Ilja Gerhardt

QUANTUM OPTICSCQT

GROUP

Ilja Gerhardt, Matthew P. Peloso, Caleb Ho, Antía Lamas-Linares and Christian Kurtsiefer

Entanglement-based Free Space Quantum Cryptography in Full Daylight

QUANTUM OPTICSCQT

GROUP

Centre fo rQu ant umTechn o lo g ies , Sing a p o re

Ilja Gerhardt

QUANTUM OPTICSCQT

GROUP

Introduction to QKDIntroduction to QKD

• Quantum Cryptography =Quantum Key Distribution–Have a Random Number at two

locationsOne-time pad for encryption

• 1980ies: Prepare and Send–Bennet Brassard, BB84–Send single Photons, non-cloning

theorem

• 1990ies: Entanglement Based… why?

Ilja Gerhardt

QUANTUM OPTICSCQT

GROUP

Prepare and SendPrepare and Send

• Need for Random Numbers

• Different Photons, different Colors?–Dimensionality of Hilbert space needs to

be known for security

Ilja Gerhardt

QUANTUM OPTICSCQT

GROUP

Entanglement based QKDEntanglement based QKD

• Ekert 1991 (E91)–Let’s measure two qubits of an

entangled entity–Perfect (Anti)correlation–Bell’s inequality violated= eavesdropper knowledge can be

eliminated via PA

• Photons for convenience…• Freespace: Ad-hoc set-up

Ilja Gerhardt

QUANTUM OPTICSCQT

GROUP

Entanglement based QKDEntanglement based QKD

• Pairsource: BBO, lean & compact–Strong temporally correlated –Broader than dimmed lasers

19’’10’’

Ilja Gerhardt

QUANTUM OPTICSCQT

GROUP

Detection of PhotonsDetection of Photons

50:50PBS

PBS

APD, +45°

l/222.5

°

V

-45°H

• Detection: Polarization analyzer

J.G. Rarity et al., J. Mod. Opt. 41, 2345 (1994)

Ilja Gerhardt

QUANTUM OPTICSCQT

GROUP

SetupSetup

P B S

R b

H WB S P B S

T U

P C

B o b

telesco peSending

telesco peR eceiving

R bT U

P C

PA

C CB BO

W P

350 m

350 m

C lassical C hannel

A lice

M. Peloso et al., New J. Phys. 11 045007 (2009)

Ilja Gerhardt

QUANTUM OPTICSCQT

GROUP

Problems in DaylightProblems in Daylight

• Destruction of detectors• Non-linearity of the detector

–Saturation

• Accidental clicks–Confused with real coincidence events

(QBER)

• Fluctuations of ~30dB (day/night)

0 2 4 6 8 100.0

0.5

1.0

1.5

2.0

2.5

Ideal Counts [10 cps]6

De

tect

ed C

oun

ts [1

0cp

s]6

M. Peloso et al., New J. Phys. 11 045007 (2009)

Ilja Gerhardt

QUANTUM OPTICSCQT

GROUP

Expected Rates at daylightExpected Rates at daylight

M. Peloso et al., New J. Phys. 11 045007 (2009)

Ilja Gerhardt

QUANTUM OPTICSCQT

GROUP

Sun & SolutionsSun & Solutions

blacko utco ating

fo cusing

sh ield

fo cusing

inter ferencefilter

bafflespinho le

to detecto r s

sun

- 3-4 dB-12 dB100µradFOV=10cm

Dt=1ns

M. Peloso et al., New J. Phys. 11 045007 (2009)

6.7nm

Ilja Gerhardt

QUANTUM OPTICSCQT

GROUP

SetupSetup

Alice

BobM. Peloso et al., New J. Phys. 11 045007 (2009)

Ilja Gerhardt

QUANTUM OPTICSCQT

GROUP

Measured rates at daylightMeasured rates at daylight

M. Peloso et al., New J. Phys. 11 045007 (2009)

• Sync possible at daylight

Ilja Gerhardt

QUANTUM OPTICSCQT

GROUP

Experimental resultsExperimental results

coincidences(cps)

accidentalcoincidencerate ra (cps)

QBER (%)

raw key rate(s-1) key rate after

error correction (s-1)

total detection rateat receiver (kcps)

total 'local' detectorrate at source (kcps)

M. Peloso et al., New J. Phys. 11 045007 (2009)

Ilja Gerhardt

QUANTUM OPTICSCQT

GROUP

ConclusionsConclusions

• Entanglement based QKD in daylight, 4 days run without interruption• Synchronization works at ambient

conditions• Noise Suppression

–Spectral–Temporal–Spatial

• Brighter Sources…70 pairs/(s mW MHz)

M. Peloso et al., New J. Phys. 11 045007 (2009)

Ilja Gerhardt

QUANTUM OPTICSCQT

GROUP

http://qolah.org

Thanks for your attention