Multi-functional quantum bits for future computers

Contrary to the classical computer chip, the smallest memory element of a quantum computer chip, the quantum bit or qubit, does not only store binary information - 0 or 1, off or on, but also values in between, called quantum mechanics superpositions. With the help of these quantum properties, many computation processes can be executed in parallel and, hence, computing capacity of a quantum computer is enhanced significantly. "A promising approach to the next generation of quantum computers is based on materials, single systems of which are not larger than an atom and the quantum properties of which can be accessed and controlled optically", stated Professor David Hunger of KIT's Physikalisches Institut, who coordinates the SQUARE project.

For this, rare earth ions, electrically charged atoms of rare-earth metals, exhibit an outstanding potential that will be studied in SQUARE. The ions can store quantum states for a very long time and can be activated separately by light in special solid-state crystals. Hence, a large number of ions is accessible as qubits. "Thanks to their special electronic structure, rare earth ions are shielded against interference fields", David Hunger stated. "Moreover, they can strongly interact with each other, which is a major basis for quantum circuits." Quantum properties can be read out directly by light and quantum states can be transmitted to photons. This enables interconnection of distant quantum nodes and their use for quantum communication.

Research relating to single rare earth ions still is in an early stage, but "the project concentrates on a highly promising platform that offers many advantages over established approaches, such as ions captured in vacuum devices and superconducting quantum circuits", David Hunger said.

SQUARE is aimed at establishing rare earth ions that can be addressed separately as basic building blocks for scalable quantum technologies. In particular, it is planned to demonstrate functional elements of a multi-qubit quantum register that can be read out optically and to realize building blocks of a quantum network. This will significantly enhance computation capacity of quantum computers in the future. In cooperation with industry partners, central technological components required for scalable implementation will be developed.

The European Commission funds SQUARE within the framework of the Quantum Technology Flagship. Over a duration of ten years, it is planned to invest a billion euros into research relating to quantum technologies. With this, development of novel technologies based on elemental quantum effects will be accelerated and closer cooperation with industry will be initiated to speed up development to maturity.