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Blue LEDs and a revolution in light

Morning in Lindau: great scenery and wonderful talks
Morning in Lindau: great scenery and wonderful talks.

By Hamish Johnston at the Lindau Nobel Laureate Meeting in Germany

It has been a great morning of physics talks this morning here at the 66th Lindau Nobel Laureate Meeting. Hiroshi Amano, who shared the 2014 Nobel Prize for Physics for the development of the blue LED, spoke first about the practical aspects of his creation.

Electronic displays and low-energy lighting are two obvious applications for blue LEDs. Amano pointed out that LED lighting uses 1/8 the energy of incandescent bulbs and 1/2 that of fluorescent lights. But perhaps more importantly, he says that this low-energy operation means that light can be introduced to remote and poor parts of the world. This has the potential to boost education because it enables children in areas with no mains electricity to read and study at night.

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Talking about immigration with Nobel laureates in Lindau

Lakeside view: Lindau's harbour on Lake Constance
Lakeside view: Lindau’s harbour on Lake Constance.

By Hamish Johnston at the Lindau Nobel Laureate Meeting in Germany

I arrived in the German town of Lindau yesterday evening expecting it to be a sleepy little burg where I would struggle to find somewhere open to get a bite to eat. Instead I was greeted at the station by a cacophony of car horns and singing as Germany had just beat Slovakia and claimed its place in the next round of the Euro 2016 football tournament.

I’m here in the far south of Germany for the 66th Nobel Laureate Meeting. Tomorrow I will be hosting a “press talk” about how immigration continues to shape the scientific world. Last week’s momentous decision by the UK to leave the European Union is sure to come up in the panel discussion, which will include input from two chemistry Nobel laureates – Martin Karplus and Daniel Shechtman. I will also be joined on the panel by two early-career physicists: Winifred Ayinpogbilla Atiah from Ghana and Ana Isabel Maldonado Cid from Spain.

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Cats and causality: is your moggy an Isaac Mewton?

Causal connection: are cats feline physicists? (Courtesy: CC BY/David Corby)
Causal connection: are cats feline physicists? (CC BY David Corby)

By Hamish Johnston

It’s been a very difficult week for some UK-based physicists for reasons that you can read about here. Therefore I thought this week’s Red Folder should be a bit of a tonic, so here’s a combination that’s guaranteed to put smile on even the glummest face: cats, physics and the Internet.

“Cats seem to grasp the laws of physics,” at least according to Saho Takagi and colleagues at Kyoto University in Japan. It seems that our feline friends have a firm understanding of causality, as shown by their ability to recognize that an effect (an object falling out of an overturned container) is preceded by its cause (the noisy shaking of the object in the upright container). The cats quickly realized that a noisily shaken container would yield an object, but the silent shaking of an empty container would not.

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UK physics faces huge uncertainty after EU referendum result

Physicists in the UK are coming to terms with the shock decision taken by the UK electorate to leave the European Union (EU). The withdrawal follows yesterday’s national referendum, which resulted in 51.9% of voters choosing to leave. The dramatic verdict throws up many deep and worrying questions about how UK physics will fare as a non-EU member.

Voters in the UK referendum were asked one simple question: “Should the United Kingdom remain a member of the European Union or leave the European Union?” Now that the UK has voted to leave, the UK’s prime minister will be expected to notify the European Council of the UK’s intention to withdraw. This will then set in place Article 50 of the EU’s governing Lisbon Treaty that will give the remaining 27 member states up to two years to decide on the arrangements for the withdrawal, including details of the UK’s future relationship with the EU.

Many scientists and officials – both within the UK and Europe – have voiced their fears about the damaging impact the vote will have on physics. Roy Sambles, president of the Institute of Physics (IOP), which publishes physicsworld.com, says there are “wide-ranging implications” of the UK leaving the EU, adding that membership has had a significant positive impact on UK science. He adds that membership has led to “some of the continent’s greatest scientific minds working in the UK” and that the current success and strength of UK physics and the wider scientific community “is very much entwined with the strength of research within Europe and around the world”.

Rolf-Dieter Heuer, president of the German Physical Society (DPG), says that the society “regrets” the decision. “Europe risks losing a strong and valuable research partner at a time when cross-border collaboration in science is needed more than ever,” says Heuer. “The solution to today’s demanding global challenges requires more internationally oriented science, not less. Rather than going our separate ways, we should be strengthening our international scientific ties.”

Heuer, a former director-general of the CERN particle-physics laboratory, says that the withdrawal will have “far-reaching consequences” for UK science and universities, with access to EU funding programmes becoming “significantly more difficult, if not impossible”, and that the mobility of UK researchers within Europe will be “compromised”. However, he adds that the DPG will continue to “strengthen and develop the very positive relationship” with the IOP and that he is convinced that UK researchers “will continue to show their strong commitment to European science”.

Changing times ahead

While EU cash for science in the UK only represents about 3% of the UK’s annual R&D expenditure across industry and academia, which stood at £30.6bn in 2014, UK science has traditionally done well out of EU funding. The UK’s Office for National Statistics estimates that the UK contributed €5.4bn between 2007 and 2013 to research spending in the EU and received back €8.8bn – an overall gain.

UK researchers were also successful when it came to winning grants from the EU Seventh Framework Programme, which ran from 2007 to 2013. Scientists in the UK came seventh overall, with a success rate of 22.6%. Over the same period, researchers from the UK won €1.7bn in grants from the European Research Council – 22.4% of the total and more than any other nation. The UK also receives about €1.1bn of Marie Skłodowska-Curie Actions – awards that enable researchers to work in different countries, sectors or disciplines.

Venki Ramakrishnan, president of the Royal Society, warns that any failure to maintain the free exchange of people and ideas between the UK and the international community, including Europe, “could seriously harm UK science”. He adds that many global challenges can only be tackled by countries working together. “In negotiating a new relationship with the EU we must ensure that we do not put unnecessary barriers in place that will inhibit collaborations,” says Ramakrishnan, who shared the 2009 Nobel Prize for Chemistry. “We must make sure that research, which is the bedrock of a sustainable economy, is not short-changed, and the government ensures that the overall funding level of science is maintained. One of the great strengths of UK research has always been its international nature, and we need to continue to welcome researchers and students from abroad.”

Free movement concerns

Paul Coxon, a materials scientist at the University of Cambridge, told physicsworld.com that the freedom of movement of researchers within the EU has allowed the UK to maintain its position as “one of the world’s science superpowers” but that is now at risk following the decision to leave. “The EU is now developing its own science policy to address the major challenges facing us in the future such as climate change, renewable energy and health. Leaving the EU means the UK will lose its role in helping set the agenda,” he adds.

The risk of losing the free movement of people to and from the UK also worries Sarah Main, director of the Campaign for Science and Engineering – a non-profit organization that promotes science and engineering in the UK. “The free movement of people in the EU made it easy for scientists to travel, collaborate and share ideas with the best in Europe and for companies and universities in the UK to easily access top talent from Europe,” she says, adding that it is now vital that science is “on the table” when discussions about the future of the UK’s relationship with the EU are made.

Remaining issues

Another vital issue is the UK’s contribution to major facilities in Europe such as CERN and the European Southern Observatory. It is possible for individual nations in these organisations to be non-EU members, but again that position would require a complex renegotiation. More difficult still is membership of facilities such as ITER (of which the EU as a whole is a member) as well as the European Space Agency, which is an independent intergovernmental organization, but which maintains close ties with the EU.

There is also a huge question mark over how quitting the EU will affect UK universities. According to the Higher Education Statistics Agency, some 14% of research and teaching staff in UK university departments are from non-UK EU countries. In physics departments, this figure rises to an average of 24%, although in some departments up to 50% of staff come from elsewhere in the EU.

Britain’s withdrawal will in addition affect the opportunities for international students coming to the UK. Non-UK EU students currently make up 5% of all students at UK universities and 17% of physics PhD students. EU students are treated as UK students for funding purposes, meaning that they are charged annual “home” fees of up to £9000, rather than the £18,000 or more paid by students from outside the EU. Recruitment of EU students could now become more complicated and more expensive, especially if EU students are subject to visa regulations.

Wendy Piatt, director-general of the Russell Group – a collection of 24 leading UK universities – warns that leaving the EU has now created “significant uncertainty” for UK universities. “The UK has not yet left the EU so it is important that our staff and students from other member countries understand that there will be no immediate impact on their status at our universities,” she says. “However, we will be seeking assurances from the government that staff and students currently working and studying at our universities can continue to do so after the UK negotiates leaving the EU.”

Julia Goodfellow, president of Universities UK, says leaving the EU will create “significant challenges for universities”. “Our first priority will be to convince the UK government to take steps to ensure that staff and students from EU countries can continue to work and study at British universities and to promote the UK as a welcoming destination for the brightest and best minds,” she says. “They make a powerful contribution to university research and teaching and have a positive impact on the British economy and society. We will also prioritize securing opportunities for our researchers and students to access vital pan-European programmes and build new global networks.”

Sambles, meanwhile, says that the IOP will continue to remind the UK government of the importance of science. “Whether the UK is in or out of the EU, science creates jobs and supports economic growth,” says Sambles. “We will be working with the government and the science community to ensure that these issues are being dealt with during the difficult negotiations of the next few months.”

How LIGO’s merging black holes formed from two massive stars

Physicists in Poland and the US have carried out computer simulations that shed light on the events leading up to the famous creation of gravitational-wave GW150914, which was detected last year by the LIGO observatory in the US. Their research paints a vivid picture of how two huge stars could have evolved over just a few million years into a pair of black holes that then merged, emitting a gravitational wave that eventually reached Earth. Such mergers, the team concludes from the simulations, are more common than thought, suggesting that the next generation of gravitational-wave detectors could rack up hundreds of similar observations every year.

Announced by members of the LIGO collaboration in March, the first-ever detection of a gravitational wave was made on 14 September 2015 by LIGO’s two giant interferometers. Astrophysicists reckon that GW150914 was created by a black hole about 36 times as massive as the Sun merging with a smaller black hole of about 29 solar masses. The resulting black hole is expected to be 62 solar masses, which makes these objects the three largest “stellar-mass” black holes known to astronomers (ignoring the much bigger supermassive black holes at the centre of many galaxies, including our own Milky Way).

Krzysztof Belczynski and Tomasz Bulik from the University of Warsaw have now joined forces with Daniel Holz of the University of Chicago and Richard O’Shaughnessy of the Rochester Institute of Technology to use a numerical model of stellar evolution to sketch out the events that led up to the creation of GW150914. Their simulations suggest that the story of GW150914 began about 12 billion years ago – just two billion years after the Big Bang – with the birth of two huge stars, each having about 40–100 solar masses.

Biggest and brightest ever

These stars, which formed from mostly hydrogen gas, were able to grow to huge sizes and would have been some of the most massive and brightest stars ever to exist in the universe. Stars like the Sun, in contrast, formed much later in the history of the universe, and could not get so big because they incorporated heavier elements created by previous generations of stars.

The simulations suggest that these stars orbited each other in a binary system for just four million years before one became a black hole without first exploding in a supernova. While the idea that a huge star could turn into a black hole without a supernova is controversial, it could help to explain why binary black holes as massive as those that created GW150914 came into existence.

No supernovae

The new black hole, the researchers believe, was then engulfed by the remaining star, such that the two objects shared the same envelope of gas. At this point, the astronomers surmise that the remaining star became a compact, hot core that then made the transition to a black hole – again without a supernova.

The team reckons that the conversion of the stars into black holes happened relatively fast – in just over five million years. This binary black-hole system then existed for a further 10 billion years, with the two objects slowly spiralling into each other before the merger occurred. This created GW150914, which then took 1.2 billion years to reach Earth.

While the regions of “pristine” hydrogen gas where the two massive stars formed are relatively uncommon in the universe, they should be highly visible to gravitational-wave detectors such as LIGO. “Because LIGO is so much more sensitive to these heavy black holes, these regions of pristine gas that make heavy black holes are extremely important,” explains O’Shaughnessy. “These rare regions act like factories for building identifiable pairs of black holes.”

Despite the rarity of the progenitors of GW150914, O’Shaughnessy and colleagues calculate that once the LIGO detectors are operating at their maximum sensitivity, they could be detecting as many as 1000 black-hole mergers per year in the 20–80 solar-mass range. Indeed, just last week, LIGO announced its second discovery of a gravitational wave, this time from a black-hole merger at about 21 solar masses.

The simulations are described in Nature.

Could on-chip quantum frequency combs lead to scalable quantum computers?

Quantum mechanics offers fundamentally new opportunities for processing information. In contrast to the classical notion of a bit, where the state of a physical system such as a transistor corresponds to either zero or one, a quantum bit (qubit) is in a superposition of zeros and ones. By combining multiple qubits it is possible to form entangled qubit states, in which the unique, non-classical correlations of qubits enable quantum computation.

Manipulating a single qubit changes the whole ensemble and therefore the properties of each entangled qubit, which means that a quantum computer can work in parallel on many computations at once. So, for certain types of problem, a quantum computer can solve tasks exponentially faster than a classical computer, with revolutionary consequences for fields ranging from cryptography and nanotechnology to pharmaceuticals and artificial intelligence.

For certain types of problem, a quantum computer can solve tasks exponentially faster than a classical computer, with revolutionary consequences

Generating multiple entangled qubit states that are both stable and controllable, however, is a massive technological challenge. Systems considered so far have tended to rely on complex and expensive technology to generate quantum superposition among agglomerations of atoms, ions or electrons. But these only work at cryogenic temperatures and have to be extensively shielded from electromagnetic radiation, which limits their scalability and thus their commercial potential.

A more practical qubit candidate is the photon, which interacts less with the surrounding environment and so allows a longer decoherence time, which is the average time after a quantum state is destroyed under normal operating conditions. The other advantage of light is that a perfect framework for sharing photon qubits already exists: the optical fibre network currently used for classical communication. The challenge to enable practical applications is to generate entangled photon states simultaneously in a compact, scalable and cost-effective way.

Towards photonic chips

Ever since Einstein’s explanation of the photoelectric effect more than a century ago, photons have served as powerful quantum laboratories. In the past few decades, optics experiments have revealed the intriguing “non-local” character of entangled photons. It allows phenomena such as quantum teleportation and cryptography, and numerous different entanglement schemes have been investigated and demonstrated. The photon offers several different degrees of freedom that can be successfully entangled, such as polarization, frequency, time, position and orbital angular momentum. Each has advantages and disadvantages depending on the particular application for which it will be used.

While polarization is probably one of the first and most studied degrees of freedom for entanglement, its applications are limited because it is restricted to two perpendicular values, such as horizontal and vertical, and because it is difficult to transport polarization states faithfully through optical fibres. Since scalability is the key to building a quantum computer that can do useful tasks, the ability to generate tens or hundreds of entangled qubits is vital. This could, in principle, be achieved using bulk optical set-ups such as those found in many research laboratories, but it would require hundreds or thousands of optical elements such as mirrors, beamsplitters and phase-shifters – all of which must maintain a certain degree of coherence, therefore hampering this approach.

A promising alternative, which is being investigated by many groups worldwide, is to miniaturize bulk optical set-ups by fabricating them on a photonic chip. Such integrated approaches have only become realistic in recent years thanks to the availability of single splitters and phase shifters at such scales, but it is still not clear how long it will take to reach the fabrication capabilities necessary for building on-chip optical set-ups with the required complexity.

One promising way to remedy this situation is to exploit the frequency domain, where many photons at different wavelengths can be transmitted within the same fibre or waveguide. By combining this with the temporal degree of freedom – whereby information is encoded in the arrival time of single photons (so-called time-bin entangled qubits) – it is possible to generate more interconnected quantum states. This frequency-domain approach is exactly the route currently being investigated by means of quantum frequency combs.

Multiphoton entanglement

Classical frequency combs are ingenious devices that generate a series of equally spaced frequency components, a bit like the teeth of a comb. They can be produced, for example, in the optical regime by mode-locked lasers, where the phase of each spectral component is controlled to generate a train of ultrashort pulses in the temporal domain. The ability to manipulate these structures allows frequency combs to serve as extremely precise optical rulers. These instruments have revolutionized metrology and spectroscopy, and were honoured by the 2005 Nobel Prize for Physics.

Frequency combs are now attracting attention in the quantum regime, where their multifrequency-mode characteristics could create many entangled optical quantum states at once. Initially, these investigations were based on bulky and complex set-ups using nonlinear effects in crystals implemented in optical free-space cavities. But thanks to parametric nonlinear effects – namely spontaneous four-wave mixing (SFWM) – in micro-scale cavities, we can now make classical frequency combs much less complex. An immense research effort over the last 10 years has led, in the past year, to the generation of equidistantly spaced and phase-locked frequency components in a compact, integrated format (Science 351 357).

This advance led our group at the INRS in Canada to combine integrated frequency combs and photon-pair generation in order to generate complex optical quantum states that address the issues of scalability, practicality and compactness for future quantum technology implementations (Science 351 1171). The result is the culmination of a six-year-long effort to combine recent advances in integrated nonlinear optics with quantum optics (Opt. Express 22 6535; Nat. Commun. 6 8236). Specifically, our approach uses a ring waveguide microcavity that exhibits nearly equidistantly spaced resonances (i.e. frequencies that are allowed to oscillate within the cavity), thus defining many accessible frequency channels. Importantly, the device is in an integrated format and can be fabricated using chip-scale semiconductor technologies, thereby allowing it to be incorporated into established electronics platforms.

Powering up the device sets in motion a chain of events that ultimately leads to the generation of entangled states. First, a coupled laser field induces nonlinear frequency conversion, arising from the cavity-enhanced light–matter interaction inside the microring. The resulting nonlinear effect, SFWM, annihilates two photons of the excitation field and generates two new photons on a resonance pair symmetric to the excitation frequency. The microring was designed so that the SFWM process operates over a large bandwidth, leading to several tens of resonance pairs with entangled qubits. Using time-bin entanglement, which encodes quantum information in terms of the relative arrival times of light pulses, a single photon that passes through an interferometer with unbalanced arms will be in a superposition of early and late arrival times, representing the superposition of 0 and 1 for a standard bit. This constitutes a robust single-photon qubit that is compatible with waveguide and fibre network implementations.

Remarkably, the unique multimode structure of the quantum frequency comb readily allows us to combine two entangled photon pairs to generate larger multiphoton qubit states, as required by a useful quantum computer. By selecting four resonances and post-selecting the simultaneous arrival of four photons, we have generated entangled four-photon qubit states within the quantum frequency comb. Furthermore, our results pave the way to a system that can in principle directly generate many qubits. We were able to verify the generation of these entangled states using quantum-interference measurements, which revealed a degree of interference higher than any possible classical implementation.

Outlook

Over the past few years various research groups have been working towards integrating quantum light sources on-chip, while others have been striving to generate complex quantum states in a frequency-comb arrangement based on bulk and elaborate optical set-ups. We have managed, for the first time, to bring together these two very different and seemingly disconnected approaches.

Further improvements of our scheme will let us generate and control multiphoton entangled states, which is a necessary step towards practical quantum computing. This will require us either to properly tailor the pump laser or to introduce quantum gates for manipulating the time-bin qubits on-chip. In addition, while the main photon source (the nonlinear microcavity) is already integrated on-chip, other components, such as the interferometers and filters, are currently fibre-based. To reach practical detection rates, we need to integrate such components directly on-chip, while a complete on-chip source will require hybrid technology to also integrate the laser and the single-photon detectors.

This is an ongoing challenge for the integrated optics community, and different groups around the world are working hard on semiconductor lasers and detectors to reach this goal. Provided that funding is sustained, we predict the first commercial applicability of our scheme in the next 10 years, opening a new era in information processing.

China’s tunnel vision

On 4 July 2012 the world took note of particle physics. The announcement that the existence of the Higgs boson had finally been confirmed (at the prerequisite “5 sigma” level of significance) by researchers working on the Large Hadron Collider (LHC) at CERN was met with cheers and heartfelt congratulations, both inside the European particle-physics laboratory and out. This was a momentous result, given that the Higgs boson and its associated field represent an essential cornerstone of the Standard Model of particle physics.

In their book From the Great Wall to the Great Collider, science journalist Steve Nadis and Harvard physicist Shing-Tung Yau embrace that momentum and the community’s great excitement to make a compelling case for investment in the next-generation particle accelerator. While construction of the LHC was a “no-lose” project (because the machine would either find evidence of the Higgs’ existence, or rule it out and force physicists to develop new theories), the arguments in support of an even bigger – and more costly – collider are more subtle. Now that the Higgs has been found, the foremost mysteries on particle physicists’ minds include the nature of the elusive “dark matter” and the question of why the universe contains vastly more matter than antimatter – a conundrum known as charge–parity violation. In the grand scheme of our understanding of the fundamental basis of nature, these questions are every bit as exciting as the Higgs, and answering them may, similarly, require a mammoth new machine.

In their book, Nadis and Yau attempt to merge two stories into one, providing both a fairly detailed account of the history of particle physics and a description of how particle physicists all over the world (not just in China) hope to embark on the next big project in our quest to uncover the remaining secrets of the universe. Using the Great Wall of China as a metaphor, they paint a picture of China’s scientific development that shows a nation full of ambition and aspirations, one that is rapidly catching up with the international community and taking on a leading role where possible. Whereas the Great Wall was constructed to keep foreign invaders away from the Chinese heartland, the proposed “Great Collider” would embrace the international scientific community and propel Chinese science to a leading role on the world’s stage.

Having recently re-read Dennis Overbye’s brilliantly written history of cosmology and particle physics, Lonely Hearts of the Cosmos, I was keen to learn about the latest developments in this fast-moving field and read first-hand where we might be going from here. As this book is aimed at a general readership, the authors clearly felt that they had to provide a detailed background to the field of particle physics, as well as a proper international context. This is where I got confused. First, as a professional physicist, I found their introduction to particle physics tedious and scholarly, requiring a significant level of prior knowledge. A concise summary of definitions and particle types would have been useful indeed. The narrative reads like a scientific manuscript that has been toned down somewhat to the level of us mere mortals. I am afraid that this feels a little forced. Certainly, it is a far cry from Overbye’s eloquence, and the authors’ composition falls into many of the pitfalls outlined in Steven Pinker’s The Sense of Style – arguably the current best style guide for aspiring authors.

All of this genuine excitement about particle physics is wrapped in an overtly political message to Chinese policy-makers

This leads me to wonder who the authors considered as their intended audience. On the one hand, the introduction to particle physics appears aimed at undergraduate physics students. On the other, the excitement of the Higgs discovery comes across palpably, and so does the scientists’ ambition to uncover the remaining secrets of the universe by embarking on an even bigger adventure. The reader gets a great impression of where the cutting edge is in this field, where key open questions cover such diverse topics as the conditions that prevailed a mere fraction of a second after the Big Bang; whether the Standard Model is a one-size-fits-all theory or may need further and significant modifications (such as the addition of much more massive fundamental particles); and the scientific surprises and returns much higher-energy accelerators could yield. Another key question is whether multidimensionality – most notably, string theory – could overcome the so-called Higgs “mass hierarchy” problem, which is the discrepancy of 16 orders of magnitude between the mass of the Higgs (measured independently by both the ATLAS and the CMS detectors at CERN) and the Planck mass predicted by the Standard Model.

Yet all of this genuine excitement is wrapped in an overtly political message to Chinese policy-makers, suggesting that the development of a 70–100 TeV electron–positron accelerator, followed by a proton–proton collider using the same basic infrastructure, would cement China’s place in the international scientific pecking order. It is unfortunate that the authors chose to start their narrative with such a political statement in the guise of the book’s introduction, which was clearly composed after the main text had been completed. As a consequence, a number of key technical terms – symmetry, string theory, M theory – are introduced from the outset but not addressed or defined until later chapters.

Nevertheless, Nadis and Yau make a very compelling case for particle physics as a clear technology driver, one that has tremendous potential for spawning numerous (as yet unknown) spin-off applications, both technical and in terms of China’s position as a leading scientific nation. With the country’s gross domestic product robustly on the rise, China will soon overtake the US as the leading economy in the world. Its scientific workforce is incredibly keen to take on and indeed lead large international projects, a perspective broadly supported by the country’s technologically savvy leadership. Time is of the essence, however. Research and development to get a gargantuan effort such as the Circular Electron–Position Collider and its proposed successor, the Super Proton–Proton Collider, on its way towards construction and operation by the time CERN’s LHC runs out of steam must start soon. The world is looking for Chinese leadership, and this seems like an excellent opportunity to step up to the challenge. This is the key message I take away from Nadis and Yau’s plea for the world to get prepared for the next half-century of excitement in particle physics.

  • 2015 International Press $29.95hb 214pp

Web life: Gravity and Levity

So what is this site about?

Gravity and Levity is the personal blog of Brian Skinner, a theoretical condensed-matter physicist who began it back in 2009, when he was a PhD student at the University of Minnesota, US. He’s now a postdoc at the Massachusetts Institute of Technology, where his research focuses on the behaviour of strongly correlated electronic systems, such as low-dimensional electron gases and materials such as graphene.

What topics does it cover?

Skinner takes the view that “physics as a whole is much more interesting than my own meagre contributions to it”, and he isn’t really into physics news, politics or gossip either. So, if you’re interested in reading about the latest theoretical condensed-matter research, or about day-to-day life as an academic, you’ve come to the wrong place. Instead, Gravity and Levity focuses on some of the “big ideas” in physics, taking deep but mathematically simple looks at topics such as the size of an electron, the relationship between physics and maths, and why elements containing more than 137 protons cannot exist (at least according to current theory). If you think that sounds like a pretty eclectic list, you’re right, and it doesn’t end there. Skinner has also written posts about the mathematics of basketball, Cooper pairs and even “the field theory of swords”.

Swords? As in, big metal pointy things?

The same. At the end of 2015, Skinner wrote a series of posts about the nature of particles and fields, and in the series’ final post, he began by observing that one could use this same basic framework to describe the behaviour of macroscopic objects – such as swords. The rest of the post explains that a metal sword can be modelled as an orderly lattice of atoms, and describes how its strength and rigidity arises from defects (more specifically, disordered dislocations) in that lattice. Hence, when a blacksmith is forging a sword, “his effort is largely going into creating a tangled knot of dislocations inside the metal”. It’s a beautiful, fascinating and eminently relatable explanation, and that’s exactly what you can expect from reading Gravity and Levity.

Can you give me a sample quote?

From an April 2014 post about the role of symmetry in physics: “I would like to suggest using the phrase ‘to be a perfectly symmetric ass’ as a description of someone who is being paralysed into inaction by symmetry. For example, suppose someone asked you to predict what will happen if you apply a large voltage between a small inner sphere and a large outer sphere that is filled with a weakly conducting plasma. Most of us who had Gauss’s law arguments trained into us would immediately say that an electrical current will flow out from the inner sphere in a radially symmetric way, and consequently that the total current flow will be very small. But most of us would be wrong, because what actually happens is the system figures out very quickly that there is a much lower-energy way to move its current from inner to outer surface. Namely, by creating sharp (symmetry-breaking) pathways with intense current, which produce dielectric breakdown of the plasma and allow the current to flow easily. If you allow symmetry to fool you into thinking that the current will flow slowly and radially, then you…have ‘made a perfectly symmetric ass of yourself’. I say it lovingly, of course, because I make a perfectly symmetric ass of myself all the time.”

From physics degree to Hollywood

By James Dacey

Rob Pieké. (Courtesy: Manisha Lalloo)

This summer many of you will watch smoke billowing out of buildings as yet another villain wreaks havoc on the New York skyline in the latest Hollywood blockbuster. I’m willing to bet that as you eat your popcorn you won’t be thinking about the Navier–Stokes equations of fluid dynamics. (Well, perhaps you will now that I’ve mentioned it!)

In fact, part of the reason that virtual smoke in films looks so realistic is because visual effects (VFX) specialists have applied the Navier–Stokes equations to their graphics. This was one of the interesting tidbits I learned from a talk yesterday in London by Rob Pieké, head of software at Moving Picture Company (MPC).

Pieké was speaking as part of a half-day event on “physics and film” organized by the Institute of Physics, which publishes Physics World. The gist of his presentation was that basic physics principles are used in a variety of ways to create special effects that capture viewers’ attention. “The audience wants to see something fantastical but grounded in reality,” said Pieké. Another example he gave was how naturally bouncing hair in computer-generated characters is modelled on mass—spring systems. Each individual hair could be modelled on as many as 30 masses connecting by springs.

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Experiment is first to see kicking photons heat up nanoparticles

The warming effect of tiny “kicks” from photons colliding with a microscopic particle has been measured for the first time by physicists in Switzerland. These kicks put a fundamental limit on how particles can be cooled when confined in optical traps. Gaining a better understanding of this effect could lead to experiments in which a trapped particle is put into a well-defined quantum state. Such particles could then be used to study how such states evolve under the influence of gravity.

A particle can be confined in an optical trap by a potential-energy well created by the electromagnetic field of laser light. The confined particle undergoes harmonic motion with amplitude that depends on its energy. Individual atoms and ions can be addressed separately by a laser and their quantum states can be manipulated by photons. However, in most cases, the dominant heating mechanism for trapped macroscopic particles is thermal collisions with residual gas molecules that remain in the trap after it is evacuated.

Feedback cooling

In this latest experiment, a team led by Lukas Novotny of ETH Zurich confined silica nanoparticles of approximately 50 nm radius in an optical trap using an infrared laser, before sucking out almost all of the air to create an ultrahigh vacuum in the chamber. The researchers used a process called feedback cooling, in which the trapped particle’s position is monitored and the trap frequency modulated accordingly, to draw energy out of the particle’s motion and reduce its temperature to the microkelvin level.

At pressures greater than 10–9 atmospheres, they found that the lowest temperature they could reach was determined by pressure. Below this, however, the researchers found that the lowest achievable temperature approached a lower limit, irrespective of pressure. This, they say, means that at very low pressures, the dominant limitation on the cooling is electromagnetic noise present in the trap.

Recoil energy

To investigate further, the team switched off the feedback cooling at very low pressure, allowing the photons that trapped the particle to heat it freely. The gap between the energy states of the oscillating particle is orders of magnitude larger than the recoil energy from each individual photon collision, so most of the photons simply scattered off the particle elastically with no effect on its energy. Occasionally, however, photons managed to kick the nanoparticle into a higher oscillation state, gradually warming it up. The warming process was random, but, by repeating it many times and averaging their results, the researchers managed to generate smooth warming curves showing that, when trapped with a higher-power laser, the particle heated up faster.

“This really shows the difference compared to thermal heating [from collisions with gas molecules],” says Novotny. “Surely thermal heating would not depend on the laser power.” The researchers also showed that the rate of warming depended more strongly on particle volume than expected from thermal heating.

“High-hanging fruit”

The team is now looking at ways to make the particle even cooler, until its quantum nature becomes genuinely measurable. In such a situation, it would be interesting, says Novotny, to switch off the laser and track the particle’s behaviour. “We know where the particle should end up and when…any deviation from that would indicate that there must be additional terms that generate heating or decoherence or whatever.” The ultimate ambition, Novotny says, would be to look at the effect of gravity on the quantum state of the particle. “We have here a massive particle that feels gravity: if we release it in the trap, it falls. We could prepare the particle in a desired quantum state and measure how the quantum state evolves under the influence of the gravitational field. I think this is the true Holy Grail, but this is a very high-hanging fruit!”

Andrew Geraci of the University of Nevada in Reno, who was not involved in the research, says the effect has long been predicted. “Having seen a demonstration where the theory is in pretty good agreement with the experiment, one can predict that this will be a limitation at the level they’ve shown,” he explains. “One of the goals of their group, our group and many other groups in the field, is to try and cool the particle to its quantum ground state. One needs to overcome this sort of mechanism, and now it’s been measured and understood better, I think it will guide the community in what really has to be done to circumvent this limitation.”

The research is described in Physical Review Letters.

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