1 A Remote Bell-State-Analyzer a.k.a. Robert´s Diploma–Project

©Robert Prevedel2 Overview Introduction Facts about Fibers Polarization Compensation Hong-Ou-Mandel Dip Dispersion Cancelation Tools for Realisation Applications of a Remote BSA

©Robert Prevedel3 Experimental BSA 1 D 2 D BS PBS Only will leave in different output modes More complete BSA distinguishable because of different polarization

©Robert Prevedel4 QM at the Beamsplitter BS term only vanishes if completely indistinguishable No Coincidences  perfect Photon-Bunching  HOM-Dip

©Robert Prevedel5 Indistinguishability Photons need to be indistinguishable in every degree of freedom: Polarization Frequency Arrival Time Spatial Mode (not relevant in fibers) Methods for achieving this: Polarization Compensation Narrow Filtering (3.2 nm) Adjusting Path Length (Hong-Ou-Mandel-Dip)

©Robert Prevedel6 Some Facts about Fibers For our 780nm Single-Mode-Fibers 4 - 4,5 dB loss per km (~3.5 dB for 800m) Dispersion 100 ps km -1 nm -1 Temperature Sensitivity Expansion-coeff. ~ 4mm K -1 km -1 Phase variation -90 rad K -1 m -1 Phase stability requires temperature stability of K External stress (bending, twisting) induces birefringence and leads to rotation of Poincaré-Sphere

©Robert Prevedel7 Setup BBO Fiber-Squeezer Pol QWP HWP LD 785nm Setup similiar to Mach-Zehnder Path-lengths equalized within coherence length (~0.4mm) Fiber-Squeezer for linear pol. QWP + HWP for circular pol. Det I Det II BS I Phase- Modulator BS II Coupler 800m fiber under Danube

©Robert Prevedel8 Polarization Compensation induce phase modulation (0.5 Hz) fiber-squeezer for rotation along degree of latitude QWP+HWP for rotation along degree of longitude parallel polarizers  max. fringes orthogonal polarizers  no fringes Poincaré-Sphere

©Robert Prevedel9 Polarization Compensation in the Lab

©Robert Prevedel10 HOM-Dip-Search Mirror BBO &-Logic BS Fiber-Squeezer Pol QWP HWP both 45°, H,... scan Delay-Mirror-Position Dip-width corresponds to size of wavepackets (~170 µm) Beamstopper 800m

©Robert Prevedel11 Dispersion Cancelation Dispersion in one arm does not lead to distinguishable detection scheme – no information about creation time of photon Only if center of wavepacket is delayed – bunching disappears Dispersion cancelation only works for photons originating from the same source (energy correlation) Active Compensation for very long fibers (>30 km) Polarization Mode Dispersion negligible for high quality fibers

©Robert Prevedel12 Tools for Realisation Measuring fiber-length-difference with Stanford SR620 (± 1 cm; maybe even more due to detector-jitter) Compensation with Free-Space-Delay Polarization Compensation via Internet and Fieldpoint HOM-Dip-Scan with Delay-Mirror Expected major difficulties: Temperature and hence length stability of fiber Poor-Dip-Visibility due to Polarization Drifts, etc… Low Count-Rates require even longer scans…..

©Robert Prevedel13 Applications of a Remote BSA Long Distance Entanglement Swapping Quantum Repeater Delayed Choice Entanglement Swapping Teleportation “Reversed”

©Robert Prevedel14 The End…

©Robert Prevedel15 2x25km MZ-Interferometer BS ΔΦ calculation for 2 gaussian-wave-packets fringes caused by 1-photon-interference dotted line represents average over one measurement LD

©Robert Prevedel16