Altered States and Logic Gates
Whether it’s quantum-mechanical or classical, information processing must consist of stupendous numbers of very simple actions performed by “logic gates” that obey if-then, and-or rules. For example: If X has some value, then change Y to the same value. Or change Y to the opposite value. And so forth. In conventional computers, transistors perform those tasks and pass the results along to other transistors through electronic connections, building up elaborate logic chains seriatim.
In a quantum computer, however, that kind of direct contact between components could easily destroy fragile quantum states. So scientists are searching for ways to create uniquely quantum logic gates in which one object can switch the state of another object with minimal destructive interaction.
That’s not easy. Transistors transfer cleanly defined, fixed-value (0 or 1, on or off, high or low voltage,etc.) information units between gate components. Quantum computer logic gates, however, will have to transfer superpositions of multiple simultaneous states (0 and 1 in some proportion) from one object to another. Very few objects or systems are suitable.
Current favorites include trapped ions, “cavity QED” (in which single atoms and photons are coupled in microscopic enclosures) and trapped electrically neutral atoms. The last are particularly promising because neutral atoms have relatively long coherence times (in which they can remain in the superposition of states) and scientists have developed proven procedures to control them through cooling and trapping.
But the “down-side of neutral atoms,” says JQI Fellow Steve Rolston, “is that they don’t interact much. And because they’re neutral, it’s usually a comparatively weak interaction.”
Moreover, getting them to influence one another usually requires some sort of collision, which runs the risk that mechanical motion from the impact will hasten collapse of the superposition, a condition called “decoherence.”
So about 10 years ago, Rolston and a few collaborators began looking at a novel notion for how to construct a two-atom logic gate. They were interested in taking advantage of peculiar phenomena that occur in Rydberg atoms. An atom in the Rydberg state, named for Swedish physicist Johannes Rydberg (1854-1919), has been super-excited; its outermost electron has absorbed so much energy that it has nearly left the atom. It remains attached, but orbits at an exaggerated distance from the nucleus, giving those atoms certain special properties.
One is electric polarizability, and hence a heightened response to electrical and magnetic fields -- itself a potentially exploitable feature. But perhaps even more useful is a behavior called the “Rydberg blockade.” It works like this: Take two adjacent atoms of the same element, and put one in a Rydberg state by hitting it with just the right frequencies of laser light. Then give the other atom the same treatment. Even though it has been excited identically, the second atom will not enter the Rydberg state. The presence of an existing Rydberg atom prevents the creation of a second one within a certain radius.
The blockade effect provides three considerable advantages as a basis for future logic gates: It happens very fast; the interaction is strong (orders of magnitude stronger than the attractive van der Waals forces that typically bind neutral atoms); and it does not require physical contact between atoms. Rolston, Peter Zoller of the University of Innsbruck and collaborators there and elsewhere proposed a scheme for using Rydberg states as quantum gates in 2000. Now several labs around the world are studying ways to make that happen.
Eventually, Rolston’s group at the University of Maryland (UMD) -- working with JQI colleagues at the National Institute of Standards and Technology (NIST) and the Condensed Matter Theory Center at UMD -- hopes to be able to confine Rydberg atoms in an optical lattice.
Optical lattices are grids formed when laser beams overlap and interfere with one another. The result is a geometrical pattern of energy gradients. When atoms are placed in the lattice, they naturally settle into the minimum-energy spots with neat, regular spacing. At that point, researchers could use very carefully directed laser beams to manipulate the atoms into controlled interactions for use as logic gates.
“But first,” Rolston says, “there’s the matter of making it actually work experimentally.” That will entail several different lines of research. One involves learning how to fine-tune control of Rydberg atoms, which are so highly excited that they can easily lose their outer electrons, either spontaneously or by overstimulation from the laser beams. That process, called ionization, destroys
the atoms’ utility as logic-gate components.
In addition, investigators have to develop the ability to target and hit only selected atom pairs in the lattice, and to create a lattice with the right spacing interval to make targeting possible. At present, Rolston’s group is working with rubidium atoms.
Like its chemical cousins sodium, potassium and the other alkali metals, rubidium has a single electron in its outermost orbital, and the specific laser frequencies necessary to excite that lone electron are obtainable in the laboratory.
The final experimental configuration has not been determined. But the general arrangement of the apparatus is depicted below.
Isolating and testing Rydberg atoms begins in a high vacuum chamber  where a gas of approximately 500 million rubidium atoms is caught in a magneto-optical trap . On the “ceiling” of the trap space is a device (photo at left) that produces a magnetic field gradient which herds the atoms into the desired location. Many evaporate away.
Once in position and cooled to a few hundred-billionths of a degree above absolute zero, about 40 percent of the atoms remain, and are exposed to laser beams that enter through the side ports .