2. Quantum Cryptography:
Cryptographic technique based on the laws of physics.
Transmits photons(qubits) instead of electrical signals(bits).
In theory, it achieves totally secure communications.
Once photons have been observed, they are irrevocably changed.
Quantum key distribution is the most well known example.
3. Quantum key distribution (QKD):
Two distinct channels.
Quantum key channel.
Public data channel.
5. Eavesdropping detection:
Error rate < E max No eavesdropping
Error rate > E max Eavesdropping or the channel is
noisy.
Alice and Bob should then discard the whole key and start
over.
6. SO what is the state
of the art in quantum
cryptography?
7. Current limitation:
Quantum keys are faint and delicate.
They require dedicated expensive dark fibers .
Trying to use such low-level signals
over 'lit fiber' has been rather like
trying to see the stars whilst staring at
the Sun
Quantum and data signals have extreme contrast in
their intensities.
So plucking the quantum key photons out of the fibre is
impossible.
8. Toshiba research:
Transmit the quantum photons and data signals at
different wavelengths So signals won’t clash.
Develop detectors catch just one photon at a time.
Gate opens for just a tenth of a billionth of a second.
Quantum key signal photons arrive, one by one.
9. Results:
Transmit binary data at 1Gbps in both directions.
Perform QKD at 500Kbps at the same time over a 90km.
This is 50,000 times faster than the previous best QKD.
Alice sends a crypto key to Bob via a stream of single photons (quantum key distribution, QKD). If a man-in-the-middle attacker somehow manages to intercept the photons, this interrupts the transmission in such a way that can be detected.that snoops on the quantum channel will cause a measurable disturbance to the flow of single photons.
Alice uses a light source to create a photon.The photon is sent through a polarizer and randomly given one of four possible polarization and bit designations — Vertical (One bit), Horizontal (Zero bit), 45 degree right (One bit), or 45 degree left (Zero bit).The photon travels to Bob’s location.Bob has two beamsplitters — a diagonal and vertical/horizontal - and two photon detectors.Bob randomly chooses one of the two beamsplitters and checks the photon detectors.The process is repeated until the entire key has been transmitted to Bob.Bob then tells Alice in sequence which beamsplitter he used.Alice compares this information with the sequence of polarizers she used to send the key.Alice tells Bob where in the sequence of sent photons he used the right beamsplitter.Now both Alice and Bob have a sequence of bits (sifted key) they both know
The probability Eve chooses the incorrect basis is 50% (assuming Alice chooses her basis randomly), and if Bob measures this intercepted photon in the basis Alice sent he will get a random result, i.e. an incorrect result with probability of 50%. The probability an intercepted photon generates an error in the key string is then 50% x 50% = 25%.If Alice and Bob publicly compare n of their key bits (thus discarding them as key bits, as they are no longer secret) the probability they find disagreement and identify the presence of Eve isSo to detect an eavesdropper with probability Pd = 0.999999999 Alice and Bob need to compare n = 72 key bits.
The main challenge for the coexistence of quantum and data signals on the same fiber arises from the extreme contrast in their intensities.Each quantum signal typically contains approximately 0.5 photons per pulsewhile a data-laser pulse may contain 106 photons or more for a Gb=s link.
