By James Dacey in Mexico
Yesterday was day three of the Physics World Mexican adventure and it turned out to be a really exciting 24 hours. Matin Durrani and I visited Teotihuacan – the “City of the Gods”– located 30 miles north-east of Mexico City. We were there to witness some of the closing moments of a 15-year particle physics experiment designed to “see” inside the Sun Pyramid, the world’s third biggest pyramid by volume.
One of the “quietest” magnetic environments in the Milky Way has been unveiled at the Technical University of Munich (TUM). Built by physicists based in Germany, the US and Switzerland, the shielded chamber is claimed to be the most effective for its size, and is able to reduce magnetic fields by a factor of more than one million. It could be used to measure the charge distribution within the neutron and, ultimately, determine whether the particle has an electric dipole moment (EDM). The shield could also be used in biomedical applications such as brain scanning and treating cancer using magnetic nanoparticles.
Lab-based tests of fundamental physical parameters often require the near-total exclusion of electromagnetic disturbances. Some of the main customers of shielding apparatus are those who make high-precision measurements based on a particle’s intrinsic angular momentum, or “spin”, because this is highly influenced by stray magnetic fields. By measuring spin precession – the cycling orientation of spin in an applied magnetic field – researchers can, for example, test whether the neutron has an EDM. The Standard Model of particle physics allows the neutron to have a tiny EDM as a result of the violation of charge–parity (CP) symmetry. However, measuring a larger EDM could point to new physics that explains why there is much more matter than antimatter in the universe.
In 2006 a magnetic shield at the Institut Laue-Langevin in Grenoble, France, reduced external magnetic fields by a factor of about 10,000, allowing for an extraordinary experimental precision that could, in principle, separate two fundamental charges at a distance of less than 10–28 m. That experiment did not reveal any EDM inside the neutron, but it did place an upper limit on the parameter that has yet to be surpassed.
Improving on this limit is what motivated Peter Fierlinger of the TUM and colleagues to design a better magnetic shield. In principle, creating a magnetic shield is straightforward: a volume simply needs to be surrounded by a magnetizable material, so that any stray magnetic field lines are guided around sensitive instruments, like water flowing around a stone. In practice it is more difficult, requiring Maxwell’s equations of electromagnetism to be solved for a specific material and shape – and this requires a lot of computation.
The team’s shield consists of concentric shells of aluminium and a proprietary, highly magnetizable alloy known as Magnifer. The entire shield occupies a volume of 4.1 m3 and the shielded volume is about 1 m3. It excludes magnetic fields – depending on their frequency – by a factor of roughly one million, which equates to an internal magnetic field of about 0.5 nT (0.5 × 10–9 T). In contrast, the average magnetic field throughout the Milky Way is thought to be about 0.6 nT.
“We are, so far, the first collaboration that has realized such an apparatus with such a small magnetic field, suitable for a hundred times improved measurement,” says Fierlinger, whose group calls the shield the new “state of the art”.
While the team intend to use the shield to measure the neutron’s EDM, not everyone is convinced that it will dramatically improve the result. Werner Heil of Johannes Gutenberg University in Mainz, Germany, says that the shield should not be “overvalued”. “Who benefits from whether the residual field is now less than 0.5 nT or, say, only 1.5 nT?” he asks. “Fundamental-physics experiments such as the search for an EDM of the neutron do not work at such low magnetic fields; instead, one has to apply an additional magnetic holding field inside the shield that is about a factor of 1000 bigger than the residual field. The real challenge is to make the applied magnetic field as homogeneous as possible – and that’s not addressed in the work.”
Whatever the implications for the neutron EDM, the new shield could find applications elsewhere. Magnetic shields are also used in the detection of “biomagnetic” signals from the brain, for instance, as well as in the development of injectable magnetic nanoparticles that target cancerous cells in medicine.
“I am sure some clever people will eventually find other applications,” says Barry Taylor, a scientist at the National Institute of Standards and Technology in the US who helps to set the internationally accepted set of values for the fundamental physical constants. “I am reminded of the two following old saws: ‘Necessity is the mother of invention’ and ‘Build a better mousetrap and the world will beat a path to your door’.”
The shield is described in the Journal of Applied Physics.
A microscope that can see up to 1000 individual fermionic atoms has been developed by a team of physicists in the US. Using two laser beams, the research team traps a cloud of potassium atoms in an optical lattice, cools the atoms and then simultaneously images them. The new technique allows researchers to clearly resolve single fermions, directly observe their magnetic interactions and even detect entanglement within the ensemble.
Fermions are particles that have half-integer spin, and therefore are constrained by the Pauli exclusion principle, which dictates that no two identical fermions can occupy the same quantum state simultaneously. Fermions include many elementary particles – quarks, electrons, protons and neutrons – as well as atoms consisting of an odd number of these elementary particles. As a result, the collective behaviour of fermions is responsible for the structure of the elements in the periodic table, high-temperature superconductors, colossal magnetoresistance materials, the properties of nuclear matter and much more. Despite their importance, however, we still do not have a complete picture of strongly interacting systems of fermionic particles because they are notoriously difficult to image and study.
Researchers have been studying bosons – particles that have integer spin and can occupy the same quantum state – by cooling clouds of bosonic atoms down to temperatures near absolute zero to form a Bose–Einstein condensate and then studying their interactions. But doing the same with fermions is no mean feat – the exclusion principle does not allow two fermions to be in exactly the same state. Therefore, as more fermions are added to a system, each succesive one comes in at an increasingly higher energy, making the system very tricky to cool. Furthermore, ultracold atoms are easily perturbed by just the light from a single photon, which makes it difficult to confine atoms for long enough to obtain a clear image.
To get round these problems, Lawrence Cheuk, Martin Zwierlein and colleagues at the Massachusetts Institute of Technology have developed a microscopy technique that involves imaging the atoms with the same light that cools them. The fermions are first cooled to a temperature of just above absolute zero using standard methods, including laser cooling, magnetic trapping and evaporative cooling of the gas, until the temperature of all of the atoms is just above absolute zero. At this point the atoms settle into the wells of an optical lattice, thereby stopping any contact between neighbouring fermions and preventing them from interacting with each other. The optical lattice is located just 7 μm from the microscope’s imaging lens, and is made of criss-crossing laser beams that form an “egg carton” structure with a fermion trapped in each well.
The atoms are then cooled even more by using two lasers, each at a different wavelength. This method makes use of Raman transitions: an atom absorbs one photon, is immediately stimulated to emit another and so drops down one vibrational level in the process. The location of each of the atoms is identified by the stimulated photon that it emits as it cools. These photons are captured by the microscope lens above the lattice, and this allows the team to detect the fermion’s exact position within the lattice to an accuracy better than the wavelength of the light.
Using this method, Zwierlein and colleagues were able to cool and image more than 95% of the atoms in a potassium-40 gas cloud. The team was surprised to find that the fermions remained cold even after the imaging was complete. “That means I know where they are, and I can maybe move them around with a little tweezer to any location, and arrange them in any pattern I’d like,” says Zwierlein. To make sure that their experiment did not suffer any light-assisted losses, the researchers looked at how the atoms move around between successive images, and at the statistics of how the atoms are distributed around the lattice. The team found that it was not losing a significant number of atoms.
Chad Orzel, a physicist at Union College in the US who was not involved in the work, is impressed with the research because it opens up the possibility of using fermionic atoms to create a wider range of condensed-matter analogues. “If you look at the behaviour of bosons in an optical lattice, that’s analogous to the behaviour of superconductors, where electrons have paired up to act like bosons. But a system of fermions in a lattice is more analogous to a normal conductor, where the electrons are subject to Pauli exclusion, and you can see other fun behaviours that way.” He adds that with fermionic systems, “you can also think about using light fields to manipulate the interactions between atoms in interesting ways, and watch how particles move around”. Orzel told physicsworld.com that Zwierlein’s work is a nice addition to the cold-atom experimental toolbox. “Because the atoms are out in the open and directly imaged, you have all sorts of freedom to change parameters without needing to make whole new samples,” he adds.
The research is published in Physical Review Letters.
By Hamish Johnston
High-temperature (high-Tc) superconductivity has given hope and heartbreak in equal measure to physicists since the phenomenon was first discovered in 1986.
The hope is two-fold: that we will soon understand why superconductivity arises in this complex group of materials; and that this knowledge will lead us to a material that is a superconductor at room temperature. The former would be a triumph of the physics of highly correlated systems and the latter would spark a technological revolution.
Doing physics research costs money and today most of it comes from government funding agencies. Grant applications are reviewed by expert scientists and funding policies are shaped by bureaucrats and politicians. This inevitably leads to mountains of paperwork, and Jackson argues that this wastes valuable time that could be spent on actually doing research.
His solution is for physicists to appeal directly to the public for research money by using Fiat Physica, which he launched late last year. Jackson tells physicsworld.com editor Hamish Johnston about how crowd-funding works and describes some of the projects that have used his service. He also explains how Fiat Physica will avoid paying for crackpot research on topics such as perpetual motion.
By Matin Durrani in Mexico City
It’s one of the biggest universities in the world with several hundred thousand students, but the Universidad Nacional Autonóma de México (UNAM) is certainly not the oldest. In fact, the first person to get a degree and PhD in physics at UNAM – Fernando Alba – is still alive. Aged 95, he studied at UNAM’s Institute of Physics shortly after it opened its doors in 1939.
By James Dacey in Mexico City
When you visit an unfamiliar city, you can often discover some hidden gems by just wandering the streets with your eyes wide open. This is what happened to Physics World editor Matin Durrani and me yesterday here in Mexico City when we stumbled across the Museo de la Luz (Museum of Light) in the backstreets of the historic city centre.
Located in an old Jesuit college with a beautiful courtyard, the exhibits are spread over three floors covering a wide spectrum of themes, from human vision to the history of the theories of light. What I loved about the place is that it really did offer something for everyone. Too often I find that museums can be great for kids or great for the type of serious adult who loves to leaf through tea-stained archives. El Museo de la Luz manages to hit a sweet spot, being informative and interactive but not too whizz-bang – that is certainly not what I needed yesterday with this jetlag!
The delay in the construction of the Thirty Meter Telescope (TMT) on Hawaii’s tallest mountain, Mauna Kea, is continuing to cause turmoil within the astronomy community. First, the Office of Hawaiian Affairs (OHA) Board of Regents announced in April that it had withdrawn its support for the telescope. Then, last month, an e-mail forwarded to some 200 astronomy faculty, researchers and students sparked outrage when it claimed that the telescope was being “attacked by a horde of native Hawaiians”.
Construction of the TMT – featuring a primary mirror 30 m across that will be housed in a structure 66 m wide and 56 m tall – had been halted in early April, following protests by native Hawaiians. Mauna Kea is currently home to 13 telescopes, and TMT supporters maintain the newest and largest observatory will be constructed with care for the environment and the mountain’s cultural importance. TMT members say they have obtained all of the necessary permits for the observatory, and that they have the legal right to proceed. But indigenous Hawaiians claim that Mauna Kea – their spiritual and cultural pinnacle – is being desecrated, and a growing number of astronomers are now at odds with the project, too.
On 20 April, the situation became tense when an e-mail by University of California astronomer Sandra Faber to a group of astronomers – which was then forwarded to 200 astronomy faculty, researchers and students – sparked outrage. In the e-mail, Faber stated that the TMT is “in trouble, attacked by a horde of native Hawaiians who are lying”. Faber has since apologized.
On 6 May, Megan Urry, president of the American Astronomical Society (AAS), released a statement in which she underlined the diversity in the astronomical community. “I tell all of you, very clearly,” she wrote, “that racism is unacceptable, that referring to groups as monolithic is not acceptable, and that the AAS is firmly committed to an inclusive, welcoming, professional environment.” Urry added “Astronomers may have a range of opinions and perspectives on various matters, but we speak as one on the principle of respectful discourse at all times.”
Urry’s statement came a week after the OHA Board of Regents voted to rescind its 2009 decision to support Mauna Kea as the site of the TMT. The OHA is a public agency that is responsible for improving the well-being of native Hawaiians. The decision on 30 April, which followed some four hours of testimony, is a neutral stance because the board could have voted to oppose the construction – indeed, some were upset that it did not do so. The OHA board says that the neutral stance provides it with a bargaining chip of sorts in future negotiations regarding how Mauna Kea is used.
By Matin Durrani in Mexico City
I don’t know about you, but my trick whenever flying halfway across the world is to shoehorn myself as fast as possible into the new time zone I’m in. Having travelled from the UK to Mexico City with my colleague James Dacey yesterday, that tactic seems to have worked…so far. After staying up till midnight following a mini-feast of fabulous spicy tacos at a nearby restaurant while a thunderstorm broke, I woke up on cue at 7 a.m. as dawn broke in one of the biggest urban areas in the world.
We’re both here to gather material for a Physics World special report on physics in Mexico, which is due out in September. Following fast on the heels of recent reports on India, Brazil, Korea, India (again), Japan and China, the report will shine a light on some of the exciting physics research going on in the country and highlight some of the challenges and opportunities the country’s physicists face, too.
Imagine that you were to visit a distant planet and find its surface blanketed with sophisticated machines. These machines sense and respond to their environment, diagnose and repair themselves, and create their own fuel from their surroundings. As a technology-savvy earthling, you would be incredulous to learn that the only value seen in such machines was to grind them up, process and eat them – yet this is precisely what we do every day. Plants on Earth possess all of these diverse functions and more, but only now are we beginning to consider the potential of plants for new technologies.
The nascent field of “plant nanobionics” seeks to harness known properties of plants to augment or reinvent human technology. By treating living plants as technological platforms we can learn how to integrate nanoparticles with plant-based materials to impart novel functions to devices. Sensors in the form of plants could sample their environment through transpiration and report the result via radio-frequency signals, for example, or we can imagine self‑repairing, plant-based photonic devices that serve as communications networks. Plants even have their own power source – photosynthesis, which has the added benefit of consuming carbon dioxide – and are made of cheap materials that are naturally recyclable.
The possibilities of plant nanobionics are potentially far-reaching. A world in which materials repair themselves using sunlight or where buildings in cities act as active carbon sinks would be transformative. Nanoelectronic devices parasitically wired and integrated into a plant’s internal machinery could draw power, store energy and communicate sensory information relating to water stress, chemical exposure, nutrient stress or ambient illumination. How realistic is this vision? While there are definite limitations to plant nanobionics, this is a new field with much unexplored territory and scope for surprise.
Consider photosynthesis from an engineering perspective. A plant chloroplast (the photosynthetic engine of the organism) can produce sugars at an average rate of 40 µg per square centimetre per hour. That is equivalent to 10% of the energy stored in a watch battery each day – not enough to power your smartphone or tablet, but sufficient to drive an active radio-frequency identification circuit to export information, an electrochemical sensor to generate information, or a luminescent beacon for signalling.
The stems of plants, which transport sap from the roots to the leaves via a bundle of conduits each being 10–100 μm across, are another natural engineering marvel that could be exploited for devices. The electrolyte-containing conduits have an electrical conductivity of 0.5 millisiemens per centimetre, which is more than 30 times higher than that of silicon at room temperature, therefore providing channels for parallel communications from the ground to the tips of leaves.
Furthermore, pressure drops induced in the xylem from evaporating water inside leaves can reach values of more than –3 MPa. This is similar in magnitude to the pressure drops that power the entire field of microfluidic devices, which today are typically provided by bulky external pumps.
These are just a few examples that reveal the potential of plants as engineering materials, but there is much more infrastructure within the plant that could be tapped for applications. Plant nanobionics is distinct from the now well-established field of biomimetics, in which engineers learn from natural systems to create new synthetic materials, because it seeks to incorporate living plants into the final device. In plant systems, the use of nanotechnology for this purpose has no precedent.
Working at the intersection of plant physiology and nanotechnology, we became interested in plant nanobionics in 2010 thanks to a research project in which we studied plant self-repair mechanisms during photosynthesis. Our goal was to mimic such repair in synthetic devices, and we successfully built self-assembled photo-electrochemical devices by combining carbon nanotubes into a plant’s photosystem. However, the project motivated us to investigate whether we could exploit existing functions of the plant’s natural machinery – such as self-repair and the conversion of solar energy into fuels – to create high performance, self-repairing solar cells.
Last year we reported a series of new techniques that allow nanoparticles to be delivered and localized within living plants and plant organelles, and we were able to demonstrate several novel functions that could emerge as a result (Nature Materials 13 400). Using extracted spinach chloroplasts and leaves from Arabidopsis plants, we found that single-walled carbon nanotubes (SWCNTs) augment both the light reactions of photosynthesis and biochemical detection functions in these species. Furthermore, we discovered that, under certain conditions, SWCNTs can be made to assemble within the chloroplast’s photosynthetic machinery, which has proven to be a powerful enabling technique.
Surprisingly, SWCNTs that are thousands of times longer than the thickness of a lipid bilayer penetrate the outer lipid envelopes of chloroplasts and are left trapped on the inside. Wrapped in highly charged molecules, the SWCNTs assemble within the chloroplast photosynthetic machinery via a mechanism we call lipid exchange envelope penetration (LEEP): the SWCNTs become coated with lipids that form the chloroplast envelopes, pulling nanotubes in as the membrane repairs itself and thereby trapping them inside. LEEP could potentially be used to make many new types of hybrid photosynthetic materials in plants, including those that absorb light at wavelengths across the electromagnetic spectrum or those that have chemo-protective capabilities against photo-damage. We demonstrated that SWCNTs can enhance the rates of photo-induced electron transport both in chloroplasts extracted from the plant-cell host and in leaves of living plants by up to 30% relative to controls.
The delivery of SWCNTs to living plants was performed by infiltration through the stomata, which are the pores that control gas exchange between leaves and the atmosphere. We also exploited this mechanism to deliver nano-sized particles of cerium oxide (known as “nanoceria”) inside chloroplasts, significantly reducing the levels of reactive oxygen species to values 28% lower than in control chloroplasts. Nanoceria particles act catalytically as potent scavengers of these damaging molecules, a bit like supercharged vitamin C, thus protecting the chloroplast protein complexes and significantly extending the lifetime of the plant.
We also showed that plants assembled with carbon nanotubes can act as chemical sensors that communicate by fluorescent signalling. These nanobionic plants report changes in the concentration of nitric oxide (a plant-signalling molecule and environmental pollutant) by modulating the near-infrared emission of nanoparticle sensors embedded within the leaf lamina. The modified plants are able to respond within seconds of exposure and are capable of detection sensitivities below one part per million in aqueous media. We envision such nanobionic plants replacing more expensive inorganic sensors based on electronics and plastics for the detection of explosives or environmental pollutants, for instance.
The discovery that SWCNTs can assemble within a plant’s photosynthetic machinery raises the possibility of biocompatible electrodes patterned within and on the surfaces of leaves and stems. These could be interfaced with plant tissues to produce electrical circuits for computation, electrochemical detection of molecules inside the plant, or external communication. This merging of synthetic and natural infrastructures is one of the central visions of plant nanobionics. A laccase–glucose oxidase electrode pair, for example, creates a biofuel cell that could syphon off stored glucose to electrically power the circuits. This could allow the circuit to monitor the plant’s photosynthetic output directly by quantifying the sugars that are the main products of photosynthesis.
Monitoring other plant signals is similarly intriguing. Abscisic acid, for instance, is a hormone produced by roots in response to dry soil conditions that controls plant transpiration by closing the stomatal aperture. We can therefore imagine nanoelectronic circuits that respond to plant chemical signals and control the water content of their environment, for example by generating a radio-frequency signal that activates an irrigation device in response to water stress.
We can even consider incorporating plant systems directly into our own building materials to provide added functionality. Since chloroplasts can perform the basic function of converting sunlight and carbon dioxide into sugars even when they are removed from a living plant cell, materials containing transplanted chloroplasts could potentially capture unwanted carbon dioxide from the atmosphere. For this to be possible, however, we first need to prevent the natural degradation of the “naked” chloroplasts caused by reactive oxygen species and other mechanisms when they are removed from the plant cell. At the Massachussetts Institute of Technology we have recently been working on the concept of a “hyperstable chloroplast” as an engineering material, perhaps based on chemo-protective nanoparticles such as nanoceria.
Such visions might at first seem a bridge too far. Plant nanobionics requires interdisciplinary research teams and strong collaboration between plant scientists and nanotechnology researchers to make it reality, but there is a wealth of scientific knowledge and technological potential to be gained on the way towards this goal. Enhancing crop yields and the productivity of algae biofuel, or creating novel hybrid photovoltaic and optical communication materials, are already widely studied technological goals. But some applications, such as authentic plant cyborg tissue, require completely new avenues of exploration.
There are many different scientific and engineering challenges for plant nanobionics in the decade ahead. We have shown that nanoparticles introduced to a plant can be trafficked to the chloroplasts in leaves via vascular infusion, but what about directing other nanoparticles to other plant organs or tissues to boost or introduce additional functions? Does a given nanoparticle with particular properties and coatings affect its transport within the plant? Such questions are still poorly understood, but will help to develop plant biocompatible circuits and optical communication materials.
To bridge the world of electronics and plants, we also need to understand the physical limitations imposed by the plant. We have to determine, for example, how electromagnetic interactions within and between nanomaterials affect the way that visible, infrared or radio-frequency waves interact with living plants. As with every new technology, safety studies should also be thoroughly conducted before taking nanobionic plants outside the laboratory. Nanotoxicity studies demonstrate that the behaviour of nanomaterials in living tissue depends a lot on the surface chemistry, aspect ratio, nanoparticle size and other properties. While studies in this area will help engineers to design additional biocompatible materials for the plant interface, many nanobionic applications – such as those designed to replace or enhance silicon, plastic and metal devices for communications, photonics or self-powered systems – do not involve ingestible nanoparticles.
There are seemingly endless opportunities and challenges in using nanotechnology to enhance and exploit the diverse functions of plants. One certainty, however, is that plant nanobionics requires the engagement of multidisciplinary teams of plant biologists, chemists, engineers and physicists alike.
