Researchers from the Yokohama National University have demonstrated reliable quantum state transfer of photon polarization into a carbon nuclear spin coupled to a nitrogen-vacancy (NV) center in diamond. The transfer of quantum information into an otherwise inaccessible space — in this case, carbon atoms in diamond — could be applied to the transfer of sensitive information into a quantum memory without revealing or damaging the stored quantum information. The lattice structure of diamond contains a nitrogen-vacancy (NV) center with surrounding carbons. A carbon isotope (green) is first entangled with an electron (blue) in the vacancy. They then wait for a photon (red) to be absorbed, resulting in quantum teleportation-based state transfer of the photon into the carbon memory. Courtesy of Yokohama National University. Surrounded by carbon atoms, the nucleus structure of a nitrogen atom creates a nanomagnet. The researchers used the nanomagnet to anchor an electron. The researchers attached a nanowire to the surface of a diamond and applied a microwave and a radio wave to the wire to build an oscillating magnetic field around the diamond. They shaped the microwave to create the optimal, controlled conditions for the transfer of quantum information within the diamond. Using microwaves and radio waves, the researchers forced the electron spin to entangle with a carbon nuclear spin. The electron spin broke down under the magnetic field, making it susceptible to entanglement. Once entanglement occurred, a photon with quantum information was applied and absorbed by the electron. Detection of the electron after relaxation into the spin ground state allowed for the post-selected transfer of arbitrary photon polarization into the carbon memory. The quantum state transfer scheme could enable individual addressing of integrated quantum memories to realize scalable quantum repeaters. “Our ultimate goal is to realize scalable quantum repeaters for long-haul quantum communications and distributed quantum computers for large-scale quantum computation and metrology,” professor Hideo Kosaka said. The research was published in Communications Physics (https://doi.org/10.1038/s42005-019-0158-0).