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David Hunger
Project Coordinator

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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€.




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.


5 July 2019: Presentation of SQUARE at 69th Lindau Nobel Laureate Meeting

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

The SQUARE Flagship project has been presented in the context of quantum research  at the 69 Lindau Nobel Laureate Meeting during a boat trip to the island of Mainau. Ulrich Steinbach from the BW Ministry of science and several Nobel Laureates visited our booth.

26 February 2019: First SQUARE Publication online

© Kiilerich and Molmer 2019

The first SQUARE publication is online: Alexander Kiilerich and Klaus Molmer present a formalism for the evolution of quantum states of a light pulse after its interaction with a quantum scatterer - a new tool to describe quantum networks. 

Link to publication

18-22 February 2019: Flagship conference Grenoble

Flagship conference Grenoble © D. Hunger

EQTC 2019, the first Quantum Flagship conference 18-22 February 2019, gathered around 400 people from academia, industry, policy makers, funders and educators.

All flagship projects were presented, including SQUARE, and several audience discussions on QT, innovation, governance, education, standardization etc took place.

Link to EQTC 2019