Optical and hyperfine parameters of the 3H4–1D2 transition are investigated in Pr3 +:La2(WO4)3 for doping concentrations ranging from 0.02 to 3 at.%, at liquid helium temperatures. This includes optical inhomogeneous and homogeneous linewidths, peak absorption, excited-state population decays, spectral hole lifetime and inhomogeneous and homogeneous hyperfine linewidths. Optical inhomogeneous broadening with increasing Pr3 + concentration is limited due to close atomic radii between Pr3 + and La3 +. The peak absorption coefficient increases with increasing Pr3 + doping, reaching up to 28 cm−1. At the highest Pr3 + concentration, optical coherence lifetime is 4.6 μs and hole lifetime 2 s, which should allow one to transfer coherences and tailor the optical inhomogeneous linewidth, as required for rare earth-based optical quantum memories. Raman echo experiments show that hyperfine inhomogeneous linewidth and coherence lifetime (50 kHz and 250 μs respectively) are independent of Pr3 + concentration. These results suggest that highly doped crystals are useful for efficient and long-storage time quantum memories.
Optical and hyperfine parameters of the 3H4–1D2 transition are investigated in Pr3 +:La2(WO4)3 for doping concentrations ranging from 0.02 to 3 at.%, at liquid helium temperatures. This includes optical inhomogeneous and homogeneous linewidths, peak absorption, excited-state population decays, spectral hole lifetime and inhomogeneous and homogeneous hyperfine linewidths. Optical inhomogeneous broadening with increasing Pr3 + concentration is limited due to close atomic radii between Pr3 + and La3 +. The peak absorption coefficient increases with increasing Pr3 + doping, reaching up to 28 cm−1. At the highest Pr3 + concentration, optical coherence lifetime is 4.6 μs and hole lifetime 2 s, which should allow one to transfer coherences and tailor the optical inhomogeneous linewidth, as required for rare earth-based optical quantum memories. Raman echo experiments show that hyperfine inhomogeneous linewidth and coherence lifetime (50 kHz and 250 μs respectively) are independent of Pr3 + concentration. These results suggest that highly doped crystals are useful for efficient and long-storage time quantum memories.
Most of the experiments related to quantum information applications, involving rare-earth doped inorganic crystals, are performed on yttrium orthosilicate single crystals. The work presented here is motivated by the search of new compounds which can be used in the field of quantum computing and/or quantum storage. Relaxation times and hyperfine structure of the $^{3}H_{4}(0)\ensuremath{\rightarrow}^{1}D_{2}(0)$ transition in 1.4% Pr3+:La2(WO4)3 at 4K have been measured by photon-echo and spectral-hole-burning techniques. The hyperfine splittings of the ground 3H4(0) and the excited 1D2(0) states are $14.9\ifmmode\pm\else\textpm\fi{}0.1\phantom{\rule{0.3em}{0ex}}\mathrm{MHz}$, $24.6\ifmmode\pm\else\textpm\fi{}0.1\phantom{\rule{0.3em}{0ex}}\mathrm{MHz}$ and $5.0\ifmmode\pm\else\textpm\fi{}0.1\phantom{\rule{0.3em}{0ex}}\mathrm{MHz}$, $7.3\ifmmode\pm\else\textpm\fi{}0.1\phantom{\rule{0.3em}{0ex}}\mathrm{MHz}$, respectively. An inhomogeneous linewidth of $18.8\ifmmode\pm\else\textpm\fi{}0.1\phantom{\rule{0.3em}{0ex}}\mathrm{GHz}$ was measured. A homogeneous linewidth of $25.3\ifmmode\pm\else\textpm\fi{}2.0\phantom{\rule{0.3em}{0ex}}\mathrm{kHz}$ was obtained with or without an external magnetic field of about 14mT. The fluorescence dynamics of the 1D2 level obtained by a direct excitation in the $^{3}H_{4}\ensuremath{\rightarrow}^{1}D_{2}$ transition gives a nonexponential decay which indicates energy-transfer processes. This decay can be accurately fitted by the Inokuti-Hirayama model [J. Chem. Phys. 43, 1978 (1965)] with a radiative lifetime of $61\ifmmode\pm\else\textpm\fi{}1\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{s}$ giving a minimal homogeneous linewidth of 2.6kHz. The spectral-hole lifetime due to population redistribution within the ground hyperfine levels is $16\ifmmode\pm\else\textpm\fi{}2\phantom{\rule{0.3em}{0ex}}\mathrm{s}$. The results obtained for the La2(WO4)3 compound make this crystal an interesting host for quantum applications.
We studied the hyperfine structure and hyperfine coherent properties of the ${^{3}\text{H}}_{4}(0)\ensuremath{\rightarrow}{^{1}\text{D}}_{2}(0)$ transition of Pr3+ ions in a tungstate single crystal La2(WO4)3 by hole-burning and photon-echo techniques. This work is motivated by the search of an efficient three level $\ensuremath{\Lambda}$ system in this new compound with which we could build up a quantum memory. By nonconventional hole-burning experiments, the ordering of the hyperfine splittings in the 3H4(0) ground state and in the 1D2(0) excited state is obtained. The hyperfine splittings are thus ordered: 24.6 and 14.9 MHz for the 3H4(0) level and 5.0 and 7.3 MHz for the 1D2(0) level. The relative and absolute transition strengths of individual hyperfine transitions are determined by comparing absorption strengths and by measuring the Rabi flopping frequency as the transition is coherently driven. Free induction and Raman echo decays give inhomogeneous and homogeneous hyperfine linewidths of $57\ifmmode\pm\else\textpm\fi{}2$ and $1.25\ifmmode\pm\else\textpm\fi{}0.1\text{ }\text{kHz}$, respectively.
The nature of inhomogeneous broadening in rare earth systems plays an important role in the dynamical optical properties of ions in crystals. For this reason much of the work on the nature of the inhomogeneous broadening in rare earth doped systems has concentrated on the investigation of ion-ion interactions such as spectral transfer and up conversion processes.1,3 Variations in homogeneous optical dephasing times across the inhomogeneous optical line have also been reported.4 These studies concentrate on the variations of collective effects of ion- ion interactions at different positions in the inhomogeneous optical line in relatively heavily doped rare earth systems. We report variations in optically detected NQR (ODNQR) linewidths and shifts of Pr3+ hyperfme transitions in the optical ground state as a function of position in the inhomogeneous optical line and for satellite transitions of the 1D2-3H4 transition in lightly doped (0.5 at. %) Pr3+:LaF3.
Rare-earth ions in dielectric crystals are interesting candidates for storing quantum states of photons. A limiting factor on the optical density and thus the conversion efficiency is the distortion introduced in the crystal by doping elements of one type into a crystal matrix of another type. Here we investigate the system Pr3+:La2(WO4)3, where the similarity of the ionic radii of Pr and La minimizes distortions due to doping. We characterize the praseodymium hyperfine interaction of the ground-state (3H4) and one excited state (1D2) and determine the spin Hamiltonian parameters by numerical analysis of Raman-heterodyne spectra, which were collected for a range of static external magnetic-field strengths and orientations. On the basis of a crystal-field analysis, we discuss the physical origin of the experimentally determined quadrupole and Zeeman tensor characteristics. We show the potential for quantum memory applications by measuring the spin coherence lifetime in a magnetic field that is chosen such that additional magnetic fields do not shift the transition frequency in first order. Experimental results demonstrate a spin coherence lifetime of 158 ms --- almost 3 orders of magnitude longer than in zero field.
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 presents a spectroscopic study of Pr3+ ions in a novel passive waveguide architecture. To make this structure, the high refractive glass TeO2 was selected as the thin film and it was deposited on a Pr3+:Y2SiO5 crystal. In this waveguide, the 3H4 to 1D2 transition of Pr3+ ions were probed by the optical evanescent field extending into the substrate. The main concern in assessing the suitability of this material for quantum information applications was ensuring that the coherence properties of rare-earth ions, making them suitable for quantum information purposes, are preserved in this architecture. After which, to make low loss devices with these waveguides, efficient coupling techniques had to be developed. To prove that the coherence properties of the rare-earth ion doped crystal were preserved, the critical parameters of inhomogeneous linewidth, the absorption of the ions, the coherence time and spin lifetimes of the Pr3+ ions were studied. The inhomogeneous linewidth of evanescent coupled ions was about 10.0 GHz, which was consistent with the linewidth of bulk samples with the same Pr3+ doping concentration (Hedges, 2011). The absorption due to the evanescent coupling was 9.38 dB, approximately 90% of what was expected with respect to the bulk crystal with the same doping concentration. Therefore, despite using the evanescent field, the absorption is high enough for quantum memory applications. An optical coherence time of about 121 microseconds was measured, which corresponded to a homogeneous linewidth of about 2.6 kHz. This is very close to bulk sample measurements of 111 microseconds, with the same temperature and doping concentration (R. W. Equall, 1995). The spin state lifetime observed was about 9.8 s, which is also very close to the bulk sample measurement of 8.67 s (Mieth, 2016). Initial Stark shifting experiments were performed to determine whether the active ions in the substrate of the passive waveguides could be electrically controlled by applying a small voltage to electrodes on the thin film. In these experiments with a voltage change of 100 mV, the measured holewidth broadening was increasing about 0.55 MHz, which was similar to the calculated values of 0.45 MHz. The Stark coefficient for site 1 was 51.6 kHzcm/V along the D2 axis of the crystal (site 1 will be explained in Section 4.3). (F.R. Graf, 1997). Prism coupling and grating coupling were used to couple light to the ions in the substrate. Prism coupling is an easy and quick method to couple light into a waveguide and observe the properties of the system. However, grating coupling is much more practical when moving towards building a device using this method. The measurements described above indicated that the properties of ions interacting with the evanescent tail of the waveguide mode were consistent with those of bulk ions. This investigation also showed that depositing a glass thin film on a rare-earth ion doped crystal was not affecting the good coherence properties of the substrate. These results establish the foundation for large, integrated, controllable and high performance rare earth ion quantum waveguide systems.