The cavernous Royal Hall of Industries in Sydney is decked out in plush drapes and coloured lights. A huge screen looms behind a stage at one end, casting a pale wash over the hundreds of people inside. The floor of the hall is packed with tables, around which business and industry leaders, politicians, media pundits and intellectuals talk with good humour, affected by the nervous excitement that seems to crackle through the room. People look around, noting respected and familiar faces. It could be the Oscars, but here the crowd are lauding smarts, not celebrity.

The annual Australian Museum Eureka Prizes are some of the highest awards for science in Australia. Winning one is more than just a pat on the back – it’s also a chance to convey your achievements to the wider public and network with people who might take your scientific success to the next level. Although prize winners are notified ahead of the ceremony, it’s still a high-pressure environment for researchers who are often more comfortable in studious solitude than in the spotlight.

“It was incredibly exciting,” explains ANU physicist Vikram Sharma. “Usually you beaver away in your own lab. To gain recognition at a national level is quite humbling, particularly when you see all these very talented scientists around you working across a spectrum of fields, each of which is incredibly complex.”

Sharma was part of a team that won the $10,000 Eureka Prize for Scientific Research for developing a means to transmit ‘unhackable’ information. The laser technology is now in the process of being commercialised via QuintessenceLabs, a Canberra-based company established by the researchers. The work has already attracted funding from the Department of Defence and other information-dependent agencies have shown keen interest. There’s mounting excitement about a new approach to information security that could spell the end of code-breakers.

From concept to prototype

Edgar Allen Poe is remembered as a master of macabre fiction, but it wasn’t just ravens tapping in his world. Poe loved code-breaking and his writings on the subject helped crack the codes used by Germany during the Great War. Supremely confident of his own abilities, and generous enough to own that others might equal them, he once ventured that it wasn’t possible for “human ingenuity” to create a code that it could not also decipher.

In Poe’s time, cryptography largely involved the encoding of texts through transition or substitution. In recent times, this classic approach has evolved considerably, employing complex mathematics. Modern encryption is based upon mathematical operations on randomly generated large prime numbers, which are extremely difficult to reverse with existing technology. But the prospect of quantum computers that are many times more agile and powerful than their conventional counterparts means that information security can’t be guaranteed in the longer term. It looks like Poe’s prophecy would hold true were it not that quantum physics could also be used in encryption.

Dr Ping Koy Lam heads up the Quantum Optics Group, part of the Department of Physics at ANU. A high-achieving early-career researcher, he won his first Eureka Prize in 2003 for work on quantum teleportation. The group’s quantum cryptography project grew out of earlier success with stabilising laser beams. Lam and his colleagues quickly realised that their ability to perform minute manipulations on lasers could be used to convey information too.

“Where traditional cryptography is based on complex mathematics, we instead use the laws of physics to guarantee communication security,” Lam says. The concept was to encode secret ‘keys’ onto laser beams by manipulating the beams of light at the quantum level. The message could not be intercepted undetected, as any attempt to eavesdrop would disturb the beam and become apparent to both sender and receiver. A series of clean-up steps ensured that any intercepted information would be rendered useless.

Such a project required expertise in a range of disciplines. PhD candidate Andrew Lance and Dr Thomas Symul brought their skills in quantum optics, while Professor Timothy Ralph and PhD candidate Christian Weedbrook from the University of Queensland contributed much to the theoretical framework. Sharma brought the computer science skills needed to help generate the final secret keys.

During two years of experiments, the group gradually achieved more and more of the stages involved in transmitting a secret key via laser. It was a challenging process but eventually the group achieved their goal. Their work means that encryption keys can be generated with absolute secrecy so that only the intended recipient can use them.

“Although several groups around the world have quantum cryptographic technology, our group was one of the first in the world to demonstrate the transmission of a completely secret key via bright laser beams and common optics,” Symul says.

Selling the idea

It looks forbiddingly technical to the unacquainted – a complex arrangement of boxes, wires, lenses and beams, all laid out on a grid about the size of a household door. But according to Sharma, who heads up the newly incorporated QuintessenceLabs, the experiment is “relatively simple”, at least for those with an interest in optical science and engineering.

“The simple approach means you can build the technology at a reasonable cost, and that it’s much more robust. We use technology from the telecoms industry, so it’s tested and tried and available at a reasonable cost.”

In the short term, the group plans to develop a commercial version of the technology that would be able to transmit secret keys along optic fibres over distances up to 100 kilometres. This would be suitable for interagency communications within most cities. In the longer term, the technology could be adapted to make use of satellite communications to transmit secret keys all over the world. With global spending on security expected to reach over two trillion dollars this year, the quantum cryptographers hope their breakthrough will continue to get the red carpet treatment worldwide.

How it works

Vikram Sharma explains how the group’s quantum key distribution system works. In classical cryptography, the sender is referred to as Alice, the recipient as Bob, and the eavesdropper as Eve.

Step [1]

A laser beam has billions of photons travelling per second. We call this the carrier frequency. We make sure that beam is as precise and noise-free as possible. From the carrier frequency, we shift a small number of photons out of the billions, and drop them at frequencies that are displaced, plus or minus, from the carrier frequency. In this way, we encode the random number ones and zeroes on the side bands of the laser. Using this method we’ve setup an experimental prototype to simulate transmissions over 50 kilometres of fibre.

Step [2]

Bob’s receiver station makes very sensitive measurements of the laser beam to strip out the carrier and look at what was coded on the sidebands. This gives him the raw key, but the encoded data has been jiggled around by quantum noise. The key that Bob has received differs from that which Alice has sent. Also, in quantum cryptography we are totally paranoid, so we always assume that Eve is omnipresent and omniscient. We assume that all the losses in transmission are somehow captured by Eve and she is able to make optimal use of this information.

Step [3]

The raw data that Bob has received is manipulated in various ways to adjust for characteristics of the electronics and the transmission channel. We then carry out a procedure called post selection. This involves capitalising on the random nature of quantum noise by making the most of those events where Bob received more information than Eve. The idea is for Bob and Alice to capture those transmissions where Bob did really well in his measurements, and discard the rest.

Step [4]

Via a process called advantage distillation, we amplify the advantage that Alice and Bob have eked through the post selection procedure. This comes at the expense of discarding more bits of the
raw key.

Step [5]

We then engage in a process of secret key reconciliation, which is essentially a procedure for error correction. Recall that the data is noisy. Bob makes his key match Alice’s through a series of indirect disclosures that are somewhat analogous to the game Master Mind. But if Bob discloses information about his bits, even if it is indirect, then an eavesdropper could also get to know this information. Again we assume that Eve is eavesdropping and is trying to maximise her knowledge of the secret key using a composite of all the information she has gained from all her eavesdropping activities.

Step [6]

After the key reconciliation step we’ve got a perfectly clean key. The final stage is to negate the usefulness of all the information that Eve has. We draw on abstract algebra to carry out an operation that is somewhat analogous to kneading dough. Imagine that the information that Alice and Bob share is represented by a piece of pizza dough. We assume that Eve has gained considerable knowledge of the key as well from her eavesdropping, however she still has some uncertainty about some of the bits. If we represent those bits that she doesn’t know anything about by a dot of blue ink on the dough, by kneading it we spread the ink throughout her dough. Alice and Bob don’t have a problem because their bits have been perfectly aligned prior to this operation – even if you jumble them, it doesn’t matter. And just to be sure that she can’t reverse engineer the process, we chop off a bit of the dough and use only this small portion for the final key. Even if Eve had some incredibly powerful computer, she’d never be able to reconstruct Alice and Bob’s final secret key.

^^

ANU Reporter 
Summer 2007