Abstract Rare-earth ion-doped crystals are of great interest for quantum memories, a central component in future quantum repeaters. To assess the promise of 1 % Tm3+-doped yttrium gallium garnet (Tm:YGG), we report measurements of optical coherence and energy-level lifetimes of its 3H 6 ↔ 3 H4 transition at a temperature of around 500 mK and various magnetic fields. Using spectral hole burning (SHB), we find hyperfine ground-level (Zeeman level) lifetimes of several minutes at magnetic fields of less than 1000 G. We also measure coherence time exceeding one millisecond using two-pulse photon echoes. Three-pulse photon echo and SHB measurements reveal that due to spectral diffusion, the effective coherence time reduces to a few µs over a timescale of around two hundred seconds. Finally, temporal and frequency-multiplexed storage of optical pulses using the atomic frequency comb protocol is demonstrated. Our results suggest Tm:YGG to be promising for multiplexed photonic quantum memory for quantum repeaters.
We investigate the relevant spectroscopic properties of the 795 nm $^{3}{\mathrm{H}}_{6}\ensuremath{\leftrightarrow}^{3}{\mathrm{H}}_{4}$ transition in 1% ${\mathrm{Tm}}^{3+}\ensuremath{\mathbin:}{\mathrm{Y}}_{3}{\mathrm{Ga}}_{5}{\mathrm{O}}_{12}$ at temperatures as low as 1.2 K for optical quantum memories based on persistent spectral tailoring of narrow absorption features. Our measurements reveal that this transition has uniform coherence properties over a 56 GHz bandwidth, and a simple hyperfine structure split by $\ifmmode\pm\else\textpm\fi{}44\text{ }\text{ }\mathrm{MHz}/\mathrm{T}$ with lifetimes of up to hours. Furthermore, we find a
3F4 population lifetime of 64 ms---one of the longest lifetimes observed for an electronic level in a solid---and an exceptionally long coherence lifetime of $490\text{ }\text{ }\ensuremath{\mu}\mathrm{s}$---the longest ever observed for optical transitions of
Tm3+ ions in a crystal. Our results suggest that this material allows realizing broadband quantum memories that enable spectrally multiplexed quantum repeaters.
The $^{2}F_{7/2}\ensuremath{\leftrightarrow}^{2}F_{5/2}$ optical transitions of
Yb3+ doped into
Y3Al5O12 (YAG) were studied for potential quantum information and photonic signal processing applications. Absorption and fluorescence spectroscopy located the energy levels of the ground
2F7/2 and excited
2F5/2 manifolds, allowing inconsistencies between previous assignments of crystal field splittings in the literature to be resolved. These measurements reveal an unusually large splitting between the first and second levels in both the ground and excited multiplets, potentially providing for reduced sensitivity to thermally induced decoherence and spin-lattice relaxation. Spectral hole burning through two-level saturation was observed, determining the excited state lifetime to be 860 \ensuremath{\mu}s and resolving ambiguities in previous fluorescence measurements that were caused by the large radiation trapping effects in this material. Optical decoherence measurements using two-pulse photon echoes gave a homogeneous linewidth of 18 kHz for an applied magnetic field of 1 T, narrowing to 5 kHz at 2.5 T. The observed decoherence was described by spectral diffusion attributed to $\mathrm{Y}{\mathrm{b}}^{3+}\mathrm{\ensuremath{-}}\mathrm{Y}{\mathrm{b}}^{3+}$ magnetic dipole interactions. Laser absorption determined an inhomogeneous linewidth of 3.6 GHz for this transition in this 0.05%-doped crystal, which is narrower than for any other rare-earth-ion transition previously studied in the YAG host. The temperature dependence of the transition energy and linewidth of the lowest
2F7/2 to lowest
2F5/2 transition centered at 968.571 nm measured from 4 K to 300 K was well described by phonon scattering at higher temperatures, with an additional anomalous linear temperature-dependent broadening at temperatures below 80 K. Two magnetically inequivalent subgroups of
Yb3+ ions were identified when a magnetic field was applied along the $\ensuremath{\langle}111\ensuremath{\rangle}$ axis, as expected for the
D2 sites in the cubic symmetry crystal, with ground and excited state effective
g-values of
gg=3.40 (3.34) and
ge=1.04 (2.01), respectively. Together with the convenient diode laser wavelength of this transition, our study suggests that
Yb3+:YAG is a promising material system for spectral hole burning and quantum information applications.
We characterize spectroscopic and coherence properties of the 1451.37 nm excited-state zero-phonon line (ZPL) between the
3F4 and the
3H4 manifolds of a thulium-doped yttrium aluminum perovskite (Tm
3+:YAlO
3) crystal at temperatures around 1.5 K. We measure the absorption spectrum between the
3H6 -
3F4 and
3F4 -
3H4 manifolds, the inhomogeneous broadening of the
3F4 -
3H4 (excited-state) ZPL, and the lifetimes of the higher-lying and lower-lying excited states. We also investigate level shifts caused by the quadratic Zeeman interaction as well as spectral hole-burning spectra with varying magnetic fields, providing insights into hyperfine interactions. Using again spectral holes but also optical free induction decays (FIDs), we assess optical coherence times, finding a maximum of
4.75±0.07 μs at B=2T and low ion concentration in the
3F4 level. Our results -- the first to demonstrate coherence of an excited-state transition in a rare-earth crystal -- suggest the possibility of exploiting such transitions for quantum technology.
We report the energy level structure of the
3H
6 and
3H
4 multiplets for Tm
3+ doped congruent LiNbO
3, as well as the decoherence properties and their temperature dependencies for the
3H${}_{6}(1){\ensuremath{\leftrightarrow}}^{3}$H
4(1a) transition at 794 nm. It is shown that this material provides very significant improvements in bandwidth, time-bandwidth product, and sensitivity for spatial-spectral holographic signal processing devices and quantum memories based on spectral hole burning. The available signal processing bandwidth for 0.1
Tm
3+:LiNbO
3 is 300 GHz versus 20 GHz for Tm
3+YAG. The peak absorption coefficient for 0.1
Tm
3+:LiNbO
3 is 15 cm${}^{\ensuremath{-}1}$ at 794.5 nm compared with 1.7 cm${}^{\ensuremath{-}1}$ for 0.1
Tm:YAG at 793 nm, and the total absorption strength is eighty times stronger. The oscillator strength for Tm
3+:LiNbO
3 is about twenty-five times larger than that for Tm
3+:YAG, making the material five times more sensitive for processing high-bandwidth analog signals. The homogeneous linewidth, which determines processing time or spectrum analyzer resolution, is 30 kHz at 1.6 K and 350 kHz at 6 K, as measured by photon echoes. Those values establish potential time-bandwidth products of 10
7 and 7 $\ifmmode\times\else\texttimes\fi{}
10{}^{5}
,respectively.Thetemperaturedependenceofthehomogeneouslinewidthwasexplainedbyobservationofa7.8cm{}^{\ensuremath{-}1}$ crystal field level in the ground multiplet and direct phonon coupling. The excited state
3H
4 lifetime
T1 is 152 $\ensuremath{\mu}$s and the bottleneck lifetime of the lowest
3F
4 level is 7 ms from photon echo measurements. These factors combine to provide a surprisingly large increase in key parameters that determine material performance for spatial-spectral holography, quantum information, and other spectral hole burning applications.
We report detailed studies of the optical decoherence and spectroscopic properties of the 1536-nm optical transition of the bandwidth-engineered material 0.02
Er:1
Eu:Y
2SiO
5. This work is motivated by the need for comprehensive understanding of new resonant optical materials for spatial-spectral holography and quantum information applications. The dependence of optical decoherence on excitation frequency, applied magnetic field strength and orientation, crystal temperature, measurement time scale, and optical excitation density was studied using stimulated photon echo spectroscopy and time-resolved spectral hole-burning techniques. The effects of weak disorder generated by the introduction of Eu
3+ ions into the lattice was probed by comparing measurements across the static disorder-induced distribution of transition frequencies as well as by comparing with measurements on 0.02
Er:Y
2SiO
5. The results reveal that the coherence properties of Er:Y
2SiO
5 are preserved in the disordered system; moreover, the increased inhomogeneity produced by the disorder acts to suppress optical decoherence by inhibiting the electronic spin diffusion process, improving material properties for low magnetic field strengths and at increased temperatures desirable for practical applications.