\subsection*{Results}

Here, we overcome these obstacles by mixing small amounts of nanodiamonds from solution directly into a known 
photoresist \cite{Fischer2011} (See Methods). This special photoresist allows for the fabrication
of transparent optical elements made from an acrylate polymer containing nanodiamonds. The nanodiamonds contain
NV-centres which are photostable even after exposure by the DLW laser. After the DLW process they serve as
integrated single-photon sources. It is especially noteworthy that our technique is not limited to NV-centres in
diamond nanocrystals. It can be applied to any photostable single-photon emitter in nanocrystalline material,
e.g. other diamond defect centres, such as Silicon vacancy-centres \cite{Neu2011}, or other deep defect centres
 in large band gap material \cite{Weber2010}.

In the following, we demonstrate the fabrication, functionality, and interaction of key building blocks for quantum photonic
circuitry. These are interconnects (i.e., waveguides) and functional elements like resonators and emitters. We
measure single-photon emission from a diamond nanocrystal in a waveguide-coupled resonator which demonstrates
the functional combination of all these building blocks and paves the way to more complex devices.

As a first component for integrated photonic circuits, disc resonators with a disc diameter of
\unit{20}{\micro\metre} and a disc thickness of approximately \unit{1.2}{\micro\metre} on a stem with diameter
 \unit{10}{\micro\metre} were fabricated using the photoresist functionalised with nanocrystals (see Methods).
The resonator supports whispering gallery modes which are confined in the
disc close to the rim with high quality factors (Q-factors). 
We measured the Q-factor using a tunable laser coupled into the resonator via a tapered
fibre\cite{Vahala2003}. We found Q-factors as large as $10^4$ at a wavelength of around \unit{770}{\nano\metre} (see Figure
\ref{fig:tapercoupling} and Methods). The observed free spectral range of \unit{6.5}{\nano\meter} of the
resonator corresponds well to the expected value of \unit{6.4}{\nano\meter} for a disc with diameter \unit{20}{\micro\meter}
derived from geometrical considerations assuming an index of refraction of 1.5.


\begin{figure*}
  \includegraphics{Figure_2}
  \caption{\textbf{Mode measurements}. \textbf{a}, A tunable laser is coupled to a disc resonator via a tapered fibre. Upon sweeping the
  laser frequency, the transmitted light is modulated by the modes of the resonator. \textbf{b}, Resonator mode with a quality factor Q of
  $1.2 \times 10^4$. \textbf{c}, Scan over many modes with a free spectral range of \unit{6.5}{\nano\meter}
  indicated by the red arrows.}
  \label{fig:tapercoupling}
\end{figure*}

Next, a homebuilt confocal microscope (see Methods) was used to raster scan the fabricated resonator discs  (see Figure 3a) and
to identify single NV-centres by measuring the second-order autocorrelation function $g^{(2)}(\tau)$ in a
Hanbury Brown and Twiss (HBT) interferometer. In parallel, a grating spectrometer was used to resolve the
emission spectra of the individual emitters. Figure \ref{fig:diainresonator}b shows a confocal scan of a
resonator where fluorescent defects can be identified as bright spots on the resonator. Encircled is a spot on
the resonator's outer rim where coupling to the disc's whispering gallery modes is expected. Figure
\ref{fig:diainresonator}d displays an autocorrelation measurement of fluorescence collected from that spot. A
clear antibunching dip is observed reaching a value of $g^{(2)}(0)=0.31\pm0.04$ (standard error) as deduced from the fit shown as red
curve. No background correction was applied to these data. This measurement shows that the bright spot indeed
corresponds to a single NV-centre in the disc resonator.

The fluorescence spectrum of an NV-centre at room temperature is typically broadened over \unit{200}{\nano\meter} from approx.
\unit{600}{\nano\meter} to \unit{800}{\nano\meter} by phonon sidebands. This corresponds well to the measured emission spectrum shown in Figure
\ref{fig:diainresonator}c. The peak at approximately \unit{630}{\nano\meter} stems from fluorescence of the
photoresist and bleaches after long excitation. For confocal scans and correlation measurements this background
light was suppressed by spectral filtering. In addition, there is a fine modulation of the spectrum due to the
cavity resonances. It is attributed to photons that were initially emitted into the resonator modes and
afterwards scattered out by the diamond nanocrystal. Hence, these photons are detected in addition to the flat
unmodulated spectrum emitted directly out of the disc.

\begin{figure*}
  \includegraphics{Figure_3}
  \caption{\textbf{DLW resonator containing single NV-centres.} A diamond nanocrystal containing a single NV-centre is coupled to whispering gallery modes
  of a DLW disc resonator. \textbf{a}, Measurement scheme. Detection and excitation take place at the same point in a confocal configuration.
  \textbf{b}, Scanning confocal image of the resonator disc. The circle indicates a bright spot identified as single NV-centre. Its
  fluorescence is analysed in \textbf{c} and \textbf{d}. Scale bar is \unit{5}{\micro\metre}. \textbf{c},
Spectrum of the collected fluorescence. The resonator modes are seen on the broad NV-centre phonon sidebands. The
peak at \unit{630}{\nano\meter} stems from the photoresist and can be bleached over time.
  \textbf{d}, Autocorrelation function of the fluorescence from the NV-centre. A clear antibunching behaviour can be seen. The red curve is a fit to the data.}
  \label{fig:diainresonator}
\end{figure*}

As a second photonic element, arc waveguides  from the same material with a width of \unit{1.8}{\micro\meter} and
a length of \unit{40}{\micro\meter} were built. The waveguides were fabricated close to the disc resonator
in order to enable evanescent coupling to the resonator's modes (see Figure \ref{fig:waveguide}a). Their ability
to guide light within a broad spectral range is demonstrated by illuminating one end of the waveguide with light from a halogen lamp as seen
in Figures \ref{fig:waveguide}b and c. The blue and red circular spots correspond to the input and output of
the arc waveguide, respectively.

Coupling via the waveguides to the resonator modes was confirmed by shining an excitation laser (wavelength of
\unit{532}{\nano\metre}) into one port of the waveguide and, after blocking excitation light by a dichroic
mirror, analysing the light coming out of the other port. Obviously, the \unit{532}{\nano\metre}-laser excites
background fluorescence in the resonator, which is then coupled back into the waveguide. The measured
fluorescence spectrum in Figure \ref{fig:waveguide}e thus shows the characteristic modulation by resonator
modes and proves a good coupling between guided modes in the arc waveguide and confined modes in the resonator
(see Supplementary Information).

\begin{figure*}
  \includegraphics{Figure_4}
  \caption{\textbf{Arc waveguide coupled to resonator.}
  \textbf{a}, Scanning electron micrograph of a 3D arc waveguide/resonator assembly. Length of the scale bar is \unit{5}{\micro\metre}.
  \textbf{b}, The waveguide resonator assembly as seen through the cover slip. The upper half of the image is illuminated from below by white light.
  Efficient light guiding through the arc waveguide can be seen. Scale bar is \unit{10}{\micro\metre}. \textbf{c}, Sketch of the
  region shown in \textbf{b} with the illuminated area marked yellow. \textbf{d}, Sketch how the spectrum in \textbf{e} was acquired.
  \textbf{e}, Spectrum of the fluorescence detected at one of the waveguide's ports while exciting through the other.
  Resonator modes can be clearly identified. \textbf{f}, Experimental configuration for the autocorrelation measurement in \textbf{g}.
  A second objective is used for excitation.
  \textbf{g}, Measured autocorrelation function of a NV-centre inside a waveguide. It is acquired at both output ports of the waveguide
  in a cross-correlation configuration.}
  \label{fig:waveguide}
\end{figure*}


By moving the excitation spot of a second objective over a waveguide (see Figure \ref{fig:waveguide}a), individual nanodiamonds inside 
can be addressed and identified as spots with higher fluorescence. The intensity cross correlation
between the waveguide's two outputs shows a clear antibunching behaviour. This result highlights a way of
coupling single emitters to thin waveguides, which can possess very high collection efficiencies, 
as demonstrated recently using  tapered optical fibres
\cite{Fujiwara2011,Schroder2012}. Here, we show a direct on-chip integration of such devices with both waveguide
ends accessible. Detection at the waveguide's ends can yield count rates of 
 \unit{29,000}{\per \second} after passing a 50/50 beam splitter and spatial and spectral
filtering. The autocorrelation function at zero time delay of the fluorescence collected in this way is
$g^{(2)}(0)=0.37\pm0.13$ (standard error) showing the single-photon character of the light (see Figure \ref{fig:waveguide}g). When
accounting for the 50/50 beam splitter, at high excitation powers the number of background corrected counts
from the defect centre is measured to be \unit{130,000}{\per \second}.

\begin{figure*}
  \includegraphics{Figure_5}
  \caption{\textbf{Waveguide coupling of a single NV-centre inside a resonator.} \textbf{a}, Sketch of the experimental configuration.
  The excitation spot is scanned over the resonator disc. Photons are detected at both waveguide outputs simultaneously.
  \textbf{b}, Photon counts collected at
  one end of the waveguide while scanning the excitation spot with a second objective. The circle
  highlights the position of a single NV-centre. Shape distortions are due to non closed-loop piezo-scanning. Scale bar is \unit{5}{\micro\metre}.
  \textbf{c}, Cross correlation (background corrected) between the waveguide output ports when the NV-centre marked by a
  circle in \textbf{b} is excited. The bin size corresponds to the repetition rate of \unit{40}{\mega\hertz}}
  \label{fig:res16dia}
\end{figure*}

Finally, we verified active quantum functionality of a coupled 3D system, i.e., single photon emission to
resonator modes, as well as subsequent collection and routing via a coupled waveguide. In order to do so, raster
scans of the excitation spot over the resonator disc were performed while the photons from one of the
waveguide's ends are collected (see Figure \ref{fig:res16dia}a). When the excitation laser hits the outer rim of
the resonator, the created fluorescence light is coupled to the resonator modes and subsequently out to the
waveguide, as can bee seen in Figure \ref{fig:res16dia}b. Therefore, the outer rim of the resonator appears
bright in the raster image. Bright spots indicate possible positions of NV-centres. To identify NV-centres via
their non-classical light emission, an HBT setup, with the laser-written arc waveguide serving as beam splitter, is
used.
Figure \ref{fig:res16dia}b shows this correlation measurement for the NV-centre encircled in Figure
\ref{fig:res16dia}a. The data shown were acquired with a pulsed laser at \unit{40}{\mega\hertz} repetition rate and background
corrected (see Supplementary Information). The value of $g^{(2)}(0)=-0.18\pm0.21$ (standard deviation) is far below the threshold 
$g^{(2)}(0)=0.5$ for having 
equal contributions of two centers and proves that the main contribution
to the signal stems from a single NV-centre. This clearly indicates that the NV-centre emits single photons
into resonator modes, which are then coupled out and redirected by the arc waveguide. With a single source in a resonator
coupled to a waveguide, this device represents a key integrated 3D quantum photonic circuit.

\subsection*{Discussion}

In conclusion, we have shown an approach which allows for the direct incorporation of single quantum emitters
into true 3D photonic structures of nearly arbitrary shape. The one step fabrication scheme is easy to implement
and requires no elaborate clean-room environment. We have pointed out that our method is not limited to
NV-centres in nanodiamond, but can be extended to any other stable quantum emitter embedded in nanocrystals. The
flexibility of DLW enables immediate scaling-up to more complex structures (see Figure \ref{fig:dlw}b) where
single photons are collected, sent through beam splitters, interferometers, or other optical elements
inter-connected by waveguides in a 3D architecture. 
Also tree-dimensional multi-arm quantum interferometers~\cite{} can be implemented.
Tuning of individual components, e.g. the resonator, is 
possible as well (see Supplementary Information). With (partial) metallisation of the structures 
embedding single photon emitters in 3D plasmonic structures or in metamaterials \cite{Rill2008} can be envisioned. 
A next challenging, but rewarding step is to perform DLW of perfectly aligned 3D structures around emitters of known position. 
This would enable on-demand fabrication of arbitrary 3D quantum photonic architectures.



\subsection*{Methods}

\textbf{Direct laser writing.} The DLW setup consists of a Ti:sapphire oscillator (SpectraPhysics MaiTai HP)
delivering \unit{100}{\femto\second} pulses centred around \unit{810}{\nano\metre}. The beam is focussed through
an oil-immersion lens (Leica HCX PL APO 100x/1.40-0.70 OIL CS). The sample is moved with a 3D piezo stage (PI
P-527.3CL). The writing was performed with \unit{6}{\milli\watt} average writing power and at
\unit{50}{\micro\metre\per\second} scan velocity. The samples were developed in isopropyl alcohol for 30
minutes, afterwards in acetone for 30 minutes, and finally dried super-critically (Leica CPD030).

The new photoresist is based on the monomer pentaerythritol tetraacrylate (PETTA) which contains 350~ppm
monomethyl ether hydroquinone as inhibitor. Next, 0.25~\% wt of the photoinitiator
7-diethylamino-3-thenoylcoumarin were added. Last, 2~\% wt of an ethanol-based nanodiamond suspension were added
and stirred overnight. Nanodiamonds were of type 1b and had a median diameter of \unit{25}{\nano\metre} (Microdiamant AG).
Spatial dimensions of the fabricated structures were extracted from scanning electron micrographs.

\textbf{Confocal microscope.} The homebuilt confocal microscope is based on the body of a Zeiss Axiovert 200.
Either a frequency doubled Nd:YAG continuous wave laser or a pulsed laser (LDH-P-FA-530, PicoQuant) were used to
create the excitation light around \unit{532}{\nano\metre}. A large-numerical-aperture air microscope objective
mounted on a 1D piezo stage (MIPOS 100, piezosystem jena) focussed the excitation light and collected the
fluorescence. The sample was scanned by a 2D piezo stage (PXY 80 D12, piezosystem jena). Collected fluorescence
light was separated from the excitation laser by a dichroic mirror. Then, it was either spatially filtered and
sent
to a grating spectrometer 
or it was spectrally filtered by RG630 glass, split by a 50/50 beam splitter, and directed towards two pinholes.
These pinholes could be independently adjusted to spatially filter out different spots of the image plane, so
that cross correlations could be measured. After the pinholes the light was directed to avalanche photodiodes
(SPCM-AQR-14/SPCM-AQRH-14, Perkin Elmer).

To perform correlation measurements at the waveguide ends, an oil-immersion microscope objective (UPlanSApo
60XO, Olympus) was used for collecting the light, while different air objectives were used to focus the
excitation laser onto the sample from the other side. The air objectives were mounted on a 3D piezo system
(PXY100 ID and MIPOS 100, piezosystem jena) to enable scanning of the excitation spot while the sample was kept
at a fixed position. Also, the 50/50 beam splitter within the HBT setup could be replaced by a D-shaped mirror
in an image plane in cases where cross-correlation measurements with high count rates were needed.


\textbf{Q-factor measurements.} The mode structure of the disc resonators was analysed by means of coupling in a
tunable external-cavity diode laser (Velocity Series, New Focus) at around \unit{770}{\nano\metre} via the
evanescent fields of a tapered optical fibre.
By tuning the frequency of the laser over the distinct whispering gallery modes, light was coupled into the
resonator and different modes could be observed as Lorentzian shaped dips in the transmitted power. The
polarization of the incoming light was chosen to maximize the coupling depth. For normalizing the data sets, we
first performed a reference scan with an uncoupled fibre taper and compared the results to the situation when
the same taper was coupled to the resonator. This provided direct access to the different coupling depths and
cancelled out overlaying power modulations caused by slightly non-adiabatic taper transitions.

The Q-factors were calculated from the dips by a Lorentzian fit function with an additional linear term to
better match the local environment of the resonances. The fibre taper used had a waist diameter of
\unit{1.5}{\micro\metre}. Measurements were performed in full contact.

\subsection*{Acknowledgements}
The Berlin team acknowledges support by DFG (FOR1493 and SFB951).
The Karlsruhe team acknowledges support by the
DFG-Center for Functional Nanostructures (CFN) via
subprojects A1.4 and A1.5 and by the Karlsruhe School of Optics \& Photonics (KSOP).
J.W. acknowledges funding by the state of Berlin (Elsa-Neumann). 

\subsection*{Author contributions}
A.W.S. wrote the manuscript and conceived the experiment with J.F., J.K. and J.F. produced the samples, A.W.S., R.H. and J.W.
performed the optical measurements and data analysis. M.W. and O.B. supervised the study.
All authors discussed the experiment, the results and the manuscript.

\subsection*{Additional Information}
The authors declare no competing financial interests.


