In this work, after a brief review of what has been already obtained in the field of quantum information using rare earth ions, we will discuss recent results on memory protocols on Kramers and non-Kramers ions and in particular on a new Pr3+doped tungstate La2(WO4)3. This includes electromagnetically induced transparency or photon echo based schemes. Materials requirements for efficient memories will also be discussed.
In this work, after a brief review of what has been already obtained in the field of quantum information using rare earth ions, we will discuss recent results on memory protocols on Kramers and non-Kramers ions and in particular on a new Pr3+doped tungstate La2(WO4)3. This includes electromagnetically induced transparency or photon echo based schemes. Materials requirements for efficient memories will also be discussed.
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.
Quantum information uses special properties of quantum systems to manipulate or transmit data. This results in new processes, which are impossible to obtain with classical devices. For example, quantum computing and quantum storage, which are two important fields in quantum information research, aim respectively at performing very fast calculations and at storing quantum states of photons. These two applications could be obtained in solid-state systems using rare earth doped crystals. In this context, the most important property of these materials is the long coherence lifetimes of rare earth ion optical and hyperfine transitions. This allows one to create long-lived superposition states, which is a fundamental requirement for efficient quantum computing and storage. Promising results have already been demonstrated in rare earth doped crystals but it will be difficult to improve them with current materials. In this paper, we discuss the general and specific requirements for rare earth ions and crystals in order to perform quantum computing with a large number of quantum bits as well as all solid-state quantum storage. We also present the properties of a few recently studied crystals: Ho3+:YVO4, Ho3+:LuVO4 (quantum computing) and Tm3+:Y3Al5O12 (quantum storage).
This thesis describes a number of coherent processes, such as quantum information processing, superradiance and electromagnetically induced transparency, which have been experimentally implemented using rare-earth-ion-doped crystals. The rare-earths are a class of elements that have in common, an atomic structure that allows for very long lifetimes as well as coherence times, both on optical transitions and on spin transitions. These ions can be naturally trapped inside host crystals, and with the phonon vibrations removed by cooling down to cryogenic temperatures (< 4 K), they can be used for a number of different quantum information processes. Most experiments were carried out using PrYSO, but also other crystal types were investigated, such as Nd:YVO4, and La2(WO4)3. The characterization of the ions were done with a variety of methods, including among others, photon echo techniques and electromagnetically induced transparency. Quantum computing is a rapidly growing field and there are still many potential candidates for its implementation. In our work we have utilized the spin states of the Pr rare-earth ion as a qubit, and demonstrated arbitrary single qubit gates, which are important pieces towards a quantum computing realization in these systems. Most of the experimental work done for the thesis was carried out using an ensemble approach, that has the advantage of giving a strong readout signal, but for future scaling to multiple qubits, single instances are more promising, which is discussed. Quantum memories are crucial components in applications such as quantum networks and long distance quantum communication. In this thesis, work has been done to investigate how quantum memories can be implemented in rare-earth crystals. In particular, there are two protocols, CRIB and AFC, which were suggested with rare-earth crystals directly in mind, and both of these require high optical depth, for maximum recall efficiency. In this thesis, implications of being in the high optical depth regime, with phenomena such as superradiance, a collective effect that could cause the stored light to be immediately reemitted, and slow light effects that come from performing the storage inside spectrally structured materials, were investigated. In order to carry out the phase-sensitive experiments, a laser system with a very narrow linewidth of ~1 kHz at 606 nm was constructed, by locking the laser to a semi-persistent spectral hole. In addition, many experiments required advanced pulse shapes, such as complex sechyp pulses or pulses obtained from optimal control theory. In order to be able to accurately create such shapes, an elaborate system using an arbitrary waveform generator and two well calibrated AOMs, controlled from a computer, was also built.
Optical waveguides in rare-earth-doped crystals are one of the most promising systems for practical implementations of quantum memory. To further develop these systems, detailed understanding of these materials is required. We report experimental studies of the optical properties and decoherence for Tm3+:LiNbO3, Pr3+:LiNbO3, Er3+:LiNbO3 and Er3+:KTiOPO4 (KTP) bulk crystals for quantum memory applications and discuss potential differences between the properties of the bulk single crystals and optical waveguides. These systems include the rare-earth ions most commonly exploited in quantum memory studies incorporated into two of the most technologically significant waveguide host materials. Photon echo methods were used to study decoherence as a function of temperature, applied magnetic field strength, dopant concentration and excitation wavelength. Decoherence mechanisms were investigated and modelled to interpret the observed behaviour. Spectral hole burning was used to characterize Stark and Zeeman effects. Bulk crystal properties were compared and contrasted to the properties of doped waveguides.
Since quantum information technologies are expected to offer communication security and high computational capacities, research in the field is currently attracting a lot of attention. Among the materials studied so far, rare earth doped inorganic insulators are one of the most promising. With the different available trivalent rare earth ions, the visible and the IR range including the telecom wavelength at 1.5 μm can be covered. Transitions are usually narrow, and at low temperatures, long optical and spin coherence times can often be observed. Investigations using bulk single crystals have already led to many promising results. Recently, spectroscopic studies have been extended to other forms of inorganic materials, such as transparent ceramics, thin films, and nanoparticles for single rare‐earth qubits. Progress in these areas is expected to offer many new possibilities for the design of quantum light‐matter interfaces and scalable quantum memories and processors.
Spin dynamics are investigated in several rare-earth-ion-doped crystals, both for so-called Kramers ions and non-Kramers ions. I have studied two illustrative examples of such systems, Neodymium (Kramers) and Europium (non-Kramers). The main motivation behind these studies is the search for long relaxation and coherence times in solid state atomic ensembles. To this end I employ spectroscopic techniques to probe different kinds of interactions and relaxation mechanisms taking place in the samples. I perform spectroscopy of Neodymium (Kramers) in two different crystal lattices, yttrium orthosilicate and yttrium orthovanadate. I propose new methods to improve the relaxation lifetimes, and make precise characterizations of the interactions taking place within the atomic ensemble. In particular we achieve the most precise measurement of a kind of interaction which has received considerable attention recently in other areas of physics, the Dzyaloshinskii-Moriya interaction. These spectroscopic studies are of course of fundamental interest, but they can also be applied to quantum communication and computation. In this context, we were able to increase the efficiency of a quantum memory based on a Kramers ion (Nd) to be on par with the best non-Kramers (Eu,Pr) memories. Using Neodymium ions, I have certified that multi-dimensional entanglement can be stored in a quantum memory. The second part of this thesis concerns the study of Europium (non-Kramers), starting with a characterization of the Zeeman and quadrupole interactions. I believe these studies will pave the way for long duration (100 ms or more) quantum storage in a rare-earth-ion-doped crystal.