Rare Earth Atoms See the Light

Tiny units of matter and chemistry that they are, atoms constitute the entire universe. Some rare atoms can store quantum information, an important phenomenon for scientists in their ongoing quest for a quantum Internet.

New research from UC Santa Barbara scientists and their Dutch colleagues exploits a system that has the potential to transfer optical quantum information to a locally stored solid-state quantum format, a requirement of quantum communication. The team’s findings appear in the journal Nature Photonics.

“Our research aims at creating a quantum analog of current fiber optic technology in which light is used to transfer classical information — bits with values zero or one — between computers,” said author Dirk Bouwmeester, a professor in UCSB’s Department of Physics. “The rare earth atoms we’re studying can store the superpositions of zero and one used in quantum computation. In addition, the light by which we communicate with these atoms can also store quantum information.”

Atoms are each composed of a nucleus typically surrounded by inner shells full of electrons and often have a partially filled outer electron shell. The optical and chemical properties of the atoms are mainly determined by the electrons in the outer shell.

Rare earth atoms such as erbium and ytterbium have the opposite composition: a partially filled inner shell surrounded by filled outer shells. This special configuration is what enables these atoms to store quantum information.

However, the unique composition of rare earth atoms leads to electronic transitions so well shielded from the surrounding atoms that optical interactions are extremely weak. Even when implanted in a host material, these atoms maintain those shielded transitions, which in principle can be addressed optically in order to store and retrieve quantum information.

Bouwmeester collaborated with John Bowers, a professor in UCSB’s Department of Electrical and Computer Engineering, and investigators at Leiden University in the Netherlands to strengthen these weak interactions by implanting ytterbium into ultra-high-quality optical storage rings on a silicon chip.

“The presence of the high-quality optical ring resonator — even if no light is injected — changes the fundamental optical properties of the embedded atoms, which leads to an order of magnitude increase in optical interaction strength with the ytterbium,” Bouwmeester said. “This increase, known as the Purcell effect, has an intricate dependence on the geometry of the optical light confinement.”

The team’s findings indicate that new samples currently under development at UCSB can enable optical communication to a single ytterbium atom inside optical circuits on a silicon chip, a phenomenon of significant interest for quantum information storage. The experiments also explore the way in which the Purcell effect enhances optical interaction with an ensemble of a few hundred rare earth atoms. The grouping itself has interesting collective properties that can also be explored for the storage of quantum information.

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