Aurelia Butler

An international team of scientists have demonstrated a leap 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 will be transformative to 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 measurements of fundamental physical phenomena requiring precise time synchronisation among the sensors.
Spin-photon interfaces are elementary building blocks for quantum networks that allow converting stationary quantum information (such as the quantum state of an ion or a solid-state spin qubit) into light, namely photons, that can be distributed over large distances. A major challenge is to find an interface that is both good at storing quantum information and efficient at converting it into light. Optically active semiconductor quantum dots are the most efficient spin-photon interface known to date but extending their storage time beyond a few microseconds has puzzled physicists in spite of decade-long research efforts. Now, researchers at the University of Cambridge, the University of Linz and the University of Sheffield have shown that there is a simple material’s solution to this problem that improves the storage of quantum information beyond hundred microseconds.
Quantum Dots are crystalline structures made out of many thousands of atoms. Each of these atoms’ nuclei has a magnetic dipole moment that couples to the quantum dot electron and can cause the loss of quantum information stored in the electron qubit. The research team’s finding, reported in Nature Nanotechnology, is that in a device constructed with semiconductor materials that have the same lattice parameter, the nuclei ‘felt’ the same environment and behaved in unison. As a result, it is now 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 as opposed to, say, electrical noise in the device. This is really a great position to be in because the nuclear ensemble is an isolated quantum system, and the coherent electron will be a gateway to quantum phenomena in 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 was initially anticipated, and there is room for further improvement in 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, co-author of this paper at the University of Linz. “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 Cavendish PhD student, Leon Zaporski — the first author of the paper. “Most researchers approach the problem of isolating qubit 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” explains Professor Mete Atatüre, co-author of this 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.” More

A handful of hyper-productive fisheries provide sustenance to a billion people and employ tens of millions. These fisheries occur on the eastern edges of the world’s oceans — off the West Coast of the U.S., the Canary Islands, Peru, Chile, and Benguela. There, a process called upwelling brings cold water and nutrients to the surface, which in turn supports large numbers of larger sea creatures that humans depend on for sustenance.
A new project led by researchers at Texas A&M University is seeking to understand how changes to the climate and oceans will impact fisheries in the U.S. and around the world.
“We’re interested in how climate change is going to alter upwelling and how the sustainability of the future fisheries will be impacted,” said Ping Chang, Louis & Elizabeth Scherck Chair in Oceanography at Texas A&M University (TAMU). “It turns out that when we increase the resolution of our climate models, we find that the upwelling simulation becomes much closer to reality.”
Funded by the National Science Foundation (NSF), the project aims to develop medium to long-term fishery forecasts, driven by some of the highest-resolution coupled climate forecasts ever run. It is one of the 16 Convergence Accelerator Phase 1 projects that address the ‘Blue Economy’ — the sustainable use of ocean resources for economic growth. Convergence projects integrate scholars from different science disciplines.
The TAMU team, led by oceanographer Piers Chapman, includes computational climate modelers, marine biogeochemical modelers, fishery modelers, decision support system experts, and risk communications scholars from academia, federal agencies, and industry.
Chang and Gokhan Danabasoglu at the National Center for Atmospheric Research (NCAR) lead the climate modeling component of the research. They use the Frontera supercomputer at the Texas Advanced Computing Center (TACC) — the fastest academic supercomputer in the U.S. — to power their research.

In the 1990s, marine biologist Andrew Bakun proposed that a warming climate would increase upwelling in the eastern boundary regions. He reasoned that since land is warming faster than the oceans, the temperature gradient between land and ocean would drive a stronger wind, which makes upwelling stronger. However, recent historical data suggests the opposite might in fact be the norm.
“A lot of papers written in the past use coarse resolution models that don’t resolve upwelling very well,” Chang said. “High resolution models so far predict upwelling in most areas, not increasing. The models are predicting warmer, not colder temperatures in these waters. In Chile and Peru, the warming is quite significant — 2-3ºC warming in the worst case scenario, which is business as usual. That can be bad news for upwelling.”
The areas where upwelling occur are quite narrow and localized, but their impact on the marine ecosystem is very large. The eastern Pacific upwelling, for instance, is only about 100 kilometers wide. The climate models used by the Intergovernmental Panel on Climate Change (IPCC) have a resolution of 100 kilometers — and would therefore only produce one data point for the upwelling region, not nearly enough to predict future changes accurately.
On the other hand, the model used by Chang and his colleagues uses a resolution of 10 kilometers in each direction. These are 100 times more resolved than the IPCC models — and require roughly 100 times the compute power.
Chang’s study relies on two separate, but related, sets of simulations. The first set involves an ensemble (the same model run with a slightly different starting point to produce a statistically valid result) of high-resolution coupled Earth system models. The second incorporates observed data in the atmosphere to generate realistic ocean states that are then used to initialize the model prediction. Starting from 1982, it will perform five-year retrospective forecasts to determine the skill of the model in forecasting upwelling effects.

“There’s a limit to how far out you can make a forecast,” Chang said. “Beyond a certain time limit, the model no longer has skill. At five years, our model still shows useful skill.”
The team reported their results in Nature’s Communications Earth & Environment in January 2023.
The Blue Economy project continues the TAMU-NCAR team’s multi-decade effort to upgrade global climate models so they are higher resolution and more physically accurate. The model used by the team was one of a handful of high-resolution Earth system models that were included in the most recent IPCC report and are being explored by an IPCC subcommittee. They represent the future of global climate modeling.
At 10 kilometer resolution, researchers believe it is possible for models to realistically generate extreme weather events like tropical cyclones or atmospheric rivers, as well as more refined predictions of how climate in a specific region will change. However, models at this resolution still cannot resolve clouds, which requires models with a few kilometer resolution and can currently only be integrated for short-term, not climate, timescales.
The effort to capture the Earth system continues to improve.
The TAMU-NCAR project will be one of the first to incorporate biogeochemical models of the ocean and fisheries models into Earth system models at 10 km resolution.
“TACC is unique in providing resources for researchers like us to tackle the fundamental questions of science,” Chang said. “Our goal is not routine forecasts. What we want is a better understanding of the Earth system dynamics that are missing in current climate models to make our model and our methods better. Without Frontera, I don’t know if we could make simulations like we do. It’s critical.” More