Quantum computers have arrived at last, experts say – but instead of replacing standard computing methods, they are complementing them.

In the 1990s, quantum computing was heralded as a way to outperform even the best supercomputers. By the end of the decade the first quantum computers were up and running in laboratories. But over the following 10 years, the technology was notable by its absence outside the lab.

Now, though, the first commercial quantum systems are coming into use as peripherals that allow conventional machines to communicate more securely. However, in an era of growing cybercrime, physicists warn that even quantum network links won't be invulnerable to hackers.

Quantum net

There's a danger that people will begin to think quantum computing is all hype, says Philip Hemmer at Texas A&M University in College Station. "This is clearly not the case as quantum security systems are becoming reality."

The world's first quantum cryptography system, linking BBN Technologies and Harvard University, both in Cambridge, Massachusetts, was created in June 2004.

The speed of quantum links grew faster and by October 2007 the technology was advanced enough for id Quantique, a quantum communication company based in Geneva, Switzerland, to secure the transmission of results from the State of Geneva elections.

But assuming the technology is totally secure would be a mistake: physicists have now started to show that such systems can be subverted to reveal quantum-secured secrets.

Shared secret

Before sensitive data is sent through a quantum network, the sender and receiver first establish a secret code known only to the two of them. It is sent to the receiver as a series of quantum bits, or qubits, each encoded into the polarity of a single photon.

Any attempt by a third party to intercept and read those photons to steal the secret code is doomed to fail because reading the photons affects their polarisation state and adds noise to the signal.

Sender and receiver can quickly spot the intrusion by comparing a random subset of the secret code, and switch to a different security channel before important data is sent.

Hidden flaws

This failsafe system has been borne out in many tests, leading to a widespread belief that quantum communication systems are unconditionally secure.

But Hoi-Kwong Lo at the University of Toronto in Canada warns that there is no room for complacency.

During careful study of quantum communication systems, his team has identified flaws that supposedly shouldn't exist. In 2007, they realised that imperfections in the design of the detectors used to receive the quantum key represent a chink in the technology's armour.

Then, last year, they became the first group to experimentally demonstrate how to hack into a commercial quantum encryption system – the ID-500 sold by id Quantique

Design flaw

Most commercial quantum links have two detectors, each tuned to detect protons in one of the two different polarisation states – "1" or "0" – used to make up the secret code. The sender, "Alice", tells the receiver, "Bob", over a standard link when she is about to send a quantum signal – at which point Bob switches on his two detectors for a few hundred picoseconds.

If one of the two photon detectors registers a hit during that time, Bob lets Alice know, confirming a signal has arrived. An eavesdropper, "Eve", can listen into the conversation and knows when the quantum signal will be sent, but because she doesn't know whether Bob's "1" or "0" detector registers a hit from the quantum signal, she learns little.

Lo's team realised that tiny imperfections in the design of the photon detectors mean they aren't quite switched on at the same instant, and for a few picoseconds only one will be on.

If Eve knows, for instance, that the "1" detector is switched off slightly later than the "0" one, all she must do is direct Alice's signal through a slightly longer fibre optic cable en route to Bob. Timed right, Eve can make sure the photon arrives at Bob when only his "1" detector is open. Now, if Bob registers a click and tells Alice, Eve knows that the photon was in the "1" state.

Time shifting

"Time-shifting the optical signal can be done with an optical delay line and an optical switch – both are simple components that are commercially available," says Lo. In tests, his team can successfully hack a commercial quantum communications device 4% of the time, he says. A low success rate, but enough to show the link is far from unbreakable.

There are a number of relatively simple ways that this particular loophole can be closed, says Bing Qi, a member of Lo's team. "But the main question is how to find the security loopholes in the first place."

Grégoire Ribordy, CEO at id Quantique, says the firm takes security loopholes very seriously and has set up collaborations with leading research groups in the field. "The loophole identified by Professor Lo has been fixed and current systems are immune to it," he says.

Courtesy Computer Crime