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Home » Page 1071

Nonlinear optical quantum-computing scheme makes a comeback

A debate that has been raging for 20 years about whether a certain interaction between photons can be used in quantum computing has taken a new twist, thanks to two physicists in Canada. The researchers have shown that it should be possible to use “cross-Kerr nonlinearities” to create a cross-phase (CPHASE) quantum gate. Such a gate has two photons as its input and outputs them in an entangled state. CPHASE gates could play an important role in optical quantum computers of the future.

Photons are very good carriers of quantum bits (qubits) of information because the particles can travel long distances without the information being disrupted by interactions with the environment. But photons are far from ideal qubits when it comes to creating quantum-logic gates because photons so rarely interact with each other.

One way around this problem is to design quantum computers in which the photons do not interact with each other. Known as “linear optical quantum computing” (LOQC), it usually involves preparing photons in a specific quantum state and then sending them through a series of optical components, such as beam splitters. The result of the quantum computation is derived by measuring certain properties of the photons.

Simpler quantum computers

One big downside of LOQC is that you need lots of optical components to perform basic quantum-logic operations – and the number quickly becomes very large to make an integrated quantum computer that can perform useful calculations. In contrast, quantum computers made from logic gates in which photons interact with each other would be much simpler – at least in principle – which is why some physicists are keen on developing them.

This recent work on cross-Kerr nonlinearities has been carried out by Daniel Brod and Joshua Combes at the Perimeter Institute for Theoretical Physics and Institute for Quantum Computing in Waterloo, Ontario. Brod explains that a cross-Kerr nonlinearity is a “superidealized” interaction between two photons that can be used to create a CPHASE quantum-logic gate.

This gate takes zero, one or two photons as input. When the input is zero or one photon, the gate does nothing. But when two photons are present, the gate outputs both with a phase shift between them. One important use of such a gate is to entangle photons, which is vital for quantum computing.

The problem is that there is no known physical system – trapped atoms, for example – that behaves exactly like a cross-Kerr nonlinearity. Physicists have therefore instead looked for systems that are close enough to create a practical CPHASE. Until recently, it looked like no appropriate system would be found. But now Brod and Combes argue that physicists have been too pessimistic about cross-Kerr nonlinearities and have shown that it could be possible to create a CPHASE gate – at least in principle.

From A to B via an atom

Their model is a chain of interaction sites through which the two photons propagate in opposite directions. These sites could be pairs of atoms, in which the atoms themselves interact with each other. The idea is that one photon “A” will interact with one of the atoms in a pair, while the other photon “B” interacts with the other atom. Because the two atoms interact with each other, they will mediate an interaction between photons A and B.

Unlike some previous designs that implemented quantum error correction to protect the integrity of the quantum information, this latest design is “passive” and therefore simpler.

Brod and Combes reckon that a high-quality CPHASE gate could be made using five such atomic pairs. Brod told physicsworld.com that creating such a gate in the lab would be difficult, but if successful it could replace hundreds of components in a LOQC system.

As well as pairs of atoms, Brod says that the gate could be built from other interaction sites such as individual three-level atoms or optical cavities. He and Combes are now hoping that experimentalists will be inspired to test their ideas in the lab. Brod points out that measurements on a system with two interaction sites would be enough to show that their design is valid.

The work is described in Physical Review Letters. Brod and Combes have also teamed-up with Julio Gea-Banacloche of the University of Arkansas to write a related paper that appears in Physical Review A. This second work looks at their design in more detail.

Testing the brain’s ‘physics engine’, lawnmower aurora alert and more

https://youtu.be/1vwa8-wUJIo

 

By Hamish Johnston and Tushna Commissariat 

You may not know it, but apparently you have a dedicated region in your brain that is your “physics engine”. At least that is what cognitive researchers from Johns Hopkins University are suggesting after they have pinpointed a specific region of the human brain that intuitively understands physics – at least when it comes to predicting how objects behave in the real world. According to the team, the engine is kick-started when we observe physical events as they happen and is “among the most important aspects of cognition for survival”. Surprisingly, the region is not located in the brain’s vision centre, but is actually the same area we tap into while making plans of any type. In the video above, the team has created a little game for you to test your engine’s horsepower – go ahead and tell us how you did.

What physicist can resist making a back-of-the-envelope calculation to test an outlandish claim, especially if it involves a race between a car and gravity? In his recent Forbes blog, physicist Chad Orzel has sharpened his pencil to try to work out whether the Tesla Model S can accelerate to 100 km/h on the track in less time than it would if it was dropped from a height. The car is electric, which means that it should be much faster off the mark that a conventional vehicle. But, “Can a Tesla Model S really accelerate faster than gravity?”.

What’s the difference between a dazzling display of the aurora borealis and a lawnmower? Not much, it seems, if you use a geomagnetic sensor operated by AuroraWatch at the University of Lancaster in the UK. On Tuesday the organization sent out a “red alert” to its e-mail subscribers telling them to look out for a big show of the northern lights. But alas, the apparent huge spike in geomagnetic activity seen in the AuroraWatch sensors was actually caused by the operation of a nearby lawnmower. Can you imagine what would happen if someone accelerated nearby in a Tesla Model S? You can read all about it here: “Red alert cancelled”.

Nobel laureate James Cronin dies at 84

Photograph of James Cronin
Particle pioneer: James Cronin at the 2010 Lindau Nobel Laureate Meeting. (CC BY-SA 3.0 Markus)

American nuclear-physicist James Cronin, who shared the 1980 Nobel Prize for Physics with Val Fitch, died on 25 August, at the age of 84. Cronin and Fitch – who died in February last year – were awarded the prize for their 1964 discovery that decaying subatomic particles called K mesons violate a fundamental principle in physics known as “CP symmetry.” The research pointed towards a clear distinction between matter and antimatter, helping to explain the dominance of the former over the latter in our universe today.

Born in Chicago, Illinois, on 29 September 1931, Cronin completed his BSc in 1951 at the Southern Methodist University in Dallas, where his father taught Latin and Greek. Cronin moved to the University of Chicago, where he graduated with a PhD in physics in 1955. While there, Cronin benefited from being taught by stalwarts of the field, including Enrico Fermi, Maria Mayer and Subrahmanyan Chandrasekhar.

After his doctorate, Cronin worked as an assistant physicist at the Brookhaven National Laboratory (BNL) until 1958, when he joined the faculty at Princeton University, where he remained until 1971. He then returned to the University of Chicago to become professor of physics. Cronin met Fitch during his time at BNL and it was Fitch who brought him to Princeton. While there, the duo aimed to verify CP symmetry using BNL’s Alternating Gradient Synchrotron (AGS) by showing that two different particles did not decay into the same products.

Verified violation

They planned on doing this by colliding proton beams into a metal target to produce many millions of short-lived and long-lived K mesons. The former would always decay into two pi mesons, while the latter would not. Instead, to their surprise, they spotted a “suspicious-looking hump” in the data, which showed that, on occasion (in 0.2% of the cases), the long-lived variety also decays into two pi mesons, thereby violating CP symmetry.

In recent years, Cronin was instrumental in the development of the Pierre Auger Project, which he conceptualized in 1992 with fellow physicist Alan Watson. The $50m cosmic-ray observatory is based in Argentina, and is designed to pick up ultra-high-energy cosmic rays as they travel at near-light speeds through the Earth’s atmosphere, producing “air showers” of other particles as they interact with atmospheric nuclei. It is currently the world’s largest cosmic-ray detector, with a 3000 km2 collecting area. Despite the fact, the observatory announced in December last year that it is set for a $14m upgrade, which will allow for more precise measurements of the mass of particles that make up cosmic rays, as well as trying to pinpoint their original source.

Giant two-atom molecules are the size of bacteria

Enormous two-atom molecules about the size of ordinary bacteria have been made by two chemists in Switzerland. Comprising two caesium atoms, each “macrodimer” is about 1 μm in length – which is almost 10,000 times larger than common diatomic molecules such as oxygen. Although macrodimers were first spotted in 2009, this time the scientists were able to study the molecules more directly. They were also able to flesh out the existing theory describing these short-lived molecules and predict which types would have longer lifetimes. This allowed them to create macrodimers that could last about 1 μs before breaking apart into ions.

The macrodimers are so large because their constituent atoms are also huge – with each atom having an outermost electron that is excited into a far-flung atomic orbital. These are known as Rydberg atoms, and at room temperature they only exist for a very short time. This is because the outer electron is so weakly bound to the rest of the atom that collisions from nearby particles can easily knock it out of the atom. To minimize these collisions and extend the lifetime of the Rydberg atoms so molecules could be made, Heiner Saßmannshausen and Johannes Deiglmayr of the Swiss Federal Institute of Technology (ETH) in Zurich, Switzerland, created Rydberg atoms at extremely low temperatures.

They began with a diffuse cloud of caesium atoms that had been laser-cooled to below 40 μK. The average separation between atoms in the cloud was about 1 μm. The duo then used pulsed laser light to excite a small fraction of the caesium atoms into Rydberg states in the 44th energy level. Then they pulsed the gas cloud with a second laser, which had a photon energy slightly less than that required for caesium’s transition to the 43rd energy level. That difference in energy is equal to the binding energy of the macrodimer. That is the amount of energy that two caesium atoms in the 43rd and 44th energy levels would lose by joining together as a macrodimer.

Ions of distinction

This pulse excited pairs of atoms simultaneously into a state in which the two atoms behaved collectively as a molecule. To confirm that it had indeed created macrodimers, the team looked for the caesium ions that formed when the huge molecues break apart. The researchers found that these ions have distinctive properties that are predicted by a macrodimer model, which allowed the duo to conclude that they had indeed made the giant molecules.

The orbital overlap is basically zero

Heiner Saßmannshausen, ETH Zurich

The atoms in the micron-sized molecule interact with each other via van der Waals forces. This is a relatively weak interaction that arises when the outer electron of one atom deforms the shape of the other atom via electrostatic forces. This deformity can result in either attraction or repulsion between the two atoms, depending on the distance between them. This exotic molecular “bond” is different from the usual bonds that hold molecules together, such as covalent and ionic bonds, in which atoms in close proximity share or give up electrons to each other. “In our case, the atoms are really completely separated,” Saßmannshausen says. “The orbital overlap is basically zero.”

“This is a great achievement,” says Robin Côté, a theoretical physicist at the University of Connecticut, who was part of the team that first predicted the existence of macrodimers in 2002. This latest work expands on that prediction, which was based on a much simpler model.

Quantum gold mine

Côté says the work heralds a gold mine of new quantum-mechanical phenomena on a different scale. “The fact that these macrodimers exist is amazing,” he says. “It’s amazing that quantum mechanics is relevant between objects a micron apart. This is a new type of molecule that you could not observe under normal conditions.”

Côté and collaborators are already on to the next step: modelling a three-atom micron-scale molecule. In 2013, they published a paper predicting the existence of these macrotrimers, still yet to be created in the lab. In addition, he says that because macrodimers provide a new way to control two atoms at once, they could be used in quantum-information applications. “What’s next? Who knows?” he says. “There’s plenty of possibilities. Whenever you get a new toy, there are plenty of interesting new things to think about.”

The macrodimers are described in Physical Review Letters.

  • There is much more about the fascinating world of Rydberg atoms in this feature article by Keith Cooper: “The rise of Rydberg physics“.

Nuclear power’s ups and downs

Photo taken from the bottom of a cooling tower, with red and orange machinery in the foreground, steep grey walls, and a circle of clear blue sky, into which a small puff of cloud is escaping
Policy purview: The British nuclear-power story owes more to politics and economics than to science. (Courtesy: iStock/svedoliver)

As the First World War began, the British foreign secretary Sir Edward Grey reportedly declared “The lamps are going out all over Europe.” Judging by recent predictions of a gap between the UK’s supply of electricity and future demand, perhaps we should replace “Europe” with “Britain”. Nuclear power stations provide more than 20% of the UK’s electrical generation capacity, but most will close by the mid-2020s; only one new nuclear power station has been ordered since 1980; and plans to build more are still not finalized. The reasons for the ambivalent attitude to nuclear power are a combination of technology, politics and economics. In his book, The Fall and Rise of Nuclear Power in Britain, Simon Taylor – from the University of Cambridge’s Judge Business School – concentrates mainly on the latter.

Taylor begins by reviewing the period between 1945 and 2002, which he calls “the years of hope and disappointment”. During this period governments dithered and plans for future nuclear power stations grew and diminished. The claims of nuclear proponents – mainly scientists and engineers developing the plants – were highly optimistic, and cost estimations were confused by somewhat dubious accounting methods (for example, R&D costs were ignored). Indeed, government plans were initially determined more by the UK Atomic Energy Authority (UKAEA) than by the Central Electricity Generating Board, which was much more sceptical even though two of its heads were ex-UKAEA chairpersons. This should, perhaps, have been a warning sign.

The problems for the UK nuclear industry can be traced to the decision, after the Second World War, to develop air-cooled, graphite-moderated piles to produce plutonium for nuclear weapons. The UK had also been considering the possibility of nuclear-generated electricity, since politicians were told that coal was becoming scarce (after it was found to be more plentiful, the arguments moved from economics to security and diversity of supply). So, when the plutonium piles were found to be less efficient than hoped, the supposed coal shortage and the desire for nuclear weapons came together, leading to the birth of the Magnox reactor programme.

These CO2-cooled, graphite-moderated reactors used natural uranium fuel and operated successfully for decades: the last closed in December 2015. However, they were “developed” over the years, which, as Taylor notes, “reduced the chance of economies of scale”, as different designs were built by different consortia on different sites. Most of them overran both in time and cost. Following on were the advanced gas-cooled reactors (AGR), which were CO2-cooled, graphite-moderated but used enriched fuel and are the main type in use in the UK today.

A major problem was that the UK chose not to pursue what was, by the 1970s, rapidly becoming the global design choice: the light-water reactor (LWR). This US-developed design uses low-enriched fuel and either pressurized water or boiling water as both coolant and moderator, and US economic power meant the UK’s gas-cooled designs lost out in global competition. The last AGRs were ordered in the late 1970s, and by then the UK’s nuclear industry was already beginning to look very fragile: even in 1965 there were warnings that AGRs were not the economic choice.

As Taylor shows in the book, these problems came to a head in the 1980s, when the Conservative government privatized the electricity generation industry, exposing the nuclear sector’s financial and operational weaknesses. Only one new nuclear power station, the pressurized-water reactor at Sizewell B (Britain did, at last, catch up with global thinking on design), was planned and built in this period; it was supposed to have been the first of 10. Several attempts to privatize nuclear power stations ended with the government retaining responsibility for the Magnox reactors and, later, rescuing the privatized British Energy, which operated the AGRs. Also, in the early 1990s, the price of coal was artificially increased to protect its industry, meaning “nuclear had to bear the cost”. The extent of coal’s influence on nuclear power, in terms of availability, political influence, economics and now carbon emissions, is an intriguing theme running through the book.

The Labour governments that followed from the late-1990s were initially rather circumspect about nuclear power, partly due to ministers who saw it as a threat to Labour’s traditional coal industry base. However, by 2005 increasing pressure to reduce greenhouse gases and fossil fuel use produced a change of heart. A series of policy decisions (including, significantly, the Climate Change Act of 2008, which required a steady decrease in carbon emissions) led to the next stage: new nuclear power.

Taylor concentrates on the economic and political activities from 2002 through to 2015, when his book was completed. During this period the path for new nuclear power stations was eased somewhat via changes to planning controls and siting decisions. A system for private companies to submit designs to regulatory bodies before offering them to the operators was also introduced, reducing commercial risks. These designs were of foreign origin, though, because in this period the UK ceded control of its nuclear power industry to other countries. In particular, British Energy was sold to EDF, which is about 85% owned by the French government.

Although the government’s mantra has long been “no subsidies”, events such as the 2008 financial crash and the meltdown at Japan’s Fukushima Daiichi reactor made this impractical. To date, EDF’s plan to construct European pressurized-water reactors (EPRs – the only new design with UK regulatory approval) has received several government financial guarantees, including a guaranteed price for electricity for 35 years; government debt guarantees on construction loans; and indemnity against future policy changes. These are subsidies by other names, and required EU approval; their costs will be borne by consumers. Even with such support, EDF’s project still needed additional financing from Chinese partners. As part of the deal, these companies will receive assistance in getting regulatory approval for a Chinese-designed LWR. Similar guarantees will, no doubt, be expected by the two other potential operators, HORIZON and NuGen, which are also foreign owned and considering different designs, respectively a Japanese boiling-water reactor and a US pressurized-water reactor, assuming regulatory approval is granted.

Given all these difficulties, it is natural to consider whether alternative power sources could take nuclear’s place. But while Taylor expresses some doubts about the costs of nuclear power stations, dubbing EDF’s EPR “the world’s most expensive power station”, he finds the alternatives wanting.

His closing comment that “reliable sources of low-carbon power…[are needed]…that avoid dependence on foreign gas and which offer heat and light on a cold, still winter’s night” is worth repeating. Nuclear power is the only proven, low-carbon technology that can do this, but it needs government help. Since the book was written, new threats to EDF’s plans have surfaced. EDF and the French government seem supportive – albeit with a few dissenters, including EDF’s financial director, who resigned in March because he believed the project could jeopardize the company’s future. However, the final decision to build has still not been made and it will not occur until September at the earliest after a consultation with the French unions. All three potential new-build operators are targeting the mid-2020s for first operation.

Taylor’s book is an excellent summary of the technological, economic and political tribulations of nuclear power in the UK up to 2015. If I have one quibble it’s in the title: “fall and rise” hardly does justice to the rollercoaster ride that has been the history of nuclear power in the UK.

  • 2016 UIT Cambridge £19.99pb 256pp

Rocky planet found in habitable zone around Sun’s nearest neighbour

In a breakthrough discovery, clear evidence of at least one planet orbiting Proxima Centauri, the closest star to our Sun, has been found by the international Pale Red Dot collaboration. The exoplanet – dubbed Proxima b – has a minimum mass of about 1.3 times that of the Earth and is therefore most likely a terrestrial planet with a rocky surface, and has a short orbit of around 11.2 days. Our newly found neighbour also lies within its star’s habitable zone, meaning that it could, in theory, sustain liquid water on its surface, and may even have an atmosphere. The team suggests that the system may even contain another larger exoplanet that is much further away, or smaller companion planets, but the evidence for these is currently not conclusive.

Proxima Centauri was first spotted in 1915 by the Scottish astronomer Robert Innes, and is a red-dwarf star that is merely 4.2 light-years away from the Sun, in the constellation of Centaurus. It is too dim to be seen with the naked eye and lies relatively close to the bright Alpha Centauri binary star system. Thanks to its proximity to us, we can clearly resolve the star’s angular diameter, which is about one-seventh that of the Sun, while the star’s mass is about an eighth of the Sun’s.

Planets ahoy

Red dwarfs are small, cool, main-sequence stars, usually with a surface temperature of less than 4000 K. They abound in our galaxy, but are often difficult to observe, thanks to their low luminosity. But they are the most common stars in our stellar neighbourhood and at least 20 out of 30 of our nearest neighbours are red dwarfs. Of these stars, many are known to host exoplanets, and previous studies have found that almost 40% of red dwarfs have a “super-Earth”-class planet – with masses that are between 2 and 10 times that of the Earth – orbiting in the star’s habitable zone.

One of the methods currently used by astronomers to detect potential exoplanets is known as the “radial-velocity technique”, where they pick up tiny shifts in the wavelength of starlight caused by the presence of an exoplanet. These changes are derived from shifts in the parent-star’s spectral lines caused by the Doppler effect. While this method works very well for enormous planets in close orbit to the stars, it is extremely hard to pick up Earth-sized planets. Fortunately, red dwarfs are so small that their “wobble” is much easier to detect with our current technology.

Although there have been hints of a possible exoplanet in orbit around Proxima Centauri for the past 15 years, the signal was not convincing enough and appeared to have some strange deviations. We still do not fully understand the dynamics of red dwarfs and how they change over different timescales, and as a result, researchers could not tell for sure if the previously detected signals from Proxima Centauri were truly from a planet or due to some variability introduced during the star’s 88 day rotation, such as by a star spot, which also introduces a wobble.

Planetary signal?

Since 2013, the Pale Red Dot campaign has worked towards getting enough clear observations to tell for sure if the star hosted a planet. A way to do this was to carry out a long and continuous observational study of the star, and earlier this year, the team carried out a study of the star using the European Southern Observatory’s High Accuracy Radial velocity Planet Searcher (HARPS) spectrograph at the La Silla Observatory in Chile, together with three other telescopes around the world.

According to project co-ordinator Guillem Anglada-Escudé, from Queen Mary University in the UK, such a long observing mode is normally not allowed by ESO for a variety of reasons. But after a lot of “convincing” by the collaboration, they were allowed to study the star for 20 minutes every night for a two-month period, from 19 January to 31 March 2016.

Plot showing the 'wobble' of Proxima Centauri, from data taken in 2016
Planetary fingerprint: This plot shows how the motion of Proxima Centauri towards and away from Earth is changing with time over the first half of 2016. Sometimes Proxima Centauri is approaching Earth at about 5 km per hour – normal human walking pace – and at times receding at the same speed. This regular pattern of changing radial velocities repeats with a period of 11.2 days. Careful analysis of the resulting tiny Doppler shifts showed that they indicated the presence of a planet with a mass at least 1.3 times that of the Earth, orbiting about 7 million kilometres from Proxima Centauri – only 5% of the Earth–Sun distance. (Courtesy: ESO/G Anglada-Escude)

To ensure that the signal was in no way a false one created by Proxima Centauri, which is an active star, the team also carefully monitored its changing brightness during the campaign using telescopes at the San Pedro de Atacama Celestial Explorations Observatory in Chile and the Las Cumbres Observatory global network.

According to team-member Pedro Amado from the Instituto de Astrofísica de Andalucía in Granada, Spain, this continuous monitoring of at least five cycles of the stellar signal was crucial for the conformation. By analysing the 2016 data together with Doppler measurements collected by two ESO telescopes between 2000 and 2014, the team can confirm the existence of at least one planet in orbit around our neighbouring star.

Flashy star

The observations showed that Proxima Centauri approaches and recedes from the Earth at around 5 km/h and the pattern regularly repeats with a period of 11.2 days. By analysing the Doppler shifts, the researches determined that the planet has a minimum mass of 1.3 times that of the Earth and orbits Proxima Centauri at a distance of about 7 million kilometres – only five per cent of the Earth–Sun distance. The researchers deduce that the planet is terrestrial thanks to its Earth-like mass. It is also most likely tidally locked, but whether it has a synchronous rotation – i.e. the same side is always in the light or dark, like our Moon – is impossible to tell.

Although Proxima b orbits much closer to its star than Mercury does to the Sun, its parent star is much fainter than ours. This means that Proxima b lies well within the habitable zone around the star and has an estimated surface temperature that would allow the presence of liquid water. Despite the temperate orbit of Proxima b, the conditions on the surface may be strongly affected by the ultraviolet and X-ray flares from the star – far more intense than the Earth experiences from the Sun. But, according to team-member Ansgar Reiners at the Institut für Astrophysik, Universität Göttingen in Germany, this does not exclude the existence of an atmosphere, looking at the activity of the star today.

What will ultimately determine the actually habitability of the planet – including whether it currently has liquid water on its surface and an atmosphere – depends entirely upon its formation history. If the planet formed far away from the star and then migrated into its current orbit, it would have contained lots of water. On the other hand, it will be a dry planet, such as Venus or Mercury, if it formed close by, or so the researchers speculate.

Amado points out that we have “unmatched observing opportunities” as the exoplanet is so close to us and can, in theory, be resolved by a 3.5 m telescope. Actually being able to pick up enough light from it is another challenge that telescopes such as the ELT could attempt. “This is a planet in our neighbourhood and maybe we will finally send out a probe and take a picture from somewhere other than Earth,” hopes Amado.

The discovery is described in Nature.

Why do we need synchrotrons to study superconductors?

Superconductors conduct electricity with zero resistance below a critical temperature (Tc), leading to all manner of exciting applications. One of the dream scenarios is to discover a material that superconducts at room temperature. So-called “high-temperature superconductors” do exist but their transition point is still significantly below 0 °C. One of the factors hindering this research field is that we still have a very limited understanding of how high-temperature superconductors function.

In this video, Mark Dean of the Brookhaven National Laboratory (BNL) in New York explains how this endeavour can be aided by the bright X-rays produced at synchrotron facilities. Presenting from the beamline at the National Synchrotron Light Source II (NSLS-II), Dean introduces his own research on oxygen- and copper-based superconductors. He is investigating a mysterious weak ordering of electrons that occurs in these materials when they superconduct.

This video is part of our 100 Second Science series, in which researchers give concise presentations covering the spectrum of physics.

Magnetic bacteria target hard-to-treat tumours

Bacteria that respond to magnetic fields and low oxygen levels may soon join the fight against cancer. Researchers in Canada have done experiments that show how magneto-aerotactic bacteria can be used to deliver drugs to hard-to-reach parts of tumours. With further development, the method could be used to treat a variety of solid tumours, which account for roughly 85% of all cancers.

Cancer cells in a growing tumour consume large amounts of oxygen and parts of the tumour will become starved of oxygen – or hypoxic. It is notoriously difficult to deliver tumour-destroying drugs to these hypoxic regions using conventional pharmaceutical nanocarriers, such as liposomes, micelles and polymeric nanoparticles.

Now, a team led by Sylvain Martel of the NanoRobotics Laboratory at the Polytechnique Montréal – including researchers at McGill University – has developed a method that exploits the magnetotactic bacteria Magnetoccus marinus (MC-1) to overcome this problem.

Tiny compass needles

A MC-1 bacterium has a chain of magnetic nanoparticles that acts like a microscopic magnetic compass needle. The bacteria live in saltwater estuaries in the northern hemisphere, where they use the Earths’s geomagnetic field to point them towards deeper water with low oxygen concentrations. The microbes do this because they thrive where oxygen concentrations are low. Indeed, the oxygen levels found in hypoxic regions of a tumour – about 0.5% – are perfect for MC-1.

The researchers created an artificial environment to allow these bacteria to migrate towards the hypoxic regions of tumours in live mice with colorectal cancers. “We first produce a weak magnetic field pointing towards the tumour to guide drug-loaded bacteria and make them swim towards the tumour (a process called magnetotaxis),” says Martel. “Once inside the tumour and sufficiently close to the hypoxic zones, we remove the magnetic field to allow the bacteria to use their internal oxygen sensors (aerotaxis) and follow the decreasing oxygen gradient in the tumour until they reach the 0.5% oxygen level.”

These bacteria can be used as general transport vehicles to carry a huge variety of therapeutic agents, such as various drug molecules, radiotherapeutic agents, stem cells and immunotherapeutics. “In the short term, we will be using our technique to study how it can enhance cancer treatment,” says Martel. “The possibilities are vast, since all therapeutic agents for treating solid tumours share a common problem – the effective delivery to the site of treatment.”

Therapeutic agents

Looking further into the future, the researchers say they would like to look into the efficacy of various therapeutic agents that are delivered using their new technique. They also hope to collaborate with other research groups around the world.

In the next few months the team will begin to develop medical protocols based on the technique. Also planned is the implementation of mathematical models to improve how magnetic fields are used to guide the bacteria. They will also continue doing studies of the safety of the technique, which Martel says “are encouraging so far”.

The new technique is described in Nature Nanotechnology.

The monk and the multiverse

Robert Grosseteste was born sometime around the year 1170. By the time he died in 1253, he had gained a reputation as one of the leading scholars and philosophers of his age. However, some modern researchers have gone even further, calling him “the most brilliant scientist you’ve never heard of”.

“One idea he’s very famous for is a theory for the physical origin of the universe that, believe it or not, starts with a flash of light and expands out with a giant rapidly moving sphere – it’s a big bang theory of the universe,” says Tom McLeish, a physicist at Durham University.

McLeish is a member of the Ordered Universe Project, an interdisciplinary group of scientists and historians who are re-examining Grosseteste’s writings and, in many cases, “translating” his ideas into a modern mathematical form. This process has led the group in some unexpected and fruitful directions. For example, while the details of Grosseteste’s “big bang” are not compatible with modern theories – like other ancient and medieval scholars, he believed that the Earth was at the centre of the universe – McLeish notes that “physicists love playing with alternate realities and counterfactuals and toy models”. And as it turns out, analysing Grosseteste’s equations poses some interesting computational problems.

In this podcast, you’ll hear from McLeish and other members of the Ordered Universe Project, including:

  • medieval historian Giles Gasper on who Grosseteste was and the difficulties of reading early copies of his works;
  • physicist Brian Tanner on putting Grosseteste’s ideas into modern mathematical form, and on the differences between observing natural phenomena and conducting experiments;
  • psychologist Hannah Smithson on Grosseteste’s ideas about colour and the rainbow, and what they tell us about how people perceive the world around them.

What atomic-layer deposition could do for energy and the environment

Tools are useful when they meet all the demands of a particular objective, and invaluable when they continue to meet requirements that are ever evolving over time. Atomic-layer deposition (ALD) was already a useful tool for thin films in the 1980s, although commercial applications were then limited to electroluminescent displays. By the 1990s, use of ALD was making inroads into the microelectronics industry, but it was not until the end of that decade that the great match between the fabrication requirements in nanotechnology and the precision and control the technique can achieve became apparent.

Interest in the technique was confirmed in the early 2000s, when the American Vacuum Society began an international conference series on ALD. Since then, more than 1200 research papers, 80 reviews and two books have been published on ALD in nanotechnology. Some of the latest research using this technique to make devices for energy and environmental applications have appeared in a recent focus collection of the journal Nanotechnology from IOP Publishing, which also publishes Physics World.

ALD is a variant of the widely used technique of chemical-vapour deposition (CVD), in which a thin film is grown on a substrate by exposing it to one or more volatile gases – known as precursors – that react or decompose on the substrate to produce a required structure. The big difference with ALD is that the precursors are never present at the same time. Instead, ALD involves exposing the surface of a material to atoms of the chemicals to be deposited in separate stages and then clearing the excess between each stage.

In each of these stages, the precursor molecules react with the surface in a self-limiting way, which means that the reaction stops when all of the reactive sites on the surface are consumed. As a result, the amount of material that can be deposited on the surface after a single exposure to all of the precursors (a so-called ALD cycle) is governed by the nature of the precursor–surface interaction. By varying the number of cycles, researchers can therefore grow materials uniformly and with high precision on arbitrarily complex and large substrates.

In a paper published in the late 1990s exploring how ALD might be useful in nanofabrication (Nanotechnology 10 19), Mikko Ritala and Markku Leskelä of the University of Helsinki list the merits this tweak introduces. These include “accurate and simple film-thickness control, sharp interfaces, uniformity over large areas, excellent conformality, good reproducibility, multilayer processing capability and high film qualities at relatively low temperatures”. These attributes have proved particularly valuable for attempts to improve the efficiency and scalability of alternative-energy technologies.

Coating electrolytes on 3D structures

Many of the processes key to energy harvesting and storage devices are improved by the increased surface area that 3D structures bring. For lithium-ion batteries, a higher surface area means higher energy and power densities, as well as space to accommodate the addition and removal of lithium ions, which can introduce mechanical strain during use. Researchers studying lithium-ion batteries are therefore increasingly looking towards solid state as opposed to liquid electrolytes to avoid risks of leakage and corrosion, with sophisticated coating techniques to add electrolyte and counter-electrode layers to complex 3D nanostructures therefore being in high demand.

ALD not only provides pinhole-free coatings of solid-state electrolytes on 3D electrode structures, but it also lets researchers tailor the coating thickness, which can significantly improve performance. As Tsun-Kong Sham and Xueliang Sun and colleagues at the University of Western Ontario in Canada point out (Nanotechnology 25 504007), “It is expected that the lithium-phosphate thin films prepared by ALD can find potential applications as solid-state electrolytes for 3D all-solid-state micro lithium-ion batteries, which, as an emerging area, deserves more extensive investigation in the coming future.”

Supercapacitors are a particularly attractive energy resource in nanoelectronics, where space is at a premium. As with research on lithium-ion batteries, the advantages of 3D structures with solid-state electrolytes have been noted. Writing in the Nanotechnology focus collection (26 064002), Giuseppe Fiorentino, Frans Tichelaar and their colleagues at Delft University of Technology in the Netherlands report on supercapacitors made from carbon-nanotube bundles as high-aspect-ratio electrodes that can improve the capacitance by a factor of five. The electrode is coated with aluminium-oxide as the electrolyte, followed by titanium-nitride as the counter electrode. As Fiorentino’s team points out, the work is not only “the first known example of large-scale manufacturable nanostructured capacitors”, but also provides useful insights for coating such high-aspect-ratio nanostructures.

Solar energy panels
Sunny side up ALD has been used to coat 3D structures in solar cells, reducing how far charge carriers must travel to reach electrodes. (Courtesy: iStockphoto/gyn9038)

Electrochemical cell structures can also take on elaborate hierarchical forms, as demonstrated by Xudong Wang and colleagues at the University of Wisconsin-Madison and the Forest Products Laboratory in the US. They have combined ALD and cellulose nanofibres to produce extensively branched structures for photo-electrochemical water splitting. Such a 3D titanium-dioxide fibre-nanorod heterostructure offers, they say, “a new route for a cellulose-based nanomanufacturing technique, which can be used for large-area, low-cost and green fabrication of nanomaterials as well as their utilizations for efficient solar-energy harvesting and conversion”.

As for solar-energy harvesting, ALD is already a well-entrenched technique in the field, and has been used to coat the inverse opals and other 3D structures often employed to reduce the distances that charge carriers must travel to reach electrodes. In the focus collection of Nanotechnology (26 064001), Alfred Iing Yoong Tok and colleagues at Nanyang Technological University in Singapore and the University of New South Wales in Australia review the application of ALD in solar-power technologies, focusing on its use for surface passivation, surface sensitization and band-structure engineering.

The potential for lateral confinement

As well as providing an excellent tool for coatings, ALD offers great potential for controlled lateral confinement. This aspect is the subject of detailed scrutiny in the catalysis work of Marcel Verheijen at Philips Innovation Services and Eindhoven University of Technology in the Netherlands. Coating thickness is still key for catalysis performance, as demonstrated by Ai-Dong Li and colleagues at Nanjing University in China. However, as the work by Verheijen and his colleagues in the group of Wilhelmus Kessels at the university identifies, ALD can also help with size matters. In a study of four ALD processes for the preparation of nanoparticle catalysts made from platinum and palladium, they identify the potential for size control, as well its dependence on process conditions (Nanotechnology 27 034001).

It is the ability to accommodate innovation that makes atomic-layer deposition so invaluable

It is perhaps the ability to accommodate innovation that makes ALD so invaluable. The use of plasmonic metamaterial absorbers has only really gained notice in the past few years, and ALD is already proving invaluable for work exploring the essential mechanisms in these systems, too. Xin Chen and colleagues at the Shanghai Institute for Technical Physics, for example, exploit ALD control over dielectric layers in order to distinguish between different plasmon modes. “We have demonstrated inversed plasmonic metamaterial absorber architectures with a tuneable ALD spacer layer, and thus identified the contributions of the gap plasmon and the interference-enhanced local surface plasmon resonance to the superior absorption in a step-by-step manner,” they say.

It is typical of science to break down problems into manageable pieces that can be tackled step by step. By breaking down deposition to an atomic level with step-by-step, self-limiting stages, ALD mirrors this approach and in so doing seems to provide a multipurpose tool for a diverse range of applications.

  • The Nanotechnology focus collection on energy and environmental applications of atomic-layer deposition is available at this link.
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