The present work deals with techniques to improve the EIT-based light storage in an all solid-state memory, i.e. a rare-earth ion-doped PrYSO crystal. The performance of such a memory can be defined by its capability to store light pulses with high efficiency and for long storage durations. In general, the storage efficiency is theoretically limited by the EIT-LS protocol efficiency, while the storage duration is limited by decoherence processes in the solid-state memory. Thus, we first optimized the EIT-LS efficiency at short storage durations. We then investigated the performance of composite and adiabatic techniques for efficient and robust rephasing and finally applied composite techniques for DDC to extend the coherence lifetime in our storage medium.
Optimization of the EIT Light Storage Efficiency: In order to optimize the EIT-LS protocol we performed systematic measurements of the EIT-LS efficiency in a PrYSO crystal featuring an optical depth d of 6. The optical depth sets the theoretical limit of the EIT-LS protocol efficiency. We experimentally optimized the efficiency by systematic variations of the control pulse power and the probe pulse duration. Furthermore, we applied an iterative algorithm, to optimize the temporal shape of the probe pulse. We found a maximal protocol efficiency 36 %, comparable to the theoretical limit. In order to increase the optical depth d, we developed a multipass setup for the probe beam, consisting of a ringlike arrangement, which is compatible with the geometrical constraints given by the existing setup. This multipass setup allows the simple variation of the number of probe passes N through the crystal and thus enables a flexible change of the effective optical depth. With this setup we achieved up to 16 probe passes through the PrYSO crystal, which corresponds to an increase of the effective optical depth from 6 to 96. We experimentally optimized the EIT-LS efficiency at variable optical depths. At N=14, i.e. an effective optical depth of about 84, we achieved an EIT-LS efficiency of 76.3 % in forward retrieval configuration, reaching previous values in an EIT-driven memory of cold atoms and achieving the highest ever obtained EIT-LS efficiency in a solid-state memory. However, due to losses at the optical components, the setup efficiency was limited to about 25.2 % at N=2. As future work will focus on the storage of few photons, it will thus be necessary to improve the optical components, to achieve adequate detection efficiencies. It might also be useful, to further investigate possibilities for backward readout configurations combined with a multipass probe setup.
Composite and Adiabatic Rephasing of Atomic Coherences: In order to extend the EIT-LS duration of our memory towards the coherence lifetime, we implemented composite and adiabatic rephasing techniques, exhibiting an improved robustness regarding variations and fluctuations of experimental parameters. We investigated their performance regarding experimental variations, in an application for rephasing of atomic coherences in the inhomogeneously broadened hyperfine transition of PrYSO. First, we applied universal composite pulses (UCP). UCP were originally designed for robust and high-fidelity population inversion in a two-state system, compensating simultaneous variations in any type of experimental parameter. We have experimentally shown that UCP exhibit an enhanced robustness to variations in several experimental parameters. Using UCP we could increase the higher rephasing efficiency about 25 %, compared to diabatic π pulses. UCP can be useful whenever significant unknown experimental variations or fluctuations prevent the application of diabatic π pulses. Second, we demonstrated a first experimental implementation of composite adiabatic passage (CAP). Essentially, CAP is a composite version of RAP, consisting of a sequence of RAP pulses with appropriately chosen relative phases. CAP was proposed to improve RAP in situations of fairly fulfilled adiabaticity. We compared the performance of CAP and RAP at different degrees of adiabaticity. In particular, we systematically investigated their performance with respect to variations in pulse duration and static detuning. We found CAP to be able to compensate for weak adiabaticity, leading to constant and high rephasing efficiency, irrespectively of the exact choice of the experimental parameters. These properties of CAP can be of interest whenever robust state manipulations are required, while sufficient adiabaticity cannot be provided, e.g. due to experimental restrictions. Third, we performed a first experimental demonstration of single-shot shaped pulses (SSSP), derived from techniques on shortcuts to adiabaticity. We demonstrated the capability of SSSP for efficient and robust rephasing and compared our results with diabatic π pulses. A further comparison with other adiabatic techniques might be necessary to provide a broader insight on SSSP. All three presented techniques can be used to improve the robustness of an population inversion or rephasing process. However, different requirements on the control of the experimental parameters have to be fulfilled. UCP, and CAP rely on identical pulses and a precise relative phase control, i.e. within a few degrees. CAP in addition needs a simple control of the time-dependent detuning. The complex time-dependent Rabi frequency and detuning of SSSP require a much more sophisticate control of experimental parameters. Thus, the choice of an adequate technique strongly depends on the actual experimental situation, i.e. on the experimentally controllable parameters.
Composite Pulses for Dynamic Decoherence Control: We investigated the performance of UCP and universal robust (UR) sequences for DDC of directly RF-prepared coherences. We compared our results with well known CPMG DD and KDD sequences. We performed systematic measurements with respect to the cycling time, the phase of the coherence and the order of UCP and UR DD sequence. We found time separated UR DD sequences to be robust with respect to the phase of the coherence. We applied these DD sequences on EIT-LS coherences and compared with CPMG DD and KDD in XY4. Our experiments showed that our UR DD sequences can outperform the often considered state-of-the-art KDD in XY4 sequence, yielding about a factor of 2 longer coherence lifetimes. Combining our results on optimized EIT-LS with advanced composite DD sequences and static decoherence control by ZEFOZ might provide a major step towards the development of an all solid-state quantum memory
