Last week Physics World’s Michael Banks was at the APS March Meeting in San Antonio, and at the top of his to-do list was to belt out a few tunes at the event’s regular physics singalong. You can hear him in harmony with a roomful of physicists in a rendition of “(You Got Me) Lasing” in the video above. It is sung by Walter Smith of Haverford College to the tune of Britney Spears’ “(You Drive Me) Crazy” and his performance drives the dance floor into a frenzy of moshing physicists.
My photo opportunity: this could be the last we will see of the CMS for three years.
By Tushna Commissariat at CERN
Regular readers of Physics World will know that I am currently visiting the CERN particle physics lab in Geneva, ahead of the restart of the Large Hadron Collider (LHC) in the coming weeks. My first stop yesterday afternoon was a press conference in which CERN’s director-general Rolf Heuer and other leading physicists briefed us about “Run 2” and what researchers are hoping to discover. You can read about what they had to say here: “Large Hadron Collider fires up in a bid to overturn the Standard Model“.
I managed to squeeze in a quick last-minute visit to the Compact Muon Solenoid (CMS) detector before it is sealed up tight for the next three years. My host was CMS communications officer Achintya Rao, who took me and a few others deep underground into the bowels of the CMS – and what a sight it was!
After a two-year hiatus, CERN is set to restart the Large Hadron Collider (LHC) and its main experiments ALICE, ATLAS, CMS and LHCb over the next few weeks. After discovering the Higgs boson in 2012, the LHC was shut down in February 2013 for a major upgrade of the accelerator and its experiments. If all goes well, the LHC and its experiments will be fully operational and collecting data in late May or early June 2015.
Upgrade work – including a complete overhaul of the superconducting connections between magnets – was completed last June, and much of the LHC has now been cooled to its operating temperature of 1.9 K. On 7 March, the first proton beams were transported through some sectors of the 27 km-long collider. By May of this year, the revamped LHC is expected to be colliding protons at a collision energy of 13 TeV, heralding the beginning of what CERN has dubbed “Run 2”. While this energy is nearly double that of the previous 8 TeV run, it is below the LHC’s design energy of 14 TeV. The decision to run at 13 TeV was made because of the extra time that is required to “train” the LHC’s superconducting magnets for 14 TeV collisions.
The higher energy should allow CERN physicists to improve their understanding of the newly found Higgs boson, because the number of particles produced in collisions is expected to increase by a factor of 10. Overall, physicists will have to sift through nearly five times more data in Run 2 than were produced in the LHC’s first run. CMS spokesperson Tiziano Camporesi, says that they will not be using “brute force methods” to cope with this deluge of data, but have instead developed more efficient ways of processing it.
Another important goal of Run 2 is the search for evidence of physics beyond that described by the Standard Model of particle physics. In particular, physicists will be looking for evidence of supersymmetry (SUSY) – a theory that predicts that every fundamental particle has a so-far-undiscovered “superpartner” particle whose properties are imperceptibly different. Other signs of new physics that could be detected include evidence for extra dimensions, exotic particles and dark matter.
No theoretical beacon
Looking for these phenomena will be very different from the search for the Standard Model Higgs, says Camporesi. During the hunt for the Higgs, physicists had a “theoretical beacon guiding us” towards the particle, he says. Beyond the Standard Model, he points out that “theory is at a loss as there are too many competing hypotheses, and so in a way the experimentalists are taking over”.
I want to see the first light in the dark universe. If that happens, then nature is kind to me
Rolf Heuer, director-general of CERN
Ironically, the LHC researchers will begin this process by testing extremely precise predictions made by the Standard Model. “We know what should happen at 13 TeV, so we will look everywhere for any deviations from what is predicted,” explains Camporesi. While certain theories such as SUSY are appealing because they do make precise predictions, it is not clear if and when the LHC will be able to confirm or rule out these theories. “The parameter space of SUSY is such that we may find it in the first week [of Run 2] or in 2035,” he explains. Indeed, Camporesi points out that even if finding SUSY lies beyond the abilities of the LHC, the collider may be well equipped to make precise predictions in much the same way as its predecessor at CERN, the Large Electron–Positron Collider, helped physicists to make an accurate prediction of the mass of the top quark.
Both the CMS and ATLAS experiments will be revisiting some of the fluctuations or oddities in the data that both detectors saw in Run 1. While most of these are expected to be statistical fluctuations from the Standard Model, if they grow in statistical significance throughout Run 2 it could indicate the emergence of new physics. Indeed, ATLAS spokesperson David Charlton says that previously dismissed candidates such as B quarks may be the very particles that “break the Standard Model”.
Missing energy
The situation is different for physicists looking for hints of dark matter in Run 2, because if the elusive particles are produced at the LHC they are not expected to be detected directly. Instead, physicists will be looking for missing energy in particle collisions, energy that has been taken away undetected by dark matter. Indeed, the LHC is back as a leading contender in the race between a number of diverse experiments to see further evidence of dark matter.
Maria Chamizo-Llatas, who was the CMS run co-ordinator on Run 1, says that there is plenty of engagement and discussion between CERN physicists and astrophysicists trying to detect dark matter in astronomical sources. She says that this co-operation is fruitful and necessary, and must increase so that the two communities can guide each other in their respective searches. However, she does confess that she would prefer it if CERN were to “see it first”. Indeed, Rolf Heuer, director-general of CERN, said at the CERN press conference held yesterday for the LHC restart, “I want to see the first light in the dark universe. If that happens, then nature is kind to me.”
Droplets can be made to chase each other around a track and even self-assemble into devices, simply by mixing two everyday liquids. This remarkable discovery made by scientists in the US has already been used to create beautiful shapes and patterns, and could also be exploited to create optical components that assemble themselves and even to clean surfaces.
Surfaces such as glass have a strong attraction to water molecules and many other liquids. As a result, a drop of pure water or propylene glycol will normally spread itself out to form a very thin film on an ultraclean glass surface. On everyday surfaces, however, droplets will often remain stuck at a single point, like a raindrop stationary on a sloping car windscreen. This process is called “contact-line pinning”, and is caused by contaminants and surface roughness.
When he was an undergraduate at the University of Wisconsin-Madison, Nate Cira noticed something unexpected: droplets of water and food colouring mixed together not only endured on a surface but began to dance around each other in elaborate patterns. Cira is now working on a PhD at Stanford University, and has teamed up with Manu Prakash and others to study this curious effect.
Tension and evaporation
Prakash’s team realized that the key to understanding the phenomenon is an effect that was first described in 1865 by the Italian physicist Carlo Marangoni. He pointed out that in a liquid with a surface-tension gradient, fluid is drawn towards the region where the surface tension is higher.
The droplets have a little tornado inside, and that flux is what keeps the droplet from spreading
Manu Prakash, Stanford University
It turns out that this “Marangoni effect” is relevant to food colouring/water mixtures because the propylene glycol used in food colouring evaporates more slowly at room temperature than water. This means that as a droplet spreads on a clean surface, more water than propylene glycol evaporates from the growing surface of the droplet. This means that the concentration of water near the surface of the droplet is lower than in its centre. Propylene glycol has a much lower surface tension than water, which means that the edge of the droplet also has a lower surface tension than the centre. Fluid is therefore pulled into the centre of the droplets by the Marangoni effect, preventing the droplet from spreading further. “The droplets have a little tornado inside, and that flux is what keeps the droplet from spreading,” explains Prakash.
On a clean surface the effect overcomes any pinning caused by roughness, and the droplets are able to move freely and respond to tiny forces. This allows the droplets to interact with each other in surprising ways. If two droplets are close together, the water evaporating from the droplets makes the humidity higher in the gap between droplets than elsewhere in the surrounding air. This means that evaporation will be slower from the inward-facing sides of the drops than it is from sides facing away from the gap, the results being a net force drawing the droplets together.
Two droplets with the same composition will coalesce straight away, but when the composition of the droplets differs significantly, something different happens. They are attracted together initially, but at very short distances they begin to exchange molecules across the gap. This makes the humidity in the gap lower than at the outward-facing side of the droplet with a higher water concentration. This drop will move away from the drop containing less water, and the result is that one droplet chases the other until they both have the same water concentration.
Racetracks and lenses
Variations on these effects allow the team to have droplets chasing each other continuously around a track that is simply drawn on a surface using a marker, which creates hydrophobic lines that the droplets cannot cross. The droplets can also be made to sort themselves and even align themselves on parallel plates, to produce a fluidic lens that could focus an image (see video above). The effects could be seen in any droplets containing two fluids in which one had both a higher surface tension and a higher rate of evaporation than the other, on a variety of ultraclean substrates such as aluminium, silicon and flexible indium tin oxide.
While Prakash says that the work was “purely driven by curiosity”, the researchers believe that the system provides a useful test-bed for exploring many-body physics involving interacting particles. “The experiments are very easy to run but the outcomes are really fascinating,” Prakash says. Beyond this, they are exploring potential industrial uses for cleaning solar cells and silicon wafers without the need for harsh chemicals.
Manoj Chaudhury of Lehigh University in Pennsylvania is impressed: “I think it is definitely a very significant work – in fact it’s a beautiful work…This is a new phenomenon they have discovered – one just has to sit down and find applications.”
For most of us, life does not stop after a hard day’s work. Some people like to sit down with a good book. Others might want to study or catch up on some household chores. Often the desire is even simpler: a chance to relax and spend time with friends and family.
Such options are always open to about five and a half billion of us. However, for the remaining one and a half billion – some 20% of the world’s population – the choices are rather more limited. These are the people in the developing world who do not have access to on-grid lighting, a feature of modern life that the rest of us take for granted. “If you’re not connected to an electricity grid,” says Beth Taylor, “then at 6 p.m. when the Sun goes down, either life stops or you’re dependent on a smoky, dangerous kerosene lamp.”
Taylor is one of many individuals – others being charity workers, businesspeople, engineers and indeed former physicists – who want to improve access to alternative off-grid lighting. She is chair of the UK National Committee for the International Year of Light, and has been championing the UK effort in Study After Sunset – an initiative that is intended to bring safe off-grid lighting to school-age children in particular. Although the initiative has only just begun, and the number of affected people is huge, Taylor hopes that by the end of 2015 she and her colleagues will have been able to make a difference. “Our aim is to leave a real legacy at the end of the year,” she says.
The dark age
The disadvantages of kerosene lamps compared with electric lamps are almost too numerous to mention. They are inefficient devices that produce a dim glow, directed upwards rather than sideways or downwards where the light would be most useful. They rely on an expensive fuel. And worse still, the toxic black smoke they emit is deadly. According to the World Health Organization, the burning of kerosene contributes to indoor air pollution and respiratory diseases, which kill more than 1.5 million people every year – more than the total child deaths from HIV/AIDS and malaria combined.
It does not stop there. The United Nations Environment Programme estimates that every kerosene lamp generates on average 200 kg of carbon dioxide per year, contributing significantly to global warming. Kerosene itself is often sold in plastic drinks bottles, which children can easily mistake for actual drinks. But the most obvious problem is kerosene’s flammability: a 2012 analysis of Ugandan households by economist Chishio Furukawa at Brown University in the US, found that kerosene lamps were responsible for 70% of fires, many of them fatal.
In 2010 the then US Secretary of State Hillary Clinton launched the Global Alliance for Clean Cookstoves, which aims to reduce deaths from unsafe indoor stoves by partnering companies with charities, and by jointly talking to governments and investors. Taylor hopes that she can instigate a similar alliance in Study After Sunset, which aims to promote the manufacture and marketing of alternative light sources to kerosene lamps. “The Alliance for Clean Cookstoves has made a big impact, and that is the kind of thing I would really like the Year of Light to help with,” she says.
There have been alternatives to kerosene lamps available for a long time, but in recent years one technology has emerged that has blinded the competition: light-emitting diode (LED) lamps. Patrick Walsh began to think about the potential for such lighting in 2006 while he was a physics student at the University of Illinois at Urbana–Champaign, taking time off with the non-profit organization Engineers Without Borders USA to design and build an electricity generator that ran off vegetable oil for a village in India. While he was living in the village, Walsh quickly realized that the locals’ need for electricity mainly stemmed from lighting. “We had brought a couple of LED lamps with us, but the products were just junk,” he says. “At that time, LEDs were sky-rocketing in terms of efficiency and reliability, and it was clear that they were going to be the future, as a replacement for kerosene. The question was, who could make a product to meet that need?”
Walsh thought he could. He carried out a feasibility analysis to see if an LED lamp could be sufficiently more cost effective than a kerosene lamp to make manufacturing it a worthwhile business. He found out that it could be – but that calculation, he now admits, was the easy part. While completing his degree back in the US, Walsh took on extra studies in mechanical and electrical engineering, and then headed to China where he spent two years developing a cheap-yet-effective solar-powered LED lantern. It was only in 2009 that, together with fellow University of Illinois alumni Anish Thakkar and Mayank Sekhsaria, Walsh launched the first “Sun King”.
To anyone who is familiar with modern electrical technology, the Sun King might not seem like much. It looks like a typical portable spotlight that is mounted on a stand made out of bent wire. But, as Walsh explains, it is the details that matter – one of those details being the charging indicator. According to Walsh, many consumers were positioning their prototype lamp’s solar panel in a way that allowed the lamp to charge, but at only half the normal rate – so people would mount the panel on their wall, instead of on their roof. “A normal charging indicator would show that it’s charging, but it wouldn’t show that it’s not charging very quickly,” he adds. “You can solve that with education, but you can also just try to design the product so that it’s obvious to users. So we have on each [Sun King] a charging rate indicator, which influences the way people use the product.”
A new dawn
Walsh’s attention to detail paid off. By 2013 he was selling one million Sun Kings a year, and he estimates that for the company’s last financial year, 2014, that figure will be nearly double. He believes 15 million people worldwide are currently using the lamps, which now come in several variants – the more expensive models even contain a USB socket for charging mobile phones, for instance.
“I was recently in a market in an out-of-the-way city in Kenya,” he recalls. “There were probably 100 stalls, selling food and sundry items – and about half of the stalls were using Sun Kings. It was such a normal part of life that if you asked them about it they were like, ‘Yeah, it’s just what we use for light.’?”
Light relief For homes without electricity, the Study After Sunset initiative aims to replace dangerous and polluting kerosene lamps (Top left) with cheap and safe LED alternatives such as the solar-powered Sun King, the LuminAID designed for disaster areas and the gravity-powered GravityLight. (Courtesy: (top row) GravityLight; Greenlight Planet; (bottom row) Greenlight Planet; LuminAID; GravityLight)
The Sun King is just one of many LED lanterns designed by small companies with the developing world in mind. Most work on the principle of recharging batteries with solar power, although there is at least one interesting exception. In 2013 product designers Martin Riddiford and Jim Reeves in the UK launched GravityLight, an LED lamp that turns gravitational potential energy into electrical energy. Aiming to be as affordable as possible, all you need to do to switch on this lamp is to attach to its hook a filled ballast bag, which gradually lowers, driving a small dynamo to supply electricity to the LEDs. Lifting about 10 kg, which takes only a few seconds, provides 25 minutes of power.
Other lamps have tackled slightly different off-grid lighting problems, such as those that arise in times of natural disasters. That is the focus of LuminAID, an inflatable LED lamp that was invented by product designers Anna Stork and Andrea Sreshta at Columbia University in New York, US, while watching coverage of the aftermath of the 2010 Haiti earthquake. The key strengths of LuminAID are that it is portable and durable. Fifty of the lamps can be folded and packaged into a space that would otherwise contain eight flashlights, and deploying one is as simple as inflating it like a rubber armband. The inflated packaging diffuses the LED’s light, and creates a shockproof, waterproof and buoyant exterior.
One of the turning points for the company came last year when it worked with ShelterBox, a disaster-relief organization based in the UK, to distribute more than 30,000 LuminAID lights to the victims of Typhoon Haiyan, which struck south-east Asia in November 2013. “We heard very positive feedback from the field on this distribution and the impact these lights had,” says Stork. “We have been selling and producing this product for a little over two years and this was the first instance where we had the production capacity and everything up and running to provide lights in a large volume after an emergency.”
Many companies that manufacture off-grid LED lighting have now joined forces under the non-profit Global Off-Grid Lighting Association (GOGLA), which acts as an industry advocate. Koen Peters, the executive director of GOGLA, says that the products currently have a market penetration of 2–5%. “It’s a huge market that we’re only just starting to reach,” he adds.
But it is worth it, he explains – and not just to rid people of the obvious problems of accidental fire and respiratory disease. “Once people have light, they aspire to something beyond it,” he says. “They might spend some of the saved money on a solar electricity generator to charge their phone, rather than go to a village where they will spend half a dollar to charge it. Then they might use the saved money from that to buy a better generator that can charge a radio, or a TV. LED lighting is the first stepping stone to an electric life, to which everyone aspires.”
Illuminating lives
In a sense, says Peters, the challenge for LED lighting is easier than that for clean stoves: whereas in all cultures good light is associated with safety, some cultures tend to cling to their traditional cooking habits of lighting fires indoors. Nonetheless, there are challenges. People need to be more aware of the availability of good LED technology, he says, and of the disadvantages of cheaper competitor products that still beset the marketplace. Moreover, the supply chain is still not as well financed as it could be. “The market is growing 100% a year – but it is being held back by supply, not demand,” he notes.
After all, off-grid lighting is not a purely charitable venture. It is a business and, as it grows, there may well be benefits for everyone. Peters believes that off-grid lighting products could find steadily more customers in developed nations such as the US, which, he says, has less reliable electricity grids than much of Europe. He also believes that developments in off-grid lighting and solar panels could spawn other off-grid technologies, such as flat-screen televisions that can be powered by a solar device via a USB port – particularly once big companies find interest. “Once the Samsungs and Panasonics of this world begin to see the potential in off-grid markets,” he says, “there will be a real benefit for the developed world too. I personally would be interested in a TV that runs off USB.”
The International Year of Light – can you remind me what that’s about?
Well, we’ve had international years of physics (2005), astronomy (2009), chemistry (2012) and crystallography (2014). Now it’s the turn of light. The idea for the International Year of Light and Light-based Technologies (IYL 2015) was originally dreamt up by top brass at the European Physical Society (EPS) and it’s since been endorsed by the UN Educational, Scientific and Cultural Organization (UNESCO) with support from more than 100 partners around the world. The year seeks to show not only why light is scientifically interesting, but also how it is so essential to modern life – be it to light up our streets, in medicine, as part of communications technology, or in art and culture.
So what is this site?
This is the official IYL 2015 blog. It’s run by Jorge Rivero González – a science-communications official from the EPS who is serving as the society’s outreach officer. But he’s not the author of the blog. His job is to commission and collate blog posts from anyone with an interesting tale to tell about light, including scientists, artists and educators.
What kind of topics does it cover?
Given that light means so many things to different people, there’s a huge variety of material. Some posts are about specific scientific advances, such as the benefits of photonics technology or the work of the Diamond synchrotron-radiation facility in the UK. But quite a few examine how light and light technology can improve our daily lives. One post, for example, tackles the Liter of Light project, in which people without access to mains electricity can generate eco-friendly light simply by installing a soda bottled filled with chlorinated water on their roof. There’s also an interesting post about how you can join thousands of people in measuring how bright (or dark) the night sky is where you are on 14 March and 12 September. Plus there are weekly updates about the many hundreds of IYL 2015 events going on around the world.
Can I contribute?
Yes, of course you can. As John Dudley, current EPS president and chair of the IYL 2015 steering committee, pointed out at the year’s opening ceremony at UNESCO headquarters in Paris in January, scientists have to grab this one chance to spread the message about light. So simply e-mail Rivero with any ideas you have. After all, once this year’s over, it’s over. (And who remembers the International Year of the Potato? Exactly.)
I’ve heard Physics World has made a guest appearance.
That’s right. Matin Durrani, the editor of Physics World, wrote about the work of University of Oxford physicist Josh Silver, who has developed a set of spectacles with liquid-filled lenses that can be adjusted by the wearer. The glasses are ideal for the millions of people in developing nations without access to professional eye-care. Physics World first covered Silver’s work in 2010 and we selected that article as one of our 10 best-ever features on light, which make up a free-to-read digital edition of the magazine available online or via our app. We’ve also made a podcast about Silver’s work, available online or via iTunes.
Fractal patterns that arise when healthy human cells turn cancerous have been observed for the first time by scientists in the US. Using an atomic force microscope (AFM), Igor Sokolov and colleagues at Tufts University and Clarkson University saw the patterns while studying the surfaces of cervical epithelial cells at nanometre resolution. The work could give us a better understanding of how the surface of cells affects the progression of some cancers, which could in turn lead to new strategies for fighting the disease.
Although the origin of many cancers is still a mystery, some scientists believe that these diseases are linked to complex processes in living cells becoming unbalanced, which could lead to chaotic behaviour. Indeed, signs of chaos have already been seen in biochemical and physical studies of cancerous tissue – with the structure of some cancerous tissues, for example, having fractal properties associated with chaotic systems.
Fractal patterns had, however, never been seen before on the surfaces of single cancer cells. The new observation could be significant because scientists already know that the surface of a cancer cell plays an important role in “metastasis”. This is the process whereby cancer cells manage to leave a primary tumour – often forcing their way through healthy tissue – and travel to other parts of the body to create secondary tumours.
Immortal cells
The new study was carried out using cells cultured in the laboratory. Three types of human cervical epithelial cells were studied: normal cells taken from healthy women; malignant cancer cells taken from cancer patients; and “immortal” pre-cancerous cells that were created by treating some of the healthy cells with a human papilloma virus genome. The cells were then freeze-dried so that they could be studied with an AFM.
The researchers mapped the structural features of the surfaces of the cells at a resolution of less than about 20 nm per image pixel. In particular, the AFM measured the “stickiness” between the instrument’s tiny probe and the cellular surface. The images were then processed using a Fourier transform to identify any repeating patterns. The team then analysed this information for signs of patterns that repeat on a number of different length scales – a hallmark of a fractal pattern.
The team found that the surfaces of both healthy cells and cancer cells did not have fractal patterns, whereas such patterns were seen on the pre-cancerous cells. This finding was unexpected. “Despite previous expectations that fractal patterns are associated with cancer cells,” says Sokolov, “we found that fractal geometry only occurs at a limited period of development when immortal cells become cancerous.”
Surface transformation
According to Sokolov, the team also discovered that cells deviate more from fractal behaviour when they further progress towards cancer, while normal cells do not have fractal patterns. This could mean that the fractal pre-cancerous phase plays a role in transforming the surface of a healthy cell to that of a cancer cell.
Sokolov and colleagues hope that their discovery could help to identify “weak points” in the transition from healthy to cancerous cells that could be targeted to stop the development of cancer. Such a transition could involve instabilities in biological processes that occur in the cell and lead to chaotic behaviour at the surface. If these instabilities could be prevented from emerging, then the progression to cancer could be halted.
“We need to further our understanding as to how important the cell surface is in the development of cancer,” concludes Sokolov.
Superconducting “Cooper pairs” of electrons have been split to create entangled pairs of electrons in a new device built by physicists in Finland and Russia. The device employs two quantum dots made of graphene. Although other types of quantum dots have been used for this purpose, the latest research suggests that graphene quantum dots should deliver long-lived entangled electron pairs that could be used in quantum computers.
Entanglement is a quantum-mechanical phenomenon in which properties of fundamental particles are correlated so that making a measurement on one particle can instantaneously affect another particle – even across very large distances. In principle, a quantum computer can use this connectedness to perform certain calculations much faster than a conventional computer. Although practical quantum computers do not exist today, some potential designs involve using the intrinsic angular momenta, or “spin”, of electrons as quantum bits (qubits) of information that can be entangled.
Superconductors provide a ready source of entangled electrons because the Cooper pairs that allow these materials to conduct electricity with little or no resistance are in fact entangled pairs of electrons with opposite spin. Splitting the pairs while preserving the electrons’ entanglement can be done simply by connecting ordinary metal wires to either end of the superconductor. If the set-up is just right, each wire will carry away one electron from a pair. However, it is more often the case that both electrons will end up going down the same wire.
Boosting the odds
One way to boost the odds in favour of separation is to replace the wires with tiny blobs of semiconductor containing just several thousand atoms. These quantum dots have electron energy levels that can be set precisely by carefully adjusting their size. The two electrons from each Cooper pair can be guided to different resonant energy levels and separated as a result. This approach has already been exploited using quantum dots made from indium arsenide and, with greater efficiency, using carbon nanotubes.
The latest work, carried out by Pertti Hakonen and colleagues at Aalto University in Finland together with Gordey Lesovik of the Landau Institute for Theoretical Physics near Moscow, instead uses quantum dots made from graphene. Graphene should be able to preserve the entanglement of the separated electron pair for longer, thanks to the fact that it consists of a single layer of carbon atoms, which constrains the electrons to move in a straight line and so avoids the emission of electromagnetic radiation that interferes with the spin state.
Pair production: scanning-electron-microscope image of the patterned graphene sheet before the electrical leads have been laid out. The light-grey regions are silicon dioxide and darker grey corresponds to graphene. The graphene quantum dots are the two, small rectangular pieces at the ends of the narrow graphene ribbons. The scale bar is 1 µm in length.
The team used electron-beam lithography to carve out two rectangular quantum dots (each 200 × 150 nm) from a layer of graphene deposited on a silicon-dioxide substrate. The dots were positioned 180 nm apart, covered by a superconductor made from a thin sandwich of titanium and aluminium, and connected to two metal contacts.
Aligning energy levels
To split the entangled electrons from the superconductor, the researchers first set the resonant energy level of the quantum dots to equal the energy possessed by the Cooper pairs. They then varied the gate voltage across one of the dots and monitored the current flowing through the other. They found that across most of the voltage range there was no current, but that at certain voltages the current would suddenly increase, drop below zero and then return to the zero mark. The rise, they explain, occurs because at that voltage the energy in one dot increases very slightly, while that in the other drops by the same small amount, causing the electrons to separate and so register a current (unseparated pairs register as zero current). The negative current, meanwhile, is caused by electrons “elastic co-tunnelling” through the superconductor. “It is like having a switch where you reverse the current by aligning the energy levels either symmetrically or antisymmetrically,” says Hakonen.
This is really a beautiful experiment
Detlef Beckmann, Karlsruhe Institute of Technology
Venkat Chandrasekhar of Northwestern University in the US praises the team’s ability to “independently control the energy levels of the two quantum dots”, and so neatly distinguish Cooper-pair splitting from elastic co-tunnelling. Detlef Beckmann of the Karlsruhe Institute of Technology in Germany agrees, arguing that the group can “probe the mechanism of Cooper-pair splitting more clearly” than has been possible to date. “This is really a beautiful experiment,” he says.
There is, however, still room for improvement. Hakonen and colleagues are working to increase the device’s efficiency – it currently splits just 10% of electrons passing through it – by better controlling the quantum dots’ energy levels. They also aim to show that the device not only splits Cooper pairs, but that it does in fact preserve entanglement. They plan to do this by recording the spin of the separated electrons using contacts made from the nickel–iron magnetic alloy dubbed permalloy.
A system of nine quantum bits (qubits) that is robust to errors that would normally destroy a quantum computation has been created by researchers at the University of California, Santa Barbara (UCSB) and Google. The device relies on a quantum error-correction protocol, which the team says could be deployed in practical quantum computers of the future.
In principle, powerful quantum computers can be built from a collection of qubits. For a qubit based on an electron, for example, these states would be “spin up” and “spin down”, with one state representing a logical “1” and the other “0”. Each qubit can be in a superposition of two quantum states at the same time and N qubits could be quantum-mechanically entangled to represent 2N values simultaneously. This would lead to the parallel processing of information on a massive scale not possible with conventional computers.
Extremely fragile
However, quantum computers are extremely fragile, and a computation can be easily destroyed by “bit errors” that occur when external noise in the environment affects the values of the qubits. While it is proving very difficult to create practical qubits that are robust enough to eliminate such errors, an alternative approach is to accept that errors will occur and to try to correct for them as the quantum calculation progresses.
Now, UCSB’s John Martinis and colleagues have taken an important step forward by demonstrating repetitive error correction in an integrated quantum device that consists of nine superconducting qubits. Each qubit is a small circuit consisting of a capacitor and a Josephson junction, and is made from an aluminium film evaporated onto a sapphire substrate. The qubit can be thought of as an artificial atom with information stored in its quantum states.
“Our nine-qubit system can protect itself from bit errors that unavoidably arise from noise and fluctuations from the environment in which the qubits are embedded,” explains team member Julian Kelly. “We also show that ‘more is better’: nine qubits protect the system better than five qubits, a critical requirement when moving to more qubits in a real quantum computer of the future.”
Measuring parity
“In quantum mechanics, we cannot measure a qubit without destroying the superposition and entanglement that makes quantum mechanics work,” says team member Rami Barends, “but we can measure something called parity – which forms the basis of quantum error correction.” The parity is defined to be “0” if both qubits have the same value and “1” if they have different values. Crucially, it can be determined without actually measuring the values of both qubits.
The researchers exploited this fact and repetitively measured the parity between adjacent “data” qubits by making use of “measurement” qubits. “Each cycle, these measurement qubits interact with their surrounding data qubits using quantum logic gates and we can then measure them,” Kelly explains. “When an error occurs, the parity changes accordingly and the measurement qubit reports a different outcome. By tracking these outcomes, we can figure out when and where a bit error has occurred and correct for it.”
More is better
The more qubits that are involved in the process, the more information is available to identify and correct for errors, explains team member Austin Fowler. “Errors can occur at any time and in all types of qubits: data qubits, measurement qubits, during gate operation and even during measurements. We found that a five-qubit device is robust to any type of bit error occurring anywhere during an algorithm, but a nine-qubit device is better because it is robust to any combination of two-bit errors.”
Although still a long way off from real-world applications, the researchers say that a “self-correcting” device such as theirs could be a great platform for testing some of the ideas behind error correction – such as protecting a quantum state against so-called phase-flip errors. “We are also now busy improving the quality of our qubits and the materials we use to make them,” says Kelly.
Smiley happy people: who would not want to be a particle physicist? (Courtesy: ATLAS)
Over on the Quantum Diaries blog, Aidan Randle-Conde has put together a lovely photo-essay called “30 reasons why you shouldn’t be a particle physicist”. It is reverse psychology, of course, and the 30 images highlight the benefits of devoting your life to studying subatomic particles. As someone who chose to do condensed-matter physics, do I now think that I made a huge mistake? No, but I have shared the thrill and excitement of being at CERN when the Higg’s was discovered and seen the Large Hadron Collider and its detectors up close, so I know where he is coming from.
