The Dawn Of Quantum Applications

Unbreakable cryptography, unimaginable simulations of profoundly complex problems and super-fast networks are just some of the promise held out by quantum computing. And now European scientists are poised to deliver on that promise, thanks to the work of the Qubit Applications (QAP) project.

The integrated project has cherry-picked major obstacles in the path of quantum computing, problems that could have immediate applications and could command a ready market.

Chief among them is quantum cryptography. “Quantum computing, when it arrives, could make all current cryptographic technology obsolete,” notes QAP co-coordinator Professor Ian Walmsley.

Thankfully, researchers have developed quantum cryptography to deal with that issue.

“Quantum cryptography over short distances was demonstrated in a previous project,” explains Walmsley. “The problem is, it only works over a short distance.”

Weaving entangled webs

That is because quantum cryptography relies on entanglement. Entanglement is a concept that explains how two or more particles exhibit correlation – a relationship if you like – that would be impossible to explain unless you supposed that they belonged to the same entity, even though they might be separated by vast distance.

Imagine you were playing a game of quantum coin flipping with a colleague: you are heads and the colleague tails. You are two distinct individuals, but if the coin comes up heads your colleague loses, and you win. There is a correlation between the coin tossing. Now, with a quantum coin, it is heads the colleague wins and tails you win at the same time.

This is the extra bit that quantum mechanics gives us, and which we use in secure communications, suggests Walmsley.

That explains, with a little inaccuracy, the concept of entanglement, and it is at the core of quantum key distribution, or QKD. It is far too complex to break quantum encryption by brute force, and it is immune to eavesdropping because, at the quantum level, the act of observing an object changes the object observed. It means that encryption is guaranteed by the laws of physics.

The technique was demonstrated in Vienna 2008, but it works only over short distances. EU-funded QAP hopes to develop a quantum repeater that can maintain entanglement over large distances. It has already had considerable success up to the 200km range, and growing.

Ideal information carrier

Maintaining entanglement over long distances – so essential to QKD, but also communications and networks – is the most immediate and compelling application in the QAP programme, but it is far from the only one. Many other areas of work show signs of progress, too. Storage and memory are essential for quantum computing.

It is not too difficult to encode a piece of information on a photon, which is an ideal information carrier because of its high speed and weak interaction with the environment.

It is difficult to store that information for any length of time, so QAP is developing ways of transferring quantum information from photons to and from atoms and molecules for storage, and the project is making steady progress.

Similarly, QAP’s work to develop quantum networks is progressing well. One team within the overall research effort has managed to develop a reliable way to calibrate and test detectors, a prime element in the network system.

“This is important because it will be essential to develop reliable methods to test results if work on quantum networks is to progress,” notes Walmsley. The research group has submitted a patent application for this work.

Stimulating simulation

Quantum simulation, too, offers some tantalising opportunities. The primary goal of QAP’s Quantum Simulations and Control subproject is to develop and advance experimental systems capable of simulating quantum systems whose properties are not approachable on classical computers.

Imagine, for example, trying to model superconducting theory. It is hugely complex, and classic computers are quickly overwhelmed by the size of the problem.

But quantum methods are inherently capable of dealing with far greater complexity, because of the nature of the qubit, or quantum bit. Classical, digital bits operate on the basis of on or off, yes or no. But quantum bits can be yes, no, or both. It takes classical computing from 2D, into the 3D information world.

One could say that, while classical computers attack problems linearly, quantum computers attack problems exponentially. As a result, with just a few qubits, it is possible to do incredibly large computations, and that means that quantum simulation of complex problems could be a medium-term application.

“We are not saying we will solve all the problems in the area of simulation, but we will make a good start,” warns Walmsley.

That defines QAP nicely: a kick-start for quantum applications in Europe.

The QAP project received funding from the ICT strand of the EU’s Sixth Framework Programme for research.