How 007 could send an unbreakable coded message to Moonraker
Michael Brooks
In New Mexico the light can be truly astonishing. Not because it gives the desert a strange aesthetic beauty, but because it contains coded messages.
Researchers at the Los Alamos National Laboratory announced recently that they have written data into single photons, the tiny packets of energy that make up light. They then fired these photons through the sunny desert sky. A mile later they picked those exact same photons out from the billions of others also hitting a detector, and then pieced the message together.
This mile-long path is a crucial breakthrough. The turbulent part of the atmosphere that can disrupt signals is about a mile thick, so it is now feasible to put data into a single photon and send it to a satellite. The information it contains can then be beamed to anywhere in the world. And, even more importantly, the nature of photons makes it impossible to eavesdrop: these coded messages are absolutely unbreakable.
The technique, which is set to provide a revolution in cryptography, is known as “quantum key distribution”. It is almost entirely equivalent to the systems currently in use to exchange messages by optical links, but there is one crucial difference.
While standard keys are protected by the almost insoluble mathematical problems used to create them, the quantum key is protected by something even more fundamental than clever mathematics. “Its security is based on laws of nature,” says Richard Hughes, head of the Los Alamos team.
A single photon is a quantum particle, and the laws of nature that govern such particles are extremely useful for code- making. The photons simply can’t be copied, and they can’t be diverted without disturbing their delicate state. An eavesdropper will affect the sequence of numbers in a way that the transmitter and receiver are easily able to check.
To establish a key, the researchers first encode binary data in the photons. Each photon can handle one digit – a clockwise- spinning photon, for example, might be a 1; anti-clockwise is 0. The whole key is a sequence of such digits, which can then be added to the digitised information you wish to secretly exchange. The receiver simply subtracts the digits in the key to reveal the original message.
With quantum channels into space, it will be possible for a Central Intelligence Agency (CIA) operative in South America to communicate with a satellite to establish a quantum key. The same satellite will also establish another secure key with CIA headquarters. The satellite can then mix these keys and send the agent the result.
Using a combination of this mix and his own key, the agent can encrypt messages that only his bosses at headquarters can decode. So he can use any old radio to transmit schedules of mafia drug-running trips. It wouldn’t matter how good the mafia were at working out the CIA’s standard mathematical codes, this message would simply be impossible to read.
Eventually, Hughes says, such totally secure communication will be possible from anywhere on Earth. “Satellites would extend quantum key distribution to distances beyond those possible with fibres, and to situations with no fibre access.”
The project is now beginning to look seriously into space. “We are starting to make preliminary plans for a surface-to- satellite quantum key distribution experiment,” Hughes says.
As yet, though they do not have firm plans for putting an experiment on a particular satellite mission. Hughes believes a space experiment is probably five years away, since it will involve developing a practical and “spaceworthy” version of the system.
While they work through these issues, they are also quietly working towards perfecting quantum crypto- graphy with standard optical fibres. The Los Alamos team has already achieved record-breaking photon distribution networks: using the longest fibre available to them, they have transmitted quantum keys over 50km.
That means, Hughes points out, all of Washington’s high-security institutions could be linked by unbreachable “quantum- secure” communications lines. However, he won’t comment on whether such projects are actually under consideration.
It won’t only be a spook’s technology: quantum cryptography will undoubtedly find markets within private sector companies who demand privacy in their communications. Financial institutions will begin to use fibre-based quantum links in the next few years, and operators of satellites are likely to be more than mildly interested by the Los Alamos breakthrough.
As the world’s reliance on satellite-based communications increases, security against instruction interception and “hijacking” becomes ever more important. The tiny photon provides the world’s most reliable courier service. And it now delivers into space.
The Los Alamos researchers send one million key digits through the air every second. A truly random sequence of bits is necessary for the quantum key; they took their bits from the fluctuations of electrical noise. Depending on whether this specified 0 or 1, one of two lasers fires a single photon: the 1 laser fires photons with, say, clockwise spin; the 0 laser gives an anti-clockwise-spinning photon. The transmitter and receiver can compare the bits over a standard public communications channel to check for errors and eavesdropping and can choose which bits to use for a quantum key.
In the desert sun, the researchers lost around nine-tenths of the transmitted photons, but correctly interpreted 95% of those that arrived at the detector.