With the creation of nitrogen (NV) in 1b diamond it is common to find that the absorption and emission is predominantly of negatively charged NV centres. This occurs because electrons tunnel from the substitutional nitrogen atoms to NV to form NV-N + pairs. There can be a small percentage of neutral charge NV 0 centres and a linear increase of this percentage can be obtained with optical intensity. Subsequent to excitation it is found that the line width of the NV-zero-phonon has been altered. The alteration arises from a change of the distribution of N + ions and a modification of the average electric field at the NV-sites. The consequence is a change to the Stark shifts and splittings giving the change of the zero-phonon line (ZPL) width. Exciting the NV-centres enhances the density of close N + ions and there is a broadening of the ZPL. Alternatively exciting and ionizing N 0 in the lattice results in more distant distribution of N + ions and a narrowing of the ZPL. The competition between NV-and N 0 excitation results in a significant dependence on excitation wavelength and there is also a dependence on the concentration of the NV-and N 0 in the samples. The present investigation involves extensive use of low temperature optical spectroscopy to monitor changes to the absorption and emission spectra particularly the widths of the ZPL. The studies lead to a good understanding of the properties of the NV-N + pairs in diamond. There is a critical dependence on pair separation. When the NV-N + pair separation is large the properties are as for single sites and a high degree of optically induced spin polarization is attainable. When the separation decreases the emission is reduced, the lifetime shortened and the spin polarization downgraded. With separations of <12 A 0 there is even no emission. The deterioration occurs as a consequence of electron tunneling in the excited state from NV-to N + and an optical cycle that involves NV 0. The number of pairs with the smaller separations and poorer properties will increase with the number of nitrogen impurities and it follows that the degree of spin polarization that can be achieved for an ensemble of NV-in 1b diamond will be determined and limited by the concentration of single substitutional nitrogen. The information will be invaluable for obtaining optimal conditions when ensembles of NV-are required. As well as extensive measurements of the NV-optical ZPL observations of Stark effects associated with the infrared line at 1042 nm and the optically detected magnetic resonance at 2.87 GHz are also reported.

We obtain a low optical inhomogeneous linewidth of 25 MHz in the stoichiometric rare-earth crystal ${\mathrm{EuCl}}_3·6{\mathrm{H}}_2\mathrm{O}$ by isotopically purifying the crystal in ${}^{35}\mathrm{Cl}$. With this linewidth, an important limit for stoichiometric rare-earth crystals is surpassed: the hyperfine structure of ${}^{153}\mathrm{Eu}$ is spectrally resolved, allowing the whole population of ${{}^{153}\mathrm{Eu}}^{3+}$ ions to be prepared in the same hyperfine state using hole-burning techniques. This material also has a very high optical density, and can have long coherence times when deuterated. This combination of properties offers new prospects for quantum information applications. We consider two of these: quantum memories and quantum many-body studies. We detail the improvements in the performance of current memory protocols possible in these high optical depth crystals, and describe how certain memory protocols, such as off-resonant Raman memories, can be implemented for the first time in a solid-state system. We explain how the strong excitation-induced interactions observed in this material resemble those seen in Rydberg systems, and describe how these interactions can lead to quantum many-body states that could be observed using standard optical spectroscopy techniques.

We describe a quantum memory spectral preparation strategy that optimizes memory efficiency and bandwidth in materials such as ${}^{167}\mathrm{Er}^{3+}$:${\mathrm{Y}}_2{\mathrm{SiO}}_5$ in a high-field regime, where the hyperfine structure is resolved. We demonstrate the method in ${}^{167}\mathrm{Er}^{3+}$:${\mathrm{Y}}_2{\mathrm{SiO}}_5$ by preparing spectrally isolated 18-dB-absorbing features on a $<1$ dB background. Using these features we create an atomic frequency comb and show a quantum storage of 200-ns pulses with 22% efficiency, limited by the background absorption which arises from laser instability. We describe the experimental improvements needed to reach the material limits: $O\left(1\right)$-s spin-state storage, $O\left(100\right)\mathrm{MHz}$ bandwidth, and $>90\%$ efficiency.

We report measurements of discrete excitation-induced frequency shifts on the ${}^{7}F_0\to {}^{5}D_0$ transition of the ${\mathrm{Eu}}^{3+}$ center in $\mathrm{La}\mathrm{\text{:}}\mathrm{Lu}\mathrm{\text{:}}{\mathrm{EuCl}}_3·6{\mathrm{D}}_2\mathrm{O}$ resulting from the optical excitation of neighboring ${\mathrm{Eu}}^{3+}$ ions. Shifts of up to $46.081\pm 0.005\mathrm{MHz}$ were observed. The magnitude of the interaction between neighboring ions was found to be significantly larger than expected from the electric dipole-dipole mechanism often observed in rare earth systems. We show that a large network of interacting and individually addressable centers can be created by lightly doping crystals otherwise stoichiometric in the optically active rare earth ion, and that this network could be used to implement a quantum processor with more than ten qubits.