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The journey of a crisp

Stack of seasoned crisps — Snack stack. (Courtesy: iStock/elena moiseeva)

As I walk into the giant Walkers crisp factory on the outskirts of Leicester, UK, the first thing that strikes me is the noise. I hastily insert my bright-pink ear plugs – part of a compulsory outfit that includes a blue over-suit, a hair net and non-slip steel-toe-capped shoes. It’s warm, steamy and smells – as you might expect – like a chip shop. Tall stainless-steel structures, all jiggling or vibrating, fill the brightly lit cavernous space, the size of a football pitch.

I’m being given a tour of the biggest crisp factory in the world, which every day receives 675 tonnes of potatoes that it turns into five million packets of crisps. My guide is John Bows, a physicist at the multinational snack giant PepsiCo, who researches new processes to improve the production of the humble potato chip. Crisps are a multi-billion-dollar industry worldwide, so the firm – which owns brands such as Cheetos, Doritos, Lays and Walkers – invests in physicists to help get that competitive edge. Even an incremental improvement in production pays off in no time.

I don’t have the faintest idea how physics can be used to improve what is basically frying slices of potato in oil. But Bows’ enthusiasm is infectious and so I’m here with an open mind. I’m intrigued to find out what physics is involved as we make our way around the factory, right from the incoming-potatoes stage to the final packaged product.

Peeled and sliced

We begin at “raw materials – receiving”, where a lorry reverses in, tips its container and pours out a torrent of potatoes. A powerful hose helps them roll and float out into an open flume where they drift away, like some kind of potato water park. No-one wants anything other than potatoes accidentally entering the plant so any floating debris is skimmed off by a “de-wooder”, which is followed swiftly by a “de-stoner”. “Anything heavier, or more dense, than a potato will fall more readily and therefore will go through this tube,” says the on-duty technician, who pulls a lever to reveal what gets sieved out. A load of stones clatter down and I’m surprised to see a few small potatoes too. As Bows explains, the few millimetres near the surface of a potato are more dense than the rest, which means that small potatoes are slightly denser overall than large ones so get discarded here.

Crisps in a long metal trough oriented towards the viewer drop out of a drum in the distance, and fall off into a metal chute in the foreground — Tasty trip. (Courtesy: PepsiCo)

Walkers used to get phone calls years ago from customers complaining about solid non-potato chunks in their crisp packets. The problem was that golf balls would occasionally arrive along with the potatoes, and as these little white spheres happen to have the same density as potatoes, they didn’t get removed with the density-based sorting system. I wondered if this lumpy problem was solved by some clever change in the manufacturing process in the factory, but Bows reveals a much simpler solution: Walkers now only sources potatoes from farms that are well away from golf courses.

Walking to our next port of call I am surprised by the amount of liquid on the floor, as well as the odd mound of foam, and little piles of stray potatoes nestled into the corners of steel platforms. Holding onto a rail, I climb up some steel steps to the next processing stage, where the raw potatoes are peeled in giant metal chambers. Although I can’t see in – the chambers are sealed off – they are making a racket. As Bows explains, a batch of potatoes is being dropped in and spun around in an abrasive circumference for two or three minutes, which removes 90–95% of the peel. “How much is actually removed depends on the potato’s shape – if it’s absolutely spherical we get a good peel removal, but there’s always some left on,” he says. That might sound a bit shoddy, but consumers apparently don’t mind a bit of dark peel around the edges of a crisp. If anything, it makes them perceive the product as being more “natural”.

Talking of colour, Bows says probably the worst thing for consumers is to find a green crisp in their packet. To avoid that scenario, PepsiCo has installed a computerized vision-inspection system, which images each and every peeled potato and discards any that aren’t the right colour. “If you reject here – whole potatoes – it’s more efficient than rejecting green crisps later down the line,” Bows explains. Two lines of cameras face the oncoming stream of potatoes and an array of black rectangular “fingers” flicks any rejects down below. The action is so fast that it’s a challenge to see the fingers move, but we do hear the occasional “dunk!”, followed by the sound of a potato rolling away and falling into a vat below.

A large open-ended metal drum with vertical ridges on the inside is fed crisps from one end and deposits them from another. The crisps are about halfway up one side of the drum, indicating that it is turning — Crisps are seasoned in rotating drums. (Courtesy: PepsiCo)

Our next stop is the slicing station, where a technician bends over a squat metal cylinder (without its ends on), roughly 40 cm wide, with the curved edge vertical. Every 90 minutes or less a technician checks – and if necessary replaces – each of eight blades spaced around each cylinder. Blunt blades are a no-no as they cause excess ripping of cells at the potato surface. Beside us, several other slicers are in use. Potatoes drop down into the cylinder, which has a platform at the base spinning at about 200 rpm; this throws the potatoes sideways into the blades, where they get sliced in one go. “There’s actually a tremendous amount of physics going on in here,” says Bows. Since the potato gets cut in a curved profile, the inner and outer surfaces of the slice experience different shear forces. This causes the outer surface to be more fractured and to release more starch.

I can certainly see that going on – the slices are carried away in a flume, this time in an opaque milky-coloured liquid. Bows assures me that all the starch gets recovered at the on-site starch-recovery plant – waste of any resource, including energy and water too, is kept to a minimum. Bows and his team are currently working on how to redesign the slicers to reduce starch loss in the first place. “If you can go from 2% starch loss to 1% starch loss, that’s a big saving in the solids, which translates to heavier chips,” he explains.

Hotting up

Next we go to the warmest part of the production line: the fryer, where the cut potatoes are cooked in hot oil. The frying is hidden from view, taking place in a covered metal chamber several metres long. But Bows first draws my attention to the step just before, where the flume carrying the sliced potatoes turns a corner and fans out into a fishtail shape, depositing the slices onto a conveyor belt about two metres wide. His group is researching how to ensure that the transfer of slices from the flume to the conveyor belt results in the overlapping slices having a uniform mass distribution.

“If you were to watch for a long time, you would see that this isn’t really uniform ‘left-centre-right’ mass distribution,” Bows explains. “And if it’s not uniform here, it’s not uniform going into the fryer.” Higher-density areas of slices within the flyer can lead to soft crisps, where not enough water can escape and the slices don’t reach the optimum temperature. The researchers working on this project have to understand the physics of slices in turbulent liquid so that their models are accurate and can be used to improve the flume and fishtail designs.

After the fishtail, the slices might have 10–15% surface water, the precise amount of which is one of the biggest throughput limitations of the fryer. Another research project at PepsiCo is to understand the surface physics of removing water from a potato slice, and how best to extract it. One method is to fire jets of air at the slices, and vacuum the resulting water droplets away before they can resettle. The team is aiming for a reduction to 8% surface water, pre-fryer.

“There’s a delta-T across the fryer,” Bows says, slipping into language that marks him out as a physicist. “This is one of the control parameters of the texture.” For an ordinary crisp, which is about 10 potato cells wide, temperature is set to about 180 °C at the entrance to the fryer and 160 °C at the exit. The slices are moved through various frying zones using rotating paddles, which serve both to break up clumps, and to submerge buoyant slices that have floated to the top of the oil. As the slices move through the fryer, the starch within them undergoes several phase transitions. The starch passes from its native crystalline state through to a rubbery “melt phase”, and then through glass transition and back to a glassy phase. “If you get the timing of that process right, and if you get the right temperature curve, you get the right texture,” says Bows. “The starch conversion is critical.”

Crisps fall from above, through a large-ended metal funnel, and pile onto a large circular metal disc that has an undulating shape. Fourteen U-shaped channels feed crisps from where they land in the centre of the disc to its outer edge, from which they fall into 14 rectangular metal buckets below — Portion control. (Courtesy: PepsiCo)

To use a continuous fryer – like the ones at Walkers – to make thick, brown and crunchy gourmet-style “kettle chips”, you can simulate the original batch method of kettle-chip manufacture, in which a whole load of potato slices are dunked into a vat or “kettle” of oil, causing the temperature to lower before rising back up again. By changing the time–temperature profile along a continuous fryer so that it starts off hot, cools a little and then gets hot again, the starch is converted at a different rate to give a harder, crunchier bite, matching crisps made using a batch fryer.

As the slices exit the fryer, they still have a high internal vapour pressure, which continues to drive off water vapour. It is only once the internal vapour pressure collapses that oil is sucked into the slices. After this stage, crisps are typically about 30% oil. Walkers crisps use high oleic sunflower oil, which is low in saturated fat.

We now move on to Bows’ favourite device in the Leicester plant: the “Optisort”. This machine is similar to the vision-inspection system that discards whole green spuds, but whereas that machine imaged every individual potato, the Optisort images every single crisp. Considering that there are typically 20–30 slices per potato, the Optisort has to work a lot faster. So before they pass through the device, the crisps are arranged on the conveyor belt in a layer one crisp thick so that every crisp is visible, then sped up by making them fall down a parabolic curve. “The curve’s been designed to accelerate the potato slices, so they fall through at incredible speed,” says Bows. Moving at 3 m/s – about a third of the speed of an Olympic sprinter – I’m mesmerized by the sight of the “singulated” crisps whizzing past. Arrays of cameras photograph every single slice to check they are a nice golden-yellow colour, with green or black slices rejected. The movement is so fast that at first I can’t figure out how the rejects are removed from the stream. Bows then points out a gap about 10 cm wide which the crisps fly over horizontally. Any duds are dispatched downwards with a swift puff of air.

Flavour of the month

So now we have it: the unseasoned PepsiCo crisp. Up until this point I have seen what is a standard process in PepsiCo crisp factories throughout the world, where all that differs is the potato variety. What comes next is for these Walkers crisps to be tailored to the British market. Through a set of double doors, we leave the realm of noisy automated machinery, where we met just two people, and enter a quieter space filled with staff. Noisy vibrations are replaced with the light chatter of factory workers as a power ballad – “Unchained melody” – plays on the radio in the background.

The crisps’ journey continues as they are sent tumbling through giant rotating drums, where a curtain of seasoning powder drops onto them. Bows says that his team looks at physical parameters such as the Froude number – the ratio of inertial to gravitational forces – to optimize the drums so that breakage is minimized but coverage time is maximized. I can smell the seasoning but neither Bows nor I can quite identify it. Wandering around the drum we find several labelled cardboard boxes that solve the mystery: it’s good old cheese and onion, which Bows and I agree is our favourite flavour. We also concur on the worst: prawn cocktail.

My tour’s still not quite over. I walk up some steps to an elevated platform and am faced with a myriad of circular devices all making a rhythmic clunking noise. It is here that the crisps are weighed and bagged, using a method like nothing I had imagined. At each station, crisps fall from above, onto a metal plate shaped like a 14-petalled flower. Each “petal” funnels crisps into one of 14 hinged buckets or “heads” that are arranged in a circle below. Once every second or more, a couple of these heads – different ones each time – tip their contents into a central vertical shoot that disappears to a lower level. “These heads will take in about four or five crisps at a time, and some combination will be very close to the target weight and dispensed to the bag-maker underneath,” explains Bows. “It’s a purely statistical process.”

Below deck I see indeed that each dumped load corresponds to a bag being filled and sealed. The crisps drop into a tube of packaging material inflated by nitrogen, which feeds downwards at a rate of about a metre every 4 seconds. A heat-sealer and slicer lops off each filled packet at a rate of about 1.6 per second. The packets then fall onto a conveyor belt and go onwards and upwards to be further packaged by robots into multipacks or boxes, and on to the consumer. Another current research project, Bows tells me, is to look into how to reduce the seal from its current width of about 3 mm to 1 mm. “This would save a fortune in packaging material,” he says. Considering the volumes involved at the Leicester factory, those little enhancements would soon add up.

After leaving the factory, as we’re peeling off our hair nets, I have one last question. I’d already committed a faux pas by claiming that my favourite crisp is a cheesy puff – an affronted Bows had reminded me that cheesy puffs aren’t crisps, which by definition are slices of potato fried in oil. But I’m curious to know what happens to that starchy liquid we saw sloshing around at the slicing station. “That starch you recover on site – where does it go?” I ask. “Oh, the starch?” replies Bows. “We make Quavers out of it.” No-one can accuse PepsiCo of letting anything go to waste.

See below for a video interview with John Bows about his career

Bacteria nanospheres may help camouflage tiny crustaceans

A group of small crustaceans that live in the twilight zone of the open ocean are coated in nanospheres that reduce the amount of light the crustaceans reflect, biologists in the US have discovered. The spheres, which are smaller than the wavelength of visible light, appear to be bacteria. The researchers believe the coating may help hide the transparent creatures from predators with bioluminescent searchlights.

Hyperiids are a suborder of more than 200 species of marine crustaceans. They inhabit the pelagic zone of the open ocean, from the surface to depths of around 4000 m. The featureless habitat of this twilight zone offers few places to hide from predators, so many of its denizens – including hyperliids – have transparent bodies as a form of camouflage. In the ambient ocean light they are almost invisible.

However, many of the local predator species, such as dragonfish, squid and lanternfish, have bioluminescent searchlights to help them find prey. As these creatures scan their light through the water they can locate these “invisible” animals by the light that reflects off their body surfaces. Such reflections are a particular issue for hard-shelled crustaceans such as hyperiids, because there is a large gradient between the refractive index of their chitin shells and that of seawater – something that results in relatively large amounts of reflection.

Altered refractive index

Researchers at Sönke Johnsen’s sensory biology lab at Duke University, in North Carolina, hypothesized that such hard-shelled creatures would develop surface structures that minimize these reflections by altering the refractive index gradient. Using scanning electron microscopy, they examined seven hyperiid species, representing six families, that ranged in body length from 10 to 100 mm. “They were as distantly related as possible while still being called a hyperiid,” explains Laura Bagge, who led the research.

This is the same idea as the technology used in anti-reflective coatings in eyeglasses
Laura Bagge, Duke University

The team discovered a dense single layer of spheres on the surface of all seven species. These appear to be bacteria and are not identical across the species, ranging in diameter from 52 to 320 nm. To determine whether these spheres affect reflectance, they created optical models of a clean, flat chitinous surface and the same surface covered with monolayers of 52 nm, 110 nm and 320 nm spheres.

At angles of incidence of less than 55° a clean chitinous surface in seawater will reflect between 0.6% and 1% of incident visible light, according to the model. A monolayer of 110 nm spheres had the greatest impact on reflectance, reducing it to less than 0.1% over a broad range of wavelengths and angles of incidence.

Blue-green light

According to the researchers, the nanospheres can reduce the surface’s reflectance of blue-green light (approximately 480 nm wavelength – the most common type of bioluminescence in the ocean) to as little as one hundredth of the uncoated value. In most cases, reflectance was reduced from approximately 1% to less than 0.1% for blue-green light at angles of incidence of less than 45°. The 52 nm spheres were the least effective, reducing the reflectance of wavelengths between 400 and 500 nm to less than 0.5% at angles of incidence of less than 45°.

Bagge explains that the nanospheres reduce reflectance by acting as an intermediate refractive index layer, smoothing out the gradient between the seawater and the animal’s surface. “This is the same idea as the technology used in anti-reflective coatings in eyeglasses, but this type of thin film has never been seen on an animal before,” she says.

Bagge told Physics World that the team is “fairly confident that the spheres are bacteria” and is currently carrying out DNA analysis to confirm this. If they are bacteria, it is not difficult to imagine how this symbiotic relationship evolved. “Hyperiids that were less reflective and less visible due to having just the right size of thin bacterial layer on their surface would have done better at avoiding predators” and the bacteria would have benefited from “colonising a hard surface rather than remaining free-floating.”

The study is described in Current Biology.

Flash Physics: A superconducting non-superconductor, best of Russian physics, NPL backs instrumentation

How to make a superconductor from a non-superconductor

Calcium-iron arsenide, which is usually not a superconductor, has been made to superconduct by Paul Chu and colleagues at the University of Houston in the US. This was done using an idea first proposed in the 1970s – that superconductivity can be enhanced or even created at the interface between two materials. Chu and colleagues heated calcium-iron arsenide so that it coexists in two different structural phases, neither of which is superconducting. Then the sample is cooled carefully to preserve the two phases. When cooled to below 25 K, the material is a superconductor at the interface between the phases. While this superconducting temperature is too low to be of practical use, Chu believes that the work offers a new direction in the search for more efficient, less expensive superconducting materials. The research is described in Proceedings of the National Academy of Sciences.

Study reveals top Russian physics institutions

A bibliometric study by researchers at the National Research University Higher School of Economics (HSE) in Russia has measured the scientific impact of 39 physics institutions belonging to the Russian Academy of Sciences (RAS). Carried out by HSE sociologists Yuriy Kachanov and Natalia Shmatko, together with Yulia Markova from the American Association for the Advancement of Science, they found that the Joint Institute for Nuclear Research, the Alikhanov Institute for Theoretical and Experimental Physics, the Lebedev Physical Institute – all based in Moscow – and the Ioffe Institute in St Petersburg are the top physics research institutions in the country. The study looked at the number of researchers based at each institution, together with publication statistics. “We were able to prove that big institutions held authority on the global science scene and produced more scientific data, which was highly received by the physics community,” says Shmatko.

Advancing instrumentation at the National Physical Laboratory

Photograph of the National Physical Laboratory campus — Instrumental: the National Physical Laboratory campus in Teddington. (Courtesy: NPL)

A new initiative aimed at strengthening ties between tech firms and the UK’s National Physical Laboratory (NPL) was officially launched last night at the Institute of Engineering and Technology in London. The project, known as NPL Instruments, will see experts at the Teddington-based national measurement institute work closely with companies to develop bespoke instruments, products and related services. At the event, NPL chief-executive Peter Thompson told Physics World that the new business unit would focus on products at a moderate stage of development (equivalent to Technology Readiness Levels 4 and 5) in the areas of advanced manufacturing, environment, health and life sciences, and the digital sector. NPL’s work on instruments tends to be “hidden in plain view”, Thompson told an audience of around 100 lab personnel, industry scientists, engineers and academics at the event, adding that the new business unit is intended to help publicize and expand the lab’s role as an “instrument development partner”. Paul Shore, who leads both the new unit and NPL’s engineering measurement division, gave indoor GPS technologies and “smaller, faster, cheaper” atomic clocks as examples of products where the lab’s existing strengths in measurement and sensing could help to catalyse technical advances. The initiative comes on the heels of a transition period for NPL, which announced in August that it would make up to 50 staff members redundant as part of what Thompson called a “rebalancing” of the 116 year-old lab.

You can find all our daily Flash Physics posts in the website’s news section, as well as on Twitter and Facebook using #FlashPhysics. Tune in to physicsworld.com later today to read today’s extensive news story on bacterial nanospheres.

Neutron holograms image the interiors of objects

The first holograms of large objects made using neutrons have been unveiled by physicists in the US and Canada. Unlike conventional holograms produced with laser light, neutron-generated holograms can image the interiors of objects. The researchers say these 3D images could be useful in materials science and could even be used in quantum computers of the future.

Beams of neutrons are a good probe of the internal structure of solid objects because they are able to pass through most materials without being completely absorbed. As a result, neutrons can be used to image the interiors of objects that are too large or to dense to be studied with X-rays – which are much more prone to be absorbed or scattered by matter. Dmitry Pushin, a physicist at the University of Waterloo who led the hologram research, explains: “A heavy metal such as lead is very transparent to neutrons.” Neutrons have also been used to image materials such as hydrogen in fuel cells, which are essentially invisible to X-rays. Because neutrons interact with matter differently than X-rays or other electromagnetic radiation, images constructed with neutrons deliver a different perspective on an object than images constructed with photons.

While scientists have used neutrons to create images for several decades – including tiny holograms on the atomic scale – Pushin’s group is the first to create neutron-generated holograms of macroscopic objects. To make their holograms, the researchers used a beam of neutrons generated by a nuclear reactor at the National Institute of Standards and Technology located in Maryland in the US.

Spiral plate

These neutrons are first split into two beams called the object beam and the reference beam, in analogy to the two laser beams used in conventional holography. The object beam then passes through a 1 cm-thick piece of aluminium known as a spiral phase plate. This increases the orbital angular momentum (OAM) of the beam by a specific amount. The object beam is then recombined with the reference beam to form a 3D interference pattern on a detector. This interference pattern is the hologram image of the spiral phase plate and has a spatial resolution on the micron scale.

Pushin says that using neutron holograms to image the spiral phase plate also provides a new way of visualizing the quantum nature of OAM. The team created holograms of several different spiral phase plates that each increased the neutron beam’s OAM by a different quantity. The holograms’ interference patterns resemble zebra stripes and matched computer simulations. These simulations predicted that the hologram’s interference patterns would branch and fork uniquely, depending on the OAM of the object beam (see figure). Because OAM can be used as a way of storing and transmitting quantum information, the holograms could potentially be useful in quantum-information research such as the development of quantum computers.

Topological materials

Pushin anticipates other applications for neutron holography, including studying the internal structure of new materials. In particular, he says the technique could aid in the discovery of topological conductors and insulators.

The next step, Pushin says, is to create more tools such as diffraction gratings, which can manipulate the shape of the neutron beam in more precise ways. The team also plans to start using neutron holography to characterize new materials.

The research is described in Optics Express.

Physics of food – the November 2016 issue of Physics World is now out

By Matin Durrani

If you love crisps – and frankly who doesn’t? – you’ll relish the cover feature of the latest issue of Physics World, in which features editor Louise Mayor tours the world’s biggest crisp factory at Leicester in the UK to see how physics is improving production of this yummy salty snack. The issue is now live in the Physics World app for mobile and desktop and will also be made available on physicsworld.com later this month.

Elsewhere in this special issue on physics and food, you can find out how electric fields could help to cut the fact from chocolate and discover why sound holds the key to our appreciation of what we eat.

You can also see how physicists – being masters of data-gathering, modelling and simulation – are ideally placed to develop products that are healthier, more nutritious and make more of our resources. Find out too how soft-matter physicists are crafting “functional” foods that promote feelings of fullness and satisfaction.

(more…)

Flash Physics: Thirty Meter Telescope could go to Spain, artificial muscles flex, light switches light

Canary Islands chosen as alternative site for Thirty Meter Telescope

Originally planned for Mauna Kea mountain in Hawaii, the Thirty Meter Telescope (TMT) could be built at the Observatorio del Roque de los Muchachos in the Canary Islands. The TMT International Observatory (TIO) Board of Governors has chosen the observatory in Spain as the “primary alternative” to the controversial Hawaiian site, which is being opposed by native Hawaiians who see building the TMT on Mauna Kea as a desecration of their spiritual and cultural pinnacle. Chair of the board Henry Yang says: “Mauna Kea continues to be the preferred choice for the location of the Thirty Meter Telescope, and the TIO Board will continue intensive efforts to gain approval for TMT in Hawaii.” In July 2016, TMT deputy project-manager Fengchuan Liu said that the decision to build the TMT on an alternative site will be taken “by early 2017”. Construction of the TMT is planned to begin in April 2018, with completion in 2022.

Improving the flex of artificial muscles

A new type of artificial-muscle fibre that has high tensile strength and actuates – expands or contracts in response to a stimulus – at much cooler temperatures than previous fibres, has been developed by researchers at the Louisiana State University in the US. Guoqiang Li says the team was able to exceed the performance of other artificial muscles by focusing on the thermal properties as well as the molecular structure of their polymer fibre. According to Li, the team found that two factors are crucial for high performance – the untwisting nature of the fibre during actuation and its negative coefficient of thermal expansion. “The actuation temperature is very high in the polymer fibres used previously, for example they can go to 160 °C,” says Li. “For some applications, like medical devices, [the] actuation temperature is too high. So you need to find a way to lower it.” Li’s group managed to bring the maximum actuation temperatures down to 67 °C. This lower temperature is particularly significant when considering applications related to human body temperature. In addition to medical devices, such applications include breathable textiles and self-healing materials with structures that adapt to environmental changes. The research is described in Applied Physics Letters.

Dim light controls intense laser field

Image showing the design of the non-Hermitian photonic metamaterial — Light work: design of the non-Hermitian photonic metamaterial, which was created within a silicon waveguide. (Courtesy: H Zhao *et al./Phys. Rev. Lett.*)

A new way of switching a beam of light on and off using another light beam has been unveiled by physicists in the US. Unlike other “light–light” switching schemes, which employ intense light beams to control relatively dim beams, this latest technique uses a weak beam to control a much brighter beam. The switch makes use of a new type of optical material called a non-Hermitian photonic metamaterial, which is created within a silicon optical fibre. Tiny features in the silicon create a standing wave when infrared signal light is shone into one end of the fibre. When control light is shone into the opposite end of the fibre, destructive interference in the fibre prevents the signal light from being transmitted. Created by Liang Feng at the State University of New York at Buffalo and colleagues and described in Physical Review Letters, the new switch could – with further improvements – find use in high-speed, all-optical telecoms networks of the future.

You can find all our daily Flash Physics posts in the website’s news section, as well as on Twitter and Facebook using #FlashPhysics. Tune in to physicsworld.com later today to read today’s extensive news story on neutron holograms.

Why aye, astronomer lad

There are a lot of books out there about backyard astronomy. An Astronomer’s Tale is almost certainly the only one that will, in addition to introducing the constellations and offering tips about deep-sky observing, also teach you how to split a brick in half and describe what it feels like to fight in a Sunderland–Chelsea football gang war. The author, Gary Fildes, grew up in the north-east of England in the 1970s, when most adult men in the area worked down the mines, in the Sunderland shipyards or on a building site. After a chequered career at school, young Gary chose the third option, and trained as a bricklayer. “My future was Sunderland and I would do Sunderland things whether I liked them or not,” he writes. “More times than not, I liked them back then.”

He did have an interest in astronomy, sparked by a childhood Christmas when one of his brothers received a telescope, but after being beaten up as a teenager for talking about the Moon, it was a passion he kept to himself. Then, in his mid-30s, something changed; as Fildes puts it, “I came out. As an astronomer.” He began reading science books in the evening, bought his own telescope, and started going along to meetings of the Sunderland Astronomical Society, or SAS (motto: “Who Stares Wins”). Eventually, Fildes’ passion for astronomy completely transformed his life. Today, he is lead astronomer at Kielder Observatory, home to some of the darkest skies in the whole of the UK and a major attraction for people eager to reconnect with the night sky.

In An Astronomer’s Tale, Fildes intersperses stories from his upbringing with a series of monthly guides to the night sky. The observing tips are honed by years of experience, and Fildes excels at capturing the atmosphere at public observatories and star parties, where red light bulbs “create an atmosphere of secrecy, as if we are all children hiding beneath a duvet cover”. His book also offers a gentle reminder to professional astronomers – who spend most of their time glued to computer screens rather than telescope eyepieces – not to forget the simple pleasures of observing. “Is their inspiration, their ability to dream about our universe, getting lost in the technical jungle of scientific progression?” he wonders. If it is, places like Kielder – and books like this one – will surely help them find it again.

2016 Century £16.99hb 320pp

Science under duress

Black-and-white photo of Josef Stalin in military uniform — Triumph and tragedy: A new book looks at the science and the scientists in Russia and the Soviet Union, particularly the contrasting fates of biology and physics under Stalin.

The idea that all branches of scientific knowledge will someday unite to create a single, logical explanation of the universe has been captivating scientists and philosophers for generations. In the mid-19th century, Karl Marx made this “scientism” an important thread of his materialist philosophy, and the impulse behind it is still apparent today in efforts to develop a “theory of everything” in physics. In the first half of the 20th century, however, this dream bore bitter fruit thanks to Vladimir Lenin and Josef Stalin, whose devotion to Marx’s “one science” contributed to the deaths of millions of ordinary Russians.

How this happened is the subject of Stalin and the Scientists: a History of Triumph and Tragedy. In it, author Simon Ings, a science writer and arts editor at New Scientist magazine, sets out to cover “more or less the whole of scientific life” in Russia and the Soviet Union between 1905 and 1953. For much of the book, though, the tragic story of Soviet genetics takes centre stage. As Ings explains, the theories of Charles Darwin and Gregor Mendel dictate that change takes place over many generations and heritable characteristics are passed down more or less unaltered. However, these ideas were anathema to the Bolsheviks: the revolutionaries who overthrew the Russian tsar in 1917 wanted to change society overnight, while talk of inherited qualities smacked of support for the aristocracy. Naturally, they preferred an alternative theory in which organisms change in response to their environment and pass down these changes to the next generation.

In recent years, this “Lamarckian” theory (named after the early 19th-century French scientist Jean-Baptiste Lamarck) has been partially rehabilitated by the emerging science of epigenetics. However, the version promulgated by Stalin’s favourite geneticist, the charlatan Trofim Lysenko, had nothing at all to recommend it, and its widespread application worsened the famines that struck the Soviet Union during the 1930s and 1940s. Despite this, geneticists who persisted in Mendelian views were liable to be demoted, sent to the gulag or (at the height of Stalin’s purges in the mid-1930s) shot.

In comparison, Ings notes, Soviet physicists got off lightly. Although many of them also spent time in the gulag, there was never a physics equivalent of Lysenko, and the contrasting fates of Soviet biology and Soviet physics make interesting (and sometimes troubling) reading. Covering the entirety of Soviet science is an ambitious task, but Ings tears into it with gusto, ably recounting the careers of dozens of biologists, agronomists, physicists and Communist Party officials during one of the most tumultuous periods in world history.

2016 Faber and Faber £20.00hb 528pp

New Ising-machine computers are taken for a spin

Two independent teams of physicists in the US and Japan have each built versions of a new kind of computer called an “Ising machine”. The devices use physical systems to imitate a network of interacting magnetic spins with the aim of solving very complex optimization problems. Although the full potential of these Ising machines remains to be explored, the researchers say that their initial results show that their optical-fibre-based devices work just as expected. Furthermore, the research suggests that future Ising machines could outperform conventional, digital computers when it comes to discovering new drugs or optimizing the efficiency of factory production lines.

Ising machines are designed to find the best solution to problems that involve large numbers of competing alternatives. This involves performing “combinatorial optimizations” that are “NP-hard”, which means that the number of possible solutions increases exponentially with the number of components in a system. Examples of such problems include the “travelling-salesman problem” – which involves planning the shortest route linking a number of different cities – as well as the process of finding combinations of atoms and molecules that are good candidates for potential new drugs. Even the most powerful conventional computers are simply unable to provide practical solutions to these problems.

Ising machines are named after German physicist Ernst Ising, who studied the problem of how a set of spatially distributed magnetic moments arrange themselves in their lowest-energy state, given that each moment can assume one of two values: either spin-up or spin-down. For spins strung out along a line that only interact by pointing in the opposite direction as their two nearest neighbours, the answer is trivial: it is up, down, up, down, etc. But the problem becomes extremely difficult to solve when considering more general configurations in which each spin interacts arbitrarily with every other spin. The problem then is NP-hard.

Physical systems

The idea behind an Ising machine is to “map” a difficult optimization problem on to a specific Ising problem containing spins with certain couplings, and to find a physical system that will be able to solve a wide range of such problems. As Peter McMahon of Stanford University in the US points out, an Ising machine could be made simply by arranging bar magnets at certain nodes in a 2D grid and then looking to see how the magnets end up aligning with one another. One problem, he says, is that such a device would only be able to solve one specific Ising problem, dictated by the spacing – and hence coupling – between magnets.

Scientists have already worked on a number of more sophisticated alternatives. In the 1980s, John Hopfield and David Tank carried out work on neural networks in which each artificial neuron represented a spin. The firing or not-firing states of a neuron correspond to spin-up or spin-down and the weights of neural connections represent the coupling strengths. More recently, researchers at Canadian company D-Wave and elsewhere have looked to devices known as adiabatic quantum computers. These exploit the phenomenon of quantum tunnelling to place a set of quantum bits into their lowest energy state – in analogy with the Ising model.

When we constructed our machine there was no guarantee that we would get anything useful out of it so when we turned it on we got a pleasant surprise
Peter McMahon, Stanford University

According to McMahon, both of these options suffer from the fact that each spin usually only couples to nearby neighbours. That means that although such devices can still solve arbitrary problems, they must be made much bigger to do so. Typically, he says, to solve an n-spin Ising problem they need about n² physical spins.

Light pulses

The idea for the new Ising machines came from Stanford’s Yoshihisa Yamamoto as part of the Japan Science and Technology Agency’s ImPACT programme. Yamamoto and colleagues showed in 2014 that they could build a small Ising machine by feeding a sequence of light pulses into an optical cavity. Spins were represented by pulses’ phase and were made to interact by diverting a small part of each pulse along an optical component known as a delay line, such that the pulse fragment re-entered the cavity as the next pulse passed by. But the need for many delay lines makes the approach hard to scale up and indeed the researchers only managed to process four pulses.

Now, two separate efforts, spawned from that original research, have created very similar variations on the initial device by marrying optics and electronics. Rather than sending tapped pulses along delay lines, the researchers instead measure the pulses’ phases and send that information to an electronic circuit, which adjusts the strength of a laser beam shone into the cavity in such a way as to mimic the effect of the delay lines.

Yamamoto, McMahon and colleagues at Stanford have built an Ising machine with 100 spins, each of which couples to every other spin, and have used the device to solve or find good approximate solutions to some 4000 Ising problems. Meanwhile, a group led by Hiroki Takesue of NTT Corporation near Tokyo has managed to build a machine with 2000 spins, again with complete spin–spin coupling, but in this case testing the device against just three problems.

Wireless networks

Takesue says that he and his colleagues are now testing their machine against other Ising problems and are also investigating several candidate applications, including drug discovery and the optimization of frequency channels in wireless networks. He adds that they also plan to increase the number of spins to more than 20,000 over the next three years.

McMahon is confident, on the basis of theoretical extrapolations, that the NTT machine can, like his group’s device, solve a wide range of Ising problems. But he says it is unclear whether this machine, or an even larger version of it, will outperform classical computers. “When we constructed our machine there was no guarantee that we would get anything useful out of it so when we turned it on we got a pleasant surprise,” he says. “But it remains to be seen whether this particular approach can beat state-of-the-art classical machines in real-world applications, or whether we will need a new computing architecture.”

The Ising machines are described in separate papers in Science.

Flash Physics: Detector nears the quantum limit, why Saturn has rings, monolayer emits photon pairs

New microwave detector approaches the quantum limit

A new and extremely precise way of amplifying and measuring tiny microwave signals has been unveiled by physicists at Aalto University and the University of Jyväskylä in Finland. Mika Sillanpää, Tero Heikkilä and colleagues created their detector by combining a micron-sized mechanical resonator resembling a drum with two superconducting microwave cavities. The device is able to amplify a very weak microwave signal with a gain of 41 dB – a factor of about 12,500 – while only adding about four quanta of noise to the signal. This is close to the minimum amount of noise possible (the standard quantum limit), which is half a quanta of noise. As well as being able to amplify very weak signals so that they can be measured, the technique could be used in quantum-information systems in which quantum bits of information (qubits) are encoded into microwave signals. Another important feature of the new technology is that it can convert signals from one microwave frequency to another. Writing in Physical Review X, the team suggest that this could be useful for developing quantum-information systems that are based on several different qubit technologies.

Computer simulations shed light on planetary rings

A series of computer simulations done by scientists in Japan and France provide important insights into how the rings around Saturn and other planets formed – and why the composition of Saturn’s rings is different to that of the rings of Neptune and Uranus. Ryuki Hyodo and colleagues at Kobe University, the University of Paris Diderot and the Tokyo Institute of Technology focussed on the “late heavy bombardment” era of the solar system. This happened about four-billion years ago and is thought to have involved the inward migration of thousands of Pluto-sized objects from the outer solar system. The team first calculated the probability that some of these objects would pass close enough to Jupiter, Saturn, Uranus and Neptune such that they would be broken up by tidal forces. The researchers found that enough fragments would be created and then captured by the giant planets to account for the current rings of Saturn and Uranus. Simulations also revealed that these fragments – some of which would be several kilometres in size – would break up as they orbit the planets to become the circular rings of much smaller objects seen today. The simulations offer a suggestion as to why Saturn’s rings are made mostly of ice, whereas the rings of Uranus and Neptune contain much more rock. This, they write in Icarus, is because Saturn is less dense than Uranus and Neptune and therefore the tidal forces it exerted on the Pluto-like objects is weaker. As a result, Saturn’s gravity was only able to chip away at the ice on the surface of the passing objects whilst Uranus and Neptune were able to break up the underlying rock.

Transition-metal monolayer emits photon pairs

Artist's impression of the two-photon source — Red light, green light: the monolayer of tungsten diselenide (below in this artist’s impression) emits exactly two photons of different frequencies under suitable conditions. They are depicted in red and green. (Courtesy: Karol Winkler)

A 2D monolayer of transition metal dichalcogenides (TMDC) can be used to generate pairs of photons, say researchers at the Julius-Maximilians-Universität Würzburg in Germany. TMDCs behave like semiconductors and are often used to make ultra-small and energy-efficient chips. Christian Schneider, Sven Höfling and colleagues produced monolayers of tungsten diselenide by using a piece of tape to peel off thin layers from a multi-layer film of the TMDC. This involved repeatedly peeling the film so that thinner and thinner layers are made until the material on the tape is only one-atomic-layer thick. This layer is then cooled down to a temperature just above absolute zero and it is then excited with a laser, causing it to emit single protons under specific conditions. “We were now able to show that a specific type of excitement produces not one but exactly two photons,” says Schneider. “The light particles are generated in pairs so to speak.” Two-photon sources are of interest to those carrying out quantum cryptography and other such protocols that involve entanglement. The research is described in Nature Communications.

You can find all our daily Flash Physics posts in the website’s news section, as well as on Twitter and Facebook using #FlashPhysics. Tune in to physicsworld.com later today to read today’s extensive news story on Ising machines.

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