The SQUARE Quantum Flagship Project

Scalable Rare Earth Ion Quantum Computing Nodes

Quantum 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.


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€.



11 June 2021: Publication in Nature communications

Casabone et al.: Dynamic control of Purcell enhanced emission of erbium ions in nanoparticles
1 Chetan Desmukh/ICFO
A picture of the fiber micro-cavity, towards the right, a reflection of the fiber in the planar mirror on which erbium doped nanoparticles are placed can be seen.

A team of SQUARE members led by Hugues de Riematten achieved strong emission enhancement of Erbium ions with dynamic control of Purcell enhancement up to 100x faster than the excited state lifetime, promising for single telecom photon wavepacket shaping.

Nature communication 2021, DOI: 10.1038/s41467-021-23632-9

9 December 2020: Publication in NanoLetters - Rare-earth doped nanoparticles for quantum memories

Fossati et al.: A Frequency-Multiplexed Coherent Electro-optic Memory in Rare Earth Doped Nanoparticles
1 Diana Serrano
Measurement by optical techniques of nuclear spin properties in rare earth doped nanoparticles.

SQUARE member Professor Philippe Goldner and his team from Chimie ParisTech, PSL University, CNRS, have demonstrated that nanoscale memories of rare-earth nanoparticles crystals are capable of storing light efficiently, making them promising candidates for developing multimode and long-storage quantum memories.

Nano Letters 2020, DOI: 10.1021/acs.nanolett.0c02200

29 April 2020: New SQUARE publication in Physical Review Letters

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

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 and the Physics Synopsis

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

1 D.Hunger
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.