Quantum Cryptography: Public key distribution and coin tossingCharles Bennett & Gilles Brassard International conference on Computers, Systems & Signal ProcessingBangalore, India December 10-12, 1984

science -> tbhfodd

physics -> qgxtjdr

micro-photoluminescence?

one-time-pad: 1 0 0 1 1 1 0

transmission probability:

absorption probability:

Polarised photons:

Vertical

Horizontal

45 degree

cos2 ( a - b )

sin2 ( a - b )

How does it work?

BobAlice

10010101101001

RDDRDRDRRR

Rectilinear basis: 0o : binary 0 90o : binary 1

Diagonal basis: 45o : binary 0 135o : binary 1

45,90,135,0....

Bob's filters

DRDDRRDRDR

1 001 0 100 1 011

Bob's bitsAlice's bits Alice's bases encoded photons

0 1 1 0 1 1 0 0 1 0 1 1 0 0 1

D R D R R R R R D D R D D D R

R D D R R D D R D R D D D D R

1 1 1 0 0 0 1 1 1 0 1

R D R D D R R D D D R

Ok Ok Ok Ok Ok OK

1 1

1 0 0 1

Alice's Bits

Alice's Bases

Alice's Photons

Bob's Bases

Bob's Bits

Bob's announces bases of received bits

Random reveal:

Resulting one-time-pad:

Why is it secure?

No passive eavesdropping: you cannot split a photon.

All Bob's information carrying bits will agree with Alice's

Evil Eve the eavesdropper

BobAlice

45,90,135,0....

Bob's filters

DRDDRRDRDR

1 0010 1001 011

Bob's bitsencoded photons

Eve

RRRRRRRRRR

50/50 disagreement

Eve's random filters

Alice and Bob can check for eavesdropping by publicly comparing some random subset of the received photons

Coin tossing: 50/50 chance game between "distrustful" parties

Alice chooses one basis, Bob must guess

0 1 1 0 1 1 0 0 1 0 1 1 0 0 1

R R R R R R R R R R R R R R R

R D D R R D D R D R D D D D R

0 1 0 0 1

1 0 1 1

0 1 1 0 1 1 0 0 1 0 1 1 0 0 1

Alice's Bits

Alice's Bases

Alice's Photons

Bob's Bases

Bob's R basis

Bob's D basis

Single photons

Why? Because Alice must know how many photons she is sending to Bob!

Quantum dots!

Creates anti-bunched light. Test with autocorrelation measurement.

Perfect Real

Simulated curves

Hanbury-Brown Twiss Experiment