Quantum cryptography

Our intuition is developed by watching the world around us. This intuition works fairly well in situations that we usually encounter, but when we stray too far from these, our intuition fails miserably, and we have to fall back on mathematics and physics to understand how things work. In particular, if things get too big or go too fast, then our intuition fails, and we need to fall back on Einstein's theories of relativity to explain things accurately. GPS satellites, for example, are affected by both of these possibilities. They are near a big, heavy object (the Earth) and they go fairly fast in their orbits. Relativity tells us that time gets distorted in either of these cases, and we find that we need the framework of relativity if we want to make GPS satellites accurate enough to be useful. Without the use of relativity to correct for the slight time distortions that these satellites experience, position errors in a GPS system would accumulate at a rate of roughly 6 miles per day.

Our intuition also fails when things get very small. This is the realm of quantum mechanics, and the models that predict things accurately on this scale are nothing like what we see in our daily lives. In particular, quantum mechanics tells us that quantum systems exist in all possible states at once, and that measuring such a system collapses it into one of the possible states, losing information about the other states when it does this. So while a classical bit is either a logical 0 or a logical 1, a quantum bit can be both 0 and 1 at the same time, and if we measure its state it will turn into either a 0 or a 1, losing all of the information about the other state. This means that any information that we encode as quantum states has very different properties than the information that we encode using classical bits and bytes. It also provides the basis for three interesting technologies: quantum cryptography, quantum computing, and counterfactual computing. The most mature of these is quantum cryptography.

The term "quantum cryptography" is a bit misleading. The term describes a technology that is used to distribute cryptographic keys that are encoded as quantum information, so "quantum key distribution" is a more descriptive name for it.

An adversary who intercepts a transmission protected with quantum cryptography will destroy some of this quantum information when he tries to determine the state of what he intercepted. When this happens, he will be unable to make exact copies of the information, so he will be unable to retransmit an exact copy of what he received. Because of this, the intended receiver will be able to tell that this transmission was intercepted, and decide to not use the key that was observed by the eavesdropper. So quantum cryptography cannot stop an adversary from eavesdropping, but it can detect when such eavesdropping has happened. The first quantum cryptography protocol was invented by 1984 by Charles Bennett and François Brassard, and is commonly called the BB84 protocol.

In the BB84 protocol, for each bit that the sender needs to transmit, he needs to pick a coordinate system with which to encode the bit. This defines what states the quantum information contains. He can use coordinates based on the familiar binary 0 and 1, or he can use other sets of coordinates. He then encodes the bits using the appropriate coordinate system and transmits them. After this, he sends a list of the coordinate systems that he used for each bit. The recipient needs both the encoded bits and the coordinate systems that were used to encode them to recover the information that was sent in this way.

An eavesdropper who intercepts the encoded bits will destroy some of the quantum information in them when he checks their state. This loss of information will cause errors that will be detected by the recipient – some errors usually happen in any transmission, but too many errors indicates that eavesdropping has occurred. An eavesdropper can also intercept the list of coordinate systems that is sent, but without the information that was encoded with them, knowing the coordinate systems is useless.

Information protected by quantum cryptography needs to be encoded in quantum states, and existing implementations use individual photons that are then transmitted over a fiber-optic link. Because any hardware in a communication channel that boosts the fading signal strength needs to interact with the signal, existing quantum cryptography technologies are limited to a single fiber-optic link. Repeaters act just an eavesdropper, and destroy quantum information when they interact with it.

Quantum cryptography is an established and proven technology. There have been commercially-available quantum cryptography products since 1999, and there are now two vendors from which the technology is available. On the other hand, while the problems of key distribution and key management are indeed difficult, they have not become so difficult that quantum cryptography is an attractive alternative for most commercial deployments. So although the technology has been available for quite a while, it has not yet become a commercial success. Maybe we'll be seeing more of it in the future.