Research conducted at the University of Bristol means a number of quantum computing algorithms may soon be able to execute calculations of a complexity far beyond what today's computers allow us to do. The breakthrough involves the use of a specially designed optical chip to perform what's known as a "quantum walk" with two particles ... and it suggests the era of quantum computing may be approaching faster than the scientific establishment had predicted.

A random walk – a mathematical concept with useful applications in computer science – is the trajectory of an object taking successive steps in a random direction, be it over a line (with only two possible directions) or over a multi-dimensional space. A quantum walk is the same concept, but translated to the world of quantum computing, a field in which randomness plays a central role. Quantum walks form an essential part of many of the algorithms that make this new kind of computation so promising, including search algorithms that will perform exponentially faster than the ones we use today.

For their experiments, the researchers designed a network of optical circuits in a silicon chip, and then managed to make two photons perform a quantum walk along the network at the same time. Other researchers had previously achieved quantum walk for a single photon, but this was the first time that a quantum walk was achieved with two photons.

The task was challenging because, in order for the algorithm to perform correctly, the two particles need to be exactly identical and the researchers needed to account for how the two particles would interfere with each other as they moved throughout the circuits. Going from two to more photons, however, should be relatively straightforward as the same principles would apply.

But why go through all this trouble just to add a few more particles? The answer is that with every photon added to the system, the number of outcomes increases exponentially. So, if a single-photon quantum walk has 10 possible outcomes, a two-photon walk will have 100 possible outcomes, and so on. This allows the researchers to simulate highly complex situations that are currently outside of the number-crunching possibilities of today's fastest supercomputers.

The team is planning to apply their results to develop new and more sophisticated simulation tools, by increasing both the number of photons in the system and by using larger circuits. In the long run, a multi-photon quantum walk could be used to reliably simulate and thus better understand physical phenomena governed by the laws of quantum mechanics where photons are involved, such as photosynthesis and photon absorption in solar cells, as well as to develop exponentially faster search engines.

"Using our new technique, a quantum computer could, in less than ten years, be performing calculations that are outside the capabilities of conventional computers," commented Professor Jeremy O'Brien, Director of the Centre for Quantum Photonics. The research will be published in tomorrow's issue of the journal *Science*.

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