To make a conditional logic gate between two ion qubits, we require a coupling between them—in other words, we need them to talk to each other. Because both qubits are positively charged, their motion is strongly coupled electrically through a phenomenon known as mutual coulomb repulsion. In 1995 Juan Ignacio Cirac and Peter Zoller, both then at the University of Innsbruck in Austria, proposed a way to use this coulomb interaction to couple indirectly the internal states of the two ion qubits and realize a CNOT gate. A brief explanation of a variant on their gate goes as follows.
First, think about two marbles in a bowl. Assume that the marbles are charged and repel each other. Both marbles want to settle at the bottom of the bowl, but the coulomb repulsion causes them to come to rest on opposite sides, each a bit up the slope. In this state, the marbles would tend to move in tandem: they could, for instance, oscillate back and forth in the bowl along their direction of alignment while preserving the separation distance between them. A pair of qubits in an ion trap would also experience this common motion, jiggling back and forth like two pendulum weights connected by a spring. Researchers can excite the common motion by applying photon pressure from a laser beam modulated at the natural oscillation frequency of the trap.
More important, the laser beam can be made to affect the ion only if its magnetic orientation is up, which here corresponds to a qubit value of 1. What is more, these microscopic bar magnets rotate their orientation while they are oscillating in space, and the amount of rotation depends on whether one or both of the ions are in the 1 state. The net result is that if we apply a specific laser force to the ions for a carefully adjusted duration, we can create a CNOT gate. When the qubits are initialized in superposition states, the action of this gate entangles the ions, making it a fundamental operation for the construction of an arbitrary quantum computation among many ions.
Researchers at several laboratories—including groups at the University of Innsbruck, the University of Michigan at Ann Arbor, the National Institute of Standards and Technology (NIST) and the University of Oxford—have demonstrated working CNOT gates. Of course, none of the gates works perfectly, because they are limited by such things as laser-intensity fluctuations and noisy ambient electric fields, which compromise the integrity of the ions’ laser-excited motions. Currently researchers can make a two-qubit gate that operates with a “fidelity” of slightly above 99 percent, meaning that the probability of the gate operating in error is less than 1 percent. But a useful quantum computer may need to achieve a fidelity of about 99.99 percent for error-correction techniques to work properly. One of the main tasks of all trapped-ion research groups is to reduce the background noise enough to reach these goals, and although this effort will be daunting, nothing fundamental stands in the way of its achievement.
But can researchers really make a full-fledged quantum computer out of trapped ions? Unfortunately, it appears that longer strings of ions—those containing more than about 20 qubits—would be nearly impossible to control because their many collective modes of common motion would interfere with one another. So scientists have begun to explore the idea of dividing the quantum hardware into manageable chunks, performing calculations with short chains of ions that could be shuttled from place to place on the quantum computer chip. Electric forces can move the ion strings without disturbing their internal states, hence preserving the data they carry. And researchers could entangle one string with another to transfer data and perform processing tasks that require the action of many logic gates. The resulting architecture would somewhat resemble the familiar charge-coupled device (CCD) used in digital cameras; just as a CCD can move electric charge across an array of capacitors, a quantum chip could propel strings of individual ions through a grid of linear traps.