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Quantum Encryption - An IntroductionThe use of quantum mechanics to encrypt messages may foil eavesdroppers and code-breakers for goodAT THE speed of light, Hephaestuss sacred fire/ Blazed from beacon tower to beacon tower. Thus,

according to Aeschylus, spoke Clytemnestra, an ancient queen of Mycenae, explaining how shereceived tidings that her husband Agamemnon was on his way home from the Trojan war. PerhapsAgamemnon would have done better to have turned up unannounced. The news left the queen andher lover just enough time to plot the cuckolds assassination. When Agamemnon sent the message, though, he probably worried less about betrayal thaninterception. Using signal fires or flashes of sunlight reflected from a mirror was (and is) a cheap andsimple way to broadcast information. But although such signals were used until the early 20th centuryto co-ordinate military maneuvers, they are vulnerable to espionage: a clever onlooker can easilyobserve the flashes of light and crack the message, even if it is encoded.It may seem surprising, therefore, that some physicists at Los Alamos National Laboratories in NewMexico believe that they can transmit messages through the open air in complete secrecy, by

exploiting the quantum-mechanical properties of light. In June 2001, at the International Conferenceon Quantum Information, in Rochester, New York, they explained how to build a system that willbroadcast un-crackable messages via satellite.

All keyed up

Modern cryptography, such as that employed in a widely used program, Pretty Good Privacy (PGP),relies on the mathematical manipulation of data. To encrypt, or lock, a message, the programperforms a series of mathematical steps on it, using a number more than a hundred digits long. Only

that numbers correct mate, or key, is able to undo these steps and unpack the message. The size of these numbers, and the mathematical formula that links them, make it, in effect,impossible to work out the key by computational brute force. One of todays keys could not becalculated even if all of todays computers worked for the lifespan of the universe on the task. Surelythat is safety enough?Perhaps not (that is why the software is called pretty good and not perfect). Future advances incomputing may be enough to overwhelm the defences of PGPand its kind. More prosaically, a thiefwho stole a key could impersonate its careless ownerand nobody would ever know the difference.Richard Hughes, the leader of the team at Los Alamos, thinks that using photons (the particles, orquanta, of the quantum theory of light) to manufacture and distribute keys would correct these

weaknesses. Individual photons possess properties that are governed by one of the basic laws ofquantum mechanics, the uncertainty principle. A photons polarization, for example, can be measuredagainst any of three yardsticks: the horizontal axis, the diagonal axis, and the circular axis. However,the more that is known about one of these, the less can be known about the others. In other words, ifan exact measurement is taken of a photons horizontal polarization, nothing at all can be knownabout its polarisation on the diagonal or circular axes. Take the same measurement twice, and theanswer will remain the same; but take one and then another, and the first answer becomesworthless.

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A light in the distance

This feature of photons means that they can be used to construct single-use, self-destructing keys,

rather like the envelopes in spy movies that conveniently explode when they get into the wronghands. To agree upon a key, the sender and the recipient (by convention called Alice and Bob) mustarrange to open a channel for sending the photons. Alice makes a string of photons, measures thepolarisation of each as she pleases, and then sends them off to Bob. Bob measures the polarisationof each of the photons as it arrives, in any way he chooses.From this point on, Bob and Alice can communicate without cloak or dagger. Bob rings up Alice andtells her what sort of measurements he performed, though not his results. Some of his measurementsare likely to have overlapped with Alices. She tells him which measurements to keep, and which todiscard. They now both know which bits they share information about, and so are able to agree onthe spot upon a private key that consists of this common string of bits.Eve, an eavesdropper listening to their conversation, requires Alices original string of photons in

order to make head or tail of this exchange. This is where the natural safeguards of quantum physicsmake their advantages felt. If Alice had sent a beam of photons to Bob, Eve could have interceptedthe beam and split it into two, taking a portion and sending the rest on to Bob, who may not notice theloss. But individual photons cannot be split: any photons that Eve intercepts, Bob will surely miss.Should Eve adopt the so-called bucket-brigade strategyto intercept and resend photons asquickly as she canshe will still give her presence away. The uncertainty principle dictates that Evecannot copy Alices photons exactly. The error she introduces by making faulty copies will be enoughto trigger Bobs suspicions. Alice and Bob can then compress their common key in a way that willfrustrate Eves efforts.In theory, the system also protects against future snooping. Because the mould that made this uniquequantum keythe string of photonsis lost forever, it would remain invulnerable even if thedaughters or grand-daughters of Eve were to possess unlimited computing power. And since thequantum key is not only made and used, but also discarded, on the spot, nobody can spirit it awayand use it to impersonate Alice or Bob later on.Other teams have already sent quantum keys over long stretches of optical fibre. But Dr Hughessgroup has a loftier goal. Next month, it will test a system for distributing quantum keys under thecloudless skies of New Mexico. With a very faint laser, an outpost will emit a string of photons. Tenkilometres away, a telescope will try to discern these brief scintillations against the immensebackground noise of desert daylight. Early trials have suggested that the system will be able to sendsingle photons reliably, day or night, and will not blink out of service when weather conditions do notsuit it.

Since transmitting photons through the dense air at ground level is harder than transmitting straightup through the atmosphere to an orbiting satellite, Dr Hughess experiment opens up the possibilitythat earth-to-satellite quantum-cryptography channels could be built. Once that happens, anybodywithin range of the satellite would have a completely secure way to get a quantum key. It seems thatthe ultimate in cryptographic technology may share much in common with the primitive. Telescopeshunting for distant glimmers of light will make a comeback. The flashes will just be a bit flashier.