Cryptography – Key management – Key distribution
Reexamination Certificate
1998-03-31
2001-02-13
Swann, Tod R. (Department: 2767)
Cryptography
Key management
Key distribution
Reexamination Certificate
active
06188768
ABSTRACT:
TECHNICAL FIELD
This invention relates to secure communication channels and more particularly to a secure optical communication channel for the transmission of cryptographic key information using single photons.
BACKGROUND OF THE INVENTION
Quantum cryptographic key distribution (QKD) systems transmit cryptographic key data encoded in the quantum states of individual optical photons. QKD was first described by C. H. Bennett et al., “Quantum Cryptography: Public key distribution and coin tossing,”
Proc. Int. Conf. Computer Systems and Signal Processing
, pp. 175-179 (Bangalore 1984). The benefits of such a system are that it allows secure transmission of key data over unsecured optical links with security guaranteed by the fundamental quantum properties of light rather than by computational complexity or barriers to interception. This is possible because single photons cannot be split into smaller pieces (intercepted or diverted photons simply won't arrive at the intended destination), nor can they be intercepted and consistently regenerated in identical states since their states cannot be fully characterized by single measurements, leading inevitably to errors in the states of the replacement photons.
Practical systems for distribution of cryptographic keys using quantum cryptography protocols require transmission of single-photon optical signals through some medium, such as optical fiber. Since these protocols encode information in the phase or polarization of the photons, phase and polarization state changes due to mechanical and thermal stresses on the fiber, or to fiber imperfections, must be eliminated or compensated to a degree that permits reliable interferometric detection of the encoded information. In addition, the two parties needing to share a cryptographic key must be able to exchange timing and auxiliary information via a conventional channel.
A technique described by Martinelli in
Opt. Comm
., 72, 341 (1989) permits automatic, passive compensation for the polarization-transforming effect of the fiber. In this technique light is transmitted through an optical fiber, passes through a Faraday rotator, reflects from a mirror, and returns through the Faraday rotator and fiber. It can be shown that the polarization state of the light returning to the input end of the fiber is always orthogonal to the polarization state of the input light, independent of the polarization transformation induced by the fiber. This effect is referred to as Faraday ortho conjugation.
A quantum cryptographic key distribution system based on a long-path, time-multiplexed interferometry that utilized the Faraday ortho conjugation effect to automatically compensate uncontrolled birefringence effects has been described by Muller et al. in “Plug and play systems for quantum cryptography,”
Appl. Phys. Lett
. 70, 793-5 (1997). The system has excellent interference characteristics (>99% contrast ratio) and clearly shows the value of the auto compensation technique. However, it has several significant weaknesses, including the following:
(1) As implemented, it requires fast phase modulators capable of transmitting both polarizations of light, whereas most available waveguide modulators transmit only a single polarization.
(2) Photons carrying one of the bit values are not sent to a detector. This single detection channel arrangement reduces the data rate by one half.
(3) It requires the use of three Faraday mirrors.
(4) Optical clutter caused by the use of a standard beamsplitter with a do pair of Faraday mirrors to generate a delayed pulse, gives an infinite series of “echo” pulses. Each of these clutter pulses is a factor T
2
smaller than the last (where T is the intensity transmittance of the delay line beamsplitter). The use of a high value of T (T close to 1) means that the echo pulses take many delay line periods to die out, limiting the bit transmission rate.
(5) The single-photon detector sees a reflected laser pulse and an echo series with every shot. In addition, the 2-state protocol used will also direct strong pulses at the detector. These strong extra pulses striking the detector will increase noise counts and limit repetition rate.
The Muller et al. reference also suggests a polarization-encoded system that uses light pulses with specific polarization states, but does not describe an actual implementation.
Thus, what is needed is a practical autocompensated fiber optical system for quantum cryptographic key distribution that eliminates the weaknesses described above.
SUMMARY OF THE INVENTION
The invention is a QKD system that splits discrete light signals from a laser source into a pair of light pulses that are orthogonally polarized with respect to each other, imparts a phase shift to one or both of these separate pulses during their round trip from the sender to the receiver and back, assures that the return pulses from the receiver are attenuated to single-photon pulses, recombines the phase-shifted pulses at the sender, and then detects from the recombined signal its polarization state, which is representative of the net phase shift imparted by the sender and receiver. A polarization preparation and laser isolation stage at the sender converts the light signals from the laser into light signals with equal amplitude horizontal and vertical polarizations, while also assuring that no return light from the receiver is discarded or sent back to the laser. The phase modulator at the receiver transmits only one polarization (e.g., vertical), but is used in a manner that permits it to equally modulate both polarization components of an arriving pulse. In this arrangement, when both components of a pulse reach the phase modulator at the receiver, they are both entirely vertically polarized and a phase shift is imparted at that time. This has the advantage that the effect of any time variation or phase errors in the phase modulator will be the same for both components. The key information is decoded at a detection stage at the sender that uses two detectors, one of which detects a first polarization state corresponding to the phase difference between the two phase shifts being 0 and the other of which detects a second polarization state corresponding to the phase difference between the two phase shifts being &pgr;.
The use of a Faraday mirror at the receiver, together with the requirement that each of the separate pulses travels the same total path length back to the point where it is recombined into the recombined signal, results in an autocompensating system that eliminates the birefringence effects of the optical fiber. The invention allows the use of single polarization phase modulators, such as annealed proton-exchange waveguide modulators, and thus eliminates the need to use fast phase modulators that are capable of transmitting both polarizations of light. The use of a single Faraday mirror in combination with polarizing beamsplitters eliminates “echo” or “clutter” pulses from the system.
For a fuller understanding of the nature and advantages of the present invention, reference should be made to the following detailed description taken together with the accompanying figures.
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Schneier, Bruce, Applied Cryptography, 1996, pp. 554-557.
C. H. Bennett et al., “Eavesdrop-detecting Quantum Communcations Channel”,IBM Technical Disclosure Bulletin, vol. 26, No. 8, Jan. 1984, pp. 4363-4366.
C. H. Bennett et al., “Qu
Bethune Donald Stimson
Risk William Paul
Berthold Thomas R.
International Business Machines - Corporation
Meislahn Douglas
Swann Tod R.
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