A couple of years ago, I came across what I thought was a funny (and physics-related) video about a water slide. The slide is called “Verrückt”, which my German-speaking colleagues translate as “mad” or “crazy”, and it caught my attention because it was being built at an amusement park in my home town of Kansas City. As the video shows, the slide experienced a few problems during its testing phase.
“When the rafts are loaded with more than 1000 pounds, the slide becomes unsafe,” says the video’s announcer as the test raft goes airborne. “We’re going to have to redesign the entire slide,” an unnamed official adds.
The story began around this time last year, soon after the LHC was rebooted and began its impressive 13 TeV run, when the ATLAS collaboration saw more events than expected around the 750 GeV mass window. This bump immediately caught the interest of physicists the world over, simply because there was a sniff of “new physics” around it, meaning that the Standard Model of particle physics did not predict the existence of a particle at that energy. But also, it was the first interesting data to emerge from the LHC after its momentous discovery of the Higgs boson in 2012 and if it had held, would have been one of the most exciting discoveries in modern particle physics.
A new type of optical metasurface whose properties can be dynamically reconfigured with a laser pulse has been developed by researchers in the UK. The team believes that its technology, which has lower loss than traditional plasmonic resonators, could be useful for reconfigurable optoelectronic components.
Although metamaterials were originally developed to passively manipulate radiation to achieve perfect lenses or cloaking devices, the field has broadened to encompass materials that can be switched or tuned to modulate their properties. These have often featured plasmonic resonators – subwavelength noble-metal structures that interfere directly with the electromagnetic field of radiation and reshape a wavefront.
To reconfigure these, they can be combined with a so-called phase-change material, which alters its properties in some way in response to an external signal. However, plasmonic resonators often have high losses, especially at optical frequencies. In recent years, therefore, many researchers have produced metasurfaces from silicon or other dielectric materials, because their losses are smaller and they are easier to manufacture.
Laser switch
In the new research, Nikolay Zheludev and colleagues at the University of Southampton have produced metasurfaces purely from the chalcogenide germanium antimony telluride. Many chalcogenides – a class of compounds including sulfides, selenides and tellurides – can exist in both amorphous and crystalline phases. Heating the crystal above its melting point for a few nanoseconds destroys the crystalline structure and turns the material into an amorphous glass.
To trigger the reverse transition, the glass has to be heated to a lower temperature for a longer time (but still less than a microsecond). Chalcogenides have often been used in plasmonic metamaterials to shift the resonant frequencies of the plasmonic resonators by altering their surrounding environment. However, the optical properties of crystalline and amorphous chalcogenides themselves are very different and this phase-changing material is being used in rewritable CDs and DVDs, and is also being developed for new types of computer memory.
The researchers deposited 300 nm-thick films of amorphous germanium antimony telluride onto quartz substrates. They measured the near-infrared absorption of the film across a range of near-infrared wavelengths, finding that it was relatively transparent. Next, they used ion beams to selectively etch away the chalcogenide to produce subwavelength nanogratings. Pronounced absorption resonances appeared, with the resonance frequency dependent on the grating periodicity.
Phase changing
Zheludev’s team scanned a green laser beam over the surface. The light heated the material, causing it to crystallise. Upon measuring the optical properties of the crystalline gratings, the researchers found significant differences: after crystallization, one grating reflected only 20% as much light at 1470 nm as it had when the chalcogenide was amorphous. “We show for the first time that a dielectric metamaterial may be switched through the phase change of the dielectric itself,” says Zheludev. The researchers have not yet demonstrated the reverse transition back to the amorphous form of the grating: this will be more difficult, simply because it requires heating the material above its melting point while maintaining the structure of the grating.
Thomas Taubner of RWTH Aachen University in Germany praises the research, which he says forms part of a move towards all-dielectric, reconfigurable metasurfaces that researchers have worked towards in the past few years. He believes the absence of a reversible phase transition makes this “a first step”, but says that “in the nanophotonics community, the first goal is of course to show the concept and then later to do the engineering.”
The Rio 2016 Olympics will kick off tomorrow and over the next three weeks, while you enjoy watching the world’s top athletes compete in the huge variety of sports, spare a thought for the physics involved. From how to throw a ball to running, from pole vaulting to golf, physics and sport are fellow brethren. Head on over on the JPhys+ blog to read “The big physics of sport round-up!” post and watch our video series above, in between cheering on your favourite teams.
The George Eastman Museum in Rochester, New York, is the oldest photography museum in the world. The Victorian mansion that houses the museum was once home to Eastman himself – a pioneer in photography who in the 1880s helped bring photography to the masses after inventing roll film and designing the Kodak camera. Eastman’s passion for colour and order are reflected in the museum’s manicured gardens, where a diverse selection of blossoms sprouts from strictly symmetrical flower beds.
Within the museum’s walls, there are more than 400,000 photographic objects, including a collection of about 5000 daguerreotypes – images typically no bigger than a postcard, printed on polished copper plates and encased in glass. Sourced from all over the world, examples include a likeness of the “father” of the daguerreotype, French photographer Louis-Jacques-Mandé Daguerre. Invented by Daguerre in the 1830s, daguerreotypes became the first commercially available form of photography.
For about two decades – until other, more efficient photographic methods became available and popular – this early way of recording visual images surged in popularity worldwide. For the first time, people could buy an exact image of themselves. The method was used to capture snapshots of landmarks, famous people, ordinary citizens, as well as notable events such as burning buildings or lively street scenes. Subjects included Queen Victoria and her daughter, US president Abraham Lincoln, writer Harriet Beecher Stowe, the Acropolis in Athens and the waterfront of Cincinnati. The Eastman museum collection includes a daguerreotype of abolitionist Frederick Douglass.
“People went nuts with it,” says Patrick Ravines, a chemist-turned-conservator at Buffalo State College in New York, who studies and collects daguerreotypes. “They used to document everything around the world, and daguerreotypes show things that don’t exist any more.” He points out that researchers can study daguerreotypes to study erosion in natural settings, measure the growth and change of cities, or even see what archaeological sites – such as the Acropolis – looked like in the 19th century. These images serve as a record of their day.
History maker Louis Daguerre, pictured (left) using his daguerreotype process by Jean-Baptiste Sabatier-Blot in 1844. Daguerre’s own daguerreotype of Boulevard du Temple in Paris in 1838 (right) is the oldest surviving photograph of a living person (bottom right) but has suffered severe damage since this copy was created. (Courtesy: George Eastman House, International Museum of Photography and Film; Louis Daguerre)
Yet, like everything, daguerreotypes yield to time, and are now in trouble. Some have become cloudy; others have developed spots. Some are so damaged that they’re no longer recognizable. They’re deteriorating at a rapid clip.
In 2005 art conservator Ralph Wiegandt, then at the Eastman Museum, helped prepare a large exhibition of daguerreotypes to be exhibited at the International Center of Photography in New York City. During the exhibition, curators noticed that 30 of the objects showed signs of inexplicable degradation and were fading fast. To find ways to salvage the images, Wiegandt turned to physicists at the nearby University of Rochester.
One of them was Nicholas Bigelow, who was eager to bring his team to the project. Bigelow’s scientific interest was piqued by the fact that early photographers were using nanotechnology to create the images, even though they were unaware of it at the time. “If you were to take the nanoparticles that form the image on the daguerreotype, you’d have to have between 100 and 1000 of them stacked side by side to be as [wide] as a human hair,” he says in a video interview filmed for the University of Rochester.
To save the daguerreotypes, Bigelow and his fellow scientists would need to follow a three-step process. First, they’d have to understand the surface physics and chemistry of how the images were made. Second, they’d have to identify the processes that fuel degradation. Third, they’d have to get creative – and find ways to use their research to forestall the process, if not reverse it. Fortunately, the team had an arsenal of modern imaging tools at its disposal, including scanning electron microscopes (SEMs), transmission electron microscopes, X-ray scanners and focused ion beam milling, to get beneath the surface of the centuries-old images.
“It’s a surface where history and art and science intersect, and it’s very, very complicated,” says Brian McIntyre, who manages the Integrated Nanosystems Center at the University of Rochester. As he points out, the objects provide the first true – not drawn – images of the world. “They give us a window on the world that really is first light.”
Forming the image
Daguerreotypes aren’t like other photographs. The image is printed using a laborious process that may require hours just to get one shot. It’s printed directly onto a metal plate – which renders each daguerreotype one-of-a-kind and irreproducible. They’re unusually rich in detail. Bigelow says he was drawn to the project after seeing a daguerreotype of the Cincinnati waterfront made in 1848. Zooming in on that image with magnifying lenses reveals an astonishing level of detail. “If you blow this up, and you blow it up, and you blow it up, the level of resolution and detail in this nanotechnology photograph is fantastic,” he says.
Wiegandt, who now continues his daguerreotype work as a visiting research scientist at the University of Rochester, says that well-made daguerreotypes can be enlarged 20–30 times and still reveal minute details of their subjects – a resolution that, today, would require a 140,000-megapixel digital camera.
But the scientists wanted to zoom in even further to see what was happening at the nano level. “It was determined that looking at the surface – the surface science, the surface morphology, the surface physics – was where we ought to start,” recalls McIntyre.
1 The shape of things Transmission electron microscopy of daguerreotypes reveals clear 3D-shaped particles in the amalgam on the surface of the images, including hexagon and truncated triangle. The geometry probably corresponds to the amalgam’s make-up. (CC BY Nanoscale Research Letters 10.1186/1556-276X-7-337)
The process of making a daguerreotype begins when a silver-coated copper plate is painstakingly polished to a mirror finish. The plate is then placed in a closed box with iodine and bromine vapours, which sensitizes it to light and gives it a rose-coloured hue. Next, the picture is taken: the plate is mounted in a camera and exposed to light, usually for a few minutes, to form the image. The plate is then exposed to hot mercury vapours, which create particles of amalgam – a combination of silver and mercury – that develop the image. Finally, the silver iodide is removed, and the image may be “gilded”, which involves coating it with gold chloride.
“The fascinating aspect of daguerreotypes is that you’ve got light interacting with a material – and changing the nature of that material,” says Ravines, who led a study, published in May in the Journal of Imaging Science and Technology (60 30504), that reports what happens at the molecular level during that laborious process. He and his colleagues focused primarily on the steps during which the image was formed – after the plate was exposed to iodine and bromine, but before it was gilded.
Images taken with an SEM of a daguerreotype from Ravines’ personal collection showed that many of the larger amalgam particles – about 200–800 nm in diameter – are hexagonal in shape and pile up in the whites and light greys of the image (figure 1). Their findings corroborate studies by scientist Alice Swan in the late 1970s, and by art conservation scientist M Susan Barger in the 1980s and 1990s, that also revealed the hexagonal shape. In the new study, however, the researchers observed particles with other shapes too, including large trapezoidal solids and other 3D geometries. Ravines suspects those other geometries may correspond to a variety of amalgam species that combine mercury and silver in different proportions.
2 It’s the pits Focused ion beam microscopy dramatically shows the condition of surface disruption on a daguerreotype – likely caused by excess gold used to “gild” finish the photograph. (Courtesy: NSF/Brian McIntyre/University of Rochester)
Understanding how nanoparticles and the surface respond to airborne pollutants such as water vapour, sulphur-based gaseous pollutants and other sources of corrosion, says Ravines, could help scientists better observe how the images vanish over time. Figure 2 shows one example of the damage.
Destructive forces
In 2010, with support from a grant from the National Science Foundation, Bigelow, Wiegandt and McIntyre went looking at damage on the nanoscale. They were in for some surprises. Perhaps most startling was the fact that the daguerreotype surface can support life that, ultimately, harms it. “One thing that we never would have expected is that the daguerreotype is a biologically active surface,” says Bigelow. “We have discovered essentially on every daguerreotype we looked at, there are small colonies of fungi growing, and those fungi are damaging the surface.”
SEM images reveal networks of micron-scale filaments sprawling on the surface (figure 3). The microbes seemed to not only live on the surface, but also interact with it in some way – a discovery that McIntyre likens to finding life on Mars. The surface of a daguerreotype is “pretty darn toxic”, as he puts it. Silver and mercury have antimicrobial properties, but they don’t stop the micro-organisms from spreading across the silver surface of the images.
3 Biological danger One danger for daguerreotypes comes from the colonies of fungi that form on the surface. Shown here are some of the diverse biological forms seen with scanning electron microscopy. The microbes form sprawling networks of filaments that not only live on the surface but interact with it too. (Courtesy: NSF/Ralph Wiegandt/University of Rochester/George Eastman Museum)
The team also found problems lurking beneath the surface. “One thing we’ve observed is that below the image particles, and above the native silver surface, there seems to be a gap forming, and that gap can lead to the uppermost surface of the daguerreotype exfoliating from the underlying silver,” says McIntyre. In extreme cases, this condition leads to large flakes peeling away from the image like ripped pieces of paper – a type of degradation that’s easily visible to the naked eye. In images from SEMs, the gaps resemble networks of underground caves, with the scientists saying these look like Kirkendall voids – cavities that appear at the interface between two attached metals, such as a metal and an alloy in a solder joint. In daguerreotypes, the voids likely form as a result of the reaction between a gold (gilding) solution and the silver. Though the exfoliation mechanism isn’t well understood, those SEM images suggest that the metal seems to be moving, in part, due to it interacting with the solution used to gild the image, though McIntyre admits other factors could be involved.
Writing in the Journal of Imaging Science and Technology, Ravines says he has seen voids beneath the image that raise questions about the overall stability of the surface that the image rests on. Understanding those voids will be important for devising sound conservation treatment strategies. “We have to be careful because of the way that subsurface is almost like Swiss cheese,” he says. “Knowing the way [the daguerreotype] is built allows us to better preserve it.”
Another source of degradation comes, ironically, from historic efforts to preserve the images. Daguerreotypes are typically sealed in cases beneath a cover glass. Over time, moisture accumulates on the underside of the cover glass and dissolves it. The highly alkaline droplets can drip onto the daguerreotype. “We see patterns of degradation associated with the glass coming apart,” McIntyre says. “You try to do the right thing 150 years ago, but your best intensions are foiled by nature.”
Rescue strategy
The first priority in conserving daguerreotypes is to stabilize them in their current state. Following on from the imaging work, Wiegandt began to develop new strategies to protect the artefacts from degrading any further. One tactic included encapsulating the image in a frame filled with an inert gas, such as argon, to prevent pollutants and other corrosive elements from getting inside. That may not be a permanent solution, but it might buy more time. “If you can give the daguerreotype 20 years of life before a recharge, that’s a good investment of time and energy,” says Ravines.
Another tantalizing possibility, says McIntyre, is to undo the damage altogether. “We’ve done some things at the nano level that we think can not only stabilize but perhaps reverse some of the degradation process,” he says. One approach addresses the surface-exfoliation problem In preliminary experiments, he says, the researchers have been developing a way to physically repair that damage – a bit like a nano sewing machine that might stitch the damaged area.
McIntyre cautions, however, that so far they’ve only been able to use this approach in micron-sized areas, and scaling it up to be useful on daguerreotypes in general would be difficult. “I don’t know, frankly, if it’s possible, but it’s certainly conceivable,” he says. “It would be very cool.” Developing a useful tool would also not come cheap.
Daguerreotypes today
Daguerreotypes, in a way, have now come back into fashion. Professional photographers and curious hobbyists can learn how to make them in classes at the Eastman Museum, and successful artists including Adam Fuss have created oversized daguerreotypes many times larger than anything produced in the 19th century. In another 100 years, those works will likely face degradation, too – though perhaps, by then, conservators will have a set of sophisticated tools to keep the images from fading away.
But the benefits of saving daguerreotypes may even extend beyond the art world, with Bigelow and Wiegandt hoping their findings will have applications to other areas of nanotechnology. The amalgam nanoparticles, for example, exhibit “self-assembling” behaviour – a growing area of nanoscience research.
For McIntyre, the project not only demonstrated the interdisciplinary reach of nanoscale physics – but was a professional thrill too. “It’s probably one of the most interesting things I’ve ever worked on,” he says. “It’s not often you get the opportunity to work in the historical realm, the science realm and the art realm simultaneously.”
The UK’s National Physical Laboratory (NPL) – the country’s standards lab – is consulting on making up to 50 compulsory redundancies as it prepares to shift its research priorities towards quantum technologies and big data. The number of redundancies is due to be confirmed on 15 August, with the NPL expected to have already informed the UK government how many of the approximately 780 person workforce will lose their jobs.
Standard setting
Since it was founded in 1900 in Teddington, the NPL has set measurement standards in a range of areas from acoustics to neutron metrology. Recently, the NPL has put a greater focus on developing quantum technologies. It opened the Quantum Metrology Institute last year, which will develop the standards and tests needed for the UK to commercialize quantum technology. Other topics that have seen a more intense focus include medical physics, with an emphasis on cancer detection and treatment, and data science, including how to handle the vast volumes of measurements produced by modern techniques.
Lost experience?
According to Clive Scoggins – a negotiator for the Prospect union that represents NPL workers – the number of potential job losses could approach 70 when combined with voluntary redundancies, representing around 15% of scientific staff. Scoggins warns that Prospect members working at NPL are “extremely concerned” about the apparent lack of detail in plans so far communicated to staff. “Decades of scientific knowledge and experience may be lost before NPL is fully aware of what skills it may require to deliver an extraordinary level of impact in those new areas, as is expected of a world-leading national measurement institute.”
However, Fiona Auty, NPL’s head of communications, emphasizes that many could remain at the lab in different roles, because the lab currently has more than 50 vacancies. While Auty says that “no team is being lost”, she admits there are some areas that will be stopped, but would not say which ones due to the ongoing consultation process. “The NPL wants to invest in new areas and we cannot remain in every area, so this is not a headcount loss exercise but a changing of our science portfolio,” Auty told physicsworld.com. “We are investing where we believe metrology can make a huge difference to current and future areas of growth or challenge for the UK.”
A different mission
Glenn Martyna from the IBM Watson Research Center in New York, US, who is collaborating with the NPL in supervising a PhD student, recognizes that the revolution in big data is a challenge to manage. “The changes wrought by big data are changing NPL,” he says. “I expect both IBM and NPL to survive with slightly different missions.” Martyna’s PhD student is due to complete his course as originally intended, with Auty hoping the majority of the other students working with the NPL – including the 100 or so at its Postgraduate Institute – will likewise be unaffected. “A few will be impacted, but in every case we are looking to retain relationships and students in a way that is mutually sensible,” says Auty.
Quantum physicist Kai Bongs from the University of Birmingham, who collaborates with NPL researchers, is surprised by the changes but is happy that quantum technology will benefit. “Although in the short term there will be some pain, the longer term benefits could be substantial,” he says.
It is commonly claimed that you cannot fold a piece of paper in half more than seven times. That may be true for a standard piece of paper of A4 dimensions, but according to US teenage (at the time) mathematician Britney Gallivan the maximum number of folds is in fact dependent on the initial size of the sheet. In this video, Jack Baker from the University of Leicester, UK, answers the question “how many times could you fold a piece of paper as large as the size of the observable universe?” To find out the answer watch the video.
This is one of a collection of videos based on student projects from the University of Leicester’s “Physics Special Topics” course, in which students use their physics knowledge to define and answer a quirky or unusual research question. The videos are part of our 100 Second Science series.
With its penchant for using centrifuges, water baths and other laboratory tools to whip up novel tastes and textures, modernist cuisine – also known as “molecular gastronomy” – has a reputation for complexity. In his book Molecular Gastronomy at Home, chef Jozef Youssef aims to change that. Using simple, step-by-step guides, Youssef – an alumnus of Heston Blumenthal’s modernist restaurant the Fat Duck – shows adventurous home cooks how to take “culinary physics out of the lab and into your kitchen” by whipping up mousses, infusions and other exciting creations with a (comparatively) limited array of specialist cooking utensils and ingredients.
The book’s focus on techniques, rather than recipes per se, emphasizes the experimental nature of this type of cooking, and readers are strongly encouraged to play around until they get it right. Indeed, in the book’s introduction, Youssef rather charmingly admits that even he “took quite a few attempts to get the results [he] wanted” when trying his hand at a technique called reverse spherification. This technique – the first of 15 to feature in the book – involves adding calcium lactate to a flavoured liquid and then carefully pipetting the resulting mixture into a bath of water and sodium alginate. Sodium alginate is used in the food industry as a gelling agent, and one outcome of this gastro-chemical reaction (performed with yoghurt as the flavouring base) is shown in the photo top left.
For a complete explanation of how this reaction proceeds (and why reverse spherification is so much trickier than basic spherification, in which a flavour + sodium alginate mixture is dripped into a calcium lactate bath) you’ll need to look elsewhere. However, a few chapters towards the end of the book do go into somewhat more detail on the scientific side, and the overall effect is more than enough to whet your appetite – in more ways than one.
A five-qubit trapped-ion quantum computer, which is programmable and reconfigurable, has been demonstrated by researchers from the Joint Quantum Institute in the US. The team’s computing architecture is such that the researchers can programme multiple algorithms into their trapped-ion processor, which is a first. Although the computer is relatively small at five qubits and the algorithms they process fairly simple, the researchers say that there are a variety of ways to scale up this architecture to build a functional quantum computer in the future.
The hallmark of a quantum computer will be its ability to solve certain computational problems – such as factoring large numbers or simulating complex chemical reactions as well as the interactions between large numbers of fundamental particles – exponentially faster and more efficiently than is possible with current classical computing. There are a variety of quantum methods and technologies – including superconducting qubits and trapped ions or quantum annealers and adiabatic quantum computing – that various groups around the world are adapting in the race towards building the first true quantum computer.
Five-ion trap
Chris Monroe’s group at the Joint Quantum Institute and the Joint Center for Quantum Information and Computer Science, at the University of Maryland in the US, uses trapped ions as qubits. In this technique, information is stored in the atomic-ions’ states. Electromagnetically confining a number of such ions, or “trapping” them, the particles can then be entangled by applying appropriate laser beams. The finely tuned laser light manipulates each ion in a specific way, depending upon its state. “In this way, the collective motion of the chain of ions behaves as a data bus that allows qubits to talk to each other,” say Monroe.
These operations are the quantum logic gates in the system, and with ions, the researchers are able to execute gates between any pair of ions in the chain. Monroe further explains that the effective wiring of the quantum computer in this case is enforced from the outside – this is a unique feature of ion qubits, he adds, because “they are not hard-wired and hence their connections can be reconfigured and programmed from the outside”.
While small ion-trap quantum computers have previously been built, each was a single-purpose device, capable of running a particular algorithm or generating a fixed entangled state. Now though, Monroe, together with Shantanu Debnath and colleagues, has demonstrated that the device can be programmed with multiple algorithms.
The team’s processor is made up of five ytterbium ions confined in a linear radio-frequency trap and laser-cooled to form a line, separated by about 5 μm. All of the ions are initially prepared in a standard state and a given algorithm is executed by applying a series of 50–100 laser pulses on particular individual or pairs of ions along the chain. Monroe told physicsworld.com that this “involves a few software steps: first we compile the algorithm from ‘textbook’ operations into the operations that are native to our system”.
Primitive operations
These native operations are then further broken down into pre-calculated laser pulse shapes, which depend upon the position of the particular ions in the chain. “Each of these primitive operations has 99% accuracy or so, and indeed when we concatenate 50–100 of these operations, the net accuracy of the algorithm is in the 70–90% range, depending on the details,” he adds. The team was able to implement these gates with greater than 98% fidelity.
Different view: An ion trap with four segmented blade electrodes used to trap a liner chain of atomic ions for quantum-information processing. Each ion is addressed optically for individual control and read-out using the high optical access of the trap. (Courtesy: Shantanu Debnath and Emily Edwards)
The researchers implemented some quantum algorithms, to see how the system fared as their five qubits worked in tandem. They ran two simple quantum algorithms – the Deutsch-Jozsa and Bernstein-Vazirani algorithms – that perform mathematical functions in a single step. While these can easily be performed on a normal computer, it would take several operations. They also ran a quantum Fourier transformation (QFT), which is a fundamental step in other more complex algorithms. “The QFT involves two-qubit gates between all possible pairs of qubits, and it has never been demonstrated,” says Monroe. Debnath says that it is “a very useful protocol used in the Shor’s factorization and a variety of other phase-estimation problems”, adding that these algorithms “show the flexibility of the processor as a programmable device”.
Atomic perfection
According to Monroe, trapped-ion qubits are more flexible as compared with their solid-state counterparts – they can be replicated and scaled up with negligible variation from qubit to qubit, and can be externally reconfigured using lasers and their quantum states can be initialized and read out with near 100% accuracy. Their downside though, is that they perform a quantum operation relatively slowly (gate times of microseconds or longer). And, as with most quantum systems, the usual challenge “is making sure that the ‘quantumness’ is well preserved as the system is scaled to more qubits”, says Debnath.
More problematic though is the fact that the practical engineering of trapped-ion qubits lags behind that of silicon and solid-state platforms because there are very few industrial companies that make these ionic devices, according to Monroe. “Sandia National Laboratories is one of the few places that manufacture monolithic chips with microfabricated electrodes for the reliable loading and trapping of atomic ions, and the future of this technology will depend upon Sandia and other places like Honeywell, Inc. to produce more varieties of these chips”, says Monroe.
He adds that his Maryland group has been aided by the fact that it uses a particular atomic species of ytterbium (171Yb+) that is also used for atomic clocks, and the control laser they use is industrially produced and has been reliably engineered. “The ion-trap community is dominated by university and federal research laboratories, with students and postdocs instead of professional systems-engineering staff. It can take many years to obtain the infrastructure to get these devices to work reliably and repeatedly.” Despite these issues, Monroe, Debnath and the team are convinced that ion-trap computing has the potential to be scaled up – both by increasing the number of qubits as well as by increasing connectivity between qubits by using photons – to a fully fledged quantum computer.
A room-temperature “supercurrent” has been identified in a Bose–Einstein condensate of quasiparticles called magnons. That’s the finding of an international team of researchers, which says the work opens the door to using magnons in information processing. Other researchers, however, believe the claim is premature, arguing that less-novel explanations have not been ruled out.
The term “supercurrent” describes the resistance-free current of charged particles in superconductors. It also describes the viscosity-free current of particles in superfluid helium. The common denominator of these systems is that they can be described as Bose–Einstein condensates (BECs) – collections of bosons, such as Cooper pairs or Helium-4, that can be described by a single wavefunction.
Frictionless flow
In such systems, the frictionless particles flow continuously. What causes the particles to move is not some external force – supercurrents endure without needing any input force to drive them. Instead, the flow is caused by a gradient in the phase of the BEC wavefunction. Previously, all examples of supercurrents have been at cryogenic temperatures. That’s because heat tends to excite particles into higher quantum states, which would destroy a BEC. The collection of particles would no longer be described by a single wavefunction, which would make macroscopic quantum phenomena such as supercurrents impossible.
In 2006, however, a team led by Sergej Demokritov at the University of Münster and Burkard Hillebrands of the University of Kaiserslautern, both in Germany, made a breakthrough that would pave the way for the new result reported here. The team showed that it is possible to create a BEC of magnons – quasiparticles which are quanta of electron-spin waves – at room temperature. To do this, the researchers applied a process called parametric pumping to a crystal film of yttrium iron garnet – a technique that injected magnons into the crystal’s ground state. They found that, when the magnon density became sufficiently high, the magnons formed a BEC.
Room-temperature first
In the new research, Hillebrands and colleagues produced such a magnon BEC, then heated its centre using a laser pulse. By changing the duration of the pulse, the researchers could alter the temperature difference they created between the laser-lit spot and the rest of the material. The purpose of the heating was to affect the magnetic properties of the material, and so affect the phase of the wavefunction governing the BEC.
The researchers observed the movement of magnons away from the heated region, and found that the magnitude of this flow increased with the temperature difference. Theoreticians in Ukraine and Israel constructed mathematical models of this flow, and believe that it can only be explained by the existence of a magnon supercurrent. “When we first saw it, we didn’t understand it,” says Hillebrands. “Fortunately, we have a very good team including top-level theoreticians.”
The researchers suggest that, in addition to fundamental scientific interest, the work could potentially lead to real-world applications. They say that they have opened the door to using room-temperature magnons for information storage and processing. “If we can now move this into a device where a macroscopic quantum state can be used, then for the future this looks very promising,” says Hillebrands. “This is at the very beginning – I cannot promise a device yet, but it is certainly very worthwhile to look into this new field.”
Insufficient evidence?
Demokritov, however – who was involved in the 2006 work but not this latest research – is sceptical that the researchers have genuinely observed a supercurrent. He believes the researchers have insufficient evidence that the flow is not simply due to the parametric pumping adding energy into the system. He offers the example of a magnetic stirrer, in which a rotating magnetic field turns a bar to stir a fluid. “If you looked at this, and you saw the fluid was rotating persistently, you might say that you had superfluidity at room temperature,” he says. “Of course, this would definitely not be the case because the energy would be brought to the system by the rotating magnetic field underneath the glass. I am afraid, unfortunately, that that is the case here.”
Demokritov also claims that his own team had better experimental evidence for a supercurrent – reported in 2012 in Scientific Reports – than that presented by Hillebrands and colleagues. “We were not so brave as to call it a supercurrent because the issue of dissipation is still open,” he says.
Demokritov is not the only sceptic. In July, physicist Edouard Sonin, at the Hebrew University of Jerusalem, Israel, published a comment on the arXiv preprint server in response to Hillebrands and colleagues’ paper, a pre-print of which was also first posted to the arXiv. He wrote that their pre-print “has not provided persuasive evidence of spin supercurrent” and that the authors neither checked the criteria for existence of a spin supercurrent in yttrium-iron-garnet magnetic films, nor did they discuss the “more natural scenario” that spin transport in their experiment was purely due to dissipation.
Hillebrands maintains that standard magnon-dissipation processes cannot properly explain the phenomena that his group have observed, although he concedes that they “still need to prove” that the flow is completely free of dissipation.
