EIT Light Storage of Weak Coherent Pulses in a Doped Solid

Abstract

The research project dealt with the implementation of EIT light storage in the solid state medium Pr:YSO for storage of single photons on a microsecond timescale and weak coherent pulses on a timescale of seconds. The main challenge was to maintain a large efficiency at long storage times and suppress control pulse background by about 120 dB to enable detection of signals on the single photon level. We used a configuration with counterpropagating probe and control beams which enabled a control pulse suppression of about 80 dB and achieved the remaining suppression by spectral filtering. Since the signal and control read pulse frequencies differ only by ∼10 MHz, we built a filter based on a second Pr:YSO crystal in which we tailored the absorption spectrum to the experimental requirements by optical pumping. For short storage times of 2 μs, the filter enhanced the separation between the signal and control read pulses by 42 dB. This enabled the storage of a weak coherent pulse with an average photon number of 1.1 with an efficiency of 42% and a SNR of 3.2. The efficiency was limited by the OD of the storage medium and could be increased by a multipass setup. Residual leakage of the control pulse through the filter limited the SNR. Still, to the best of our knowledge, our result represents the first implementation of single photon storage based on EIT in a solid state medium. To prolong the coherence lifetime, we implemented ZEFOZ. The influence of the magnetic field reduced the OD of the Pr:YSO crystal which led to a lower storage efficiency. We characterized light storage under ZEFOZ conditions at a short storage time of 2 μs. By application of a multipass setup with three passes and temporal shaping of the probe pulse, we partially compensated the reduced OD and achieved efficiencies of up to 23% for storage of classical light pulses. In order to maintain a high setup transmission, we used only two passes for storage of weak coherent pulses. We demonstrated storage of a weak coherent pulse with an average of 10.2 photons for 2 μs with 16% efficiency and a SNR of 1.3. Again, leakage of the control read pulse through the spectral filter limited the SNR. We found this leakage to be due to a weak but spectrally broad pedestal in the spectrum of our OPO-SFG laser system. In order to prolong the light storage time and exploit the long coherence lifetime under ZEFOZ conditions, we applied different sequences for rephasing with RF pulses. In the design of our RF coils for rephasing, we had to consider restrictions given by the static magnetic field setup. Therefore, the rephasing π pulses had a significant inhomogeneity which led to a low rephasing efficiency of 15% when we applied two π pulses. To compensate these errors, we applied UR DD which increased the rephasing efficiency to 40%. Due to an experimental limitation, we could not reduce the separation of the rephasing π pulses below 10 ms without reducing the storage efficiency. This limited the effectiveness of DD. Nevertheless, we achieved a storage time of 7.5 s for classical light pulses and confirmed this timescale for storage of weak coherent pulses with an average of 52 photons. We stored weak coherent pulses with different photon number for 1.28 s and extrapolated a SNR of 0.09 for storage of a single photon. For storage of a weak coherent pulse with an average of 7 photons, we determined an efficiency of 6% and a SNR of 0.6. This represents the first implementation of a memory for weak coherent pulses close to the single photon level with a storage time in the regime of seconds. Compared to the current state-of-the-art single photon storage time of 100 ms, we achieve a more than one order of magnitude longer storage time. As an extension of our investigations, we developed, implemented and studied novel universal composite pulse sequences to invert superposition states or efficiently transfer population between two states. In particular, we experimentally demonstrated UCPs for population inversion which are especially suited for situations in which pulse area and detuning errors are correlated. We showed that we can rotate the excitation profile of the sequence by changing the phases of the excitation pulses. This enables adaption to the specific correlation of errors present in an experiment. Moreover, we demonstrated a composite version of STIRAP. We showed that for detuned pulses, the phases of UCPs enhance the robustness to variations in any experimental parameter. We achieved an increase in population transfer efficiency from 50% for five times repeated detuned STIRAP to 85% for UR CSTIRAP.