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\begin{poster}
\large \sf

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        \begin{center}
          {\sc \huge Experiments with Correlated Photons}\\[9mm]
          {\large
          Lijiong~Shen,
          Jianwei~Lee,
          Mathias~A.~Seidler,
          Brenda~Chng,\\
          Hou~Shun~Poh,
          Peng~Kian~Tan,
          Alessandro~Cer\`{e},
          Christian~Kurtsiefer}\\[9mm]
          {\Large August 2016 - December 2017}
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\begin{multicols}{2}
% \setlength{\parskip}{1ex plus 0.5ex minus 0.2ex}
\begin{center}
  {\bf \Large \textsf{Many-pairs non-locality}}\\[.2cm]
collaborators:~Jean-Daniel~Bancal$^1$,~Yu~Cai,~Nicolas~Sangouard$^1$,~and~Valerio~Scarani.\\[-.2cm]
\end{center}
We consider a scenario where $n$ identical entangled pairs are prepared and
measured collectively. A many-pairs Bell inequality can be violated in this scenario using two binning strategies: majority vote and parity.
The maximal number of pairs for which a violation can be observed quantifies
the high quality of the pair source. We find a violation up to 41 pairs in the
presence of majority voting and up to 12 pairs in the presence of parity binning~[1].

\begin{figurehere}
\begin{minipage}[c]{0.45\linewidth}
     \centering
         \includegraphics[width=.7\columnwidth]{manypairs/figure1.eps}\\[1cm]
         \includegraphics[width=.85\columnwidth]{manypairs/compression_setup.eps}
\end{minipage}
\begin{minipage}[c]{0.55\linewidth}
    \includegraphics[width=\columnwidth]{manypairs/majority_versus_separation_angle.eps}\\
    \includegraphics[width=\columnwidth]{manypairs/parity_versus_separation_angle.eps}
\end{minipage}
\figcaption{Top left: Many-pairs scenario.
  Bottom left: experimental setup: high purity non-collinear SPDC source of near-IR polarization entangles photon pairs.
  Right: CHSH parameter $S$ as function of the separation angle between measurement basis for majority voting (top) and parity binning (bottom).}
\end{figurehere}
\begin{center}
  {\bf \Large \textsf{Time resolution of TES for CW sources}}\\[.2cm]
collaborators:~Thomas Gerrits$^2$, Adriana~E.~Lita$^2$, Sae~Woo~Nam$^2$\\[-.2cm]
\end{center}
    Transition edge sensors (TES) can be highly efficient single photon
    detectors. For continuous wave (CW) sources with large mean photon number
    their energy resolving ability and timing accuracy is compromised by the
    slow recovery time on the order of microseconds.
    We developed a technique to
    assign arrival times to overlapping detection events.
    % in the regime of low signal to noise ratio, as in the case of TES detection of near-infrared radiation.
    For this, 
    a two-level discriminator coarsely locates pulses in time, while the exact
    timing of individual photoevents is obtained by fitting to a heuristic
    detector response function. 
    This allows to measure 
    the second-order time correlation
    of a coherent source in a single spatial mode using a single TES.\\[.2cm]
\begin{figurehere}
\begin{minipage}{0.2\linewidth}
    \centering
    \includegraphics[width=1\columnwidth]{tes_pulse/TES-setup.pdf}\\[1cm]
    \includegraphics[width=1.2\columnwidth]{tes_pulse/TesSquidIntegrator.eps}
\end{minipage}
\begin{minipage}{0.8\linewidth}
    \centering
\includegraphics[width=0.48\columnwidth]{tes_pulse/2ph_fit} %\vspace*{5mm}
\includegraphics[width=0.48\columnwidth]{tes_pulse/g2_cw}
\end{minipage}
\figcaption{Top left: experimental setup: a CW laser diode centered at 810~nm
    with a variable attenuator to control the average photon number.
    Bottom right:  TES signal detection and amplification.
    Center: overlapping pulses, corresponding to two photon detection events, with fit to the heuristic model based on average single photon response.
    Right: second order autocorrelation of the laser light obtained with the TES.}
\end{figurehere}
\begin{center}
  {\bf \Large \textsf {Photocorrelation spectroscopy}}\\[-2cm]
\end{center}
Photocorrelation spectroscopy can help determining very narrow spectral
lines found in some stellar objects. We are able to determine
the line width of a 20\,MHz wide spectral line on a thermal blackbody
background, an distinguish laser-like emission from pseudo-thermal light. This
may help to identify optical stellar lasers in the visible range~[2].

\begin{figurehere}
  % \begin{center}
    \includegraphics[width=0.42\columnwidth]{completesetup4}\hspace{2cm}
    \includegraphics[width=0.42\columnwidth]{g2_arclaser_6}
  % \end{center}
  \figcaption{Left: preparation of a Doppler-widened laser line on top of a thermal
    background. Right: resultant photopair correlation, revealing a long tail
   in $\mathsf{g^{(2)}(\tau)}$ corresponding to a narrow spectral line on top
   of a thermal photon bunching signature.}
\end{figurehere}
\begin{flushleft}
  \begin{center}{\bf \large \textsf {References}}\\[.2cm]\end{center}
  \small
  [1]~H.~S.~Poh, A.~Cer\`e, J.~D.~Bancal, Y.~Cai, N.~Sangouard, V.~Scarani, and C.~Kurtsiefer. Phys.~Rev.~A \textbf{96}, 022101 (2017)
  \newline
  [2]~P.K.~Tan and C.~Kurtsiefer, MNRAS, Vol 469, \textbf{2} (2017)
  \newline
  [3] S.~Pironio, et al.,  Nature, {\textbf{464}},1021 (2010).
  \newline
  [4]  A.E. Lita, A.J. Miller and Sae Woo Nam, Opt. Express {\textbf{16}},
  3032 (2008).
  \newline
  [5] B.~Srivathsan, G.~K.~Gulati, B.~Chng, G.~Maslennikov, D.~N.~Matsukevich, and C.~Kurtsiefer, PRL \textbf{111}, 123602 (2013).
  \end{flushleft}
\columnbreak
\begin{center}
  \begin{center} {\bf \Large \textsf {Bell tests \& Randomness generation without detection loophole}}\\[.2cm]
collaborators:~Thomas Gerrits$^2$, Adriana~E.~Lita$^2$, Sae~Woo~Nam$^2$,
Thinh~Le~Phuc, Jean-Daniel~Bancal$^1$, Valerio~Scarani.
\end{center}
\end{center}

Quantum physics can provide randomness
that can be certified as being uncorrelated to any outside process or
variable, thus provide private randomness, based on a violation of a Bell
inequality [3].
With advancements in high efficiency infrared photon detectors [4] in combination with a high efficiency pair source,
a loophole free experimental violation of the Bell inequality becomes
feasible, which can be engineered into 
a source of certified randomness.
%
% This violation can also be observed as long as the overall efficiency is
% larger than 2/3 and the state and measurement basis are chosen
% appropriately~[7].

\begin{figurehere}
\begin{minipage}{0.45\linewidth}
    \centering
    \includegraphics[width=\columnwidth]{bell_violation/Sagnac2017.pdf}
\end{minipage}
\begin{minipage}{0.55\linewidth}
    \centering
    \includegraphics[width=1\columnwidth]{bell_violation/vio_exp.pdf}
\end{minipage}\\[.5cm]
\figcaption{Left: Efficient polarization-entangled pair source and detection
  scheme  involving superconducting transition edge sensors (TES).
  Top right:
  Violation of CHSH inequality observed as a function of the time bin width.
  Bottom right:
  Rate of random bits that can be extracted as a function of the time bin width.
  }
\end{figurehere}

Our pair source can  reliably prepare non-maximally entangled states with
a high fidelity. In combination with TES, we regularly find system efficiencies  $>$80\%.
This permits to test a version of CHSH inequality that  with a
continuous source of photon pairs, with events defined by dividing the
measurement time in discrete time bins of fixed length. Numerical simulation
and initial experimental results support the intuition that this approach can
generate randomness with an efficiency exceeding that of systems based on
pulsed pair sources.\vspace*{5mm}

\begin{center}
  {\bf \Large \textsf{Six-wave mixing in cold atoms for narrowband triplets of photons}}
\end{center}
Extending earlier results in with four wave mixing [5], three-photon states
from a cold atomic ensembles can be generated with six wave mixing.
The photon wavelength and bandwidth will be compatible with other atomic
systems, and the possibility of heralding a photon pairs could improve the
scaling of quantum protocols based on light-atom interactions.

\begin{figurehere}
\begin{minipage}{0.3\linewidth}
    \centering
    \includegraphics[width=1\columnwidth]{six_wave_mixing/energy_levels_SAB}
\end{minipage}
\begin{minipage}{0.7\linewidth}
    \centering
    \includegraphics[width=.6\columnwidth]{six_wave_mixing/phase_matching_sketch}\\
    \includegraphics[width=.9\columnwidth]{six_wave_mixing/swm_seed.pdf}
\end{minipage}\\[.5cm]
\figcaption{Left: schematic of the energy levels of $^{87}$Rb for the six-wave mixing.
Top right: phase matching scheme. Non-collinear phase-matching allows for geometrical separation of modes only few hundreds of MHz away.
Bottom right: we confirm phase matching by parametrically amplifying a seed
laser beam at $795$~ns (corresponding to photon~$C$) in a five-beam
configuration, and observing the concurrent generation of light at $762$~nm as
a signature of the six-wave mixing process.
}
\end{figurehere}
\vspace*{5mm}

\begin{center}
  {\bf \Large \textsf{Toward an heralded quantum memory for single photons}}
  collaborators:~Zhang~Zhiqiang, Ravi~Kumar, Kyle~Arnold, Murray~Barrett.
\end{center}

Quantum memories promise the scalability of quantum communication protocols. A memory that confirms the effective storage of an excitation could enhance this scalability even further. We investigate a scheme based on the interaction between a cold atomic ensemble and a cavity, and test it with narrowband heralded single photons.

\begin{figurehere}
\begin{minipage}[c]{0.4\linewidth}
    \centering
    \includegraphics[width=0.98\columnwidth]{hybrid/setup_sketch.pdf}
\end{minipage}
\begin{minipage}{0.6\linewidth}
    \centering
    \includegraphics[width=0.9\columnwidth]{hybrid/cavity_efficiency/cavity_output.pdf}
\end{minipage}\\[.5cm]

\figcaption{Left: schematic of the experiment. Four-wave mixing in a cold
  cloud of $^{87}$Rb generates heralded single photons (probe) wich are sent to a second cold atomic ensemble interacting with an optical cavity.
Left: time correlation between heralding events and detections at the output of the cavity.
}
\end{figurehere}

\begin{flushleft}
  % \begin{center}
  {\centering \bf \large \textsf {Affiliations}

  }

  % \end{center}
  \small{
    $^1$Department of Physics, University of Basel, Klingelbergstrasse 82, 4056 Basel, Switzerland\newline
    $^2$National Institute of Standards and Technology (NIST),
  325 Broadway, Boulder,
  Colorado 80305, USA
  }
{\small{
The work presented here was partly supported by the National Research
Foundation, Ministry of Education. Part of the work was supported by a Tier-3
Grant from MOE on randomness.}}
\end{flushleft}

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