1. Why is QKD not used?
Towards high-performance CVQKD
Practical security considerations
Experimental Demonstration of
Continuous-Variable Quantum Key Distribution
over 80 km of Standard Telecom Fiber
Paul Jouguet, S´bastien Kunz-Jacques, Anthony
e
Leverrier, Philippe Grangier, Eleni Diamanti
SeQureNet, T´l´com ParisTech
ee
2012-09-14
1/20

2. Why is QKD not used?
Towards high-performance CVQKD
Practical security considerations
QKD: four main limitations (1/2)
Speed
Not enough for OTP: not a serious issue
Physical parameters estimation over large blocks: hardware
drifts, latency
Distance
Asymmetric crypto is not limited
Trusted nodes are not a good solution: quantum repeaters are
required
2/20

3. Why is QKD not used?
Towards high-performance CVQKD
Practical security considerations
QKD: four main limitations (2/2)
Security
Incomplete security proofs
Distance between proofs and practical implementations
Deployment
QKD requires a dark fiber: $$$
WDM compatibility: lowers QKD extra premium
3/20

4. Why is QKD not used?
Towards high-performance CVQKD
Practical security considerations
Two technologies
Discrete variables Continuous variables
medium photon phase/polar. field amplitude-phase
detection photon counters coherent detection
range 100 km 25km
rate 1Mb/s 10kb/s
components active cooling standard
integration WDM WDM
security yes yes
4/20

5. Why is QKD not used?
Towards high-performance CVQKD
Practical security considerations
Gaussian protocol
Losses and excess noise lower the SNR.
5/20

6. Why is QKD not used?
Towards high-performance CVQKD
Practical security considerations
Optical setup
6/20

7. Why is QKD not used?
Towards high-performance CVQKD
Practical security considerations
Speed limitations
Delay between classical and quantum signals (200ns)
Laser pulsed with a tunable frequency (1MHz)
Data acquisition speed (up to 5MHz)
Filtering: tradeoff between speed and electronic noise (current
cutoff at 10MHz, 100MHz feasible arxiv:1006.1257 Yue-Meng
Chi et al.)
High-speed error correction: LDPC codes (GPU) or polar
codes (CPU) (up to 10Mbit/s) (arxiv:1204.5882, P. Jouguet
and S. Kunz-Jacques)
7/20

8. Why is QKD not used?
Towards high-performance CVQKD
Practical security considerations
Range limitations
High error-correction efficiency, even at low SNRs
Finite-size effects
Low SNR Alice/Bob synchronization mechanism
Low-loss LO path
Excess noise (imperfect relative phase estimation)
New SNR regions allowed by error-correction techniques.
8/20

9. Why is QKD not used?
Towards high-performance CVQKD
Practical security considerations
Channel virtualization
Idea introduced by Leverrier (Phys. Rev. A 77, 042325
(2008))
Translates the initial problem into a channel coding problem
on a good channel
Good means very efficient error-correcting codes available
Usual suspect: BIAWGNC
9/20

10. Why is QKD not used?
Towards high-performance CVQKD
Practical security considerations
How to improve the efficiency of the multidimensional
scheme?
Improve the approximation between the virtual channel and
the target channel
Improve the efficiency of the codes on the target channel
(Phys. Rev. A 84, 062317 (2011), P. Jouguet, S.
Kunz-Jacques and A. Leverrier)
10/20

11. Why is QKD not used?
Towards high-performance CVQKD
Practical security considerations
What is a good code for the BIAWGNC?
A code is designed for a channel and a SNR
Free parameter: the code rate R = n−m , m the number of
n
parity-check equations, n the length
Low SNR / Lot of redundancy / Low rate
R
Efficiency β (SNR) = C (SNR)
Typical values:
Slice reconciliation: β = 90%, SNR= 3, @30km
Multidimensional reconciliation: β = 89%, SNR= 0.5, @50km
11/20

12. Why is QKD not used?
Towards high-performance CVQKD
Practical security considerations
Set of available codes
β SNR
93.6% 1.097
93.1% 0.161
95.8% 0.075
96.9% 0.029
96.6% 0.0145
95.9% 0.00725
12/20

13. Why is QKD not used?
Towards high-performance CVQKD
Practical security considerations
How to get more flexibility on the SNR?
Possible to design codes with lower rates
Not necessary: repetition codes
Shortening, puncturing
Optimization on the modulation variance
13/20

14. Why is QKD not used?
Towards high-performance CVQKD
Practical security considerations
Theoretical secret key rate
Parameters: VA ∈ {1, 100}, T = 10−0.2d/10 , ξ = 0.01, η = 0.6,
Velec = 0.01
1
SNR=1.097 (1)
1 SNR=0.161 (2)
0.1 SNR=0.075 (3)
2 SNR=0.029 (4)
Key rate (secret bits/pulse)
3 SNR=0.0145 (5)
0.01 SNR=0.00725 (6)
4
5
0.001
6
0.0001
10−5
10−6
0 20 40 60 80 100 120 140 160 180
Distance (km)
14/20

15. Why is QKD not used?
Towards high-performance CVQKD
Practical security considerations
Finite size effects
First analysis for discrete modulation protocols in Phys. Rev.
A 81, 062343 (2010), Leverrier et al.
Statistical uncertainty over estimated parameters (T , ξ)
n
K= N (βI (x; y ) −S PE (y ; E ) − ∆(n))
Extended analysis for Gaussian modulation, including
imperfect homodyne detection (efficiency, electronic noise)
and shot noise estimation (Phys. Rev. A 86, 032309 (2012),
poster 33)
Main effect: uncertainty on the excess noise ξ
15/20

16. Why is QKD not used?
Towards high-performance CVQKD
Practical security considerations
Experimental results
Parameters: d = 53km, η = 0.552, Velec = 0.015, SNR= 0.17,
β = 94%, = 10−10
0.002
excess noise
std dev of the experimental excess noise
std dev of the theoretical estimator (108 samples)
std dev of the theoretical estimator (106 samples)
0.0015
Excess noise (shot noise units)
0.001
0.0005
0
-0.0005
0 5 10 15 20
Time (hours)
16/20

17. Why is QKD not used?
Towards high-performance CVQKD
Practical security considerations
Experimental results
Parameters: d = 53km,80km, η = 0.552, Velec = 0.015,
SNR=0.17,0.08, β = 94%, = 10−10
100000
asymptotic theoretical key rate
nite-size theoretical key rate (108 samples)
asymptotic experimental point
nite-size experimental point (108 samples)
10000
Key rate (bit/s)
1000
100
0 20 40 60 80 100
Distance (km)
17/20

18. Why is QKD not used?
Towards high-performance CVQKD
Practical security considerations
Imperfect preparation
Thermal noise (Phys. Rev. A 81, 022318 (2010), Usenko and
Filip)
Gaussian modulation: truncated and discretized (finite
amount of randomness)
Can be taken into account into the security proof (Phys. Rev.
A 86, 032309 (2012), poster 33)
But a lot of random numbers are required (+ sifting and
multidimensional protocol)
Calibration procedures
Calibration of the homodyne detection
Calibration of the phase noise
18/20

19. Why is QKD not used?
Towards high-performance CVQKD
Practical security considerations
Summary
Long distance CVQKD with Gaussian modulation is possible
thanks to low SNR error-correction capability
Computing-power consuming because of decoding close to the
threshold
We use GPU (30× faster than CPU plus friendly Moore’s law)
Higher secure distance with virtual Gaussian post-selection?
(arxiv:1205.6933, J. Fiurasek, N. J. Cerf, arxiv:1206.0936, N.
Walk et al.)
Work on WDM coming
19/20

20. Why is QKD not used?
Towards high-performance CVQKD
Practical security considerations
Enquire about Cygnus
An open and customizable CVQKD research platform
Tests/demonstrations of integration in optical networks
20/20