Photon echo, spectral hole burning, and optically detected magnetic resonance in ${}^{171}\mathrm{Yb}^{3+}$:${\mathrm{LiNbO}}_3$ bulk crystal and waveguides

Recent progress in the realization of high-quality optical resonators and waveguides along with the possibility to incorporate rare-earth ions, make lithium niobate a promising material to build integrated platforms for quantum information processing. ${}^{171}\mathrm{Yb}^{3+}$ is a particularly attractive system because it can show long coherence lifetimes at zero magnetic field thanks to transitions insensitive to magnetic field fluctuations. In this paper, we investigate the optical and spin properties of ${}^{171}\mathrm{Yb}^{3+}$ ions in ${\mathrm{LiNbO}}_3$ bulk crystals as well as implanted waveguides. Using hole-burning spectroscopy and optically detected magnetic resonance, we studied ground and excited state hyperfine structures and probed optical and spin spectral holes. Importantly, the hole linewidths suggest that part of the ions in the waveguides are in a similar environment as in the bulk sample. We furthermore characterized spin population relaxation and coherence lifetimes of ${}^{171}\mathrm{Yb}^{3+}$ ions in the bulk crystal at temperatures between 50 mK and 9 K. At low temperatures, $T_2$ up to 9.5 $\mu \mathrm{s}$ (34 kHz homogeneous linewidth) and spin relaxation rates as long as $\approx 100$ ms were measured. Our results show that ${}^{171}\mathrm{Yb}^{3+}$:$\mathrm{Li}\mathrm{Nb}{\mathrm{O}}_3$ is a system that exhibits narrow optical homogeneous linewidths over a 50 GHz bandwidth together with an electron spin degree of freedom. This is of interest for a variety of applications in integrated quantum photonics such as quantum memories or quantum processors.

Figure 1

(a) Optical microscope image of LNOI waveguides. (b) Scanning electron microscope image of the ${45}^{\circ }$ undercut used to redirect light out of the sample plane. (c) A microwave wire was stretched parallel to the waveguide to resonantly drive ${}^{171}\mathrm{Yb}^{3+}$ spins.

Figure 2

Top: Emission spectrum recorded at 11 K exciting the 50 ppm ${}^{171}\mathrm{Yb}$:LN bulk crystal at 957 nm. Transitions are labeled according to Ref. [51]. The peak indicated by the arrow is believed to originate from the coupling with a localized phonon mode. Bottom, blue and black circles, left scale: Polarized absorption spectra at 10 K. Center frequency corresponds to 980.5 nm (vacuum). Bottom, purple circles, right scale: Photon echo amplitude as a function of the laser frequency at 1 K. The delay between the excitation and rephasing pulses was fixed at 5 $\mu \mathrm{s}$. Lines represent Lorentzian fits.

Figure 3

(a), (b) ODMR spectra of ${}^{171}\mathrm{Yb}$:LN bulk crystal and ${}^{171}\mathrm{Yb}$:LNOI waveguide. The labels refer to the energy levels in Fig. 4 (right). (c), (d) MW spectral holes burnt at 2900 MHz in the $1\to 3,4$ spin transition in ${}^{171}\mathrm{Yb}$:LN bulk crystal and ${}^{171}\mathrm{Yb}$:LNOI waveguide. Linewidths are respectively 19 and 10 MHz FWHM. Black lines represent Lorentzian fits. All experiments were performed at 5 K.

Figure 4

Left: Experimental (orange) and simulated (black) optical SHB spectra for different external magnetic fields oriented along the $c$ axis. In purple, the spectrum acquired with the maximum magnetic field (20.1 mT) directed perpendicular to the $c$ axis. A vertical shift has been added for clarity. The dotted lines highlight the linearly shifting holes attributed to ${}^{171}\mathrm{Yb}^{3+}$ ions in ${\mathrm{C}}_3$ sites or impurities of even ${\mathrm{Yb}}^{3+}$ isotopes. The narrow peak at $-160$ MHz is an artifact caused by an AOM. Experiments were performed at 1 K. Right: Simulated ${}^2\mathrm{F}_{7/2}$(0) and ${}^2\mathrm{F}_{5/2}$(0) hyperfine level energies as a function of magnetic field along the $c$ axis.

Figure 5

(a), (b) Optical spectral holes burnt in ${}^{171}\mathrm{Yb}$:LN bulk crystal and ${}^{171}\mathrm{Yb}$:LNOI waveguide. Linewidths (FWHM) determined by Lorentzian fits (black lines) are 3 MHz (a), and 1 and 9 MHz (b). Experiments were performed at 5 K.

Figure 6

Inset: Optical hole depth as a function of delay after burning ($T$ = 1 K). (Main) Temperature dependence of the two hole relaxation rates (circles) and fit using Eq. (4) (lines).

Figure 7

Inset: Two-pulse photon echo decay at three different temperatures. Exponential fits are shown as black lines. Main: Homogeneous linewidth, obtained through two-pulse photonecho measurements, as a function of temperature (circles) and fit using Eq. (5) (black line).

Figure 8

Three-pulse photon echo experiments. Main: Effective homogeneous linewidth estimated from Eq. (7) versus waiting time at three temperatures. The black lines are linear fits. Inset: Photon echo peak intensity as a function of waiting time for ${\tau }_d$ = 2.5 $\mu \mathrm{s}$ and at $T$ = 1 K. The black line is a fit using Eq. (7).