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Quantum Communication

With 1 photon: Q cryptography

With 2 photons: Q crypto, Bell tests, qutrits, plasmons

With 3 photons: Q teleportation

With 4 photons: entanglement swapping

News from the industry forehead

Nicolas Gisin Hugo Zbinden, Ivan Marcikic, Hugues de Riedmatten,

Sylvain Fasel, Jeroen van Houwelingen, Rob Thew

Group of Applied Physics, University of Geneva

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Q communication in optical fibres

The transmission depends on the wavelength- Lower attenuation : 1310 nm (0.3 dB/km) and 1550 nm

(0.2dB/km) (telecom wavelengths)

Two problems : Losses and decoherence. How to minimize them ?

Time-bin coding with

Decoherence due to birefringence : Polarization Mode Dispersion

photons at telecom wavelength

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Time-bin qubits

qubit :

any qubit state can be created

and measured in any basis

variable coupler variable coupler

1 0

h

Alice Bob

D0

D1

switchswitch

1

0

10 10 iecc

0

1

2

10

2

10

2

1i0

2

1i0

10 10 iecc

State preparation Projective measurement

W. Tittel & G. Weihs, Quant. Inf. Comput. 1, Number 2, 3 (2001)

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BA

i

BAecc 1100

10

B

0A

01

B

1A

Detectors &Coincidence

Time-bin entanglement

PDC

Laser Variable coupler

Robust against decoherence

in optical fibers

0 20 40 60 80 1000

50

100

150

200

250

Noise level

coin

cide

nces

[/1

0s]

phase[arb unit]

After 2x2km of optical fibers

2 km optical fibers

V net =96% Photon pair creation in a non-linear

crystal

Parametric down-conversion (PDC)

Energy and momentum conservationispisp kkk

p=710nms=1310nm

i=1550nm

R. Thew et al., Phys. Rev. A 66, 062304 (2002)

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1-photon: Q cryptoAlice Bob

*2-

31 km

Siftedkey rate

OpticalQBER

Accident.QBER

DetectorQBER

Dispers.QBER

TotalQBER

Compensation 23 Hz 5.5 % 1 % 4 % 0 % 10.5 %Interf. Filter 11 Hz 4 % 1 % 1.7 % 0.5 % 7.2 %

Results: (PRA 63,012309, 2001 and S. Fasel et al., EJPD 30, 143, 2004)

CDCIF

Asynchronous heraldedsingle-photon source

TDC-like measurement (normalized)

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

-5 -4 -3 -2 -1 0 1 2 3 4 5

bin

Time between a click at detector A and a click at detector B [# trigger signals]

Num

ber

of e

vent

s ( n

orm

ailz

ed)

P(1)=60% P(2)=0.02% g(2)(0)=0.0012 34 KHz(quant-ph/0408136)

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A -1

A 1

B -1

B 1

2 5 k m S O F 2 5 k m D S F

2-photons: Bell test over 50 km2-photons: Bell test over 50 km

BobAlice

Bp00

Bp11

Bp10

Bp01

Time arrival on A1

Ap00

Ap11

Ap10

Ap01

Time arrival on B1

0 = short path

1 = long path

BABA

11002

1

Type I NLCCreating photons @ 1.3 & 1.55 m

deterministic sepation with WDM coupler

)cos(1),( ijVPij

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Idea: verify from time to time the phase

10800 10820 10840 10860 10880 10900 10920 10940

0

1

2

3

4

5

Stabilisation period~ 5 [s]

Measuring period~ 100 [s]

Inte

nsi

ty [

arb

. u

nits

]

Time [s]

Stabilisation of the interferometersStabilisation of the interferometers

APD

WDM

BS

FM

FM

EPRSource

PIN

Stabilised laser

Feedback

Phase controlerSwitch

Every 100 s the phase is brought back to a given value

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Bell test over 50 kmBell test over 50 km

With phase control we can choose four different settings = 0° or 90° and = -45° or 45°

Violation of Bell inequalities:)45,90()45,0()45,90()45,0( EEEES

0.0 0.2 0.4 0.6 0.8 1.0 1.2-1.0

-0.8

-0.6

-0.4

-0.2

0.0

0.2

0.4

0.6

0.8

1.0

Co

rre

latio

n F

un

ctio

n

Time [h]

006.0518.0)45,0( E

0.0 0.2 0.4 0.6 0.8 1.0 1.2-1.0

-0.8

-0.6

-0.4

-0.2

0.0

0.2

0.4

0.6

0.8

1.0

Co

rre

latio

n F

un

ctio

n

Time [h]

005.0554.0)45,90( E

0.0 0.2 0.4 0.6 0.8 1.0 1.2-1.0

-0.8

-0.6

-0.4

-0.2

0.0

0.2

0.4

0.6

0.8

1.0

Cor

rela

tion

Fun

ctio

n

Time [h]

006.0533.0)45,0( E

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4-1.0

-0.8

-0.6

-0.4

-0.2

0.0

0.2

0.4

0.6

0.8

1.0

Co

rre

latio

n F

un

ctio

n

Time [h]

007.0581.0)45,90( E

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5-1.0

-0.8

-0.6

-0.4

-0.2

0.0

0.2

0.4

0.6

0.8

1.0

Co

rre

latio

n F

un

ctio

n

Time [h]

012.0185.2 SViolation of Bell inequalities

by more than 15

Marcikic et al., PRL, in press, quant-ph/0404124

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2-photons: Qutrit Entanglement

llecmmecssc llmm ii )(2

)(10

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Bell Violation with qutrits

I = 2.784 +/- 0.023

I(lhv) = 2 < I(2) = 2.829 < I(3) = 2.872

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2-photons: Plasmon assisted entanglement transfer

In collaboration with Prof. Erni, Zürich

SS+LL

SLLS

difference of detection tim e

eve

nts

TAC

150 m

G old 200nm th ick

700nm

300nm

Co

inci

de

nce

s

Phase Phase

phase

Reference fringesVisib ility : 90%

Plasm on assistedVisib ility : 90%

Co

inci

de

nce

s

a short lived phenomenon like a plasmon can be coherently excited at two times that differ by much more than its lifetime. At a macroscopic level this would lead to a “Schrödinger cat” living at two epochs that differ by much more than a cat’s lifetime.

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3 photons: Q teleportation & Q relays

EPR

Bell

2 bitsU

0.80

0.82

0.84

0.86

0.88

0.90

0.92

0.94

0.96

0.98

1.00

0 50 100 150 200 250 300 350

Distance [km]

Fid

eli

ty

n=1 n=2 n=3 n=4

Alice Charlie

EPR source

Bob

BSM

Classical channel

2 km 2 km 2 km

J. D. Franson et al, PRA 66,052307,2002Collins et al.,quant-ph/0311101

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Q repeaters & relays

*entanglement

*entanglement

entanglement

Bellmeasurement

. .

QND measurement+ Q memory

*entanglement

*entanglement

Bellmeasurement

.??

RE

PE

AT

ER

R

EL

AY

H. Briegel, W. Dür, J. I. Cirac and P. Zoller, Phys. Rev. Lett. 81, 5932 (1998)

J. D. Franson et al, PRA 66,052307,2002; D. Collins et al., quant-ph/0311101

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2 km of optical fiber 2 km of optical fiber

AliceAlice:creation of qubits to be teleportedAliceAlice:creation of qubits to be teleported

AliceAlice

55 m

BobBob:analysis of the teleported qubit, 55 m from Charlie

BobBob:analysis of the teleported qubit, 55 m from Charlie

BobBob

CharlieCharlie:the Bell measurementCharlieCharlie:the Bell measurement

CharlieCharliefs laser @ 710 nmfs laser @ 710 nm

Experimental setupExperimental setup

creation of entangled qubitscreation of entangled qubits

coincidence electronicscoincidence electronics

&

LBO

RG

WDM

RG

WDM

GeInGaAs

LBO

BS

InGaAs

fs l

aser

sync out

1.3

m 1.3m

1.5 m

1.5 m

2km 2km

2km

H. de Riedmaten et al.,PRL 92, 047904-1/4, 2004

I. Marcikic et al., Nature, 421, 509-513, 2003

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resultsresults

0 2 4 6 8 10 12 14 16 18

0

5

10

15

20

25

30

35

40

0 2 4 6 8 10 12 14 16 18

0

5

10

15

20

25

30

35

40

fou

r-fo

ld c

oin

cid

ence

s [1

/50

0s]

Phase [arb. units]

0

1000

2000

3000

4000

5000

6000

7000

8000

9000

Th

ree-

fold

co

inci

den

ce [

/500

s]

fou

r-fo

ld c

oin

cid

ence

s [1

/50

0s]

Phase [arb. units]

0

1000

2000

3000

4000

5000

6000

7000

8000

9000

Th

ree-

fold

co

inci

den

ce [

/500

s]

Equatorial statesEquatorial states

Raw visibility : VRaw visibility : Vrawraw= 55 = 55 ±± 5 %5 %

2V1F raweq = 77.5 = 77.5 ±± 2.5 % 2.5 %

1

0

1,0F

0,1F

= = 78 78 ± ± 3%3%

= = 77 77 ± ± 3%3%

North & south polesNorth & south poles

wrongcorrect

correct

CC

CF

mean fidelity: mean fidelity: FFpolespoles=77.5 =77.5 ±± 3 % 3 %

0 1 2 3 4 5 60.0

0.2

0.4

0.6

0.8

1.0

coin

cid

ence

[arb

un

it]

time between start and stop [ns]

3

1

3

2

peqmean FFF

77.5 ±2.5 %

Mean FidelityMean Fidelity

» 67 % (no entanglement)

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2 km of optical fiber 2 km of optical fiber

AliceAlice:creation of qubits to be teleportedAliceAlice:creation of qubits to be teleported

AliceAlice

55 m

BobBob:analysis of the teleported qubit, 55 m from Charlie

BobBob:analysis of the teleported qubit, 55 m from Charlie

BobBob

CharlieCharlie:the Bell measurementCharlieCharlie:the Bell measurement

CharlieCharliefs laser @ 710 nmfs laser @ 710 nm

Experimental setupExperimental setup

creation of entangled qubitscreation of entangled qubits

coincidence electronicscoincidence electronics

&

LBO

RG

WDM

RG

WDM

GeInGaAs

LBO

BS

InGaAs

fs l

aser

sync out

1.3

m 1.3m

1.5 m

1.5 m

2km 2km

2km

H. de Riedmaten et al.,PRL 92, 047904-1/4, 2004

On the same spool !

See Halder et alquant-ph/0408092

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A B C D

4-photons: Entanglement swapping

Bell state measurement

Entangled photons that never interacted

EPR source EPR source

BAi

BA e 1,10,0 )()( AB DC

iDC e 1,10,0 )()(

CD

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Sources of time-bin entangled photons

BAi

BAABe 1,10,0

2

1)(

BAi

BACDe 1,10,0

2

1)(

The experiment

Bell state measurement

CBCBBC0,11,0

2

1)(

CBCBi

AD)( 0,11,0e

21

Entanglement analysis

A lice B o b

Actively stabilized interferometers

1km1km

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In the experiment :Partial Bell state measurement

Entanglement swapping

)()( )()( CDABABCD

AD

i

BC

AD

i

BC

ADBC

ADBC

e

e

)()(

)()(

)()(

)()(

)2(

)2(

Four Bell states involved

in the experiment !

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Superposition basis: results

V = (80 ± 4) %F 90 %

78 hours of measurement !

100 200 300 400 500 6000

10

20

30

40

50

60

70

80

90

100

Four

-pho

ton

coin

cide

nces

[/6

h]

Phase [degrees]

0

1000

2000

3000

4000

5000

3-p

hoto

n co

inci

denc

es [

/6h]

with

out B

SM

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Results: computational basis

0

10

20

30

40

50

60

70

80

1 2 3 4

Fou

r ph

oton

coi

ncid

ence

s [

/ hou

r]

DA 0,0

DA 1,0DA 0,1

DA 1,1

%5.31.91

Total

CorrectMean Fidelity =

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News from the industry forehead

http://www.swissquantum.deckpoint.ch/embargo/index_en.php

Yesterday, September 29, id Quantique, DeckPoint and the University of Geneva officially inaugurated the first data archive site secured with Quantum Cryptography.

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Data archiving network secured byQuantum Crypto

10 km

http://www.swissquantum.deckpoint.ch/embargo/index_en.php

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Quantis: Quantum random numbers on demand (www.randomnumbers.info)

4 Mbit/s per module, up to 4 modules on one PC card

1 light source1 beam splitter2 photon countersin a few cm3

(4x5x1 cm3) !!!

www. idquantique.com

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Few « qubit » ApplicationsPhoton-counting OTDR

Coherent Q measurement of the degree of polarization:

0 20 40 60 80 100 120

0.00.10.20.30.40.50.60.70.80.91.0

symbole plein: polarimètre symbole vide: projection sur (-)

DOP=1 DOP=0.74 DOP=0.34

DO

P

temps(s)

J.Lightwave Tech, 2, 390, 2004

PRL 91, 167902, 2003

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Conclusions

Where are the applications?

Next September 29, id Quantique, DeskPoint and the University of Geneva will officially inaugurate the

first data archive site secured with Quantum Cryptography.

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Single Photon Sources

919 nm / 0.7 nm

650 nm/ >50 nm

1550 nm / 7nm

/

Quantum Dot

NVHigh pump

Low pump

271.40.12g(2)(0) [%]

0.040.0020.30.02P2 [%]

8.32.26060P1 [%]

76 MHz5.3 MHz803 kHz34 kHzNT

Beveratos et. al., PRL 89, 187901 (2002)Vuckovic et. al., APL 82, 3596 (2003)

Fasel et. al. in preparation 21

2)2(

P

P2)0(g

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Size of the classical communication

One proton in one cm3 at a temperature of 300 K:

][017.12m

pkT 10

2

mp

h

233

019.1)dim(

L

Hd

155)(ln 22 d bits

1020 protons in one cm3 at a temperature of 300 K 1020 x 155 1022 bits

To be compared to today’s optical fiber communication in labs:

1 Tbyte x 1024 WDW channels x 1000 fibers 1019 bits/sec.

1 hour !!

bits

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entangled time-bin qubit

variable coupler

non-linearcrystal B

sA

sl

B

lA

AAi

BA 11e00

depending on coupling ratio and phase , maximally and non-maximally entangled states can be createdR. Thew et al, PRA 66, 062304, 2002

Extensions to entanglement in higher dimensions:- qutrits: R. Thew et al, quant-ph/0307122- up to dimesnion 20: H. Deriedmatten et al, quant-ph/0309058 5 10 15 20

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

Vis

ibil

ity

Dimension d

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Bell state measurementBell state measurement

BABABBAAii 0110101010

21

BABABBAAii 0110101001

21

)1010(01102121 BBAA

i BABA

011001102121

H. Weinfurter, Europhysics Letters 25, 559-564 (1994)H. de Riedmatten et al., Phys. Rev. A 67, 022301 (2003)

1 2

A B