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# NOMAD CAMELS: Configurable Application for Measurements, Experiments and
Laboratory Systems

Alexander D. Fuchs  Lehrstuhl für Angewandte Physik, Department Physik,
Friedrich-Alexander Universität Erlangen-Nürnberg, Germany.
0000-0003-1896-9242 Johannes A. F. Lehmeyer  Lehrstuhl für Angewandte Physik,
Department Physik, Friedrich-Alexander Universität Erlangen-Nürnberg, Germany.
0000-0003-2041-9987 Heinz Junkes  Fritz-Haber-Institut, Berlin, Germany.
0000-0002-0218-4873 Heiko B. Weber  Lehrstuhl für Angewandte Physik,
Department Physik, Friedrich-Alexander Universität Erlangen-Nürnberg, Germany.
0000-0002-6403-9022 Michael Krieger  0000-0003-1480-9161

(20 November 2023)

## Summary

NOMAD CAMELS (short: CAMELS) is a configurable, open-source measurement
software that records fully self-describing experimental data. It has its
origins in the field of experimental physics where a wide variety of
measurement instruments are used in frequently changing experimental setups
and measurement protocols. CAMELS provides a graphical user interface (GUI)
which allows the user to configure experiments without the need of programming
skills or deep understanding of instrument communication. CAMELS translates
user-defined measurement protocols into stand-alone executable Python code for
full transparency of the actual measurement sequences. Existing large-scale,
distributed control systems using e.g. EPICS can be natively implemented.
CAMELS is designed with focus on full recording of data and metadata. When
shared with others, data produced with CAMELS allow full understanding of the
measurement and the resulting data in accordance with the FAIR (Findable,
Accessible, Interoperable and Re-usable) principles (Wilkinson et al., 2016).

## Statement of need

Research data management has piqued greater and greater interest in recent
years. Today, research funding agencies demand sustainable research data
strategies. The key criterion is to create research data following the FAIR
principles and thereby improve world-wide data-driven research (_DFG Position
Paper_ , 2018). While one ingredient, electronic lab notebooks, are an
important step towards FAIR data, it is equally important to record the
measurement data along FAIR principles as early as possible in the research
workflow.

In experimental physics many custom-built measurement setups are controlled by
very specific software written by individual researchers. This results in a
heterogeneous landscape of software fragments for measurements written in many
different languages and with often incomplete documentation, making it almost
impossible for other researchers to extend existing code. The degree to which
the stored raw data is understandable varies greatly but is often
unintelligible even for researchers from the same lab. Important metadata such
as instrument settings or the actual measurement steps performed to obtain the
final raw data are rarely recorded, making it virtually impossible to
reproduce experiments. Therefore, the documentation of experiments is
incomplete preventing FAIR research data. Although there are some tools
available (e.g.  _SweepMe!_ (_SweepMe_ , n.d.), _iC_ (Pernstich, 2012),
PyMoDAQ (_PyMoDAQ_ , n.d.)) to realise control of arbitrary measurement
instruments, they are frequently not open-source or their data output is not
compliant to the FAIR principles.

Figure 1: Visualization of CAMELS functionality and workflow. CAMELS connects
directly with local instruments and/or large-scale lab infrastructure running
network protocols, e.g. EPICS. Customizable measurements protocols are
translated into Python code and executed. The output is FAIR-compliant
measurement data.

## NOMAD CAMELS

CAMELS is an open-source tool that automatically collects all computer-
accessible experimental metadata. It features a user-friendly graphical
interface that enables the creation and customization of measurements without
the need for programming knowledge. By default, the data is stored in a
structured HDF5 file format that closely resembles the structure of the NeXus
standard (_FAIRmat NeXus Proposal_ , n.d.; Könnecke et al., 2015). The final
HDF5 file contains both the actual measurement data and metadata in a single
file, compliant to the FAIR principles.

Moreover, CAMELS allows for direct access to the _NOMAD_ repository
(Scheidgen, Brückner, et al., 2023), (Scheidgen, Himanen, et al., 2023) or its
on-premise installation called _NOMAD Oasis_ enabling direct linking to
electronic lab notebook (ELN) entries. The user can for example connect
measurements to previous experiment workflows documented in _NOMAD_ ELNs.
CAMELS can subsequently upload measurement results directly into the ELN
providing a simple and stream-lined data workflow.

CAMELS builds on _bluesky_ (Allan et al., 2019; _Bluesky_ , n.d.) initially
developed to control instruments at large-scale research institutions using
EPICS (_EPICS_ , n.d.; Knott et al., 1994). In CAMELS, _bluesky_ is employed
to orchestrate the instrument communication either directly (e.g. via PyVISA
(_PyVISA_ , n.d.)) or via using network protocols. Existing lab infrastructure
controlled by EPICS is therefore immediately accessible. A schematic overview
of the functionality of CAMELS is displayed in Figure 1.

CAMELS provides a comprehensive set of functionalities that can be split into
three primary components: instrument management, measurement protocols and
manual controls.

Figure 2: (a) The instrument manager allows to install and configure
instrument drivers from the curated instrument driver library or the user’s
hard drive. (b) The measurement protocol editor allows users to configure
arbitrary measurement sequences.

### Instrument Management

Scientific instruments can be added to CAMELS in two ways: The first involves
the _instrument manager_ (c.f. Figure 2a) to add instruments from the official
curated driver repository (“CAMELS-Drivers,” n.d.). These drivers are
installed into the Python environment via _pip_ (_Pip_ , n.d.) with each
driver being packaged individually.

The second way is to add self-built drivers by creating the necessary files
locally and placing these in the directory specified in the CAMELS settings.
To facilitate this process CAMELS provides a _driver builder_ that
automatically generates the essential structure and boilerplate code. As
CAMELS is an open-source project developed by and for the community, users are
encouraged to contribute to the driver library by creating pull requests for
new drivers on the GitHub repository (“CAMELS-Drivers,” n.d.).

In general, a CAMELS driver comprises two files: One containing the hardware
interface communication, the other one defining the available instrument
settings. Data communication to instruments is handled via _channels_ that can
be set and/or read; they correspond to an instrument’s individual
functionality or physical property.

### Measurement protocols

In CAMELS a _measurement protocol_ is a distinctive sequence of individual
steps including setting and reading instrument channels (see Figure 2b),
loops, conditional execution, running sub-protocols, PID control, etc. This
yields a measurement in a ‘recipe-style’ format, where the next step is
usually executed after the successful completion of the preceding step.
Asynchronous data acquisition is supported.

CAMELS translates the protocol created in the GUI into a Python script, which
is then executed. The script can be viewed, run independently of CAMELS, and
modified if required. CAMELS protocols and settings can be stored and shared
with colleagues enabling easy repeatability of experiments.

### Manual controls

Certain scientific instruments require manual control before starting
predefined measurement routines, e.g. adjusting stages, controlling
temperature, valves, pumps, etc. In CAMELS this is achieved through the
_manual controls_ functionality which can be applied to any writable
instrument channel.

### Data output

Figure 3: CAMELS stores measurement data together with rich metadata collected
automatically into a structured HDF5 file by default. This includes (a) a
human readable measurement protocol summary and (b) the executable Python
script that was used to actually record the data. This allows others to
understand the data acquisition and to reproduce the experiment.

After executing the measurement protocol, the time-stamped data is by default
saved to an HDF5 file with a structure similar to the NeXus standard (Könnecke
et al., 2015). Data can also be exported in CSV format with the metadata
exported in JSON.

The stored data can be divided into distinct sections:

  * •

Time-stamped raw data obtained during the execution of the measurement
protocol.

  * •

Instrument settings.

  * •

Human-readable summary of the measurement protocol information (see Figure
3a).

  * •

Complete Python script that recorded the data (see Figure 3b) as well as
information on the Python environment, i.e. a list of used packages and
versions.

  * •

User-defined metadata, e.g. sample and user information.

### Documentation

In-depth documentation and guides for installing, using and troubleshooting
can be found on the CAMELS documentation webpage (Fuchs & Lehmeyer, n.d.).

## Acknowledgements

We thank Patrick Oppermann (Fritz-Haber-Institut der Max-Planck-Gesellschaft)
for valuable discussions.

NOMAD CAMELS is being developed within the NFDI consortium _FAIRmat_ funded by
the Deutsche Forschungsgemeinschaft “DFG, German Research Foundation”, project
460197019.

## References

reAllan, D., Caswell, T., Campbell, S., & Rakitin, M. (2019). Bluesky’s Ahead:
A Multi-Facility Collaboration for an a la Carte Software Project for Data
Acquisition and Management. _Synchrotron Radiat. News_ , _32_(3), 19–22.
https://doi.org/10.1080/08940886.2019.1608121

pre _Bluesky Project_. (n.d.). Retrieved October 5, 2023, from
https://blueskyproject.io/

pre _EPICS - Experimental Physics and Industrial Control System_. (n.d.).
Retrieved October 5, 2023, from https://epics-controls.org/

pre _FAIRmat NeXus proposal_. (n.d.). Retrieved October 30, 2023, from
https://fairmat-nfdi.github.io/nexus-fairmat-
proposal/9636feecb79bb32b828b1a9804269573256d7696/fairmat-cover.html

preFAU-LAP/CAMELS_drivers: device implementation for CAMELS. (n.d.). In
_GitHub repository_. GitHub. Retrieved September 28, 2023, from
https://github.com/FAU-LAP/CAMELS_drivers

pre _Förderung von Informationsinfrastrukturen für die Wissenschaft_. (2018).
https://www.dfg.de/download/pdf/foerderung/programme/lis/positionspapier_informationsinfrastrukturen.pdf

preFuchs, A. D., & Lehmeyer, J. A. F. (n.d.). CAMELS - Configurable
Application for Measurements, Experiments and Laboratory Systems — NOMAD-
CAMELS documentation. In _NOMAD CAMELS - Documentation_. Retrieved October 31,
2023, from https://fau-lap.github.io/NOMAD-CAMELS/

preKnott, M., Gurd, D., Lewis, S., & Thuot, M. (1994). EPICS: A control system
software co-development success story. _Nucl. Inst. Methods Phys. Res. A_ ,
_352_(1-2), 486–491. https://doi.org/10.1016/0168-9002(94)91577-6

preKönnecke, M., Akeroyd, F. A., Bernstein, H. J., Brewster, A. S., Campbell,
S. I., Clausen, B., Cottrell, S., Hoffmann, J. U., Jemian, P. R., Männicke,
D., Osborn, R., Peterson, P. F., Richter, T., Suzuki, J., Watts, B.,
Wintersberger, E., & Wuttke, J. (2015). The NeXus data format. _J. Appl.
Crystallogr._ , _48_(1), 301–305. https://doi.org/10.1107/S1600576714027575

prePernstich, K. P. (2012). Instrument Control (iC) – An Open-Source Software
to Automate Test Equipment. _Journal of Research of the National Institute of
Standards and Technology_ , _117_ , 176–184.
https://doi.org/10.6028/jres.117.010

pre _Pip documentation V23.3.1_. (n.d.). Retrieved October 30, 2023, from
https://pip.pypa.io/

pre _PyMoDAQ_. (n.d.). Retrieved October 30, 2023, from
https://pymodaq.cnrs.fr/

pre _PyVISA_. (n.d.). Retrieved October 30, 2023, from
https://pyvisa.readthedocs.io/

preScheidgen, M., Brückner, S., Brockhauser, S., Ghiringhelli, L. M.,
Dietrich, F., Mansour, A. E., Márquez, J. A., Albrecht, M., Weber, H. B.,
Botti, S., Aeschlimann, M., & Draxl, C. (2023). FAIR Research Data With NOMAD:
FAIRmat’s Distributed, Schema-based Research-data Infrastructure to Harmonize
RDM in Materials Science. _Proceedings of the Conference on Research Data
Infrastructure_ , _1_. https://doi.org/10.52825/cordi.v1i.376

preScheidgen, M., Himanen, L., Ladines, A. N., Sikter, D., Nakhaee, M.,
Fekete, Á., Chang, T., Golparvar, A., Márquez, J. A., Brockhauser, S.,
Brückner, S., Ghiringhelli, L. M., Dietrich, F., Lehmberg, D., Denell, T.,
Albino, A., Näsström, H., Shabih, S., Dobener, F., … Draxl, C. (2023). NOMAD:
A distributed web-based platform for managing materials science research data.
_Journal of Open Source Software_ , _8_(90), 5388.
https://doi.org/10.21105/joss.05388

pre _SweepMe! - A multi-tool measurement software_. (n.d.). Retrieved
September 28, 2023, from https://sweep-me.net/

preWilkinson, M. D., Dumontier, M., Aalbersberg, Ij. J., Appleton, G., Axton,
M., Baak, A., Blomberg, N., Boiten, J. W., da Silva Santos, L. B., Bourne, P.
E., Bouwman, J., Brookes, A. J., Clark, T., Crosas, M., Dillo, I., Dumon, O.,
Edmunds, S., Evelo, C. T., Finkers, R., … Mons, B. (2016). The FAIR Guiding
Principles for scientific data management and stewardship. _Sci. Data_ ,
_3_(1), 1–9. https://doi.org/10.1038/sdata.2016.18

p


††thanks: Contributed equally to this work††thanks: Contributed equally to
this work††thanks: Contributed equally to this work

# Highly indistinguishable single photons from droplet-etched GaAs quantum
dots integrated in single-mode waveguides and beamsplitters

Florian Hornung<EMAIL_ADDRESS>Ulrich Pfister  Stephanie
Bauer  Institut für Halbleiteroptik und Funktionelle Grenzflächen, Center for
Integrated Quantum Science and Technology (IQST) and SCoPE, University of
Stuttgart, Allmandring 3, 70569 Stuttgart, Germany  Dee Rocking Cyrlyson’s
Institute of Semiconductor and Solid State Physics, Johannes Kepler University
Linz, 4040 Linz, Austria  Dongze Wang  Ponraj Vijayan  Institut für
Halbleiteroptik und Funktionelle Grenzflächen, Center for Integrated Quantum
Science and Technology (IQST) and SCoPE, University of Stuttgart, Allmandring
3, 70569 Stuttgart, Germany  Ailton J. Garcia Jr  Saimon Filipe Covre da Silva
Institute of Semiconductor and Solid State Physics, Johannes Kepler University
Linz, 4040 Linz, Austria  Michael Jetter  Simone L. Portalupi  Institut für
Halbleiteroptik und Funktionelle Grenzflächen, Center for Integrated Quantum
Science and Technology (IQST) and SCoPE, University of Stuttgart, Allmandring
3, 70569 Stuttgart, Germany  Armando Rastelli  Institute of Semiconductor and
Solid State Physics, Johannes Kepler University Linz, 4040 Linz, Austria
Peter Michler  Institut für Halbleiteroptik und Funktionelle Grenzflächen,
Center for Integrated Quantum Science and Technology (IQST) and SCoPE,
University of Stuttgart, Allmandring 3, 70569 Stuttgart, Germany

###### Abstract

The integration of on-demand quantum emitters into photonic integrated
circuits (PICs) has drawn much of attention in recent years, as it promises a
scalable implementation of quantum information schemes. A central property for
several applications is the indistinguishability of the emitted photons. In
this regard, GaAs quantum dots (QDs) obtained by droplet etching epitaxy show
excellent performances with visibilities close to one for both individual and
remote emitters. Therefore, the realization of these QDs into PICs is highly
appealing. Here, we show the first implementation in this direction, realizing
the key passive elements needed in PICs, i.e. single-mode waveguides (WGs)
with integrated GaAs-QDs, which can be coherently controlled, as well as
beamsplitters. We study both the statistical distribution of wavelength,
linewidth and decay times of the excitonic line of multiple QDs, as well as
the quantum optical properties of individual emitters under resonant
excitation. Here, we achieve single-photon purities as high as
$1-\text{g}^{(2)}(0)=$0.929\pm 0.009$$ as well as two-photon interference
visibilities of up to V${}_{\text{TPI}}$=$0.939\pm 0.004$ for two
consecutively emitted photons.

Several quantum technologies such as optical quantum computing, quantum
communication, quantum sensing and quantum simulation will strongly benefit
from small-footprint photonic integrated circuits (PICs) due to the high
scalability of the approach [1, 2]. A central part of these technologies are
efficient on-demand sources of single and indistinguishable photons. In this
regard, semiconductor quantum dots (QDs) are very promising candidates,
showing very pure single-photon emission [3], high count rates [4, 5, 6] and
emission linewidths close to the Fourier limit [7, 8, 9]. The compatibility of
QDs with PICs has been demonstrated in various experiments, ranging from
direct integration in fully monolithically grown samples [10, 11, 12, 13, 14,
15] to different hybrid integration techniques [16, 17, 18, 19] and fiber-to-
chip or chip-to-chip coupling [20, 21, 22, 23]. In order to increase the chip
efficiency, different cavities compatible with waveguides (WGs) were
introduced, which on the one hand increase the coupling efficiency into the WG
and on the other hand shorten the decay time via the Purcell-effect, thus
allowing higher repetition rates [24, 25, 26, 27]. Another important point
with respect to the up-scaling of quantum PICs is a tuning method to
compensate the spectral mismatch between individual emitters and enable two-
photon interference. Several tuning mechanisms have already been demonstrated
for QDs in PICs, including local temperature tuning via laser heating [28,
29], local [30] and global strain [31, 32, 33] and electrical fields [34, 35,
36]. Nevertheless, interference between two WG-integrated remote QDs has been
shown recently without an additional energy tuning knob [37].
When having a closer look on different QD platforms, GaAs QDs obtained by
droplet etching epitaxy (DEE) are very promising, as they show intrinsically
short decay times due to their large lateral extent leading to excitons in the
weak confinement regime [38], near-unity two-photon interference visibilities
for consecutive photons [38, 39] and a small wavelength distribution due to
the strain-free growth [40]. Strain tuning [41] and electrical tuning [42]
have also been demonstrated, as well as emission linewidths close to the
Fourier-limit [43]. Furthermore, record-values in single-photon purity [3] and
in two-photon interference visibility of two remote sources [44] were achieved
with DEE GaAs-QDs, but the integration of such QDs into PICs has not yet been
shown.
Here, we demonstrate the on-chip guiding and splitting of single photons from
DEE GaAs-QDs using a monolithic approach. The sample was grown via molecular
beam epitaxy with a layer structure as shown in Figure 1a. The necessary
refractive index contrast for wave-guiding is achieved by different Al-
contents in the two top layers (see supplementary for more information). A
full-vectorial finite-difference eigenmode (FDE) solver was used to obtain the
WG-dimensions for single-mode operation, giving a height of
$310\text{\,}\mathrm{nm}$ and a width of $450\text{\,}\mathrm{nm}$ (see
supplementary). The single-mode behavior of the WGs was confirmed by measuring
the degree of polarization (DOP), resulting in an average of $\approx$
$93\text{\,}\mathrm{\char 37\relax}$ (see supplementary). To simulate the
ideal 2x2 multi-mode interference splitter (MMI) dimensions, the eigenmode
expansion (EME) method was used, resulting in a length of
$58\text{\,}\mathrm{\SIUnitSymbolMicro m}$ with a width of
$4.5\text{\,}\mathrm{\SIUnitSymbolMicro m}$ (see supplementary). The WGs were
processed using electron-beam lithography with a positive resist, followed by
inductively coupled plasma reactive ion etching. To prevent oxidation, the
etching was stopped about $20\text{\,}\mathrm{nm}$ above the cladding layer.
In a last step, the sample was cleaved perpendicular to the WGs and mounted in
a helium-flow cryostat (operation temperature of $4\text{\,}\mathrm{K}$) with
access from both the side and the top of the WGs for optical characterization.
SEM-pictures of a cleaved WG facet and the top view of an MMI can be seen in
Figure 1b and c, respectively.

Figure 1: a) Sketch of the layer structure of the WG-sample with GaAs QDs
embedded in $\mbox{Al}_{0.15}\mbox{Ga}_{0.85}\mbox{As}$. b) SEM picture of a
cleaved WG-facet. c) SEM top-view of an MMI-beamsplitter.

The sample consists of several MMIs, where the length was swept from
$57\text{\,}\mathrm{\SIUnitSymbolMicro m}61\text{\,}\mathrm{\SIUnitSymbolMicro
m}$ in $1\text{\,}\mathrm{\SIUnitSymbolMicro m}$ steps, since the simulations
predict a 50/50 splitting ratio with high transmission in this range.
Additionally, straight reference WGs without MMI were also processed, which
were used to measure an attenuation of $8.15\pm
1.08\text{\,}\mathrm{dB}\text{\,}{\mathrm{mm}}^{-1}$ (in a wavelength range
from $740\text{\,}\mathrm{nm}770\text{\,}\mathrm{nm}$, see supplementary for
details). The splitting ratio of the devices was verified by comparing the
micro-photoluminescence emission at the two outputs of the MMI. Representative
spectra of a QD located in an MMI with $60\text{\,}\mathrm{\SIUnitSymbolMicro
m}$ length are shown in Figure 2a for above-bandgap excitation, as well as
phonon-assisted and strict resonant excitation of the neutral exciton (X),
demonstrating the 50/50 splitting into the two output arms. The transmission
through the MMIs was determined to $82.2\pm 1.2\text{\,}\mathrm{\char
37\relax}$ (see supplementary for more information). Note that the resonant
spectra are recorded without spectral filtering, making use of the spatial
separation of the excitation and the collection paths. Remaining laser stray
light is additionally suppressed by coupling the light into a polarization-
maintaining fiber and using a pulse-shaper to spectrally narrow the laser
pulses (resulting in a pulse length of about
$20\text{\,}\mathrm{ps}30\text{\,}\mathrm{ps}$) [45]. Interestingly, the X
transition can’t be seen in above-band excitation for this specific QD, which
we ascribe to additional charges due to the nearby WG-sidewalls and a weak
background doping (see supplementary). The good laser suppression allows to
observe clear Rabi-oscillations, as illustrated in Figure 2b. The fit to the
data gives an excitation fidelity of $58.4\pm 2.5\text{\,}\mathrm{\char
37\relax}$.

Figure 2: a) Spectra of QD1 using different excitation wavelengths, including
a $532\text{\,}\mathrm{nm}$ above-band (AB) laser, pumping via longitudinal
acoustic (LA) phonons and strictly resonant excitation. Photoluminescence was
recorded at both output arms of the MMI, demonstrating the 50/50 splitting
ratio of the device. b) Integrated intensity over excitation power for QD1 in
resonant excitation with corresponding fit, showing clear Rabi oscillations.
Figure 3: a) TCSPC measurement for the X of QD1 under resonant excitation and
corresponding fit including a deconvolution. The lack of a beating indicates
that only one fine-structure component can couple to the WG-mode, as shown in
the sketch. b) FPI-measurement of the X under resonant excitation and
corresponding Voigt-fit. c) Second-order correlation measurement of the X of
QD1 under pulsed resonant excitation using the on-chip beamsplitter. The fit
to the data includes the deconvolution with the IRF. Inset: Correlation
measurement obtained using a coarse bin size corresponding to one excitation
cycle ($6.57\text{\,}\mathrm{ns}$), plotted for delay times up to
$2\text{\,}\mathrm{ms}$ and corresponding fit to extract the bunching
parameters. d) Hong-Ou-Mandel measurement of the X of QD1, using the on-chip
MMI to divide the photon stream and a fiber-beamsplitter to recombine it. The
photons of one arm are delayed by the repetition rate of
$6.57\text{\,}\mathrm{ns}$ and a polarization control (not shown in the
sketch) allows to measure the co- and cross-polarized case. The fit to the
data includes the deconvolution with the IRF.

Next, the quantum optical properties of the X of three different QDs under
resonant $\pi$-pulse excitation were measured in more detail. An overview of
the results is given in Table 1, while the corresponding measurements of QD1
are shown in Figure 3a-d. First, the decay time was measured in a time-
correlated single-photon counting (TCSPC) experiment by correlating the signal
of an APD with $\approx$ $60\text{\,}\mathrm{ps}$ time resolution with the
laser trigger. A mono-exponential fit including the convolution of the system
response (compare Figure 3a) gives a decay time of $201\pm
1.\text{\,}\mathrm{ps}$. The absence of a beating in the measurement indicates
that either the fine-structure splitting of the X is close to zero or that the
two dipoles are exactly parallel and orthogonal to the WG as indicated by the
sketch in Figure 3a. In the latter case, which we believe to be accurate since
no beating was observed in any of the TCSPC measurements, only the orthogonal
component can couple to the TE-mode of the WG. Note that in order to excite
the orthogonal dipole component, the polarization of the excitation laser has
to be matching, which may increase the coupling to the TE-mode and thus the
amount of laser light in the WG [46]. However, while a dependence of the laser
stray light on the position as well as the wavelength was found, which we
ascribe to the presence of local stray centers, no significant increase in
laser stray light was found when switching between parallel and orthogonal
polarization. In contrast, exciting only one dipole is highly beneficial, as
it removes the influence of the fine-structure on the wave-package, which is
important for remote two-photon interference experiments. Furthermore, since
the laser is suppressed geometrically, driving only one dipole component means
no additional losses, which are inherent to excitation based on polarization-
suppression [47].

Table 1: Overview of state preparation fidelity, decay time $\tau$, linewidth $\Delta f$, and both raw and corrected g(2)-values and two-photon interference visibilities ($6.57\text{\,}\mathrm{ns}$ time delay), respectively, measured for three different QDs. QD # | Prep. Fid. | $\tau$ | $\Delta f$ | g${}^{(2)}_{\text{raw}}$(0) | g${}^{(2)}_{\text{bgc.}}$(0) | V${}_{\text{TPI, raw}}$ | V${}_{\text{TPI, full-corr.}}$
---|---|---|---|---|---|---|---
QD1 | $58.4\pm 2.5\text{\,}\mathrm{\char 37\relax}$ | $201\pm 1.\text{\,}\mathrm{ps}$ | $4.82\pm 0.09\text{\,}\mathrm{GHz}$ | $0.142\pm 0.003$ | $0.078\pm 0.010$ | $0.760\pm 0.012$ | $0.859\pm 0.015$
QD2 | $58.9\pm 2.8\text{\,}\mathrm{\char 37\relax}$ | $135\pm 1.\text{\,}\mathrm{ps}$ | $3.74\pm 0.08\text{\,}\mathrm{GHz}$ | $0.209\pm 0.002$ | $0.168\pm 0.002$ | $0.775\pm 0.014$ | $0.939\pm 0.004$
QD3 | $63.8\pm 4.9\text{\,}\mathrm{\char 37\relax}$ | $207\pm 1.\text{\,}\mathrm{ps}$ | $8.79\pm 0.20\text{\,}\mathrm{GHz}$ | $0.121\pm 0.002$ | $0.071\pm 0.009$ | $0.714\pm 0.013$ | $0.814\pm 0.016$

Key for two-photon interference experiments, in particular with remote QDs, is
the linewidth of the transition. To classify our emitters in this regard, the
linewidths of several Xs under resonant excitation were measured through high-
resolution spectroscopy, employing a Fabry-Pérot interferometer. While the
whole statistics is discussed later on, a representative scan for QD1 is
plotted in Figure 3b. In this case, a linewidth of
FWHM${}_{\text{Voigt}}$=$4.82\pm 0.09\text{\,}\mathrm{GHz}$ is obtained, which
is a factor of $\approx$ 6 to the Fourier limit ($0.79\text{\,}\mathrm{GHz}$).
Since we expect mostly inhomogeneous spectral broadening due to spectral
diffusion induced by the nearby WG-sidewalls and residual impurities in the
sample, the Lorentzian part of the Voigt fit was fixed to the Fourier-
transform limit and only the Gaussian part was fitted.
Following the demonstration of resonant excitation without requiring
additional off-chip laser-filtering, the single-photon operation of the device
needs to be proven. To do so, the g${}^{(2)}(\tau)$ is measured using the on-
chip MMI as the 50/50 beamsplitter in a Hanbury-Brown and Twiss
interferometer. The signal of the two output arms is directly sent to two
fiber-coupled APDs with timing resolutions of
$\approx$$250\text{\,}\mathrm{ps}$. The corresponding measurement is shown in
Figure 3c. A strong bunching can be observed, which is well known for DEE
GaAs-QDs without stabilization of the electronic environment [43]. In order to
correctly extract the Poisson level, the measurement was correlated up to
$2\text{\,}\mathrm{ms}$ in a binning corresponding to the repetition rate of
the laser ($6.57\text{\,}\mathrm{ns}$, see inset of Figure 3c) and was fitted
to extract the bunching parameters. From the fit, two different bunching time
scales of $\approx$ $65\text{\,}\mathrm{\SIUnitSymbolMicro s}$ and $\approx$
$125\text{\,}\mathrm{\SIUnitSymbolMicro s}$ are obtained, resulting in an
optical on-time of $50.8\pm 0.4\text{\,}\mathrm{\char 37\relax}$. Similar
values were found for all measured QDs and the bunching seemed to be stable
between different measurements. After normalization,
g${}^{(2)}_{\text{raw}}$(0)=$0.142\pm 0.003$ is extracted, by comparing the
area of the central peak to the average of 114 side-peaks (corrected for the
bunching), with an integration window of $6.57\text{\,}\mathrm{ns}$. Note that
a considerable contribution of the non-vanishing g(2)(0) stems from an
uncorrelated background, that cannot be explained by the detector dark counts.
We ascribe it to a CW-contribution of our laser, since we also see a
background when directly measuring the instrumental response function (IRF).
Fitting the data including a convolution with the system response allows a
correction for the background resulting in
g${}^{(2)}_{\text{bgc.}}$(0)=$0.078\pm 0.010$. We ascribe the rest of the non-
vanishing g(2)(0) to a small amount of pulsed laser stray light, as well as
some chance for a re-excitation process to occur within the same pulse.
After showing the generation, guiding and on-chip splitting of resonantly
excited single photons, the last step consists in benchmarking the two-photon
interference visibility of consecutive photons from the same QD. To do so,
photons with a time separation of $6.57\text{\,}\mathrm{ns}$ were generated
and brought to interfere. The on-chip MMI was again used as a part of the
setup, here as first beamsplitter in the Mach-Zehnder interferometer. One of
the two detection paths was then delayed by $6.57\text{\,}\mathrm{ns}$ before
entering a fiber beamsplitter, where the interference took place. A short free
space setup including wave plates for polarization control was implemented
before the fiber beamsplitter, which allowed switching between parallel and
orthogonal polarization. The corresponding measurements for QD1 are shown in
Figure 3d. Normalization of the data is done similarly to the
g${}^{(2)}(\tau)$-measurements by a coarse binning to extract the bunching
parameters. As mentioned before, no significant changes of the bunching
behavior between the different measurements was found. The raw two-photon
interference visibility is extracted by comparing the areas of the central
peak for the parallel and the orthogonal measurement, with each being
normalized to their respective Poisson level, again using an integration
window of $6.57\text{\,}\mathrm{ns}$. Following the formula
$\text{V}_{\text{TPI}}=1-\frac{\text{g}^{(2)}_{\parallel}(0)}{\text{g}^{(2)}_{\perp}(0)}$,
a visibility of V${}_{\text{TPI, raw}}$=$0.760\pm 0.012$ is obtained. A fit to
the data allows a full correction of the value, including the background
counts, the non-perfect g(2)(0) and a deviation of the fiber beamsplitter from
the ideal 50/50 splitting ratio, resulting in V${}_{\text{TPI, full-
corr.}}$=$0.859\pm 0.015$. This indicates that most of the line broadening
happens on time scales larger than the delay used in the measurement, as the
expected visibility for two remote sources with properties like QD1 would be
only 0.30. This value could be further improved by improving the material
quality and embedding the QDs in a diode structure [43, 44]. The measurements
on QD2 and QD3 confirm this behavior, showing high corrected visibilities of
$0.814\pm 0.016$ and $0.939\pm 0.004$, respectively, despite QD3 having more
than twice the linewidth of QD2.
Finally, statistical measurements on a large amount of QDs located in
different MMIs were performed, beginning with the wavelength distribution of
the Xs, as shown in the histogram in Figure 4a. Averaging over 104 QDs, a
central wavelength of $781.71\text{\,}\mathrm{nm}$ with a standard deviation
of $3.53\text{\,}\mathrm{nm}$ is obtained, underlining the small wavelength
distribution of DEE GaAs QDs. Next, the decay times of 25 Xs were measured,
with the corresponding histogram plotted in Figure 4b. The average decay time
is $183\text{\,}\mathrm{ps}$ with a standard deviation of
$22\text{\,}\mathrm{ps}$ and thus a bit shorter than the $\approx$
$250\text{\,}\mathrm{ps}$ given in Reference 38. A possible reason could be
additional non-radiative decay channels due to the nearby WG sidewalls or a
slightly weaker confinement of the X due to the lowered Al-content of the
barrier material. For the same 25 Xs, the linewidths were also measured and
are plotted in the histogram in Figure 4c. Here, the average linewidth is
$6.73\text{\,}\mathrm{GHz}$ with a standard deviation of
$1.77\text{\,}\mathrm{GHz}$. As mentioned earlier, we ascribe the main part of
the line broadening to spectral diffusion, which may be increased by the
nearby WG sidewalls.

Figure 4: Statistical distributions of the Xs, as well as the average and
corresponding standard deviations for a) the central wavelength, measured for
104 QDs b) the decay times, measured for 25 QDs c) the linewidth, measured for
the same 25 QDs.

In conclusion, we have shown the first integration of DEE GaAs QDs into
single-mode WGs, additionally combining them with MMIs to perform an on-chip
beamsplitting operation with 50/50 splitting ratio. Both resonant excitation
of individual emitters as well as statistical measurements were performed.
Three QDs with different linewidths were investigated closer under $\pi$-pulse
excitation, showing high two-photon interference visibilities >0.80 and low
g(2)(0)-values <0.1. Using an excitation laser that is polarized perpendicular
to the WGs allowed us to address a single dipole of the X, which is beneficial
with respect to the efficiency as well as for potential on-chip two-photon
interference experiments from remote sources in the future. When thinking in
this direction, the small wavelength distribution ($781.71\pm
3.53\text{\,}\mathrm{nm}$) and the low average decay time ($183\pm
22.\text{\,}\mathrm{ps}$) are very promising, although the spectral broadening
of the lines ($6.73\pm 1.77\text{\,}\mathrm{GHz}$) has to be lowered. The
latter is mainly attributed to spectral diffusion due to residual impurities
in the sample and the nearby WG sidewalls. A surface passivation could help in
this regard [48, 49], as well as the integration into diodes [43, 44] or
deterministic fabrication [14, 50] to maximize the distance to the sidewalls.
Deterministic fabrication would also be beneficial to preselect QDs that are
spatially and spectrally close for integration into the same MMI [51].
Nevertheless, the integration and resonant excitation of DEE GaAs QDs is an
important step towards a high-quality on-chip source of single and
indistinguishable photons with a small wavelength distribution and inherently
short decay times.

##  .1 Acknowledgments

The authors thank Christian Schweikert from the Institute of Electrical and
Optical Communications for support with the MMI-simulations and Thomas Reindl
and Marion Hagel from the Max Planck Institute for Solid State research for
support with the sample preparation. This work was financially supported by
the DFG via the project MI 500/32-1, the Austrian Science Fund (FWF) via the
project I 4320, the Research Group FG5, the European Union’s Horizon 2020
research and innovation program under Grant Agreements No. 899814 (Qurope) and
No. 871130 (Ascent+), the QuantERA II Programme that has received funding from
the European Union’s Horizon 2020 research and innovation programme under
Grant Agreement No. 101017733 via the project QD-E-QKD and the FFG Grant No.
891366, the Linz Institute of Technology (LIT), the LIT Secure and Correct
Systems Lab, supported by the State of Upper Austria.

##  .2 Associated content

The following files are available free of charge.

  * •

Supplementary material: sample growth, simulations of the single-mode WGs and
the MMIs, degree of polarization, attenuation measurements, transmission
measurements.

##  .3 Competing interests

The authors declare no competing interests.

##  .4 References

## References

  * Wang  _et al._ [2020] J. Wang, F. Sciarrino, A. Laing, and M. G. Thompson, Integrated photonic quantum technologies, Nature Photonics 14, 273 (2020).
  * Moody  _et al._ [2022] G. Moody, V. J. Sorger, D. J. Blumenthal, P. W. Juodawlkis, W. Loh, C. Sorace-Agaskar, A. E. Jones, K. C. Balram, J. C. F. Matthews, A. Laing, M. Davanco, L. Chang, J. E. Bowers, N. Quack, C. Galland, I. Aharonovich, M. A. Wolff, C. Schuck, N. Sinclair, M. Lončar, T. Komljenovic, D. Weld, S. Mookherjea, S. Buckley, M. Radulaski, S. Reitzenstein, B. Pingault, B. Machielse, D. Mukhopadhyay, A. Akimov, A. Zheltikov, G. S. Agarwal, K. Srinivasan, J. Lu, H. X. Tang, W. Jiang, T. P. McKenna, A. H. Safavi-Naeini, S. Steinhauer, A. W. Elshaari, V. Zwiller, P. S. Davids, N. Martinez, M. Gehl, J. Chiaverini, K. K. Mehta, J. Romero, N. B. Lingaraju, A. M. Weiner, D. Peace, R. Cernansky, M. Lobino, E. Diamanti, L. T. Vidarte, and R. M. Camacho, 2022 Roadmap on integrated quantum photonics, Journal of Physics: Photonics 4, 012501 (2022).
  * Schweickert  _et al._ [2018] L. Schweickert, K. D. Jöns, K. D. Zeuner, S. F. C. da Silva, H. Huang, T. Lettner, M. Reindl, J. Zichi, R. Trotta, A. Rastelli, and V. Zwiller, On-demand generation of background-free single photons from a solid-state source, Applied Physics Letters 112, 093106 (2018).
  * Wang  _et al._ [2019] H. Wang, Y.-M. He, T.-H. Chung, H. Hu, Y. Yu, S. Chen, X. Ding, M.-C. Chen, J. Qin, X. Yang, R.-Z. Liu, Z.-C. Duan, J.-P. Li, S. Gerhardt, K. Winkler, J. Jurkat, L.-J. Wang, N. Gregersen, Y.-H. Huo, Q. Dai, S. Yu, S. Höfling, C.-Y. Lu, and J.-W. Pan, Towards optimal single-photon sources from polarized microcavities, Nature Photonics 13, 770 (2019).
  * Tomm  _et al._ [2021] N. Tomm, A. Javadi, N. O. Antoniadis, D. Najer, M. C. Löbl, A. R. Korsch, R. Schott, S. R. Valentin, A. D. Wieck, A. Ludwig, and R. J. Warburton, A bright and fast source of coherent single photons, Nature Nanotechnology 16, 399 (2021).
  * Nawrath  _et al._ [2023] C. Nawrath, R. Joos, S. Kolatschek, S. Bauer, P. Pruy, F. Hornung, J. Fischer, J. Huang, P. Vijayan, R. Sittig, M. Jetter, S. L. Portalupi, and P. Michler, Bright Source of Purcell-Enhanced, Triggered, Single Photons in the Telecom C-Band, Advanced Quantum Technologies , 2300111 (2023).
  * Kuhlmann  _et al._ [2015] A. V. Kuhlmann, J. H. Prechtel, J. Houel, A. Ludwig, D. Reuter, A. D. Wieck, and R. J. Warburton, Transform-limited single photons from a single quantum dot, Nature Communications 6, 8204 (2015).
  * Pedersen  _et al._ [2020] F. T. Pedersen, Y. Wang, C. T. Olesen, S. Scholz, A. D. Wieck, A. Ludwig, M. C. Löbl, R. J. Warburton, L. Midolo, R. Uppu, and P. Lodahl, Near Transform-Limited Quantum Dot Linewidths in a Broadband Photonic Crystal Waveguide, ACS Photonics 7, 2343 (2020).
  * Strobel  _et al._ [2023] T. Strobel, J. H. Weber, M. Schmidt, L. Wagner, L. Engel, M. Jetter, A. D. Wieck, S. L. Portalupi, A. Ludwig, and P. Michler, A Unipolar Quantum Dot Diode Structure for Advanced Quantum Light Sources, Nano Letters 23, 6574 (2023).
  * Schwagmann  _et al._ [2011] A. Schwagmann, S. Kalliakos, I. Farrer, J. P. Griffiths, G. A. C. Jones, D. A. Ritchie, and A. J. Shields, On-chip single photon emission from an integrated semiconductor quantum dot into a photonic crystal waveguide, Applied Physics Letters 99, 261108 (2011).
  * Laucht  _et al._ [2012] A. Laucht, S. Pütz, T. Günthner, N. Hauke, R. Saive, S. Frédérick, M. Bichler, M.-C. Amann, A. W. Holleitner, M. Kaniber, and J. J. Finley, A Waveguide-Coupled On-Chip Single-Photon Source, Physical Review X 2, 011014 (2012).
  * Prtljaga  _et al._ [2014] N. Prtljaga, R. J. Coles, J. O’Hara, B. Royall, E. Clarke, A. M. Fox, and M. S. Skolnick, Monolithic integration of a quantum emitter with a compact on-chip beam-splitter, Applied Physics Letters 104, 231107 (2014).
  * Kiršanskė  _et al._ [2017] G. Kiršanskė, H. Thyrrestrup, R. S. Daveau, C. L. Dreeßen, T. Pregnolato, L. Midolo, P. Tighineanu, A. Javadi, S. Stobbe, R. Schott, A. Ludwig, A. D. Wieck, S. I. Park, J. D. Song, A. V. Kuhlmann, I. Söllner, M. C. Löbl, R. J. Warburton, and P. Lodahl, Indistinguishable and efficient single photons from a quantum dot in a planar nanobeam waveguide, Physical Review B 96, 165306 (2017).
  * Schnauber  _et al._ [2018] P. Schnauber, J. Schall, S. Bounouar, T. Höhne, S.-I. Park, G.-H. Ryu, T. Heindel, S. Burger, J.-D. Song, S. Rodt, and S. Reitzenstein, Deterministic Integration of Quantum Dots into on-Chip Multimode Interference Beamsplitters Using in Situ Electron Beam Lithography, Nano Letters 18, 2336 (2018).
  * Schwartz  _et al._ [2018] M. Schwartz, E. Schmidt, U. Rengstl, F. Hornung, S. Hepp, S. L. Portalupi, K. llin, M. Jetter, M. Siegel, and P. Michler, Fully On-Chip Single-Photon Hanbury-Brown and Twiss Experiment on a Monolithic Semiconductor–Superconductor Platform, Nano Letters 18, 6892 (2018).
  * Zadeh  _et al._ [2016] I. E. Zadeh, A. W. Elshaari, K. D. Jöns, A. Fognini, D. Dalacu, P. J. Poole, M. E. Reimer, and V. Zwiller, Deterministic Integration of Single Photon Sources in Silicon Based Photonic Circuits, Nano Letters 16, 2289 (2016).
  * Davanco  _et al._ [2017] M. Davanco, J. Liu, L. Sapienza, C.-Z. Zhang, J. V. D. M. Cardoso, V. Verma, R. Mirin, S. W. Nam, L. Liu, and K. Srinivasan, Heterogeneous integration for on-chip quantum photonic circuits with single quantum dot devices, Nature Communications 8, 889 (2017).
  * Kim  _et al._ [2017] J.-H. Kim, S. Aghaeimeibodi, C. J. K. Richardson, R. P. Leavitt, D. Englund, and E. Waks, Hybrid Integration of Solid-State Quantum Emitters on a Silicon Photonic Chip, Nano Letters 17, 7394 (2017).
  * Katsumi  _et al._ [2018] R. Katsumi, Y. Ota, M. Kakuda, S. Iwamoto, and Y. Arakawa, Transfer-printed single-photon sources coupled to wire waveguides, Optica 5, 691 (2018).
  * Ellis  _et al._ [2018] D. J. P. Ellis, A. J. Bennett, C. Dangel, J. P. Lee, J. P. Griffiths, T. A. Mitchell, T.-K. Paraiso, P. Spencer, D. A. Ritchie, and A. J. Shields, Independent indistinguishable quantum light sources on a reconfigurable photonic integrated circuit, Applied Physics Letters 112, 211104 (2018).
  * Singh  _et al._ [2019] A. Singh, Q. Li, S. Liu, Y. Yu, X. Lu, C. Schneider, S. Höfling, J. Lawall, V. Verma, R. Mirin, S. W. Nam, J. Liu, and K. Srinivasan, Quantum frequency conversion of a quantum dot single-photon source on a nanophotonic chip, Optica 6, 563 (2019).
  * Bauer  _et al._ [2021] S. Bauer, D. Wang, N. Hoppe, C. Nawrath, J. Fischer, N. Witz, M. Kaschel, C. Schweikert, M. Jetter, S. L. Portalupi, M. Berroth, and P. Michler, Achieving stable fiber coupling of quantum dot telecom C-band single-photons to an SOI photonic device, Applied Physics Letters 119, 211101 (2021).
  * Sund  _et al._ [2023] P. I. Sund, E. Lomonte, S. Paesani, Y. Wang, J. Carolan, N. Bart, A. D. Wieck, A. Ludwig, L. Midolo, W. H. Pernice, P. Lodahl, and F. Lenzini, High-speed thin-film lithium niobate quantum processor driven by a solid-state quantum emitter, Science Advances 9, eadg7268 (2023).
  * Arcari  _et al._ [2014] M. Arcari, I. Söllner, A. Javadi, S. L. Hansen, S. Mahmoodian, J. Liu, H. Thyrrestrup, E. Lee, J. Song, S. Stobbe, and P. Lodahl, Near-Unity Coupling Efficiency of a Quantum Emitter to a Photonic Crystal Waveguide, Physical Review Letters 113, 093603 (2014).
  * Liu  _et al._ [2018a] F. Liu, A. J. Brash, J. O’Hara, L. M. P. P. Martins, C. L. Phillips, R. J. Coles, B. Royall, E. Clarke, C. Bentham, N. Prtljaga, I. E. Itskevich, L. R. Wilson, M. S. Skolnick, and A. M. Fox, High Purcell factor generation of indistinguishable on-chip single photons, Nature Nanotechnology 13, 835 (2018a).
  * Hepp  _et al._ [2018] S. Hepp, S. Bauer, F. Hornung, M. Schwartz, S. L. Portalupi, M. Jetter, and P. Michler, Bragg grating cavities embedded into nano-photonic waveguides for Purcell enhanced quantum dot emission, Optics Express 26, 30614 (2018).
  * Łukasz Dusanowski  _et al._ [2020] Łukasz Dusanowski, D. Köck, E. Shin, S.-H. Kwon, C. Schneider, and S. Höfling, Purcell-Enhanced and Indistinguishable Single-Photon Generation from Quantum Dots Coupled to On-Chip Integrated Ring Resonators, Nano Letters 20, 6357 (2020).
  * Kim  _et al._ [2018] J.-H. Kim, S. Aghaeimeibodi, C. J. K. Richardson, R. P. Leavitt, and E. Waks, Super-Radiant Emission from Quantum Dots in a Nanophotonic Waveguide, Nano Letters 18, 4734 (2018).
  * Katsumi  _et al._ [2020] R. Katsumi, Y. Ota, A. Osada, T. Tajiri, T. Yamaguchi, M. Kakuda, S. Iwamoto, H. Akiyama, and Y. Arakawa, In situ wavelength tuning of quantum-dot single-photon sources integrated on a CMOS-processed silicon waveguide, Applied Physics Letters 116, 041103 (2020).
  * Grim  _et al._ [2019] J. Q. Grim, A. S. Bracker, M. Zalalutdinov, S. G. Carter, A. C. Kozen, M. Kim, C. S. Kim, J. T. Mlack, M. Yakes, B. Lee, and D. Gammon, Scalable in operando strain tuning in nanophotonic waveguides enabling three-quantum-dot superradiance, Nature Materials 18, 963 (2019).
  * Elshaari  _et al._ [2018] A. W. Elshaari, E. Büyüközer, I. E. Zadeh, T. Lettner, P. Zhao, E. Schöll, S. Gyger, M. E. Reimer, D. Dalacu, P. J. Poole, K. D. Jöns, and V. Zwiller, Strain-Tunable Quantum Integrated Photonics, Nano Letters 18, 7969 (2018).
  * Tao  _et al._ [2020] L. Tao, W. Wei, Y. Li, W. Ou, T. Wang, C. Wang, J. Zhang, J. Zhang, F. Gan, and X. Ou, On-Chip Integration of Energy-Tunable Quantum Dot Based Single-Photon Sources via Strain Tuning of GaAs Waveguides, ACS Photonics 7, 2723 (2020).
  * Hepp  _et al._ [2020] S. Hepp, F. Hornung, S. Bauer, E. Hesselmeier, X. Yuan, M. Jetter, S. L. Portalupi, A. Rastelli, and P. Michler, Purcell-enhanced single-photon emission from a strain-tunable quantum dot in a cavity-waveguide device, Applied Physics Letters 117, 254002 (2020).
  * Petruzzella  _et al._ [2018] M. Petruzzella, S. Birindelli, F. M. Pagliano, D. Pellegrino, Ž. Zobenica, L. H. Li, E. H. Linfield, and A. Fiore, Quantum photonic integrated circuits based on tunable dots and tunable cavities, APL Photonics 3, 106103 (2018).
  * Schnauber  _et al._ [2021] P. Schnauber, J. Große, A. Kaganskiy, M. Ott, P. Anikin, R. Schmidt, S. Rodt, and S. Reitzenstein, Spectral control of deterministically fabricated quantum dot waveguide systems using the quantum confined Stark effect, APL Photonics 6, 050801 (2021).
  * Papon  _et al._ [2023] C. Papon, Y. Wang, R. Uppu, S. Scholz, A. Wieck, A. Ludwig, P. Lodahl, and L. Midolo, Independent Operation of Two Waveguide-Integrated Quantum Emitters, Physical Review Applied 19, L061003 (2023).
  * Łukasz Dusanowski  _et al._ [2023] Łukasz Dusanowski, D. Köck, C. Schneider, and S. Höfling, On-Chip Hong–Ou–Mandel Interference from Separate Quantum Dot Emitters in an Integrated Circuit, ACS Photonics 10, 2941 (2023).
  * Reindl  _et al._ [2019] M. Reindl, J. H. Weber, D. Huber, C. Schimpf, S. F. C. da Silva, S. L. Portalupi, R. Trotta, P. Michler, and A. Rastelli, Highly indistinguishable single photons from incoherently excited quantum dots, Physical Review B 100, 155420 (2019).
  * Schöll  _et al._ [2019] E. Schöll, L. Hanschke, L. Schweickert, K. D. Zeuner, M. Reindl, S. F. C. da Silva, T. Lettner, R. Trotta, J. J. Finley, K. Müller, A. Rastelli, V. Zwiller, and K. D. Jöns, Resonance Fluorescence of GaAs Quantum Dots with Near-Unity Photon Indistinguishability, Nano Letters 19, 2404 (2019).
  * da Silva  _et al._ [2021] S. F. C. da Silva, G. Undeutsch, B. Lehner, S. Manna, T. M. Krieger, M. Reindl, C. Schimpf, R. Trotta, and A. Rastelli, GaAs quantum dots grown by droplet etching epitaxy as quantum light sources, Applied Physics Letters 119, 120502 (2021).
  * Huber  _et al._ [2018] D. Huber, M. Reindl, S. F. C. da Silva, C. Schimpf, J. Martín-Sánchez, H. Huang, G. Piredda, J. Edlinger, A. Rastelli, and R. Trotta, Strain-Tunable GaAs Quantum Dot: A Nearly Dephasing-Free Source of Entangled Photon Pairs on Demand, Physical Review Letters 121, 033902 (2018).
  * Huang  _et al._ [2021] H. Huang, D. Csontosová, S. Manna, Y. Huo, R. Trotta, A. Rastelli, and P. Klenovský, Electric field induced tuning of electronic correlation in weakly confining quantum dots, Physical Review B 104, 165401 (2021).
  * Zhai  _et al._ [2020] L. Zhai, M. C. Löbl, G. N. Nguyen, J. Ritzmann, A. Javadi, C. Spinnler, A. D. Wieck, A. Ludwig, and R. J. Warburton, Low-noise GaAs quantum dots for quantum photonics, Nature Communications 11, 4745 (2020).
  * Zhai  _et al._ [2022] L. Zhai, G. N. Nguyen, C. Spinnler, J. Ritzmann, M. C. Löbl, A. D. Wieck, A. Ludwig, A. Javadi, and R. J. Warburton, Quantum interference of identical photons from remote GaAs quantum dots, Nature Nanotechnology 17, 829 (2022).
  * Schwartz  _et al._ [2016] M. Schwartz, U. Rengstl, T. Herzog, M. Paul, J. Kettler, S. L. Portalupi, M. Jetter, and P. Michler, Generation, guiding and splitting of triggered single photons from a resonantly excited quantum dot in a photonic circuit, Optics Express 24, 3089 (2016).
  * Makhonin  _et al._ [2014] M. N. Makhonin, J. E. Dixon, R. J. Coles, B. Royall, I. J. Luxmoore, E. Clarke, M. Hugues, M. S. Skolnick, and A. M. Fox, Waveguide Coupled Resonance Fluorescence from On-Chip Quantum Emitter, Nano Letters 14, 6997 (2014).
  * Huber  _et al._ [2020] T. Huber, M. Davanco, M. Müller, Y. Shuai, O. Gazzano, and G. S. Solomon, Filter-free single-photon quantum dot resonance fluorescence in an integrated cavity-waveguide device, Optica 7, 380 (2020).
  * Liu  _et al._ [2018b] J. Liu, K. Konthasinghe, M. Davanço, J. Lawall, V. Anant, V. Verma, R. Mirin, S. W. Nam, J. D. Song, B. Ma, Z. S. Chen, H. Q. Ni, Z. C. Niu, and K. Srinivasan, Single Self-Assembled InAs/GaAs Quantum Dots in Photonic Nanostructures: The Role of Nanofabrication, Physical Review Applied 9, 064019 (2018b).
  * Manna  _et al._ [2020] S. Manna, H. Huang, S. F. C. da Silva, C. Schimpf, M. B. Rota, B. Lehner, M. Reindl, R. Trotta, and A. Rastelli, Surface passivation and oxide encapsulation to improve optical properties of a single GaAs quantum dot close to the surface, Applied Surface Science 532, 147360 (2020).
  * Pregnolato  _et al._ [2020] T. Pregnolato, X.-L. Chu, T. Schröder, R. Schott, A. D. Wieck, A. Ludwig, P. Lodahl, and N. Rotenberg, Deterministic positioning of nanophotonic waveguides around single self-assembled quantum dots, APL Photonics 5, 086101 (2020).
  * Li  _et al._ [2023] S. Li, Y. Yang, J. Schall, M. von Helversen, C. Palekar, H. Liu, L. Roche, S. Rodt, H. Ni, Y. Zhang, Z. Niu, and S. Reitzenstein, Scalable Deterministic Integration of Two Quantum Dots into an On-Chip Quantum Circuit, ACS Photonics 10, 2846 (2023).