Preserving Atomic Coherences for Light Storage in Pr3+:Y2SiO5 Driven by an OPO Laser System

This work had three objectives to improve an EIT-based, solid-state memory for light. First, we set up a solid-state-laser system for radiation at the wavelength λ = 606nm, i.e., the optical transition in our storage medium, the rare-earth-ion doped crystal PrYSO. Second, we implemented efficient rephasing of optically driven coherences after EIT-based light storage by means of rapid adiabatic passage (RAP) pulses. Last but not least we implemented a novel coherence population mapping (CPM) protocol in order to shelve fragile atomic coherences in robust and long-lived populations in PrYSO. Solid-State-Laser System: We developed a solid-state-laser system based on two nonlinear processes, optical parametric oscillation (OPO) and intra-cavity sum-frequency generation (SFG). The system is designed to generate continuous wave output in the orange part of the visible spectrum. OPO and SFG are implemented on a periodically poled lithium niobate crystal (PPLN). The crystal is divided into sections with appropriate poling periods for quasi phase matching of OPO and SFG. In addition, the poling period changes along the crystal height to allow tuning of the OPO-SFG output wavelength. The system provides output in a range between λ = 605nm and λ = 616nm with an output power P > 1W. For light storage experiments, we operate the OPO-SFG at λ = 606nm with a maximum available output power of P = 1.3W. An external Pound-Drever-Hall (PDH) frequency stabilization reduces the laser linewidth to Δν ≈ 60 kHz on a time scale of 100 ms. The OPO-SFG provides stable output for more than 30 hours with a root-mean-square power jitter below 2%. In comparison to a previously used dye laser, the OPO-SFG requires less maintenance, performs more robust against external temperature fluctuations and vibrational noise and allows easier (re-) alignment. It performs slightly better in terms of frequency stability and the two systems are comparable in terms of output power. Dye lasers on the other hand provide a larger tuning range. Future work should include a revised crystal design, i.e., a reversed section order of OPO and SFG on the PPLN crystal. This has been suggested to yield higher efficiency and output stability. In addition, we use three discrete poling periods in the SFG section, whereas the OPO section consists of a fanned poling structure. A double-fanned structure could provide enhanced tunability and help to partially suppress power fluctuations for different output wavelength. In addition, it should be possible to further reduce the laser-frequency linewidth by revising the locking loop or using a higher finesse cavity for the PDH stabilization. Adiabatic Rephasing of Atomic Coherences: We experimentally implemented rephasing of optically driven coherences in PrYSO by RAP pulses. As a feature of adiabatic pulses, the parameters for RAP are defined in loose boundaries given by the adiabaticity criterion. This makes RAP potentially robust to parameter fluctuations. We experimentally verified this property and showed that rephasing with RAP provides superior performance compared to rephasing with π pulses. In particular, RAP provides enhanced robustness against variations in the Rabi frequency, pulse detuning and spatial inhomogeneity of the driving field. In our specific (3 mm long) PrYSO crystal and for standard experimental parameters, RAP yielded a factor of 1.15 higher rephasing efficiency compared to rephasing with π pulses. This value further increased when we artificially increased experimental imperfections or performed experiments with lower maximum Rabi frequencies. Concluding, RAP provides higher efficiency and more robust performance than π pulses in inhomogeneously broadened media, in the case of low available driving field power and for driving field inhomogeneities. However, RAP pulses typically require longer pulse durations than π pulses. This can cause heating or prevent the use of RAP when fast rephasing is required. An alternative to adiabatic pulses are, e.g., composite pulses or single-shot-shaped pulses. These are investigated in the context of another Ph.D thesis. Coherence Population Mapping: We implemented a novel CPM protocol to store atomic coherences in long-lived populations in PrYSO. CPM works in any three-state system and does not require complex setups beyond a radiation source to generate a short write and read sequence. As an important feature, CPM stores arbitrary coherences equally well in the populations of a three-state system, i.e., CPM does neither require inhomogeneous broadening, nor is the storage efficiency dependent on the phase of the initial coherence. To our best knowledge, this exhibits a unique feature, which no other coherence population mapping protocol provides. Thus, CPM is an alternative to the stimulated photon echo (SPE), which requires inhomogeneous broadening to map an arbitrary initial coherence onto populations. However, the maximum retrieval efficiency with CPM is 1/3 of the initial coherence amplitude (and 1/2 with SPE). We experimentally verified the main characteristics of both protocols with RF driven mapping sequences and RF induced initial coherences. Our results confirm phase-insensitive storage with CPM and storage times reaching the minute regime, i.e., the population relaxation time. We also tested CPM in combination with EIT-based light storage. However, we obtained retrieval efficiencies below 1% and we observed reduced storage durations (for both SPE and CPM) compared to our previous experiments with RF induced coherences. A major contribution to the reduced efficiency is due to the fact that light storage works in a specific ensemble in the inhomogeneously broadened optical line in PrYSO. Our specific CPM (and SPE) pulses on the other hand couple to more ensembles and cause enhanced absorption for the retrieved signal. Future work should thus focus on the implementation of optical CPM to address only the ensemble used for light storage. Regarding a reduced storage duration, we could not yet find an explanation and suggest to implemented CPM in different storage media to further investigate the limitation of storage duration

Preserving Atomic Coherences for Light Storage in Pr3+:Y2SiO5 Driven by an OPO Laser System

This work had three objectives to improve an EIT-based, solid-state memory for light. First, we set up a solid-state-laser system for radiation at the wavelength λ = 606nm, i.e., the optical transition in our storage medium, the rare-earth-ion doped crystal PrYSO. Second, we implemented efficient rephasing of optically driven coherences after EIT-based light storage by means of rapid adiabatic passage (RAP) pulses. Last but not least we implemented a novel coherence population mapping (CPM) protocol in order to shelve fragile atomic coherences in robust and long-lived populations in PrYSO. Solid-State-Laser System: We developed a solid-state-laser system based on two nonlinear processes, optical parametric oscillation (OPO) and intra-cavity sum-frequency generation (SFG). The system is designed to generate continuous wave output in the orange part of the visible spectrum. OPO and SFG are implemented on a periodically poled lithium niobate crystal (PPLN). The crystal is divided into sections with appropriate poling periods for quasi phase matching of OPO and SFG. In addition, the poling period changes along the crystal height to allow tuning of the OPO-SFG output wavelength. The system provides output in a range between λ = 605nm and λ = 616nm with an output power P > 1W. For light storage experiments, we operate the OPO-SFG at λ = 606nm with a maximum available output power of P = 1.3W. An external Pound-Drever-Hall (PDH) frequency stabilization reduces the laser linewidth to Δν ≈ 60 kHz on a time scale of 100 ms. The OPO-SFG provides stable output for more than 30 hours with a root-mean-square power jitter below 2%. In comparison to a previously used dye laser, the OPO-SFG requires less maintenance, performs more robust against external temperature fluctuations and vibrational noise and allows easier (re-) alignment. It performs slightly better in terms of frequency stability and the two systems are comparable in terms of output power. Dye lasers on the other hand provide a larger tuning range. Future work should include a revised crystal design, i.e., a reversed section order of OPO and SFG on the PPLN crystal. This has been suggested to yield higher efficiency and output stability. In addition, we use three discrete poling periods in the SFG section, whereas the OPO section consists of a fanned poling structure. A double-fanned structure could provide enhanced tunability and help to partially suppress power fluctuations for different output wavelength. In addition, it should be possible to further reduce the laser-frequency linewidth by revising the locking loop or using a higher finesse cavity for the PDH stabilization. Adiabatic Rephasing of Atomic Coherences: We experimentally implemented rephasing of optically driven coherences in PrYSO by RAP pulses. As a feature of adiabatic pulses, the parameters for RAP are defined in loose boundaries given by the adiabaticity criterion. This makes RAP potentially robust to parameter fluctuations. We experimentally verified this property and showed that rephasing with RAP provides superior performance compared to rephasing with π pulses. In particular, RAP provides enhanced robustness against variations in the Rabi frequency, pulse detuning and spatial inhomogeneity of the driving field. In our specific (3 mm long) PrYSO crystal and for standard experimental parameters, RAP yielded a factor of 1.15 higher rephasing efficiency compared to rephasing with π pulses. This value further increased when we artificially increased experimental imperfections or performed experiments with lower maximum Rabi frequencies. Concluding, RAP provides higher efficiency and more robust performance than π pulses in inhomogeneously broadened media, in the case of low available driving field power and for driving field inhomogeneities. However, RAP pulses typically require longer pulse durations than π pulses. This can cause heating or prevent the use of RAP when fast rephasing is required. An alternative to adiabatic pulses are, e.g., composite pulses or single-shot-shaped pulses. These are investigated in the context of another Ph.D thesis. Coherence Population Mapping: We implemented a novel CPM protocol to store atomic coherences in long-lived populations in PrYSO. CPM works in any three-state system and does not require complex setups beyond a radiation source to generate a short write and read sequence. As an important feature, CPM stores arbitrary coherences equally well in the populations of a three-state system, i.e., CPM does neither require inhomogeneous broadening, nor is the storage efficiency dependent on the phase of the initial coherence. To our best knowledge, this exhibits a unique feature, which no other coherence population mapping protocol provides. Thus, CPM is an alternative to the stimulated photon echo (SPE), which requires inhomogeneous broadening to map an arbitrary initial coherence onto populations. However, the maximum retrieval efficiency with CPM is 1/3 of the initial coherence amplitude (and 1/2 with SPE). We experimentally verified the main characteristics of both protocols with RF driven mapping sequences and RF induced initial coherences. Our results confirm phase-insensitive storage with CPM and storage times reaching the minute regime, i.e., the population relaxation time. We also tested CPM in combination with EIT-based light storage. However, we obtained retrieval efficiencies below 1% and we observed reduced storage durations (for both SPE and CPM) compared to our previous experiments with RF induced coherences. A major contribution to the reduced efficiency is due to the fact that light storage works in a specific ensemble in the inhomogeneously broadened optical line in PrYSO. Our specific CPM (and SPE) pulses on the other hand couple to more ensembles and cause enhanced absorption for the retrieved signal. Future work should thus focus on the implementation of optical CPM to address only the ensemble used for light storage. Regarding a reduced storage duration, we could not yet find an explanation and suggest to implemented CPM in different storage media to further investigate the limitation of storage duration