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BaBar makes first direct measurement of time-reversal violation

The BaBar collaboration has made the first direct observation of time-reversal (T) violation. The results are in agreement with the basic tenets of quantum field theory and reveal differences in the rates at which the quantum states of the B0 meson transform into one another. The researchers say that this measured lack of symmetry is statistically significant and consistent with indirect observations.

The BaBar detector at the PEP-II facility at SLAC in California was designed to study the collisions of electrons and positrons and to determine the differences between matter and antimatter. In particular, physicists working on the experiment are interested in the violation of the charge–parity symmetry (or CP violation). Although the detector was decommissioned in the spring of 2008, data collected during the period of operation continue to be analysed.

Symmetries of the universe

Our current understanding of the universe suggests that it is governed by certain fundamental symmetries. One of these symmetries looks at the relation between charge (C), parity or “handedness” (P), and time (T) – meaning that if you apply a CPT transformation to a system, it shows no difference from the original system. However, physicists are constantly searching for any possible signs of CPT-violation, which could indicate the presence of new physics. In the realm of the weak force, however, instances of the breaking of individual symmetries have been observed in cases of parity inversion or a combination of parity inversion and charge conjugation (CP). Therefore, it was expected that these systems would also show asymmetries when time was reversed. That is, transformation from one state to another would occur at different rates when the process is reversed in time, thus showing a T-violation.

“While CP violation in the B sector is well established by both BaBar and Belle, all CPT-violation tests have always been consistent with zero,” says Patrick Koppenburg, a physicist from the Dutch National Institute for Subatomic Physics (Nikhef), and a member of the LHCb collaboration at CERN. “So, the observation of T violation is not a surprise, but it still needed to be tested.” Indeed, physicists have waited for nearly 50 years to make this direct observation since the discovery of CP violations in 1964. The discovery also comes 14 years after another experiment – the CPLEAR experiment – claimed to have the first experimental proof of the violation in 1998 but this claim proved controversial.

Probing the arrow of time

Electron–positron collisions inside BaBar are tuned to just the right energy for producing Υ(4S) mesons, which are composed of a bottom quark and its antiquark. These Υ particles swiftly decay into B mesons, such as the neutral B0 mesons used in this study.

In 10 years, BaBar detected almost half a billion pairs of B and anti-B mesons. Since these pairs are created from the same Υ, they inherit their quantum numbers from the parent Υ. This “entanglement” of the two simultaneously produced B0 mesons is crucial to observing T violations. “Since the global quantum numbers of the B0-antiB0 system are fixed by the Υ(4S) decay, the state of the first B0 meson to decay – whatever it may be – dictates the state of the other B0 meson at that time, which itself decays after some time into another state,” explains Fernando Martinez-Vidal, who is at the Institute for Particle Physics at the University of Valencia and Spain’s National Research Council (CSIC), and is one of the physicists who worked on this study. “By appropriately choosing the states into which the first and second B0 mesons decay, we can prepare the processes to be studied and compared.”

Forwards and backwards

In the world of quantum physics, the individual mesons can be expressed as superpositions, in terms of linear combinations of both B0 and anti-B0 flavour states. The transformations studied are the change of a B0 meson from a “flavour” state to a “linear-combination” state, and the time-reversed change from a “linear-combination” state to a “flavour” state. To begin with, the BaBar physicists identified the flavour of the first meson in the pair to decay (B0 or anti-B0) and used this information to “tag” the flavour of the second meson. Taking the instant this decay occurred as the starting time, they measured the time it took the second meson to transform into a linear-combination state. They then performed the measurement in reverse: if the first meson transforms into a linear-combination state, this information can be used to determine the linear-combination state of the second meson and measure the time taken for it to decay into a “flavour” state.

Thus, by exchanging the initial and final states of the transformation, the physicists could see if there were any differences in the rates of each of these transformations. Unsurprisingly, they found the difference they were looking for, with a significance of 14σ – in particle-physics experiments, a significance of 5σ and above is considered a definite discovery.

While BaBar may have gone silent nearly half a decade ago, hopefully more new results will emerge from the collected data.

The work is published in Physical Review Letters.

Toshiba sends quantum keys long distances on busy fibres

Quantum information has been transmitted for the first time at a relatively high rate down a long-distance optical fibre that is busy with other telecommunications traffic – something that would normally disrupt quantum communications. The system was developed by researchers at Toshiba’s Cambridge Research Laboratory in the UK together with engineers at Cambridge University. It relies on a detector that can be switched on for about 100 ps (10–10 s) to ensure that it detects a quantum signal efficiently and rejects most of the background noise.

While most quantum-information applications are decades away, some organizations including several banks in Switzerland are already using the technique of “quantum cryptography” to ensure the secrecy of their communications. Quantum key distribution (QKD) allows two parties (by convention called Alice and Bob) to exchange an encryption key, secure in the knowledge that the key – which codes and decodes the data – has not been read by an eavesdropper (called Eve). This guarantee is possible because the key is transmitted in terms of quantum bits (qubits) of information. If intercepted and read by a third party, such qubits are changed irrevocably, which signals to Alice and Bob that Eve has seen the key and so not to use it.

Commercial QKD systems use photons as qubits because the particles can travel long distances in optical fibres without losing their quantum nature. The difficulty, however, is picking out a qubit photon from the noisy background of random photons that are created when much more intense data signals are sent through a commercial telecommunications fibre. One solution that has been adopted is to use “dark fibre”, which does not carry any telecommunications data signal. Unfortunately, leasing or laying dark fibre is very expensive.

Precise timing

What the Toshiba and Cambridge researchers have done is to find a way of filtering out the noise and focussing on the qubit photons. Developed by Zhiliang Yuan and Andrew Shields at Toshiba along with colleagues at Cambridge, the scheme involves Alice creating a qubit photon within a 30 ps time window. The photon is then sent down a 90 km fibre to Bob, who detects the photon by switching on his detector for just 100 ps at the expected arrival time.

Although the fibre is full of noisy photons, most of the randomly arriving noise photons will not reach the detector over this extremely short duration, explains Yuan. As for Bob, he knows exactly when to look for Alice’s photon thanks to a timing system that runs over the same fibre. Based on an off-the-shelf diode laser, the clock signals are sent from Alice to Bob and allow Bob to determine the arrival of the qubit photons to within 10 ps.

The team tested its system on long sections of standard fibres, transmitting standard telecoms data at a rate of 1 Gbit/s in both directions along the fibre, which creates background noise. Despite the noise, the team was able to transmit quantum keys at a rate of 500 kbit/s over a 50 km section of fibre – which is 50,000 times the previous fastest data rate for this distance. The team was also able to send quantum keys at nearly 8 kbit/s over a 90 km fibre – the longest distance yet for an active telecoms fibre.

Perfect for metro networks

Although these distances are not good enough to allow quantum communications across oceans or continents, 90 km is more than enough for a metro network in a large city. Yuan told physicsworld.com that the Cambridge researchers are now working with colleagues in Japan to perform stringent field tests to improve the performance of the system.

As well as QKD, the temporal filtering technology could also find use in other quantum-information applications, where weak quantum signals have to coexist with intense data signals in the same transmission media. One example is distributed quantum-information processing and computing.

The work is described in Physical Review X.

Siemens and Bosch pull out of major African solar initiative

The German firms Siemens and Bosch have announced they are both leaving the Desertec Industrial Initiative (Dii) – a private industry consortium that plans to install a total of 125 GW of solar-power capacity throughout the Middle East and North Africa by 2050. The withdrawal of Siemens and Bosch from Dii, effective at the end of the year, has reignited doubts about the viability of the ambitious hundred-billion-euro project, which would involve piping the energy back to Europe via cables at the bottom of the Mediterranean Sea.

Siemens has also announced it is to completely pull the plug on its solar-energy business. Blaming “changed framework conditions, lower growth and strong price pressure in the solar markets”, the firm will instead focus on developing its wind and hydroelectric power units. Siemens says it is withdrawing from solar because its expectations for solar-energy activities “have not been met”. The firm adds that it sees renewable energy accounting for 28% of global energy use by 2030, but that solar power will make up only 9% of total renewable output, compared with 54% for hydro power and 27% for wind power.

Disappointing, but not fatal

Klaus Schmidtke, a spokesman for Dii, which is based in Munich, acknowledges that the firms’ withdrawal from the project is a disappointment. “Of course, we are not happy about this,” he says. “[But] we do not expect [this decision to have] any negative impact for our initiative.” Schmidtke notes that Siemens was just one of 21 Dii shareholders, and that other companies are still involved in the project (Bosch is only an associate member). These include Swiss industrial firm ABB, electric utility firm E.ON and Deutsche Bank. Schmidke adds that the US solar-panel maker First Solar, currently an associated member of Dii, is in talks to become a full shareholder.

Siemens says that it is already speaking with potential buyers for its photovoltaic activities in its Solar and Hydro Division, adding that the company will continue to produce steam turbines, generators, grid technology, control systems and other items for solar thermal and photovoltaic power plants.

Same old Standard Model

A simulation of an ATLAS event

(Courtesy: CERN)

By Tushna Commissariat

After the rather disappointing news for SUSY researchers from the Hadron Collider Conference in Kyoto this week, it seems as if physicists at the conference have not had anything exciting to say about the Higgs boson either. While both the CMS and ATLAS collaborations did present their latest results, from data collected since the historic Higgs discovery in July, all the current results still point to a Standard Model Higgs.

As a number of other bloggers have already pointed out, what is probably most interesting about these latest results is what is missing – both CMS and ATLAS have only updated certain channels. Conspicuous by its absence was the diphoton (gamma–gamma) channel, which was not updated by either collaboration. The reason for this seems to be some discrepancy between the analysis done by the two experiments, with concerns regarding systematic errors and calibration. Adam Falkowski, who writes the Resonances blog, explains these discrepancies in some more depth.

Papers with the new results from both CMS and ATLAS are available, but the usual blog suspects – Peter Woit, Matt Strassler and the viXra – all agree that the results are anti-climactic. It seems as though we will have to wait until the mysterious diphoton channel gives up its secrets, hopefully by sometime next year, before there is Higgs euphoria again.

Nanotube yarn flexes its muscles

An international team of researchers led by Ray Baughman of the University of Texas at Dallas has developed a new type of artificial muscle made from a “yarn” of twisted carbon-nanotube threads infused with wax. The new actuator structures are different from previously made devices in that they do not need an external electrolyte to function. The muscles can lift more than 100,000 times their own weight, can contract and expand extremely quickly and can operate over a wide temperature range. Such properties could make them ideal for use in a range of future applications, such as humanoid robots, intelligent textiles and advanced rotary motors.

The team made the artificial muscles by first growing a vertically aligned forest of carbon nanotubes. These are hollow cylinders of rolled-up carbon sheets with walls as thin as a single atom. The researchers then drew a thin sheet of nanotube bundles from the forest and twisted this sheet to make a yarn containing helices of intertwined carbon nanotubes. Next, they infused the yarn with molten paraffin wax.

Each end of a thread was then connected to a power supply. When a voltage is applied, the wax heats up and expands. The pressure subsequently produced by this expansion causes the twisted yarn to contract and to partially unwind, which creates a rotating action similar to that seen when stretching a helical spring. The yarn rotates in the opposite direction as the wax cools. Carbon nanotubes are ideal for making such yarns – the sheets are nearly as light as air (they have a density of around just 1.5 mg/cm3) but are stronger than steel, with a specific strength that can reach 560 MPa cm3/g.

Simplified design

Previous artificial-muscle designs work in a similar way but they rely on the nanotubes being immersed in a liquid electrolyte. The use of wax effectively dispenses with the need for such an external conducting liquid, so simplifying the muscle design greatly, explains Baughman.

“The torsional action of the thread can be used to rotate an attached paddle to an average speed of 11,500 revolutions per minute for more than two million reversible cycles,” he says. “The wax-infused nanothreads can also lift more than 100,000 times their own weight and can generate 85 times more mechanical power during contraction than the same size natural muscle. This equates to them being able to lift weights 200 times heavier than is possible for natural muscle of the same diameter.”

Good even without wax

Coiling the nanotube thread increases its thermal expansion coefficient by 10 times, even without the wax filling, he adds. “This thermal expansion is negative, which means that the unfilled yarn contracts as it is heated. Indeed, heating the yarn in an inert atmosphere from room temperature to about 2500 °C (a temperature much higher than the melting point of steel) provides more than 7% contraction when lifting heavy loads – something that has never been seen before for such high-work-capacity actuators.”

According to the researchers, this high thermal expansion for the coiled yarns means that they could be ideal for use in intelligent textiles that would happily function between –50 and 2500 °C. Such proposed textiles would be able to adapt to the needs of the wearer, providing protection in cold conditions, for example, by becoming less porous and cooling the wearer down by becoming more so. Taking this idea a step further might entail incorporating the yarn muscles into sportswear and perhaps even fire-fighter clothing. The artificial muscles might also be used in microfluidic circuits as pumps and for regulating valves, and even in novel applications such as window blinds that open and close in response to ambient temperatures.

“Other possible applications include medial catheters for minimally invasive surgery, nanoactuators and motors in future micromachines, zoom lenses for digital cameras, humanoid robotics, prostheses and exoskeletons,” Baughman says.

Early commercialization possible

Such applications may not just be distant dreams because the researchers have already succeeded in producing kilometre lengths of their yarns. “Since small actuators only require centimetre lengths of thread, this suggests that early commercialization of this material should not be a problem,” says Baughman. “What will be more difficult, however, is to upscale the single-thread actuators to larger ones in which hundreds or thousands of individual muscles operate in parallel.”

The team, which includes scientists from Australia, China, South Korea, Canada and Brazil, is now busy demonstrating new applications for its single-yarn muscles. It is also trying to upscale to large muscles comprising fibre arrays such as those found in natural skeletal muscle.

The current work is detailed in Science.

Should 16–18 year olds be taught modern physics such as quantum mechanics?

By James Dacey

Facebook poll

Earlier this week, my colleague Hamish Johnston wrote this blog entry about a new video that is highly critical of high-school physics education in the US. The video, presented as an open letter to President Barack Obama, bemoans the fact that current curricula in the US focus almost exclusively on classical physics and exclude modern physics such as quantum mechanics almost entirely. The narrator claims that the vast majority of high-school students are not required to learn about any physical phenomena discovered or explained more recently than 1865 (presumably a reference to the year that James Clerk Maxwell published the first version of his famous equations).

The narrator, Henry Reich, is a physicist at the Perimeter Institute for Theoretical Physics in Canada. Reich has released the video on his popular YouTube channel, Minute Physics, in the belief that physics education in the US needs a serious revamp. He argues that the US may lose its standing as the leading nation of innovation unless modern physics concepts such as photons and the structure of atoms are introduced into high-school curricula. He compares the present situation to a scenario in which high-school biology students were not taught about DNA, or geology students were not taught about plate tectonics. For those of you not familiar with the school system in the US, high school refers to students up to 18 years old.

But what do you think about Reich’s sentiments? In theory it would be lovely for all teenagers to be exposed to some of the wonderful ideas of modern physics such as the Higgs boson, antimatter or the cosmological models of how the universe evolved. But the reality is that truly getting to grips with some of these concepts requires an advanced level of maths, which has not always been reached by 18 year olds. The narrator addresses the maths question by saying that great communicators such as Carl Sagan and Neil deGrasse Tyson have triumphed at conveying the fundamental principles of physics in an engaging manner without the need for advanced maths. But, again, the reality is that these people are exceptional – one cannot expect all school teachers to be as gifted at communicating difficult physics as these celebrated TV presenters.

Let us know what you think in this week’s poll.

Should 16–18 year olds be taught modern physics such as quantum mechanics?

Yes, the whole shebang
Yes, but only the ideas not the complex mathematics
No, at this age students should focus on classical principles

To have your say please visit our Facebook page, and please feel free to post a comment to explain your decision.

In last week’s poll we asked another question relating to US politics. We asked you to grade Barack Obama’s governance of US science during his first presidential term? The spread of results was as follows.

A – Awesome 0%
B – Brave effort given the economic constraints 24%
C – Could have done better 44%
D – Dreadful 32%

So in the heads and hearts of our Facebook followers, Obama has his work cut out to meet their expectations in his second term. We hope to hear from you again in this week’s poll. And I’ll make you a promise now that next week’s poll will have nothing to do with US politics!

Millikelvin cooling of large molecules is no myth

In Greek mythology Sisyphus was condemned by the gods to repeatedly push a heavy boulder to the top of a hill, only to see it roll back down to the bottom. Now, physicists in Germany have used a similar scheme to cool a collection of fluoromethane molecules to a temperature of just a few thousandths of a kelvin. Cooling molecules with more than two atoms had proved very difficult and this latest development could lead to breakthroughs in chemistry, particle physics and even quantum computing.

Over the past few decades physicists have developed a variety of tools for cooling gases of atoms ever closer to absolute zero – with temperatures of less than a millionth of a kelvin reached. This has led to all sorts of breakthroughs, such as the creation of an unusual state of matter known as a Bose–Einstein condensate in which all of the constituent particles exist in a single quantum state.

Cooling molecules down to the same temperatures could also lead to major breakthroughs. Potential applications include the development of quantum computers, in which the necessary strong and stable interaction between quantum bits could be achieved via the long-range electrical forces between very low-energy polar molecules. Ultracold molecules might also be used in delicate processes that are impossible to carry out with warmer, more energetic particles, such as using electromagnetic fields to control chemical reactions at the molecular level or observing the tiny difference in energy between left- and right-handed chiral molecules predicted to follow from an inherent asymmetry in the electroweak force.

Unwanted rotations or vibrations

This greater complexity makes molecules much more difficult to cool than atoms using established techniques. One such technique, laser cooling, involves slowing down – and hence lowering the temperature of – atoms in a gas by making them absorb photons from laser beams pointed in opposing directions. About 10,000 such interactions are needed to cool each particle in the gas and any one interaction could cause unwanted rotation or vibration of a molecule.

Despite these difficulties, in 2010 researchers in the US managed to cool down a gas of diatomic (two-atom) molecules using lasers. And this year another American group achieved a similar result using evaporative cooling, which lowers the temperature of a gas by allowing the most energetic particles to escape. But this latest work by Gerhard Rempe and colleagues at the Max Planck Institute for Quantum Optics outside Munich published in Nature extends cooling to molecules made up of five atoms.

The team used a gas of fluoromethane, its molecules consisting of three atoms of hydrogen, one of carbon and one of fluorine. Rather than rely on the puny kicks of individual photons to slow down the particles in a gas, Rempe’s group instead uses the much greater energy available in an external electric field. The researchers’ “electric trap” consists of two parallel capacitor plates, each 4 cm by 2 cm and separated by a gap of 3 mm. The inside surfaces of the plates are patterned to create an electric field that is uniform in the centre of the gap but that grows stronger closer to the plates.

Climbing the well

Initially, pre-cooled molecules are held in the centre of the trap thanks to the potential well set up by their interaction with the electric field. After being excited to a vibrational state by an infrared laser fired through the trap, the molecules spontaneously decay to an intermediate-energy rotational state chosen so that it creates a deeper potential well than that in the molecules’ initial, lower-energy state. The molecules then figuratively “climb up” the sides of this well, losing kinetic energy as they do so, and are subsequently hit by a beam of microwaves that forces them back down to the edge of the shallower well beneath. As they fall back down the sides of this well, the molecules pick up less kinetic energy than they forego in the intermediate state. This means that, on balance, they lose energy. The idea is that by repeating this process several times the molecules can be cooled to extremely low temperatures.

In fact, the researchers were able to reduce the temperature of about one million fluoromethane molecules by more than a factor of 10, to about 30 mK, in only around a dozen cycles. In a commentary piece accompanying the paper, John Barry and David DeMille of Yale University say that this small number of cycles was crucial, since it allowed the molecules to be cooled even though about 10% of the sample was lost in each cycle because of unwanted rotational or vibrational excitations.

Other researchers contacted by physicsworld.com were also positive. Rudi Grimm of Innsbruck University in Austria says that physicists are “desperately lacking” efficient ways of cooling molecules and that the “proof-of-principle demonstration” carried out by the German group “looks very good”. Wolfgang Ketterle of the Massachusetts Institute of Technology, meanwhile, says he is “impressed” by the latest work, arguing that it and “other recent progress” on molecular cooling “opens the door to ultracold chemistry”.

Breaking the 1 mK barrier

Zeppenfeld says that his group’s next step is to try and get below 1 mK, at which point, he believes, the molecules could be used for applications such as quantum computing. Achieving this, he says, might involve making detection of the cooled molecules more efficient or employing molecules that decay in less time than the roughly 0.1 s typical of fluoromethane, in order to limit unwanted collisions with background gas in the trap.

The cooling scheme is described in Nature.

A strange cat in Dublin

Not many life stories in physics involve Nazis, illicit sex, a strange cat and the genetic code. Thus, a new biography of the great Austrian physicist Erwin Schrödinger is always of interest, and with Erwin Schrödinger and the Quantum Revolution, veteran science writer John Gribbin does not disappoint.

Many Physics World readers will be aware of Walter Moore’s 1992 biography Schrödinger: Life and Thought, which remains the definitive text on this colourful quantum pioneer. In fact, Moore also published an edited (and sadly neglected) version of his book for a popular audience, and Gribbin’s book is pitched more at this level. The new biography offers little new historical material, but Gribbin’s lucid style makes for an excellent introduction to this intriguing scientist and, indeed, to the world of quantum physics.

Gribbin sets the stage with a brief introduction to classical physics, followed by a description of the first quantum revolution. The work of Planck, Einstein and Bohr is described accurately yet succinctly in the author’s characteristic clear prose. The story continues with a description of the second quantum revolution, from Louis de Broglie’s hypothesis of wave–particle duality to Schrödinger’s brilliant wave mechanics (with Heisenberg’s matrix mechanics along the way). This is a familiar story for physicists, but one we never tire of reading.

The debate concerning the philosophical implications of the new theory is explained carefully, with a clear description of what became known as the “Copenhagen” interpretation. Einstein’s distrust of this interpretation is well known, but it is often forgotten that Schrödinger shared his views. As Gribbin points out, it is interesting that the father of wave mechanics had no faith in the idea of a wavefunction that collapses on observation, as posited by the Copenhagen camp. This objection is best exemplified by Schrödinger’s famous thought experiment of a cat that is neither dead nor alive before observation. Gribbin has written on Schrödinger’s interpretation of the quantum wavefunction many times before, notably in In Search of Schrödinger’s Cat (Bantam 1984) and Schrödinger’s Kittens and the Search for Reality (Phoenix 1996). Still, the wavefunction pioneer’s objection to the Copenhagen interpretation is worth restating.

The fascinating story of Schrödinger’s life and career is skilfully interspersed with the science. This is no easy task, given that he spent chunks of his career in Vienna, Jena, Zurich, Berlin, Oxford, Graz and Dublin, but Gribbin manages to maintain the reader’s interest throughout these sojourns without sacrificing accuracy. A good example is Gribbin’s description of the infamous Graz episode, when – having unwisely returned to Austria from Oxford in 1936 – Schrödinger penned a cringeworthy letter of apology to the Nazis, who had come to power in Austria following the Anschluss with Germany. As Gribbin explains, the publication of this letter damaged Schrödinger’s reputation abroad, while doing little to allay the Nazis’ suspicions of him.

Einstein’s distrust of the Copenhagen interpretation is well known, but it is often forgotten that Schrödinger shared his views

Into this crisis came a life-saving offer from neutral Ireland. Impressed by the Institute of Advanced Studies in Princeton, the Irish premier Éamon de Valera had decided to set up a similar institute in Dublin, with Schrödinger at the helm. The peripatetic professor accepted with alacrity and with the help of De Valera, he arrived safely in Dublin in October 1939.

He and his ménage, that is. Schrödinger set up house in Dublin with his wife Anna, his mistress Hilde and Ruth, his daughter by Hilde. As Gribbin points out, this arrangement was quite unusual in holy Catholic Ireland, yet it is a curious fact that the Schrödingers felt much more at home in Ireland than they had in Oxford. Gribbin offers the explanation that in Ireland “there was a marked contrast between what was officially approved and what people actually did”, which I think is about right. In any event, Schrödinger indulged in numerous romantic affairs in Dublin without sanction, producing two further children out of wedlock.

The description of Schrödinger’s years in Dublin is the most enjoyable part of Gribbin’s story and there are many moments of humour. For example, Gribbin describes how, in its early years, the Dublin Institute for Advanced Studies attracted the attention of the Irish Times satirist Myles na Gopaleen, who caused a stir when he observed that “Professor Schrödinger has been proving lately that you cannot establish a first cause. The first fruit of the institute, therefore, has been to show that there is no God.” The institute’s authorities were furious; Schrödinger himself was unperturbed. At the same time, Gribbin is careful not to underestimate the work Schrödinger did in Dublin, from his research in general relativity to his attempts at a unified field theory, from his work on the interpretation of quantum theory to his speculations in molecular biology.

The fact that the Dublin institute became a leading centre for the study of relativity forms an important part of Schrödinger’s legacy, but it is his work on molecular biology that is surely the most extraordinary aspect of his career. In 1943 Schrödinger gave a series of public lectures in Dublin in which he asked how hereditary information might be encoded in living cells. While much of the work he spoke about was not original, a book based on the lectures –- called What is Life? – went on to be a major influence in the field of genetics.

In the last chapter of the book, Gribbin considers Schrödinger’s interpretation of quantum theory from a modern perspective. He reviews several important developments of the past half-century, from the theoretical work of David Bohm and John Bell to the experiments of Alain Aspect and Anton Zeilinger. He then introduces the “many-worlds” interpretation and draws an intriguing connection between it and Schrödinger’s philosophy. There is an interesting point here, but the discussion is coloured by Gribbin’s own dislike of the Copenhagen interpretation.

This is a lucid biography of a brilliant scientist whose life and philosophy continues to intrigue. Although it contains little new historical material (apart from some lovely photographs and a nice surprise in the epilogue), Erwin Schrödinger and the Quantum Revolution is a cracking good read that will be enjoyed by physicists and non-physicists alike.

Web life

So what is the site about?

Ask Nature is a site devoted to biomimicry, an interdisciplinary field in which practitioners study how animals and plants solve problems, and then use those solutions to develop better human technologies. The site lists many instances of technology imitating life, including a surgical bandage inspired by gecko feet, a fog-harvesting mesh inspired by a desert beetle and a ceiling fan inspired by the seed pod of a sycamore tree. In total, there are nearly 200 examples of actual bio-motivated inventions described on the site, but they are just the tip of an iceberg of possibilities. Ask Nature also contains an astoundingly large catalogue of animal and plant strategies that might inspire solutions to human technological problems. Known as the “Biomimicry Taxonomy”, this catalogue contains around 1500 entries.

Can you give me some examples?

The Morpho butterfly keeps itself dry and clean in its rainforest environment thanks to nanostructures on its wings that make them both extremely hydrophobic and self-cleaning. Such structures have inspired new types of paint, textiles and glass that require less labour and fewer chemicals to keep clean. Another rainforest denizen – a medium-sized bird called a toucan – has an outsized beak that can be up to a third of its length, while making up only 5% of its weight thanks to the beak’s foam-like interior structure and thin outer layer. This light-but-strong construction might prove useful in ultralight aircraft components, or perhaps the panels in cars that protect people from injury during crashes. Not all of the taxonomy’s entries are rainforest species, but the richness and sheer biodiversity of these areas does seem to promote the development of novel adaptations. Yet another reason, if one were needed, to be concerned about their disappearance.

How is the taxonomy organized?

Each entry in the Biomimicry Taxonomy is assigned to one of eight “function groups”, which are in turn divided into 30 sub-groups and 162 separate functions. For example, the high-level function group “move or stay put” is split into two sub-groups, called “attach” and “move”. Attachment is further divided between permanent and temporary stickiness, while movement is grouped by travel that takes place in (or on) gases, liquids and solids. This seems sensible enough, but taxonomy is not always an exact science, and the system used in Ask Nature occasionally throws up a few anomalies. For instance, it seems odd that the hairy footpads of the fennec fox (which help it move over desert sand without slipping) are classed under “movement”, while the cloven hooves of the mountain goat (which help it move over rocky terrain without slipping) are categorized as “attachment”.

How should I use the site?

If your interest in animal science has been piqued by this special issue of Physics World, the Ask Nature site is a great place to learn more about the amazing adaptations that animals (and plants) rely on to survive. For casual browsers, the two dozen or so “featured strategies” on the site are a good place to start. These entries are more complete than most others, with detailed explanations, references and photos as well as basic explanations of strategies and their possible applications to human technologies. If you are looking for inspiration on a particular design challenge, though, you would be better off using the site’s extensive search function. To get the best results, you may need to do some lateral thinking. As the site puts it, you might want to ask “How would nature reduce drag?” or “How does nature move through air?” rather than something more direct, such as “How would nature design an efficient wind turbine blade?” In the process, you might even find that merely looking at the problem in a different way helps lead to a solution.

Quantum dots entangled with single photons

Two independent teams of physicists are the first to have entangled a single photon with a single electron spin held in a quantum dot. Thanks to the ease with which quantum dots can be fabricated and controlled, the breakthrough could lead to practical quantum computers and quantum communication systems.

Entanglement is a quantum effect that allows particles such as photons and electrons to have a closer relationship than predicted by classical physics. For instance, a photon–electron pair can be created experimentally such that if the photon polarization is measured to be in the vertical direction, a measurement of the electron spin would find its spin pointing in the same direction. This occurs in spite of the fact that a measurement on the photon (or electron) alone will reveal a random value.

This close relationship could be put to use in quantum computers, which could in principle outperform today’s classical computers. Photons are expected to play an important role in quantum computation because they can carry bits of quantum information (qubits) over long distances. However, photons cannot by their very nature stand still and stationary qubits such as quantum dots are needed to store quantum information. While researchers have already shown that trapped ions and defects in diamond crystals can be entangled with single photons, these systems can be difficult to work with on a practical level.

Colour or polarization

One research team included Kristiaan de Greve and colleagues at Stanford University and focused on quantum information stored in the polarization of the photons. The other team, headed by Atac Imamoglu at ETH Zurich, took a different approach that involves information stored in the wavelength of photons.

Quantum dots are tiny pieces of semiconductor that are compatible with conventional electronics and therefore offer a practical way forward. Both teams used quantum dots formed at the interface between two different semiconductors. A single electron can become trapped in the dot – and because the dot is so small, the electron inhabits a set of atom-like energy levels.

Quantum information can be stored in the spin of the electron – with “0” corresponding to spin up and “1” to spin down for example. In both the Stanford and ETH experiments the value of the qubit was set to the spin-up state by firing a “pump” laser pulse at the dots. Then, a second laser pulse is fired at the dot, which pops the electron into a higher energy state. This state can then decay to either a spin-up state with the emission of a “blue” vertically polarized photon or a spin-down state and a “red” photon that is horizontally polarized. Red and blue simply refer to the wavelengths of the photons, with the latter shorter than the former.

Too much entanglement

The process leaves the quantum dot entangled with both the colour and polarization of the photon. For the entangled states to be useful for quantum computing applications, it must involve only one property of the photon. Therefore an important challenge for both teams is how to destroy one type of entanglement without affecting the other.

De Greve and colleagues addressed this by lowering the energy of the photon in a process called “down conversion”. This is done by sending the photon through a special crystal that is pumped by an infrared laser. This process has the effect of “smearing together” the two colour states of the photon and removing that aspect of the entanglement – while preserving the polarization. An added benefit of the down-conversion process is that the photon emerges at a wavelength compatible with optical telecommunications systems

Meanwhile in Switzerland, Imamoglu’s team faced the opposite problem of erasing the polarization entanglement while sparing the colour. To do this they relied on the fact that a plane-polarized photon – with horizontal or vertical polarization – can be expressed as a superposition of circularly polarized photon states (clockwise and anticlockwise). The team passed their photon through a polarizing filter puts all photons into an anticlockwise state erasing the polarization entanglement. The laser pulses used to drive the entanglement process were set to have circular polarization, which means that the polarizing filter also prevented this light from swamping the detection of single entangled photons.

Measuring spin

In both set-ups the spin of the quantum dot is measured by firing a second laser pulse at the quantum dot. The result involves the emission of a photon with a polarization that is related to the spin state of the quantum dot. By measuring the correlations between the spin-measurement photon and either the colour or polarization of the qubit photon, both teams were able to prove entanglement.

Physicists have already shown that photons can remain entangled after travelling distances of over 100 km in air and therefore this latest development could offer a way to link quantum computers over large distances. Because entangled states are destroyed if an eavesdropper tries to intercept a message, the quantum-dot system could also find use in quantum encryption systems.

Both experiments are described in Nature.

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