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University of Cambridge physicists find 'Holy Grail of quantum dot research'


By Paul Brackley


Cambridge physicists have found the Holy Grail of quantum dot research.

A quantum dot. Image: University of Cambridge (7394384)
A quantum dot. Image: University of Cambridge (7394384)

Their breakthrough could have a significant impact on the long-term future of computing.

Quantum dots are semiconductor crystals only a few nanometres in size. When electricity or light is applied, they emit light, and this can be tuned by altering the dot’s size - a property that has already proved useful in a range of applications.

Samsung and others, for example, uses quantum dots in their TVs, for example, to provide a superior display to LED screens.

Quantum dots are comprised of atoms that interact magnetically with a trapped electron.

This interaction, however, poses a problem for those working on quantum computing.

While standard computers uses binary ‘bits’ for memory, storing everything as 0s and 1s, quantum computers use quantum bits or ‘qubits’ which can be in a superposition of states. Think of a qubit like a spinning coin, for example, that is both heads and tails at the same time.

The potential of quantum computing is vast: Microsoft chief executive Satya Nadella said it has the the potential to “radically reshape the world”.

But the interaction of the electron in a quantum dot with the nuclear spins has limited its usefulness as a qubit, or unit of quantum information - until now.

A research group at the Cavendish Laboratory at the University of Cambridge, led by St John’s College fellow Professor Mete Atatüre has found a way to control the sea of nuclei in quantum dots so that they “dance in unison”, meaning they can operate as a quantum memory device.

Prof Atatüre said: “Quantum dots offer an ideal interface, as mediated by light, to a system where the dynamics of individual interacting spins could be controlled and exploited. “Because the nuclei randomly ‘steal’ information from the electron they have traditionally been an annoyance, but we have shown we can harness them as a resource.”

The research group exploits the laws of quantum physics and optics to investigate computing, sensing and communication applications.

The quantum dot research team from the University of Cambridge The research team, from left, Mete Atatüre, Dorian Gangloff, Emil Denning, Claire Le Gall, Daniel Jackson, Jonny Bodey. Picture: Mete Atatüre (7394382)
The quantum dot research team from the University of Cambridge The research team, from left, Mete Atatüre, Dorian Gangloff, Emil Denning, Claire Le Gall, Daniel Jackson, Jonny Bodey. Picture: Mete Atatüre (7394382)

In this latest work, they exploited the interaction between the electron and the thousands of nuclei using lasers to ‘cool’ the nuclei to less than one milliKelvin, or a thousandth of a degree above absolute zero.

They were then able to control and manipulate the thousands of nuclei as if they formed a single body in unison, like a second qubit.

This proves the nuclei in the quantum dot can exchange information with the electron qubit and can be used to store quantum information as a memory device.

Fundamental concepts of quantum physics, such as entanglement and superposition principle, are exploited in quantum computing.

But a quantum computer still needs a processor, memory and a bus to transport the information, just like a classical computer.

Qubits are the processor - and these can be an electron trapped in a quantum dot - and the bus is a single photon that these quantum dots generate, which is ideal for exchanging information. So the missing link for quantum dots has been quantum memory.

Prof Atatüre said: “Instead of talking to individual nuclear spins, we worked on accessing collective spin waves by lasers. This is like a stadium where you don’t need to worry about who raises their hands in the Mexican wave going round, as long as there is one collective wave because they all dance in unison.

“We then went on to show that these spin waves have quantum coherence. This was the missing piece of the jigsaw and we now have everything needed to build a dedicated quantum memory for every qubit.”

The photon, the qubit and the memory need to interact with each other in a controlled way. Typically, this has been achieved by interfacing different physical systems to form a single hybrid unit, but this can be inefficient.

The new research, published in Science, shows that in quantum dots, the memory element is automatically there with every single qubit.

Dr Dorian Gangloff, one of the first authors of the paper and a fellow at St John’s, said the discovery will renew interest in these types of semiconductor quantum dots.

“This is a Holy Grail breakthrough for quantum dot research – both for quantum memory and fundamental research; we now have the tools to study dynamics of complex systems in the spirit of quantum simulation,” he said.

The world’s first commercial quantum computer - the Q System One - was launched by IBM last month and is available for use to companies via the internet,

Thanks to the carefully controlled environment required - freezing temperatures, with minimal sound or vibrations - you won’t be picking one up any time soon at PC World.

Prof Gangloff said: “The impact of the qubit could be half a century away but the power of disruptive technology is that it is hard to conceive of the problems we might open up – you can try to think of it as known unknowns but at some point you get into new territory. We don’t yet know the kind of problems it will help to solve which is very exciting.”


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