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Quantum measurement is for the birds, but is not essential for plants

A new general approach for evaluating the “quantumness” of biological processes such as the ability of some birds to sense the Earth’s magnetic field has been developed by physicists in Switzerland and the US. It involves describing the process as a “quantum meter” that uses quantum coherence to measure magnetic-field strength or light intensity. Atac Imamoglu of ETH Zürich and Birgitta Whaley of the University of California, Berkeley, have applied their framework to bird navigation and photosynthesis, and have concluded that only the former is completely dependent on quantum coherence.

Scientists believe that some species of birds navigate using Earth’s magnetic field – an idea known as magnetoreception that is backed up by experiments that show that captive birds will respond to changing magnetic fields. Understanding how this happens is tricky. Although electron spins in biological molecules are affected by the Earth’s magnetic field, the size of the effect is so small that it should be completely washed out by thermal fluctuations. However, some quantum systems can be extremely sensitive to external magnetic fields, and this is why scientists believe that some birds could navigate by making quantum measurements.

Radical measurements

One such bird is the European robin, which appears to have magnetoreceptor molecules located in its visual system. Physicists believe that the measurement process is triggered when a “cryptochrome” protein absorbs light. This causes a flavin adenine (FAD) nucleotide on the protein to form an excited singlet spin state, which involves two electron spins with a combined spin of zero. This state then decays in picoseconds to a “radical pair state” in which the spin of one of the FAD electrons is transferred to an amino acid that is located about 1.5 nm away along the length of the protein.

This transfer is believed to preserve quantum coherence and because each spin is isolated from its surroundings, the resulting radical pair remains in a coherent quantum state for times greater than 10 ns. This, physicists believe, should be long enough for a robin to make a quantum measurement.

The direction in the protein along which this separation occurs provides a spatial reference for measuring the Earth’s magnetic field. In particular, the relative orientation of the separation direction and the Earth’s field affects the rate at which the radical pair will decay to a protonated state that provides a signal to the bird’s nervous system. Scientists believe that it is this protonated state – or subsequent chemical reactions – that links the bird’s sensory system to the magnetic-field measurement.

Success hinges on coherence

In this new work, Imamoglu and Whaley developed a general approach for looking at the interactions involved in the magnetic-field measurement, to work out whether the system is indeed a quantum meter. In the case of magnetoreception, they conclude that the measurement process hinges on the long-lived quantum coherence of the radical pair. Indeed, Imamoglu told physicsworld.com that quantum coherence boosts the ability of the system to measure magnetic fields by many orders of magnitude.

However, when Imamoglu and Whaley applied their analysis to photosynthesis, they came to a very different conclusion. In this case, the quantum meter is a collection of chromophore molecules, which transfer energy from absorbed sunlight to a “reaction centre” where the energy is extracted in the form of mobile electrons. Therefore, the quantum meter measures the intensity of the sunlight in terms of the rate at which electrons are produced.

The measurement process begins with sunlight “pumping” the chomophores from their electronic ground state into an excited state. Energy is then transferred from this state to the reaction centre by excitons (electron–hole pairs) that must first find their way through a labyrinth of chromophores. This involves hopping from molecule to molecule in a process similar to a random walk. This transfer occurs more rapidly and more efficiently than expected. This has led some physicists to suggest that the excitons travel through the chromophores via a coherent quantum superposition of all possible pathways, which could allow the excitons to find the most efficient route to the reaction centre with very few excitons being lost along the way.

Minor improvement

To decide whether coherent transfer makes a difference, Imamoglu and Whaley looked at the relevant timescales. If the excitons remained coherent for relatively long periods of time, they should be more likely to reach their destination and therefore boost the performance of the quantum meter. What the researchers found, however, is that this enhancement is at best 5–10%, and therefore photosynthesis could function without the need for quantum coherence.

Gregory Scholes of Princeton University told physicsworld.com that the role of quantum coherence in photosynthesis is still a matter of scientific debate. “My opinion is that coherence (whether or not it’s quantum I don’t specify) is the unavoidable consequence of fast energy transfer in a compact light-harvesting complex,” he explains. “So nature may not be targeting coherence, but it may use it ‘unknowingly’ by optimizing energy transfer rates.”

The research is described in Physical Review E.

Zombie outbreaks in San Antonio

Where are you going to run to? (Courtesy: iStockphoto/Renphoto)

By Michael Banks in San Antonio, Texas

If you ever find yourself in the unfortunate position of trying to survive a zombie apocalypse in the US, what should you do?

Well, according to Alex Alemi of Cornell University and colleagues, you should head to the Rocky Mountains or the Nevada desert.

Using 2010 US census data for population levels around the country, Alemi and colleagues used statistical mechanics to model how a zombie outbreak would spread.

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Gravitational lensing creates ‘Einstein’s cross’ of distant supernova

Multiple images of a supernova created by gravitational lensing have been captured for the first time by an international team of astronomers using the Hubble Space Telescope (HST). The “Einstein cross” pattern comprises four images of a distant supernova created by the gravitational lensing of its light as it passed a distant galaxy within a cluster of galaxies on its way to Earth. In addition to giving us a closer look at the dynamics of distant supernovae, the team says that its discovery will help to improve our understanding of the distribution of dark matter in the lensing galaxy and galaxy cluster, as well as to test Einstein’s general theory of relativity and measure the rate of cosmic expansion in the universe.

A gravitational lens is a large galaxy or group of galaxies that bends or “lenses” light from a distant source as it travels towards an observer. The effect was predicted by Einstein’s general theory of relativity and the first such lens was discovered in 1979. Sometimes, the distant light source, lensing galaxy and the observer line up precisely, and we can see an “Einstein ring” – a perfect loop of light from the source encircling the lensing mass. But if there is any misalignment along the way, we observe partial arcs or spots. Depending on the relative positions of the bodies, four such spots can be seen, forming an Einstein cross. The lensing effect serves as a “natural telescope” for astronomers, who can determine the mass of the lensing galaxy and its dark-matter content based on the amount of distortion observed.

Long search

“It’s a wonderful discovery,” says Alex Filippenko of the University of California, Berkeley, who is part of the team that found the latest quadruple-lensed supernova image, explaining that researchers have been “searching for a strongly lensed supernova for 50 years, and now we’ve found one”. Thanks to the many conditions that need to be fulfilled for a gravitational lens to be seen from Earth, and the relatively short lifetime of a supernova, such a lensed supernova with four images has never been seen before.

Even more interesting, thanks to an understanding of the peculiarities of gravitational lensing, the team already knows that a fifth image will appear in the next decade. This will give astronomers a “replay” of the supernova, because light can take various paths around and through a gravitational lens and therefore arrive at Earth at different times. This is particularly rare and useful, because astronomy is not normally a predictive science. “The longer the pathlength, or the stronger the gravitational field through which the light moves, the greater the time delay,” says Filippenko.

The team used a computer model to predict the pathways that the light from the supernova can take around the lensing cluster, which also suggests that we already missed out on seeing earlier images of the exploding star 10 and 50 years ago. The team has dubbed the distant supernova SN Refsdal (after the late pioneering astrophysicist Sjur Refsdal), and it is located about 9.3 billion light-years away (redshift 1.5), near the edge of the observable universe, while the lensing galaxy is about 5 billion light-years (redshift 0.5) from Earth.

Multiple replays

“Basically, we get to see the supernova four times and measure the time delays between its arrival in the different images, hopefully learning something about the supernova and the kind of star it exploded from, as well as about the gravitational lenses,” says team member Patrick Kelly, also at Berkeley, who discovered the supernova while looking through infrared images taken by the HST last November.

The galaxy that splits the supernova’s light is part of a large cluster – MACS J1149.6+2223 – that was discovered more than 10 years ago. In 2009 astronomers reported that the cluster created the largest known image of a spiral galaxy ever seen through a gravitational lens. The more distant galaxy appears in multiple images around the foreground lensing cluster and it hosts the supernova in one of the galaxy’s spiral arms. “We get strong lensing by a red galaxy, but that galaxy is part of a cluster of galaxies, which is magnifying it more. So we have a double lensing system,” explains Kelly.

Kelly hopes that measuring the time delays between the phases of the supernova in the four images will let them put better constraints on the mass distribution of the foreground galaxies, as well as the expansion and geometry of the universe. If the researchers identify it as a Type Ia supernova (these have relatively standard brightness) by studying its spectrum, they could place even stronger limits on both the matter distribution and cosmological parameters.

The work is published in Science.

Rediscovering Marie Curie and the pioneering women of science

The panel of speakers at the women in physics conference. (Courtesy: Institute of Physics)

This Sunday, as the world celebrates International Women’s Day, I’ll be thinking of some amazing women who had a huge impact on the world of physics, helping shape the field as we know it today. Indeed, yesterday I was at the Institute of Physics in London, attending a day-long conference on “The lives and times of pioneering women in physics” hosted by the Institute’s Women in Physics group along with its History of Physics group. While there were a host of interesting speakers at the event, undoubtedly the star of the day was French nuclear physicist Hélène Langevin-Joliot, granddaughter of one of the 20th-century’s most famous female physicists – Marie Curie.

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Daresbury holds a ‘wedding for microscopists’

Photograph of Quentin Ramasse
Quentin Ramasse in the driver’s seat at SuperSTEM 3. (Courtesy: Stuart Eyres/EPSRC)

Last month on a rainy grey morning in north-east England I headed to the Daresbury Laboratory as the SuperSTEM lab there celebrated the installation of its latest world-class microscope. Industrial and academic microscopists from around the world gathered for the inauguration, which was described as a “wedding for microscopists” because so many people from the tightly knit microscopy community were there. You can hear the excitement in the audio piece below, where SuperSTEM lab director Quentin Ramasse and other researchers at the event tell me their plans for the new instrument.

Celebrating SuperSTEM 3
Celebrating SuperSTEM 3

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Celebrating a year of light

Shuji Nakamura talks to delegates about blue light-emitting diodes.

By Michael Banks in San Antonio, Texas

With 2015 being the International Year of Light it is perhaps the perfect opportunity to have a session at this year's American Physical Society meeting in San Antonio dedicated to the forefront of optics research.

Yesterday afternoon saw a number of light pioneers update delegates about their research. The session boasted three of last year's Nobel-prize winners: Stefan Hell of the Max Planck Institute for Biophysical Chemistry in Gottingen, Germany; William Moerner of Stanford University; and Shuji Nakamura of the University of California, Santa Barbara.

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Virus mapped in 3D using X-ray pulses

Intense, extremely short pulses of X-rays have been used for the first time to reconstruct 3D images of individual virus particles. Developed by an international team of physicists, the diffraction technique could make it possible to map the structure of infectious viruses such as HIV, influenza and herpes, shedding light on possible ways to combat diseases caused by these infectious agents.

Scientists have used X-ray diffraction to image live cells, viruses and simple nanostructures in 2D using a technique called nanocrystallography. This involves incorporating a number of identical particles (viruses, for example) into a crystal – a time-consuming and expensive process that does not work for some particles. Now, a team led by Janos Hajdu at Uppsala University in Sweden has tested a technique that avoids crystallization with the added benefit of delivering 3D images.

We outrun radiation damage but we only get one shot per sample
Janos Hajdu, Uppsala University

Brighter than the Sun

The team focused on the Acanthamoeba polyphaga mimivirus, which is about 450 nm across. The researchers injected virus particles into an aerosol stream, which was then subjected to high-energy pulses from an X-ray free-electron laser at Stanford University. Each pulse lasted just 70 fs and delivered a peak power density more than 1018 times that of sunlight hitting the Earth.

Such an extremely intense pulse will vaporize a mimivirus particle, but not before the X-rays scatter from the virus and create a diffraction pattern that is recorded by a bank of detectors. "We outrun radiation damage but we only get one shot per sample," explains Hajdu. Because X-rays primarily scatter off of electrons, the diffraction patterns that Hajdu and his team recovered can be used to calculate the distribution of electrons within the mimivirus particles.

Hajdu and his team scattered X-rays off nearly 200 identical mimivirus particles, collecting one 2D diffraction pattern from each event. To make sure that the particles were not altered by being injected into an aerosol, the team showed that mimivirus particles that did not intersect the laser pulses were still infectious.

Adding a dimension

With nearly 200 diffraction patterns in hand, Hajdu and his colleagues next set about adding the patterns together to produce a single 3D image. Combining data from different observations not only increases the signal – necessary for small viruses that scatter X-rays weakly – but also gives insights about viral structure that are only possible with 3D observations.

Series of 24 diffraction patterns captured from 24 different virus particles
Diffraction patterns: series of 24 diffraction patterns captured from 24 different virus particles. (Courtesy: Tomas Ekeberg et al./Phys. Rev. Lett.)

One important complication that the researchers had to overcome is that they were not all oriented in the same way with respect to the X-ray pulses. Instead, their orientations were randomly distributed. Therefore, it is necessary to retrieve the relative orientation of each mimivirus particle before adding the diffraction patterns together. Hajdu and his team did this using a mathematical optimization algorithm developed by another group of physicists in 2009. Because this algorithm presumes that the particles differ only in orientation, identical particles must be used. Fortunately, many pathogenic viruses that affect humans are reproducible, such as HIV, influenza and herpes.

The researchers achieved a spatial resolution of approximately 125 nm in their final reconstructed image of the mimivirus's electron density. Better resolutions have been recorded in other 2D studies, but this investigation is important because it shows that 2D diffraction patterns can be combined to produce 3D images of biologically important samples. "We now look forward both to pushing towards higher resolution and studying both smaller and larger samples," says Hajdu.

This research is described in Physical Review Letters.

Supporting industrial physicists

By Michael Banks in San Antonio, Texas

Here is a stat for you: around 50% of US physics graduates (both undergraduates and postgraduates) go on to work in industry.

Whether you think that is good or bad, the American Physical Society (APS) wants to do more to support those physicists who don't pursue a career in academia.

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Physicists urged to back plans for Africa’s first synchrotron

Scientists in Africa are being urged to support construction of their continent's first synchrotron light source. The call to back plans for an African light source has been made by Herman Winick of the SLAC National Accelerator Laboratory, who is speaking this week at a meeting of the American Physical Society in Texas. Winick first unveiled his vision for such a facility last year and believes that, if built, it could help researchers in Africa to tackle some of the continent's big issues, such as Ebola and other life-threatening diseases.

There are almost 50 synchrotron light sources in 23 different countries around the world, but Africa is currently the only habitable continent without such a facility. These machines work by accelerating electrons to high energies and then injecting them into a circular storage ring where they emit powerful beams of X-rays. The X-rays can then be used to study the structure and properties of materials in a range of disciplines, from condensed-matter physics to biology.

Early days

Although the African light source is still in the early stages of planning, Winick anticipates that it would have an energy of around 3 GeV. That is similar to other existing facilities such as the Laboratório Nacional de Luz Síncrotron in Brazil, which was the first synchrotron to be built in the southern hemisphere and is currently the only light source operating in Latin America.

Winick wants the African facility to be modelled on the SESAME light source, which is currently being built near Amman, Jordan. Costing $110m, SESAME will open to users next year and, in addition to carrying out top-quality science, seeks to build bridges between researchers from 10 nations in the Middle East and beyond, including Egypt, Jordan, Iran, Israel, Pakistan and Palestine. "Africa should follow the SESAME model," Winick insists.

Taking ownership

Researchers working on plans for the African light source have already established an interim steering committee, which will help to set up a more permanent international advisory committee to oversee the formation of a scientific case for the synchrotron. "The plan is to bring together those working on synchrotron radiation in Africa," says Winick, who serves on the interim steering committee.

One potential host for the African light source could be South Africa, which already has around 40 synchrotron users in the country and has been considering hosting such a facility for the last decade. Those plans were boosted in 2012, when scientists on the country's Synchrotron Research Roadmap Implementation Committee submitted a proposal to the government, followed by a more detailed business plan in 2014.

Role for the ESRF

South Africa is also an associate member of the European Synchrotron Radiation Facility (ESRF) in Grenoble, France, paying around 0.3% of its budget. Indeed, Winick thinks that the ESRF could play a similar role in supporting the African light source as the United Nations Educational, Scientific and Cultural Organization (UNESCO) did in getting SESAME off the ground – namely by bringing together all of the founding members to form a governing council.

The next stage for the fledgling project is for the interim committee to meet in November at the ESRF, in what Winick dubs "t = 0" for the African light source. Winick now calls on researchers across the continent to get behind the project. "They need to get involved and take ownership of it," he says. The path is unlikely to be quick, with Winick thinking that it will take at least another 15 years before the facility is open for business.

Weighty matters

March maze. The conference centre for the APS meeting.

By Michael Banks in San Antonio, Texas

After finally getting my head round the maze-like Henry B Gonzalez Convention Center in San Antonio, it was straight into a packed schedule at this year's American Physical Society (APS) meeting.

One topic that always causes concern among researchers is the crunch in helium supply.

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