Long‑lived quantum optical memories are indispensable core components for quantum communication networks and quantum relay links. This work establishes a coherence‑scale‑evolution phenomenological framework for hour‑class coherent storage in rare‑earth crystals Eu³⁺:Y₂SiO₅ and Pr³⁺:Y₂SiO₅. Theoretical characteristic constants are calibrated against published benchmark experimental observations, circumventing complex numerical solutions of ultrafast microscopic energy‑level dynamics. We provide multiple quantitatively verifiable forward‑looking predictions: storage lifetime under ultra‑low‑temperature conditions, global optimal operating window for control‑laser power, performance zoning for rare‑earth doping schemes, and the engineering trade‑off between storage duration and coherence fidelity. The model is built upon macroscopic physical constraints of lattice thermal vibration, light‑medium coupling and dopant‑mediated medium stability. Detailed microscopic mathematical derivations are omitted, and this manuscript focuses entirely on experimentally testable outputs. The results offer theoretical guidance for parameter optimization of rare‑earth‑based quantum memories, experimental workflow adjustment and novel doped‑system design, and help reduce high‑cost trial‑and‑error in low‑temperature cryogenic experiments.
Long‑lived quantum optical memories are indispensable core components for quantum communication networks and quantum relay links. This work establishes a coherence‑scale‑evolution phenomenological framework for hour‑class coherent storage in rare‑earth crystals Eu³⁺:Y₂SiO₅ and Pr³⁺:Y₂SiO₅. Theoretical characteristic constants are calibrated against published benchmark experimental observations, circumventing complex numerical solutions of ultrafast microscopic energy‑level dynamics. We provide multiple quantitatively verifiable forward‑looking predictions: storage lifetime under ultra‑low‑temperature conditions, global optimal operating window for control‑laser power, performance zoning for rare‑earth doping schemes, and the engineering trade‑off between storage duration and coherence fidelity. The model is built upon macroscopic physical constraints of lattice thermal vibration, light‑medium coupling and dopant‑mediated medium stability. Detailed microscopic mathematical derivations are omitted, and this manuscript focuses entirely on experimentally testable outputs. The results offer theoretical guidance for parameter optimization of rare‑earth‑based quantum memories, experimental workflow adjustment and novel doped‑system design, and help reduce high‑cost trial‑and‑error in low‑temperature cryogenic experiments.
Nuclear‑spin‑induced magnetic noise is one dominant decoherence source limiting hour‑scale coherent optical storage in Eu3+:Y2SiO5 (Eu:YSO). Appropriately oriented static external magnetic fields can suppress spin‑flip fluctuations and extend coherent retention time, yet unified quantitative predictions for usable field window, crystal axis preference and ZEFOZ (Zero‑First‑Order‑Zeeman) working points remain scarce. Built upon the phenomenological theory of coherence‑scale evolution established in our preceding work, spin‑magnetic perturbation is incorporated as an additional macroscopic decoherence channel. System characteristic parameters are calibrated against published low‑temperature spectroscopic benchmarks for Eu:YSO at 1.7 K. All internal microscopic evolution routines are not disclosed; only measurable forward‑looking outputs are presented. Quantitative experimentally‑testable predictions are reported: the favorable static‑magnetic‑field amplitude window is 0.11 T - 0.36 T with maximum lifetime enhancement factor up to 2.61 relative to zero‑field baseline; high‑field degradation threshold locates near 0.42 T; among three principal crystallographic axes, the D1 axis delivers optimal performance, D2 axis gives intermediate improvement, and D3 axis is unfavorable for long‑time storage. Under the preferred D1 orientation, the primary ZEFOZ working point occurs at 0.227 T, bringing an extra 32.4% suppression of residual magnetic noise and further lifting theoretical storage‑time upper bound. Practical tuning suggestions for cryogenic rare‑earth quantum‑memory setups are provided, together with explicit validity limits for the present phenomenological framework.
Abstract Quantum memory is a core enabling device for large-scale quantum networks, serving as the fundamental building block for quantum repeaters and transportable quantum memories. Among various physical platforms, rare-earth-ion-doped crystals stand out as one of the most promising solid-state candidates for long-lived photonic quantum memories, owing to their exceptionally long spin coherence time and their compatibility with integration. In this review, we first introduce the physical mechanisms underlying long-lived spin coherence in rare-earth systems, with particular emphasis on the zero-first-order-Zeeman transition and dynamical decoupling techniques. We then review the experimental progress in this field, highlighting three distinct lifetimes—the spin coherence time, the light storage time, and the quantum storage time—and covering experiments that have achieved hour-scale spin coherence, minute- to hour-scale light storage, and minute-scale single-photon-level quantum storage. Finally, we discuss the remaining challenges that hinder the realization of practical long-lived quantum memories, explore potential strategies to overcome these obstacles, and outline the prospects for their future applications in quantum networks.
This thesis describes experiments aimed at developing a key component in applications such as quantum networks and long-distance quantum communication systems: quantum memories. Quantum memories for light using photon-echo and coherent rephrasing techniques have been developed. The work is carried out on rare-earth-ion-doped inorganic crystals. Several properties are associated with these crystals such as long coherence times and long-lived ground state sublevels that are particularly interesting in connection applications in the area of quantum information science. It is demonstrated the preparation of optical inhomogeneous absorption profiles such that the light only interacts with a specific transition at a selected frequency. Narrow absorbing structures with widths of <100 kHz have been prepared with no absorption in the surrounding spectral interval. Most of the experiments were carried out using Pr3+:Y2SiO5. However, in our search for suitable a Λ-system in the material under consideration, we also performed spectral hole-burning spectroscopy in Nd:Y2SiVO4. As part of the search for host materials for Pr, Pr:La2(WO4)3 crystal was characterized using a variety of methods, such as, hole burning and photon-echoes. Electromagnetically induced transparency was also observed in this crystal. The most important criteria for evaluating quantum memory performance and the most important physical requirements are discussed. After investigating the standard photon echo techniques and controlled reversible inhomogeneous broadening (CRIB) for quantum state storage, we considered an efficient multimode quantum memory protocol; the atomic frequency comb (AFC) protocol. Combs with peaks of widths 100-300 kHz and an optical depth of~ 10 were created by optical pumping inside an emptied region of the inhomogeneous profile of Pr3+:Y2SiO5, allowing us to improve the efficiency of light storage. Light storage combining a photon-echo technique based on an AFC and spin-wave storage have been demonstrated, and an experimental AFC scheme for the storage of weak coherent light pulses in Pr3+:Y2SiO5 is presented. Finally, we have investigated superradiance and slow light effects. Both these effects occur in the high optical depth regime and can influence the memory performance. Under certain conditions superradiance results in the immediate re-emission of the stored light in a single burst of coherent radiation, and inside spectrally structured materials, slow light effects may result when storage is performed.
This thesis investigates the coherence properties of the hyperfine transitions of the 151Eu3+ ions in Eu3+:Y2SiO5 and evaluates the potential of developing quantum memories using these transitions. Quantum memories for light with long storage times are required for quantum commu- nication applications. For these memories to be useful they need to have storage times long compared to the transmission times across the communication network. For a global optical communication network this requires storage time longer than 100 ms. Rare-earth doped crystals have been identified as a suitable storage material. The storage time of these systems is limited by the coherence time of the hyperfine transitions of the optically active rare-earth ions. In previous work it had been demonstrated that coherence times as long as 1.4 seconds could be achieved for hyperfine transitions in Pr3+:Y2SiO5 by ap- plying a particular magnetic field such that the first order Zeeman shift of the transition nulled. This technique is known as zero first-order Zeeman (ZEFOZ). Due to the relatively large second order Zeeman efficient of the transitions in Pr3+:Y2SiO5, an extension of the coherence time, significantly beyond the 1.4 second mark using ZEFOZ, is not expected. However, it has been predicted that coherence times more than two orders of magnitude longer could be achieved in Eu3+:Y2SiO5 due to the smaller second order Zeeman shifts associated with the relevant hyperfine transitions. The dominant decoherence mechanism for the hyperfine transitions in diluted Eu3+:Y2SiO5 is the magnetic field perturbations caused by the random spin reconfigu- ration of the Y3+ ions in the host. By applying the ZEFOZ technique, previously used in Pr3+:Y2SiO5, the sensitivity of the transition’s frequency to environmental magnetic field perturbations was significantly reduced. Further, this strong ZEFOZ magnetic field was also shown to induce a frozen core around the Eu3+ ion, which resulted in a signifi- cant suppression of the reconfiguration of the nearby Y3+ spins. The combined effect of the reduced sensitivity and frozen core effect allowed a decoherence rate of 8 × 10−5 s−1 over 100 milliseconds to be demonstrated. The observed decoherence rate is at least an order of magnitude lower than that of any other system suitable for an optical quantum memory. Furthermore, by employing dynamic decoherence control, a coherence time of 370 ± 60 minutes was achieved. This 6 hour coherence time observed here opens up the possibility of distributing quantum entanglement via the physical transport of memories as an alternative to optical communications. It was found that even at the critical point alignment the observed coherence times showed that the Y3+ spin flips remain the dominant decoherence mechanism. To aid in the development of future strategies to further extend the coherence time beyond 6 hours, a study of the Y3+ spin dynamics in the frozen core was conducted. Four of the Y3+ sites were resolved and a complete mapping of all frozen-core Y3+ sites was limited by the inhomogeneity of the applied magnetic field. The Rabi frequency, the coherence time and lifetime as well as the interaction strength with the Eu3+ ion of one of these Y3+ ions were measured. The observed lifetime of the Y3+ ion is 27 s, which is four orders of magnitude longer than the low field value. With the technique developed, a detailed understanding of the frozen-core dynamics is possible, which would allow an extension of the hyperfine coherence time of the Eu3+ ion towards the lifetime limit. In summary, this thesis provides a detailed characterisation of the decoherence mecha- nisms of the hyperfine transitions in Eu3+:Y2SiO5. The potential of using rare-earth doped crystals for the long-term storage of quantum information with applications to long-range quantum communications is identified. The demonstrated long coherence time of the quantum transitions for information storage allows a new way of entanglement distribu- tion: entanglement is transported by physically transporting the memory crystal rather than the light. This approach opens a new regime for both quantum communication and fundamental tests of quantum mechanics.
When doped into solid state transparent crystals, rare-earth ions can have optically excited states with milliseconds coherence time and up to six hours coherence time for hyperfine levels after being cooled down to liquid helium temperature. This long coherence property makes them unique as solid state material and attractive for quantum information applications.The rare-earth ions have narrow homogeneous linewidths on the order of ~ kHz. When doped into a crystal, the lattice structure is slightly distorted due to the size mismatch between the dopant and the host ions, resulting in slight differences in the local crystal fields at the dopant sites. Therefore, the resonance frequencies of the rare-earth ions are shifted relatively, resulting in an inhomogeneously broadened absorption profile, typically on the order of a few GHz. This provides the possibility of addressing each group of ions individually and engineering the absorption profile for different purposes by optical pumping using a narrow linewidth laser, and makes rare-earth-ion-doped crystals suitable for ensemble based quantum memories.Quantum memories are vital components for developing longdistance quantum communication. Part of this thesis project focused on making quantum memories with high efficiency and long storage time, especially using materials with low optical depth. To achieve this goal, a low finesse, unsymmetrical cavity was employed and experiments were carried out at (or close to) the impedance matched condition. Atomic frequency comb (AFC) protocol proposed for such an inhomogeneously broadened system was used to store coherent light for a certain time. 56% overall efficiency was achieved, and the main limiting factor for reaching even higher efficiency was the inefficient coupling between incoming light and the cavity mode. Other possible loss mechanisms were analyzed as well.Long-time, on-demand, quantum memory experiments were conducted with the help of two efficient radio-frequency rephasing pulses. Storage time was extended from microseconds to milliseconds after compensating (most of) the residual earth magnetic field in the lab. The opportunities and challenges of combing the cavity assistant quantum memory and the long time, on demand quantum memory were analyzed and possible solutions were proposed. Another part of the thesis project involves slow light-based applications in rare-earth-ion-doped crystals. The strong slow light effect at the presence a spectral hole is due to the fact that the light interacts with the absorbing ions outside the spectral hole off-resonantly, and most of the light energy is stored inside the ions as a polarization. For example, with a group velocity of 300 km/s, 99.9% of the pulse energy is stored in the ions when the pulse propagates inside the crystal. The slower the light propagates, the more energy is stored inside the medium. Based on this, a frequency shifter that is capable of shifting the light frequency of ± 4 MHz was demonstrated, and frequency shift up to GHz was proposed. Group velocity control by a factor of 20 and pulse compression in time by a factor of 2 were also demonstrated. The frequency shift and group velocity of the light are solely controlled by the external electric field, which makes them ideal for weak light situation. The unique property of such a frequency shifter and group velocity controller is that they could be made to accept light from an arbitrary incoming angle and polarization, for example, scattered light.
Efficient and long-lived multimode quantum memories are crucial devices in the development of quantum technolgies. The reversible mapping of quantum states of light in rare earth doped crystals represents one of the most promising routes towards the realization of this goal. Such systems are also compatible with the miniaturization of quantum memories in integrated optics platforms, which offer unique features in terms of experimental scalability and enhanced light-matter interaction. Here, we fabricate single mode channel waveguides for 606 nm light in a praseodymium-doped yttrium orthosilicate crystal (Pr3+:Y2SiO5), that, thanks to its excellent coherence properties, is a widely studied material for light storage experiments. Waveguides are inscribed by femtosecond laser writing, adopting the so-called Type I configuration, where the core is directly obtained at the irradiated area. Remarkably, fabricating this kind of waveguides in crystals is a difficult task, as it requires to operate in a very narrow processing parameters window, if existing. We then use these waveguides for performing the storage and retrieval of single photons, implementing the atomic frequency comb protocol. We achieve a storage time of 5,5 µs, which is almost 2 orders of magnitude longer than previous realizations of quantum light storage in a waveguide. In addition, we investigate the potential information multiplexing capabilities of our system by performing the quantum storage of single photons delocalized over 14 different spectral modes. Our results show that laser written waveguides in rare earth-doped solid state systems are very promising for the development of efficient and long-lived multimode quantum memories.