Artist’s impression of electron spin in a quantum dot, which is interfaced with light and strongly coupled nuclear spins. Image: Leon Zaporski – University of Cambridge.
Artist’s impression of electron spin in a quantum dot, which is interfaced with light and strongly coupled nuclear spins. Image: Leon Zaporski – University of Cambridge.

An international team of scientists has demonstrated an important advance in preserving the quantum coherence of quantum dot spin qubits as part of the global push for practical quantum networks and quantum computers.

These technologies have the potential to transform a broad range of industries and research efforts: from the security of information transfer, through the search for materials and chemicals with novel properties, to measuring fundamental physical phenomena requiring precise time synchronisation between sensors.

Spin-photon interfaces are elementary building blocks for quantum networks. They are designed to convert stationary quantum information (such as the quantum state of an ion or a solid-state spin qubit) into light, namely photons, that can be transported over large distances. A major challenge, however, is finding an interface that is both good at storing quantum information and efficient at converting it into light.

The optically active semiconducting nanomaterials known as quantum dots are the most efficient spin-photon interfaces known to date, but extending their storage time beyond a few microseconds has confounded physicists in spite of decade-long research efforts. Now, researchers at the universities of Cambridge and Sheffield in the UK and the University of Linz in Austria have discovered a simple solution to this problem that improves the storage of quantum information to well beyond a hundred microseconds.

In a quantum dot spin qubit, quantum information is stored in the spin state of a confined electron. But a single quantum dot is made up of many thousands of atoms, each of which has a magnetic dipole moment that couples to the confined electron, resulting in the rapid loss of its quantum information. In a paper in Nature Nanotechnology, the researchers report constructing a device using quantum dots with the same lattice parameter. Because of this, the atomic nuclei in these quantum dots 'felt' the same environment and behaved in unison, making it possible to filter out this nuclear noise and achieve a near two-order magnitude improvement in storage time.

“This is a completely new regime for optically active quantum dots where we can switch off the interaction with nuclei and refocus the electron spin over and over again to keep its quantum state alive,” said Claire Le Gall from Cambridge’s Cavendish Laboratory, who led the project. “We demonstrated hundreds of microseconds in our work, but really, now we are in this regime, we know that much longer coherence times are within reach. For spins in quantum dots, short coherence times were the biggest roadblock to applications, and this finding offers a clear and simple solution to that.”

While exploring the hundred-microsecond timescales for the first time, the researchers were pleasantly surprised to find that the electron only sees noise from the nuclei, rather than, say, electrical noise from the device. This means the nuclear ensemble is an isolated quantum system, allowing the confined electron to be a gateway to quantum phenomena in a large nuclear spin ensemble.

Another thing that surprised the researchers was the 'sound' that was picked up from the nuclei. It was not quite as harmonious as they anticipated, which means there is great potential for improving the system’s quantum coherence through further material engineering.

“When we started working with the lattice-matched material system employed in this work, getting quantum dots with well-defined properties and good optical quality wasn’t easy,” says Armando Rastelli from the University of Linz, a co-author of the paper. “It is very rewarding to see that an initially curiosity-driven research line on a rather ´exotic´ system and the perseverance of skilled team members Santanu Manna and Saimon Covre da Silva led to the devices at the basis of these spectacular results. Now we know what our nanostructures are good for, and we are thrilled by the perspective of further engineering their properties together with our collaborators.”

“One of the most exciting things about this research is taming a complex quantum system: a hundred thousand nuclei coupling strongly to a well-controlled electron spin,” explains Leon Zaporski, a PhD student in the Cavendish Laboratory and first author of the paper. “Most researchers approach the problem of isolating qubits from the noise by removing all the interactions. Their qubits become a bit like sedated Schrödinger’s cats, that can barely react to anyone pulling on their tail. Our ‘cat’ is on strong stimulants, which – in practice - means we can have more fun with it.”

“Quantum dots now combine high photonic quantum efficiency with long spin coherence times,” says Mete Atatüre, a professor in the Cavendish Laboratory and a co-author of the paper. “In the near future, we envisage these devices to enable the creation of entangled light states for all-photonic quantum computing and allow foundational quantum control experiments of the nuclear spin ensemble.”

This story is adapted from material from the University of Cambridge, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.