The nascent industry of quantum communications could suffer a fatal blow if the U.S. enacts sweeping new regulations to provide wiretapping access to law enforcement.

The weirdness of quantum mechanics makes it possible for two parties to share an encryption key and be sure that no one else can copy it. Any attempt to eavesdrop on the communication of the quantum key would irreversibly disturb its quantum state, thus revealing that the channel is being wiretapped.

In recent decades, the development of quantum communication and encryption has motivated significant advances in basic research in mathematics, physics and engineering.

The first of such futuristic systems are already commercially available and have been installed by a few banks and government agencies. And quantum data security could one day be available over the Internet to anybody with a fiber-optic connection, as MIT physicist Seth Lloyd wrote in Scientific American [see "Privacy and the Quantum Internet," October 2009,].

The New York Times now reports that the U.S. government is seeking to establish new regulations that would outlaw encryption systems that don’t provide a built-in way for a third party to intercept and decrypt the data. Law enforcement would use such "backdoor" access, subject to court approval.

The details of the proposal are still being debated among various branches of government, but according to the Times—which first broke the story on September 27—the Obama administration plans to introduce a new bill next year.

The bill would affect all sorts of services that use encryption, from BlackBerrys to Skype. These services would have to be redesigned to enable wiretapping, which could put a considerable burden on the companies that provide them and on any future start-up company attempting to bring innovation to the field.

Critics have also pointed out that similar government efforts have been tried in the past, only to be abandoned when authorities found them to be unenforceable, in part because it is relatively easy to write and distribute encryption software. A document sent over a BlackBerry, for example, could have an additional level of encryption added by the user. Factory-provided backdoor access would then not be of much help to a wiretapper; it would be like unlocking a safe only to find another safe inside.

As the Times article noted, it is hard to see how such laws could be enforced on encryption software that users have already installed on their computers, or that is sold by foreign companies—to which U.S. law would not apply—or made freely available by amateur programmers. "There would be a black market for cryptosystems without a back door," says Lloyd.

But quantum encryption systems would face stark consequences. "This would probably be the end of the quantum key distribution industry," says physicist Norbert Lütkenhaus of the University of Waterloo in Ontario.

Quantum key distribution requires special hardware to produce the encryption keys. And the laws of physics make it impossible for a third party to copy or store the key.

Two types of quantum weirdness make quantum encryption possible: quantum superposition and quantum entanglement. Here’s how it works: the device that produces quantum keys emits pairs of photons, and sends one photon from each pair to Alice and the other one to Bob. Each photon is in a superposition of states—for example, a photon’s polarization can be simultaneously horizontal and vertical—thus representing a "0" and a "1" at the same time. Only the act of detecting the photon can make it settle on one of the possible choices.

Moreover, the two photons in each pair are "entangled," which means that once Alice finds a photon to be, say, a 1, she knows that the corresponding photon received by Bob must necessarily be a 0. Alice and Bob can then use their strings of correlated bits as a common encryption key. Any act of eavesdropping will destroy the entanglement, so it’s easy for Alice and Bob to detect the breach.

Once Alice and Bob have their shared encryption key, Alice can then use it to encrypt her message and send it to Bob by any means of communication: to anyone who intercepts it, the encrypted message will look like a string of totally random bits, and only Bob, who has the key, will be able to decrypt it.

Trouble is, there is no way for the designers of the quantum encryption device to store a copy of the key or to provide it to a third party: after the device has sent out the photons, even the device itself has no way of knowing which of the bits will turn into 1s and which will turn into 0s. "Nature kind of has a guaranteed right of privacy already built in," Lloyd says.

It is possible that new quantum encryption protocols could be designed to allow a "trusted intermediary," which could be a company such as Skype, to oversee the process, for example using triplets of entangled photons instead of pairs, says Artur Ekert of the National University of Singapore, a pioneer of the field. But secure communication that relies on a third party can also be established using classical physics, Lütkenhaus points out; the whole point of quantum encryption was to guarantee secure communications without the need to trust a third party.

Thus any legal requirement for all communication technology to provide backdoor access could effectively outlaw the entire quantum encryption industry. Ironically, governments, which are themselves customers of the technologies, could end up suffering too.

There is another way that regulators could get around the problem, Lloyd says: "Maybe they could try to legislate the laws of physics."