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Quantumhttp://square.phi.kit.edu/img/EuSpinCavityCoupling_website.png technologies rely on materials that offer the central resource of quantum coherence, that allow one to control this resource, and that provide suitable interactions to create entanglement. Rare earth ions (REI) doped into solids have an outstanding potential in this context and could serve as a scalable, multi-functional quantum material. REI provide a unique physical system enabling a quantum register with a large number of qubits, strong dipolar interactions between the qubits allowing fast quantum gates, and coupling to optical photons – including telecom wavelengths – opening the door to connect quantum processors in a quantum network.

SQUARE's Goals

This project aims at establishing individually addressable rare earth ions as a fundamental building block of a quantum computer, and to overcome the main roadblocks on the way towards scalable quantum hardware. The goal is to realize the basic elements of a multifunctional quantum processor node, where multiple qubits can be used for quantum storage, quantum gates, and for coherent spin-photon quantum state mapping. Novel schemes and protocols targeting a scalable architecture will be developed. The central photonic elements that enable efficient single ion addressing will be engineered into deployable technologies.

The Team

The project gathers 6 leading experimental and theoretical European research groups and two technology companies. The expertise of the consortium includes inorganic chemistry, solid-state and atomic physics, quantum optics and quantum information processing. The two companies strengthen the technology development aspects of the consortium.


SQUARE has received funding from the European Union’s Horizon 2020 research and innovation programme within the Flagship on Quantum Technologies in the sector basic science under grant agreement No 820391. It started on 1.10.2018 and will last for 3 years with a budget of 3 M€.




New SQUARE publication in Physical Review Letters

Thomas Kornher et al.: Sensing Individual Nuclear Spins with a Single Rare-Earth Electron Spin
Confocal scan of cerium centers under pulsed laser excitation, taken at the focus of an SIL

Thomas Kornher and the team around Roman Kolesov in Jörg Wrachtrup's group at the university of Stuttgart managed to get an important step ahead for rare earth ions: They could sense a single silicon nuclear spin by optically reading out a single Cerium ion. Such nuclear spins are promising as long-lived quantum bits of larger multi-qubit registers. See the article at PRL https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.124.170402 and the Physics Synopsis https://physics.aps.org/articles/v13/s56.

17.-19. Feb 2020: SQUARE Consortium Meeting at ICFO, Barcelona

SQUARE Consortium at ICFO (February 2020)

The SQUARE consortium met for an intense meeting at ICFO, where significant progress was presented and discussed: Highlights include the dynamic control of Purcell enhancement of Erbium ions (link), sensing of individual nuclear spins with a single rare-earth electron spin (link), and an extended theory for quantum interactions with pulses of radiation (link).

10-11 July 2019: SQUARE Summer School in Karlsruhe

Lecture at SQUARE Summer School 2019 in Karlsruhe

The summer school of the Quantum Flagship project SQUARE provided an introduction to quantum computing and quantum networks with a focus on implementations with optically adressable rare earth ion spins, specifically the approaches that are persued within the SQUARE project.
The meeting gathered 37 researchers and students for 2 full days, and all SQUARE partners were present. All SQUARE members were present, and guests from the most related flagship projects OpenSuperQ, AQTION, and QIA, as well as from other groups in the field of REI quantum memories, networks, and materials were present.


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Prof. Hunger at SQUARE booth 69th Lindau Nobel Laureate Meeting