Communications Quantiques

Hugo ZbindenGroupe de Physique Appliquée

“Quantum Technologies”Université de Genève

Cryptographie QuantiqueGénération de nombres aléatoires

Stéganographie basée sur du bruit quantique

What’s Cryptography?

Secure communication between Alice and Bob

The spy, Eve, tries to read the encoded message

Eve

BobAlice

“Alice Bob”

asuektüds&l

“Alice Bob”

Plain Text

Key Key

Cipher Text

Plain Text

Classical Cryptography

Based on Complexity

DES, AES (secret key)

RSA (public key)

Security unproven

One-way functionsInteger factorisation

107 53 = x

5671 = y z

Classical Cryptography based on Information Theory

one time pad (Vernam)

plaintext : 001010010010011101010001101001key: +101011011011001010100111010101cyphertext: 100001001001010111110110111100

security provenproblem: key distribution

1) Quantum Key Distribution• Quantum Crpytography is not a new coding method

• Send key with individual photons (quantum states)

• The eavesdropper may not measure without perturbation (Heisenbergs uncertainty principle)

• Eavesdropping can be detected by Alice and Bob!

QKD is proven information theoretically secure!

BB84 protocol (Bennett, Brassard, 1984)

Alice's Bit Sequence

0 1 0 - 0 1 1 1 1 - 1 0

- 1 - - 0 1 - - 1 - 1 0

Bob's Bases

Bob's Results

Key

Alice

Bob

Polarizers

Horizontal - Vertical

Diagonal (-45 , +45 )

H/V Basis

45 Basis

8

Eavesdropping

50% 50%

50%50%

50% 50% 50%50%

Bob

Eve

Ok Ok OkErrorError

Alice

…

Error with 25 % probabilityIAE = 2 QBER (quantum bit

error rate)

Eve’s attacks: information curves

9

0.40.0

Shan

non I

nfo

rmat

ion

0.1 0.2 0.30.0

0.2

0.4

0.6

0.8

1.0

QBER

)(1 QBERHIAB

IAEProbabilistic I-R

IAE = 2 QBER

Secret key rate

Incoherent attacks: information curves

0.40.0

Shan

non I

nfo

rmat

ion

0.1 0.2 0.30.0

0.2

0.4

0.6

0.8

1.0

QBER

)(1 QBERHIAB

IAEProbabilistic I-R

IAE

IAE = 1 - H(1/2 + Sqrt(QBER(1-QBER))

IAE = 2 QBER

Secret key rate

The steps to a secret keyAlice Bob

Quantum channel

Public channel

(losses)

Sifted key

Raw key

Key Key

+ Authentication!!!

Smolin and BennettIBM 1989

Swiss QCRYPT project (2013)

Nature Photonics 9, 163–168 (2015)

Efficient protocol

Finite key analysis

Low noise detectors

Low loss fibres

Ingredient 1: efficient and simple QKD scheme

QBER

Visibility

Reveals action of eavesdropperInput for key distillation

Coherent One Way (COW) Characteristics

• 1.25 GHz clock (625 MHz bit generation

rate)

• No active elements at Bob, robust bit

measurement basis

• Robust against PNS attacks

• Security proof for collective attacks

Ingredient 2: tight finite key analysis

Comparison of secret key rate using different postprocessing blocksizes

(10⁴, 10⁵, 10⁶, 10⁷ left to right)

Solid red: New tail inequality

Dashed blue: Previous tail inequality

Allows around an order of magnitude reduction of post-processing block size

Ingredient 3: low noise single photon detectors

1 cps

100 cps

System requirements:

Low dark count rate of SPD

Compact ( no SNSPD)

APDs: afterpulsing!

Optical QBER

afterpulsing

APD’s in photon counting mode

Bias over breakdown voltage

U

IUBias

A single photon can generate a

macroscopic pulse

How to stop the avalanche

– Passive quenching

– Active quenching

– Active gating +

-

---

+

++

-

50 RQAPD

UBias

SignalU

t

UBias

UBreakdown

Afterpulsing in APDs

Absorption region

Photon

Macroscopic current

Multiplication region

Absorption region

Macroscopic current

Multiplication region

AfterpulseDetection

Absorption region

Multiplication region

Trapped charges

Trapped charges

More current flow = More trapped charges = More afterpulsing

Free running Negative Feedback Aavalanche Photo Diode

• Rapid passive quenching + hold-off time-> low afterpulsing

• 1 darkcount/s( 10% eff, 160 K)

M. Itzler et al., Proc. SPIE 2009, 7222, 72221K-1 B. Korzh et al., Appl. Phys. Lett. 104, 081108 (2014)

Tradeoff: Temperature and noise

Temperature

Afterpulse Dark counts

Compensate with

hold-off

Afterpulse mitigation with longer hold-off time

Specs: dark count rate vs temperature

B. Korzh et al., “Free-running InGaAs single photon detector with 1 dark count per second at 10%

efficiency,” Appl. Phys. Lett. 104, 081108 (2014)

1.2 cps

reduction

of 2 o.m.

Reduction due

to lower

breakdown

voltage

=> smaller

field

Trap-assisted

tunneling

InP

Thermal

generation

InGaAs

Stirling coolers

3.5 kg

153 K

220 g

110 K

Jean-Yves Martin et al., “Thales Cryogenics rotary cryocoolers for HOT applications,” Proc. of

SPIE Vol. 8353 (2012)

Ingredient 4: Low Loss Optical Fibres

Total attenuation of an optical fiber:

BLIMOHTMUVIRRS 　

Intrinsic Extrinsic

Fiber design

Coating materials

Eliminated

by CVDReduced

by Cl dry

Not major

contribution

Not major

contributorsReduced

stresses

© 2015 Corning Incorporated

Rayleigh scattering is dominant: density and dopant fluctuations minimizedby choosing optimum (small) dopant concentration.

Ultra low loss fibers

Terrestrial applications

Submarine applications

Attenuation (dB/km)

0.16 0.17 0.18 0.19 0.20

SMF28®ULL

80 m2

SMF28®Ultra

80 m2SMF28®e+

80 m2

Vascade®

EX3000

150 m2

Vascade®

EX2000

112 m2

Vascade®

EX1000

76 m2

© 2015 Corning Incorporated

….putting all together:

FPGA is essential!

Results: Secret (finite key) rates vs distance

η = 22%toff = 114 µsµ = 0.075ppbs = 6.6x105

tpp = 17245 s

εQKD= 4x10-9

η = 27%toff = 42 µsµ = 0.1ppbs = 1.1x107

tpp = 308 s

η = 22%toff = 9 µsµ = 0.06ppbs = 2x107

tpp = 537 s

3 b/s

13 kb/s

1kb/s

Stability over 70h (200km)

Automatic tracking:

QBER Temporal alignment:Quantum signal clock recovery with 10 psresolutionExtinction ratio:Modulator bias voltage

VisibilityAdjust Laser current (wavelength)

Summary: Notable QKD demonstrations

B. Korzh, C. W. Lim et al., “Provably Secure and Practical Quantum Key Distribution over

307 km of Optical Fibre,” Nature Photonics 9, 163-168 (2015)

First long distance experiment with finite key analysis and quantifiable

security statement

First long distance experiment with APDs

Current developments

Make it smaller (ATCA Telecom standard)

Make it cheaper

Make it faster longer distances (quantum repeater, satellite)

2) Quantum Random NumberGenerator

Why RNG?Game/Simulation/Classical Cryptography (RSA, DSA …)/

Quantum Key Distribution

Why Physical RNG?"Anyone who considers arithmetical methods of producing random digits is,

of course, in a state of sin.“ John von Neumann (1951)

Why Quantum RNG?Random classical noise could be predictable

Possibility to estimate/certify the entropy

Realisations of QRNGs

using single photons

Rate: 4 Mbit/s per module

If

Possibility to

extract quantum randomness

Example with a Nokia N10

Sanguinetti B., et al. 2004 Phys. Rev. X 4 031056

• Exploiting photon statistics (shot noise)

PART 3: Quantum Secure Steganography

arXiv 1509.07106

Disclaimer: We are physicists….

from Greek steganos, or "covered," and graphie, or

"writing"): hiding of a secret message within an

ordinary message

Cryptography guarantees secrecy, but not privacy.

Steganography important in countries with

untrustworthy, totalitarian regimes

Universal Declaration of Human Rights: Art. 19

Steganography

Hiding secret information in a picture

Example with a Nokia N10

Sanguinetti B., et al. 2004 Phys. Rev. X 4 031056

• Steganography exploiting shot noise

Naive idea

Use least significant bit to transmit (OTP) encoded data

Simulated Histogram of the pixel values of a homogeneous area

Better idea

Take photographs of a static object in rapid succession

Assumptions:

1. state of object and camera unchanged between to consecutive pictures K and C

2. Each pixel is statistical independent (no crosstalk).

Protocol: given Text T, create a new picture S as follows:

S cannot be distinguished from any real photograph

Private key steganography

Experimental realisation

Tests with scientific mono-chrome and consumer colour cameras with raw image files

8 Mpix 16 bit tiff files

error-correction applied (Reed-Solomon code)

Results

It works!

no cross-pixel correlations

stability depends on experimental situation

colour camera needs more investigations

works also for jpeg files (less bits can behidden)

Conclusions

«Practical» QKD over 300 km range(reasonable limit 400km)

«True Random Numbers» have quantum origin

Provable secure steganography is possible(more work needed to test it in more «practical»

situations)

Quantum Communication:some quantum physics - lots of high tech