines the speed of events in the world. The theory of relativity sets the ultimate speed limit, that of light, but in practice the limit is usually much lower. What sets it? It turns out that quantum mechanics has a self-policing tendency, the Lieb-Robinson bound. Bloch and colleagues have observed it for the first time. They began with an insulator, dialed up the interaction energy, and watched the atoms start to self-organize. A wave of activity spread though the system at twice the speed of sound. What governed the velocity was that atoms did not passively roll on the wave, but actively contributed to it. Some quantum gravity theorists have speculated that the speed of light represents the Lieb-Robinson bound of some underlying quantum system out of which space and time emerge.
To me, the atomic hubbub suggests another meaning for the horse-race metaphor. For would-be nanoengineers, atoms are not tiny nuts and bolts. They are like animals, with a mind of their own. You don't screw them together, but cajole them. If you set the right conditions, they'll do all the work for you. They are not passive, but a creative force capable of things you might never expect.
Figures courtesy of Immanuel Bloch, Christof Weitenberg, and Peter Schauß, Max Planck Institute for Quantum Optics