One of the more inspiring stories we have come across this week was the tale of a resourceful inventor in the West African nation of Togo. Kodjo Afate Gnikou has managed to build a 3D printer at the meagre cost of $100 by mainly using parts he found in a scrap yard in the capital city Lomé. The story is described oninhabitat.com, which says the machine has been constructed from broken scanners, computers, printers and other e-waste.
On the subject of 3D printing, Wired magazine ran a story about how the UK supermarket chain Asda is planning to trial a 3D printing service at its store in York. They will be offering customers the chance to take a break from their shopping to have a full body scan, which will be used to create miniature dolls of themselves. Prices apparently start at £40 and Asda boasts about how lifelike these dolls can be: “The technology produces highly realistic ‘mini me’ figurines at whatever scale you like!”
Portrait of Ada Lovelace (1838)
From a shop in York to the next story that involved celebrations all round the world. Tuesday was Ada Lovelace Day 2013. The annual celebrations, which are now in their fifth year, are held to recognize the achievements of women in science, technology, engineering and maths (STEM). The annual event was founded in 2009 by the social technologist and writer Suw Charman-Anderson “as a response to online discussions about the lack of women on stage at tech conferences”.
This year events included a mass Wikipedia “editathon” at the University of Oxford in an attempt to raise the profile of women’s contributions to science, as described in this article in the Guardian.
The most distant gravitational lens yet has been found at a colossal distance of 9.4 billion light-years. The chance discovery by an international team of astronomers not only allowed the team to directly measure the mass of the distant galaxy that caused the lensing but has also led to questions about the more distant object whose light was lensed. The magnified object is a type of dwarf galaxy that is thought to be rare and the chances that such a peculiar galaxy would be gravitationally lensed are small. Therefore its observation suggests that current theories have underestimated the number of such galaxies in the early universe.
A gravitational lens is a large galaxy or group of galaxies that bends or “lenses” light from a distant source as it travels towards an observer. The effect was predicted by Einstein’s general theory of relativity. In rare cases the lens, the distant light source and the observer line up precisely and the result is an “Einstein ring” – a perfect circle of light around the lensing mass. However, if there is any misalignment along the way, astronomers observe partial arcs, spots and other such distorted images, depending on the relative positions of the bodies.
Useful rings
Such a lensing effect has proved to be useful – researchers can determine the mass of the lensing galaxy including its dark matter content – thanks to the amount of distortion or lensing observed. The lensing effect also acts as a “natural telescope” of sorts, magnifying details of distant galaxies that would be difficult to observe otherwise. Indeed, ever since the first evidence of such lensing was seen in 1979, astronomers and cosmologists have used the phenomenon to find distant objects and supernovae and to even map the dark-matter content of our universe.
Now, a team led by Arjen van der Wel from the Max Planck Institute for Astronomy in Heidelberg, Germany, along with colleagues in Italy and the US, has, rather accidently, detected the furthest such lens. Van der Wel was reviewing observations made with the Large Binocular Telescope in Arizona that were part of another study that looked at the spectra of massive, old galaxies.”[I] noticed a galaxy that was decidedly odd. It looked like an extremely young galaxy, and at an even larger distance than I was aiming for,” says Van der Wel. Intrigued by the anomalous object, he looked at other images of the object taken with the Hubble Space Telescope as part of the CANDELS and COSMOS surveys and once more the object looked like an old galaxy but with some irregular features. As a result, Van der Wel suspected that he might be looking at a gravitational lens. He combined all the available images of the object and corrected for the haze of the lensing galaxy’s stars to see a “quadruple lens” that formed an almost perfect Einstein ring.
The researchers found that after being deflected, light from the lens travelled nearly 9.4 billion years to reach us, corresponding to a redshift of z = 1.53. This puts the lens much further away than other lenses discovered to date. From the amount of distortion observed, the researchers calculated that the lens galaxy has a total mass of 8 × 1010 solar mass. From that, nearly 75% of this is made up of stars, meaning that the rest of the mass could consist of dark matter. But Van der Wel explains that uncertainties are such that all mass in the lens can be accounted for by stars only.
Hitting the bullseye
Seeing an Einstein ring also means that both the lens and the background light source are aligned to better than 0.01 arcseconds – that is equivalent to a 1 mm separation at a distance of 20 km. Van der Wel tells physicsworld.com that this was indeed a rare alignment – “If the light rays are darts thrown in New York City, then they managed to hit the bullseye of a dartboard in Boston. Now, the universe throws many darts around (there are many background galaxies) and has many dartboards (many potential foreground lenses), but not that many. The chances of hitting a dartboard are not so small, but the chances of hitting the bullseye…” exclaims Van der Wel.
But the distance and fortuitous alignment were not the only surprises from this study. The background light-source galaxy (even more distant at z = 3.41) itself proved to be a “star-bursting dwarf galaxy”. This is a comparatively low-mass galaxy (only about 100 million solar mass worth of stars) that is extremely young (only about 10–40 million years old) and produces new stars at an enormous rate. Such dwarf starburst galaxies are thought to be rare and the chance of these peculiar galaxies being lensed is small. Yet this is the second starbursting dwarf galaxy found to be lensed. This might force astronomers to re-think their models of galaxy evolution as starbursting dwarf galaxies might be much more common than previously thought. “Perhaps only one in a hundred faint galaxies is a starbursting dwarf galaxy. That combined with the chance alignment makes this a very unlikely object,” says Van der Wel.
Raman images showing the presence of three different molecules. (Courtesy: Max Planck Institute for Quantum Optics)
Frequency combs have been used by physicists in Germany and France to improve the performance of Raman spectroscopy – allowing the technique to identify several different molecules at the same time. The researchers say that it could be used to speed up the microscopic mapping of chemical species in a sample or be used to follow chemical reactions in real time. However, the new analytical method must be improved before it can be used routinely in the lab.
In coherent anti-stokes Raman spectroscopy, a sample is irradiated repeatedly by pairs of ultrashort laser pulses. The first pulse sets chemical bonds in the molecules vibrating, which causes the refractive index of the sample to vary periodically. This change in refractive index modulates the frequency of the second pulse. The gap between the two pulses is increased slightly with each subsequent pair, so the second pulse sees a slightly later point in the bonds’ vibration periods and its frequency is shifted by a slightly different amount. By looking at the sinusoidal variation in frequency shift with time lag, scientists can work out the vibration frequencies of the chemical bonds. As each type of molecule has a distinct signature of vibration frequencies, Raman spectroscopy can identify the presence of specific molecules in a sample.
Time consuming
The pair of pulses is usually generated from a single laser pulse that is split into two. One pulse is sent on a longer path, causing it to hit the sample slightly later. Changing the path length of that pulse varies the time lag. This works well if the aim of the measurement is to detect the presence of a known molecule. However, if there are several different target molecules – or if the goal is to identify unknown molecules – the entire measurement process must be repeated at multiple frequencies and this can be time consuming. The technique becomes more unwieldy if it is used to create a 2D image by scanning the light across a sample and acquiring Raman spectra at each pixel.
Now, researchers at the Max Planck Institute for Quantum Optics and Ludwig Maximilians University (LMU) and colleagues at University Paris-Sud have used frequency combs to get around this problem. Frequency combs produce an extremely rapid and stable series of femtosecond laser pulses, each containing a broad and highly regular spectrum of frequencies. Theodor Hänsch – the leader of the LMU and Max Planck groups – shared the 2005 Nobel Prize for Physics for inventing the frequency comb.
A thousand times faster
The team used two frequency combs, the second pulsing at a slightly slower rate than the first. As a result the combs moved in and out of phase, generating a series of pairs of pulses with a time gap that changes slowly. Because the technique does not involve the mechanical adjustment of a path length, it can acquire a single Raman spectrum about 1000 times faster than the traditional method. Furthermore, as each pulse contained a broad spectrum of frequencies, they could excite multiple bond vibrations simultaneously. Mathematical analysis of the complex relationship between the refractive index and the time lag between the pulses told the researchers what sinusoidal frequencies the pattern contained, and thus what chemical bonds – and therefore chemical species – had given rise to it.
There is a catch, however. The bond vibrations die out in picoseconds, so once the gap between the pulses has grown longer than this, the second pulse does not see any variation in refractive index at all until the time gap returns again to smaller values. “One of the limitations of our technique at the moment,” says team member Nathalie Picqué, “is that we can measure one full spectrum covering the entire fingerprint region [the electromagnetic region in which the bond vibrations occur] very quickly, but the time from one spectrum to the next is rather long.” The researchers plan to reduce this by using frequency combs that pulse more rapidly. Indeed, combs producing up to a billion pulses per second are available now and even faster combs are being developed.
Elegant combination
“Most of the elements here were done before,” says Yaron Silberberg of the Ultrafast Optics Group at the Weizmann Institute of Science in Israel. “People have used short pulses for Raman microscopy, and also people have used frequency combs for doing all kinds of tricks, but the combination is very elegant. They’ve combined two things and made you think, ‘Hey, how come nobody did this before?'”
Picqué hopes that the research will have applications beyond microscopy. “You could, for example, imagine that you would like to monitor a chemical reaction as a function of time. So you would want to measure the spectrum every ten microseconds in order to see how some product develops.”
We look at the rise of quantum computing in this second film from our series about technology spin-offs from fundamental physics research. Such devices, which would exploit superposition, entanglement and other quantum phenomena to perform super-fast calculations, have the potential for some amazing feats. But in the pursuit of a practical quantum computer, big challenges still remain.
The film was recorded at the University of Sussex in the UK where physicists are developing an approach known as ion trap quantum computing. “A normal computer has bits and these bits encode information, so numbers and words are encoded into bits which are zero or one,” explains Winfried Hensinger, head of the Ion Quantum Technology Group. “In a quantum computer, instead of having bits you have quantum bits, which can be in a superposition of zero and one – so they can be zero and one at the same time.” In the film, Hensinger provides a tour of his lab, describing how his group cools atoms down to temperatures approaching absolute zero, then traps them inside vacuum systems.
This series of films is concerned with technologies that have the potential to transform lives and societies. In the case of quantum computing, some of the most exciting applications might lie in the fields of biology and biochemistry, an idea that is explored in the film by the science writer John Gribbin. “When we have a quantum computer, we will be able to make very accurate computer models of processes involving chemical reactions, up to and including biological reactions,” he explains. “We will much better understand how proteins work, how genes work, and that will have tremendous implications in developing both the chemical and the biochemical technology of the future”.
This is a view shared by Laurence Pearl, a structural biologist at Sussex, who believes that the development of quantum computers is likely to attract the attention of pharmaceuticals companies, which could use them in the development of new drugs. “If it’s going to cost you £10 million to predict the drug that was going to work rather than a billion to actually develop the drug that’s going to work, I think drugs companies would go for that,” he says.
This film is one of a three-part series exploring some of the most promising technologies that are emerging from physics research. You can read about other physics spin-offs by reading the free special 25th anniversary issue of Physics World in our digital magazine or app.
Talk about going down a rabbit hole. There I was bent nearly in half, not quite on my knees, crawling through a narrow tunnel barely four feet tall that sloped down at a steep angle. Donning blue-grey overalls, waterproof boots and a hard hat with a miner’s lamp, I trod carefully to avoid catching myself on the sharp bits of rock protruding from all sides of the poorly lit, claustrophobic cavity. The sweltering heat and stifling humidity made breathing a chore, but I was not going to complain. It was a privilege to join Tullis Onstott and Maggie Lau of Princeton University, and Tom Krieft of the New Mexico Institute of Mining and Technology, on this scientific adventure. Nearly two kilometres underground, we were deep inside an active gold mine located near Johannesburg, South Africa. Our mission: to collect samples of ground water seeping through cracks in the bedrock, which Onstott’s team would later examine for living organisms that thrive where the Sun never shines.
In deep places of the Earth such as these, Onstott’s team and others have identified varieties of bacteria that challenge what we thought we knew about biology. Rather than relying directly or indirectly on photosynthesis, they instead feed off hydrogen gas and exist in underground ecosystems that have been totally disconnected from the biological cycles on the Earth’s surface for possibly tens of thousands of years. In 2011 Gaetan Borgonie from the University of Ghent in Belgium and his colleagues spotted roundworms (nematodes) living kilometres below ground level in several South African mines – the first multicellular organisms to be recovered from such depths. These discoveries have extended the biosphere of our planet considerably – and added to its biomass. But more interesting still, they might even provide clues to the biology of the early Earth before the evolution of photosynthesis, or to the nature of life on other worlds that have a different atmospheric make-up from our own.
Extreme beings
Organisms found in the deep subsurface of the Earth are among the many so-called “extremophiles” that scientists have come across over the past few decades. Others include microbes that live close to volcanic vents on the ocean floor, or on salt flats near the Red Sea. Yet more are found beneath the permafrost of the Canadian Arctic, within parched soils of the Atacama Desert in South America and even at the edges of the stratosphere. The very existence of these creatures affirms that life is a hardy phenomenon, capable of adapting to a remarkable range of environmental conditions.
Still, despite their magnificent and bewildering variety, all of these organisms are intimately connected to each other: they share the same biochemistry, inhabit the same evolutionary tree and trace their origins to a common ancestor that probably existed over three billion years ago. But to date, scientists have not uncovered a “shadow biosphere” on Earth, comprised of a radically different sort of life. Nor have they found compelling evidence of extraterrestrial life – yet.
Tiny hints Extremophiles can survive conditions lethal to most life forms on Earth. This tardigrade, for example, can survive at temperatures near absolute zero, at pressures six times those on the deepest ocean floor, and even in the vacuum of space under cosmic radiation. Their existence could give clues to life on other worlds. (Courtesy: Eye Of Science/Science Photo Library)
What researchers have done is to confirm that the ingredients of life, as well as potential habitats, exist beyond the Earth and are ubiquitous in our cosmic neighbourhood. Laboratory measurements show that amino acids – building blocks of proteins – are common in meteorites and comets. Some carbon-rich meteorites even contain components of DNA called nucleobases. Astronomical spectroscopy at optical, infrared and radio wavelengths has revealed a number of complex organic molecules in interstellar gas clouds – the birth sites of stars and their planetary retinue.
Closer to home, our neighbouring world Mars remains a prime target in the search for life beyond Earth, with growing evidence of past water flows raising the prospect of habitability sometime in its history. Likewise, the big moons of Jupiter and Saturn, especially those that might harbour subsurface oceans, continue to intrigue us.
Beyond our solar system
In recent memory, the most dramatic development in the quest to understand our place in the universe has been the identification of thousands of planets orbiting stars other than the Sun, known as extrasolar planets, or exoplanets. Using ground-based telescopes and spacecraft such as NASA’s Kepler observatory, astronomers commonly find such alien worlds by measuring a star’s wobble as unseen planets tug on it, or by registering a star’s periodic dip in brightness as a planet transits in front of it. That is a big change from merely 20 years ago, when we were certain of just one planetary system – our own. The pace of discovery has been astounding and the incredible diversity of worlds has surprised us many times over.
What is more, thanks to a suite of remarkable new instruments, we have taken the temperature of distant planets, espied water in their atmospheres and even captured the first direct pictures of alien worlds. A number of “super-Earths” have been found already – those more massive than Earth but less so than our ice giants Uranus and Neptune – and astronomers expect to find Earth-sized planets by the dozen within the next few years. Some of these will likely be in the so-called habitable zone, where the temperatures are just right for liquid water. That will inevitably bring questions about alien life to the fore. But detection will not come easy. It will take a new generation of telescopes to pin down molecules that we associate with life – such as oxygen, ozone, methane, water and carbon dioxide – in the atmosphere of a distant terrestrial world. Even if and when we succeed in identifying such telltale signs of life, we probably will not know for a while what sort of creatures might inhabit that world.
The Earth is special among its siblings in the solar system as the only planet with surface oceans and life on a planetary scale. However, it seems absurd, if not arrogant, to think that ours is the only life-bearing world in the galaxy, given hundreds of billions of other suns, the veritable cornucopia of planets and the apparent abundance of life’s ingredients. It may be that life is fairly common, but “intelligent” species are not. In any case, as the history of science has proven time and again, generalizing from a single instance often leads to misguided, if not dangerous, conclusions. So we will have to find at least one other example of life elsewhere before we can discern what is and is not unique about life on this precious bit of reformed cosmic debris.
Fantastical knot-like structures of light could soon be created in the lab thanks to calculations made by physicists in the US, Poland and Spain. They have discovered a new family of solutions to Maxwell’s equations that are knots of light that do not disperse or lose their specific topological properties as they propagate. The researchers say such knots, if made for real, could be used to trap atoms or create similar knots in plasmas or quantum fluids.
Identified by Hridesh Kedia at the University of Chicago, along with colleagues at the Polish Academy of Sciences in Warsaw and the Spanish National Research Council in Madrid, the new family of solutions to Maxwell’s equations have field lines describing all “torus knots” and “links”. Torus knots are those knots that can lie on the surface of a torus, whereas a link is a collection of such knots.
One solution involves magnetic-field lines that trace out a familiar “trefoil” knot around a torus that is aligned in the plane perpendicular to the direction of propagation of the light (see figure). As the light propagates, the knot is distorted but retains the topological property of being a trefoil knot. The electric-field lines have the same structure as the magnetic-field lines but are rotated about the propagation axis by an angle that depends upon the knot. Other solutions include cinquefoil knots and linked rings.
Knotty problem
Kedia and colleagues believe that these knots could be made in the lab using tightly focused Laguerre–Gaussian beams. These beams have been created and studied extensively because – unlike most other beams of light – they carry orbital angular momentum.
If these optical knots can be made in the lab, they could have a number of scientific applications. Physicists are already exploring how focussed Laguerre–Gaussian beams can be used to trap ultracold atoms and this latest theoretical development could lead to new ways of trapping them. Firing such knots into a plasma or quantum fluid could also result in knot-like entities propagating through those materials, thereby offering new ways of studying these states of matter.
Once the preserve of mathematicians, knot theory is playing an increasingly important role in how physicists describe the behaviour of physical systems, ranging from liquid crystals to superconductors. Most of these descriptions arise from numerical simulations of complex systems, rather than the exact solution of the equations describing the system of interest.
As you may have gathered (and if not, where have you been?) this month marks the 25th anniversary of Physics World – the member magazine of the Institute of Physics (IOP).
The issue has been available in print, online and via our apps (from the App Store and Google Play) since the start of the month to all members of the IOP, but because we want to celebrate our birthday with as many people as possible, we’re now making available a free PDF download of the entire issue to members and non-members alike. The PDF doesn’t have all the great multimedia you’ll find in the online and app versions, but it is still worth checking out.
The issue looks back at some of the highlights in physics of the last 25 years and also forward to where the subject is going next. We’ve split the bulk of the issue into five sections, each with five items (five times five being 25, of course):
Science songs in London and a series of “lightning talks” in the Equadorian capital Quito are among the many events being held today around the world to mark Ada Lovelace Day 2013. The annual celebrations, which are now in their fifth year, are held to recognize the achievements of women in science, technology, engineering and maths (STEM).
The day’s namesake Ada Lovelace is often referred to as the world’s first computer programmer. Born in 1815, Lovelace was a child of the Romantic poet Lord Byron, and was raised by her mother who encouraged her daughter to develop an interest in science, logic and mathematics. Lovelace excelled and became friends with the mathematician Charles Babbage at the University of Cambridge, who had already started drawing up plans for his famous calculating machines.
Physicists have taken another step forward in exploiting light’s vast unused potential as an information carrier. Researchers in Germany have calculated how “twisted” light beams can influence electrons inside hydrogen atoms via the beam’s orbital angular momentum. While the theory has not yet been confirmed in the lab, the team says that its work could lead to the development of a new way of storing and retrieving quantum information – something that could play a crucial role in the operation of future quantum computers.
Spin angular momentum is a familiar property of electromagnetic waves and it gives rise to light’s polarization. But electromagnetic waves – and indeed matter waves – can also possess what is known as orbital angular momentum (OAM), which means that a beam’s wavefront spirals around its propagation axis (in contrast to a plane wave, whose wavefront remains at right angles to this axis). Such a beam has an undefined phase and therefore has zero intensity at its centre.
First observed in 1992, OAM-carrying beams are known as twisted beams. They can now be routinely created in the laboratory using a variety of techniques that include the use of special holograms and phase plates. These beams are used in a number of areas of research, including optical communications. Indeed, researchers have shown that information-carrying beams with different degrees of twistedness can be transmitted simultaneously through a glass fibre, so increasing the transmission capacity of the link.
Twisted qubits
More futuristically, twisted beams might be used in quantum computers. These are devices that exploit quantum mechanics to operate on qubits – superpositions of 0 and 1. The idea would be to create qubits that are superpositions of two OAM states. Writing quantum information to the computer would involve firing a laser beam made up of photons in OAM superposition at a collection of atoms, so transferring the OAM to the atoms and storing the data. Reading out those data would then involve shining a second laser beam onto the atoms, in order to encode that beam with OAM information about the atomic state.
In the latest work, Oliver Matula of the University of Heidelberg and colleagues provide a general theoretical description of the interaction between twisted light beams and atoms. To do so, they calculate the angular distribution of emitted electrons when a twisted light beam impinges on and ionizes a hydrogen atom.
The researchers found that when the atom is placed close to the centre of the beam, the angular distribution is markedly different to that which would be produced by an incoming plane wave. In other words, the beam transfers its OAM to the electrons. However, when the atom is far from the beam’s centre, it turns out that the electron distribution is similar to that produced a by a plane wave. In this case, the researchers concluded, the twisted light beam exchanges only spin angular momentum with the electron.
Bessel beams
A theoretical description of interactions between beams of twisted light and atoms had already been published in 2010 by Jordi Mompart of the Autonomous University of Barcelona and colleagues in Spain. But that work considered a more limited class of “paraxial” waves that have a small transverse momentum. The latest work is more general because it describes the effect of so-called Bessel beams, which do not spread as they propagate and have a characteristic intensity profile made up of a series of concentric rings. These beams can, in principle, have arbitrary transverse momentum. Matula’s group calculated that when close to the paraxial limit an ultraviolet Bessel beam can transfer OAM to hydrogen atoms no more than about 100 Bohr radii (one Bohr radius being about 0.1 nanometres) from the centre of the beam, while in a non-paraxial state it can do so for atoms up to 10,000 Bohr radii from the centre.
James McGuire of Tulane University in New Orleans, US, who also works on the theory of twisted-photon–matter interactions, believes that the latest work is likely to contribute to applications in areas such as quantum information. But he cautions that experimental proof of this and related theoretical research will not come easily. “We are not yet at the point of testing these calculations, although I am optimistic that this may happen within the next few years,” he says, adding that “it is unfortunate that in this field there is not a closer relationship between people working in theory and experiment.”
Intense laser pulses
Mompart also praises the latest work, which he describes as an “excellent paper”, and agrees that it will be difficult to verify experimentally. “The ionization of atoms close to the vortex of an OAM light beam requires very intense laser pulses far beyond the limits of today’s technology,” he says. However, he adds that the Extreme Light Infrastructure, being developed in Eastern Europe, will have the required intensity.
Even with this experimental demonstration under their belts, physicists would still face significant additional hurdles in exploiting such OAM transfer inside a quantum computer. For one thing, Matula points out, the current work only considers the interaction of a twisted beam with a single atom and ignores the effect of the many other atoms that would be present inside a quantum computer. In addition, he says, thought would need to be given as to exactly how such interactions could be applied to specific quantum algorithms, such as the identification of a number’s prime factors.
