%!TEX root = clock_main.tex
\begin{frame}\frametitle{Summary}
	\begin{block}{Synchronization with time-correlated photons\footnote[1]{Appl. Phys. Lett. 114, 101102 (2019)}}
	\begin{itemize}
		\item Independent of clock separation
		\item Intrinsic clock instabilities can be resolved
			% \begin{itemize}
			% \item Does not require trustworthy estimate of separation
			% \item Secure against Symmetric Delay Attacks
			% \end{itemize}
		% \item Secure against symmetric attacks
	\end{itemize}
	\end{block}
	% \vfill
	\begin{block}{Security with polarization-entanglement}
	\begin{itemize}
		\item Enables signal verification
	\end{itemize}
	\end{block}
	\begin{block}{Ongoing research}
	\begin{itemize}
		\item Detecting circulator-based asymmetric delay attacks\footnote[2]{arXiv:1907.09661 (2019)}
		\item e.g. cyclic evolution of quantum state $\rightarrow$ Berry phase\footnote[3]{arXiv:1808.09019 (2018)}
	\end{itemize}
	\end{block}
	% \end{itemize}
	% \end{block}
	% \footnotetext[1]{\tiny Appl. Phys. Lett. 114, 101102 (2019)}
	% \footnotetext[2]{\tiny arXiv:1907.09661}
	% \begin{block}{Next:\\\vspace*{+15pt}Two polarisation-entangled sources for secure~synchronisation}
	% \begin{itemize}
	% 	\item Bell test authenticates pairs
	% 	\item Open problem: Detection of asymmetric delay attacks 
	% 	% \item Sufficient brightness for tracking time-dependent offsets
	% % 	\item Improve pair rates to track frequency drifts between clocks \\
	% % 		\item \textcolor{gray}{> 100 pairs / sec for clocks with \\frequency stability of  < $10^{-8}$ over 100 ms.}
	% \end{itemize}
	% \end{block}
	\note{
	In conclusion, we have demonstrated a clock synchronization protocol with correlated photon pairs.

	THe protocol is robust against changes in clock separation,
	we can resolve the intrinsic instabilities of independent Rb clocks with relatively modest rates of 200 pairs per sec.
	The performance can still be improved by an order of magnitude with faster detectors.

	% The protocol is suitable for mobile parties as it does not rely on knowing the physical separation between clocks beforehand.

	% Using widely available Si APDs, and modest pair rates (200/s) we synchronized two independent Rb clocks to a precision where we resolved their intrinsic frequency instabilities.

	Security can be improved with entangled photons.
	Counterfeiting of the quantum states of the photons the the synchronization channel are no allowed by the no-cloning theorem.

	% With entangled photons, signal verification is possible by monitoring unexpected changes in the distributed entanglement. 

	% Although this technique is suitable for detecting counterfeiting attempts,

	However, attacks that do not disturb the polarization states, such as those with the circulators presented, cannot be detected with th present protocol,
	and is the subject of ongoing research.

	% in its present form the protocol is unsuitable for attacks that do not disturb polarization entanglement,
	% such as with the circulators presented beforehand. 
	% optical elements, such as the circulator that do not affect the distributed entanglement can be used to hack the system. 
	% For the unlikely scenario where an advasary is capable of deploying a circulator based attack between two mobile parties,

	Detecting such an attack remains an ongoing research. 
	% However, this form of attack is more easily achieved in fibre networks than in free-space platforms where this protocol is more likely to be adopted.
	% In conclusion, we have demonstrated a symmetric clock synchronization procedure with two SPDC sources where we have measured a clock error and found it to be independent of separation.

	% With modest pair rates of about 200 per second, 
	% we also achieved a precision that allowed us to resolve fluctuations arising from the intrinsic frequency instability of individual Rb clocks.
	% The precision is expected to improve by an order of magnitude using superconducting nanowire detectors.
	% It is secure as far as symmetric attacks are considered.

	% In the next iteration, we would like to implement the same scheme with two entangled photon sources so that the origin of the pairs can be authenticated.

	% It remains an open problem as to whether the presence of the optical circulators can be detected via polarization correlation measurements, or via some other methods.

	Thank you.
	% The trustworthiness of the clock error that is being estimated 
	}
\end{frame}