Erbium-167-doped yttrium orthosilicate (167Er3+:Y2SiO5) is a rare-earth-ion-doped crystal which possesses unique properties that would make it an ideal microwave-addressed quantum memory. It has an optical transition at 1.5 µm, which lies in the telecom C-band where much of the optical telecommunications infrastructure is already operating. It also possesses both nuclear and electron spin states, and hyperfine structure with transitions at microwave frequencies. The hyperfine structure exhibits microwave frequency transitions with or without an applied magnetic field, which would allow for a memory compatible with superconducting systems. However, the complicated hyperfine structure has prevented the transitions of 167Er3+:Y2SiO5 from being fully utilised.\nWith the recent publication of new spin Hamiltonian parameters for the ground state of 167Er3+:Y2SiO5, we can predict which transitions should have the longest coherence time at zero field. Based on calculations using these spin Hamiltonian parameters, we used Raman heterodyne spectroscopy to investigate the hyperfine structure of 167Er3+:Y2SiO5 for small magnetic fields and for energy level differences from 600 to 1200 MHz. We observed many transitions from the 4I15/2 ground state, as well as the 4I13/2 excited state that until now has had little investigation regarding its hyperfine structure.\nBy comparing our spectra to the existing ground state spin Hamiltonian parameters, and unpublished excited state parameters, we identified the origin of many of the transitions. We observed differences between the predicted transition frequencies from both sets of parameters and the experimental data. The data obtained using Raman heterodyne was used to improve both ground and excited state parameters to give a better description of the zero-field hyperfine splittings.\nIn addition, we identified transitions from both the 4I15/2 ground and 4I13/2 excited state that have a small dependence on magnetic field. These transitions, with zero field frequencies of 879.4 MHz and 896.7 MHz respectively, yielded coherence times of 67 µs and 300 µs at 3.2 K, when measured with a two-pulse spin echo sequence. By using a dynamic decoupling sequence, we extending the coherence times of the transitions at 879.4 MHz to 380 µs and for the transitions at 896.7 MHz, to 1.4 ms.\nThese coherence time measurements demonstrate an improvement of a previous zero field coherence measurement in 167Er3+:Y2SiO5. By operating at lower temperatures, reducing the 167Er3+ ion concentration or using a different transition, it should be possible to extend the coherence time of 167Er3+:Y2SiO5 even further.
We report a low-temperature spectroscopic characterization of the ground-state hyperfine transitions in
151Eu3+:Y2SiO5 via coherent Raman scattering. Inhomogeneous linewidths of 21 and 38 kHz were measured for the ${I}_{z}=\ifmmode\pm\else\textpm\fi{}\frac{1}{2}$ $\ensuremath{\leftrightarrow}$ $\ifmmode\pm\else\textpm\fi{}\frac{3}{2}$ and the $\ifmmode\pm\else\textpm\fi{}\frac{3}{2}$ $\ensuremath{\leftrightarrow}$ $\ifmmode\pm\else\textpm\fi{}\frac{5}{2}$ ground-state transitions, respectively. Spin-echo decays were recorded for the ${I}_{z}=\ifmmode\pm\else\textpm\fi{}\frac{1}{2}$ $\ensuremath{\leftrightarrow}$ $\ifmmode\pm\else\textpm\fi{}\frac{3}{2}$ transition at zero field and for a small static magnetic field. In the latter case, analysis by a simple model gave an amplitude of 284 Hz and a correlation time of 3.5 ms for the transition frequency fluctuations. Application of a dynamical decoupling sequence resulted in coherence lifetimes up to 474 ms, an 18-fold increase compared to the spin intrinsic phase memory time of 26 ms.
In many important situations, the dominant dephasing mechanism in cryogenic rare-earth-metal-ion--doped systems is due to magnetic field fluctuations from spins in the host crystal. Operating at a magnetic field where a transition has a zero first-order Zeeman (ZEFOZ) shift can greatly reduce this dephasing. Here we identify the location of transitions with a zero first-order Zeeman shift for optical transitions in Pr
3+:YAG and for spin transitions in Er
3+:Y
2SiO
5. The long coherence times that ZEFOZ can enable would make Pr
3+:YAG a strong candidate for achieving the strong-coupling regime of cavity QED, and would be an important step forward in creating long-lived telecommunications wavelength quantum memories in Er
3+:Y
2SiO
5. This work relies mostly on published spin-Hamiltonian parameters, but Raman heterodyne spectroscopy was performed on Pr
3+:YAG to measure the parameters for the excited state.
Although superconducting qubits are one of the most promising systems for quantum information applications, a separate interface is required to allow for information distribution through optical links. Here, the authors investigate the suitability of
167Er
3+:Y
2SiO
5 as a microwave-to-optical memory interface. This material holds considerable promise as it has an optical transition in the telecommunication band. The authors identify hyperfine transitions with long coherence times without an external magnetic field, which are suitable for interfacing with superconducting qubit systems.
Er
3+:Y
2SiO
5 crystal is a promising candidate with a great variety of its potential applications in quantum information processing and quantum communications ranging from optical/microwave quantum memories to circuit QED and microwave-to-optics frequency converters. Some of the above listed applications require ultra-low temperature environment, i.e., temperatures
T≲0.1 K. Most of the experiments with erbium doped crystals have been so far carried out at temperatures above 1.5~K. Therefore, only little information is known about Er
3+:Y
2SiO
5 coherence properties at millikelvins. Here, we investigate optical decoherence of
167Er:Y
2SiO
5 crystal by performing 2- and 3-pulse echo experiments at millikelvin temperature range and at weak and moderate magnetic fields. We show that the deep freezing of the crystal results in an increase of optical coherence time by one order of magnitude compared to temperature of 1.5 Kelvin and magnetic field of $\sim$0.2~T, taken as a reference point. We further describe the detailed investigation of the decoherence mechanisms in this regime.
Erbium-doped solids are a promising platform for quantum technology due to the
Er3+ 1.5-\textmu{}m optical transition in the telecom C band that can offer long coherence times. To characterize the intrinsic
C2 site of
Er3+ in
Y2O3 for quantum information and photonic signal-processing applications, absorption and site-selective fluorescence spectroscopy were used to assign the
Er3+4I15/2$\ensuremath{\leftrightarrow}$
4I13/2 transitions. The lowest-to-lowest transition occurs at 1535.608 nm and laser absorption determined an inhomogeneous linewidth of 780 MHz in a 0.005%-doped crystal at 3.4 K. Time-resolved fluorescence established an upper limit for the excited-state lifetime of 8.5 ms. Angular-dependent Zeeman absorption was used to determine ground- and excited-state
g tensors. The
C2-site
g tensor components for the ground state are
gx=1.60,
gy=4.71, and
gz=11.93 with a tipping angle of \ensuremath{\alpha} = $2.{06}^{\ensuremath{\circ}}$, whereas for the excited state the components are
gx=1.08,
gy=4.36, and
gz=10.07 with a tipping angle of \ensuremath{\alpha} = $\ensuremath{-}11.{7}^{\ensuremath{\circ}}$. Optical decoherence can be suppressed by the application of a magnetic field parallel to $\ensuremath{\langle}111\ensuremath{\rangle}$ crystal axes where all
C2 sites become nearly magnetically equivalent with large
g values so that spin flips of neighboring
Er3+ ions can be simultaneously suppressed. Using this magnetic field direction, two-pulse photon echoes gave a homogeneous linewidth of 5 kHz for an applied magnetic field of 0.5 T, narrowing and saturating at 2.5 kHz at 3.5 T. Decoherence over intermediate magnetic fields was described by spectral diffusion due to $\mathrm{E}{\mathrm{r}}^{3+}\text{\ensuremath{-}}\mathrm{E}{\mathrm{r}}^{3+}$ magnetic dipole interactions driven by the direct phonon process. With
Er3+:
Y2O3 absorption at a convenient telecom laser wavelength and the ability to fabricate waveguides, our study suggests that this is a promising material system for quantum information and spectral hole-burning signal-processing applications.