A research team at Oxford University working on quantum technologies that will sit at the heart of the world’s first scalable quantum computer, has made a major breakthrough in the field of quantum particle research.

The team at the Department of Physics, led by Professor Ian Walmsley, has for the first time successfully demonstrated an effect called three-photon interference, a curious behaviour of fundamental particles of light that could have a significant impact on the future of quantum computing.

With millions of times the processing power and speed of conventional computers, quantum computers have the potential to revolutionise every area of our lives.
The breakthrough comes on the 30th anniversary of the landmark discovery of two-photon interference. Light is made up of particles or energy packets called photons, and ordinarily these particles do not interact with each other. However, it was found that the dynamics of two photons of light can become interconnected, even though there is no physical interaction between them. Known as the Hong-Ou-Mandel (HOM) effect, the experiment uses a beam splitter that splits one beam of light into two, transmitting one half of the light and reflecting the other. Scientists showed that two identical photons arriving at the same time into a beam splitter which has two exit ports will always emerge from the same port together, a process known as interference. They can emerge from either port but will always leave as a pair bunched together. The bunching effect is stronger the more characteristics the photons share, such as colour, polarization, shape, and timing.

Now for the first time this interference effect has been achieved using a three-particle version of the experiment. The research team prepared three pure photons using precision microfabrication techniques, transmitting them to a fibre-optic interferometer with three entry and three exit ports, revealing some surprising results. It was found that the collective effects of the multiple particles enhanced the strength of interference even if their properties seemed very different, suggesting new interference mechanisms exist. 
The research is significant as it not only expands current understanding of the foundations of quantum physics, of which quantum interference is a fundamental property, but also opens up new possibilities for more robust quantum computation using photons. It also has the potential to reduce some of the significant engineering challenges researchers and technology developers face to bring quantum computing into reality.

The UK government has invested £270 million to establish a quantum technology industry, and the Networked Quantum Information Technologies (NQIT) Hub led by Oxford University is the largest of four technology hubs in the UK National Quantum Technology Programme. Although the reality of a scalable quantum computer is still some way in the future, once developed it will be one of the biggest scientific and engineering achievements in history. With super-fast processing speeds and the ability to solve problems on a scale currently impossible with conventional computing power, it could help accelerate research into diseases like cancer, discover new drugs and enable personalised treatments, create ultra-secure encryption for the finance industry and communications, and help save lives through accurate weather forecasting and predictive climate change modelling.

Read the publication in Physical Review Letters:
Distinguishability and Many-Particle Interference, Adrian J. Menssen, Alex E. Jones, Benjamin J. Metcalf, Malte C. Tichy, Stefanie Barz, W. Steven Kolthammer, and Ian A. Walmsley, Phys. Rev. Lett. 118, 153603

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