# Recent News

## Bell Inequalities with One Bit of Communication

In our most recent publication for the special issue on nonlocality of the Journal of Entropy, we study Bell scenarios supplemented by one bit of communication. The main motivation is to close a longstanding open problem concerning the communication complexity of quantum correlations.

The problem is the following: “Can the statistics obtained by performing projective measurements on two-qubit states be reproduced through the communication of one bit between the parties?”. We study the case of three projective measurements for each party and show that quantum statistics can be reproduced in this scenario, in the worst cases by sending one bit at least half of the time. Our results also expose a general structure behind the facet inequalities of Bell scenarios with an added bit of communication, based on the facet inequalities of the (communication-less) Bell scenarios.

## $^{171}$Yb$^{3+}$:YSO - The quantum memory of tomorrow

In our recent paper, published in Nature Materials, we study new solid-state material ($^{171}$Yb$^{3+}$:YSO crystal) containing electronic spins and showing promising properties for future quantum technologies.

This work was done in collaboration with Institut de Chimie du CNRS. The discussion of main experimental results can be found in Phys.org.

The strong enhancement of the coherent properties simultaneously for optical and microwave transitions had been observed for zero and low external magnetic fields, allowing future realisation of broadband quantum memories, microwave-optical transducers and an interface with superconducting qubits.

This material opens up a new field of possibilities for creating a global quantum network; it also underlines the importance of pursuing fundamental research in parallel with more applied research, such as devising a quantum memory.

## Efficient optical pumping using hyperfine levels in $^{145}$Nd$^{3+}$:Y$_2$SiO$_5$

In the rare-earth ion community, non-Kramers ions are traditionally thought to achieve larger quantum memory efficiencies. This is due to the fact that the best storage efficiencies (around 30-35% in bulk) were achieved in non-Kramers ion-doped solids, while Kramers ions have been limited to less than 30% until now. In our recent paper, published in NJP, we show that the secret to improve the efficiency of Kramers ions relies on exploiting the optical pumping with nuclear spin states. Thanks to a detailed study of the relaxation processes limiting optical pumping in $^{145}$Nd$^{3+}$:Y$_2$SiO$_5$ , we achieve efficiencies comparable to the best state-of-the-art bulk non-Kramers quantum memories. This paves the way towards high efficiency optical quantum memories for quantum repeaters, taking advantage of the many interesting properties of Kramers ions, such as the frequency of the relevant optical transition and broadband capabilities.

## Characterization of the hyperfine properties for quantum memory application

Quantum repeaters have strong requirements for quantum memory properties, namely on the efficiency, storage time, and fidelity (a measure of how close the output state is to the input). In our group, we use rare-earth-ion-doped crystals as quantum memories. In a previous work, we have demonstrated storage times up to 1.5 ms in our Europium-doped yttrium orthosilicate crystal. In a new study recently published in Physical Review B, we have characterized fully the ground and excited spin state properties of the relevant optical transition under external magnetic fields. This paves the way towards the realization of a quantum memory with longer storage times by using dynamical decoupling. These developments are very exciting as they should take our memory storage time beyond the minimal requirement for a quantum communication.

## Quantum entanglement between 16 million atoms in a solid

Quantum theory is unequivocal: it predicts that a vast number of atoms can be entangled and intertwined by a very strong quantum relationship even in a macroscopic structure. Until now, however, experimental evidence has been mostly lacking, although recent advances have shown the entanglement of 2,900 atoms. Our group together with Prof. Nicolas Brunner have recently demonstrated the entanglement between 16 million atoms in a crystal. This was done based on the novel theoretical approach of certifying geniune multipartite entanglement for multiatomic ensembles interacting with light. The research is published in Nature Communications while the brief explanation of main results can be found in the University press realease.

In a parallel work by groups of Prof. Christoph Simon and Prof. Wolfgang Tittel from University of Calgary an entanglement between many large groups of atoms has been demonstrated.