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Breakthrough UK research paves way for quantum circuits

Dual wave / particle nature of light. Image courtesy University of Cambridge©

A team from the University of Cambridge has built a semiconductor chip that converts electrons into a quantum state that emits light, but is large enough to see by eye.

The researchers claim that, because the quantum superfluid is simply set up by shining laser beams on the device, it can lead to practical ultrasensitive detectors. The research is published in Nature Physics.

According to Dr Gab Christmann, Professor Jeremy Baumberg and Dr Natalia Berloff from the University of Cambridge, quantum mechanics normally shows its influence only for tiny particles at ultralow temperatures. However, the team mixed electrons with light to synthesise supersized quantum particles the thickness of a human hair, that behave like superconductors. Building microscopic cavities which tightly trap light into the vicinity of electrons within the chip, they produced new particles called 'polaritons' which weigh very little, encouraging them to 'roam widely'.

The team, working with a team in Crete, produced the new samples needed which allow the polaritons to flow around at will without getting stuck. When injecting them in two laser spots, they found that the resulting quantum fluid spontaneously started oscillating backwards and forwards, in the process forming some of the most characteristic quantum pendulum states known to scientists, but thousands of times larger than normal.

"These polaritons overwhelmingly prefer to march in step with each other, entangling themselves quantum mechanically," said Christmann. The resulting quantum liquid is said to have some peculiar properties, including trying to repel itself. It can also only swirl around in fixed amounts, producing vortices laid out in regular lines. By moving the laser beams apart, the team directly controlled the 'sloshing' of the quantum liquid, forming a pendulum beating 'a million times faster than a human heart'.

"This is not something we ever expected to see directly, and it is miraculous how mirror perfect our samples have to be," added Christmann. "We can steer our rivers of polariton quantum liquid on the fly by scanning around the laser beams that create them." Increasing the number of laser beams creates even more complicated quantum states.

The goal of the work is to make such quantum states using an electrical battery and at room temperature, which would allow a new generation of ultrasensitive gyroscopes to measure gravity, magnetic field, and create quantum circuits. Christmann concluded: "Just to see and prod quantum mechanics working in front of your eyes is amazing."

The research was funded by the Engineering and Physical Sciences Research Council and the EU.

Chris Shaw

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