Entanglement, that most counterintuitive quantum phenomenon by which particles share an unseen link that aligns their properties, is looking more mundane all the time. Just last week two groups of researchers reported entangling a photon with a crystal-based device, potentially paving the way for solid-state memories that can store and then release entangled particles as needed.
Another week, another advance. In a paper published online January 19 in Nature a team of physicists announced that they have developed the capability to churn out pairs of entangled particles, billions at a time. (Scientific American is part of Nature Publishing Group.) The advance might someday allow for the streamlined development of quantum processors with a large number of quantum bits working in parallel.
Stephanie Simmons, a graduate student at the University of Oxford, and her colleagues created entangled states in phosphorus atoms embedded in a silicon crystal, entangling the spin of each atom's nucleus with the spin of one of its electrons. (Spin is a quantum property analogous to the pointing of a tiny bar magnet—an atomic nucleus or an electron can spin up or spin down.) "This is the first demonstration of repeatable, on-demand entanglement of an ensemble of spins," she says.
Phosphorus-doped silicon holds promise for solid-state quantum computing and quantum information processing, because each phosphorus atom has a relatively free electron that is open to manipulation, and because information can be encoded on both the spin of that electron and on the spin of the nucleus itself. And quantum devices based on silicon offer a relatively clear path to integration with existing, classical electronics. But no one had demonstrated the ability to mass-produce entanglement in such silicon crystals, which is necessary for some implementations of solid-state quantum information processing.
With a series of radio-frequency and microwave pulses, Simmons and her colleagues set the approximately 10 billion phosphorus atoms in the silicon crystal into an entangled state wherein each atom was in a superposition of two states at once—both nucleus and electron in a spin-up configuration in one state, or both spin-down in the other. Measuring the spin of either the nucleus or the electron collapses the atom's superposition into one of the two possible states. "If you measure up on a given electron, you know that the nucleus is up," Simmons says. (The entangled state was not completely pure, so some atoms did not cooperate.)
Ultimately, ensembles of entangled particle pairs could find use as quantum bits, or qubits, in quantum computers. A qubit's capacity to be both 0 and 1 simultaneously, and to interact with its neighbors via entanglement, would give quantum computers a huge leg up on ordinary machines. A massively powerful quantum computer based on an ensemble of entangled spins would need nowhere near 10 billion qubits, but it would require some way to move quantum information from one qubit to the next—perhaps by inducing the phosphorus electrons to jump between atoms. That feat, if it could be achieved, would help sidestep the difficulty of adding entangled qubits one by one, Simmons says, making for a more scalable approach to quantum computing.