News analysis: Doing the quantum walk

There has been a lot of research undertaken on quantum computing since the concept was outlined in 1982 by Nobel Laureate Richard Feynman. But despite all the work, progress has been slow and, until recently, quantum computers remained largely theoretical. Rather than using binary mathematics as its basis, the quantum computer takes advantage of the strange world of quantum mechanics, where things aren't always what they appear. While everyday computers use the concept of bits with a value of 0 or 1 as the basis for their calculations, quantum computers use qubits. These building blocks not only exist as the 0s and 1s with which we are familiar, they can also exist as so called 'superpositions'. Any operation on a qubit is applied to both states, so one operation is applied to two values, two operations to four, and so on. Increasing the number of qubits increases the number of operations exponentially. In theory, complex calculations can be completed in a fraction of the time needed by current computers. The gap between the theory of quantum computing and its reality is large; many researchers are pursuing ways of building such a device, but none has done so as yet. However, some recent developments are encouraging scientists to suggest that the first quantum computers could be operating in less than a decade. One of the latest pieces of work to be published comes from the University of Bristol's Centre for Quantum Photonics (CQP). "It is widely believed that a quantum computer will not become a reality for at least another 25 years," said Professor Jeremy O'Brien, CQP director. "However, we believe, using our new technique, a quantum computer could, in less than 10 years, be performing calculations that are outside the capabilities of conventional computers." An international research group led by scientists from CQP has developed a chip which it believes could be used to perform complex calculations and simulations using quantum particles. As such, the team believes its device represents a new route to quantum computing. The technique uses two identical photons moving along a network of circuits in a silicon chip to perform what is known as a 'quantum walk'. A quantum walk is an algorithmic approach which provides a basis for universal quantum computation. The team says that quantum walk experiments using one photon have been done before and can be modelled by classical wave physics. However, this is said to be the first time a quantum walk has been performed with two particles and the implications, according to Prof O'Brien, are 'far reaching'. "Using a two photon system, we can perform calculations that are exponentially more complex than before," he said. "This is very much the beginning of a new field in quantum information science and will pave the way to quantum computers that will help us understand the most complex scientific problems." The leap from using one to two photons is complex: not only because the two particles need to be identical in every way, but also because of the way in which the particles interact. "Now we can directly realise and observe two photon quantum walks, the move to a three photon or multiphoton device is relatively straightforward," said Prof O'Brien. "Each time we add a photon, the complexity of the problem we are able to solve increases exponentially, so if a one photon quantum walk has 10 outcomes, a two photon system can give 100 outcomes and a three photon system 1000 solutions and so on." In the short term, the team expects to use the results to help them develop new simulation tools. In the longer term, a quantum computer based on a multiphoton quantum walk could be used to simulate processes governed by quantum mechanics, such as superconductivity and photosynthesis.