̽��ֱ�� of Cambridge - spin

Through the looking glass: artificial ‘molecules’ open door to ultrafast devices

sc604 — Wed, 03 Mar 2021 00:00:01 +0000

Polaritons are quantum particles that consist of a photon and an exciton, another quasiparticle, combining light and matter in a curious union that opens up a multitude of possibilities in next-generation devices.

̽��ֱ��researchers have shown that geometrically coupled polariton condensates, which appear in semiconductor devices, are capable of simulating molecules with various properties.

Ordinary molecules are groups of atoms bound together with molecular bonds, and their physical properties differ from those of their constituent atoms quite drastically: consider the water molecule, H₂O, and elemental hydrogen and oxygen.

“In our work, we show that clusters of interacting polaritonic and photonic condensates can form a range of exotic and entirely distinct entities – ‘molecules’ – that can be manipulated artificially,” said first author Alexander Johnston, from Cambridge Department of Applied Mathematics and Theoretical Physics. “These artificial molecules possess new energy states, optical properties, and vibrational modes from those of the condensates comprising them.”

Johnston and his colleagues – Kirill Kalinin from DAMTP and Professor Natalia Berloff, who holds joint positions at Cambridge and Skoltech – were running numerical simulations of two, three, and four interacting polariton condensates, when they noticed some curious asymmetric stationary states in which not all of the condensates have the same density in their ground state.

“Upon further investigation, we found that such states came in a wide variety of different forms, which could be controlled by manipulating certain physical parameters of the system,” said Johnston. “This led us to propose such phenomena as artificial polariton molecules and to investigate their potential uses in quantum information systems.”

In particular, the team focused on an ‘asymmetric dyad’, which consists of two interacting condensates with unequal occupations. When two of those dyads are combined into a tetrad structure, the latter is, in some sense, analogous to a homonuclear molecule – for instance, to molecular hydrogen H₂. Furthermore, artificial polariton molecules can also form more elaborate structures, which could be thought of as artificial polariton compounds.

“There is nothing preventing more complex structures from being created,” said Johnston. “We’ve found that there is a wide range of exotic, asymmetric states possible in tetrad configurations. In some of these, all condensates have different densities, despite all of the couplings being of equal strength, inviting an analogy with chemical compounds.”

In specific tetrad structures, each asymmetric dyad can be viewed as an individual ‘spin,’ defined by the orientation of the density asymmetry. This has interesting consequences for the system’s degrees of freedom, or the independent physical parameters required to define states. ̽��ֱ��spins introduce a separate degree of freedom, in addition to the continuous degrees of freedom given by the condensate phases.

̽��ֱ��relative orientation of each of the dyads can be controlled by varying the coupling strength between them. Since quantum information sem.

“In addition, we have discovered a plethora of exotic asymmetric states in triad and tetrad systems,” said Berloff. “It is possible to seamlessly transition between such states simply by varying the pumping strength used to form the condensates. This property suggests that such states could form the basis of a polaritonic multi-valued logic system, which could enable the development of polaritonic devices that dissipate significantly less power than traditional methods and, potentially, operate orders of magnitude faster.”

Reference:
Alexander Johnston, Kirill P. Kalinin, and Natalia G. Berloff. ‘Artificial polariton molecules.’ Physical Review Letters B (2021). DOI: 10.1103/PhysRevB.103.L060507

Adapted from a Skoltech press release.

Researchers from the ̽��ֱ�� of Cambridge and Skoltech in Russia have shown that polaritons, the quirky particles that may end up running the quantum supercomputers of the future, can form structures that behave like molecules – and these ‘artificial molecules’ can potentially be engineered on demand. Their results are published in the journal Physical Review B Letters.

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Yes

A new spin on organic semiconductors

sc604 — Tue, 26 Mar 2019 00:00:59 +0000

̽��ֱ��international team from the UK, Germany and the Czech Republic has found that these materials could be used for ‘spintronic’ applications, which could make cheap organic semiconductors competitive with silicon for future computing applications. ̽��ֱ��results are reported in the journal Nature Electronics.

‘Spin’ is the term for the intrinsic angular momentum of electrons, which is referred to as up or down. Using the up/down states of electrons instead of the 0 and 1 in conventional computer logic could transform the way in which computers process information.

Instead of moving packets of charge around, a device built on spintronics would transmit information using the relative spin of a series of electrons, known as a pure spin current. By eliminating the movement of charge, any such device would need less power and be less prone to overheating – removing some of the most significant obstacles to further improving computer efficiency. Spintronics could therefore give us faster, energy-efficient computers, capable of performing more complex operations than at present.

Since organic semiconductors, widely used in applications such as OLEDs, are cheaper and easier to produce than silicon, it had been thought that spintronic devices based on organic semiconductors could power a future computer revolution. But so far, it hasn’t worked out that way.

“To actually transfer information through spin, the electron’s spin needs to travel reasonable distances and live for a long enough time before the information encoded on it is randomised,” said Dr Shu-Jen Wang, a recent PhD graduate of the ̽��ֱ�� of Cambridge’s Cavendish Laboratory, and the paper’s co-first author.

“Organic semiconductors have not been realistic candidates for spintronics so far because it was impossible to move spins around a polymer circuit far enough without losing the original information,” said co-first author Dr Deepak Venkateshvaran, also from the Cavendish Laboratory. “As a result, the field of organic spintronics has been pretty quiet for the past decade.”

̽��ֱ��internal structure of organic semiconductors tends to be highly disordered, like a plate of spaghetti. As such, packets of charge don’t move nearly as fast as they do in semiconductors like silicon or gallium arsenide, both of which have a highly ordered crystalline structure. Most experiments on studying spin in organic semiconductors have found that electron spins and their charges move together, and since the charges move more slowly, the spin information doesn’t go far: typically only a few tens of nanometres.

Now, the Cambridge-led team say they have found the conditions that could enable electron spins to travel far enough for a working organic spintronic device.

̽��ֱ��researchers artificially increased the number of electrons in the materials and were able to inject a pure spin current into them using a technique called spin pumping. Highly conductive organic semiconductors, the researchers found, are governed by a new mechanism for spin transport that transforms them into excellent conductors of spin.

This mechanism essentially decouples the spin information from the charge, so that the spins are transported quickly over distances of up to a micrometre: far enough for a lab-based spintronic device.

“Organic semiconductors that have both long spin transport lengths and long spin lifetimes are promising candidates for applications in future spin-based, low energy computing, control and communications devices, a field that has been largely dominated by inorganic semiconductors to date,” said Venkateshvaran, who is also a Fellow of Selwyn College.

As a next step, the researchers intend to investigate the role that chemical composition plays in an organic semiconductor’s ability to efficiently transport spin information within prototype devices.

̽��ֱ��research was coordinated by Professor Henning Sirringhaus at the Cavendish Laboratory and funded through a European Research Council (ERC) Synergy Grant jointly held by the ̽��ֱ�� of Cambridge, Imperial College London, ̽��ֱ�� of Mainz, Czech Academy of Sciences and Hitachi Cambridge Laboratory.

Reference:
Shu-Jen Wang, Deepak Venkateshvaran et al. ‘Long spin diffusion lengths in doped conjugated polymers due to enhanced exchange coupling.’ Nature Electronics (2019). DOI: 10.1038/s41928-019-0222-5

Researchers have found that certain organic semiconducting materials can transport spin faster than they conduct charge, a phenomenon which could eventually power faster, more energy-efficient computers.

Organic semiconductors have not been realistic candidates for spintronics so far because it was impossible to move spins far enough without losing the original information

Deepak Venkateshvaran

Deepak Venkateshvaran and Nanda Venugopal

Hand sketch of an organic lateral spin pumping device

Yes

Researchers road-test powerful method for studying singlet fission

tdk25 — Mon, 17 Oct 2016 15:00:00 +0000

Physicists have successfully employed a powerful technique for studying electrons generated through singlet fission, a process which it is believed will be key to more efficient solar energy production in years to come.

Their approach, reported in the journal Nature Physics, employed lasers, microwave radiation and magnetic fields to analyse the spin of excitons, which are energetically excited particles formed in molecular systems.

These are generated as a result of singlet fission, a process that researchers around the world are trying to understand fully in order to use it to better harness energy from the sun. Using materials exhibiting singlet fission in solar cells could make energy production much more efficient in the future, but the process needs to be fully understood in order to optimize the relevant materials and design appropriate technologies to exploit it.

In most existing solar cells, light particles (or photons) are absorbed by a semiconducting material, such as silicon. Each photon stimulates an electron in the material's atomic structure, giving a single electron enough energy to move. This can then potentially be extracted as electrical current.

In some materials, however, the absorption of a single photon initially creates one higher-energy, excited particle, called a spin singlet exciton. This singlet can also share its energy with another molecule, forming two lower-energy excitons, rather than just one. These lower-energy particles are called spin "triplet" excitons. Each triplet can move through the molecular structure of the material and be used to produce charge.

̽��ֱ��splitting process - from one absorbed photon to two energetic triplet excitons - is singlet fission. For scientists studying how to generate more solar power, it represents a potential bargain - a two-for-one offer on the amount of electrical current generated, relative to the amount of light put in. If materials capable of singlet fission can be integrated into solar cells, it will become possible to generate energy more efficiently from sunlight.

But achieving this is far from straightforward. One challenge is that the pairs of triplet excitons only last for a tiny fraction of a second, and must be separated and used before they decay. Their lifespan is connected to their relative "spin", which is a unique property of elementary particles and is an intrinsic angular momentum. Studying and measuring spin through time, from the initial formation of the pairs to their decay, is essential if they are to be harnessed.

In the new study, researchers from the ̽��ֱ�� of Cambridge and the Freie Universität Berlin (FUB) utilised a method that allows the spin properties of materials to be measured through time. ̽��ֱ��approach, called electron spin resonance (ESR) spectroscopy, has been used and improved since its discovery over 50 years ago to better understand how spin impacts on many different natural phenomena.

It involves placing the material being studied within a large electromagnet, and then using laser light to excite molecules within the sample, and microwave radiation to measure how the spin changes over time. This is especially useful when studying triplet states formed by singlet fission as these are difficult to study using most other techniques.

Because the excitons' spin interacts with microwave radiation and magnetic fields, these interactions can be used as an additional way to understand what happens to the triplet pairs after they are formed. In short, the approach allowed the researchers to effectively watch and manipulate the spin state of triplet pairs through time, following formation by singlet fission.

̽��ֱ��study was led by Professor Jan Behrends at the Freie Universität Berlin (FUB), Dr Akshay Rao, a College Research Associate at St John's College, ̽��ֱ�� of Cambridge, and Professor Neil Greenham in the Department of Physics, ̽��ֱ�� of Cambridge.

Leah Weiss, a Gates-Cambridge Scholar and PhD student in Physics based at Trinity College, Cambridge, was the paper's first author. "This research has opened up many new questions," she said. "What makes these excited states either separate and become independent, or stay together as a pair, are questions that we need to answer before we can make use of them."

̽��ֱ��researchers were able to look at the spin states of the triplet excitons in considerable detail. They observed pairs had formed which variously had both weakly and strongly-linked spin states, reflecting the co-existence of pairs that were spatially close and further apart. Intriguingly, the group found that some pairs which they would have expected to decay very quickly, due to their close proximity, actually survived for several microseconds.

"Finding those pairs in particular was completely unexpected," Weiss added. We think that they could be protected by their overall spin state, making it harder for them to decay. Continued research will focus on making devices and examining how these states can be harnessed for use in solar cells."

Professor Behrends added: "This interdisciplinary collaboration nicely demonstrates that bringing together expertise from different fields can provide novel and striking insights. Future studies will need to address how to efficiently split the strongly-coupled states that we observed here, to improve the yield from singlet fission cells."

Beyond trying to improve photovoltaic technologies, the research also has implications for wider efforts to create fast and efficient electronics using spin, so-called "spintronic" devices, which similarly rely on being able to measure and control the spin properties of electrons.

̽��ֱ��research was made possible with support from the UK Engineering and Physical Sciences Research Council (EPSRC) and from the Freie Universität Berlin (FUB). Weiss and colleague Sam Bayliss carried out the spectroscopy experiments within the laboratories of Professor Jan Behrends and Professor Robert Bittl at FUB. ̽��ֱ��work is also part of the Cambridge initiative to connect fundamental physics research with global energy and environmental challenges, backed by the Winton Programme for the Physics of Sustainability.

̽��ֱ��study, Strongly exchange-coupled triplet pairs in an organic semiconductor, is published in Nature Physics. DOI: 10.1038/nphys3908.

In a new study, researchers measure the spin properties of electronic states produced in singlet fission – a process which could have a central role in the future development of solar cells.

Future research will focus on making devices and examining how these states can be harnessed for use in solar cells

Leah Weiss

Spin, an intrinsic property of electrons, is related to the dynamics of electrons excited as a result of singlet fission – a process which could be used to extract energy in future solar cell technologies.

̽��ֱ��text in this work is licensed under a Creative Commons Attribution 4.0 International License. For image use please see separate credits above.

Yes

Cambridge to research future computing tech that could “ignite a technology field”

tdk25 — Thu, 14 Apr 2016 23:01:34 +0000

A project which aims to establish the UK as an international leader in the development of “superconducting spintronics” – technology that could significantly increase the energy-efficiency of data centres and high-performance computing – has been announced.

Led by researchers at the ̽��ֱ�� of Cambridge, the “Superspin” project aims to develop prototype devices that will pave the way for a new generation of ultra-low power supercomputers, capable of processing vast amounts of data, but at a fraction of the huge energy consumption of comparable facilities at the moment.

As more economic and cultural activity moves online, the data centres which house the servers needed to handle internet traffic are consuming increasing amounts of energy. An estimated three per cent of power generated in Europe is, for example, already used by data centres, which act as repositories for billions of gigabytes of information.

Superconducting spintronics is a new field of scientific investigation that has only emerged in the last few years. Researchers now believe that it could offer a pathway to solving the energy demands posed by high performance computing.

As the name suggests, it combines superconducting materials – which can carry a current without losing energy as heat – with spintronic devices. These are devices which manipulate a feature of electrons known as their “spin”, and are capable of processing large amounts of information very quickly.

Given the energy-efficiency of superconductors, combining the two sounds like a natural marriage, but until recently it was also thought to be completely impossible. Most spintronic devices have magnetic elements, and this magnetism prevents superconductivity, and hence reduces any energy-efficiency benefits.

Stemming from the discovery of spin polarized supercurrents in 2010 at the ̽��ֱ�� of Cambridge, recent research, along with that of other institutions, has however shown that it is possible to power spintronic devices with a superconductor. ̽��ֱ��aim of the new £2.7 million project, which is being funded by the Engineering and Physical Sciences Research Council, is to use this as the basis for a new style of computing architecture.

Although work is already underway in several other countries to exploit superconducting spintronics, the Superspin project is unprecedented in terms of its magnitude and scope.

Researchers will explore how the technology could be applied in future computing as a whole, examining fundamental problems such as spin generation and flow, and data storage, while also developing sample devices. According to the project proposal, the work has the potential to establish Britain as a leading centre for this type of research and “ignite a technology field.”

̽��ֱ��project will be led by Professor Mark Blamire, Head of the Department of Materials Sciences at the ̽��ֱ�� of Cambridge, and Dr Jason Robinson, ̽��ֱ�� Lecturer in Materials Sciences, Fellow of St John’s College, ̽��ֱ�� of Cambridge, and ̽��ֱ�� Research Fellow of the Royal Society. They will work with partners in the ̽��ֱ��’s Cavendish Laboratory (Dr Andrew Ferguson) and at Royal Holloway, London (Professor Matthias Eschrig).

Blamire and Robinson’s core vision of the programme is “to generate a paradigm shift in spin electronics, using recent discoveries about how superconductors can be combined with magnetism.” ̽��ֱ��programme will provide a pathway to making dramatic improvements in computing energy efficiency.

Robinson added: “Many research groups have recognised that superconducting spintronics offer extraordinary potential because they combine the properties of two traditionally incompatible fields to enable ultra-low power digital electronics.”

“However, at the moment, research programmes around the world are individually studying fascinating basic phenomena, rather than looking at developing an overall understanding of what could actually be delivered if all of this was joined up. Our project will aim to establish a closer collaboration between the people doing the basic science, while also developing demonstrator devices that can turn superconducting spintronics into a reality.”

̽��ֱ��initial stages of the five-year project will be exploratory, examining different ways in which spin can be transported and magnetism controlled in a superconducting state. By 2021, however, the team hope that they will have manufactured sample logic and memory devices – the basic components that would be needed to develop a new generation of low-energy computing technologies.

̽��ֱ��project will also report to an advisory board, comprising representatives from several leading technology firms, to ensure an ongoing exchange between the researchers and industry partners capable of taking its results further.

“ ̽��ֱ��programme provides us with an opportunity to take international leadership of this as a technology, as well as in the basic science of studying and improving the interaction between superconductivity and magnetism,” Blamire said. “Once you have grasped the physics behind the operation of a sample device, scaling up from the sort of models that we are aiming to develop is not, in principle, too taxing.”

A Cambridge-led project aiming to develop a new architecture for future computing based on superconducting spintronics - technology designed to increase the energy-efficiency of high-performance computers and data storage - has been announced.

Superconducting spintronics offer extraordinary potential because they combine the properties of two traditionally incompatible fields to enable ultra-low power digital electronics

Jason Robinson

10515 images via Pixabay

Growing quantities of data storage online are driving up the energy costs of high-performance computing and data centres. Superconducting spintronics offer a potential means of significantly increasing their energy-efficiency to resolve this problem.

̽��ֱ��text in this work is licensed under a Creative Commons Attribution 4.0 International License. For image use please see separate credits above.

Yes

New state of matter detected in a two-dimensional material

sc604 — Mon, 04 Apr 2016 15:04:40 +0000

An international team of researchers have found evidence of a mysterious new state of matter, first predicted 40 years ago, in a real material. This state, known as a quantum spin liquid, causes electrons – thought to be indivisible building blocks of nature – to break into pieces.

̽��ֱ��researchers, including physicists from the ̽��ֱ�� of Cambridge, measured the first signatures of these fractional particles, known as Majorana fermions, in a two-dimensional material with a structure similar to graphene. Their experimental results successfully matched with one of the main theoretical models for a quantum spin liquid, known as a Kitaev model. ̽��ֱ��results are reported in the journal Nature Materials.

Quantum spin liquids are mysterious states of matter which are thought to be hiding in certain magnetic materials, but had not been conclusively sighted in nature.

̽��ֱ��observation of one of their most intriguing properties — electron splitting, or fractionalisation — in real materials is a breakthrough. ̽��ֱ��resulting Majorana fermions may be used as building blocks of quantum computers, which would be far faster than conventional computers and would be able to perform calculations that could not be done otherwise.

“This is a new quantum state of matter, which has been predicted but hasn’t been seen before,” said Dr Johannes Knolle of Cambridge’s Cavendish Laboratory, one of the paper’s co-authors.

In a typical magnetic material, the electrons each behave like tiny bar magnets. And when a material is cooled to a low enough temperature, the ‘magnets’ will order themselves over long ranges, so that all the north magnetic poles point in the same direction, for example.

But in a material containing a spin liquid state, even if that material is cooled to absolute zero, the bar magnets would not align but form an entangled soup caused by quantum fluctuations.

“Until recently, we didn’t even know what the experimental fingerprints of a quantum spin liquid would look like,” said paper co-author Dr Dmitry Kovrizhin, also from the Theory of Condensed Matter group of the Cavendish Laboratory. “One thing we’ve done in previous work is to ask, if I were performing experiments on a possible quantum spin liquid, what would I observe?”

Knolle and Kovrizhin’s co-authors, led by Dr Arnab Banerjee and Dr Stephen Nagler from Oak Ridge National Laboratory in the US, used neutron scattering techniques to look for experimental evidence of fractionalisation in alpha-ruthenium chloride (α-RuCl₃). ̽��ֱ��researchers tested the magnetic properties of α-RuCl₃ powder by illuminating it with neutrons, and observing the pattern of ripples that the neutrons produced on a screen when they scattered from the sample.

A regular magnet would create distinct sharp lines, but it was a mystery what sort of pattern the Majorana fermions in a quantum spin liquid would make. ̽��ֱ��theoretical prediction of distinct signatures by Knolle and his collaborators in 2014 match well with the broad humps instead of sharp lines which experimentalists observed on the screen, providing for the first time direct evidence of a quantum spin liquid and the fractionalisation of electrons in a two dimensional material.

“This is a new addition to a short list of known quantum states of matter,” said Knolle.

“It’s an important step for our understanding of quantum matter,” said Kovrizhin. “It’s fun to have another new quantum state that we’ve never seen before – it presents us with new possibilities to try new things.”

Reference:
A. Banerjee et al. ‘Proximate Kitaev quantum spin liquid behaviour in a honeycomb magnet.’ Nature Materials (2016). DOI: 10.1038/nmat4604

Researchers have observed the ‘fingerprint’ of a mysterious new quantum state of matter in a two-dimensional material, in which electrons break apart.

It’s an important step for our understanding of quantum matter.

Dmitry Kovrizhin

Genevieve Martin, Oak Ridge National Laboratory

Excitation of a spin liquid on a honeycomb lattice with neutrons.

̽��ֱ��text in this work is licensed under a Creative Commons Attribution 4.0 International License. For image use please see separate credits above.

Yes

Baby mantises harness mid-air ‘spin’ during jumps for precision landings

fpjl2 — Thu, 05 Mar 2015 16:48:31 +0000

̽��ֱ��smaller you are, the harder it is not to spin out of control when you jump. Miniscule errors in propulsive force relative to the centre of mass results in most jumping insects – such as fleas, leafhoppers and grasshoppers – spinning uncontrollably when they jump.

Until now, scientists worked under the hypothesis that such insects can’t control this, and spin unpredictably with frequent crash landings.

But new high-speed video analysis of the jumps of wingless, baby praying mantises has revealed a technique which actually harnesses the spinning motion, enabling them to jump with accuracy at the same time as repositioning their body mid-air to match the intended target – all in under a tenth of a second.

Researchers used a thin black rod distant from the platform on which the mantises sat as a target for them to jump at.

During the jumps, the insects rotated their legs and abdomen simultaneously yet in varying directions – shifting clockwise and anti-clockwise rotations between these body parts in mid-air – to control the angular momentum, or ‘spin’. This allowed them to shift their body in the air to align themselves precisely with the target on which they chose to land.

And the mantises did all of this at phenomenal speed. An entire jump, from take-off to landing, lasted around 80 milliseconds – literally faster than the blink of a human eye.

At first, scientists believed the mantis had simply evolved a way to mitigate the natural spin that occurs when such small insects jump at speed.

On closer inspection, however, they realised the mantis is in fact deliberately injecting controlled spin into the jump at the point of take-off, then manipulating this angular momentum while airborne through intricate rotations of its extremities in order to reposition the body in mid-air, so that it grasps the target with extreme precision.

For the study, published today in the journal Current Biology, the researchers analysed a total of 381 slowed-down videos of 58 young mantises jumping to the target, allowing them to work out the intricate mechanics used to land the right way up and on target virtually every time.

“We had assumed spin was bad, but we were wrong – juvenile mantises deliberately create spin and harness it in mid-air to rotate their bodies to land on a target,” said study author Professor Malcolm Burrows from Cambridge ̽��ֱ��’s Department of Zoology, who conducted the research with Dr Gregory Sutton from Bristol ̽��ֱ��.

“As far as we can tell, these insects are controlling every step of the jump. There is no uncontrolled step followed by compensation, which is what we initially thought,” he said.

In fact, when the researchers moved the target closer, the mantises spun themselves twice as fast to ensure they got their bodies parallel with the target when they grasped it.

For Sutton, the study is similar to accountancy, only with distribution of momentum instead of money. “ ̽��ֱ��mantis gives itself an amount of angular momentum at take-off and then distributes this momentum while in mid-air: a certain amount in the front leg at one point; a certain amount in the abdomen at another – which both stabilise the body and shift its orientation, allowing it to reach the target at the right angle to grab on,” he said.

̽��ֱ��researchers tested what would happen if they restricted the ability of the mantis to harness and spread the ‘spin’ to its extremities during a jump. To do this, they glued the segments of the abdomen together, expecting the mantis to spin out of control.

Intriguingly, the accuracy of the jump wasn’t impeded. ̽��ֱ��mantises still reached the target, but couldn’t rotate their bodies into the correct position – so crashed headlong into it and bounced off again.

̽��ֱ��next big question for the researchers is to understand how the mantis achieves its mid-air acrobatics at such extraordinary speeds. “We can see the mantis performs a scanning movement with its head before a jump. Is it predicting everything in advance or does it make corrections at lightning speed as it goes through the jump? We don’t know the answer between these extreme possibilities,” said Burrows.

Sutton added: “We now have a good understanding of the physics and biomechanics of these precise aerial acrobatics. But because the movements are so quick, we need to understand the role the brain is playing in their control once the movements are underway.”

Sutton believes that the field of robotics could learn lessons from the juvenile mantis. “For small robots, flying is energetically expensive, and walking is slow. Jumping makes sense – but controlling the spin in jumping robots is an almost intractable problem. ̽��ֱ��juvenile mantis is a natural example of a mechanical set-up that could solve this,” he said.

Professor Malcolm Burrows and Dr Gregory Sutton

High-speed videos reveal that, unlike other jumping insects, the juvenile praying mantis does not spin out of control when airborne. In fact, it both creates and controls angular momentum at extraordinary speeds to orient its body for precise landings.

As far as we can tell, these insects are controlling every step of the jump

Malcolm Burrows

Malcolm Burrows and Greg Sutton

A juvenile praying mantis

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Yes

Gone in 45 nanoseconds – but a new opportunity for quantum control?

tdk25 — Mon, 22 Dec 2014 04:42:05 +0000

A new study has successfully measured the coherence of electron spin – the period of time in which the particle’s elusive quantum state can be read and manipulated – for an electron trapped in conditions that could form the basis of a future quantum internet.

̽��ֱ��study, reported in the journal Physical Review Letters, was carried out by researchers at the Universities of Cambridge and Saarbrücken. It reveals the coherence time of an electron trapped in a silicon-based colour centre within a microscopic fragment of diamond. This is a gap, manufactured inside the diamond’s lattice structure, and designed to snare an electron so that it can be manipulated.

At just 45 nanoseconds, the time period for which the electron’s spin is visible seems a miniscule fraction, but for scientists trying to bring this under control, it is, in relative terms, an age.

̽��ֱ��“spin” of a particle is its intrinsic angular momentum and can point either up or down. Physicists at numerous leading research universities, including Cambridge, are currently engaged in research which is trying to utilise spin to develop advanced quantum technologies.

In the future, electron spin could be used to represent data and move large amounts of information much faster than is currently possible. This means that better control of spin might well underpin future computing, enable the creation of an entirely new quantum network (or quantum internet), and provide the foundations for a huge range of other technologies, such as advanced sensing devices.

One problem that hinders scientists who are attempting to gain greater command over electron spin for this purpose, however, is that spins in solids cannot be seen, or manipulated, for very long. After a tiny fraction of a second has passed, the spin’s quantum state decays beyond the point of visibility. Therefore, it needs to be retained for long enough for information about the spin to be registered and manipulated.

In the new study, the researchers successfully demonstrated the extent of the coherence of an electron trapped in a “silicon-vacancy” – an impurity in the lattice of carbon atoms that make up diamond. A silicon-vacancy centre provides highly promising conditions for the manipulation of electron spin.

Building on previous research, the researchers put the electron into a “superposition” state, using a technique which involves targeting it with two lasers with carefully-tuned frequencies. In this quantum state, the spin of the electron is potentially both up and down, and it is useful because it provides a basic position from which they can then observe and measure changes using laser pulses. ̽��ֱ��vision for future spin-based technologies involves creating chains of electrons whose spin will change relative to one another based on this initial superposition concept.

When applied to the electron in the silicon vacancy centre, the method achieved a coherence period of tens of nanoseconds – a fraction of time which, for scientists trying to control spin, is actually ample.

Dr Mete Atature, a researcher at the Cavendish Laboratory and St John’s College, ̽��ֱ�� of Cambridge, who led the study with Professor Christoph Becher in Saarbrcüken, said: “This is incremental research, but it essentially deals with the elephant in the room for these colour centres, which was whether there was long-living coherence for the electron spin or not, and whether we had time to see its quantum state?”

“Arguably this is the most pressing challenge for these colour centres right now. We established that we can not only access the electron spin states, but also sustain an arbitrary superposition of them for 45 nanoseconds. When you bear in mind that it will take us picoseconds to execute laser-based operations to manipulate the spin, it becomes clear that just a fraction of this period is required. So this gives us a lot of possibilities to work with.”

̽��ֱ��vacancy centre was created by substituting a silicon atom and a gap in place of two neighbouring carbon atoms in the carbon lattice of a fragment of diamond. Research earlier this year showed that a silicon-based vacancy has the potential to be used for this purpose because the photons – or light particles – emitted by an electron trapped in such conditions are sufficiently bright, and on a sufficiently narrow bandwidth, to be attractive for various applications. ̽��ֱ��research adds to a growing realisation among scientists that silicon-vacancy centres could provide advantageous conditions for spin and photon control, simultaneously.

“Now we know that silicon vacancies provide an alternative colour centre that has spin coherence, optical detectability and superior optical qualities,” Atature added. “ ̽��ֱ��next challenge is to see if we can extend this spin coherence time by various techniques and, in parallel, see if we can entangle the spin with a single photon with sufficiently high fidelity.”

In a breakthrough study scientists have revealed the coherence, or the visibility lifespan, of the spin of an electron in an emerging colour centre in diamond. This could provide a potential component for future quantum networks.

We established that we can not only access the electron spin states, but also sustain an arbitrary superposition of them for 45 nanoseconds. When you bear in mind that it will take us picoseconds to execute laser-based operations to manipulate the spin, it becomes clear that just a fraction of this period is required.

Mete Atature

Atomic structure of the SiV- color center, consisting of an Si impurity (red) situated on an interstitial position along the bond axis and surrounded by a split-vacancy (transparent) and the next-neighbor carbon atoms (grey).

Yes

Spin with a new twist

tdk25 — Fri, 10 Oct 2014 06:56:28 +0000

A new method of controlling the “spin” of an electron, one of the fastest-developing research topics in quantum-based technologies and widely seen as the potential foundation of numerous future advances, has been demonstrated by scientists.

In quantum physics, the “spin” of a particle refers to its intrinsic angular momentum. This can be controlled so that it is aligned with one of two directions, typically referred to as “up” or “down”.

Usually, researchers define the direction by applying a magnetic field to orientate the electron, called the “quantisation axis”. ̽��ֱ��process can, however, be distorted by the natural magnetic environment around the electron itself, which is usually seen as one of the biggest obstacles to controlling spin.

Uniquely, the new study, by a team at the ̽��ֱ�� of Cambridge and the Joint Quantum Institute (JQI) in the USA, instead turned this magnetic field into a natural advantage which allowed the electron to be held in place. ̽��ֱ��researchers did this by firing two precisely-tuned lasers at the particle, creating what they call a “dark state” from which it could be manipulated and measured.

̽��ֱ��implications for controlling quantum systems are significant because, since the 1990s, researchers have theorised that a particle’s spin could be used to store and manipulate information. Using the “up” or “down” as an alternative to the binary coding of 0s and 1s that characterises computers today, spin-based quantum computers would be able to compute difficult problems and vast amounts of data much more efficiently.

Any such development, however, depends on finding ways to bring electron spin under control in the first place. To date, researchers have had to find ways to do this in spite of the randomising effect that the magnetic field around an electron has on the orientation of its spin.

̽��ֱ��spin of an electron cannot be observed continuously without altering it, so it has to be measured before and after an attempt to manipulate its quantum state. This measurement reveals whether the spin is up or down, but the surrounding magnetic environment can also take effect at any time. If it does so, the quantisation axis of the electron is altered, and the whole picture is distorted. ̽��ֱ��effect is similar to trying to measure longitude and latitude in a world where the positions of the north and south poles are changing randomly all the time.

Dr Mete Atatüre, a researcher at St John’s College, Cambridge who led part of the new study, said: “In order to perform reliable measurements, we constantly have to fight against this fluctuating magnetic environment. In fact, most research is about trying to keep electrons detached or isolated from it. What is unique about this experiment is that we did the opposite and used this environment as a resource. We created a quantum state that wouldn’t be accessible if the magnetic field wasn’t there.”

̽��ֱ��electron was trapped inside a self-assembled “quantum dot”, a tiny structure made from a 10 nanometre-thick indium arsenide droplet, surrounded by gallium arsenide. While both materials are semiconductors, an electron can have a lower energy inside the “quantum dot” than in gallium arsenide. “This forms a natural and stable trap for single electrons within a semiconductor device, providing the desired conditions for defining a spin quantum bit” explained Carsten Schulte, a Cambridge graduate student who worked on the project.

An electron isolated in this fashion can then be targeted with lasers to manipulate its spin. If a laser strikes the quantum dot at certain wavelengths, the electron is optically excited and emits light, or fluoresces, which changes its spin. If, however, the magnetic environment interferes with this the change becomes uncontrollable.

To resolve this, the researchers fired two separate lasers at the quantum dot – one tuned to excite the “up” spin state, the other to excite the “down” state. These interfered with each other destructively, preventing any fluorescence at all and creating a so-called “dark state”.

“You would expect two lasers to raise the level of optical excitation even more, but in fact when this is done no light comes out of the quantum dot,” Jack Hansom, another graduate student in the research team, said. “Optical excitation ceases and the electron finds a unique quantum superposition state, which is neither up nor down.”

By changing the relative phase between the two lasers, they were able to redefine the specific dark state and force the electron into it. This showed that the electron could be manipulated in its own up/down coordinate system without the researchers ever knowing the orientation of up and down during the whole process.

“What is profound is that the electron is always in the same quantum superposition state, but the basis in which it is represented evolves with the nuclear field that remains unknown to us,” Atatüre added. “This research shows that the magnetic environment around the quantum dot does not need to remain a problem, but can be utilized for the definition and control of a quantum bit.”

̽��ֱ��full report appears in the October issue of Nature Physics.

Scientists have successfully demonstrated a new way to control the “spin” of an electron – the natural intrinsic angular momentum of electrons which could underpin faster computing in the future. ̽��ֱ��technique counterintuitively makes use of the ever-changing magnetic field of the electron’s environment - one of the main obstacles to traditional methods of spin control.

Most research is about trying to keep electrons isolated from the magnetic environment. What is unique about this experiment is that we did the opposite and used it as a resource

Mete Atature

Carsten Schulte

Spin manipulation in a noisy environment. In the quest for ever more precise manipulation of quantum systems, any uncontrolled interaction with the environment is usually considered detrimental.

Yes