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Eavesdrop-Detecting Quantum Communications Channel

IP.com Disclosure Number: IPCOM000041438D
Original Publication Date: 1984-Jan-01
Included in the Prior Art Database: 2005-Feb-02
Document File: 4 page(s) / 54K

Publishing Venue

IBM

Related People

Bennett, CH: AUTHOR [+4]

Abstract

This communications channel is impossible to eavesdrop on without a high probability of being detected. The security against eavesdropping follows from the uncertainty principle, and would not be compromised by any improvement in technology. The channel consists of a straight black tube about 1 meter in diameter and up to 100 km long through which are sent faint standard pulses of polarized light (0Œ, 45Œ, 90Œ, and 135Œ polarizations), in a beam with so little divergence (less than 10-5) that transmission losses are only a few per cent. The magnitude of the pulses is such that the receiver, with the best available photomultipliers (about 25 per cent quantum efficiency), detects an expected one photon per pulse.

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Eavesdrop-Detecting Quantum Communications Channel

This communications channel is impossible to eavesdrop on without a high probability of being detected. The security against eavesdropping follows from the uncertainty principle, and would not be compromised by any improvement in technology. The channel consists of a straight black tube about 1 meter in diameter and up to 100 km long through which are sent faint standard pulses of polarized light (0OE, 45OE, 90OE, and 135OE polarizations), in a beam with so little divergence (less than 10-5) that transmission losses are only a few per cent. The magnitude of the pulses is such that the receiver, with the best available photomultipliers (about 25 per cent quantum efficiency), detects an expected one photon per pulse. The uncertainty principle prevents an eavesdropper (even one with perfectly efficient photomultipliers) from reliably reading or copying such pulses if he does not know beforehand which pulses are rectilinearly (0OE or 90OE) and which pulses are diagonally (45OE or 135OE) polarized. The sender and receiver do know this information, having agreed on it beforehand (the 'quantum key' information), and can therefore use the pulses to communicate, a 0- or 45-degree pulse standing for a binary zero and a 90- or 135-degree pulse standing for a binary one. Because the receiver's photomultipliers are not perfectly efficient, and because of smaller losses in the quantum efficiency due to beam divergence, losses at the mirrors, etc., about 1/e = 0.368 of the pulses fail to be counted at all. An error-correcting encoding step, described in more detail below, permits the receiver to reconstruct the message despite these lost pulses. On the other hand, if too many pulses are missing, or if the decoder for the error-correcting code reports more than a very few polarization errors among the pulses that were received, the receiver concludes that the message has been subjected to eavesdropping and rejects it. In the drawing, mechanical shutters 10 and movable mirrors 29,31 are used to choose the optical paths in the sending and receiving subsystems, but the actual light pulses should probably be briefer than can be achieved with mechanical means, to reduce the number of spurious counts from the photomultiplier's dark current.

This is done by electronically pulsing the laser 14 during the time window when the mechanical shutters 10 are in position (i.e., one open and three closed), and, at the receiving end, discriminating electronically against pulses that arrive outside the briefer electronic time window determined by the pulse time and propagation delay. The neutral density filter 16 in the sending subsystem serves to reduce the intensity of the standard pulses produced by the shutter 10 and polarizer 18 until they yield an expected one detected photon per pulse for the receiver. The optics of the sending subsystem aims to achieve uniformity of pulse intensity and narrow beam divergenc...