http://www.quantumlah.org

DSTAA*STAR

Quantum Cryptography

EQCSPOT, QuComm (EU)

http://xqp.physik.uni−muenchen.de

VDI / BMBF, DFG$$:

Christian Kurtsiefer

Quantum Information and QuantumControl Conference, Toronto July 2004

Tools for Experimental

udwig− aximiliansniversität München

L MU

LMULMU

Overview

a free space implementation of BB84

a relatively simple single photon source

tools for implementing the Ekert protocol

simple setup!

try to bridge urban area sites without laying out dedicated fibers

dream about key exchange with satellites

Why Free Space Cryptography?

( )Systems !Fiber−based

small & compact setup

Previous Art

BB84 protocol with polarisation encoding

faint pulse source

C. Bennet, IBM 1992

J. Franson, Baltimore

R. Hughes, Los Alamos

J. Rarity & friends, Qinetiq 2001

~100m

~30 cm

(0.1 photons / pulse)

1.9 km

~ km, now: 10 km

A free−space QKD implementation

bridge a useful distance

Technical Challenges

preparation of single photons

detection of single photons

transport

no amplification possible low losses

random numbershigh rate, high entropy

background suppression

synchronisation of Alice / Bob

p(0)p(1)p(n>1)

= 90.48 %= 9.05 %= 0.47 %

<n>=0.1:λn

n!e

−λp(n) =

laserdiode

weakcoherentpulse

potentially insecure lower signal bandwidth

��

Weak coherent Pulses

+−

much simpler to prepare than "true" single photons

attenuator

coherent pulses instead of single photons

laser emit coherent state light fields

t

I

t

τ

1/r

Timing...

τ

transmitter:

receiver:

time window for attention detector event

background suppression by narrow time windows

clock synchronisation necessary

ηdf0r = × µ × × T÷ 2

PD = d × τ10−7

10−5InGaAs:

Si:

f0PDηd

PD=×

µ × × T

×2

rQBER =

right basis

detector efficiencyphotons / pulse

primary send rate

BB84 raw key rate

probability for a background event

dark count rate open time window

detector induced bit error rate

Practical Estimations

channel transmission

C. Bennett, F. Bessette, G. Brassard, L. Savail and J. Smolin

J. Cryptology 5, 3 (1992)

Prior Art

Charlie Bennett’s Aunt Martha (1984, 1992)

Source miniaturisation

laser diodes : intrinsicpolarization better 1:1000

value basisH +45°

V

−45°

mirror

spatial filter

4 different sources instead of polarization modulators

EO modulators

Transmitter module

5 cm

J. Mod. Optics 41, 2345 (1994)

J.G. Rarity, P.C.M. Owens, P.R. Tapster,

Receiver simplification

50/50 beam splitter instead of polarization modulators

/ 2λ

H

V

−45°

+45°

PBSdet V/+45°

det H/−45°modulator

22.5°

PBS

50/50 PBS

Receiver Module

telescopemonitordetector

spatialfilter

faintpulsesource

PC

randomnumbergenerator

10 MHzclock

timestampunit

spectralfilter

PC

System Setup

polarisationanalyzer(4 APD)

receivertelescope(D=250mm)

channel ""classical

westlicheKarwendelspitze

(2244m)

Zugspitze(2950 m)

23.4 km

"Bob"

S

"Alice"

The Trial Site

(40 mm FWHM) (222 mm FWHM)23.4 km

Why on a mountain top?

optical path far above ground −− lower turbulence of air

actual beam

diffraction limit

low absorption (sometimes...)

low background light

Transmitter

relay optics spatial filter source module

relay optics

reference detektor

auxiliaryguidelaser

exit lens

Transmitter Unit 2

Receiver telescope & polarisation analyzer

0

50

100

150

200

4.54 % (2.6%)

5.05 % (2.3%)

5.38 % (2.6%)

τ = 1.4 ns

<n>

#1

#2

#3

0.18

0.096

0.081

−1

−1

−1

−1

−1

−1

5578 s

4510 s

4360 s

1490 s

1365 s

1162 s

QBER

Experimental Results

polarisation transfer from Alice to Bob:

23.4 km through air

detection time window

700 ms

H+45

V−45

V+45

H

−45

events in

source diode detector

run background (same base)raw key rate

(from background)

4 cm 1.5..2m

T = 85% T = 50% T= 76.7%T = 1%

~1000...1500 cps

ηT = 92%

= 45%

10 MHzrepetition rate

per pulse0.1 photons

observed:

−0.7 dB −20 dB −3 dB −1.15 dB −3.8 dB

Ø 25 cm

Loss budget

pulsedsource

telescope 1 free propagation telescope 2 spectralfilter

detektor

s−1

~106 s−1

30−40 dB

~500 ps

~50−50k

20 − 100 Mbit/s

Current Technological Limits

timing jitter

dark count rate

repetition rate

transmission of thr optical channel

random number source

photodetectors

Urban Area Link

source

pinhole

photodidoe

21 22 23 0 1 2 3 4 5 6 7 8 9 10 11

0.25

0.5

0.75

1

1.25

1.5

1.75

Theresienstraße/Amalienstraße (ca. 500 m)

min

max

22.8.02 23.8.02

time of day [MESZ]

pow

er @

rec

eive

r

within 10 s (100 µs sample interval)

power fluctuations

Transmission Fluctuations in an Urban Link

Effects of turbulence

0 ms 100 ms 200 ms

300 ms 400 ms

d ~ 9 kmdowntown Munich

intensity distribution vs. time

10−20m over ground

25 cm

)(

Urban Receiver Setup

detectorset

mechanicaldrift adjusters

spatial filter& relay optics

80mm receive telescope

alignment laser

2

3 4

5

7

78

5

7

6

8

1 7

5

7clock

sifting

privacy amplification

sifting

privacy amplification

8: DoS

soft / algo

3: side channels

host

Bob

host

Alic

e

Practical attack schemes

generatorrandom photon

sourcephoto−detector

timestampunit

memoryclock

raw key

error correction

final key

raw key

error correction

final key

physical electrical

1: bad random numbers / backdoor2: no single photons

4: optical intercept of detectors

5: vagabonding raw key6: too optimistic assumptions on

eavesdroppers’ knowledge7: electrical eavesdropping

842 844 846 848 850

5

10

15

20

25

30

H −45°+45° V

spec

tral

pow

er [c

ps]

Spectral Indistinguishability of Laser Diodes

pulsed laser diodes

resolution

free running laser diodes indistinguishable through their wavelength

emission wavelength [nm]

+

−

brI

when an APD undergoes an avalanche,light is emitted:

possible eavesdropping attack:

single photon

Eve

Alice Bob

spectral distribution of breakdown lightof Si APD

600 800 1000 12000

0.2

0.4

0.6

0.8

1.

APD breakdown flash − an eavesdropping backdoor?

measured emission:~40 photons /sr for Si APD in usualoperation mode

no problem with spectral & spatialfiltering

inte

nsity

wavelength [nm]

A typical DoS attack....

Overview

a free space implementation of BB84

tools for implementing the Ekert protocol

a relatively simple single photon source

NV centers in diamond

276A. Grubner et al., Science , 2012 (1997)

...exhibit radiative decay probability ~1

...similar to atoms

...are stable

Fluorescence from color centers in solids

Use these centers to create single photons:

2.) Emission1.) Excitation

37 38 39 40 41 42 43 44

1000

2000

3000

4000

36 37 38 39 40 41 42 43 442

3

4

5

6

7

8

9

10

532 nmlaser

dichroicmirror

single modeoptical fiber

FWHM = 548 nm

y / µ

m

x / µm

diamond

objecive color filter

to photodetector

PZT−table

Confocal Microscope

NV−center

Experimental Setup

-50 0 50 100

0.5

1.

1.5

0.5

1.

1.5

2.

1

1.5

2.

t2

t1

∆τ = 60 ns

model:

TDC

50/50

D2

D1

Observation of the photon statistics

frommicroscope

beam spitter g(2)

mg(2

)m

g(2)

m

1

23

Intensity correlation function

P = 0.5 mW

P = 10 mW

P = 2 mW

/ nsτ2

−τ / τ2= 1 + c e(τ)g(2) + c e−τ / τ

33

Overview

a free space implementation of BB84

a relatively simple single photon source

tools for implementing the Ekert protocol

/ 2λ

H

V

−45°

+45°

/ 2 λ

H

V

−45°

+45°

22.5°22.5°

PBS

50/50 PBS

PBS

50/50PBS

−45°+45°

Polar.

HV

11

00b2 e

10101

010

b1 w

11

00

−45°+45°

Polar.

HV

Ψ−source for

for every detection:

additional eavesdropping tests (check Bell inequalities)

no external random number source!

Alice: Bob:

BobAlice

continue like in BB84

EPR / Ekert Protocoll

axisoptical

uniaxialnonlinearopticalcrystal

ΘP

θ s,i

s,iφ

pump beam

idler

signal

iζe12

= (Ψ HV + VH )

P. Kwiat et al., Phys. Rev. Lett. 75 , 4337 (1995).

direction 1 direction 2

Type−II Parametric Down Conversion

polarisation−entangled photon pairs along directions 1 & 2indistinguishability of photons leads to a

energy & momentum conservation:2 cones

Experimental Setup I

/ 2λ

/ 2λ

/ 2λ

/ 2λ

w = 82 µm FWHM

wp

A ∆θ

BBO

PBS

PBS

351 nm

C

CUV light

arm 1

arm 2

polarisationcontrol

detector 1

detector 2

targeted bandwidth: 4 nm FWHM no interference filter

acceptance angle: 0.22 deg FWHM

conversion diameter in the crystal:

0 100 200 300 4000

50

100

150

200

250

300

350

0

500

1000

1500

310

coin

cide

nces

per

sec

310

sing

le c

ount

s pe

r se

c

cτ = 6.8 ns)(

Count Rates vs. Pump Power

UV pump power (mW)

coincidence count rate: from a 2 mm thick BBO crystal

coincidence /single ratio: (Si APD, actively quenchend)

thanks to N. Gisin & friends

360 800 cpsIdentifying correlated photon pairs

28.6 %

( 85 )σ

coincidence rate:10 000 cps

J. Volz, Ch. Kurtsiefer, H. WeinfurterAppl. Phys. Lett. 79, 869 (2001)

S = 2.63±0.0074

Diode Laser Based Photon Pair Source

high entanglement:

in 5 sec integration timeper point

(banks, gov orgs, cell phonebase stations?)

experimentally implementnew entanglement−basedQKD protocols(with D. Kaszlikowski, B.G. Englert, LC. Kwek et al.)

improve singlet pair sources

establish a free−space link

understand / compensateatmospheric transmissionfluctuations

Experimental Activities in Singapore.....

simple single photon sources still subject of research

operation under ambient light conditions

Summary

Next steps

think about satellite links?

urban area quantum key distribution

BB84−type quantum key distribution systems became technology

Ekert protocol QKD systems / pair sources under development

implement new protocols

collaborationsMunich group

Pho

tons

Sin

gle

Paul R. Tapster

Phil M. Gorman

John G. RarityQin

etiq

Jürgen Volz

Christian Schmidt

Markus Oberparleiter

Pavel Trojek

Oliver Schulz

Ruprecht SteinhüblNew

Pro

toco

lsE

ntan

gled

pai

rs

Artur Ekert

Oh Choo Hiap

Singapore group

Matthäus Halder

Patrick Zarda

Henning Weier

Tobias Schmitt−Manderbach

Sonja Mayer

Chunlang Wang

Fre

e S

pace

Opt

ics

Foo Pei Yih

Darwin Gosal

Tey Meng Khoon

Alexander Ling

Ivan Marcikic

Antia Lamas−Linares

Ch. K.

Ajay Gopinathan

Dagomir Kaszlikowski

B.G. Englert

Kwek Leong ChuanExp

erim

enta

l

The

ory

People here & there

Christian Kurtsiefer Harald Weinfurter