1

A Remote Bell-State-Analyzer

a.k.a. Robert´s Diploma–Project

22.10.2004 ©Robert Prevedel 2

Overview

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

22.10.2004 ©Robert Prevedel 3

Experimental BSA

1VD

2VD

1HD

2HD

1D

2D

BS

BS

PBS

PBS

Only will leave in different output modes

More complete BSA

distinguishable because of different polarization

22.10.2004 ©Robert Prevedel 4

QM at the Beamsplitter

BS

term only vanishes if completely indistinguishable

No Coincidences perfect Photon-Bunching HOM-Dip

22.10.2004 ©Robert Prevedel 5

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)

22.10.2004 ©Robert Prevedel 6

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 10-5 K

• External stress (bending, twisting) induces birefringence and leads to rotation of Poincaré-Sphere

22.10.2004 ©Robert Prevedel 7

Setup

BBO

Fiber-Squeezer

Pol

QWP

HWP

LD785nm • Setup similiar to Mach-Zehnder

• Path-lengths equalized within coherence length (~0.4mm)

• Fiber-Squeezer for linear pol.

• QWP + HWP for circular pol.

De

t I

De

t II

BS I

Phase-Modulator

BS II

Coupler

800m fiber under Danube

22.10.2004 ©Robert Prevedel 8

Polarization Compensation

H

V

2

VH

2

VH

2

ViH

2

ViH

• 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

22.10.2004 ©Robert Prevedel 9

Polarization Compensation in the Lab

22.10.2004 ©Robert Prevedel 10

HOM-Dip-Search

MirrorBBO

&-Logic

BSFiber-Squeezer

Pol

QWP

HWP

• both Polarizers @ 45°, H, ...

• scan Delay-Mirror-Position

• Dip-width corresponds to size of wavepackets (~170 µm)

Beamstopper

20,3 20,4 20,5 20,6 20,7 20,8 20,9 21,0

1500

2000

2500

3000

3500

4000

FWHM=170 µm

HOM-Dip-Search 11.10.2004

Data: MCLOCALSTABLE_NModel: Gauss Chi^2/DoF = 9872.00506R^2 = 0.97685 y0 3572.31363 ±20.66431xc 20.56451 ±0.00135w 0.08396 ±0.00297A -206.86844 ±7.37063

Cou

nts

2-fo

lds

per

sec.

Delay-Mirror-Position [mm]

800m

22.10.2004 ©Robert Prevedel 11

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

22.10.2004 ©Robert Prevedel 12

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…..

22.10.2004 ©Robert Prevedel 13

Applications of a Remote BSA

Long Distance Entanglement Swapping Quantum Repeater Delayed Choice Entanglement Swapping Teleportation “Reversed”

22.10.2004 ©Robert Prevedel 14

The End…