Abstract We present the experimental demonstration of light storage towards the single photon level at a long storage time by electromagnetically induced transparency in a rare-earth ion-doped Pr3+:Y2SiO5 crystal. We apply decoherence control by static magnetic fields and appropriately designed radio-frequency composite pulse sequences to prolong the storage time in the memory. A rare-earth ion-doped filter crystal prepared by optical pumping serves to efficiently separate the signal at the single photon level from optical noise. Multipass setups around the memory and the filter crystal improve the storage efficiency and filter selectivity. Already without decoherence control, the setup permits storage of single photons in the microsecond regime at a storage efficiency of 42%. With decoherence control we demonstrate storage of weak coherent pulses containing some 10 photons for up to 10 s at a storage efficiency of several percent. The experimental data clearly demonstrate the applicability of EIT light storage to implement a true quantum memory in Pr3+:Y2SiO5 at long storage times. The scientific findings and technical developments are of relevance also to other protocols and media for quantum information storage.
Abstract We present the experimental demonstration of light storage towards the single photon level at a long storage time by electromagnetically induced transparency in a rare-earth ion-doped Pr3+:Y2SiO5 crystal. We apply decoherence control by static magnetic fields and appropriately designed radio-frequency composite pulse sequences to prolong the storage time in the memory. A rare-earth ion-doped filter crystal prepared by optical pumping serves to efficiently separate the signal at the single photon level from optical noise. Multipass setups around the memory and the filter crystal improve the storage efficiency and filter selectivity. Already without decoherence control, the setup permits storage of single photons in the microsecond regime at a storage efficiency of 42%. With decoherence control we demonstrate storage of weak coherent pulses containing some 10 photons for up to 10 s at a storage efficiency of several percent. The experimental data clearly demonstrate the applicability of EIT light storage to implement a true quantum memory in Pr3+:Y2SiO5 at long storage times. The scientific findings and technical developments are of relevance also to other protocols and media for quantum information storage.
Abstract Efficient and long-term storage of quantum information encoded in single photons is crucial for applications of optical quantum technologies, e.g. quantum repeaters in communication networks. Obviously, the maximal storage time is an important benchmark for such memories, as it defines the distances covered by the network or the applicability of quantum communication protocols therein. In this paper, we present the implementation of an optical memory driven by electromagnetically induced transparency, permitting the storage of weak coherent pulses containing on average a single photon with a signal-to-noise ratio of 1.3 ( 3 ) for a long storage time of one second. To achieve this goal, we apply decoherence control by static magnetic fields and robust dynamical decoupling sequences to prolong the coherence time in a rare-earth ion doped crystal to 14 s. A novel optical preparation scheme serves to increase the optical depth of the medium, which enables a light storage efficiency of 12.7 ( 5 ) % at the single photon level and one second storage time.
We demonstrate efficient storage and retrieval of light pulses by electromagnetically induced transparency (EIT) in a Pr^{3+}:Y_{2}SiO_{5} crystal. Using a ring-type multipass configuration, we increase the optical depth (OD) of the medium up to a factor of 16 towards OD≈96. Combining the large optical depth with optimized conditions for EIT, we reach a light storage efficiency of (76.3±3.5)%. In addition, we perform extended systematic measurements of the storage efficiency versus optical depth, control Rabi frequency, and probe pulse duration. The data confirm the theoretically expected behavior of an EIT-driven solid-state memory.
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.
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.
Abstract Long-duration quantum memories for photonic qubits are essential components for achieving long-distance quantum networks and repeaters. The mapping of optical states onto coherent spin-waves in rare earth ensembles is a particularly promising approach to quantum storage. However, it remains challenging to achieve long-duration storage at the quantum level due to read-out noise caused by the required spin-wave manipulation. In this work, we apply dynamical decoupling techniques and a small magnetic field to achieve the storage of six temporal modes for 20, 50, and 100 ms in a 151Eu3+:Y2SiO5 crystal, based on an atomic frequency comb memory, where each temporal mode contains around one photon on average. The quantum coherence of the memory is verified by storing two time-bin qubits for 20 ms, with an average memory output fidelity of F = (85 ± 2)% for an average number of photons per qubit of μin = 0.92 ± 0.04. The qubit analysis is done at the read-out of the memory, using a type of composite adiabatic read-out pulse we developed.
We report on storage of images in atomic coherences driven by electromagnetically induced transparency in a doped solid. We demonstrate image storage times up to the regime of milliseconds (i.e., more than two orders of magnitude larger than in gaseous media). Our data also reveal an improvement in the spatial resolution of the retrieved images by a factor of 40. The long storage times become possible by applying additional radio frequency pulse sequences to drive rephasing of the atomic coherences. Moreover, the perturbing effect of atomic diffusion (which significantly limits image storage times in gases) is absent in the solid. In addition, we monitored pronounced oscillations in the intensity of the retrieved image versus the storage time. These oscillations are due to the beating of dark-state polaritons. All of these results demonstrate the superior properties of coherently driven optical data storage in solids.