Kvantekryptografi med fotoner i optiske fibre

1
Lecture in "Fiberkomponenter" course, November
13, 2003
Vadim Makarov www.vad1.com/qcr/
2
Classical vs. quantum information

• Classical information

Perfect copy
Unchanged original

• Quantum information

Imperfect copy
Broken original
3
Qubit polarization stateof a single photon
Measure?
50
Measure?
50
4
What is the problem with classical cryptography?

• Secret key cryptography
• Requires secure channel for key distribution
• In principle every classical channel can be
  monitored passively
• Security is mostly based on complicated
  non-proven algorithms
• Public key cryptography
• Security is based on non-proven mathematical
  assumptions (e.g. in RSA cipher, difficulty of
  factoring large numbers)
• We DO know how to factorize in polynomial time!
  Shors algorithm for quantum computers. Just wait
  until one is built.
• Breakthrough renders messages insecure
  retroactively

5
The holy grail One-time pad

• The only cipher mathematically proven
• Requires massive amounts of key material
• (key of same length as message, used only
  once)

m
m
c
k
k
6
Key distribution

• Secret key cryptography requires secure channel
  for key distribution.
• Quantum cryptography distributes the key by
  transmitting quantum states in open channel.

7
Quantum key distribution
Bob
Alice
Diagonal detector basis
Diagonal polarization filters
Horizontal-vertical detector basis
Horizontal-vertical polarization filters
Light source
Alices bit sequence 1 0 1 1 0 0 1 1 0
0 1 1 1 0
Bobs detection basis
Bobs measurement 1 0 0 1 0 0 1 1 0 0
0 1 0 0
Retained bit sequence 1 1 0 0 1 0
0 1 0
Image reprinted from article W. Tittel, G.
Ribordy, and N. Gisin, "Quantum cryptography,"
Physics World, March 1998
8
Eavesdropping with wrong reference system
9
Interferometric QKD channel
? 1 0? or 90? - "1"
Reference systems ? 2 0? ? 2 90?
? 1 180? or 270? - "0"
10
Implementation interferometer structure
Alice
Variable Ratio PM Coupler
Polarization Combiner
Phase Modulator 1
Variable Delay Line
Polarizer
Laser
PM fiber
Attenuator
1300 nm (or 1550 nm) Pulse Rate 10 MHz
Alice's PC
Line Standard SM fiber
Public Communication Channel
Eve's Territory
Bob
Bob's PC
Phase Modulator 2
Polarization Controller
PM Coupler 50/50
APD
'0'
Polarization Combiner
Polarizing Splitter
'1'
PM fiber
11
Photo 1. Alice (uncovered, no thermoisolation
installed)
12
Photo 2. Bob (uncovered, no thermoisolation
installed)
13
20 GHz travelling-wave phase modulator, 1300
nm(manufacturer Uniphase)
14
Results of electrical breakdown in the
waveguide(microphotograph)
15
Real-time phase trackingin the interferometer

• Due to thermal drift, interferometer needs
  automatic phase adjustment every few seconds.
• Phase adjustment takes ?0.3 s and requires no
  additional components, e.g. no variable
  attenuator.

16
Photo 4. Bob (left) and Alice (right),
thermoisolation partially installed
17
Single-photon detectorAPD in Geiger mode
tgate down to 1ns gate pulse rate 20 MHz
18
Timing resolution of photon detection
Photon counts due to 75 ps laser pulse FWHM ?
200 ps
Dark counts within 5ns gate
19
Recovery from errors
Eves information
Bobs information
QBER limit

• Individual attacks 15
• All theoretically possible attacks 11(?)

20
Distance limitation
21
Components of security
1. Conventional security 2. Security against
quantum attacks 3. Security against Trojan horse
attacks - ones that dont deal with quantum
states, but use loopholes in optical scheme
22
Practical security large pulse attack
Alice
Phase Modulator
Attenuator
Alice's PC
Line
Eves Equipment
- interrogating Alices phase modulator with
powerful external pulses (can give Eve bit values
directly)
23
Typical values of reflection coefficients for
different fiber-optic components (courtesy
Opto-Electronics, Inc.)
24
Eavesdropping experiment
Alice
4 reflection
Phase Modulator
Laser
Vmod
Eve
L1
OTDR
Out
Variable attenuator
In
L2
Fine length adjustment to get L1 L2
4.1
8.2
0
Vmod, V
25
Photo 3. Artem Vakhitov tunes up Eves setup
26
Re-keying satellites/Global key distribution
network
1.9 km 10 km 23.4 km
27
Quantum key distribution infiber-optic network

• Multi-user key distribution

Bob 1
Passive splitter
Alice
Bob 2
Bob 3

• Multiplexing with telecom traffic

1300 nm
28 km
Bob
Alice
WDM
WDM
Data transmitter
Data receiver
1550 nm 1.2 Gbit/s
28
Commercial status

• Two small companies trying to sell QKD systems
• id Quantique (Geneva) 2002
• MagiQ Technologies (Boston) November 2003
• several telecom/ electronics companies,
  research groups keep close eye on commercializing
  but don't develop a production version yet.

Navajo
29
Faglærere Dag R. Hjelme, Astrid Dyrseth
DiplomaThesisAvailable!