# High-Speed Tunable Microcavities Coupled to Rare-Earth Quantum Emitters

Kangwei Xia<EMAIL_ADDRESS>3\. Physikalisches Institut,
University of Stuttgart, 70569 Stuttgart, Germany  Fiammetta Sardi
<EMAIL_ADDRESS>
These authors contributed equally: Kangwei Xia, Fiammetta Sardi 3\.
Physikalisches Institut, University of Stuttgart, 70569 Stuttgart, Germany
Colin Sauerzapf  3\. Physikalisches Institut, University of Stuttgart, 70569
Stuttgart, Germany  Thomas Kornher  3\. Physikalisches Institut, University of
Stuttgart, 70569 Stuttgart, Germany  Hans-Werner Becker  RUBION, Ruhr-
Universität Bochum, 44780 Bochum, Germany  Zsolt Kis  Wigner Research Center
for Physics, Institute for Solid State Physics and Optics, H-1121 Budapest,
Hungary  Laszlo Kovacs  Wigner Research Center for Physics, Institute for
Solid State Physics and Optics, H-1121 Budapest, Hungary  Roman Kolesov  3\.
Physikalisches Institut, University of Stuttgart, 70569 Stuttgart, Germany
Jörg Wrachtrup  3\. Physikalisches Institut, University of Stuttgart, 70569
Stuttgart, Germany  Max Planck Institute for Solid State Research, 70569
Stuttgart, Germany

###### Abstract

Electro-optical control of on-chip photonic devices is an essential tool for
efficient integrated photonics. Lithium niobate on insulator (LNOI) is an
emerging platform for on-chip photonics due to its large electro-optic
coefficient and high nonlinearity[1]. Integrating quantum emitters into LNOI
would extend their versatile use in classic photonics to quantum computing and
communication[2,3]. Here, we incorporate single rare-earth ions (REI) quantum
emitters in electro-optical tunable lithium niobite (LN) thin films and
demonstrate control of LN microcavities coupled to REI over a frequency range
of 160 GHz with 5 µs switching speed. Dynamical control of the cavities
enables the modulation of the Purcell enhancement of the REIs with short time
constants. Using the Purcell enhancement, we show evidence of detecting single
Yb3+ ions in LN cavities. Coupling quantum emitters in fast tunable photonic
devices is an efficient method to shape the waveform of the emitter[4]. It
also offers a platform to encode quantum information in the integration of a
spectral-temporal-spatial domain to achieve high levels of channel
multiplexing, as well as an approach to generate deterministic single-photon
sources[5,6].

Integrated optics is of paramount importance, especially for modern optical
information technology. It enables versatile routing of photons, wavelength
conversion and in combination with electro-optical material, an interface to
electronic components. In this regard, lithium niobate (LN) is a key optical
material due to its broad transmission window, large electro-optic
coefficients and nonlinearity[7], with various applications, such as electro-
optical modulators, coherent photon conversion and quantum memories[8,9]. The
recent commercialization of lithium niobate on insulator (LNOI) offers an
attractive platform to highly integrate photonic devices on-chip[1]. The low
operation voltage and fast electro-optical modulation speed result in a
variety of attractive applications, ranging from broadband frequency comb
generation[11] and fast electro-optic modulators[12,13] to high-quality factor
cavities[14], micro-lasers[15] and more. However, LNOI has not shown similar
functionality in integrated quantum technology. While it has been used as
classic electro-optical element in optical quantum technology, such as photon
pair sources and single photon wavelength converters[16], a direct quantum
functionalization in e.g. quantum nano mechanics or via doping with quantum
emitters has not yet been achieved. Specifically, the latter method would
allow for the realization of photonically interfaced quantum nodes with the
ability to dynamically control quantum emitters[17-19]. Such a versatile
control of individual qubits in the spectral, temporal and spatial domain
respectively, would further facilitate construction of the scalable multi-
qubit quantum networks[5,6].

Figure 1:  Optical and electro-optical characterization of LN microdisks. a)
Schematics of our device. On a layer of amorphous SiO2 a microdisk with a
waveguide is fabricated by e-beam lithography and etching. Photonic structures
are capped with a layer of SU-8 on which we deposited a 30 nm layer of ITO.
The right graph is an explored view of the device design. b) Optical images of
the fabricated microdisks on a LN thin film with top ITO electrodes. The right
graph is a SEM image of the microdisk. Incarnadine represents the ITO
electrodes and the green pattern shows the LN microdisk with coupling
waveguide. c) Linewidth of one of the monolithic LN microdisk fabricated on-
chip with a Q-factor of 244,400. Insert shows the energy level of Yb:LN. d)
Electro-optic tuning of cavity mode depending on the bias voltage applied to
the device with a coefficient of 270 MHz/V.

One option to achieve quantum-functionalized LNOI is to dope the material with
rare earth ions (REI)[20-22]. REIs in solids have recently emerged as
promising systems. They offer advantages in both optical and spin aspects:
various emission wavelengths including telecom band compatible with fiber
quantum networks, spin transitions with long coherence time that enables long
lasting quantum memory[23-25]. However, the 4$f$-4$f$ dipole forbidden optical
transitions of REIs make optical readout at single ion level challenging[26].
The incorporation of REIs into photonic cavities is an efficient approach to
enhance quantum light-matter interface, where the lifetime of REIs reduces due
to the Purcell effect[27],[17]. Currently, the coupling of single REIs to
photonic devices is based on evanescent tail coupling between photonic devices
and bulk YSO crystals or on direct fabrication of nanophotonic devices in YVO4
bulk crystals[28,19]. This is mostly due to the fact that it has proven to be
challenging to observe optically active single REIs in electro-optical thin
LNOI films. Typically, the incorporation of REIs into LNOI requires a smart
cut of the REI-doped LN bulk crystal[20,21]. Another approach is directly
doping REIs in LN thin films by ion implantation[30]. The implantation is
followed by high-temperature annealing, required to stabilize the dopants in
the correct lattice sites and heal the lattice damage. However, this process
forms cracks in LNOI and substantially reduces the optical quality of the
material.

Here, we directly doped REIs in LN thin films by means of ion implantation. We
perform hole-burning spectroscopy of Yb-implanted LNOI and show that post-
annealing at 650 ∘C is sufficient to achieve spectrally narrow excitation of
Yb3+ ions, as well as excellent charge stability. This procedure is also
compatible with fabricating photonic elements, like electro-optically tunable
high-Q microcavities. With these elements, we are able to modulate the Purcell
effect of Yb3+ ions dynamically. By coupling the REIs to the cavity and hence
increasing their emission rate, we provide evidence of detecting single Yb3+
ions.

In experiments, we implant Yb ions with an energy of 1.5 MeV and a fluence of
1012 ions/cm2 into LN films. After ion implantation, we further post-anneal
the Yb-implanted LN thin film at 650 ∘C (see SI Note 2) without observing any
damage to the film. The annealing temperature is still 500 °C lower than
required for Yb doping of LN during crystal growth. Thus, before using the
REI-implanted thin film, further characterization is needed to evaluate their
optical properties. We perform hole burning spectroscopy (HBS) to assess the
quality of the implanted Yb ions in the LN waveguide and Yb-doped bulk LN
crystal at cryogenic temperatures (the experimental details are described in
SI Note 4). Yb-doped LN bulk crystals show spectral hole widths of
2$\mathrm{\pi}$×58 MHz, while the Yb-implanted LN long waveguide shows
2$\mathrm{\pi}$×82 MHz (details see SI Note 4). The results suggest that post-
annealing at 650 ∘C is sufficient to activate and stabilize the implanted Yb
ions in LN thin films, showing a similar optical performance to the one in a
bulk crystal. This gives us a simple approach to directly incorporate REIs in
pre-fabricated photonic devices, based on commercially available LN thin film.

Figure 2:  Yb3+ in the LN microcavity. a) Lifetime of implanted Yb ions on-
and out of resonance with the cavity. The blue curve shows a long decay time
of 430 µs for off-resonance ions and the red curve shows a shorted on-
resonance decay lifetime. b) Schematics of the fast tuning protocol, showing
how the cavity stores energy, when not on-resonance with Yb ions. c) High
resolution measurements to show the time constant of our device for frequency
tunig. Switching is shown to occur on the order of 5 µs, mainly limitated by
electronics.

We further shape the Yb-implanted LN thin film to microcavities (the
fabrication protocol is described in Methods and SI Note 3). A scanning
electron microscope (SEM) image of one of the cavities is shown in Fig. 1 (b).
The microdisk resonator has a size of 7 µm radius. A waveguide positioned next
to the microdisk evanescently couples light into and out of the cavity. To
enhance its output efficiency, a 45° undercut is milled at the ends of the
waveguide (see SI Note 3). The typical Q-factor of the fabricated cavities is
in a range of 50,000-150,000 with a mode volume
$\mathrm{V_{mode}=50(\lambda/n)^{3}}$. The highest Q-factor of the cavity is
244,730, as shown in Fig. 1(c). According to simulations, the Q-factor of the
microdisk can exceed 109 when only radiative loss is taken into account (see
SI Note 3). The Q-factor of the current cavities is mainly limited by
scattering losses that can be further improved by optimizing the fabrication
protocol.

To achieve electro-optic tunability, an additional layer of indium tin oxide
(ITO) is deposited on the LN cavity serving as the top electrode, with a
negative 1 µm photoresist (SU-8) serving as a spacer. The Si substrate builds
the bottom electrode. By applying a bias voltage between the electrodes, the
reflective index of the LN cavity is modulated, resulting in the tuning of the
cavity resonance frequency, as shown in Fig. 1(d). We further estimate the
device’s electro-optic coupling by applying ±60 V external voltage with a step
width of 15 V. The response of the cavity turns out to be linear, indicating
the absence of hysteresis from our device, with a coefficient of 270 MHz/V. In
the experiment, a maximum voltage of $\pm$300 V is applied corresponding to
the tuning range of $\pm$81 GHz.

With electro-optical tuning, the cavity frequency can be brought in and out of
resonance with Yb3+:LN. For cavity QED experiments, a microdisk with a
Q-factor of 79,833 and high coupling between the cavity and the waveguide is
further investigated. When the frequency of the cavity is tuned on resonance
with the ZPL of Yb3+:LN, we observe a shortening of the Yb3+ lifetime in the
cavity, as shown in Fig. 2 (a). A double-exponential-decay-function yields a
lifetime of 51$\pm$4 µs and 182$\pm$40 µs. The different shortening of the
lifetime suggests that Yb3+ ions couple to the cavity with different strengths
because of their different positions in the cavity. The lifetime in bulk Yb:LN
is 430$\pm$2 µs. Hence Yb3+ coupling to the cavity results in a Purcell factor
of 7.6. The lifetime shortening is three times lower than the theoretical
estimation, based on Q/V ratio. This is presumably caused by inappropriate Yb
dipole orientation in the $z$ cut LNOI.

Figure 3:  Evidence of detecting single Yb ions in LN. a) Inhomogeneous line
of implanted Yb3+ in LN with a linewidth of 2$\mathrm{\pi}$×64±0.6 GHz. The
green rectangle marks the position where the PLE spectra of b) are taken. The
inserted graph is a schematic of the experimental protocol. The frequency of
the cavity is fixed to a certain position. The laser frequency is swept around
the resonance frequency of the cavity. b) PLE spectrum recorded at the tail of
the inhomogeneous line profile of Yb implanted in LN marked. A cavity with a
Q-factor of 64,065 is optically tuned from the center of the spectrum to the
tail with a step-width of 2.7 GHz and a total tuning range of 10 GHz. The
discrete peaks have an average linewidth of 2$\mathrm{\pi}$×247±10 MHz.
Overlapping of peaks shows that the cavity can be electro-optically tuned back
and forth across the spectrum.

In the following, fast electro-optic dynamic control of the microcavity
coupled to REIs is demonstrated: (1) The frequency of the cavity is tuned in
resonance with the Yb ions. (2) A laser pulse is generated to resonantly pump
the Yb ions to the excited states. (3) The excited Yb ions start to decay back
to their ground state with 50 µs lifetime due to the cavity QED. (4) Before
the Yb ions completely decay to the ground state, the cavity resonance
frequency is tuned off-resonance (54 GHz frequency detuning) from the excited
Yb ions, thus prolonging their lifetime from 50 µs to 430 µs. Since this
emission from Yb ions coupled to the cavity is now off-resonance, the
excitation is stored in the cavity and is not coupled to the waveguide (5).
After a certain waiting time, the cavity is tuned back on resonance with the
excited Yb ions, resulting in the restoration of fast decay of the Yb ions.
The experiment has been carried out with different waiting times, as shown in
Fig. 2(b).

To determine the electro-optical switching time of the microcavity,
measurements with fast time resolution are performed (see Fig. 2c). According
to the rising and falling slopes of the fluorescence signals, the switching
time of the microcavity is $\sim$ 5 µs, limited by the bandwidth of the high-
voltage operational amplifier in use.

We further investigated the detection of a single ion, based on the increased
fluorescence signal due to Purcell enhancement. In the experiment, a cavity
with a Q factor of 64,065 is used. Instead of at the center of the
inhomogeneous line of Yb3+, the cavity resonance frequency is tuned along the
tail of the inhomogeneous line of 980 nm Yb3+ optical transition (more than
140 GHz away from the center of the inhomogeneous line, as shown in Fig. 3
(a)). A laser power of 22 nW is applied to excite single Yb ions. The
respective photoluminescence excitation spectrum (PLE) is shown in Fig. 3(b),
where discrete lines are observed. In total, 24 discrete lines are fitted with
an average linewidth of $\mathrm{2\pi\times 247\pm 9~{}MHz}$ (SI Note 5). We
further plot one of the peaks, as shown in Fig. 4, with a fitted linewidth of
2$\mathrm{\pi}$×215±22 MHz. The linewidth is twice as large as the spectral
hole width that we obtained by hole-burning spectroscopy of Yb3+ implanted in
a LN waveguide (see Fig. 4 (a)). We suspect that the broadening is caused by
spectral diffusion.

Figure 4:  Spectroscopy of single Yb ions in LN. a) Hole burning spectroscopy
of Yb implanted LN waveguide with a spectral hole width of 2$\mathrm{\pi}$×97±
2.3 MHz. b) Plot of an individual PLE peak marked as a rectangle with green
solid lines in Fig.3 (b). The linewidth is 2$\mathrm{\pi}$×215± 22 MHz. c)
Single PLE spectrum of a sweep marked as a rectangle with black dash lines in
Fig. 3 (a). The sweep is from lower frequency to higher frequency (upper
curve) and reverse (lower curve). Both spectra show identical features,
demonstrating that discrete peaks are not experimental artefacts but
reproducible spectral features. d) Acquisition of PLE spectra for more than
800 s over the same range as c) showing no obvious spectral diffusion of ions
inside the cavity

Another important feature of emitters coupled to cavities is their spectral
stability. Often, the fabrication of optical cavities results in spectrally
unstable emitters. We investigate the spectral stability by observing
consecutive PLE measurements over an extended period of time (Fig. 4(d)).
Remarkably, we do not observe a strong spectral diffusion. Moreover, due to
the flexible electro-optical tunability, we can detune the cavity resonance
frequency in steps of 2.7 GHz and obtain the PLE spectra in different ranges,
as shown in Fig. 3(b). The total sweeping range is 10 GHz, which is two times
broader than the intrinsic linewidth of the cavity.

According to the PLE measurements, the count rate of the discrete peaks is
20-30 counts/s, which is consistent with our estimation (see Methods). This
suggests that the discrete peaks are corresponding to single Yb ions in the
LNOI microcavity. Due to the low count rate, we are not able to perform e.g.
second-order correlation measurements to unambiguously support our assumption.

##  .1 Conclusion

In conclusion, we incorporate REI quantum emitters into LN thin film with
optical properties similar to native REIs. The electro-optic control of the LN
microcavities enables couplings of REIs to be controlled at 5 µs switching
speed and over a 160 GHz frequency range. Facilitated by the Purcell
enhancement, we further observe single Yb3+ ions in the cavities. The
demonstration of dynamic control of the on-chip photonic devices enables the
shaping of a single photon waveform. The spectral, temporal and spatial
selectives of the single ions coupled to the cavities will be a flexible and
scalable approach to encode quantum information by employing high levels of
channel multiplexing. Controlling the cavities in and out of the resonance of
single ions will be a highly desirable method to generate deterministic single
photon sources. Furthermore, LN thin films and an additional spin-free
material, such as a YSO thin film, can form a hybrid quantum system. The long-
lived quantum memory can be dynamical controlled by the electro-optical tuning
of the LN photonic devices, which enables spectral multiplexing of quantum
memories.

##  .2 Sample preparation

After the implantation and annealing, the samples are further cleaned in a
Piranha solution to then evaporate a 30 nm Titanium layer through E-gun
evaporation. Subsequently a double layer of PMMA(400 nm and 80 nm) is spin-
coated directly onto the sample. The e-beam lithography is then performed with
a RAITH E-line machine. A layer of 120 nm Nickel/Chromium alloy with a
percentage of 80 to 20 is evaporated using e-gun and specific structure are
created by lift-off procedure in NEP at 90 ∘C. Micro-cavities are then
transferred to the LNOI by Plasma etching in a Plasma Oxford Instruments
machine with a recipe consisting of different steps. In the first part SF6 gas
is utilized in order to have anisotropic plasma and optimize the transfer in
LN with help of smoothing leverage of no ICP AR milling. Then a hard mask of
NiCr and Ti are wet etched with the use of Nitric acid and Piranha solution.
Lastly, the last 70 nm of LN are removed by Ar milling, leaving a 10 nm of
thin film before SiO2 and Si chip.

##  .3 Single Yb3+ ions’ photon count rate estimation

According to the Purcell enhanced lifetime we measured, the count rate of
single Yb3+ in LN cavity is calculated to be 9,000 photon/s. The total amount
of the detected photons is estimated to be:

$9000\times\eta_{c}(30\%)\times\eta_{o}(30\%)\times\eta_{e}(50\%)\times\eta_{m}(70\%)\times\eta_{chopper}(50\%)\times\eta_{d}(60\%)$
=40 counts/s,

where $\eta_{c}$ is the coupling efficiency between the microcavity to the
waveguide; $\eta_{o}$ represents the output efficiency at the end of the
waveguide; $\eta_{e}$ indicates the signal collected from only one end of the
waveguide; $\eta_{m}$ is the collection and transmission efficiency of the
optical microscopy; $\eta_{chopper}$ is the duty cycle of the chopper; and
$\eta_{d}$ is the efficiency of the superconducting single photon detector.

##  .4 Quality of micro-cavities

For the cavity, we studied in Fig. 2 (and Fig. 3), the linewidth is 3.83 GHz
(and 4.9 GHz); the Q factor is 79,833 (and 64,065); the REI lifetime reduction
is measured to be 50.0 us (and 62.7 µs). The Purcell factor is C=
$\gamma_{0}$/ $\gamma_{c}$-1=7.6 (and 5.9), where $\gamma_{c}$, $\gamma_{0}$
represent the decay rates of Yb:LNOI coupled to the cavity and the natural
lifetime of the bulk Yb:LN respectively. These measurements further allow us
to estimate the QED parameters for our cavities as ($g$,$\kappa$, $\Gamma_{c}$
)=2 $\mathrm{\pi}$×(1.64 MHz,3.83 GHz,370 Hz) and (2 $\mathrm{\pi}$ ×(1.62
MHz,4.9 GHz,370 Hz)), where $g$,$\kappa$ are related to the single photon Rabi
frequency and cavity decay rate. We further estimated for two cavities, 88%
(85.6%) of the Yb:LNOI fluorescence is emitted into the cavity mode and the
branching ratio of emission into 980 nm transition is improved from 25% to
91.5% (89.1%).

##  .5 Acknowledgments

We would like to thank Arkadiy Kolesov and Frank Thiele for their support with
the experiment, and Dr. Matthias Widmann for helping with artwork design. We
would like to thank the fruitful discussion with Dr. Qi-chao Sun, Prof. Kai-
Mei Fu, and Dr. Philippe Goldner. We would like to thank Sabrina Jenne for
language corrections. R. K. acknowledges financial support by the DFG (Grant
No. KO4999/3-1), and R. K. and J. W. acknowledge financial support by the FET-
Flagship Project SQUARE, the EU via SMeL and QIA, as well as the DFG via FOR
2724 and GRK 2642. J. W. acknowledges BMBF, Q. Link. X. Z. K. and L. K.
acknowledges the financial support by the National Research, Development and
Innovation Fund of Hungary within the Quantum Technology National Excellence
Program (Project Nr. 2017-1.2.1-NKP-2017-00001) .

##  .6 Author contributions

K. X. and F. S. contribute equally. K. X., F. S, R. K. and J. W. conceived and
designed the experiments. F. S., R. K., K. X., T. K. fabricated the
microstructures. H. B., Z. K. and L. K. provided the samples. K. X., F. S., C.
S. and R. K. built the experimental setups.K. X., F. S., C. S. and R. K.
performed the experiment and analysed the data. K. X., F. S., C. S., T. K., R.
K., and J. W. wrote the manuscript with input from all authors and R. K. and
J. W. supervised the whole project.

##  .7 Reference

[1] A. Boes, B. Corcoran, L. Chang, J. Bowers, A. Mitchell, Status and
potential of lithium niobate on insulator (LNOI) for photonic integrated
circuits, Laser Photonics Rev. 12 1700256, 2018.

[2] D. Awschalom, R. Hanson, J. Wrachtrup, B. Zhou, Quantum technologies with
optically interfaced solid-state spins, Nat. Photon. 12 516, 2018.

[3] H. J. Kimble, The quantum internet, Nature 453 1023, 2008.

[4] S. Cundiff, A. Weiner, Optical arbitrary waveform generation, Nat. Photon.
4 760, 2010.

[5] D. Awschalom, K. Berggren, H. Bernien, et.al.,, Development of quantum
interconnects (QuICs) for next-generation information technologies, PRX
Quantum 2 017002, 2021.

[6] F. Kaneda, F. Xu, J. Chapman Kwiat, Quantum-memory-assisted multi-photon
generation for efficient quantum information processing, Optica 4 1034, 2017.

[7] R. Weis, T. Gaylord, Lithium niobate: Summary of physical properties and
crystal structure, Appl. Phys. A 37 191, 1985.

[8] N. Curtz, R. Thew, C. Simon, N. Gisin, H. Zbinden, Coherent frequency-
down-conversion interface for quantum repeaters, Opt. Express 18 22099, 2010.

[9] N. Sinclair, E. Saglamyurek, H. Mallahzadeh, J. Slater, M.George, R.
Ricken, M. Hedges, D. Oblak, C. Simon, W. Sohler, W. Tittel, Spectral
multiplexing for scaable quantum photonics using an atomic frequency comb
quantum memory and feed-forward control, Phys. Reiv. Lett. 113 053603, 2014.

[10] J. Lin, F. Bo, Y. Cheng, J. Xu, Advances in on-chip photonic devices
based on lithium niobate on insulator, Photonic Res. 8 1910, 2020.

[11] M. Zhang, B. Buscaino, C. Wang, A. Shams-Ansari, C. Reimer, R. Zhu, J.
Kahn, M. Loncar, Broadband electro-optic frequency comb generation in a
lithium niobate microring resonator, Nature 568 373, 2019.

[12] C. Wang, M. Zhang, X. Chen, M. Bertrand, A. Shams-Ansari, S.
Chandrasekhar, P. Winzer, M. Loncar, Integrated lithium niobate electro-optic
modulators operating at CMOS-compatible voltages, Nature 562 101, 2018.

[13] M. Li, J. Ling, Y. He, U. Javid, S. Xue, Q. Lin, Lithium niobate
photonic-crystal electro-optic modulator, Nat. Commun. 11 4123, 2020.

[14] M. Zhang, C. Wang, R. Cheng, A. Shams-Ansari, M. Loncar, Monolithic
ultra-high-Q lithium niobate microring resonator, Optica 4 1536, 2017.

[15] Z. Wang, Z. Fang, Z. Liu, W. Chu, Y. Zhou, J. Zhang, R. Wu, M. Wang, T.
Lu, Y. Cheng, On-chip tunable microdisk laser fabricated on Er3+-doped lithium
niobate on insulator, Opt. Lett. 46 380, 2021.

[16] O. Alibart, V. D’Auria, M. Micheli, F. Doutre, F. Kaiser, L. Labonte, T.
Lunghi, E. Picholle, S. Tanzilli, Quantum photonics at telecom wavelengths
based on lithium niobate waveguides, J. Opt. 18 104001, 2016.

[17] B. Casabone, C. Deshmukh, S. Liu, D. Serrano, A. Ferrier, T. Hümmer, P.
Goldner, D. Hunger, H. de Riedmatten, Dynamic control of Purcell enhanced
emission of erbium ions in nanoparticles, arXiv 2001.08532, 2020.

[18] D. Wang, H. Kelkar, D. Martin-Cano, D. Rattenbacher, A. Shkarin, T.
Utikal, S. Götzinger, V. Sandoghdar, Turning a molecule into a coherent two-
level quantum system, Nat. Phys. 15 483, 2019.

[19] D. Najer, I. Söllner, P. Sekatski, V. Dolique, M. Löbl, D. Riedel, R.
Schott, S. Starosielec, S. Valentin, A. Wieck, N. Sangouard, A. Ludwig, R.
Warburton, A gated quantum dot strongly coupled to an optical microcavity,
Nature 622 575, 2019.

[20] N. Sinclair, E. Saglamyurek, M. George, R. Ricken, C. Mela, W. Sohler, W.
Tittel, Spectroscopic investigations of a Ti : Tm : LiNbO3 waveguide for
photon-echo quantum memory, J. Lumin. 130 1586 , 2010.

[21] S. Dutta, E. Goldschmidt, S. Barik, U. Saha, E. Waks, Integrated photonic
platform for rare-earth ions in thin film lithium niobate, Nano Lett. 20 741,
2020.

[22] Z. Kis, G. Mandula, K. Lengyel, I. Hajdara, L. Kovacs, M. Imlau,
Homogeneous linewidth measurements of Yb3+ ions in congruent and
stoichiometric lithium niobate crystals, Opt. Mater. 37 845, 2014.

[23] F. Bussieres, C. Clausen, A. Tiranov, B. Korzh, V. Verma, S. Nam, F.
Marsili, A. Ferrier, P. Goldner, H. Herrmann, C. Silberhorn, W. Sohler, M.
Afzelius, N. Gisin, Quantum teleportation from a telecom-wavelength photon to
a solid-state quantum memory, Nat. Photon. 8 775, 2014.

[24] M. Zhong, M. Hedges, R. Ahlefeldt, J. Bartholomew, S. Beavan, S. Wittig,
J. Longdell, M. Sellars, Optically addressable nuclear spins in a solid with a
six-hour coherence time, Nature 177 517, 2015.

[25] E. Saglamyurek, J. Jin, V. Verma, M. Shaw, F. Marsili, S. Nam, D. Oblak,
W. Tittel, Quantum storage of entangled telecom-wavelength photons in an
erbium-doped optical fibre, Nat. Photon. 9 83, 2015.

[26] R. Kolesov, K. Xia, R. Reuter, R. Stöhr, A. Zappe, J. Meijer, P. Hemmer,
J. Wrachtrup, Optical detection of a single rare-earth ion in a crystal, Nat.
Commun. 3 1029, 2012.

[27] D. Ding, L. Pereira, J. Bauters, M. Heck, G. Welker, A. Vantomme, J.
Bowers, M. Dood, D. Bouwmeester, Multidimensional Purcell effect in an
ytterbium-doped ring resonator, Nat. Photon. 10 385, 2016.

[28] A. Dibos, M. Raha, C. Phenicie, J. Thompson, Atomic Source of Single
Photons in the Telecom Band, Phys. Rev. Lett. 120 243601, 2018.

[29] T. Zhong, J. Kindem, J. Bartholomew, J. Rochman, I. Craiciu, V. Verma, S.
Nam, F. Marsili, M. Shaw, A. Beyer, A. Faraon, Optically Addressing Single
Rare-Earth Ions in a Nanophotonic Cavity, Phys. Rev. Lett. 121 183603, 2018.

[30] X. Jiang, D. Pak, A. Nandi, Y. Xuan, M. Hosseini, Rare earth-implanted
lithium niobate: Properties and on-chip integration, Appl. Phys. Lett. 115
071104, 2019.