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Fractal patterns seen on emerging cancerous cells

Fractal patterns that arise when healthy human cells turn cancerous have been observed for the first time by scientists in the US. Using an atomic force microscope (AFM), Igor Sokolov and colleagues at Tufts University and Clarkson University saw the patterns while studying the surfaces of cervical epithelial cells at nanometre resolution. The work could give us a better understanding of how the surface of cells affects the progression of some cancers, which could in turn lead to new strategies for fighting the disease.

Although the origin of many cancers is still a mystery, some scientists believe that these diseases are linked to complex processes in living cells becoming unbalanced, which could lead to chaotic behaviour. Indeed, signs of chaos have already been seen in biochemical and physical studies of cancerous tissue – with the structure of some cancerous tissues, for example, having fractal properties associated with chaotic systems.

Fractal patterns had, however, never been seen before on the surfaces of single cancer cells. The new observation could be significant because scientists already know that the surface of a cancer cell plays an important role in “metastasis”. This is the process whereby cancer cells manage to leave a primary tumour – often forcing their way through healthy tissue – and travel to other parts of the body to create secondary tumours.

Immortal cells

The new study was carried out using cells cultured in the laboratory. Three types of human cervical epithelial cells were studied: normal cells taken from healthy women; malignant cancer cells taken from cancer patients; and “immortal” pre-cancerous cells that were created by treating some of the healthy cells with a human papilloma virus genome. The cells were then freeze-dried so that they could be studied with an AFM.

The researchers mapped the structural features of the surfaces of the cells at a resolution of less than about 20 nm per image pixel. In particular, the AFM measured the “stickiness” between the instrument’s tiny probe and the cellular surface. The images were then processed using a Fourier transform to identify any repeating patterns. The team then analysed this information for signs of patterns that repeat on a number of different length scales – a hallmark of a fractal pattern.

The team found that the surfaces of both healthy cells and cancer cells did not have fractal patterns, whereas such patterns were seen on the pre-cancerous cells. This finding was unexpected. “Despite previous expectations that fractal patterns are associated with cancer cells,” says Sokolov, “we found that fractal geometry only occurs at a limited period of development when immortal cells become cancerous.”

Surface transformation

According to Sokolov, the team also discovered that cells deviate more from fractal behaviour when they further progress towards cancer, while normal cells do not have fractal patterns. This could mean that the fractal pre-cancerous phase plays a role in transforming the surface of a healthy cell to that of a cancer cell.

Sokolov and colleagues hope that their discovery could help to identify “weak points” in the transition from healthy to cancerous cells that could be targeted to stop the development of cancer. Such a transition could involve instabilities in biological processes that occur in the cell and lead to chaotic behaviour at the surface. If these instabilities could be prevented from emerging, then the progression to cancer could be halted.

“We need to further our understanding as to how important the cell surface is in the development of cancer,” concludes Sokolov.

The research is described in the New Journal of Physics.

Graphene quantum dots split Cooper pairs

Superconducting “Cooper pairs” of electrons have been split to create entangled pairs of electrons in a new device built by physicists in Finland and Russia. The device employs two quantum dots made of graphene. Although other types of quantum dots have been used for this purpose, the latest research suggests that graphene quantum dots should deliver long-lived entangled electron pairs that could be used in quantum computers.

Entanglement is a quantum-mechanical phenomenon in which properties of fundamental particles are correlated so that making a measurement on one particle can instantaneously affect another particle – even across very large distances. In principle, a quantum computer can use this connectedness to perform certain calculations much faster than a conventional computer. Although practical quantum computers do not exist today, some potential designs involve using the intrinsic angular momenta, or “spin”, of electrons as quantum bits (qubits) of information that can be entangled.

Superconductors provide a ready source of entangled electrons because the Cooper pairs that allow these materials to conduct electricity with little or no resistance are in fact entangled pairs of electrons with opposite spin. Splitting the pairs while preserving the electrons’ entanglement can be done simply by connecting ordinary metal wires to either end of the superconductor. If the set-up is just right, each wire will carry away one electron from a pair. However, it is more often the case that both electrons will end up going down the same wire.

Boosting the odds

One way to boost the odds in favour of separation is to replace the wires with tiny blobs of semiconductor containing just several thousand atoms. These quantum dots have electron energy levels that can be set precisely by carefully adjusting their size. The two electrons from each Cooper pair can be guided to different resonant energy levels and separated as a result. This approach has already been exploited using quantum dots made from indium arsenide and, with greater efficiency, using carbon nanotubes.

The latest work, carried out by Pertti Hakonen and colleagues at Aalto University in Finland together with Gordey Lesovik of the Landau Institute for Theoretical Physics near Moscow, instead uses quantum dots made from graphene. Graphene should be able to preserve the entanglement of the separated electron pair for longer, thanks to the fact that it consists of a single layer of carbon atoms, which constrains the electrons to move in a straight line and so avoids the emission of electromagnetic radiation that interferes with the spin state.

Scanning electron microscope image showing the main components of the graphene-based device used to split Cooper pairs
Pair production: scanning-electron-microscope image of the patterned graphene sheet before the electrical leads have been laid out. The light-grey regions are silicon dioxide and darker grey corresponds to graphene. The graphene quantum dots are the two, small rectangular pieces at the ends of the narrow graphene ribbons. The scale bar is 1 µm in length.

The team used electron-beam lithography to carve out two rectangular quantum dots (each 200 × 150 nm) from a layer of graphene deposited on a silicon-dioxide substrate. The dots were positioned 180 nm apart, covered by a superconductor made from a thin sandwich of titanium and aluminium, and connected to two metal contacts.

Aligning energy levels

To split the entangled electrons from the superconductor, the researchers first set the resonant energy level of the quantum dots to equal the energy possessed by the Cooper pairs. They then varied the gate voltage across one of the dots and monitored the current flowing through the other. They found that across most of the voltage range there was no current, but that at certain voltages the current would suddenly increase, drop below zero and then return to the zero mark. The rise, they explain, occurs because at that voltage the energy in one dot increases very slightly, while that in the other drops by the same small amount, causing the electrons to separate and so register a current (unseparated pairs register as zero current). The negative current, meanwhile, is caused by electrons “elastic co-tunnelling” through the superconductor. “It is like having a switch where you reverse the current by aligning the energy levels either symmetrically or antisymmetrically,” says Hakonen.

This is really a beautiful experiment
Detlef Beckmann, Karlsruhe Institute of Technology

Venkat Chandrasekhar of Northwestern University in the US praises the team’s ability to “independently control the energy levels of the two quantum dots”, and so neatly distinguish Cooper-pair splitting from elastic co-tunnelling. Detlef Beckmann of the Karlsruhe Institute of Technology in Germany agrees, arguing that the group can “probe the mechanism of Cooper-pair splitting more clearly” than has been possible to date. “This is really a beautiful experiment,” he says.

There is, however, still room for improvement. Hakonen and colleagues are working to increase the device’s efficiency – it currently splits just 10% of electrons passing through it – by better controlling the quantum dots’ energy levels. They also aim to show that the device not only splits Cooper pairs, but that it does in fact preserve entanglement. They plan to do this by recording the spin of the separated electrons using contacts made from the nickel–iron magnetic alloy dubbed permalloy.

The research is described in Physical Review Letters.

How to make a tougher quantum computer

A system of nine quantum bits (qubits) that is robust to errors that would normally destroy a quantum computation has been created by researchers at the University of California, Santa Barbara (UCSB) and Google. The device relies on a quantum error-correction protocol, which the team says could be deployed in practical quantum computers of the future.

In principle, powerful quantum computers can be built from a collection of qubits. For a qubit based on an electron, for example, these states would be “spin up” and “spin down”, with one state representing a logical “1” and the other “0”. Each qubit can be in a superposition of two quantum states at the same time and N qubits could be quantum-mechanically entangled to represent 2N values simultaneously. This would lead to the parallel processing of information on a massive scale not possible with conventional computers.

Extremely fragile

However, quantum computers are extremely fragile, and a computation can be easily destroyed by “bit errors” that occur when external noise in the environment affects the values of the qubits. While it is proving very difficult to create practical qubits that are robust enough to eliminate such errors, an alternative approach is to accept that errors will occur and to try to correct for them as the quantum calculation progresses.

Now, UCSB’s John Martinis and colleagues have taken an important step forward by demonstrating repetitive error correction in an integrated quantum device that consists of nine superconducting qubits. Each qubit is a small circuit consisting of a capacitor and a Josephson junction, and is made from an aluminium film evaporated onto a sapphire substrate. The qubit can be thought of as an artificial atom with information stored in its quantum states.

“Our nine-qubit system can protect itself from bit errors that unavoidably arise from noise and fluctuations from the environment in which the qubits are embedded,” explains team member Julian Kelly. “We also show that ‘more is better’: nine qubits protect the system better than five qubits, a critical requirement when moving to more qubits in a real quantum computer of the future.”

Measuring parity

“In quantum mechanics, we cannot measure a qubit without destroying the superposition and entanglement that makes quantum mechanics work,” says team member Rami Barends, “but we can measure something called parity – which forms the basis of quantum error correction.” The parity is defined to be “0” if both qubits have the same value and “1” if they have different values. Crucially, it can be determined without actually measuring the values of both qubits.

The researchers exploited this fact and repetitively measured the parity between adjacent “data” qubits by making use of “measurement” qubits. “Each cycle, these measurement qubits interact with their surrounding data qubits using quantum logic gates and we can then measure them,” Kelly explains. “When an error occurs, the parity changes accordingly and the measurement qubit reports a different outcome. By tracking these outcomes, we can figure out when and where a bit error has occurred and correct for it.”

More is better

The more qubits that are involved in the process, the more information is available to identify and correct for errors, explains team member Austin Fowler. “Errors can occur at any time and in all types of qubits: data qubits, measurement qubits, during gate operation and even during measurements. We found that a five-qubit device is robust to any type of bit error occurring anywhere during an algorithm, but a nine-qubit device is better because it is robust to any combination of two-bit errors.”

Although still a long way off from real-world applications, the researchers say that a “self-correcting” device such as theirs could be a great platform for testing some of the ideas behind error correction – such as protecting a quantum state against so-called phase-flip errors. “We are also now busy improving the quality of our qubits and the materials we use to make them,” says Kelly.

The research is described in Nature.

Pioneering women of physics, why you should become a particle physicist and a BICEP2 scientist on all that dust

Smiley happy people: who would not want to be a particle physicist? (Courtesy: ATLAS)

Over on the Quantum Diaries blog, Aidan Randle-Conde has put together a lovely photo-essay called “30 reasons why you shouldn’t be a particle physicist”. It is reverse psychology, of course, and the 30 images highlight the benefits of devoting your life to studying subatomic particles. As someone who chose to do condensed-matter physics, do I now think that I made a huge mistake? No, but I have shared the thrill and excitement of being at CERN when the Higg’s was discovered and seen the Large Hadron Collider and its detectors up close, so I know where he is coming from.

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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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