Communications - July 2008

(now at Duke) is an ion-trap analogue to the electron-based charge-coupled device chips used in digital cameras. “This design gives addressability to an enormous number of ions,” says MIT’s Isaac Chuang.

Quantum dots are a popular solid-state host for qubits. Lieven Vandersypen’s research group in Delft, The Netherlands, reported last November that it had used an alternating electrical field to control single electrons contained in gallium arsenide quantum dots. Electrical control is more selective than the previously used magnetic fields. Their fidelity of flipping a single-electron spin (a simple gate) is a little lower than the 73% attained with magnetic-field control but is expected to increase as they gain experience. Future research will venture into no-spin hosts, such as silicon and carbon ( car-bon- 12, nanotubes, graphene), that are expected to have much longer decoherence times.

Linear optic qubits are created by simultaneously producing forward and backward photons and encoding their logical states into vertical and horizontal polarizations. This approach has the advantage of long decoherence times and compatibility with fiber optics but needs higher photon-creation and -de-tection efficiencies. Last December, Andrew White’s group from the University of Queensland reported that it had used a linear optic circuit involving four qubits to find the prime factors of 15 ( 5 and 3) thus demonstrating that system’s ability to perform the core processes required for implementing Shor’s algorithm.

In April, Prem Kumar of Northwestern University announced a quantum gate created within an optical fiber. A few years ago, Kumar showed that photons can remain entangled within a fiber for a distance of 100 kilometers. The recent result will be useful in creating quantum repeaters for a distributed quantum information network.

Superconducting qubits can be made in three types: charge, flux, and phase. Each uses excitation states of Josephson junctions: two superconductor pieces separated by an insulator thin enough for Cooper pairs of electrons to tunnel across. This approach is scalable, since superconductivity enables fast control and readout and large,

more than a dozen
different ways
of creating qubits
have been developed
to date.

controllable couplings between widely separated qubits. However, it also requires extremely low temperatures— milliKelvins—and tends to have short decoherence times (to date only a few microseconds). John Martinis’s group at the University of California, Santa Barbara, recently measured single-qubit fidelity of 98% in a phase-qubit system. And Robert Schoelkopf’s group at Yale developed a “transmon” qubit 300 microns long and very stable against noise.

Hybrid approaches combine the best features of their parents. Chuang’s research group is integrating ion traps into superconducting qubits. “Ion traps are very hard to connect to anything,” Chuang says. “It would be really nice to have an ion trap with wires coming in and out.” Vandersypen says that as ion traps get smaller they might eventually look similar to quantum dots.

D-Wave Systems, a venture-backed company based in Vancouver, BC, made news twice last year when it announced the operation of a 16-qubit (in February) and 28-qubit (in November) “adiabatic” quantum computer. However, many scientists are skeptical of both the claims and their importance. The adiabatic method, which is a quantum version of simulated annealing, involves slowly evolving a system toward the solution. Skeptics say this approach will prove to be neither fault-tolerant nor scalable. Although the company revealed scant scientific detail about its approach, its president and CEO, Geordie Rose, says it has no proof yet of entanglement—the hallmark of quantum computation.

Diamond-based systems are an intriguing recent entrant. The qubit is the spin state of a nitrogen impurity adjacent to a vacancy in the carbon crystal.

Stemming from Tom Kennedy’s 2003 research at the Naval Research Laboratory, “nitrogen-vacancy color centers” have two compelling advantages: their spin state can be both initialized and read out optically at room temperature and weak spin-orbit coupling in diamond makes this qubit well-decoupled from its environment. Several groups have jumped in to study this system, and its decoherence time has increased from about 50 microseconds to nearly a millisecond.

Challenges include devising ways to control individual qubits and couple them together. But David Awschalom of the University of California, Santa Barbara, is optimistic. “If you had told me,” he says, “a few years ago that you were going to try to control a single electron at gigahertz frequencies in a solid-state material at room temperature, I’d have said, ‘Good luck!’ Now we’re doing just that.”

This system is also useful as an accessible test bed for studying spin interaction in solid-state materials that may contribute to the success of other systems and quantum physics knowledge in general. Awschalom’s group recently watched quantum information from a single “nitrogen-vacancy center” spin disappear into the “bath” of spins associated with the much more common nitrogen impurities not associated with vacancies…then reappear. “In quantum physics, this is a big deal,” Awschalom says. “It’s an age-old problem. There have been 1,000 theory papers on this, but no experiments.”

The recent across-the-board progress, however, has stimulated optimism in the eventual success and impact of quantum computing inconceivable 15 years ago. For example, IBM Research’s David DiVincenzo, who proved in 1995 that quantum algorithms could be executed using only two-qubit operations and later devised seven widely accepted criteria for a practical quantum computer, says, “I’m confident that the quantum computer will eventually change the world and will deeply influence how information processing will be done in the future.”