Physicists have succeeded in entangling two quantum bits, or qubits, within a semiconductor trap — a basic building block of a quantum computer. Joining two qubits together is not new; last year, for instance, scientists entangled a record 14 using calcium atoms in a laser ion trap. But the particular materials approach for these qubits — each comprised of two pairs of electrons manipulated within gallium arsenide semiconductors — is proof of principle for a solid-state approach to quantum computing that the authors say is ripe for scaling up.
“It establishes a very important benchmark,” says Amir Yacoby, a physicist at Harvard University in Cambridge, Massachusetts, who led a team publishing the result today in the journal Science.
Quantum computers are being sought for their ability to pursue complicated problems beyond the ken of classical computers, such as decryption, and would employ the peculiar qualities of qubits. Unlike classical bits, qubits take advantage of the quantum-mechanical phenomenon of superposition, which allows them to store a 0 and 1 at the same time. Entangling two qubits allows simultaneous operations on the equivalent of four classical bits; 14 entangled qubits would store 16,384 bits; and so on, exponentially. According to IBM, 250 qubits would contain more bits than there are atoms in the Universe.
The problem, however, is doing anything with the fragile systems. On their own, qubits have a tendency to “decohere” into a 0 or 1 at the slightest provocation. Conjoining them is an equally delicate task.
But many are betting that solid-state qubits will end up being more robust than other classes of qubits, such as those created with ion traps and nuclear magnetic resonance (NMR) machines. Yacoby says that, just as the transistor quickly displaced vacuum tubes, so too will solid-state qubits, propelled by massive investment in semiconductor technologies, win out over the other types. “We want to ride that wave,” he says. Rival solid-state qubit approaches include ideas based on superconducting materials, and others based on indium arsenide nanowires.
Yacoby trumpets his team’s particular solid-state approach for its scale-up potential. His qubits are each defined by the spin state of a pair of electrons. A qubit comprised of two electrons, rather than just one, adds extra immunity from decoherence. Entangling the qubits, however, requires the tricky introduction of charge to the system. But Yacoby says that if that can be made more robust, then the fragile qubits could be entangled over much larger distances — crucial if the qubits are to be daisy-chained together into anything resembling an actual circuit. Already, Yacoby’s team has entangled the qubits over a few hundred nanometres. And he says he sees a path forward to entangling them across distances as great as a micrometre.
Image credit: Science/AAAS