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Home » Page 1299

Quantum magnetism simulated using ultracold fermions

Quantum magnetism has been mimicked – or simulated – using ultracold fermionic atoms for the first time. Researchers in Switzerland and France placed atoms on a 2D square lattice created by criss-crossing laser beams. By controlling the interactions between atoms, the team put pairs of atoms into antiferromagnetic configurations. While quantum magnetism plays an important role in a range of solid-state phenomena, it can be difficult to calculate its effect on materials such as high-temperature superconductors. As a result, quantum simulations should lead to better theoretical models of a range of solids.

Quantum magnetism involves a subtle effect called the exchange interaction. This is a quantum interaction between pairs of identical fermions – such as electrons – that tends to prevent neighbouring fermions from having their spin magnetic moments pointing in the same direction. As well as being responsible for the magnetic properties of everyday materials such as iron, quantum magnetism is also believed to play an important role in high-temperature superconductivity and other exotic states of matter such as spin liquids.

Criss-crossing laser beams

Quantum simulations using ultracold atoms allow physicists to create artificial materials in which the atoms play the role of electrons in a solid. However, unlike real materials, where it can be difficult to vary the interactions between electrons, the forces between atoms in a quantum simulator can be fine-tuned by adjusting lasers and magnetic field.

These latest simulations were done by Tilman Esslinger and colleagues at ETH Zürich and the University of Bordeaux. The team began with an ultracold cloud of potassium-40 atoms, which are fermions. The cloud is a mixture in which half of the atoms are in the –9/2 spin state and the other half in the –7/2 state. This two-state system simulates –1/2 and 1/2 spin states of the electron. The criss-crossing laser beams are then switched on, creating a 2D square lattice wherein each lattice site contains one potassium-40 atom. The exchange interaction is then simulated by applying a magnetic field to the lattice, which makes atoms with the same spin repel each other.

The next experimental step involves solving a thermodynamics problem. Even at the extremely low lattice temperatures there is too much entropy – or disorder – for quantum magnetism to emerge. To get round this problem, Esslinger and colleagues came up with a way of “stashing” entropy at the edges of the lattice so that quantum magnetism could emerge in the centre.

This is done by tweaking the properties of the optical lattice so that the interactions between nearest-neighbour atoms alternate between strong and weak in the x and y directions. An atom with a strong interaction with a nearest neighbour will form a pair (or dimer) in which the spins point in opposite directions – and the lattice of 5000 atoms becomes a collection of antiferromagnetic dimers.

Merging dimers

Ideally, the next step would be to measure the spin of each atom in the lattice to confirm that antiferromagnetic dimers have formed; however, this would be extremely difficult to do. Instead, the team uses an indirect measurement technique that involves applying a magnetic-field gradient and merging each dimer into one lattice site. The team can then measure the number of lattice sites that contain two atoms and this information can be used to calculate the number of antiferromagnetic dimers that were present in the original lattice. When the strong and weak interactions were created, a strong majority of the atom pairs were antiferromagnetic dimers.

According to Esslinger, one feature of the quantum simulation is that its parameters are in a region where it is possible to perform calculations – and the team is collaborating with theorists with the aim of improving models of quantum magnetism. Esslinger also hopes that his team can address the entropy problem by creating an optical lattice with two layers and stashing entropy in the second layer.

“One of our next goals is to address unsolved questions in the context of spin liquids,” says Esslinger.

The research is reported in Science.

‘Quantum microscope’ peers into the hydrogen atom

The first direct observation of the orbital structure of an excited hydrogen atom has been made by an international team of researchers. The observation was made using a newly developed “quantum microscope”, which uses photoionization microscopy to visualize the structure directly. The team’s demonstration proves that “photoionization microscopy”, which was first proposed more than 30 years ago, can be experimentally realized and can serve as a tool to explore the subtleties of quantum mechanics.

Information flow

The wavefunction is a central tenet of quantum theory – put simply, it contains the maximum knowledge that is available about the state of a quantum system. More specifically, the wavefunction is the solution to the Schrödinger equation. The square of the wavefunction describes the probability of where exactly a particle might be located at a given time. Although it features prominently in quantum theory, directly measuring or observing the wavefunction is no easy task, as any direct observation destroys the wavefunction before it can be fully observed.

In the past, “Rydberg wavepacket” experiments have tried to observe the wavefunction using ultrafast laser pulses. In these experiments, the atoms are in a superposition of their highly excited “Rydberg states”. These experiments show that the periodic electron orbitals around nuclei are described by coherent superpositions of quantum-mechanical stationary states. The wavefunction of each of these states is a standing wave with a nodal pattern (a “node” is where there is zero probability of finding an electron) that reflects the quantum numbers of the state. While previous experiments have attempted to capture the elusive wavefunction or the nodal patterns, the methods used were not successful. Direct observation of the nodal structure of a single atom being most difficult to achieve.

Plotting waves

In the new work, Aneta Stodolna, of the FOM Institute for Atomic and Molecular Physics in the Netherlands, along with Marc Vrakking at the Max-Born-Institute in Berlin, Germany, and other colleagues in Europe and the US have shown that photoionization microscopy can directly obtain the nodal structure of the electronic orbital of a hydrogen atom placed in a static electric field. In the experiment, the hydrogen atom is placed in the electric field E and is excited by laser pulses. The ionized electron escapes from the atom and follows a particular trajectory to the detector – a dual microchannel plate (MCP) detector – that is perpendicular to the field itself. Given that there are many such trajectories that reach the same point on the detector, interference patterns can be observed, which the team magnify by a factor of more than 20,000 using an electrostatic zoom lens. The interference pattern directly reflects the nodal structure of the wavefunction. The experiments were carried out with both resonant ionization involving a Rydberg state and non-resonant ionization.

The team chose the hydrogen atom thanks to its unique properties. “These [hydrogen atoms] are very peculiar…as hydrogen has only one electron, which interacts with the nucleus via a purely Coulombic interaction, it has a particular structure when we place it in a DC electric field,” says Vrakking. He goes on to explain that thanks to its single-electron status, hydrogen’s wavefunction can be written as the product of two wavefunctions, which describe how it changes as a function of two coordinates – the so-called parabolic coordinates. That is, the Hamiltonian of the hydrogen atom (in an external electric field) describes a splitting of its energy levels, which is known as the “Stark effect”. More importantly, though, this “Stark Hamiltonian” is exactly separable in terms of the two parabolic coordinates, which are linear combinations of the distance of the electron from the hydrogen nucleus r and the displacement of the electron along the electric-field axis z.

Vrakking told physicsworld.com that the shape of the two parabolic wavefunctions is therefore “completely independent of the strength of the field, and so it is invariable – it stays the same as the electron travels for more than half a metre in the experiment – all the way from where the ionization occurs up to the 2D detector”. This, he explains, is crucial to scaling up the spatial distribution to magnify the nodal patterns to millimetre-scale dimensions, where they can be observed with the naked eye on the 2D detector and recorded with a camera system. “What you see on the detector is what exists in the atom,” he says. The group observed several hundreds of thousands of ionization events to obtain the results, with the same preparation of the wavefunction for each.

What lies within

The figure at the top of this article shows the team’s main result – the raw camera data for four measurements, where the hydrogen atoms were excited to states with zero, one, two and three nodes in the wavefunction for one of the parabolic coordinates. “If you look at the measured projections on the detector, you can easily recognize the nodes, and see their radial, ring-like structure,” says Vrakking.

Three charts comparing resonant and non-resonant nodes
Eye of the atom Evidence for on-resonance ionization by tunnelling through the Coulomb field potential. Comparison is given between a measurement taken for one resonant and two non-resonant nodes. Images (A) and (C) were taken after non-resonant ionization, while for the central image, (B), the laser was tuned to a resonance with two nodes in the wavefunction. (Courtesy: Stodolna et al. Phys. Rev. Lett.)

He also points out the “striking difference” between images recorded following resonant excitation and images recorded following non-resonant excitation – this is seen in the image to the right, where a comparison is given between a measurement taken for one resonant and two non-resonant nodes. Images (A) and (C) were taken after non-resonant ionization, while for the central image, (B), the laser was tuned to a resonance with two nodes in the wavefunction. For the resonant ionization, the outermost ring extends significantly further radially, compared with the other two images – something that could be explained by a special kind of tunnelling effect taking place.

Vrakking says that the ultimate goal of the research was to study and visualize the hydrogen atom. Future experiments may look at how the atom would react within a magnetic field, study time-resolved electron dynamics, investigate holographic interference microscopy and perhaps even observe molecules using photoionization microscopy.

Helium under the microscope

Currently, however, the researchers are studying and analysing a helium atom using photoionization microscopy, and a paper on this will be published in the coming months. “As there are two electrons in a helium atom, we are getting some very interesting information,” says Vrakking. He says that while in some aspects the responses of the helium atom are very similar to that of hydrogen, there are also some major differences. “Although one of the helium electrons is very tightly bound to the nucleus, and the other one is very highly excited, we can see that the electrons know of each other’s existence and that they ‘talk to each other’,” says Vrakking, explaining that this could allow the team to “see” entanglement of the electrons.

The research is published in Physical Review Letters.

South Korea – day two

The physics department at KAIST
The physics department at KAIST.

By Matin Durrani

Today was the first full day in South Korea for myself and Physics World news editor Michael Banks and it saw us head off by car from our hotel in downtown Daejeon to the massive science and technology zone in the north of the city. Home to more than 1000 research institutes, universities and start-ups, the zone is called Daedeok Innopolis; it’s a kind of putative Silicon Valley, if you like.

(more…)

Which Nobel-prize-winning physics invention has had the most profound impact on society?

By James Dacey

Lightbulb and fibre optics
Physics has brought transformative inventions. (Courtesy: iStockphoto/Péter Mács)

Earlier this week my colleague reported the death of Heinrich Rohrer, the Swiss condensed-matter physicist who shared the 1986 Nobel Prize for Physics for the invention of the scanning tunnelling microscope (STM) at IBM’s Zürich Research Laboratory. Rohrer shared one half of the prize with his IBM colleague Gerd Binnig, while the other half went to the West German Ernst Ruska for his invention of the electron microscope (EM).

By bringing into view the atomic world, EMs and STMs have undoubtedly had a huge impact on science. Before their invention, optical microscopy had been a truly transformative technology. But it had been fundamentally limited to seeing things that are (roughly speaking) larger than the wavelength of the light used to produce the image. And since the wavelength of visible light is some 10,000 times larger than the typical distance between two atoms, we could not see individual atoms.

(more…)

Can we see the motion of electrons on the atomic scale?

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The electrical human

The image of God extending his hand towards a newly created Adam forms the centrepiece of the Sistine Chapel ceiling. It is a familiar image, and probably one that comes to mind for many people when they hear the phrase “the spark of life”. Yet Michelangelo’s fresco predates the serious study of electricity, so there is no animating spark passing between the famous outstretched fingers. In the 400 years since the artwork was completed, our view of electricity has changed dramatically, and from our current vantage point we know that it is vital (literally!) to the functioning of every living system on the planet. It is not, however, some mysterious and unknown life force. Rather, it is open to careful and quantitative study, and the aim of The Spark of Life is to take us on an exciting tour of the roles that bio-electricity plays in the human body.

The book’s author, Frances Ashcroft, is a University of Oxford physiologist who is well known for her work on ion channels – the remarkable molecules that regulate the flow of ions (in other words, electric current) across otherwise impermeable cell membranes. As gatekeepers to the cell’s interior, ion channels mediate the “spark of life”, and the story of their structure, their function and the consequences of their malfunction is the unifying theme of her book.

The opening chapter provides a short history of the generation and storage of electric charge, and how in the 18th century various new and exciting electrical “toys” were used to induce the contraction of frogs’ legs and give the illusion of re-animating corpses. Subsequent chapters move on to describe ion channels and the role they play in our nervous system, muscles, heart, sense organs and even our consciousness. Along the way there are diversions describing how certain species of fish use their electric organs to generate high voltages, and the ways that electricity has been used both to heal and to harm the human body.

This book is aimed at a general readership, and is written in a lively, accessible and engaging style, with plenty of anecdotes to entertain the reader while also conveying the science. Much of the complexity of living systems arises from interactions across length and time scales that range from microns and microseconds to metres and lifetimes, and Ashcroft does not shrink from dealing with this intricacy. Rather than reducing all bio-electric phenomena to the activities of ion channels, she instead uses ion channels as the jumping-off point to explain them.

There are significant differences between electricity in a living system and in conventional electric circuits

One example is her discussion of diabetes, a disease which, like many that affect the whole person, results from the abnormal behaviour of ion channels. Diabetes is the focus of Ashcroft’s scientific interest, and she vividly relates her excitement at the discovery of a molecular-scale explanation for it: in some people who – thanks to a genetic mutation – are born with diabetes, an ion-channel malfunction prevents the release of the hormone insulin (required to regulate the level of glucose in the bloodstream) from cells in the pancreas. The integrative link across spatial scales from molecule to patient is explored further with a detailed discussion of the active mechanisms of certain drugs used to treat patients born with diabetes. On the flip side, various types of lethal poison also act by interfering with ion-channel operation, and the book is enlivened by thoughtful and occasionally gruesome anecdotes.

Another strength of the book lies in its clear and inspiring accounts of the scientific process. One that stands out is the story of the sustained effort made by the Nobel-prize-winning University of Cambridge physiologists Alan Hodgkin and Andrew Huxley in figuring out the mechanism by which impulses travel along nerve fibres. Despite the sheer difficulty of their experiments, the interruption of the Second World War and competition from across the Atlantic, the eventual outcome was not only a mechanistic explanation of the nerve impulse, but also a mathematical model that articulated it.

Physicists will find much of interest in this material. Living systems, of course, operate according to physical laws, and so in principle are amenable to systematic and quantitative description. In nerve and muscle cells, the potential difference across the cell membrane varies by around 100 mV. Given that a cell membrane is about 10 nm thick, the electric field in which the ion channel operates is enormous; it is hardly surprising that the changes in molecular structure that cause ion channel pores to open and close are regulated by membrane voltage. In the heart, cells are connected so that electrical impulses are rapidly conducted from one cell to its neighbours, enabling electricity to initiate and synchronize contraction. In the brain, these connections are, of course, much more intricate.

There are, however, some traps for unwary physicist readers. Early on, Ashcroft makes an important point when she notes that there are significant differences between electricity in a living system and in conventional electric circuits. For example, in the human body it is ions rather than electrons that carry electrical current, and these ions move in solution and across membranes rather than through wires and other components. This is of more than trivial importance, because the conductance of a membrane may depend not only on the ion channel’s conductance, but also on both the ability of ion channels to distinguish between ionic species and the concentration of ions inside and outside the cell. For many interdisciplinary scientists, the complexity and intricacy of living things is precisely what makes them so appealing, and this book is a good way in for those wishing to dip a toe in the water.

This is a popular-science book, so its coverage is broad rather than deep. The Spark of Life does not tell the whole story of electricity in the human body; there are, for example, no equations. However, it does provide a superb and entertaining introduction to this area from the perspective of a physiologist, and makes valuable background reading for the increasing number of physicists who are developing an interest in the physics of life.

  • 2012 Allen Lane/W W Norton £20.00/$28.95hb 352pp

Why some galaxies age before their time

Composite image of the European Space Agency's Herschel space telescope
Composite image of the European Space Agency’s Herschel space telescope, which was used to discover HXMM 01. (Courtesy: ESA)

Like reckless rock stars, some galaxies live fast and get old long before their time. Now an international team of astronomers is watching this metamorphosis as it occurred billions of light-years away. Two distant galaxies are crashing together, turning all their gas into stars so rapidly that they will soon appear to be much older than they really are.

In the modern universe, giant galaxies – those that emit about as much light as the Milky Way – come in two main types: spiral and elliptical. Spiral galaxies are graceful, having formed their stars little by little over time, so they harbour stars from young to old. In contrast, giant elliptical galaxies converted all of their gas into stars long ago, so today they consist only of old stars with little or no gas to make new stars.

When astronomers gaze into space billions of light-years away, they witness the universe as it was billions of years ago. Yet they still see giant elliptical galaxies that consist of old stars – even as far away as 10 billion light-years from Earth. However, at that time the universe was less than four-billion years old, giving giant elliptical galaxies little time to form.

Astronomers have long been puzzled by this age problem – but now Hai Fu of the University of California at Irvine and colleagues think they may have the answer. The team has discovered a collision between two spiral galaxies 11 billion light-years away in the constellation Cetus.

Violent transition

“We believe that this object is a transitional object between a spiral galaxy and an elliptical galaxy,” he says. “Because these transitions happen so dramatically and violently, over a very short period of time, it’s really hard to actually catch them in action.”

The two galaxies are only about 62,000 light-years apart – slightly more than twice the Sun’s distance from the Milky Way’s centre – and have torn material out of each other, a sign they are colliding and will merge. As gas clouds in one galaxy smash into those in the other, they spawn lots of new stars.

Called HXMM-01, the system is bright at submillimetre wavelengths, because dust in the gas absorbs starlight and re-emits it at infrared and submillimetre wavelengths. In fact, Fu’s team says the collision is the most luminous and gas-rich merger of submillimetre bright galaxies ever seen. It emits 10 trillion times more infrared energy than the Sun does over all wavelengths. In a single hour, the system produces as much energy as the Sun will during the next two billion years.

“The star formation is really intense,” says Fu. From the high infrared luminosity, his team deduces that the collision is converting about 2000 solar masses of gas into stars each year, roughly 1000 times greater than the Milky Way’s rate.

Prolific star creation

The galaxies possess a lot of gas. They have as much mass in gas as in stars. But their prolific creation of new stars will use all the gas up in just 200 million years. The end result will be a giant elliptical galaxy with roughly 10 trillion solar masses – about 10 times more massive than the Milky Way. If we could see the galaxy as it is today, 11 billion years later, it would consist solely of old stars.

“This is an extremely rare galaxy – if the interpretation is correct,” says Abraham Loeb, who chairs the astronomy department at Harvard University. “It’s an unusual animal in the zoo of galaxies.”

But Loeb cautions that conditions may be less extreme than they seem if the collision is funnelling gas into a supermassive black hole at either galaxy’s centre. Then the black hole would heat the gas and spark a quasar, an object that emits far more light than the Milky Way but from a region as small as the solar system.

“We know that mergers usually trigger activity in the centre of a galaxy,” Loeb says. “So we would be seeing a quasar being born.”

Buried quasar?

In this case, though, the quasar is enshrouded in thick dust that blocks its intense light. The dust would heat up, emitting infrared radiation. Since Fu’s team used the immense infrared luminosity to derive the high star formation rate, Loeb says the presence of a buried quasar that produced most of the radiation would imply a more modest rate of star formation.

Fortunately, there’s a way to discern even a dust-enshrouded quasar, because quasars emit high-energy X-rays that penetrate the dust. Fu’s team is hoping to conduct such observations later this year. Meanwhile, Fu says he finds no sign of a quasar: quasars normally have hot dust, and he sees none in this object.

The research is described in Nature.

Hangout with Physics World

By James Dacey

This year Physics World is celebrating its 25th birthday. The first issue of the magazine was published in October 1988, so for October this year we are producing a special anniversary issue. It will celebrate the big physics stories from the first quarter of a century of our existence, but it will also have a strong focus on the exciting new physics that await us in the near future. To discuss our plans, I joined editor of Physics World Matin Durrani in this Google Hangout, recorded yesterday.

It was the first time we had attempted one of these fancy new hangouts; this was something of a pilot run. But with the likes of Barack Obama, CERN and the BBC all attempting this new, accessible way of video broadcasting, I reckon we’re in good company.

South Korea – day one

By Matin Durrani

Flag of South Korea
South Korea: physics rising star.

Hello from South Korea, where I’m on a week-long tour with Physics World news editor Michael Banks. We’re here to visit a series of top physics institutes and research organizations in a trip that’s taken several months of careful planning to arrange.

There are three main reasons for coming here. The first is to gather material for a Physics World special report on physics in South Korea, which will be published in September. This report will follow on from our previous special reports on India, Japan and China.

(more…)

What do plastic bags have in common with metal?

In less than 100 seconds, Adrian Sutton explains why metals and plastic bags deform in different but related ways.

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