arXiv subjects

Stefan Kröll

Publications and source records attributed to Stefan Kröll.

24 records · Page 2Linked to original sources

arXiv2013

Three orders of magnitude cavity-linewidth narrowing by slow light in a rare-earth-ion-doped crystal cavity

Three orders of magnitude cavity-linewidth narrowing in a rare-earth-ion-doped crystal cavity, induced by strong intra-cavity dispersion caused by off-resonant interaction with dopant ions is demonstrated. The strong dispersion is created by semi-permanent but rapidly reprogrammable changes of the rare earth absorption profiles using optical pumping techniques. Several cavity modes are shown within the spectral transmission window. Potential applications are discussed.

quant-ph↗

arXiv2013

Cavity enhanced storage - preparing for high efficiency quantum memories

Cavity assisted quantum memory storage has been proposed [PRA 82, 022310 (2010), PRA 82, 022311 (2010)] for creating efficient (close to unity) quantum memories using weakly absorbing materials. Using this approach we experimentally demonstrate a significant (about 20-fold) enhancement in quantum memory efficiency compared to the no cavity case. A strong dispersion originating from absorption engineering inside the cavity was observed, which directly affect the cavity line-width. A more than 3 orders of magnitude reduction of cavity mode spacing and cavity line-width from GHz to MHz was observed. We are not aware of any previous observation of several orders of magnitudes cavity mode spacing and cavity line-width reduction due to slow light effects.

quant-ph↗

arXiv2013

Efficient quantum memory using a weakly absorbing sample

A light-storage experiment with a total (storage and retrieval) efficiency $η=58 \pm 5%$ is carried out by enclosing a sample, with a single pass absorption of 10%, in an impedance-matched cavity. The experiment is carried out using the Atomic Frequency Comb (AFC) technique in a praseodymium-doped crystal ($0.05%Pr^{3+}:Y_2SiO_5$) and the cavity is created by reflection coating the crystal surfaces. The AFC technique has previously by far demonstrated the highest multi-mode capacity of all quantum memory concepts tested experimentally. We claim that the present work shows that it is realistic to create efficient, on-demand, long storage time AFC memories.

quant-ph↗

arXiv2011

Hyperfine characterization and coherence lifetime extension in Pr3+:La2(WO4)3

Rare-earth ions in dielectric crystals are interesting candidates for storing quantum states of photons. A limiting factor on the optical density and thus the conversion efficiency is the distortion introduced in the crystal by doping elements of one type into a crystal matrix of another type. Here, we investigate the system Pr3+:La2(WO4)3, where the similarity of the ionic radii of Pr and La minimizes distortions due to doping. We characterize the praseodymium hyperfine interaction of the ground state (3H4) and one excited state (1D2) and determine the spin Hamiltonian parameters by numerical analysis of Raman-heterodyne spectra, which were collected for a range of static external magnetic field strengths and orientations. On the basis of a crystal field analysis, we discuss the physical origin of the experimentally determined quadrupole and Zeeman tensor characteristics. We show the potential for quantum memory applications by measuring the spin coherence lifetime in a magnetic field that is chosen such that additional magnetic fields do not shift the transition frequency in first order. Experimental results demonstrate a spin coherence lifetime of 158 ms - almost three orders of magnitude longer than in zero field.

quant-ph↗

arXiv2010

Demonstration of atomic frequency comb memory for light with spin-wave storage

We present a light-storage experiment in a praseodymium-doped crystal where the light is mapped onto an inhomogeneously broadened optical transition shaped into an atomic frequency comb. After absorption of the light the optical excitation is converted into a spin-wave excitation by a control pulse. A second control pulse reads the memory (on-demand) by reconverting the spin-wave excitation to an optical one, where the comb structure causes a photon-echo type rephasing of the dipole moments and directional retrieval of the light. This combination of photon echo and spin-wave storage allows us to store sub-microsecond (450ns) pulses for up to 20 microseconds. The scheme has a high potential for storing multiple temporal modes in the single photon regime, which is an important resource for future long-distance quantum communication based on quantum repeaters.

quant-ph↗

arXiv2009

Experimental superradiance and slow light effects for quantum memories

The effects of high optical depth phenomena, such as superradiance, are investigated in potential quantum memory materials. The results may have relevance for several schemes, including CRIB, AFC and EIT-based quantum memories, which are based on using ensembles as storage media. It is shown that strong superradiant effects, manifested as decay rates larger than 1/T2*, are present even for moderate values of alphaL < 5, and increases as a function of alphaL. For even higher alphaL, effects like off-resonant slow light is demonstrated and discussed, and finally, the efficiency of time-reversed optimized input pulses are tested. A maximum retrieval efficiency of ~20% is reached, and agreement with the theoretically expected result is discussed.

quant-ph↗