Experimental Non Locality Proof of Quantum Teleportation and Entanglement Swapping

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Experimental Nonlocality Proof of Quantum Teleportation andEntanglement Swapping

Thomas Jennewein, Gregor Weihs,Jian-Wei Pan, Anton Zeilinger

PRL 88, 017903-2(2002)

This work was supported by the AustrianScience Fund (FWF) and the “QuComm”IST-FET project of the EuropeanCommission.



Entangled Photons

n Not factorizablen Strong correlations between the systemsn Individual systems have no properties, only joint properties

( )

( )

( )2121

2121

2121

2

1

21

2

1

+---+=

=-=

=-=

R LLR

H V V H y One of fourBell-states

-



Correlation Measurement

Correlation: E( a , b ) = P ++ + P -- - P + - - P - + = - cos(2 a -2 b ) always perfect anticorrelations for parallel Analyzers ( a = b).

( ) ( ) ( )b a b a b a -== --++2sin

2

1,, P P

( )H V V H -=2

1y

Source

-

a b



Bell’s InequalityBased on EPR: Locality and Realism

Locality: Particles act independtly in measurementRealism: Carried information ( l ) defines result

CHSH-Inequality:

( ) ( ) 2,,),(),( £¢¢+¢+¢- b a b a b a b a E E E E

QM: E( a , b ) = - cos(2 a -2 b )

222 £ /



Source of Entangled Photonsn Spontaneous Parametric Down-conversion

n non-linear optical process ( c2)n one UV-photon decays into two new photons

obeying energy and momentum conservation

extra ordinary(vertical)

ordinary(horizontal)

BBO-Crystal

UV-

Pump

+| Ò | ÒH VA A| Ò | ÒV HB B

AB



The Bi-Photon Staten The full state

n This state resembles the |Y-Òfor a narrowpump linewidth

n Contrast is reduced for finite pumplinewidth



Quantum State Teleportationn Quantum state is transferred between different

particles, as long as no information about thestate is found out.

n Classically impossible due to:n Heisenberg Uncertaintyn No-cloning Theorem

n Basic ingreedient for quantum repeater,quantum networks, quantum computers.



Teleportation Scheme1993 PRL, Bennet et al. :Teleporting an unknownquantum state via dualclassical and EPR channels.

Input State



Entanglement Swappingn The most general test of QST

n The input state is undefinedin a fundamental way

n Entanglement survivesperfectly if no information isgained

ALICE

VICTOR

Polarization Analysis

Bell-State Analyzer

EntangledPhotonSource

EntangledPhotonSource

BOB

1 2 30

Y-F+

Y-Y+F-Y-

III

Polarization Analysis



Previous Experimentsn Bouwmeester et al, Nature (1997)

n Teleportation of a well defined input staten

Observed visibility: ~ 70% ± 3%n Pan et al., PRL (1998)

n Entanglement swappingn Observed visibility: ~ 65 % ± 2%

n Violation of Bell‘s inequality requires atleast 71 %!



Experimental Setupn Pulsed

downconversion fortwo pairs

n UV Pulses: 200 fs,76 MHZ, 394 nm

n 1 event in 100sn Single mode fibersn Correlation:



Active Beam Stabilizationn CCD measures beam positionn two motorized mirrors steer the beam

0 3 6 9 12 150,0

0,10,2

0,3

0,4

0,5

0,6

0,7

0,8

0,9

1,0

1,1

without laser controlwith laser control

n o r m

a l i z e

d c a

l c u

l a t e d 4 - f o

l d r a

t e

Time [h]

Stability Comparison:

Stability sufficient for the required times (16000 s per point)!



Coincidence Logicn NIM-Logic n Programmable

Logicn VHDLn Reprogrammable



High Visibility Teleportation

n Visibility tests

n All effects limiting the visibilityconsideredn Theory:

@45° Vis=0,817@0° Vis=0,897

n Experiment:@45° Vis=0,812 ± 0,103@0° Vis=0,883 ± 0,030



Test of Bell’s Inequalityn CHSH version of Bell‘s inequality:

n S = |E(0°,22.5°) - E(0°,67.5°)| +|E(45°,22.5°) + E(45°,67.5°)| £ 2

n Experimental test of the CHSH-inequality:n S = |-0.6281 - (+ 0.6766)| + |-0.5748 - 0.5407|

= 2.420 £ 2n DS = 0.091 -> 4.6 s above the classical limit.

n Violation of Bell‘s inequaliy for particles thatnever interacted!

(QM: S=2,82...)



Fidelity of the Teleportation

n

QM: Relative timing of the detection events doesnot influence results (A. Peres)n In the Bell-measurements: simultaneously (2ns)n Delayed-Choice : BSA measurement delayed by 50 ns!

0,0 22,5 45,0 67,5 90,00,500,550,600,650,700,750,800,850,900,951,00

F i d

e l i t y

Bell Inequality Violation Limit

Classical Limit

FidelityFidelity with Delayed Choice

Polarizer Orientationf 0=f 3 [°]

Fidelity:



Space-Time Arrangement for

the Entanglement Swappingn Time-ordering of detection events

Normal Entanglement Swapping: Delayed Choice:

x

t2 ns

20 ns

2 m (~ 6 ns)x

t

50 ns

20 ns

2 m (~ 6 ns)

Alice Bob

Alice

Bob



Independence of the Photons?n The two pairs (0-1 and 2-3)

independently violate Bell‘s inequality!n

Phase drift of BS-arms would wash outany “phase coherence”.

0 2 4 6 8 10 12-20

-15

-10

-5

0

5

10

P h a s e

[ m u

l t i p l e s o

f l ]

time [h]10 100 1000

0,1

1

s ( t ) [ m

u l t i p l e s o

f l ]

t [s]

Mach-Zehnder Interferometer

Drift: 1 l with 1 s in 400 s

-> any phase relation betweenthe photons clearly washed outwithin 16000 s!



Conclusionn Entangled Photons allow “proof of principle”

experiments in quantum information processing:n Teleportation of an entangled state shows the first violation of

Bell‘s inequality for photons that never interacted. (PRL, Vol 88,017903 (2002)).

n Additional tests adressing the timing in teleportation and theindpendence of the photons were performed.

n Outlook:n Teleportation will be enhanced with more-complete BSA and

active switching of receiver photon in a long-distanceteleportation scheme (600 m).

Experimental Non Locality Proof of Quantum Teleportation and Entanglement Swapping

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