Short answer: A quantum memory stores the quantum state of light and releases it later, which quantum networks need. Crystals doped with rare-earth ions are one of the main solid-state candidates. Reviews from 2015, 2018 and 2022 describe the storage protocols, the performance measures that matter, and challenges such as low optical depth and keeping coherence in a solid. This is laboratory research on memories for single photons and entangled photons.
Why a memory for light
A 2018 topical review in Chinese Physics B says the reversible transfer of unknown quantum states between light and matter is essential for building large-scale quantum networks. Over the last decade, various physical systems were proposed for this. The review says solid-state memories based on rare-earth-ion-doped solids offer reduced setup complexity and high robustness for scalable applications. It describes methods to spectrally prepare the memory and to release the stored photonic excitation on demand, and reviews applications in quantum information science and fundamental tests of quantum physics.
A 2015 book chapter explains the background. Quantum memories for light were first demonstrated in atomic gases, both room-temperature gases and laser-cooled atoms. Several solid-state systems also look interesting: rare-earth doped crystals, nitrogen-vacancy centres in diamond, phonons in diamond and quantum dots. The chapter says solid-state memories have general advantages, including the absence of atomic motion and the prospect of integrated devices that may ease large-scale deployment in future networks. It also says that controlling light-matter interaction in solids poses challenges, such as preserving quantum coherence in a solid-state environment.
What rare-earth crystals offer
The 2015 chapter focuses on ensembles of rare-earth ions embedded in dielectric crystals. It describes how to create quantum light, such as heralded single photons and entangled photons, that is spectrally compatible with the narrow atomic transitions, and protocols based on dipole rephasing that reversibly map the light onto collective atomic excitations. It reviews the experimental state of the art and describes in more detail storage experiments in neodymium-doped and praseodymium-doped crystals.
A 2022 review in Frontiers of Physics, titled Rare-earth quantum memories: the experimental status quo, describes rare-earth doped crystals as carrying great prospects for ensemble-based solid-state memories for remote quantum communication and fast quantum processing. It says remarkable storage performance has been realised in recent years and more applications exploited, while the technical challenges are also significant. It organises the field by storage protocol, with a focus on experiments, and analyses challenges and feasible solutions.
Protocols and performance criteria
The 2022 review introduces the basic performance criteria a memory must meet in a quantum network: storage fidelity, storage time, storage efficiency, storage mode capacity and storage bandwidth. It then covers absorptive memories, including those based on electromagnetically induced transparency and on photon-echo protocols. For rare-earth crystals, the review notes that a low concentration of ions sits in the crystal lattice, so atomic diffusion is not a concern but low optical depth is still a challenge. It cites the first reported EIT storage in a rare-earth crystal as being in praseodymium-doped yttrium orthosilicate.
What this does not tell you
These are reviews, and they describe laboratory experiments with their own conditions. They do not claim a quantum network built from rare-earth memories is operating, and each performance criterion trades off against the others.
FAQ
What is a quantum memory?
A device that stores the quantum state of light and releases it on demand.
Why rare-earth-doped crystals?
A 2018 review cites reduced setup complexity and high robustness for scalable application.
What are the main performance measures?
Storage fidelity, time, efficiency, mode capacity and bandwidth, according to a 2022 review.
What is a known challenge?
Low optical depth, and preserving coherence in a solid-state environment.
Is there a working network?
These reviews describe laboratory demonstrations, not a deployed network.
All field notes