Quantum cryptography becomes a reality
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Computer Science Clay
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01-03-2009, 09:56 AM

Quantum cryptography becomes a reality
According to reliable sources from NEC, Commercial quantum cryptography, a revolutionary system that can produce quantum keys at a speed of 100Kbit/s and then broadcast it up to 40 kilometres along the commercial fibre optic lines will be available in the markets by the second half of 2005. Speaking in line with Kazuo Nakamura, senior manager of NEC's quantum information technology group at the company's Fundamental and Environmental Research Laboratories, it can be considered as a world record as it is a rare blend of speed and distance. As put by Akio Tajima, the assistant manager at the laboratory, this innovative concept has gone through several improvisations after it was successfully tested in April at the companys laboratories in Tokyo.

The system permits the users to swap the keys with a prior idea that they have not been disordered up during the transmission. The whole system works on the concept that the system works by implanting the encryption key on photons, which can be either in the receiver end or with an eavesdropper, as the photons cannot be cracked. Akio Tajima said that until last April the round-trip quantum cryptography method at NEC where it had a laser as well as a receiver at one end and also a mirror at the other end, faced some troubles regarding the high speed over long distances.

Earlier the detector that turns the photons to electrons once they collide with it functioned very slowly. This created a problem in registering these photons, as there will be an avalanche of electrons with every collision. The team lead by Tajima has rectified that disadvantage now by developing a new detector that can work reliably at 100Kbit/s. This fast pace helps in clearing this whole bunch of electrons produced by the collision from the device, quickly so that they can register the next photon. The NEC scientists have also rectified the problem with the mirror used earlier in the system called the faraday mirror. The performance of this mirror, which can reflect the light in a 90-degree rotation from the input light, changes with temperature leading to quality loss. NEC today has improvised this concept of mirror, by producing a mirror that works efficiently with temperature variations.

Another advantage of NEC system is that it has a conventional laser, which can transmit the photons through the fibre optic cables over a long distance with very less noise. Although there were powerful lasers that could trigger the propagation of photons over long distances, they all resulted in more noises leading to efficiency loss. According to Nakamura 'This is the world's fastest key generation technology at 40 kilometres'. He confirms his statement with various proofs. He said that the University of Geneva has achieved quantum transmission over a distance of over 60 kilometres, but at a much lower speed, while a system developed by Japan's National Institute of Advanced Industrial Science and Technology, a major government laboratory, has achieved nearly the same speed as NEC's system, but only at about half the distance.

According to Toshiyuki Kanoh, chief manager of the company's System Platforms Research Laboratories, this break through system invented in collaboration with the Japan Science and Technology Agency's Exploratory Research for Advanced Technology and Japan's National Institute of Information and Communications Technology, will take an year to be launched in the commercial market as its software is still on the developing stage. He also added that they are going to create a commercial market for the system which it lacks now and is expecting the police, banks and financial institutions etc to be its clients by the mid of 2005. There is also a move to demonstrate this system in various exhibitions and seminar and presentations.
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Dr. Janusz Kowalik

.ppt   Introduction to quantum cryptography.ppt (Size: 589.5 KB / Downloads: 156)
Introduction to Quantum Cryptography
• Transmitting information with access restricted to the intended recipient even if the message is intercepted by others.
• Cryptography is of increasing importance
in our technological age using broadcast, network communications, Internet ,e-mail,
cell phones which may transmit sensitive information related to finances, politics,
business and private confidential matters.
• The process
• Sender
• The classic cryptography
• Encryption algorithm and related key are kept secret.
• Breaking the system is hard due to large numbers of possible keys.
• For example: for a key 128 bits long
• there are
• PKC :the modern cryptography
• In 1970s the Public Key Cryptography emerged.
• Each user has two mutually inverse keys,
• The encryption key is published;
• The decryption key is kept secret.
• Anybody can send a message to Bob
but only Bob can read it.
• The most widely used PKC is the RSA algorithm based on the difficulty of
• factoring a product ot two large primes.
• Easy Problem Hard Problem
• Factoring a product of two large primes
• The best known conventional algorithm requires the solution time proportional to:
Quantum Computing algorithm for factoring.
• In 1994 Peter Shor from the AT&T Bell Laboratory showed that in principle a quantum computer could factor a very long
product of primes in seconds.
• Shor’s algorithm time computational complexity is
• Elements of the Quantum Theory
• Light waves are propagated as discrete quanta called photons.
• They are massless and have energy, momentum and angular momentum called spin.
Spin carries the polarization.
• If on its way we put a polarization filter
a photon may pass through it or may not.
• We can use a detector to check of a photon has passed through a filter.
• Photon polarization
• Heisenberg Uncertainty Principle
• Certain pairs of physical properties are related in such a way that measuring one property prevents the observer from knowing the value of the other.
When measuring the polarization of a photon, the choice of what direction to measure affects all subsequent measurements.
• If a photon passes through a vertical filter
it will have the vertical orientation regardless of its initial direction of polarization.
• Photon Polarization
• Polarization by a filter
• A pair of orthogonal filters such as vertical/horizontal is called a basis.
• A pair of bases is conjugate if the measurement in the first basis completely randomizes the measurements in the second basis.
• As in the previous slide example for =45deg.
• Sender-receiver of photons
• Suppose Alice uses 0-deg/90-deg polarizer sending photons to Bob. But she does not reveal which.
• Bob can determine photons by using
filter aligned to the same basis.
• But if he uses 45deg/135 deg polarizer to measure the photon he will not be able to determine any information about the initial polarization of the photon.
• The result of his measurement will be completely random
• Eavesdropper Eve
• If Eve uses the filter aligned with Alice’s she can recover the original polarization of the photon.
• If she uses the misaligned filter she will receive no information about the photon .
• Also she will influence the original photon and be unable to retransmit it with the original polarization.
• Bob will be able to deduce Ave’s presence.
• Binary information
• Each photon carries one qubit of information
• Polarization can be used to represent a 0 or 1.
• In quantum computation this is called
To determine photon’s polarization the recipient must measure the polarization by ,for example, passing it through a filter.
• Binary information
• A user can suggest a key by sending a stream of randomly polarized photons.
• This sequence can be converted to a binary key.
• If the key was intercepted it could be discarded and a new stream of randomly polarized photons sent.
• The Main contribution of Quantum Cryptography.
• It solved the key distribution problem.
• Unconditionally secure key distribution method proposed by:
• Charles Bennett and Gilles Brassard in 1984.
• The method is called BB84.
• Once key is securely received it can be used to encrypt messages transmitted
by conventional channels.
• Quantum key distribution
• (a)Alice communicates with Bob via a quantum channel sending him photons.
• (b) Then they discuss results using a public channel.
• © After getting an encryption key Bob can encrypt his messages and send them by
any public channel.
• Quantum key distribution
• Both Alice and Bob have two polarizers each.
• One with the 0-90 degree basis (+) and one with 45-135 degree basis ( )
• (a) Alice uses her polarizers to send randomly photons to Bob in one of the four possible polarizations 0,45,90,135 degree.
• (b)
• Example of key distribution
• Security of quantum key distribution
• Quantum cryptography obtains its fundamental security from the fact that each qubit is carried by a single photon, and each photon will be altered as soon as it is read.
• This makes impossible to intercept message without being detected.
• Noise
• The presence of noise can impact detecting attacks.
• Eavesdropper and noise on the quantum channel are indistinguishable.
• (1) Malicious eavesdropper can prevent communication.
• (2) Detecting eavesdropper in the presence of noise is hard.
• State of the Quantum Cryptography technology.
• Experimental implementations have existed since 1990.
• Current (2004) QC is performed over distances of 30-40 kilometers using
optical fiber.
In general we need two capabilities.
(1) Single photon gun.
(2) Being able to measure single photons.
State of the QC technology.
• Efforts are being made to use Pulsed Laser Beam with low intensity for firing single photons.
• Detecting and measuring photons is hard.
• The most common method is exploiting Avalanche Photodiodes in the Geiger mode where single photon triggers a detectable electron avalanche.
State of the QC technology.
• Key transmissions can be achieved for about 80 km distance ( Univ of Geneva 2001).
• (2)For longer distances we can use repeaters. But practical repeaters are a long way in the future.
• Another option is using satellites.
• Richard Hughes at LOS ALAMOS NAT LAB (USA) works in this direction.
• The satellites distance from earth is in hundreds of kilometers.
NIST System
• Uses an infrared laser to generate photons
• and telescopes with 8-inch mirrors to send and receive photons over the air.
• Using the quantum transmitted key
messages were encrypted at the rate
1 million bits per second.
The speed was impressive but the distance between two NIST buildings was only 730 meters.
Commercial QC providers
• id Quantique, Geneva Switzerland
• Optical fiber based system
• Tens of kilometers distances
MagiQ Technologies, NY City
• Optical fiber-glass
• Up to 100 kilometers distances
• NEC Tokyo 150 kilometers
• QinetiQ Farnborough, England
• Through the air 10 kilometers.
• Supplied system to BBN in Cambridge Mass.

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