Differential Phase Shift Quantum Key Distribution Hiroki Takesue, Toshimori Honjo, Kiyoshi Tamaki, and Yasuhiro Tokura NTT Basic Research Laboratories, NTT Corporation [email protected] S8.2
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Differential Phase Shift Quantum Key Distribution
Hiroki Takesue, Toshimori Honjo, Kiyoshi Tamaki, and Yasuhiro Tokura
NTT Basic Research Laboratories, NTT Corporation
S8.2
Acknowledgements
K. Inoue (Osaka University)
Y. Yamamoto，E. Waks, E. Diamanti, Q. Zhang, K. WenM. M. Fejer， C. Langrock, R. V. Roussev(Stanford University)
S. W. Nam, R. H. Hadfield (NIST)
S. Inoue, N. Namekata, G. Fujii (Nihon University)
Collaborators
FundsNational Institute of Information and Communication (NICT)CREST, Japan Science and Technology Agency
Scientific American Oct 19, 2007.
QKD used for Geneva election
Outline
1.What is quantum key distribution (QKD) ?
2.Differential phase shift quantum key distribution (DPS-QKD) protocol
3.QKD experiments with various single photon detectors: • Up-conversion detector• Superconducting single photon detector: SSPD• Sine-wave-gated InGaAs/InP Avalanche photodiode
4.Summary
Background
Sender (Alice) Receiver (Bob)
Public line + Public cryptosystemPossible eavesdropping ?
Private line High cost, possible eavesdropping
Secret keys
QKD channel
Secret keys Photons
Public line + QKDSecure !
Security system for users who require high-level security.
Alice Bob
Eavesdropping is revealedfrom increase of errors.
Eve
Quantum key distribution (QKD)
Quantum state(0/1 encoded)
Information Change of state Error
Measurement
Outline
1.What is quantum key distribution (QKD) ?
2.Differential phase shift quantum key distribution (DPS-QKD) protocol
3.QKD experiments with various single photon detectors: • Up-conversion detector• Superconducting single photon detector: SSPD• Sine-wave-gated InGaAs/InP Avalanche photodiode
4.Summary
Differential phase shift QKD (DPS-QKD)
Phase difference
TimeDetector
t2 t4 t5Det2 Det1 Det2
π 0 π
TimePhase Difference
Phase difference
Raw keybits 1 0 1
t1 t2 t3 t4 t5 t6 t7
０ π ０ ０ ０ π ０
Time t2 t4 t6
Raw keybits
1 0 1
BobAlice
０ π π０ π π
０ π π
T
T
. . . .Attenuation
Coherent pulsesource
Phasemodulation
Average photon number < 1 photon per pulse
Det1
Det2
Alice Bob
Inoue, Waks Yamamoto., PRL, 89, 037902 (2002).
One photon spreads over many pulses!
Merits of DPS-QKD
1. Easy implementation (simple configuration)
2. Easy to increase key rate by increasing clock frequency
3. Secure against a specific attack called “photon number splitting attack” that limited the key distribution distance of previous QKD systems.
Outline
1.What is quantum key distribution (QKD) ?
2.Differential phase shift quantum key distribution (DPS-QKD) protocol
3.QKD experiments with various single photon detectors: • Up-conversion detector• Superconducting single photon detector: SSPD• Sine-wave-gated InGaAs/InP Avalanche photodiode
4.Summary
Comparison between InGaAs APD and Si APD
Small continuous counting
Large Gated mode operation (up to 10 MHz)
Afterpulse probability
50 (typ)10000 (typ)Dark count rate[Hz]
～70 %～10 %Quantum efficiency
300-9001300-1600Wavelength [nm]SiInGaAs
Can we detect a 1.5-μm photon with a Si-APD?
1.5 μm signal photon
0.7 μm photon
Si-APD
1.3 μm pump
Filters
Noise photon suppression
Sum frequency generation
pωSFGω
sω
•The wavelength of a 1.5 μm photon is converted to 0.7 μm, and the converted photon is detected by Si-APD.
Fast, highly efficient single photon counting in the 1.5 μm band.
Periodically poled lithium niobate (PPLN) waveguide
↑↓↑↓↑↓↑
C. Langrock et al., Opt. Lett., 30, 1725 (2005).
Frequency up-conversion detector
0
10
20
30
40
50
0
5 105
1 106
1.5 106
2 106
2.5 106
0 50 100 150 200
CharacteristicsPeak quantum efficiency: 46 %
Continuous mode photon counting.
Timing jitter : 30 ps FWHM, but with long tail.Applicable to 1 GHz clock QKD system (but not to10 GHz clock)
Noise photons due to spurious nonlinear effects.
SNR improved when quantum efficiency is low.
Coupled pump power [mW]
Qua
ntum
effi
cien
cy [%
]
Dark count rate [H
z]
(λ=1551nm)
Alice Bob
Experimental setup (1-GHz clock)
CWlaser
Pulse width: 70 psAverage photon number per pulse: 0.2
Phasemodulator
D1
D2
Dispersion sifted fiber
Low-jitter up-conversion detectorsQuantum efficiency: 6%. 0.4%
ATT
PLC interferometerwith 1 ns delay
Intensitymodulator
1-GHz clock QKD experiment result
166 bit/s secure key at 100 km.2 Mbit/s sifted key at 10 km.
E. Diamanti et al, Opt. Express 14, 13073 (2006).
Superconducting single photon detector (SSPD)
Red line: low-jitter up-conversion detector
Pictures provided by NIST
Principle of SSPD
Cryogenic emvironment (3 K) Low dark count (about 10 Hz)
Fast response of NbN Low jitter (65 ps FWHM,well fitted with Gaussian)
Sapphiresubstrate
NbN
100 nm
3.5 nm
Bias CurrentIncidentPhoton
Hot spot(R > 0)
Bias Current
Current densityabove critical
Hot spot(R > 0)
Bias Current
R > 0 → Voltage Pulse Out
Bias Current
R = 0 → Voltage Drop = 0
Quantum efficiency Currently about 1 %
G. N. Gol’tsman et al, Appl. Phys. Lett. 79, 705 (2001).
(λ=1557.4nm)
Laser EAmodulator
Phasemodulator ATT
Alice Bob
SSPD1
SSPD2
10 GHzSinusoidalsignal
10 GHzRandom bit pattern
0.2 photons per pulsePulse width ～15 ps, Repetition10 GHz
Optical fiber
50 ps / div.
Electro-absorption (EA) modulator output waveformobserved with sampling oscilloscope
100 ps delayinterferometer
TIA
10-GHz clock DPS-QKD with SSPDs
10-GHz clock QKD result
Previous recordat 100 km:
166 bps
17 kbps@105 km
The first key distribution over 200 km (12 bps)
H. Takesue et al., Nature Photonics 1, 343 (2007).
InGaAs/InP APD with sine-wave gating
Sine wave gate
Photon input timing
Photon
Filter
Gate and avalanche are easily distinguished in frequency domain.
Smaller avalanche signal detected. (resulting in smaller afterpulsing).
Up-conversion detector : fragile optical componentsSSPD: cryogenic environment
Very expensive!
Semiconductor-based detector is desirable for low-cost QKD systems.
Sine-wave gated APD
0 5 10 15
102
104
106
Transmission loss (dB)
Key
rat
e (b
ps)
500-MHz clock DPS-QKD with sine-wave gated APDs
N. Namekata et al., Appl. Phys. Lett. 91, 011112 (2007).
Sifted key rate
Secure key rate
331 kbit/s at 15 km
Maximum distance 65 km
Deadtime: 200 ns (afterpulsing probability: 2.5%)
Comparison of single photon detectors
Low1 GHz～104～10 %Sine-wave gated InGaAs APD
High10 GHz～10～1 %SSPD
High1 GHz350 at QE=0.4%
< 10 %Up-conversion
Low10 MHz～104～10 %Gated mode InGaAs APD
CostMaximum clock rate
Dark count rate (in Hz)
Quantum efficiency
Green: fair, red: poor, others: moderate
Summary
DPS-QKD protocol:Easy implementation, suitable for high-clock rate system, PNS secure
Experiments with various high-speed single photon detectorsUp-conversion, SSPD, Sine-wave gating APD200 km key distribution (distance record)2 Mbit/s sifted key at 10 km (bit rate racord)
Future worksSecurity proofDevelopment of high-speed electronicsQuantum repeaterDevelopment as a real system
Open issues related to DPS-QKD
1.Unconditional security is not proven yet. Theorists are now preparing the first proof.
2.High-speed electronics designed for QKD.
3.Further increase of key distribution distance.“quantum repeaters” using quantum entanglement.
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