Modern quantum mechanics has been around for a century or so and has proven to be an incredibly useful tool for both understanding nature and creating practical technologies. Therefore, it might come as a surprise that different “attitudes” to quantum mechanics still exist among experts in the field.
These are highlighted in a paper recently uploaded to the arXiv preprint server. It describes a poll of 33 leading physicists, philosophers and mathematicians that asks their opinions on quantum theory. The survey was done at a conference in 2011 in Austria that was called “Quantum Physics and the Nature of Reality”.
Delegates were asked which is their preferred interpretation of quantum mechanics. Not surprisingly, “Copenhagen” was the winner with 42% of the vote, followed by an “information-based” approach” with 24%.
On the subject of quantum information – that is, the ongoing development of quantum computing, cryptography etc. – 76% of the respondents agreed that “it’s a breath of fresh air for quantum foundations”, whereas only 12% thought it was not relevant to the study of the foundations of quantum mechanics.
And what about that question that physicists are hearing a lot of these days: “When will we have a working and useful quantum computer?” While only 9% said within 10 years, 42% chose within 10–25 years.
The paper is by Maximilian Schlosshauer of the University of Portland, Johannes Kofler of the Austrian Academy of Sciences and Anton Zeilinger of the University of Vienna. As well as presenting the results, the trio look at correlations between different responses to try to build a picture of participant’s overall view of quantum theory. They also compare the responses to a similar poll done in 1997. You can read the paper here .
The lower image is an illustration of the optical phased array, while the upper two panels are scanning electron microscope images of the array (left) and an individual element of the array (right). (Courtesy: Jie Sun)
The first large-scale array of optical antennas on a silicon chip has been made by researchers in the US. The structures, which can generate accurate pre-defined patterns of light, could be used in a host of new application areas, including 3D holography displays and advanced medical imaging.
Antennas have long been used to transmit radio and television waves, but researchers have only recently begun to extend this concept to visible light. All antennas work by oscillating charges along their structure, which means that the size of the antenna must fit to a resonant mode for the wavelength of the electromagnetic radiation it supports. To make an antenna work at optical frequencies, it must thus be scaled down to nanometre dimensions.
The idea of connecting up multiple radio antennas, fed from a common source, has been around for a long time too. Here, the antennas are aligned in phase to enhance the emission of radio waves in a given direction. The technique is also routinely employed in astronomy, where information from multiple telescopes is collected in phase to improve the resolution of the entire set-up.
Thousands of antennas
Now a team led by Michael Watts at the Massachusetts Institute of Technology (MIT) has extended the concept to infrared light and has succeeded in fabricating an integrated optical phased array containing more than 4000 antennas on a single chip that has an area of little more than 0.5 × 0.5 mm. This is equivalent to an array of 64 × 64 antenna units, or pixels, with each pixel covering 9 × 9µm.
The antennas are made from a strong, high-index contrast dielectric grating and all work at the same power level. They are all aligned in phase to produce a sophisticated light pattern – in this case, the MIT logo, in the far field. “To our knowledge, this demonstration represents the largest coherent combination of silicon nanophotonic elements ever produced,” says Watts.
The researchers say that they are able to accurately control the direction in which light is emitted from the array and steer an emitted light beam in two dimensions.
Watts and colleagues made their phased array in a conventional 300 mm complementary metal-oxide–semiconductor (CMOS) fabrication facility using state-of-the-art tools, such as optical immersion lithography.
Imaging and 3D holography
According to the team, the arrays could be used for beam steering in sensing applications, such as light detection and ranging (LIDAR) and interferometry. The devices could also find use in medical images because they could be used to take images through light-scattering materials such as biological tissue. Such imaging makes use of “adaptive optics” techniques that automatically adjust the phase of an optical wave to compensate for the distortion caused by the surrounding medium – something that calls for accurate phase control of the light beam being employed and a huge number of pixels, two characteristics that the new optical phased arrays possess. One immediate application would be in intravascular surgery, to steer laser beams and image vascular walls, says Watts.
I believe that 3D holographic displays are not only possible now, but [are] within our grasp
Michael Watts, MIT
“However, I think the most interesting application for these arrays is in 3D holographic displays,” he says. “I believe that 3D holographic displays are not only possible now, but [are] within our grasp. This is because our array allows for separate control over the phase and amplitude of the light wave emitted as well as single-point excitation of the nanophotonic emitters, enabling truly arbitrary holograms to be generated entirely on-chip for the first time.”
He adds that such an application would be even better if the arrays operated at visible optical wavelengths, which are shorter than the near-infrared frequencies demonstrated in the current work. “To achieve this, we will now be working on reducing the arrays’ pixel size even further and using a material other than silicon for the waveguide in the structure, since silicon absorbs light in the visible part of the spectrum.”
A new theory of how plant photosynthesis involves quantum coherence has been suggested by physicists in the UK, Germany and Spain. This latest research is based on the study of organisms that live deep under the sea yet are able to convert sunlight into energy. The study suggests that molecular vibrations do not destroy the coherence – as previously thought – but rather perpetuate and even regenerate coherence. The discovery provides a better understanding of how as much as 99% of the energy of light absorbed by photosynthesis cells is successfully transferred to locations in the cells where electric energy is converted to chemical energy. The work opens up the possibility of using nature-inspired designs in quantum devices.
Until recently, living systems were thought to be “too wet and warm” to rely on delicate quantum properties such as entanglement and coherence. The problem is that these properties decay rapidly via random interactions with things in the outside world, such as vibrating molecules. However, over the past decade physicists have begun to suspect that quantum properties play important roles in biochemical processes – including photosynthesis.
This latest work was done by Alex Chin (now at Cambridge University) and colleagues at the Institute of Theoretical Physics in Ulm and the Technical University of Cartagena. The team looked at organisms called green sulphur bacteria that live 2000 m below the ocean surface. There is so little sunlight down there that the bacteria cannot afford to lose a single photon – indeed, almost 100% of the light they absorb is turned into food.
Excited states
When sunlight hits the surface of the plant, energy is transferred via chains of pigments to a reaction centre, where it is converted into chemical energy. Those pigments are held in place by proteins, which together create pigment–protein complexes, or PPCs. The PPCs effectively act as corridors and the energy itself travels in the form of molecular excited states, or molecular excitons. These excitons are able to move along the PPC by hopping from one molecule to the next.
In 2007 Graham Fleming and colleagues in the US showed that these excitons exhibit quantum coherence, which means that the excitons may exist simultaneously in a superposition of several quantum states with varying probabilities. Coherence also allows the exciton to explore multiple pathways to the reaction centre simultaneously, ultimately choosing the fastest, most efficient option. As is demonstrated in man-made solar cells (which also rely on excitons), the longer this trip takes, the more likely it is that the energy will dissipate before it reaches its destination.
Optimizing function
The presence of quantum effects in photosynthesis surprised both physicists and biologists, and left them wondering how a fragile quantum state could survive in a living organism. More specifically, research groups found that the coherenent states exist for 100-times longer than the coherence time of the energy states of an exciton. Something was helping these wave states survive long enough to ensure the safe passage of nearly 100% of the photon energy that the organisms absorbed.
This latest research suggests that the answer lies in the proteins in the PPCs, which provide structural support for the pigment molecules. The new calculations reveal that these proteins are more active participants in the transport system than was previously thought. The natural vibration frequencies of the proteins resonate with the exciton waves, and like a parent pushing a child on a swing, the protein structures keep the excitons oscillating without dampening. In fact, the exciton may pass its vibration into the protein structures, which then return it to the exciton, thus restoring its coherence.
“People have not viewed this protein structure as something that actively helps quantum phenomenon to take place in biological systems,” says team member Martin Plenio. “This is really a new way of thinking about things.”
Definitely not noise
The team’s conclusions come from precise analysis of the protein vibrations, using data from Markus Wendling and colleagues in the Netherlands, who in 2000 examined the PPC structures from green sulphur bacteria. Previous efforts to study the protein vibrations used rougher approximations and usually concluded that the vibrations were noise.
“The main difference in terms of the paradigm for doing this simulation was to not separate the system into the exciton and the environment, but to treat them all together as one large many-body system,” says Chin. “We took a completely holistic approach. This makes it very complicated in terms of variables and things that one has to keep track of, which means that computationally it is very tough.”
Understanding these protein structures could assist in building similar structures in quantum devices. If similar structures are used in the conversion of electrical energy to chemical energy, it could shed light on how to mimic photosynthesis’s high efficiency rates in man-made solar cells.
A good hypothesis
Greg Scholes of the University of Toronto in Canada praises the detailed analysis conducted by the team, and says the conclusions “fit with some of the pieces of the puzzle that have been emerging in more recent experiments”.
While Scholes believes the analysis is “sufficient proof of the idea in principle”, he says direct experiments will need to be done to confirm the conclusions. “From that perspective this work really contributes something important. Because it gives us a hypothesis, [and now] we can go and test it,” he says.
A specialized polymer film that can generate electricity thanks to its response to moisture has been created by a team of researchers in the US. The composite polymer absorbs minute amounts of evaporated water from its environment, causing it to curl up and down repeatedly. This continuous motion is converted into electrical energy that can be stored in small generators, and eventually could be harnessed to power micro- and nanoelectronic devices, such as remote sensors or actuators, as well as having biological applications.
Ways and means
With the ever-increasing demand for power, scientists are keen to develop alternative ways to harvest energy, especially from the ambient environment. But Mingming Ma, a postdoc at the David H Koch Institute for Integrative Cancer Research at Massachusetts Institute of Technology (MIT), and colleagues were studying something quite different – they were developing a polymeric electrode for medical purposes as a way to simulate muscles for nerve regeneration. “It [the discovery] was not intentional at all,” says Ma. “During our experiments, we found the film expanded when it came into contact with water vapour and realized that this caused its movement.”
Ma told physicsworld.com that the team’s newly designed 20-µm-thick film is made by interlocking two different polymers to form a network – like a mesh. One of the polymers, known as polypyrrole, forms a hard but flexible matrix that provides structural support. The other – polyol-borate – is a soft gel, and it is this polymer that swells when it absorbs water and contracts when the water is expelled.
Curling mechanics
When the material is in an environment that contains even a small amount of moisture, it absorbs some of the water, causing the film to curl upwards. This exposes the underside of the film to air, making it quickly release the moisture and somersault forward. This continuous motion converts the chemical energy of the water gradient into mechanical energy.
Twist and curl An illustration showing the continuous movement of the film. (Courtesy: Ning Zhang/MIT)
Ma is quick to point out that the device does need a water gradient – place the film on a water surface, for example, and it will absorb too much moisture to be able to flip over. “It does not need a lot of water,” Ma says. “A very small amount of moisture is enough.”
Ma says that the team was inspired by the network structure present in the skin of many animals. “They [animals] have two layers – a tough fibre layer and a softer flexible and elastic layer, which together make a sturdy and flexible material,” he says. So the researchers approached designing their film in a similar way, hoping to produce a better water-responsive actuator.
Two better than one
In the past, other water-responsive films have been made only from polypyrrole, which has a much weaker response on its own. “By incorporating the two different kinds of polymers, you can generate a much bigger displacement, as well as a stronger force,” says Liang Guo, another team member.
The team says the film can generate “contractile stress” up to 27 MPa. It claims that a 25 mg film could lift a load 380 times its own weight and transport a load 10 times its own weight.
To turn it into a generator, Ma and colleagues coupled the polymer film to a piezoelectric material – which converts mechanical stress to an electric charge – to generate an average power of 5.6 nW. This energy can be stored in capacitors to power ultra-low-power microelectronic devices such as temperature and humidity sensors. It could also power microelectricalmechanical systems (MEMS) or even smaller devices such as nanoelectronics.
Big ideas, small films
Other applications could include attaching the film to clothing worn while exercising, where sweat evaporating from the body could power small devices. Ma even suggests placing it above or near a large body of water, such as a lake, to generate electricity. But this would involve greatly increasing the mechanical-to-electrical energy conversion to make it much more efficient. “We want smaller films to power larger devices. For this we need a better piezoelectric material developed for the purpose,” explains Ma. The researchers plan to investigate this, as well as other applications for their film, in the coming months.
Scientists have long wondered where the observed magnetization of the interstellar medium came from, given that the fully ionized gas of the early universe contained no magnetic particles. According to new research by an astrophysicist in Germany, the answer lies in magnetic fluctuations within this plasma. Although these fluctuations initially summed to zero, he calculates, they would have left a positive excess of field once compressed by energetic phenomena such as supernovae explosions.
Permanent magnetism is a property of only a few materials, such as iron, in which the spins of individual electrons naturally line up in the same direction and create a residual magnetic field. In the early universe, before iron and other magnetic materials had been created inside stars, permanent magnetism did not exist. Nevertheless, the proto-interstellar medium, a plasma consisting of a few light nuclei along with free protons and electrons and which formed when the universe was less than a billion years old, did have a non-zero magnetic field.
Magnetic seeds
Astrophysicists believe that the explosive collapse of massive stars known as supernovae or the streams of charged particles referred to as galactic winds could have provided the energy needed to compress small and disordered, or “seed”, magnetic fields so that they became unidirectional and as strong as the fields observed in the interstellar medium – that is, having an energy density roughly equal to that caused by the medium’s thermal pressure. The question is: where did these seed fields come from?
To answer this question, Reinhard Schlickeiser of Ruhr University in Bochum considered the proto-interstellar plasma shortly after it came into being – an era known as “reionization” when something, probably the light from the first stars, provided the energy needed to break up the previously neutral gas that existed in the universe. The protons and electrons inside the plasma would have moved around continuously, simply by virtue of existing at a finite temperature. And, like any charged particles in random motion, they would have created random magnetic fields – which would have cancelled each other out. Nevertheless, it was the finite variance of the resulting magnetic “fluctuations”, says Schlickeiser, that subsequently led to the creation of a permanent magnetism across the universe.
To work out the field-strength variance of the fluctuations, Schlickeiser used a theory he developed in 2012 with Peter Yoon of the University of Maryland. The fluctuations are “aperiodic”, which means that, unlike the variations in magnetic and electric fields that give rise to electromagnetic radiation, they do not propagate as a wave. Indeed, their wavelength – the spatial distance over which the fluctuations occur – and their frequency – dictating how long these fluctuations last – are uncorrelated, in contrast to light, for which the values of wavelength and frequency are tied to one another via the wave’s velocity.
Much weaker than a fridge magnet
Schlickeiser summed over all possible wavelengths and frequencies for the magnetic fluctuations in a gas at 10,000 K, which would have been roughly the temperature of the proto-interstellar medium at the time of reionization. The calculation revealed field strengths of about 10–12 G inside very early-stage galaxies and around 10–21 G in the void surrounding the galaxies. These values compare with the roughly 0.5 G of the Earth’s magnetic field and the 100 G typical of a strong refrigerator magnet.
Schlickeiser points out that he is not the first person to put forward a seed mechanism for the interstellar magnetic field. Indeed, as far back as 1950 the German astronomer Ludwig Biermann proposed that the centrifugal force generated in a rotating plasma cloud will separate out heavier protons from lighter electrons, thereby creating a separation of charge that leads to tiny electric and magnetic fields. According to Schlickeiser, however, this scheme suffers from a lack of suitable rotating objects, meaning that it could only ever generate the magnetic fields in a small portion of the interstellar medium.
Observational evidence needed
Schlickeiser’s next step is to find observational evidence to back up his idea. One option, he says, would be to look at the cosmic microwave background, the very faint long-wavelength radiation that fills the universe and which was emitted about 400,000 years after the Big Bang, when electrons and protons had cooled to the extent that they could combine via mutual attraction and leave photons to propagate freely through space. The idea would be to measure variations in the polarization of this radiation, which could be done using data from the European Space Agency’s Planck satellite, given that magnetic fields rotate the plane of polarization of electromagnetic waves. “It is not clear at the moment whether these fluctuations would have measureable effects on the background radiation,” he says. “But I think it would be worth finding out.”
Massimo Stiavelli of the Space Telescope Science Institute in Maryland is positive about the latest work, arguing that “the mechanism described could indeed provide the seeds to primordial magnetic fields”. And he suggests an alternative line of evidence, from before reionization – that any magnetic fluctuations would have tended to fragment the universe’s second generation of stars as they formed. “Finding somewhere in the local universe a small-mass star with a magnetic field and primordial chemical composition would provide evidence that a mechanism like the one described was at play,” he says.
If you are looking for a nice, relaxing job that is reasonably well paid with excellent job security, then university professor is the career for you. At least that is according to a new ranking exercise on the website careercast.com, which names “university professor” as the least stressful job of 2013 – followed by seamstress/tailor, then medical-records technician. The survey is based on criteria such as “physical demand” and “deadlines”, and is part of a more extensive categorization of the best and worst jobs that will be released in April.
Since the list was published last week there has been a mighty backlash from some members of the academic community, who feel their working life has been falsely characterized. A large dose of this anger was directed at this article in the magazine Forbes, which gleefully endorsed the results. Forbes journalist Susan Adams described the life of an academic with several gems, including “Working conditions tend to be cozy and civilized and there are minimal travel demands, except perhaps a non-mandatory conference or two.” However, after the article appeared, it received so many comments from disgruntled academics that Adams felt moved to write an addendum to reflect these sentiments and to clarify her position.
Let us know what you think about the debate by taking part in this week’s Facebook poll.
Do university professors have one of the least stressful jobs?
Yes No
Please feel free to explain your answer by posting a comment on the poll.
In last week’s poll we asked you a question that involved a scientist whose fame now extends far beyond his academic research. We asked whether Stephen Hawking’s appearance in a recent advert for a price-comparison website was good for the communication of science. In the advert, Hawking is seen to create a black hole on a UK high street to destroy the comedy character known as Gio Compario. The poll was tightly contested, with 46% of respondents saying yes the advert is good for science communication, and the remaining 54% saying no it is not.
Of course, it was a very open question, so the poll attracted many comments. “It helps raise the profile of scientists in a jokey way. More and more people are now familiar with Hawking, Jim Al-Khalili and Brian Cox as TV personalities, and are enjoying and benefiting from their appearances on TV,” wrote Paul Londale. Another commenter, Raul Raúl, also has no qualms with Hawking taking part in the advert. “Isaac Asimov, a PhD in biochemistry and icon science-fiction writer and science popularizer, used to advertise IBM PC machines in late 1980s. So, let them do it,” he wrote.
Thank you for all your participation and we hope to hear from you again this week.
Pop physics: can you explain fermion identity better than using twins occupying the same space? (Courtesy: iStockphoto/Christophe Delvallé)
Of all features of the quantum realm, the forms that particles take are surely among the most bizarre. There are only two possibilities: identical objects that can mash together (bosons), and identical objects that cannot (fermions). Bosons obey a mathematics called Bose–Einstein statistics, while fermions follow Fermi–Dirac statistics. These two possibilities were found independently in 1924–1925 with Satyendra Nath Bose and Albert Einstein discovering the properties of bosons, and Wolfgang Pauli’s exclusion principle articulating the basic behaviour of fermions.
In 1926 Paul Dirac synthesized these possibilities in a mathematical framework. Crudely, if you exchange two identical objects and the “phase factor” of the wave function is the same, or “symmetric”, the objects can occupy the same quantum state and physical position and are bosons. (A group of bosons falling into the lowest possible energy state creates a Bose–Einstein condensate.) If the phase factor is negative, or “antisymmetric”, the objects cannot occupy the same position and are fermions.
The mathematics of the quantum world restricts objects to these two possibilities, which means that bosons and fermions play vastly different roles in the quantum realm. Bosons serve as force-carrying particles – photons, for example, are particles corresponding to electromagnetic forces. As for fermions, Pauli’s exclusion principle and the uncertainty principle regulate the atom’s energy levels, which determine the chemical properties of elements and the structure of matter.
Bosons and fermions are familiar to physicists. But writing in the journal Philosophy of Science in 1944 – the year before Pauli won the Nobel prize – the physicist and philosopher of science Henry Margenau said it was “strange” that there was “so little discussion of the exclusion principle in the philosophical literature”. He could have added that there was scant presence of it – or of Bose–Einstein condensation – in popular culture either.
Popular culture often exploits quantum weirdness, finding such terms as quantum leap, complementarity, superposition and parallel worlds packed with creative force. US President Barack Obama has even invoked Heisenberg’s uncertainty principle to explain his occasional reluctance to confront advisers directly about their opinions, while his recent opponent – former governor Mitt Romney – was satirically labelled a “quantum politician” for the way he articulated randomly fluctuating views, held superposed positions and asserted complementary statements on subjects depending on the context.
Yet the Pauli principle and Bose–Einstein condensation are all but absent from popular culture. Identity is the quantum shoe that hasn’t dropped.
But why not?
Tag-ability
Classical mechanics depends on the distinguishability of even primal bits of matter. In other words, you could, if you wanted, put tags on things and follow them around. Sure, atoms and molecules of the same type were thought to be identical, but each one was in principle “tag-able”. Those scientists, such as James Clerk Maxwell, who stopped to ponder the marvel of a universe full of identical yet distinguishable objects, had no explanation. Some cosmic factory had produced perfect duplicates of atoms, and all Maxwell could figure was that God was somehow responsible.
But one hint that something might be amiss came with a puzzle uncovered in the late 19th century by the American scientist Josiah Willard Gibbs. He imagined two adjacent boxes containing equal amounts of a gas at the same temperature. What happens, he asked, if a partition between the two boxes were removed? The molecules mix, of course. Indeed, if you label each molecule by a different number – even numbers, say, for molecules from the first box and odd numbers for molecules from the second – the evens and odds start together and eventually mix uniformly.
As an irreversible process, this should – according to classical physics – increase entropy. But because the entropy actually stays the same, the molecules must therefore be indistinguishable. In other words, they are different from all macroscopic objects. But until the appearance of the quantum – which early on was recognized to imply indistinguishability – nobody knew what to make of this puzzle.
The critical point
Both versions of quantum identity are bizarre. On the macroscopic level, fermion identity is like fraternal but non-identical twins being able to co-inhabit a space. Boson identity is like that old vaudeville gag of a crowd of people emerging from a small space like a phone booth – except that an infinite number of people could do so.
Given our cultural obsession with cloning and identity, why has popular culture, which celebrates the bizarre, not noticed quantum identity? Why isn’t it on T-shirts or coffee mugs, or mentioned in TV shows that appeal to geeks?
It is true that the exclusion principle seems almost commonsensical – a quantum extension of the classical reality that no two things can be in the same place at the same time; it may seem, that is, simply like saying that you cannot do on the microscopic scale what you cannot do on the macroscopic scale. Yet this misses the weirdness that what cannot be in the same place at the same time is something identical.
There must be more to it. Non-scientists often apply scientific language to experiences for which ordinary words are inadequate. So does the absence of quantum identity in popular culture signify our satisfaction with our present language of identity? Or have our imaginations shrivelled? More positively, what kinds of experiences do we humans have that might cry out for metaphors involving quantum identity?
Here, then, is a challenge for readers. Can you devise a situation drawn from the everyday world in which the phrases “quantum identity”, “Pauli exclusion”, Bose–Einstein behaviour”, or some variant of those phrases, would be poetic, enlightening or simply meaningful? I will write about responses in a future column.
The tablet revolution began on 27 January 2010, when Apple’s then chief executive, Steve Jobs, stood in a packed lecture hall in San Francisco and unveiled “a third category of device”. With the arrival of the iPad tablet, technology finally caught up with science fiction – keyboards, mice, printers and disk drives had all been replaced with a simplified touchscreen interface and a wireless network connection. Apple’s device quickly gained competitors, as tablets’ portability and ease of use made them an instant favourite among people who use their computers mainly for e-mailing, browsing the web, watching films and playing games. It took a little longer to adapt more serious desktop tools such as word processors and spreadsheets for use with a simplified touchscreen interface, but before long, almost all common desktop applications had tablet siblings.
There was, however, one major exception: LaTeX. Despite impassioned pleas from the academic world, the typesetting program used by tens of thousands of mathematicians, physicists and other scientists seemed to have been left out of this technological revolution. That began to change in September 2012, when two native LaTeX “apps” finally made it to the iPad – one of which, Texpad, was the result of a year-long development project for me and my business partner, Jawad Deo. However, the new apps still lack many of the packages and tools that users have come to expect from LaTeX, so although LaTeX has now joined the tablet revolution, it continues to lag behind. Why?
Stuck in the sandbox
LaTeX is an offshoot of a typesetting program called TeX, which was introduced by the computer scientist Donald E Knuth in 1978. In Knuth’s words, it was “intended for the creation of beautiful books – and especially books that contain a lot of mathematics”. LaTeX was born when a set of extension packages for TeX was released in the early 1980s. TeX and LaTeX differ from word processors such as Microsoft Word in that they replace cumbersome equation editors and layout options with a simple “typesetting language” (figure 1). For example, to make boldface text in LaTeX, you just place the text within the brackets of a “textbf{ }” command. Many common mathematical symbols are particularly easy to create: inserting an integration sign requires the command “int”, and the command for a subscript “s” is simply “_s”. Newcomers to LaTeX face a steep learning curve, but once they have become accustomed to the program’s high-quality typography, they rarely switch back.
1 Beautiful mathematics This block of LaTeX “code” (left) and the corresponding typeset document (right) shows how LaTeX produces several features of mathematical textbooks, including equations, lists and numbered sections. Although it takes a while for beginners to grasp the program’s structure and master its commands, LaTeX is more flexible than the “what you see is what you get” equation editors used in word processing programs, and most users feel that the results are worth the effort.
In the years since its creation, TeX migrated painlessly from mainframe to desktop to laptop, so it is not obvious why it is taking so long to progress to tablets. Nothing about the TeX typesetting language is incompatible with a touchscreen interface. Instead, the obstacles lie below the tablets’ sleek metal-and-glass exteriors, in the operating systems powering the tablet revolution.
A traditional desktop operating system has a single file system that is visible to the user and accessible by all installed software. This architecture makes it possible for a malicious or malfunctioning program to “run loose” and attack other files, but it also allows a single task to be distributed across multiple applications and tools. LaTeX typesetting is a great example of this distributive process. For example, to produce the first draft of this article, I typed it in the desktop version of our Texpad LaTeX editor, which passed the data first to LaTeX, then to a program called dvips that converts files from one format (DVI) to another (PostScript), then to the ps2pdf PostScript to PDF converter, and finally back to Texpad for display (figure 2).
2 Step by step Producing a LaTeX document is a multi-step process that requires an editor (such as Texpad) to share information first with the typesetter, LaTeX, then with conversion programs such as dvips and ps2pdf.
Tablet operating systems such as those found on the iPad and Android-based tablets, however, force all software to be self-contained: every application is restricted to a subset of the file system and hardware. This subset is called a “sandbox” and it acts like a virtual cage within which an application’s files are both contained and protected from other applications. Just as cages ensure that zoos don’t consist of a large pile of animal carcasses and one well-fed lion, sandboxes create a safe environment for the user where any single malicious or malfunctioning application is limited to damaging its own files.
This is great for security, but it inhibits software such as LaTeX, which relies on a co-operative style of software architecture. If a tablet LaTeX editor needs to communicate with a tablet LaTeX typesetter, then it must include that typesetter within its sandbox, along with all the tools and packages the typesetter requires. iOS, the iPad’s operating system and as such the most widely used tablet operating system, goes even further and requires all software to be packaged as a single program. In almost every case this makes sense, but it is a headache when you consider the four different programs in the simple typesetting example described above.
Consequently, for the iOS version of Texpad we combined the editor, LaTeX, a bibliography program called BibTeX and a basic set of LaTeX packages, all in a single program. Contrast this with a desktop installation where these components would be all be distributed, updated and operated as separate entities.
A complex ecosystem
The problem with packaging LaTeX in this monolithic manner is that after nearly 35 years, the TeX ecosystem consists of a mind-boggling number of packages, and almost every day we get an e-mail from a user asking us to add some esoteric package or tool. The youngtab package is a perfect example: it allows you to draw collections of boxes or cells called Young tableaux in your LaTeX document, and although it is indispensable for some mathematicians and theoretical physicists, it is useless for the majority of users. For now, we are happy to keep expanding what we offer, but we rarely get the same request twice, and if we added every package our customers want, our app would outgrow a tablet’s limited storage space. For example, the current edition of the most widely used distribution of LaTeX, TeX Live 2012, consumes 4 GB of space on my hard disk. Although tolerable on a laptop, that equates to almost a third of my iPad’s usable space – an unacceptable size for an application. The bloated nature of LaTeX also slows down the typesetting process. While a web browser can lay out a large document almost instantaneously, typesetting even the simplest document on my laptop takes 2 s, and my PhD thesis took well over a minute. This isn’t a problem on a fast desktop computer, but on a low-power, battery-conscious tablet, it can be.
The biggest problem for developers of a tablet-friendly LaTeX is not the number of LaTeX tools and packages, or even the sandboxed tablet operating systems, but the fractured nature of TeX itself. To illustrate this, suppose an experimental physicist is writing a paper, and she wants to include both a diagram and a photograph of her experiment. How should she do this? One common choice of package for drawing diagrams in LaTeX is PSTricks. However, PSTricks only works with the LaTeX/dvips/ps2pdf chain of typesetting tools described above, and her photo is in JPEG format, which will not typeset with that chain. JPEGs will typeset with a different version of LaTeX, known as pdfLaTeX – but PSTricks, of course, won’t.
To get around this apparent impasse, our experimental physicist could re-draw her experiment using a different package that is compatible with pdfLaTeX, or she could use a package to convert her photo to a format acceptable to the original typesetting chain. I don’t want to bore you with all her other options, but on the version of LaTeX currently running on my laptop, I count a choice of six typesetting chains, two diagram packages, and innumerable tools to knit all these choices and their incompatible file formats together.
The tangled web
Why, you may ask, is TeX structured in such a tortuous fashion? After all, it was written by Knuth, a man who is held in the same esteem among computer scientists as Richard Feynman is among physicists. When Knuth began TeX in the 1970s he started from scratch. There was no font system suitable for TeX, and no suitable file format for the final typeset product either. Knuth’s solution was to create a new font system, Metafont, and his friend David Fuchs created a new document format, DVI. All of this was written in WEB, a programming system Knuth also created. The resulting system was so far ahead of its time that it took 20 years for a general-use font format (OpenType) to emerge with support for all the typographical tricks supported by TeX and Metafont, such as kerning and ligatures (figure 3), and to bring these techniques into wide usage.
3 Spot the difference Certain combinations of letters, such as those shown above, are easier to read when parts of them are merged (ligatures) or the spaces between them are adjusted to account for their shapes (kerning).
Few people have the creativity, programming ability and endurance required to construct a system as capable as TeX from nothing, and Knuth deserves the renown it has brought him. He certainly has my gratitude. Nevertheless, over time, Knuth’s groundbreaking technologies aged and disappeared. DVI gave way to PostScript in the 1980s and PDF in the 1990s; Metafont did not survive the arrival of PostScript either; and as for the WEB programming system – well, as far as I know, Metafont, TeX and the occasional TeX-related tool, such as the bibliography-generating BibTeX, are the only pieces of software written in it. Even the LaTeX logo (below right) has aged. Initially, it was a demonstration of TeX’s powerful support for superscripts, subscripts and kerning. Now, it is just a hassle to have to type the extra capital letters every time.
Ups and downs The LaTeX logo in its official format, complete with kerning, ligatures and multi-level type.
In the face of all these changes, Knuth decided to preserve TeX’s core in its original state, and he only consented to alterations that fix bugs in this core code. So although DVI, Metafont and WEB usage have declined, TeX continues to produce files in the defunct DVI format – forcing developers to write external utilities to convert the DVI file to a PDF. Users also came to expect colour and images in their documents, but once again TeX was not modified; instead, extra features were embedded in the ancient DVI format as “special strings” for a different tool to interpret and render further down the pipeline.
Inevitably, a PDF-producing variant of TeX, called pdfTeX, was written. But thanks to Knuth’s prohibition against altering TeX, pdfTeX soon became a separate and competing typesetting tool. Many packages will run on both systems, but some run on pdfTeX only, and others on the original TeX only. Unfortunately, this “forking” set a pattern in TeX development, splitting not only the code but also the efforts of its community of developers. Today there are five widely used typesetting engines – TeX, pdfTeX, XeTeX, luaTeX and pTeX – as well as some less common ones, such as kerTeX.
Out of many, one?
At this point, non-LaTeX users may be wondering why anyone bothers with such a complex and clunky system. LaTeX’s longevity rests on one simple fact: it produces beautiful documents. For most users, that is all that matters. Not everyone needs to know about what is going on “under the hood”, and for those physicists who do take a peek, the intricate nature of LaTeX can be appealing. Its interlinking architecture conjures up images of a patchwork of tools running in harmony – the typesetting equivalent of Charles Babbage’s difference engine.
Man of principle? Donald E Knuth, original creator of LaTeX, lecturing at Case Western University in Ohio in 2010. (Courtesy: Dasha Slobozhanina)
However, a difference engine won’t fit in your pocket, and those of us working behind the scenes also recognize that this particular engine would spin more smoothly if it had fewer cogs. The incompatibility between LaTeX’s heritage and structure and the sandboxing on most tablet devices has merely heightened an existing problem. For us, the only solution has been to do what the LaTeX community should have done long ago: choose.
To create our LaTeX tablet app, we have selected a single typesetting engine, kerTeX, and a single file format, PDF. We have merged this with the BibTeX bibliography tool into a single software component, or “library”, that is plugged into Texpad, our editor. The result is a pleasantly snappy typesetter, and a starting point from which we are modernizing LaTeX’s internal architecture to make it both compatible with tablets and amenable to further development.
It is often said that “the path to hell is paved with good intentions” and today’s fractured LaTeX lies at the end of a long trail of well-intentioned rewrites. We are mindful that an ecosystem blighted by incompatible standards cannot be cured with another incompatible standard – even if it does come in tablet-friendly form. To avoid this trap, we are following XeTeX’s example of supporting standards rather than creating them. XeTeX (and the related XeLaTeX) have discarded the TeX-specific character encodings in favour of Unicode, an encoding system containing virtually all characters ever written – from the familiar Latin alphabet to ancient Egyptian hieroglyphics. Unicode has long since been the standard encoding in all other areas of the computer world, so having supported Unicode, XeTeX is also capable of working with modern font standards, such as the OpenType format mentioned above, rather than just the antiquated, and TeX-specific, Metafont files.
Other approaches are possible, especially on tablets based on the more loosely sandboxed Android operating system, on which a single application can consist of multiple interacting programs. As this article was being prepared for publication at the end of 2012, an Android developer, Vu An Hoa, released the TeXPortal application in which he packaged the entirety of TeXLive in its original multiple-program form within a single application sandbox. There is a great deal of effort being spent on adapting LaTeX for tablet computers, and this effort is incontrovertible proof of the importance and superiority of Knuth’s system.
As important as it is for TeX to keep abreast of changes in the computer world, the typographical quality of documents produced by the 1978 version of TeX still stands up against today’s word processors. That is why LaTeX has survived several technology revolutions already, and it is why it will also survive the advent of tablet computers.
The rate at which protons capture muons has been accurately measured for the first time by the MuCap collaboration at the Paul Scherrer Institute (PSI) in Switzerland. This process, which can be thought of as beta decay in reverse, results in the formation of a neutron and a neutrino. The team has also determined a dimensionless factor that influences the rate of muon capture, which was found to be in excellent agreement with theoretical predictions that are based on very complex calculations.
Muons are cousins of the electron that are around 200 times heavier. Beta decays demonstrate the weak nuclear force in which a neutron gets converted into a proton by emitting an electron and a neutrino. Now, replace the electron with the heavier muon and run the process backwards: a proton captures a muon and transforms into a neutron while emitting a neutrino. This process – known as ordinary muon capture (OMC) – is crucial to understanding the weak interaction involving protons.
The proton and the weak force
The proton’s interaction with the weak force is explained by the chiral perturbation theory (ChPT) – an approximation of quantum chromodynamics (QCD) applicable at low particle energies. At such energies, the weak interaction inside a proton is affected by the presence of the strong force. The strength of this weak interaction is determined by certain coupling constants, which must be experimentally established.
“Essentially, these constants represent the basic properties of the proton, and describe the fact that it is not point-like but has a complex internal structure,” says Peter Kammel from the University of Washington, Seattle, one of the physicists involved in this research. Three of these dimensionless parameters have been previously measured, but attempts at measuring the fourth, known as “pseudoscalar coupling”, provided conflicting results, until now. “Of course,” continues Kammel, “the pseudoscalar coupling constant could be calculated quite precisely by using chiral perturbation theory, which predicts a value of 8.26 ± 0.23.” In order to measure its value, the physicists had to first determine with a high accuracy the rates at which muon capture takes place.
Capture versus decay
The muons used in the study were produced by smashing protons into carbon targets at an energy of 590 MeV. These collisions produce both positive and negative pi-mesons (or pions), which promptly decay into positive and negative muons, respectively. Muons with an energy of 5.5 MeV are then fired into a MuCap time projection chamber (TPC), which contains ultrapure hydrogen gas at 10 bar.
The negative muons supplant the electrons that orbit the hydrogen nuclei to form a proton–muon bound state, while the positive muons remain free. A small fraction of the bound muons – around 0.16% – will get captured by the proton and disappear, as they form a neutron and a neutrino. All of the remaining muons, both positive and negative, will decay after about two millionths of a second into electrons and neutrinos. The decay times were calculated precisely by measuring the time between the muons entering the TPC and the electrons from the decay exiting the chamber. These decay times were then compared to the well-known free muon decay rate and the difference between the two determines the elusive muon-proton capture rate.
A total of around 12 billion decay events involving negative muons were detected, corresponding to 30 TB of raw data that were analysed. The analysis was performed blind to prevent any unintentional bias from distorting the results and, following the unblinding, the measured muon-capture rate was found to be 714.9 ± 5.4 (stat) ± 5.1 (syst) s–1.
Confirming predictions
Using this figure, the team could calculate the value of the pseudoscalar coupling constant, which worked out to be 8.06 ± 0.48 ± 0.28, consistent with the predictions of ChPT. Although experimental methods to determine the pseudoscalar coupling constant started in the 1960s, it was not until the MuCap experiment that the objective was achieved.
“The nucleon weak-interaction coupling constants played a significant role in understanding the weak and strong interactions,” continues Kammel. “The modern description of the process we have investigated is based on ideas proposed by Yoichiro Nambu, for which he won the Nobel Prize for Physics in 2008.” The approximate methods of calculation presented by ChPT agree very well with the experimental result, confirming yet another prediction of the Standard Model of particle physics.
With fully fledged quantum computers still potentially decades off, several groups of physicists around the world have found an alternative way of exploiting the processing power of quantum mechanics. They have built a relatively simple photonic device to carry out one specific calculation that is very difficult to perform using classical computers and which, they say, might demonstrate the greater inherent speed of quantum-based devices within the next 10 years.
Quantum computers process quantum bits – or qubits – which can exist in two states at the same time. This could, in principle, lead to an exponential increase in the processing speed of a quantum computer compared with classical devices. This quantum processing could be used, among other things, to rapidly factorize large numbers into their constituent primes and so break codes that are, in practice, uncrackable using conventional computers. However, many technical challenges remain for those trying to develop quantum computers and today the best that a quantum computer can do is to factor small numbers such as 15 and 21.
Some physicists believe that an intermediate quantum computer called a “boson sampling” machine could offer a shortcut to achieving the greater speed of quantum computing. This does not involve what is known as a universal quantum computer, but instead carries out one fixed task. The device consists of a network of beam splitters that converts one set of photons arriving at a number of parallel input ports into a second set leaving via a number of parallel outputs. Its task is to work out the probability that a certain input configuration will lead to a certain output.
Bosonic properties
In 2011 Scott Aaronson and Alex Arkhipov of the Massachusetts Institute of Technology showed that calculating that probability becomes exponentially more difficult using a classical computer as the number of input photons and the number of input and output ports increases. That difficulty is due to the unusual behaviour of photons, which belong to a class of fundamental particles known as bosons, any number of which can occupy a given quantum state. When two photons reach a beam splitter at exactly the same time, they will always follow the same path afterwards – both going either left or right – and it is that behaviour that is so hard to model classically. The MIT researchers found that predicting the machine’s output in fact requires the calculation of a series of “permanents” – single numbers associated with specific matrices that are similar to determinants but which are much harder to work out.
“Having to work out these permanents means that even the best desktop computers would struggle to get above about 30 photons,” says Ian Walmsley of the University of Oxford in the UK. “But a boson sampling machine instead is a kind of analogue computer that uses the physical properties of bosons themselves to work out the answer.”
Four independent groups of researchers have now backed up Aaronson’s theoretical work with experimental results. One is led by Walmsley and includes researchers from Oxford and Southampton universities. Another is an Australian/US group led by Andrew White of the University of Queensland that includes Aaronson. Both of these groups have had their results published in Science, while the other two groups – based in Austria/Germany and Italy/Brazil – have released their results on the arXiv preprint server. All groups used similar experimental set-ups based on either custom compact optical chips or commercial photonics with five or six inputs and outputs.
Injecting photons
The tests involved injecting up to three photons, or four in the case of the Oxford group, into specific individual inputs and then registering at which outputs photons emerged. Repeating this process over and over again, the researchers were able to work out what fraction of the time specific output configurations appeared, and therefore the probability that those configurations would occur.
Comparing these results with the probabilities calculated using matrix permanents, the groups found experiment and theory to be in very good agreement, once the theory had been corrected to account for experimental errors arising from photons that sometimes were not indistinguishable (which they must be) and which sometimes entered the system as pairs rather than individually. The finite time over which data were collected also limited the accuracy of the results.
“The bottom line is that we have built boson sampling machines, that they work and that the errors are not fatal,” says Walmsley.
Direction is clear
Being able to account for these errors suggests that the devices can be scaled up to the point where they overtake classical computers, according to Walmsley. The aim, he explains, is to improve the technology in order to process around 30 photons, which is generally considered to be about the limit for a present-day desktop classical computer, and then up towards 100, which would put the boson samplers in a league of their own. “To do that you need to make sure that the photons are all exactly the same, that they arrive at the beam splitters at exactly the same time, and that the detector efficiency allows you to sample enough data,” he says. “There are challenges in overcoming all of these things and that will mean a lot of work, but I think the direction we need to take is clear.”
White agrees. “I think that within a decade we can get there,” he says.
In fact, Walmsley believes that boson sampling machines “might not simply be a test exercise” for universal quantum computers and that they may, in fact, be used to compute additional, useful algorithms – although he points out that no such algorithms have been discovered yet.
However, Raymond Laflamme of the University of Waterloo in Canada is a little more cautious. He says that he read about the latest results “with enthusiasm” and believes that boson sampling “is teaching us something important about quantum information processing – that we might not need a fully operational quantum computer to have an interesting speed up from quantum mechanics”. But he is not sure the current devices really can be scaled up. “It is not clear to me that the experimental results will not be swamped by the imperfections when trying to reach 20 or 30 photons,” he says. “But on the other hand I can’t prove it, and that is the challenge for the experimentalists.”
