Every so often, a new technology emerges from fundamental physics research that has a genuinely transformative effect on society – X-rays and the Internet being two obvious examples from history. As commentators, the tricky part is trying to predict which of today’s emerging technologies has the potential to make such an important impact on the world. But for the 25th anniversary of Physics World magazine we have attempted to do just that. We have selected a handful of spin-offs from physics research that we believe will most alter our everyday lives over the next 25 years.
This short film profiles one of these emerging technologies: the idea of using graphene to produce drinking water. Graphene – or the “wonder material” as it has become known – was first isolated just nine years ago by Andre Geim and Konstantin Novoselov at the University of Manchester in the UK. Much of the hype surrounding this 2D honeycomb of carbon atoms has focused on its extraordinary electronic properties – who could resist the lure of an ultrathin bendable smartphone? But we think that another of graphene’s physical properties could be more important still. It turns out that despite being just one atom thick, graphene appears to be completely impervious to almost every liquid and gas. By drilling holes of the appropriate size in graphene – or creating membranes of graphene flakes stuck together with just the right-sized gaps between flakes – the material can be used to filter impurities from water.
The film is shot mainly at the University of Manchester, where scientists explain why nanoporous graphene has such great potential. “Graphene is very impermeable, even very light gases don’t pass through,” explains materials scientist Sarah Haigh. “If we were able to tailor the size of the pores within the graphene lattice, it might be possible to produce a very selective filter.” Also featured in the film is Vincent Casey, technical support manager at the charity WaterAid, who believes that graphene membranes could help meet water demand in the developing world. “Over the last 40 years there has been a growing trend towards the use of reverse osmosis and membrane technology in the provision of desalinated water, whereby you move the water across a semi-permeable membrane,” he explains.
Despite realizing the vast potential that graphene membranes hold, the scientists at the University of Manchester also understand that many challenges still lie ahead in the pursuit of a robust technology. “There can be a world of difference between what happens on a small scale in the lab and what happens when you have football-fields’ worth of membrane area in a real plant,” says Peter Budd, a membrane researcher based in the university’s chemistry department. For now though, Budd remains optimistic.
Quantum theory is over a century old, yet physicists continue to be perplexed and delighted by the weirdness of the quantum world. Whereas the laws of classical physics successfully explain the phenomena we experience every day, atoms and other tiny objects obey quantum laws that sometimes seem to defy common sense, baffling our feeble human minds. In the 21st century, we hope to put this weirdness to work by building quantum computers capable of performing amazing tasks.
To appreciate how the classical and quantum worlds differ, it is helpful to recall how information gets encoded and processed by physical systems. Just as digital information can be expressed in terms of bits, information carried by quantum systems can be expressed in terms of indivisible units called quantum bits, or “qubits”. A qubit is just a quantum system with two distinguishable states, and it can be realized physically in many possible ways; for example, by the spin of a single electron. But to get to the crux of how qubits differ from classical bits, let us view them more abstractly.
Boxing clever
We can picture a bit as a box with a ball inside that can be coloured either red or green. The box has a single door we can open to find out the ball’s colour. A qubit is also such a box, but with two doors marked 1 and 2. Whenever we open the box, we must choose either door 1 or door 2; we cannot open both. However, opening a door not only reveals the colour inside but also unavoidably disturbs what is inside.
If we put a red ball in door 1 and later open door 2, the ball that comes out has a random colour: red with probability ½ and green with probability ½. Although we often use probability to describe classical systems, the randomness exhibited by quantum systems is different. If a classical box has a ball inside and we do not know the ball’s colour with certainty, we assign probabilities to the two possible colours, reflecting our incomplete knowledge. But for the quantum box, we may be powerless to predict what will happen when we observe the colour through door 2, even though we have complete knowledge of how the box was prepared (for example, by opening door 1).
Curious affair A qubit can be viewed as a box containing a ball that is either red or green, the colour of which can be viewed by opening either of two doors (1 and 2). Strangely, we cannot predict what will happen when we observe the colour through, say, door 2, even though we know exactly how the box was prepared, for example, by opening door 1. (IOP Publishing)
The deepest differences between classical and quantum information can be fully appreciated only if we consider systems with more than one part. So consider two qubits: Alice’s in London and Bob’s in New York. This qubit pair can be prepared in a state such that if Alice opens either door of her box in London she sees a random colour, and the same is true for Bob in New York. So neither party acquires any information by measuring his or her qubit. Instead, information is hidden in correlations between what Alice sees when she opens a door in London and what Bob sees when he opens a door in New York – in this particular state Alice and Bob are guaranteed to find the same colour if they both open the same door. There are four distinguishable ways in which boxes in London and New York could be perfectly correlated – Alice and Bob could see either the same colour or different colours when both open door 1 or both open door 2. By choosing one of those four ways, we have stored two bits in the boxes.
Classical systems can also be correlated, of course, but this is different. What’s strange is that the information is completely inaccessible locally; it is entirely stored in the correlations. Though the whole system is in some definite state, the parts of the system are not. That is “quantum entanglement”.
Stranger and stranger
Entanglement gets stranger still for systems with many parts. Picture a 100-page book. If the book were classical, then by reading one page we could learn 1% of the content of the book. But a highly entangled quantum book is different. Looking at any one page we see only random gibberish, learning almost nothing about the content of the book. That is because information does not reside on the individual pages; instead it is recorded in the correlations among the pages. Only by performing a complex collective observation on many pages at once can we discern the differences between one highly entangled book and another.
For a highly entangled state of a few hundred qubits, the correlations among the qubits are so complex that describing them completely using classical information would require an unthinkable number of bits – more in fact than the number of atoms in the visible universe. This extravagant complexity of the quantum world points toward a highly plausible but unproven conjecture: classical systems cannot in general simulate quantum systems efficiently. If true, this statement has extraordinary implications. It means that by building highly controllable, many-qubit quantum systems, we should be able to perform some information-processing tasks far faster than would be feasible if we lived in a classical – rather than a quantum – world.
The technology for controlling quantum systems is advancing rapidly, fuelling the hope that in a few decades human civilization will enter an age of quantum supremacy, in which quantum computers solve problems that are beyond the reach of classical digital computers, such as factoring large numbers and simulating the physics of complex molecules. But to realize that dream, we must overcome a formidable obstacle: that of “decoherence”, which ordinarily makes large quantum systems behave classically. Entanglement among the qubits in a quantum computer is the source of its power, but entanglement between the computer and its unobserved environment is our enemy, driving decoherence.
In a classical computer an error occurs if interactions with the environment flip a bit. But a qubit is more delicate – it suffers an error if any information at all about its state leaks to the environment. That is decoherence. So for a quantum computer to work effectively, the information it processes must be perfectly concealed from the outside world until the computation is completed and the result is announced.
What weapon shall we wield to battle decoherence? Entanglement! The best way to resist decoherence is to encode information in highly entangled states. The state stored in the computer is like an entangled quantum book. The environment, interacting with the pages one at a time, acquires no information about the content of the book, because the information resides not in the individual pages but rather in the correlations among the pages. This principle, dubbed “quantum error correction”, will guide the design of future quantum computing hardware and software.
Today’s scientists and engineers are fortunate to live in an age of emerging quantum technologies. Indeed, our imaginations are poorly equipped to anticipate the many potential rewards to be gained by manipulating highly entangled quantum states. We should expect the unexpected.
The 2013 Nobel Prize for Chemistry has been awarded to Martin Karplus, Michael Levitt and Arieh Warshel for their development of computer models of complex chemical systems. All three researchers have close links to physics. Karplus, who is a US and Austrian citizen, originally studied physics and chemistry at Harvard University and is now based there and at the University of Strasbourg. Levitt, who has a physics degree from King’s College London, is a US and UK citizen working at Stanford University, while Warshel is a US and Israeli citizen based at the University of Southern California. The trio will share the SEK 8m (£775,000) and will receive their medals at a ceremony in Stockholm on 10 December.
Karplus, Levitt and Warshel won the prize for developing computational techniques that use both classical and quantum physics to describe complex chemical processes. Chemical models based on classical physics are relatively easy to compute and can therefore be used to simulate some aspects of the behaviour of large molecules such as proteins. The problem, however, is that these classical models cannot describe crucial aspects of chemistry such as how reactions proceed. To do so requires models based on quantum mechanics, which in turn need huge amounts of computing power. Quantum simulations can therefore only be applied to relatively small molecules.
Focusing on free electrons
In the late 1960s Karplus was developing quantum-based computer models that could simulate chemical reactions. Meanwhile, Levitt and Warshel were both working at the Weizmann Institute of Science in Israel where they developed a classical computer model that could simulate certain properties of large biological molecules. Warshel joined Karplus at Harvard in 1970 and the pair started to combine their classical and quantum approaches. They developed the first ever computer program to use quantum physics to model the behaviour of free electrons during a chemical reaction, while using classical physics to describe the rest of the atoms and electrons in a molecule.
Over the next few years, Levitt and Warshel worked together at the Weizmann Institute and the University of Cambridge with the aim of developing models of enzymes – long-chain molecules that play crucial roles in just about every biochemical process. This they achieved in 1976, but an important feature of the techniques developed by Karplus, Levitt and Warshel is that they can be applied to all types of chemistry. As a result, they are now not only being used to study molecules that are important for life, but also to develop new industrial processes, build better solar cells and synthesize new drugs.
The theoretical chemist Alán Aspuru-Guzik told physicsworld.com: “Karplus, Levitt and Warshel are true pioneers of modern computational chemistry.” Aspuru-Guzik, who is at Harvard, added “The quantum-mechanics/molecular-mechanics approach that they introduced is now a commonplace tool that helps scientists understand important problems related to life and, for example, understand how drug molecules work.”
Physics and chemistry
Karplus was born in Vienna in 1930 and immigrated to the US with his family in 1938. He studied physics and chemistry at Harvard before completing a PhD in chemistry at Caltech in 1953 working under Linus Pauling. After stints at the universities of Oxford and Illinois, he joined Harvard in 1966. In 1996 he took a second appointment at Strasbourg.
I’m a physicist. But that’s okay
Michael Levitt, Stanford University
Levitt was born in 1947 in Pretoria, South Africa and obtained a bachelor’s degree in physics from King’s College London in 1967. After spending a year at the Weizmann Institute working on the theory of molecules he did a PhD on the conformational analysis of proteins at Cambridge. He then worked at the MRC Laboratory of Molecular Biology in Cambridge and the Weizmann Institute before arriving at the Stanford University School of Medicine in 1987. In a Tweet issued today by Stanford, Levitt is quoted as saying: “I never studied chemistry, actually; I’m a physicist. But that’s okay.”
Warshel, meanwhile, was born in 1940 at Kibbutz Sde-Nahum in Israel. He studied chemistry at Technion – Israel Institute of Technology before doing a Master’s and a PhD in chemical physics at the Weizmann Institute. After stints at Harvard and the Weizmann Institute, he joined the University of Southern California in 1976.
The 2013 Nobel Prize for Physics has been awarded to François Englert and Peter Higgs for the theoretical discovery of the Higgs boson. The prize is worth SEK 8m (£775,000) and will be shared by the pair, who will receive their medals at a ceremony in Stockholm on 10 December.
Englert is a Belgian citizen and is Emeritus Professor of Theoretical Physics at the Université Libre de Bruxelles. Higgs is a British citizen and is Emeritus Professor of Theoretical Physics at the University of Edinburgh.
According to the prize citation, the pair are honoured “for the theoretical discovery of a mechanism that contributes to our understanding of the origin of mass of subatomic particles, and which recently was confirmed through the discovery of the predicted fundamental particle, by the ATLAS and CMS experiments at CERN’s Large Hadron Collider”.
In 1964 Englert and Higgs published papers independently of each other that set in motion the 48-year search for the Higgs boson that ended with its discovery last year at CERN.
Winners didn’t meet until 2012
Speaking on the telephone to journalists in Stockholm, Englert said that winning the prize was “Not very unpleasant, of course. I am very, very happy to have the recognition of this award.” Englert also mentioned that the first time he met his co-winner Higgs was at CERN on 4 July 2012 when physicists working on the Large Hadron Collider announced the discovery of the Higgs particle.
Permanent secretary of the Royal Swedish Academy of Sciences, Staffan Normark, said that the committee has so far been unable to contact Higgs. However a statement from Higgs has been issued by the University of Edinburgh. He says: “I am overwhelmed to receive this award and thank the Royal Swedish Academy. I would also like to congratulate all those who have contributed to the discovery of this new particle and to thank my family, friends and colleagues for their support. I hope this recognition of fundamental science will help raise awareness of the value of blue-sky research.”
Brout–Englert–Higgs mechanism
Commenting on the award, CERN’s director-general Rolf-Dieter Heuer says “The discovery of the Higgs boson at CERN last year, which validates the Brout–Englert–Higgs mechanism, marks the culmination of decades of intellectual effort by many people around the world.” Heuer’s comment highlights the contributions of the late US/Belgian physicist Robert Brout, who co-authored Englert’s 1964 paper and who was his long-time colleague at the Université Libre de Bruxelles. Brout died in 2011 and the Nobel prize is not awarded posthumously.
Several other physicists are also associated with the discovery of the Higgs mechanism, including Carl Hagen, Gerald Guralnik and Tom Kibble, who together published a paper in late 1964 in which they independently came to the same conclusions as Brout, Englert and Higgs. Commenting on the award, Kibble acknowledges that “our paper was unquestionably the last of the three to be published in Physical Review Letters in 1964…and it is therefore no surprise that the Swedish Academy felt unable to include us”. In an interview with Physics World in 2012, Higgs was also keen to flag the contributions of the US theorist Philip Anderson.
Symmetry-breaking event
The existence of a Higgs-like mass-generating mechanism plays an essential role in the Standard Model of particle physics. It arises from a symmetry-breaking event that occurred in the very early universe that created a uniform scalar field known as the Higgs field that pervades all space. Elementary particles such as leptons, quarks and the W and Z bosons conveying the weak force “acquire” their distinctive masses by virtue of their unique and different couplings to this field.
Wave–particle duality at the heart of quantum mechanics dictates that vibrations in this field should give rise to a spin-0 particle (or particles) known as the Higgs boson(s). Just as vibrating the electromagnetic field generates waves corresponding to photons, so should shaking the Higgs field create such bosons. The question that faced particle physicists over the past decades was how hard did the Higgs field need to be shaken in order to create detectable quantities of Higgs bosons? The answer came in 2012, when physicists analysed vast numbers of proton–proton collisions at 8 TeV and found very strong evidence for the Higgs boson.
Peter Higgs was born in 1929 in Newcastle-upon-Tyne, UK. He attended Cotham Grammar School in Bristol, which also counts Nobel laureate Paul Dirac as one of its former pupils. He enrolled as a physics undergraduate at King’s College London, where he went on to do a PhD on the theory of molecules. Higgs then worked at several British universities before settling at the University of Edinburgh, where he has been since 1960.
François Englert was born in Belgium in 1932 in the Brussels suburb of Etterbeek. He received a degree in electrical engineering from the Université Libre de Bruxelles before completing a PhD in natural sciences in 1959 at the same university. Englert then worked at Cornell University in the US for two years before returning to the Université Libre de Bruxelles, where he has been since 1961.
A new material that once deformed will automatically return to its original shape when heated has been developed by researchers in the US. While this is not the first such “shape-memory metal”, the latest material can go through 16,000 shape-shifting cycles without significant degradation – making it far more robust than existing materials. The metal was created using a theoretical model that predicts which types of materials should have superior shape-memory properties. The team believes its model will lead to the creation of new types of materials that could have a range of technological applications.
Shape-memory alloys are reversible phase-change materials that can exist in two crystal-lattice structures: one that is more stable above a certain transition temperature and the other favoured at lower temperatures. If the material is cooled through the transition temperature, the lattice reconfigures itself to the low-temperature structure. If it is then heated above the transition temperature, it reverts back to the high-temperature structure.
Deforming a shape-memory metal in the low-temperature phase produces a distorted version of the low-temperature lattice structure. When heated, this distorted low-temperature lattice cannot directly reconfigure itself into the high-temperature version. Instead, the metal reverts to its original shape, which allows it to adopt the high-temperature crystal structure. Applications for these materials include temperature-sensitive switches and guide wires used in catheters.
Highly stressed phase
Traditional phase-change materials undergo this shape change by passing through a highly stressed intermediate phase that introduces cracks and dislocations into the material. As this damage builds up, the shape-shifting abilities of the material degrade and sometimes the material can lose strength and fail altogether.
In 2005 Richard James and Zhiyong Zhang at the University of Minnesota outlined a set of theoretical requirements for a phase-change material to be free of this stressed transition state. If material could be made with these desired “cofactor conditions”, its structural phase could be flipped repeatedly without damaging the material.
Now, James and colleagues have designed and manufactured such a material. They began with a known shape-memory material – the alloy Zn2AuCu – and used computer modelling to find candidate alloys with slightly different proportions of zinc, gold and copper that would better satisfy the cofactor conditions. When they had arrived at their three best candidates, they prepared the alloys in the lab to see which one had the best properties.
Unpredictable array of structures
The researchers found that one of the candidates – Zn45Au30Cu25 – satisfied the theoretical requirements almost perfectly. Furthermore, the team found that the material could be put through 16,000 hot and cold cycles without any damage to its phase-change properties. On the atomic scale, the researchers found that, whereas traditional phase-change materials return to the same, increasingly imperfect, lattice structure every time the material is cooled, the lattice of their material was a complex, unpredictable array of different structures that was reset every time the material cooled through the transition temperature. This verifies a prediction made in 2007 by James and colleagues that multiple lattice structures would be possible in a material that satisfied the cofactor conditions. The meandering domain boundaries between areas of different lattice structure led the researchers to jokingly christen the new structure “riverine”.
This latest research is reported in Nature and the paper simply deals with the preservation of phase-change properties over a large number of cycles. However, James told physicsworld.com that his team has also established that the shape-memory effect is preserved as well. The researchers are also looking for other examples of phase-change materials that satisfy the cofactor conditions. “We would love to try this on a ceramic system where it would be really unexpected to have this reversibility of a big first-order phase transformation,” he says.
New generation of alloys
Ryusoke Kainuma from Tohuku University in Japan, an expert on shape-memory metals, believes that the paper “opens the door to a new generation of shape-memory alloys”, although he suggests that the difficulty of manufacturing the Zn45Au30Cu25 alloy together with the high cost of gold is likely to rule out any direct industrial application.
Meanwhile, materials physicist James Morris of the Oak Ridge National Laboratory in Tennessee is most impressed by the unconventional way that the researchers have used abstract concepts to design a real material. “Without saying what the degradation mechanisms are and without looking at the structures of the interfaces, they’ve said ‘this geometrical thing should minimize the stress that leads to degradation, and so we’re going to try that’,” he says. “And it looks reasonably convincing.”
Welcome to the second instalment of the Physics World at 25 Puzzle. The first puzzle was released last week and your second challenge lies below. #PW25puzzle
Is Schrödinger’s cat alive or dead?
1. Schrödinger’s cat is alive. 2. Schrödinger’s cat is dead. 3. Exactly one of statements 6 and 9 is true. 4. Exactly one of statements 2 and 6 is false. 5. Statements 4, 5 and 10 are all false. 6. Exactly one of statements 1 and 10 is false. 7. Exactly 5 statements are true. 8. Exactly one of statements 3 and 10 is false. 9. Exactly one of statements 6 and 10 is true. 10. Exactly one of statements 1 and 2 is false. 11. Statements 1, 8 and 11 are all false.
Enter your answer as a list, in numerical order, of the number(s) of the statements that are definitely true, as a single string with no spaces, such as, for example, 25811.
The National Institute of Standards and Technology website is on furlough this week.
By Hamish Johnston
This week the magazine and journal Science published an article called “Who’s afraid of peer review?“. It describes a remarkable “sting” operation by the journalist John Bohannon, who submitted a spoof scientific paper to 300 or so open-access scientific journals. The paper claimed to offer evidence for the anti-cancer properties of a naturally occurring compound. It contained several fundamental errors that should have been caught by the peer-review process, not to mention made-up authors working at fictitious institutes. Instead of being rejected by all the journals, more than half of the submissions (157 in total) were accepted for publication.
Entangled beats: the team at the JILA lab. (Courtesy: Baxley/JILA)
Researchers in the US have entangled the motion of a tiny mechanical drum with a microwave field. This is the first time that a macroscopic oscillator has been entangled and the work extends the observation of quantum behaviour to larger objects than before. The researchers hope to apply their results in the creation of a quantum computing circuit.
Cool beats
In early 2010, researchers at the University of California in the US were the first to have observed true quantum behaviour in a macroscopic object that was cooled down to its quantum ground state. Then, in 2011, Konrad Lehnert and colleagues at JILA – a joint institute of the University of Colorado at Boulder and the National Institute of Standards and Technology (NIST) – followed on from that work and cooled a similar microdrum down to the ground state with a technique known as “sideband cooling”, which uses microwaves instead of laser light. They cooled the drum to below 400 μK, steadily lowering its energy to just one-third of one quantum.
Then in March this year, Lehnert’s group was the first to store and retrieve quantum information from their oscillator by connecting it to a microwave circuit. In that experiment, the researchers transferred the states of a microwave field into their mechanical oscillator and then converted the state of the oscillator back into a microwave field. Now, Lehnert, along with his postdoctoral researcher Tauno Palomaki and colleagues have extended this by creating an entangled state between the oscillator and a microwave field by exciting the circuit above its resonance frequency. “During the entanglement process a microwave pulse emerges from the circuit but unlike the previous work, it’s not a state that was stored in the mechanical oscillator. That pulse is generated spontaneously by the entanglement process,” explains Lehnert. The microwave field that emerges from the circuit as a result of their experiment is known as the “pulse”, while the microwave field that they impose on the electromechanical circuit is known as the “pump”.
Perfect tuning
Lehnert tells physicsworld.com that if the pump is tuned above the circuit’s resonance frequency, a spontaneous process creates a pulse and places the mechanical oscillator in a state that is correlated with that pulse. “If the mechanical oscillator is near its ground state when that pump is applied, the oscillator and pulse won’t just be correlated, they should be entangled,” explains Lehnert.
In the experiment, the first pump is tuned below the circuit’s resonance, transferring the thermal state of the oscillator into a pulse that removes the thermal energy from the drum, cooling it to near its ground state. The second pump tone is tuned above the circuit’s resonance and this creates a pulse that is entangled with the oscillator. The third and final pump is, once more, tuned below resonance and it converts the state of the oscillator into a third pulse. During this process, a train of three pulses emerges from the circuit. The first contains the thermal state of the oscillator but it is the second and third pulses that are of interest to the researchers. “We show that the second and third pulses are entangled and because we know that the third pulse contains the state of the oscillator immediately after the entanglement was generated, the oscillator must have been entangled with the second pulse,” says Lehnert.
Subtly entangled
Lehnert is clear that that there is, undoubtedly, some subtlety when it comes to a demonstration of entanglement itself. “Evidence of entanglement comes from asking how well we can anticipate the measurement of the third pulse, based on our measurement of the second. That’s a good operational notion of correlation,” he says. If the pulses are indeed entangled, a measurement of the second pulse should allow the team to anticipate the outcome of the measurement of the third, with an uncertainty that is smaller than the fluctuations associated with the quantum vacuum fluctuations (a temporary change in the amount of energy in a point in space that comes about due to Heisenberg’s uncertainty principle).
Lehnert explains that the subtlety comes into play because the state of a pulse is specified by two numbers – the real and imaginary components. These two numbers obey the uncertainty principle – either can be measured with arbitrary precision but if both are measured simultaneously, the measurement itself introduces noise that is at least as large as the vacuum fluctuations. But for a demonstration of entanglement, the team must measure the correlations both in the real part and in the imaginary parts and it did so by measuring both simultaneously. “As such we must carefully characterize the noise added by the measurement itself. Having done so, we can show that the second and third pulses are entangled,” says Lehnert. According to him, the team has a high degree of confidence that the oscillator and the microwave pulse are entangled each time the experiment is carried out.
The US researchers are now looking to combine their circuit with superconducting qubits to store and retrieve a qubit state from the oscillator, as their work demonstrates an essential requirement for using compact and low-loss micromechanical oscillators for quantum processor. “In addition, we’d also like to use devices of this type to build quantum-enhanced force sensors. That is, we’d like to show that we can use entanglement to circumvent the quantum noise that would otherwise limit the sensitivity of a force measurement,” says Lehnert.
Two independent teams of physicists have used small pieces of glass etched with tiny gratings to accelerate electrons through enormous electric-field gradients. One team boosted the kinetic energy of the electrons at about the same rate as a conventional particle accelerator, while the other achieved 10 times that rate. The technology could one day be used to build table-top accelerators that are much smaller than conventional devices, bringing the benefits of particle-beam therapy to a wider range of cancer patients.
Laser-driven particle acceleration has been the subject of intense research over the past two decades, having been used to accelerate electrons, protons and other charged particles. Although several different techniques can be used, they all involve firing an intense pulse of laser light at a target. The intense electric field of the pulse separates electrons from the positively charged nuclei, creating a very strong electric field that can then be used to accelerate charged particles.
This latest breakthrough was made independently by two groups: one in the US and the other in Germany. The US team was led by Robert Byer of Stanford University and included physicists at the SLAC National Accelerator Laboratory, the University of California, Los Angeles and the Tech-X Corporation. In their set-up, a beam of electrons is first accelerated to a kinetic energy of about 60 MeV moving at near to the speed of light using the Next Linear Collider Test Accelerator Facility at SLAC. The other team was led by John Breuer and Peter Hommelhoff of the Max Planck Institute of Quantum Optics in Garching, whose device works for much less energetic 28 keV electrons travelling at about one-third the speed of light.
Pillars and trenches
In the US experiment, the laser acceleration is carried out by first firing the electrons into a 500-μm-long device made from silica glass. The electrons travel along a narrow channel, the two opposing walls of which are covered by gratings of pillars and trenches (see figure above). As the electrons zoom down this channel, a pulse of intense 800 nm infrared light is fired at the gratings – twice the wavelength of the gratings themselves. The pulse interacts with the gratings such that the phase of its electric field is rotated by 180° as the light passes a grating pillar. The strength of the electric field is also enhanced in a similar periodic manner.
Schematic showing a cross-section of the device created by the team at Stanford University. The blue material is silica glass and the electrons travel from right to left through the gap between the top and bottom sections. The pillars and trenches of the grating can be seen at the top and bottom of the gap. (Courtesy: E A Peralta et al./Nature)
Some electrons enter the channel at just the right moment to experience a strong electric field that accelerates them in the forward direction. And because they are travelling at very nearly the speed of light, these electrons are synchronized with the pulse as it travels through the device – and therefore they enjoy maximum acceleration throughout their journey. The US team calculates that the electrons encounter an acceleration gradient of about 300 MV/m in the device – which is more than 10 times higher than that achieved in today’s conventional accelerators.
Non-relativistic challenges
The German device, in contrast, works for much slower electrons that travel a shorter distance in one oscillation cycle of the light pulse. This means that the grating spacing in the chip should be about 250 nm. As this was too small for the team to achieve, Breuer and Hommelhoff settled on a 750 nm spacing, which meant that the electrons got an accelerating kick once every three cycles. Furthermore, because the initial speed of the electrons is much lower than the speed of light, their speed will increase significantly when accelerated. So to be effective, the grating spacing must increase as the electron travels along the device.
Despite these problems, Breuer and Hommelhoff were able to create an acceleration gradient of about 25 MV/m, which is on a par with conventional accelerators. While these accelerators-on-a-chip could lead to compact sources of high-energy electrons for scientific, commercial and medical use, there are still challenges. In particular, the Stanford device works extremely well but requires a source of relativistic electrons – which is large and expensive. While Breuer and Hommelhoff have shown that it is possible to accelerate non-relativistic electrons, much more work would be needed to create a practical system that uses lasers to make a truly compact high-energy source.
However, that has not deterred Byer. “Our ultimate goal for this structure is 1 GV/m, and we are already one-third of the way there in our first experiment,” he says.
The work by Byer and colleagues is reported in Nature and Breuer and Hommelhoff describe their research in Physical Review Letters.
This month is the 25th anniversary of Physics World – the member magazine of the Institute of Physics – and in addition to a special celebratory issue, we’ve decided to set you a challenge.
In fact, we have teamed up with GCHQ – one of the UK’s three Intelligence Agencies and home to some of the country’s hottest code-breaking talent – to create with us a set of five physics-themed puzzles. The puzzles have been devised by three GCHQ members of staff, who today we still know only as Colin, Nick and Pete. (Thank you, guys!)
Below is Puzzle 1, the first of the five. The rest will be released on successive Tuesdays throughout October on this blog. The first is the easiest – they only get harder from here on in!
