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Light used to detect quantum information stored in 100,000 nuclear quantum bits

Mon, 15/02/2021 - 15:18

The researchers, from the University of Cambridge, were able to inject a ‘needle’ of highly fragile quantum information in a ‘haystack’ of 100,000 nuclei. Using lasers to control an electron, the researchers could then use that electron to control the behaviour of the haystack, making it easier to find the needle. They were able to detect the ‘needle’ with a precision of 1.9 parts per million: high enough to detect a single quantum bit in this large ensemble.

The technique makes it possible to send highly fragile quantum information optically to a nuclear system for storage, and to verify its imprint with minimal disturbance, an important step in the development of a quantum internet based on quantum light sources. The results are reported in the journal Nature Physics.

The first quantum computers – which will harness the strange behaviour of subatomic particles to far outperform even the most powerful supercomputers – are on the horizon. However, leveraging their full potential will require a way to network them: a quantum internet. Channels of light that transmit quantum information are promising candidates for a quantum internet, and currently there is no better quantum light source than the semiconductor quantum dot: tiny crystals that are essentially artificial atoms.

However, one thing stands in the way of quantum dots and a quantum internet: the ability to store quantum information temporarily at staging posts along the network.

“The solution to this problem is to store the fragile quantum information by hiding it in the cloud of 100,000 atomic nuclei that each quantum dot contains, like a needle in a haystack,” said Professor Mete Atatüre from Cambridge’s Cavendish Laboratory, who led the research. “But if we try to communicate with these nuclei like we communicate with bits, they tend to ‘flip’ randomly, creating a noisy system.”

The cloud of quantum bits contained in a quantum dot don’t normally act in a collective state, making it a challenge to get information in or out of them. However, Atatüre and his colleagues showed in 2019 that when cooled to ultra-low temperatures also using light, these nuclei can be made to do ‘quantum dances’ in unison, significantly reducing the amount of noise in the system.

Now, they have shown another fundamental step towards storing and retrieving quantum information in the nuclei. By controlling the collective state of the 100,000 nuclei, they were able to detect the existence of the quantum information as a ‘flipped quantum bit’ at an ultra-high precision of 1.9 parts per million: enough to see a single bit flip in the cloud of nuclei.

“Technically this is extremely demanding,” said Atatüre, who is also a Fellow of St John’s College. “We don’t have a way of ‘talking’ to the cloud and the cloud doesn’t have a way of talking to us. But what we can talk to is an electron: we can communicate with it sort of like a dog that herds sheep.”

Using the light from a laser, the researchers are able to communicate with an electron, which then communicates with the spins, or inherent angular momentum, of the nuclei.

By talking to the electron, the chaotic ensemble of spins starts to cool down and rally around the shepherding electron; out of this more ordered state, the electron can create spin waves in the nuclei.

“If we imagine our cloud of spins as a herd of 100,000 sheep moving randomly, one sheep suddenly changing direction is hard to see,” said Atatüre. “But if the entire herd is moving as a well-defined wave, then a single sheep changing direction becomes highly noticeable.”

In other words, injecting a spin wave made of a single nuclear spin flip into the ensemble makes it easier to detect a single nuclear spin flip among 100,000 nuclear spins.

Using this technique, the researchers are able to send information to the quantum bit and ‘listen in’ on what the spins are saying with minimal disturbance, down to the fundamental limit set by quantum mechanics.

“Having harnessed this control and sensing capability over this large ensemble of nuclei, our next step will be to demonstrate the storage and retrieval of an arbitrary quantum bit from the nuclear spin register,” said co-first author Daniel Jackson, a PhD student at the Cavendish Laboratory.

“This step will complete a quantum memory connected to light – a major building block on the road to realising the quantum internet,” said co-first author Dorian Gangloff, a Research Fellow at St John’s College.

Besides its potential usage for a future quantum internet, the technique could also be useful in the development of solid-state quantum computing.

The research was supported in part by the European Research Council (ERC), the Engineering and Physical Sciences Research Council (EPSRC) and the Royal Society.

 

Reference:
D. M. Jackson et al. ‘Quantum sensing of a coherent single spin excitation in a nuclear ensemble.’ Nature Physics (2021). DOI: 10.1038/s41567-020-01161-4

Researchers have found a way to use light and a single electron to communicate with a cloud of quantum bits and sense their behaviour, making it possible to detect a single quantum bit in a dense cloud.

We don’t have a way of ‘talking’ to the cloud and the cloud doesn’t have a way of talking to us. But what we can talk to is an electron: we can communicate with it sort of like a dog that herds sheepMete AtatüreGerd Altmann from Pixabay Quantum particles


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‘Magnetic graphene’ forms a new kind of magnetism

Mon, 08/02/2021 - 15:21

The researchers, led by the University of Cambridge, were able to control the conductivity and magnetism of iron thiophosphate (FePS3), a two-dimensional material which undergoes a transition from an insulator to a metal when compressed. This class of magnetic materials offers new routes to understanding the physics of new magnetic states and superconductivity.

Using new high-pressure techniques, the researchers have shown what happens to magnetic graphene during the transition from insulator to conductor and into its unconventional metallic state, realised only under ultra-high pressure conditions. When the material becomes metallic, it remains magnetic, which is contrary to previous results and provides clues as to how the electrical conduction in the metallic phase works. The newly discovered high-pressure magnetic phase likely forms a precursor to superconductivity so understanding its mechanisms is vital.

Their results, published in the journal Physical Review X, also suggest a way that new materials could be engineered to have combined conduction and magnetic properties, which could be useful in the development of new technologies such as spintronics, which could transform the way in which computers process information.

Properties of matter can alter dramatically with changing dimensionality. For example, graphene, carbon nanotubes, graphite and diamond are all made of carbon atoms, but have very different properties due to their different structure and dimensionality.

“But imagine if you were also able to change all of these properties by adding magnetism,” said first author Dr Matthew Coak, who is jointly based at Cambridge’s Cavendish Laboratory and the University of Warwick. “A material which could be mechanically flexible and form a new kind of circuit to store information and perform computation. This is why these materials are so interesting, and because they drastically change their properties when put under pressure so we can control their behaviour.”

In a previous study by Sebastian Haines of the Cavendish Laboratory and the Department of Earth Sciences, researchers established that the material becomes a metal at high pressure, and outlined how the crystal structure and arrangement of atoms in the layers of this 2D material change through the transition.

“The missing piece has remained however, the magnetism,” said Coak. “With no experimental techniques able to probe the signatures of magnetism in this material at pressures this high, our international team had to develop and test our own new techniques to make it possible.”

The researchers used new techniques to measure the magnetic structure up to record-breaking high pressures, using specially designed diamond anvils and neutrons to act as the probe of magnetism. They were then able to follow the evolution of the magnetism into the metallic state.

“To our surprise, we found that the magnetism survives and is in some ways strengthened,” co-author Dr Siddharth Saxena, group leader at the Cavendish Laboratory. “This is unexpected, as the newly-freely-roaming electrons in a newly conducting material can no longer be locked to their parent iron atoms, generating magnetic moments there - unless the conduction is coming from an unexpected source.”

In their previous paper, the researchers showed these electrons were ‘frozen’ in a sense. But when they made them flow or move, they started interacting more and more. The magnetism survives, but gets modified into new forms, giving rise to new quantum properties in a new type of magnetic metal.

How a material behaves, whether conductor or insulator, is mostly based on how the electrons, or charge, move around. However, the ‘spin’ of the electrons has been shown to be the source of magnetism. Spin makes electrons behave a bit like tiny bar magnets and point a certain way. Magnetism from the arrangement of electron spins is used in most memory devices: harnessing and controlling it is important for developing new technologies such as spintronics, which could transform the way in which computers process information.

“The combination of the two, the charge and the spin, is key to how this material behaves,” said co-author Dr David Jarvis from the Institut Laue-Langevin, France, who carried out this work as the basis of his PhD studies at the Cavendish Laboratory. “Finding this sort of quantum multi-functionality is another leap forward in the study of these materials.”

“We don’t know exactly what’s happening at the quantum level, but at the same time, we can manipulate it,” said Saxena. “It’s like those famous ‘unknown unknowns’: we’ve opened up a new door to properties of quantum information, but we don’t yet know what those properties might be.”

There are more potential chemical compounds to synthesise than could ever be fully explored and characterised. But by carefully selecting and tuning materials with special properties, it is possible to show the way towards the creation of compounds and systems, but without having to apply huge amounts of pressure.

Additionally, gaining fundamental understanding of phenomena such as low-dimensional magnetism and superconductivity allows researchers to make the next leaps in materials science and engineering, with particular potential in energy efficiency, generation and storage.

As for the case of magnetic graphene, the researchers next plan to continue the search for superconductivity within this unique material. “Now that we have some idea what happens to this material at high pressure, we can make some predictions about what might happen if we try to tune its properties through adding free electrons by compressing it further,” said Coak.

“The thing we’re chasing is superconductivity,” said Saxena. “If we can find a type of superconductivity that’s related to magnetism in a two-dimensional material, it could give us a shot at solving a problem that’s gone back decades.”

 

Reference:
Matthew J. Coak et al. ‘Emergent Magnetic Phases in Pressure-Tuned van der Waals Antiferromagnet FePS3.’ Physical Review X (2021). DOI: 10.1103/PhysRevX.11.011024

 

Researchers have identified a new form of magnetism in so-called magnetic graphene, which could point the way toward understanding superconductivity in this unusual type of material.

Cavendish LaboratoryIllustration of the magnetic structure of FePS3


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‘Multiplying’ light could be key to ultra-powerful optical computers

Mon, 08/02/2021 - 10:26

An important class of challenging computational problems, with applications in graph theory, neural networks, artificial intelligence and error-correcting codes can be solved by multiplying light signals, according to researchers from the University of Cambridge and Skolkovo Institute of Science and Technology in Russia.

In a paper published in the journal Physical Review Letters, they propose a new type of computation that could revolutionise analogue computing by dramatically reducing the number of light signals needed while simplifying the search for the best mathematical solutions, allowing for ultra-fast optical computers.

Optical or photonic computing uses photons produced by lasers or diodes for computation, as opposed to classical computers which use electrons. Since photons are essentially without mass and can travel faster than electrons, an optical computer would be superfast, energy-efficient and able to process information simultaneously through multiple temporal or spatial optical channels.

The computing element in an optical computer – an alternative to the ones and zeroes of a digital computer – is represented by the continuous phase of the light signal, and the computation is normally achieved by adding two light waves coming from two different sources and then projecting the result onto ‘0’ or ‘1’ states.

However, real life presents highly nonlinear problems, where multiple unknowns simultaneously change the values of other unknowns while interacting multiplicatively. In this case, the traditional approach to optical computing that combines light waves in a linear manner fails.

Now, Professor Natalia Berloff from Cambridge’s Department of Applied Mathematics and Theoretical Physics and PhD student Nikita Stroev from Skolkovo Institute of Science and Technology have found that optical systems can combine light by multiplying the wave functions describing the light waves instead of adding them and may represent a different type of connections between the light waves.

They illustrated this phenomenon with quasi-particles called polaritons – which are half-light and half-matter – while extending the idea to a larger class of optical systems such as light pulses in a fibre. Tiny pulses or blobs of coherent, superfast-moving polaritons can be created in space and overlap with one another in a nonlinear way, due to the matter component of polaritons.

“We found the key ingredient is how you couple the pulses with each other,” said Stroev. “If you get the coupling and light intensity right, the light multiplies, affecting the phases of the individual pulses, giving away the answer to the problem. This makes it possible to use light to solve nonlinear problems.”

The multiplication of the wave functions to determine the phase of the light signal in each element of these optical systems comes from the nonlinearity that occurs naturally or is externally introduced into the system.

“What came as a surprise is that there is no need to project the continuous light phases onto ‘0’ and ‘1’ states necessary for solving problems in binary variables,” said Stroev. “Instead, the system tends to bring about these states at the end of its search for the minimum energy configuration. This is the property that comes from multiplying the light signals. On the contrary, previous optical machines require resonant excitation that fixes the phases to binary values externally.”

The authors have also suggested and implemented a way to guide the system trajectories towards the solution by temporarily changing the coupling strengths of the signals.

“We should start identifying different classes of problems that can be solved directly by a dedicated physical processor,” said Berloff. “Higher-order binary optimisation problems are one such class, and optical systems can be made very efficient in solving them.”

There are still many challenges to be met before optical computing can demonstrate its superiority in solving hard problems in comparison with modern electronic computers: noise reduction, error correction, improved scalability, guiding the system to the true best solution are among them.

“Changing our framework to directly address different types of problems may bring optical computing machines closer to solving real-world problems that cannot be solved by classical computers,” said Berloff.

 

Reference:
Nikita Stroev and Natalia G. Berloff. ‘Discrete Polynomial Optimization with Coherent Networks of Condensates and Complex Coupling Switching.’ Physical Review Letters (2021). DOI: 10.1103/PhysRevLett.126.050504

 

New type of optical computing could solve highly complex problems that are out of reach for even the most powerful supercomputers.

Gleb Berloff, Hills Road Sixth Form CollegeArtist's impression of light pulses inside an optical computer


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Quantum projects launched to solve universe’s mysteries

Wed, 13/01/2021 - 09:00

UK Research and Innovation (UKRI) is supporting seven projects with a £31 million investment to demonstrate how quantum technologies could solve some of the greatest mysteries in fundamental physics. Researchers from the University of Cambridge have been awarded funding on four of the seven projects.

Just as quantum computing promises to revolutionise traditional computing, technologies such as quantum sensors have the potential to radically change our approach to understanding our universe.

The projects are supported through the Quantum Technologies for Fundamental Physics programme, delivered by the Science and Technology Facilities Council (STFC) and the Engineering and Physical Sciences Research Council (EPSRC) as part of UKRI’s Strategic Priorities Fund. The programme is part of the National Quantum Technologies Programme.

AION: A UK Atom Interferometer Observatory and Network has been awarded £7.2 million in funding and will be led by Imperial College London. The project will develop and use technology based on quantum interference between atoms to detect ultra-light dark matter and sources of gravitational waves, such as collisions between massive black holes far away in the universe and violent processes in the very early universe. The team will design a 10m atom interferometer, preparing the construction of the instrument in Oxford and paving the way for larger-scale future experiments to be located in the UK. Members of the AION consortium will also contribute to MAGIS, a partner experiment in the US.

The Cambridge team on AION is led by Professor Valerie Gibson and Dr Ulrich Schneider from the Cavendish Laboratory, alongside researchers from the Kavli Institute for Cosmology, the Institute of Astronomy and the Department of Applied Mathematics and Theoretical Physics. Dr Tiffany Harte will co-lead the development of the cold atom transport and final cooling sequences for AION, and Dr Jeremy Mitchell will co-lead the data readout and network capabilities for AION and MAGIS, and undertake data analysis and theoretical interpretation.

“This announcement from STFC to fund the AION project, which alongside some seed funding from the Kavli Foundation, will allow us to target key open questions in fundamental physics and bring new interdisciplinary research to the University for the foreseeable future,” said Gibson.

“Every physical effect, known or unknown, leaves its fingerprint on the phase evolution of a coherent quantum system such as cold atoms; it only requires sufficiently sensitive detectors,” said Schneider. “We are excited to contribute our cold-atom technology to this interdisciplinary endeavour and to develop atom interferometry into a powerful detector for fundamental physics.”

The Quantum Sensors for the Hidden Sector (QSHS) project, led by the University of Sheffield, has been awarded £4.8 million in funding. The project aims to contribute to the search for axions, low-mass ‘hidden’ particles that are candidates to solve the mystery of dark matter. They will develop new quantum measurement technology for inclusion in the US ADMX experiment, which can then be used to search for axions in parts of our galaxy’s dark matter halo that have never been explored before.

“The team will develop new electronic technology to a high level of sophistication and deploy it to search for the lowest-mass particles detected to date,” said Professor Stafford Withington from the Cavendish Laboratory, Co-Investigator and Senior Project Scientist on QSHS. “These particles are predicted to exist theoretically, but have not yet been discovered experimentally. Our ability to probe the particulate nature of the physical world with sensitivities that push at the limits imposed by quantum uncertainty will open up a new frontier in physics.

“This new window will allow physicists to explore the nature of physical reality at the most fundamental level, and it is extremely exciting that the UK will be playing a major international role in this new generation of science.”

Professor Withington is also involved in the Determination of Absolute Neutrino Mass using Quantum Technologies, which will be led by UCL. The project aims to harness recent breakthroughs in quantum technologies to solve one of the most important outstanding challenges in particle physics – determining the absolute mass of neutrinos. One of the universe’s most abundant particles neutrinos are a by-product of nuclear fusion within stars, therefore being key to our understanding of the processes within stars and the makeup of the universe. Moreover, knowing the value of the neutrino mass is critical to our understanding of the origin of matter and evolution of the universe. They are poorly understood however, and the researchers aim to develop pioneering new spectroscopy technology capable to precisely measure the mass of this elusive but important particle.

Cambridge researchers are also involved in the Quantum Simulators for Fundamental Physics project, led by the University of Nottingham. The project aims to develop quantum simulators capable of providing insights into the physics of the very early universe and black holes. The goals include simulating aspects of quantum black holes and testing theories of the quantum vacuum that underpin ideas on the origin of the universe.

Researchers will use cutting-edge quantum technologies to transform our understanding of the universe and answer key questions such as the nature of dark matter and black holes.

NASA Goddard Space Flight CenterNew Simulation Sheds Light on Spiraling Supermassive Black Holes


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Hidden symmetry could be key to more robust quantum systems, researchers find

Wed, 09/12/2020 - 03:04

The researchers, from the University of Cambridge, have shown that microscopic particles can remain intrinsically linked, or entangled, over long distances even if there are random disruptions between them. Using the mathematics of quantum theory, they discovered a simple setup where entangled particles can be prepared and stabilised even in the presence of noise by taking advantage of a previously unknown symmetry in quantum systems.

Their results, reported in the journal Physical Review Letters, open a new window into the mysterious quantum world that could revolutionise future technology by preserving quantum effects in noisy environments, which is the single biggest hurdle for developing such technology. Harnessing this capability will be at the heart of ultrafast quantum computers.

Quantum systems are built on the peculiar behaviour of particles at the atomic level and could revolutionise the way that complex calculations are performed. While a normal computer bit is an electrical switch that can be set to either one or zero, a quantum bit, or qubit, can be set to one, zero, or both at the same time. Furthermore, when two qubits are entangled, an operation on one immediately affects the other, no matter how far apart they are. This dual state is what gives a quantum computer its power. A computer built with entangled qubits instead of normal bits could perform calculations well beyond the capacities of even the most powerful supercomputers.

“However, qubits are extremely finicky things, and the tiniest bit of noise in their environment can cause their entanglement to break,” said Dr Shovan Dutta from Cambridge’s Cavendish Laboratory, the paper’s first author. “Until we can find a way to make quantum systems more robust, their real-world applications will be limited.”

Several companies – most notably, IBM and Google – have developed working quantum computers, although so far these have been limited to less than 100 qubits. They require near-total isolation from noise, and even then, have very short lifetimes of a few microseconds. Both companies have plans to develop 1000 qubit quantum computers within the next few years, although unless the stability issues are overcome, quantum computers will not reach practical use.

Now, Dutta and his co-author Professor Nigel Cooper have discovered a robust quantum system where multiple pairs of qubits remain entangled even with a lot of noise.

They modelled an atomic system in a lattice formation, where atoms strongly interact with each other, hopping from one site of the lattice to another. The authors found if noise were added in the middle of the lattice, it didn’t affect entangled particles between left and right sides. This surprising feature results from a special type of symmetry that conserves the number of such entangled pairs.

“We weren’t expecting this stabilised type of entanglement at all,” said Dutta. “We stumbled upon this hidden symmetry, which is very rare in these noisy systems.”

They showed this hidden symmetry protects the entangled pairs and allows their number to be controlled from zero to a large maximum value. Similar conclusions can be applied to a broad class of physical systems and can be realised with already existing ingredients in experimental platforms, paving the way to controllable entanglement in a noisy environment.

“Uncontrolled environmental disturbances are bad for survival of quantum effects like entanglement, but one can learn a lot by deliberately engineering specific types of disturbances and seeing how the particles respond,” said Dutta. “We’ve shown that a simple form of disturbance can actually produce – and preserve – many entangled pairs, which is a great incentive for experimental developments in this field.”

The researchers are hoping to confirm their theoretical findings with experiments within the next year.

The research was funded in part by the Engineering and Physical Sciences Research Council (EPSRC).

Reference:
Shovan Dutta and Nigel R. Cooper. ‘Long-range coherence and multiple steady states in a lossy qubit array.’ Physical Review Letters (2020). DOI: 10.1103/PhysRevLett.125.240404

Researchers have found a way to protect highly fragile quantum systems from noise, which could aid in the design and development of new quantum devices, such as ultra-powerful quantum computers.

Until we can find a way to make quantum systems more robust, their real-world applications will be limitedShovan DuttaShovan DuttaEntanglement


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Magnetic vortices come full circle

Mon, 30/11/2020 - 16:03

Magnets often harbour hidden beauty. Take a simple fridge magnet: somewhat counterintuitively, it is ‘sticky’ on one side but not the other. The secret lies in the way the magnetisation is arranged in a well-defined pattern within the material. More intricate magnetisation textures are at the heart of many modern technologies, such as hard drives.

Now, an international team of scientists from the University of Cambridge, the Paul Scherrer Institute (PSI), ETH Zurich, the Donetsk Institute for Physics and Engineering in Ukraine and the Institute for Numerical Mathematics RAS in Moscow have discovered unexpected magnetic structures inside a tiny pillar made of the magnetic material GdCo2.

The researchers observed sub-micrometre loop-shaped configurations, which they identified as magnetic vortex rings. Far beyond their aesthetic appeal, these textures might point the way to further complex three-dimensional structures arising in the bulk of magnets and could one day form the basis for new technological applications. Their results are reported in the journal Nature Physics.

Determining the magnetisation arrangement within a magnet is highly challenging, in particular for structures at the micro- and nanoscale, for which studies have been typically limited to looking at a shallow layer just below the surface. That changed in 2017 when researchers at PSI and ETH Zurich introduced a new X‑ray method for the nanotomography of bulk magnets, which they demonstrated in experiments at the Swiss Light Source. That advance opened up a window into the inner life of magnets, providing a tool for determining three-dimensional magnetic configurations at the nanoscale within micrometre-sized samples.

Using these capabilities, the researchers ventured into new territory. The stunning loop shapes they observed appear in the same GdCo2 micropillar samples in which they had before detected complex magnetic configurations consisting of vortices — the sort of structures seen when water spirals down from a sink — and their topological counterparts, antivortices.

That was a first, but the presence of these textures has not been surprising in itself. Unexpectedly, however, the scientists also found loops that consist of pairs of vortices and antivortices. That observation proved to be puzzling. With the implementation of novel sophisticated data-analysis techniques they eventually established that these structures are so-called vortex rings — in essence, doughnut-shaped vortices.

Vortex rings are familiar to everyone who has seen smoke rings being blown, or who has watched dolphins producing loop-shaped air bubbles, for their own amusement as much as to that of their audience. The newly discovered magnetic vortex rings are captivating in their own right. Not only does their observation verify predictions made some two decades ago, settling the question whether such structures can exist. They also offered surprises. In particular, magnetic vortex rings have been predicted to be a transient phenomenon, but in the experiments now reported, these structures turned out to be remarkably stable.

“One of the main puzzles was why these structures are so unexpectedly stable – like smoke rings, they are only supposed to exist as moving objects,” said Dr Claire Donnelly from Cambridge’s Cavendish Laboratory, and the paper’s first author. “Through a combination of analytical calculations and considerations of the data, we determined the root of their stability to be the magnetostatic interaction.”

The stability of magnetic vortex rings could have important practical implications. For one, they could potentially move through magnetic materials, as smoke rings move stably though air, or air-bubble rings through water.

Learning how to control the rings within the volume of the magnet can open interesting prospects for energy-efficient 3D data storage and processing. There is interest in the physics of these new structures, too, as magnetic vortex rings can take forms not possible for their smoke and air counterparts. The team has already observed some unique configurations, and going forward, their further exploration promises to bring to light yet more magnetic beauty.

Reference:
Claire Donnelly et al. ‘Experimental observation of vortex rings in a bulk magnet.’ Nature Physics (2020). DOI: 10.1038/s41567-020-01057-3

Adapted from a PSI press release.

 

The first experimental observation of three-dimensional magnetic ‘vortex rings’ provides fundamental insight into intricate nanoscale structures inside bulk magnets and offers a fresh perspective for magnetic devices.

One of the main puzzles was why these structures are so unexpectedly stable – like smoke rings, they are only supposed to exist as moving objectsClaire DonnellyClaire DonnellyReconstructed vortex rings inside a magnetic micropillar


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