The document provides an overview of quantum cryptography and quantum key distribution. It discusses how quantum physics allows for more secure key exchange compared to classical cryptography. The principles of quantum cryptography are explained, including how single photons can be used to transmit encryption keys in a way that detects eavesdropping. Practical quantum cryptography systems are also summarized, highlighting components like single-photon sources and detectors. Potential applications and future directions are outlined, such as extending the transmission distance using quantum relays or satellites.

1. Quantum Cryptography
Beyond the buzz
Grégoire Ribordy
CERN, May 3rd 2006

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Outline
 Quantum physics and information technology
 The limits of classical cryptography
 The principles of quantum cryptography
 Practical systems and applications
 Future directions

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Moore’s law and quantum physics
Oui
Non
1
10
100
1000
10000
100000
1970 1980 1990 2000 2010 2020
Year
Transistors
per
chip
[x1000]
0
10
20
30
40
50
Size
of
transistor's
grid
[atoms]
1
10
100
1000
10000
100000
1970 1980 1990 2000 2010 2020
Year
Transistors
per
chip
[x1000]
0
10
20
30
40
50
Size
of
transistor's
grid
[atoms]
Quantum Limit

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Classical and Quantum physics
Classical physics
 … - 1900
 Describes the macroscopic world
 Deterministic
 Intuitive
Quantum physics
 1900 - …
 Description of the microscopic
world
 Probabilistic
 Central role of the observer
 Not very intuitive
Quantum physics  Novel information processing possibilities
 Quantum Information Theory (QIT)

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Generating random numbers with quantum physics
 High bit rate
• 4 or 16 Mbits/s
 Continuous monitoring
 Main OS’s supported

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Outline
 Quantum physics and information technology
 The limits of classical cryptography
 The principles of quantum cryptography
 Practical systems and applications
 Future directions

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
Message
Public Key
Scrambled
Message

Message
Private Key
Different Keys

Message
Secret Key
Scrambled
Message

Message
Secret Key
Identical keys
Key Exchange ?!?
Introduction: Classical Cryptography
 Secret Key Cryptography
 Public Key Cryptography
Alice
Bob
Different keys
 Key exchange solved
Vulnerabilities!!!

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Key length
Computing
time
Decryption
(without key)
Security of public key cryptography
Encryption
Decryption (with key)
Selected Key Length

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Vulnerabilities of public key cryptography
Key length
Computing
time
Classical computer
Selected Key Length

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Vulnerabilities of public key cryptography
Key length
Computing
time
Quantum computer
Classical computer
& Theoretical progress
Selected Key Length

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Where does Quantum Cryptography fit in?

Message
Secret Key
Scrambled
Message

Message
Secret Key
Alice
Bob
Quantum Cryptography is a key distribution technique!
Quantum Key Distribution is a better name!!!
Secret key exchange by quantum cryptography

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Outline
 Quantum physics and information technology
 The limits of classical cryptography
 The principles of quantum cryptography
 Practical systems and applications
 Future directions

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Physical implementation of a data channel
Fragile !
"0"
"1"
"1"
Classical communication Quantum communication
Security guaranteed by the laws of quantum physics

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Quantum Cryptography: rules of the game
1. Details of the protocole publicly known
2. Goal: to produce a secret key or nothing
 « Eve cannot do better than cutting the line »
Alice and Bob: to estimate Eve’s information on key
IAE small: Produce a key
IAE large:
|...> sk’
Eve
Quantum channel
Classical channel
QUANTUM KEY DISTRIBUTION

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Polarization of Photons
 Direction of oscillation of the electric field associated to a lightwave
 Polarization states
 What can we do with it ?
50 %
E


50 %

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Irreversibility of Measurements
Incoming photon polarized at 90
Incoming photon polarized at 45
50 %
Rotation of polarizer
50 %

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Quantum communications
 Transmitting information with a single-photon
 Use a quantum property to carry information
Liner States
= "0" = |0>
= "1" = |1>

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Eavesdropping (1)
 A single-photon constitutes an elementary quantum system
It cannot be split
 Semi-transparent mirror
?
?
50%
50%

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Eavesdropping (2)
 Communication interception
 Use quantum physics to force spy to introduce errors in the
communication
?
|0> |0>
Eve
Bob
Alice

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Quantum Cryptography Protocole
 BB84
 A better name: Quantum Key Distribution
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

0
0
0
0

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Eavesdropping (3)
50% 50%
50%
50%
50% 50% 50%
50%
Bob
Eve
Ok Ok
Ok
Error
Error
Alice

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Key Distillation (ideal case)
Alice Bob
Quantum channel
QBER =
0 : no eavesdropping
> 0 : eavesdropping
Sifted key
Reveals rather than prevents eavesdropping
A better name: quantum key distribution

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Key Distillation (realistic case)
Alice Bob
Quantum channel
Public channel
(losses)
Sifted key
Raw key
Key Key

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Information curves
0.4
0.0
Shannon
Information
0.1 0.2 0.3
0.0
0.2
0.4
0.6
0.8
1.0
QBER
)
(
1 QBER
H
IAB 

IAE
Opt. indiv. attack
Secret key rate

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The Principles of Quantum Cryptography: Summary
Quantum Communication
Raw key exchange
Integrity Verification
Key Distillation
Key Use
Quantum Cryptography
Conventional Symmetric
Cryptography
Point-to-point optical link
Future-proof key exchange
with security guaranteed by
the laws of physics

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Outline
 Quantum physics and information technology
 The limits of classical cryptography
 The principles of quantum cryptography
 Practical systems and applications
 Future directions

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Building a Quantum Key Distibution System
 Necessary components
 “System approach”
Channel
Single-Photon Source
Single-Photon Detector

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Polarization Coding
 Typical system
LD 1
LD 2
LD 3
LD 4
Quantum
Channel
Alice Bob
Basis 1
Basis 2
/2
PBS
PBS
"0"
"1"
"0"
Waveplates
BS
BS
BS F "1"
APD
APD
Public Channel

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Interferences

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Interferometer
 Classical interference

Port 1
Port 2
E

1
E

2
E

0
0.5
1
0 2 4 6
Phase [radians]
Sortie 1
Sortie 2

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Phase encoding
 Quantum optics: single-photon
A
B
Alice
Bob
D2
D1
0
0.5
1
0 2 4 6
Phase [radians]
Output 1
Output 2
Base 1: A = 0; p
Base 2: A = p/2; 3 p/2
Basis choice: B = 0; p/2
Compatible: Alice A  Di
Bob Di  A
(A-B = np)
Bases
Incompatible: Alice and Bob ??
(A-B = p/2)

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Phase encoding (2)
 Stability of such system ???
 In practice
A
B
Alice
Bob
D2
D1
10 km
10 km ± /10 (100 nm)
Alice
A B
Bob
Time (ns)
LL
0
20
40
60
80
CC
-3 -2 -1 0 1 2 3
CL + LC
+

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Auto-compensated set-up
 Time multiplexing
Alice
Bob
A
B
Att.
Short arm
Long arm
MF
SP

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Practical requirements
 Distance limitation < 100 km
Current range is sufficient for a vast majority of MAN/SAN applications
 Point-to-point dark fiber
• Amplifiers
• Opto-electro-opto conversion
perturbation of the quantum state of the photon
Distance
Signal, Noise
pnoise
psignal

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Link Encryptors with QKD
 Network Appliance
• Point-to-point link encryption
• Layer 2 device
• Network protocole independent
• Compatible with higher layer encryption
Specifications
- Encryption: AES (128, 192, 256 bits)
- Key rate as high as 100 keys / s
- Distance < 100 km (60 miles)
- Pair of dark fiber
Target Applications
MAN or SAN encryption

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« Swiss Quantum » Pilot Site

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Outline
 Quantum physics and information technology
 The limits of classical cryptography
 The principles of quantum cryptography
 Practical systems and applications
 Future directions

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Extending the key distribution distance
 Chaining links
 Better components
 Free space links to low-earth-orbit (LEO) satellites
 Quantum relays and repeaters
Tokyo Geneva
A B
B' A' B" A" B'" A'"
Telco Infrastructure

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Compatibility with conventional optical networks
 Optical switching
 WDM Links

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Thank you very much for your attention
id Quantique SA
Chemin de la Marbrerie, 3
CH-1227 Carouge
Switzerland
Ph: +41 22 301 83 71
Fax: +41 22 301 83 79
info@idquantique.com
www.idquantique.com

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Optical Taps
 Optical taps are cheap and simple to use
« Tapping a fibre-optic cable without being detected, and making sense of the
information you collect isn’t trivial but has certainly been done by intelligence
agencies for the past seven or eight years. These days, it is within the range of a
well funded attacker, probably even a really curious college physics major with
access to a fibre optics lab and lots of time on his hands. »
John Pescatore, former NSA Analyst
The submarine « USS Carter » worth $4.1 bn will be able to tap and eavesdrop
undersea cables.

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Key use
 The key produced by a quantum cryptography system is used with
conventional symmetric encryption algorithms
• One-time pad  « unconditional security »
• Other symmetric algorithms (AES, Tripe-DES, etc.)  enhanced security
by frequent key change
 Why is Quantum Cryptography not used to transmit data?
1) Quantum Cryptography cannot guarantee that one particular bit will
actually be received.
With a random key, it is not a problem. With data, it is.
2) Quantum Cryptography does not prevent eavesdropping, but reveals it a
posteriori. Sending a key and verifying its secrecy allows to prevent
information leakage.

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Device Authentication
Initial key
Quantum Cryptography Session n: key material
Authentication key
Encryption/decryption key
Session n
Authentication key n
Authentication
key
refreshed