Taming the Qubit

The trouble with qubits is they don't always behave.

Scientists are, however, learning to control these weird "quantum bits," particles that someday may store and process data in ultrafast computers harnessing the laws of quantum mechanics.

A team of NIST scientists recently demonstrated a way of precisely controlling a "logic gate" (or computational operation) between two qubits 97 percent of the time, compared to just 80 percent with previous techniques. Fewer errors makes large-scale, functional quantum computers much more plausible.

The researchers are working on the small scale now, using ions (charged atoms) as qubits. They demonstrated the use of qubits for just one computational operation, whereas millions or more would be needed in a real computer calculation. However, with further improvements in reliability of the gate operation, this design should enable the construction of a device based on a scalable architecture.

Qubits represent information as 1s and 0s, like digital bits in today's computers. The NIST scientists use lasers to manipulate beryllium ions trapped in a vacuum so that they "spin" up or down to represent 1 or 0. The quirks of quantum mechanics allow ions to exist in blends of the two spin states and also to become "entangled," so that the properties of two ions are intertwined. Qubits are, therefore, able to represent multiple values simultaneously and to link these values in logical ways, such that a quantum computer could perform tasks like factoring very large numbers that are impossible with today's technology.

The recent NIST experiments, reported in the journal Nature, focused on precise manipulation of qubits and the computation they perform to control entanglement. Logic gates that control entanglement are essential elements of quantum computers, which also would contain other components, such as memory regions, already demonstrated by the NIST team. In this case, depending on whether the first qubit is a 0 or 1, the gate may flip the spin of the second qubit (0 means no, 1 means yes).

To make the logic gate, the NIST scientists use two laser beams positioned at right angles to apply an oscillating force to a pair of ions. The lasers are tuned so the difference between their frequencies is very close to the frequency of the ions' natural vibrations. If both ions are in the same spin state, the lasers have no effect. However, if the ions are in different spin states, they feel an opposing laser force that causes the ions to stretch apart. Each ion traces a tiny path just 10 nanometers long and returns to its original position in physical space in 39 microseconds. During this process, the two qubits become entangled and change their quantum mechanical spin characteristics in subtle but predictable ways that depend on their starting points.