In less than 100 seconds, John Dainton argues the importance of giving academics the freedom to explore their intellectual curiosities. Many huge developments to benefit society – including the electrification of technologies and the World Wide Web – have emerged from the pursuit of fundamental answers, Dainton explains.
Watch more from our 100 Second Science video series.
By Hamish Johnston
Just in time for Christmas, researchers at the Massachusetts Institute of Technology (MIT) have unveiled the ultimate “cocktail accessory”. It’s an edible self-propelled boat that whizzes around on the surface of an alcoholic drink.
Researchers in the US have observed room-temperature ferromagnetism in a graphene nanostructure for the first time. The result, until now only predicted by theory, suggests that graphene could be used to create spintronics devices, which are circuits that use the spin of the electron to process and store information.
Graphene, a sheet of carbon just one atom thick, is a promising material for making molecular electronic devices of the future thanks to its unique electronic and mechanical properties. These include extremely high electrical and thermal conductivity plus exceptional mechanical strength. Room-temperature ferromagnetism can now be added to this already impressive list.
Sakhrat Khizroev at Florida International University and colleagues made their discovery by making a number of different measurements of the magnetic properties of graphene samples that had been functionalized with nitrophenyl (NP) groups. This involves the attachment of NP groups to the surface of graphene (see figure). The resulting graphene-based material appears to become an organic molecular magnet with ferromagnetic and antiferromagnetic ordering that persists at temperatures above 400 K.
The researchers, who include Jeongmin Hong at the University of California, Berkeley, Robert Haddon at University of California, Riverside and Walt de Heer at the Georgia Institute of Technology, have been working on these experiments since 2008. “We believe that the NP groups act to unpair electron spins at periodically spaced carbon sites along certain graphene orientations, known as ‘armchair’ and ‘zigzag’,” Hong says. “It is the interactions between these unpaired spins that lead to the magnetic order we observed.” Graphene functionalized with hydrogen also appears to have similar magnetic properties, he adds.
Ours is a “gentle chemistry” approach
Jeongmin Hong of the University of California, Berkeley
“Ours is a ‘gentle chemistry’ approach that makes use of functionalization rather than introducing defects into graphene, which is a much more aggressive strategy,” Hong explains. “Although previous research mainly looked at heavily defected material, large numbers of defects in graphene can hinder the formation of the pure zigzag edges needed for magnetism here.”
According to the researchers, the NP-functionalized graphene could be used as a new type of single-layer magnet. It might also be used to make new types of spintronics devices based entirely on carbon that exploit the unpaired spins that are present. Spintronics is a relatively new technology that exploits the spin of an electron as well as its charge.
More details about the research can be found in ACS Nano.
By Michael Banks
Not many school pupils can boast having had a world-champion physics teacher, so say hello to Julie McGavigan, who teaches physics at Eastwood High School near Glasgow and bagged a gold medal at the World Karate Championships in Denmark in October.
The 27 year old, who says the win in Denmark came as “quite a shock”, is a 3rd Dan in Shotokan karate and has taught physics for five years after studying the subject at the University of Glasgow.
McGavigan also teaches karate at evening classes at Eastwood High, where she puts physics principles to good use. “Physics helps me understand why certain stances, moves and combinations work when practising karate,” McGavigan told physicsworld.com.
By Tushna Commissariat
“Wizzing” to the top of the Red Folder this week is a group of physicists at the “Splash Lab” at Brigham Young University who have studied the physics of “splashback” that occurs when people urinate. Using high-speed cameras the researchers filmed jets of liquid from a “synthetic urethra” striking toilet walls. They found that the stream of liquid breaks up into droplets when it is about 15 cm from the urethra exit. “Wizz kids” Tadd Truscott and Randy Hurd suggest that apart from sitting down on the toilet (and risk being called Sitzpinklers by their German friends), men should get nice and close when doing their business to eliminate splashback. Take a look at their video about “Urinal dynamics” above.
Quantum mechanics and special relativity have been used to implement a protocol that ensures that a “sealed envelope” is not opened ahead of time – according to its creators in Switzerland and Singapore. Sealed envelope systems, whether literal or metaphorical, allow information to remain temporarily inaccessible to both author and intended recipient, and are used in processes such as secure voting. The latest experimental results could lead to improvements in certain kinds of financial transaction, say the researchers.
Quantum mechanics is already exploited commercially in cryptography to carry out what is known as quantum key distribution. This involves two parties, known conventionally as Alice and Bob, sharing a secret cryptography key in the form of a string of quantum particles. Any eavesdropper trying to make measurements of the particles will reveal their presence by destroying the particles’ quantum state.
Rather than protecting against intrusive third parties, a sealed envelope – or “bit commitment” – ensures mutual trustworthiness. A bit of information (0 or 1) is deposited and neither changed by Alice, the creator, nor looked at by Bob, the receiver, prematurely. Research in the early 1990s appeared to show that quantum mechanics could be used to achieve bit commitment by ensuring that any tampering by either Alice or Bob would be revealed in the altered state of the quantum particles. But in 1997 Dominic Mayers of the Université de Montréal in Canada, as well as Hoi-Kwong Lo and Hoi Fung Chau of the Institute of Advanced Study in Princeton, US, poured cold water on the idea, showing, in fact, that even with the hypothetical envelope in Bob’s hands, Alice will always be able to make it look as if she created a “one” when she actually created a “zero”, and vice versa.
The latest work, carried out by Hugo Zbinden and colleagues at the University of Geneva together with researchers at the National University of Singapore, provides experimental demonstration of a scheme that overcomes this problem. The underlying concept was proposed last year by Adrian Kent of the University of Cambridge in the UK, and exploits special relativity as well as quantum mechanics. It splits up the roles of Alice and Bob so that each works with two distant agents spaced far apart from one another. The idea is that the speed of light imposes a minimum time for any causal influence to travel between Alice or Bob and their respective agents. This time delay removes the possibility for the kind of tampering that impairs a non-relativistic approach.
In Kent’s scheme, it is receiver Bob who initiates the information transfer. He sends a string of photons to Alice, polarizing each one as he chooses, either horizontally, vertically, or along one of two different diagonal axes. Alice then declares her choice of bit value via the type of polarizer she uses to measure the state of the photons. For the sake of argument, if she plumps for “zero” she uses a horizontal–vertical polarizer, whereas if she opts instead for “one” she uses a diagonal polarizer.
The result of each photon measurement is sent at close to the speed of light to Alice’s two agents, who in turn communicate the results to Bob’s agents (positioned close by). Since no information can travel faster than light, this set-up guarantees that in the time it takes for data to travel from Alice to her agents, none of the six parties could have tampered with those data. That is the time that the metaphorical envelope remains closed.
To find out whether Alice has told him the truth about her bit choice, Bob compares Alice’s results with his polarizations for all those photons (roughly half) that he happened to polarize along one of the two axes of Alice’s polarizer. If Alice is honest, there will be a 100% match. But if instead she tries to make him believe that she chose the other bit value, there will only be about a 50% match. That is because in re-measuring the photons using the other set of polarizers, to try and hoodwink Bob, she will no longer have access to the original polarization information, having destroyed it with her first set of measurements.
However, it is still possible that Alice cheated in a different way, forwarding the photons to her agents without making a measurement. By getting her agents to make the measurement instead, she would be able to make her bit choice at a later time than she claimed. Bob, however, can check for this particular trick thanks to the fact that each of them has two agents. Being spaced so far apart, the first of Alice’s agents wouldn’t be able to communicate the result of his or her measurement to the second agent before the envelope is opened. The deceit would therefore be revealed in a disparity between the two agents’ results.
The first experimental test of this scheme was actually reported earlier this year by Yang Liu of the University of Science and Technology of China in Hefei and colleagues. But by sending laser beams through free space they could not transmit data beyond the horizon, limiting the distance between the various parties to about 20 km and the commitment time to only 30 µs. They also sent a limited number of photons – only 107 in all – giving any cheats a 5% chance of success.
The Swiss–Singapore team has achieved better results by modifying Kent’s scheme slightly. They allow Alice to communicate with her agents via a fibre-optic cable, separating out the bit decision from the polarization measurements. This allows the two sets of agents to be separated by more than 9000 km, resulting in a whopping commitment time of 15 milliseconds. The probability of successful cheating using this set up, they calculate, was a tiny 1 in 18 million.
Kent speculates that the scheme could be used in financial markets, allowing traders to commit to buy or sell something – be that gold or shares, for example – before declaring that commitment. “The idea is that you have an extra mechanism for controlling how information propagates,” he explains, “so damping down the arms race in which everyone is fighting to get a nanosecond ahead of everyone else.”
Kent is confident that there are no loop holes in his scheme, claiming that the team’s analyses “characterize every possible attack and show that none of them can work”. He points out that his calculations do not account for the effects of general relativity, but says those effects are likely to be very small. “Were there a portal connecting Geneva to Singapore it is possible that someone could break the scheme,” he adds, “but that is not something that I lie awake at night worrying about.”
Chau, now at the University of Hong Kong, believes that the latest work will have “addressed a lot of the practical issues”, identified by himself and others, needed to turn quantum cryptography into a real-world technology. But he argues that the new experiments still leave “room to improve and develop”, adding that detection using non-identical detectors remains an outstanding problem.
The research is reported in Physical Review Letters.
By Hamish Johnston
There’s an interesting preprint on the arXiv server that proposes a new way of detecting dark-matter particles. I’ve been thinking about dark matter because last week physicists working on the LUX experiment announced that the underground detector had failed to find any dark-matter particles in the first three months of its operation. LUX was designed to look for WIMPs (weakly interacting massive particles), but WIMPs are not the only game in town when it comes to dark matter. There are also axions, which are the quarry of this latest proposal by three physicists in the US.
Axions are hypothetical particles that were first postulated in the 1970s to help explain puzzling aspects of quantum chromodynamics, which is the theory that describes interactions between quarks and gluons. Axions are also interesting from a cosmological point of view because they have mass but do not interact strongly with electromagnetic radiation. These properties make them prime candidates for dark matter, a mysterious substance that appears to make up most of the matter in the universe.
Invisibility cloaks might actually make the objects they aim to hide more visible, according to researchers in the US. While existing cloaking concepts might have the potential to render objects invisible to specific electromagnetic frequencies, a recent study has shown that, when integrating over the entire spectrum, the combined scattering of the cloak is always greater than the original uncloaked object. When exposed to short broadband pulses, these cloaks might therefore be turned instead into easier-to-see “beacons”. The team proposes two solutions to this problem: a passive approach using thin shells of superconducting material; and an active solution based on metamaterials.
The topic of practical invisibility cloaks – a staple of much fantasy and science fiction – has received a great deal of scientific and media interest of late, especially the possibility of achieving cloaking at visible light frequencies. One promising avenue of enquiry has been with metamaterials, an appropriately designed shell of which can be used to drastically suppress the scattering of light from an object (for a given wavelength), making it almost undetectable. A successful demonstration of this principle, which rendered an object invisible to microwaves, was undertaken in 2006.
According to Andrea Alù and his colleague Francesco Monticone of the University of Texas at Austin, most cloaking techniques used today, including popular ones such as transformation cloaks and plasmonic cloaks, are fundamentally limited by causality and passivity to actually scatter more than the uncloaked object, if you integrate over the entire spectrum, instead of looking at just the wavelength being cloaked. “This means that if you excite the cloak with a pulse, you would actually see it more easily than the uncloaked object it is trying to hide,” says Alù. The researchers go on to explain that, apart from the scientific significance of solving the scattering problem, it is equally important for a variety of situations – from warfare to commercial uses – where it is essential that a cloaked object at a given frequency does not become a beacon in a range of the other frequencies.
In the new work, the researchers first looked at three different basic types of passive cloaks: a plasmonic cloak; a mantle cloak; and a transformation-optics cloak. The plasmonic cloak showed the most scattering, followed by the mantle cloak with slightly less scattering and the transformation-optics cloak showed the least scattering, but overall they found that all three cloaked objects scattered drastically more than the uncloaked object, over a range of frequencies.
Nonetheless, the team has used its results to propose a number of possible workarounds to this global-scattering issue. The first approach uses passive, and suitably tailored, diamagnetic or superconducting thin shells, providing up to a 25% reduction in the integrated scattering by providing a near-zero permeability for static magnetic fields. A second, active approach would instead use metamaterials with specifically positioned, powered amplifiers. Current cloaking designs have a fundamental constraint on the frequency dispersion of their passive components (described by Foster’s reactance theorem) in which the impedance of passive surfaces always grows with frequency, resulting in narrow bandwidths of cloaking and an increased global scattering. By including operational amplifiers in specific positions along the cloaking surface, the team believes it should be possible to break this limit, creating a surface impedance that decreases with frequency, allowing for cloaking over a significantly larger bandwidth.
“I don’t think that the paper asks the right question,” comments Ulf Leonhardt, a physicist from the University of St Andrews in the UK, who argues that while perfect cloaking may be physically impossible, imperfect invisibility might be perfectly sufficient. In a hypothetical perfect cloak, the speed of light would need to be infinite across all frequencies to create the effect that the light had gone around the cloaked object in the same time that it would have taken to pass through it. In an imperfect cloak, the light would take slightly longer to cross the cloak object than the equivalent amount of empty space. But, Leonhardt proposes, only very sensitive equipment would be able to detect this. “In a scattering analysis, such as the present study,” he adds, “the difference between free-space propagation and propagation with the device is considered. If the light takes longer, then this amounts to a big difference that, however, is just an artefact of the analysis. It would tell you that the cloaking device performs rather badly, whereas in reality it works just fine.”
The research is published in Physical Review X.
A common blue dye used in £5 Bank of England notes could hold the key to spintronic devices after physicists in the UK and Canada discovered that its electron spins have surprisingly long quantum decoherence times. Marc Warner and colleagues at University College London and the University of British Columbia have found that copper phthalocyanine (CuPc), which has been used as a dye since the 1930s, can have a decoherence time of as long as 3 μs. While this is still shorter than the times available using the spins associated with nitrogen vacancy (NV) impurities in diamond, the latter structures are difficult to work with.
Spintronics is all about exploiting the spin of an electron as well as its electrical charge, with, for example, spin-up corresponding to a binary “1” and spin-down to “0”. In principle, spintronic circuits could be made much smaller and more energy efficient than conventional electronics. The concept can also be extended to single electrons that could function as quantum bits of information – or qubits – which could form the basis of quantum computers. A big challenge, however, is to find materials in which the spin state lasts long enough to store and process information in practical devices.
In this latest study, Warner and colleagues used a molecular-deposition technique to create a film comprising about 1000 individual layers of CuPc and hydrogen phthalocyanine (H2Pc). They kept the ratio of CuPc to H2Pc molecules relatively low – between 0.1 and 10% in different samples – to ensure that the average distance between the copper atoms is large enough to prevent their spins from interacting with each other as this would shorten their relaxation times.
The team measured how long the spin states endure by using electron spin resonance (ESR), which involves placing the sample in a magnetic field and then using microwaves to cause transitions between spin states. The researchers were interested in two particular time parameters related to the spin states. One is the “population relaxation time” (T1), which indicates how long it takes for an ensemble of spins pointing in the same direction to reach the state where the spins are pointing in random directions. T1 is an important parameter for spintronics applications that do not involve quantum information. The other parameter is the “phase memory time” or decoherence time (T2), which says how long quantum information could be stored in the spin of copper atom.
When the team measured T1 in films, it found a rapid drop in the relaxation time as the amount of CuPc in the sample increased. In samples where the ratio of CuPc to H2Pc is about 0.1%, T1 was about 0.1 s. But when that ratio rose to about 10%, T1 fell to about 10 μs. These measurements were made at a chilly 5 K but when the temperature was increased to 80 K, the 0.1% T1 value also dropped to about 10 μs.
When it came to T2, the team found that spins in the 0.1% sample retained their coherence for about 3 μs at 5 K, dropping slightly to about 1 μs at 80 K. This performance is not quite as good as the spins associated with NV impurities in diamond, which some physicists believe could prove very useful for building quantum computers. However, CuPc is an easier material to work with than NV impurities and some other materials mooted for quantum computing.
“Our research shows that a common blue dye has more potential for quantum computing than many of the more exotic molecules that have been considered previously,” explains Warner, who is now at Harvard University in the US.
As well as having relatively long relaxation times, CuPc has several other properties that make it attractive for use in quantum computers. Unlike NV impurities in diamonds, CuPc interacts strongly with visible light: a property that could be used to create quantum devices that use both spins and light to process quantum information. The material is also easy to modify both physically and chemically, which means its properties can be changed to suit a range of applications.
The research is reported in Nature.
The discovery of graphene is truly one of the “eureka moments” of our time. It is the story of how Andre Geim and Konstantin Novoselov realized that the discarded strips of Scotch Tape routinely used to produce clean surfaces on blocks of graphite were not useless – but might actually be covered with a type of carbon only previously spoken of in scientific fables. Having demonstrated the production of graphene for the first time, Geim and Novoselov worked with speed and purpose to show that this material possess all kinds of wonderful properties, including its unprecedented strength, electrical conductivity and complete impermeability. For their studies of this one-atom-thick form of carbon, the scientists were awarded the 2010 Nobel Prize for Physics just seven years after they had described their initial discovery.
Physics World invited Geim to Bristol to give a public talk about the discovery of graphene, as part of a special lecture series to celebrate 25 years of the magazine’s publication. At the sell-out event, attended by more than 400 people, the Nobel laureate talked about why he is still so passionate about fundamental research. He told the story of graphene and discussed some of the exciting applications of the material that are beginning to emerge. The audience was also treated to a description of Geim’s earlier work on levitating frogs, which led to him sharing the Ig Nobel prize in 2000.
Date: Wednesday 23 October 2013
Speaker: Andre Geim is a condensed-matter physicist at the University of Manchester in the UK. He shared the 2010 Nobel Prize with Konstantin Novoselov for “groundbreaking experiments regarding the two-dimensional material graphene”.
