Achieving quantum computing is not just a sheer matter of improving computational speed: it is a radically different paradigm that has attracted physicists and engineers for decades and that would make it possible to more efficiently solve problems across a number of domains — from database searches to prime number factorization and artificial intelligence.

Now in a major breakthrough in the field of quantum computing and quantum metrology, a research team from the University of Bristol led by Prof Jeremy O’Brien has obtained highly precise control of up to four photons on a silicon chip for the very first time.

The journal Nature recently described the achievement as exciting and promising particularly because of its great scalability.

The importance of quantum entanglement

Unlike traditional computing, in which a bit can only be assigned one out of two possible states at a time, quantum computing harnesses the unique properties of subatomic particles to obtain further configurations.

One of such properties is a direct consequence of superposition, a quantum mechanics law stating that a particle can assume more than one state at the same time. The other main property is known as quantum entanglement: when entanglement is obtained across two or more subatomic particles, the current state of each particle can only be determined in relation with its counterpart. These two counter-intuitive principles are what makes quantum computing so powerful in the eyes of scientists and engineers.

The scientific community had already obtained important results in the field of quantum optics, which entails using photons to achieve quantum computing. Photons are considered a great choice because a single particle is relatively simple to manipulate and already moves at the fastest achievable speed — the speed of light.

Manipulating two or more light particles to obtain entanglement is however considerably harder, and the team led by Prof. O'Brien was the first to demonstrate such ability on a silicon chip by achieving entanglement with up to four photons.

With its recent progress, the British research team is now inching closer to achieving true quantum computation on a chip. "That is something that we are working towards," Prof. O'Brien told Gizmag. But a number of issues still need to be tackled in order to make this possible.

"A major issue facing large scale implementations are high-efficiency photon sources. There are major efforts world-wide, including in Bristol, in this area. Another issue is efficient photon detection, which again is being addressed in Bristol and elsewhere."

Achieving manipulation of photons on a chip

To achieve its goal, the team employed a microscopic metal electrode that was patterned onto a silicon chip and chose an approach reminiscent of the working principle of fiber optics, in which light is transmitted in a cable through continuous reflections on a silicon-based material.

The team applied a voltage across the metal electrode, which changed the temperature of a silica waveguide placed directly underneath it: this, in turn, changed the path of the photons in a very predictable and controllable way.

"The really exciting thing about this result is that it will enable the development of reconfigurable and adaptive quantum circuits for photons. This opens up all kinds of possibilities," said Prof. O'Brien. The circuits that were demonstrated were manufactured using standard industry processes but, Prof. O'Brien explained, the integration of sources and detectors might introduce sophisticated components and processing that could ultimately raise the cost of such a chip.

The research team is currently working on building larger and more sophisticated devices and tackling the remaining issues that separate us from a feasible mass-production of quantum computing-capable chips.

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