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