One important goal of the technique is to provide the basis for so-called quantum repeaters, which would make long-distance quantum communications possible. Photon-based entanglement fades after a few hundreds of kilometers, because fiber-optic cable absorbs the light, and boosting the signal destroys the entanglement. But entangling chains of quantum repeaters could link qubits over longer distances.
Although ion and atom systems are more advanced than diamond in terms of linking close qubits for quantum computing, holding a record of 14 entangled qubits, diamond has distinct advantages for linking remote processors in networks, says Hanson. Unlike ions trapped in high vacuum, qubits in diamond can be maintained at room temperature, because the material’s surrounding carbon lattice shields them so well from stray magnetic fields or vibrations that might upset their superposition.
Researchers showed last year that diamond-vacancy qubits, which last for tens of milliseconds, can even be transferred to the nuclei of neighboring carbon or nitrogen atoms, creating an array of ‘memory’ qubits that can exist for seconds—an eternity in quantum-computing terms. Also, building assemblies of solid diamond chips sounds more approachable than creating hundreds of ion traps, says Hanson.
At this early stage, with quantum processors decades away, no one is ready to bet on which system will prevail. “You can't pick a winner right now,” says Joshua Nunn, a physicist at the University of Oxford, UK.