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“Calmer” qubit designed to lengthen lifespan of information for quantum technologies

Researchers have significantly prolonged the lifetime of information carried by an electron in a microscopic structure known as a “quantum dot”. The study represents another early step towards the realisation of quantum computing, a hugely powerful possible future technology, but one that presently remains a long way off, and for which possible systems are still being studied at a very fundamental level.

The research team, led by academics at the University of Cambridge, devised a way to preserve the information represented by the “spin” state of an electron for 10 times longer than is normally possible in their quantum systems. Ironically, they did so by manipulating the electron’s interaction with its surrounding environment – the very issue that causes information to be lost in these types of system.

Their experiment is another early-stage advance towards the long-term aim of developing quantum computers, which would use the principles of quantum mechanics to transform computing and vastly increase its capacity to process and handle data.

The idea of quantum computing is to use microscopic objects, sometimes single particles, like electrons, to create qubits: atomic-scale systems that represent and manipulate information.

Dorian Gangloff, a Research Fellow at St John’s College and the Department of Physics, University of Cambridge, was one of the study’s main authors. “What we are trying to do at the moment is to control systems at an atomic level in order to build the components that would go into that type of technology,” he said.

“In this study we turned on its head the idea that the environment of a quantum bit is necessarily a ‘foe’. We used light to pin the spin state of the electron, which by its interaction with the surrounding environment calmed it down. Having prepared this quiet place for our quantum bit, we had more time to perform the sort of operations and measurements that we hope a quantum computer will eventually do.”

One potential approach to quantum computing, which is being widely studied, utilises the property of electrons known as spin. This refers to the intrinsic magnetism of an electron which causes it to behave like a tiny bar magnet capable of pointing in one of two directions, up or down. In a quantum computer, these two positions would correspond to the ones and zeros in the binary logic of present-day devices.

Critically, however, the act of measuring an electron in a qubit is what determines whether it is up or down. Until that point, the aim is to hold it in what is known as a superposition state, in which it is both simultaneously. That state, a counter-intuitive feature of how matter behaves at the quantum level, exponentially increases the information-handling capacity of a set of quantum bits compared with conventional, binary, Boolean logic devices. In the latter, the system must always be in one of a set of distinct states (for example, either one or zero in a single bit), rather than all of them at the same time.

“It means that a quantum computer would outperform today’s laptops, smartphones, PCs and supercomputers by an inconceivable factor,” Gangloff said. “Some problems, that would literally take longer than the time since the universe began to solve on a normal machine, could be solved in a matter of hours or days on a quantum computer. It would be a fundamental, qualitative change in the way that information is handled. In an increasingly information-driven age, we are talking about a technological singularity.”

A quantum dot is a basic system in which an electron is trapped in a tiny semiconductor nanostructure so that it can be manipulated and tested for this purpose. An off-resonant pulsed laser can be used to change the state of the trapped electron, so that, for example, it goes from a superposition state to one where its spin is up or down. The result can be read using a pulse of resonant laser light. If the system scatters light this signals one state, and if it does not, it means the other.

For the electron to have equal potential to be up or down, the system has to be able to rotate the electron through a preset angle to achieve a guaranteed qubit state. The theory is that a quantum computer would constantly perform many of these rotations on the exponentially large superposition states of multiple qubits, and then measure the final state of the quantum bits as the output of a computation, thereby processing huge amounts of information stored on multiple quantum bits simultaneously.

But all such systems currently suffer from a problem known as decoherence. After just a few billionths of a second, the resting state of the electron is distorted by its interaction with the wider surrounding nanostructure. Magnetic fluctuations created by the nuclei within this structure cause the electron spin to jitter, making it impossible to control accurately.

In the new study, however, the researchers managed to influence the environment by manipulating the electron itself. This helped them to preserve the information represented by the quantum dot for longer.

“Using light, we pinned the state of the electron into one configuration,” Gangloff explained. “Environmental fluctuations lead to deviations in the state of the electron, but the light then pins it back and that also pins the environment. It’s a stabilising feedback loop, creating an environment that agrees with the electron’s own state.”

Having calmed the environment within the nanostructure, the researchers were able to perform more of their standard rotation and measurement cycles, each of which takes roughly a microsecond. The environment gradually returned to its initial noisy state over one millisecond, after which it had to be prepared in a quiet state again before computation could go on.

“Having improved our qubit coherence time by a factor of 10, to tens of nanoseconds, may sound like little difference, but it nearly eliminates some important roadblocks in our quantum dot system,” Gangloff added. “Our experiment is an enabler – it improves things by an order of magnitude so that we can explore new physics that may turn out to improve things further. Eventually, if we can reach a critical threshold of capability, we will be able to start engineering quantum processors. This research is the beginning of that; we are like scouts out in the field before the road is really mapped.”

The study, which was co-authored by St John’s College Fellow, Professor Mete Atatüre, as well as Gabriel Éthier-Majcher, Robert Stockill, and Claire Le Gall from the University’s Cavendish Laboratory, is published in  Physical Review Letters.