Quantum computers can theoretically blow away conventional ones at solving important problems. But they face major hurdles: their basic computational units, called quantum bits or qubits, are difficult to control and are easily corrupted by heat or other environmental factors. Now researchers have designed two kinds of qubits that may help address these challenges.
Conventional computer bits represent either a one or a zero. But thanks to an eerie quantum effect known as superposition—which allows an atom, electron or other particle to exist in two or more states, such as “spinning” in opposite directions at once—a single qubit made of a particle in superposition can simultaneously encompass both digits. When multiple qubits become “entangled” (referring to a quantum property that links one particle's actions to those of its partners), computing capacity can rise exponentially with the number of qubits. In principle, a 300-qubit quantum computer could perform more calculations at once than there are atoms in the observable universe.
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Credit: Brown Bird Design; Source: “Silicon Quantum Processor with Robust Long-Distance Qubit Couplings,” by Guilherme Tosi et al., in Nature Communications, Vol. 8, Article No. 450; September 6, 2017
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Currently qubits based on a particle's spin direction must be positioned about 15 nanometers apart—any more, and their entanglement fails. But quantum engineer Andrea Morello of the University of New South Wales in Australia and his colleagues now claim to have designed qubits that can be separated by up to 500 nanometers. This provides much more room for vital apparatus to control the qubits. To create one of these so-called flip-flop qubits (graphic), an electron is pulled some distance from an atom's nucleus. This causes the atom to exhibit positive and negative electric poles that can interact over relatively large distances, the researchers reported in September in Nature Communications.
Another proposed qubit design is based on “quasiparticles,” which are formed from negatively charged electrons interacting with positively charged “holes” in superconducting material. In work reported in August in Nature, scientists at the Delft University of Technology and Eindhoven University of Technology, both in the Netherlands, and their colleagues created structures in which a pair of separated quasiparticles can “braid,” or exchange places, acting as a single qubit. The distance between them would decrease the chance that environmental effects could perturb both particles at once, which potentially makes such qubits highly stable, says study co-lead author Hao Zhang, a quantum physicist at Delft.
Both teams say they hope to create working versions of the new qubits soon. “I think it's very exciting that scientists are still pursuing new roads to build large-scale quantum computers,” says quantum physicist Seth Lloyd of the Massachusetts Institute of Technology, who did not take part in either study.