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Home » Page 1144

Do atoms going through a double slit ‘know’ if they are being observed?

Does a massive quantum particle – such as an atom – in a double-slit experiment behave differently depending on when it is observed? John Wheeler’s famous “delayed choice” Gedankenexperiment asked this question in 1978, and the answer has now been experimentally realized with massive particles for the first time. The result demonstrates that it does not make sense to decide whether a massive particle can be described by its wave or particle behaviour until a measurement has been made. The techniques used could have practical applications for future physics research, and perhaps for information theory.

In the famous double-slit experiment, single particles, such as photons, pass one at a time through a screen containing two slits. If either path is monitored, a photon seemingly passes through one slit or the other, and no interference will be seen. Conversely, if neither is checked, a photon will appear to have passed through both slits simultaneously before interfering with itself, acting like a wave. In 1978 American theoretical physicist John Wheeler proposed a series of thought experiments wherein he wondered whether a particle apparently going through a slit could be considered to have a well-defined trajectory, in which it passes through one slit or both. In the experiments, the decision to observe the photons is made only after they have been emitted, thereby testing the possible effects of the observer.

For example, what happens if the decision to open or close one of the slits is made after the particle has committed to pass through one slit or both? If an interference pattern is still seen when the second slit is opened, this would force us either to conclude that our decision to measure the particle’s path affects its past decision about which path to take, or to abandon the classical concept that a particle’s position is defined independent of our measurement.

Photon first

While Wheeler conceived of this purely as a thought experiment, experimental advances allowed Alain Aspect and colleagues at the Institut d’Optique, Ecole Normale Supérieure de Cachan and the National Centre for Scientific Research, all in France, to actually perform it in 2007 with single photons, using beamsplitters in place of the slits envisage by Wheeler. By inserting or removing a second beamsplitter randomly, the researchers could either recombine the two paths or leave them separate, making it impossible for an observer to know which path a photon had taken. They showed that if the second beamsplitter was inserted, even after the photon would have passed the first, an interference pattern was created.

The wave–particle duality of quantum mechanics dictates that all quantum objects, massive or otherwise, can behave as either waves or particles. Now, Andrew Truscott and colleagues at Australian National University carried out Wheeler’s experiment using atoms deflected by laser pulses in place of photons deflected by mirrors and beamsplitters. The helium atoms, released one by one from an optical dipole trap, fell under gravity until they were hit by a laser pulse, which deflected them into an equal superposition of two momentum states travelling in different directions with an adjustable phase difference. This was the first “beamsplitter”. The researchers then decide whether to apply a second laser pulse to recombine the two states and create mixed states – one formed by adding the two waves and one formed by subtracting them – by using a quantum random-number generator. When applied, this final laser pulse made it impossible to tell which of the two paths the photon had travelled along. The team ran the experiment repeatedly, varying the phase difference between the paths.

Double pulse

Truscott’s team found that when the second laser pulse was not applied, the probability of the atom being detected in each of the momentum states was 0.5, regardless of the phase lag between the two. However, application of the second pulse produced a distinct sine-wave interference pattern. When the waves were perfectly in phase on arrival at the beamsplitter, they interfered constructively, always entering the state formed by adding them. When the waves were in antiphase, however, they interfered destructively and were always found in the state formed by subtracting them. This means that accepting our classical intuition about particles travelling well-defined paths would indeed force us into accepting backward causation. “I can’t prove that isn’t what occurs,” says Truscott, “But 99.999% of physicists would say that the measurement – i.e. whether the beamsplitter is in or out – brings the observable into reality, and at that point the particle decides whether to be a wave or a particle.”

Indeed, the results of both Truscott and Aspect’s experiments shows that a particle’s wave or particle nature is most likely undefined until a measurement is made. The other less likely option would be that of backward causation – that the particle somehow has information from the future – but this involves sending a message faster than light, which is forbidden by the rules of relativity.

Aspect is impressed. “It’s very, very nice work,” he says, “Of course, in this kind of thing there is no more real surprise, but it’s a beautiful achievement.” He adds that, beyond curiosity, the technology developed may have practical applications. “The fact that you can master single atoms with this degree of accuracy may be useful in quantum information,” he says.

The research is published in Nature Physics.

The museum exhibit that you inspired

Robert P Crease at the Mind Museum in Manilla, the Philippines
Inspired thinking – Robert P Crease in front of an exhibit at Manila’s Mind Museum that was inspired by Physics World readers.

By Robert P Crease in Singapore

It’s not often that you come across a museum exhibit based on a Physics World article. But I did on Saturday at the Mind Museum – an extraordinarily beautiful and original science museum in Taguig, on the outskirts of Manila in the Philippines.

Not only that, the exhibit is right at the entrance. You may recall that I once asked Physics World readers for their thoughts on the 10 most beautiful experiments and wrote up the results in an article in September 2002. The project turned into a book, The Prism and the Pendulum: The Ten Most Beautiful Experiments in Science, which came out the following year and which Physics World reviewed.

Maria Isabel Garcia, who was planning exhibits for the then-future Mind Museum, saw the article and book, and created an exhibit based on it, consisting of videos and explanations of each of the 10 experiments, along with a sculpture designed by the Philippine artist Daniel de la Cruz.

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New maze-like beamsplitter is world’s smallest

An ultracompact beamsplitter – the smallest one in the world – has been designed and fabricated by researchers in the US. Using a newly developed algorithm, the team built the smallest integrated polarization beamsplitter to date, which could allow computers and mobile devices of the future to function millions of times faster than current machines.

Beamsplitters divide light waves into two separate channels of information, and will be crucial for the development of so-called silicon photonic chips that compute and shuttle data using light instead of electrons. “Light is the fastest thing you can use to transmit information,” says Rajesh Menon, an electrical and computer engineer at the University of Utah. “But that information has to be converted to electrons when it comes into your laptop. In that conversion, you’re slowing things down. The vision is to do everything in light.”

Light maze

Silicon photonics could significantly increase the power and speed of machines such as supercomputers and data-centre servers. In theory, devices employing such chips should not only be faster, but also consume far less power as well. Measuring only 2.4 × 2.4 μm2, the new beamsplitter is nearly 50 times smaller than any beamsplitter created to date. It also has an unorthodox maze-like shape. “Most polarizing beamsplitters do not look like this today,” says Menon. “We wanted our device to be as easy as possible to fabricate using existing techniques, and to be as efficient as possible.”

To do this, Menon and his team created an algorithm that tried various geometries, until it found the smallest and most efficient design. Menon described this process as a “smart” search, explaining that “the ‘smartness’ is important because there are too many possibilities to try and the alternatives would take a very long time.” As the design takes existing manufacturing techniques into account, the team says its beamsplitter could be produced on an industrial scale almost as inexpensively as electronic transistors are today.

Tech library

Menon noted, however, that many other technological advances will be needed before a fully photonic computer is possible. “For our device to be utilized fully, we need a whole library of other complementary devices, all of which are highly miniaturized and efficient. These devices will enable different functions such as bending the light, splitting the light, transporting light, modulating the light, and so on,” Menon says. “Once such a library is available to designers, one can expect them to put such devices together into functional circuits. That’s when the most fun and unexpected results will come.”

Andrea Alù, a researcher at the University of Texas, Austin, who did not participate in the research, thinks the team’s algorithm could have other uses as well. Last year, Alù and his collaborators proposed the idea of designing artificial materials that can perform mathematical operations as light propagates through them. “I believe the concept pushed forward by Menon and his colleagues is not limited to polarization control,” says Alù. He told physicsworld.com that it may also be “a viable platform to imprint mathematical operations of choice on a CMOS compatible chip” – a widely used type of semiconductor.

Zongfu Yu, an electrical and computer engineer at the University of Wisconsin-Madison who was also not involved in the study, called the new beamsplitter design a significant advance in silicon photonics. He adds that the work shows, for the first time, that “computational optimization can be used to achieve ultra-compact devices that are absolutely beyond the intuition of even the most experienced device designer”.

The research is published in Nature Photonics.

On top of the volcano – part two

 

By Matin Durrani at Sierra Negra, Mexico

Just as my Physics World colleague James Dacey mentioned earlier, neither of us felt super-wonderful yesterday visiting the Large Millimeter Telescope (LMT), which sits at a height of 4600 metres above sea level.  Spectacular though the facility is, the air pressure is roughly 60% of that at sea level and there is so little oxygen that even walking up a flight of stairs made me feeling pretty light-headed.

So, James and I were both quite glad to descend with LMT director David H Hughes to a height of 4100 metres, where it was time to visit another leading Mexican astronomy facility – the High-Altitude Water Cherenkov (HAWC) gamma-ray observatory.

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On top of the volcano – part one

 

By James Dacey at Sierra Negra, Mexico

Friday was the final full day of the Physics World Mexican adventure and we ended with a breathtaking experience, quite literally.

Matin and I rose early in Puebla to travel over a hundred kilometres east to the ominously named Sierra Negra volcano. This extinct beast is home to two of Mexico’s finest astrophysics facilities.

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The Dark Matter Garden, gravitational atoms, boys and girls with toys, and more

Gravitational gardening: the Dark Matter Garden at Chelsea
Gravitational gardening: the Dark Matter Garden at this year’s RHS Chelsea Flower Show. (Courtesy: National Schools’ Observatory)

By Hamish Johnston

Gardening is something that the British take very seriously and this week’s RHS Chelsea Flower Show is the pinnacle of that obsession. Indeed, it is so popular that it is covered live on television by the BBC. One highlight of the show is the garden competition, in which designers transform an empty plot into a dazzling garden in just 10 days. This year’s entries include the Dark Matter Garden, which “brings the mysteries of the universe to Chelsea”. That’s the claim of the designers of the garden (including several astronomers), who built it for the UK’s National Schools’ Observatory. The team says that its gold-medal-winning design includes “innovative structures and planting, and represents the effect of dark matter on light”.

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Nanomachine pumps molecules ‘uphill’

A new molecular pump capable of pumping other small molecules up an energy gradient has been developed by researchers at Northwestern University in the US. The new pump is very much like the protein pumps in living cells, and might be used to design artificial molecular machines similar to those found in nature. Such machines could be important for a range of applications, including synthetic muscles, tiny robots and advanced mechanical motors.

Molecular machines are ubiquitous in nature and have evolved over billions of years to exploit energy from sunlight or complex chemical reactions in the body. They are made up of complicated assemblies of proteins that are responsible for a host of processes in living organisms, such as ion transport, ATP synthesis and cell division. In fact, our muscles are controlled by the co-ordinated movement of thousands of these machines.

“Our new molecular pump is, in a sense, reminiscent of the pump proteins in our cells, which are vital components of life involved in transferring energy from food to a form that is compatible with our cells,” explains Paul McGonigal, who is part of Fraser Stoddart’s team at Northwestern. “We have designed a relatively simple small molecule that can also drive a system away from equilibrium with chemical energy from redox (oxidation-reduction) reactions.”

One-way valves and rings

The new pump is based on a molecule called a rotaxane, which has already been used to create other molecular machines. The molecule contains a linear axle capable of restricting the motion of a ring-shaped component threaded onto it. The chemical structure of the axle is such that the rings can move in one direction via a complex mechanism that involves two one-way valves (see figure above).

The machine contains several components. The first is a positively charged pyridinium unit (red) that acts as the first one-way valve. The second is a viologen unit (orange) that acts as the pump. The third is a bulky isopropylphenyl chemical group that acts as the second one-way valve (purple). Finally, the fourth component is an alkyl chain (green) that acts as the collection unit. This chain contains a chemical group at its end that is big enough to stop the rings from de-threading.

Pumping process transfers and stores energy

“The machine works thanks to reduction-oxidation cycles and precisely organized non-covalent bonding interactions,” explains team member Chuyang Cheng. “It pumps positively charged rings from solution and ensnares them around an oligomethylene chain. The redox-active viologen unit at the heart of this dumb-bell-shaped molecular pump plays a dual role in first of all attracting and then secondly repelling the rings during redox cycling,” he says.

“The pumping process is actually a way of transferring and storing energy at the molecular level,” he continues. “Part of the energy released during a reaction is siphoned off and stored in the high-energy molecules produced. In the long term, we could imagine that the energy stored by such an artificial molecular pump might be used to power another molecular machine – perhaps one that is part of an artificial muscle, for example.”

The team, reporting its work in Nature Nanotechnology, says that it would now like to be able to anchor its molecular pump in a membrane so that it pumps molecules from one side to another during operation. “Such a pump would be directly inspired by nature’s molecular machines, and especially carrier proteins,” adds McGonigal.

Creating craters, Mexican style

By Matin Durrani in Puebla, Mexico

So it’s day five of the Physics World Mexican adventure and today we’ve been to the Benemérita Universidad Autónoma de Puebla (BUAP), which is one of the oldest universities in the country. After taking a peek at a new facility containing one of the most advanced supercomputers in Latin America, we headed over to the Institute of Physics, where we bumped into Felipe Pachecho Vázquez.

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Spin currents endure at room temperature in germanium

Currents of electron spin can travel more than half a micron through germanium at room temperature, according to researchers in Japan and the UK. While physicists already know that germanium is a good conductor of spins at very low temperatures, this is the best measurement yet of its ability to transport spin at room temperature. The results suggest that the semiconductor could be used to create spintronic devices, which make use of the spin magnetic moment of the electron to store and process information.

The idea of spintronics has been around for several decades, and the nascent technology promises to deliver devices that are smaller and more energy efficient than conventional electronics. Another potential application of spintronics is to use individual electron spins – which can point up or down – to store and transfer information in quantum computers.

However, practical spintronic devices have proven to be very difficult to build, because electron spin does not travel very far in most materials and therefore the information is quickly lost. The main challenge is overcoming a well-known effect in physics called the “spin–orbit interaction”. As the electron travels through a material, the relative motions of the positively charged atoms create magnetic fields that tend to rotate the electron’s spin. In most materials, this results in the rapid destruction of a spin current across very short distances. Fortunately, some semiconductors already used in electronics – including silicon and germanium – have very weak spin–orbit interactions, and so a lot of effort has been put into studying the spin-transport properties of these materials.

Electrical injection

Before you can measure how far a spin current will travel, you have to inject it into a semiconductor, which is not easy to do. One technique involves shining circularly polarized light onto the semiconductor, which tends to spin-polarize the conduction electrons. However, this requires a light source, and is therefore not very practical for miniaturization and mass production of spintronic components. Another option is to place a ferromagnetic material next to the semiconductor so that a spin current can be driven out of the magnet and into the semiconductor. While this “electrical injection” sounds like a great idea, in practice, spin–orbit and other interactions at the interface between the two materials tend to scramble most of the spins before they make it into the semiconductor.

Several studies have suggested that electrical injection should be possible for germanium at room temperature. However, these measurements have not been conclusive because of experimental difficulties. Now, Sergei Dushenko of Osaka University, Masashi Shiraishi of Kyoto University and colleagues have used a microwave “spin-pumping” technique to inject a spin current into germanium.

Their experiment comprises a flat piece of n-doped germanium with a piece of ferromagnetic iron/nickel alloy at one end and a piece of the non-magnetic metal at the other end. An external magnetic field aligns the spins in the alloy along a specific direction. Microwave radiation is then shone onto the alloy, which causes the spins to rotate about the direction of the applied field. This causes a current of spins to be “pumped” into the germanium and flow a short distance to the metal. When the spin current enters the metal, it encounters a strong spin–orbit interaction that creates a voltage across the metal – a phenomenon called the “inverse spin Hall effect”. This voltage is then detected and related to the size of the spin current.

Theory backed up

From these measurements, the team was able to show that the spin current travels about 660 nm before it begins to suffer significant degradation. This applies at room temperature (about 290 K), and when the germanium was cooled down to 130 K, the spin current could travel about twice as far. This backs up a new theory of spin transport that was proposed last year by Yang Song, Oleg Chalaev and Hanan Dery of the University of Rochester in the US.

While 660 nm may not seem like very far, it is much larger than the size of a feature in a modern integrated circuit. In principle, this means that spins could move from one tiny spintronic device to the next without suffering degradation. Furthermore, this distance is on a par with other candidate materials for spin circuits, giving scientists another building block to create spintronic devices.

The research is described in Physical Review Letters.

Examining precious artefacts without breaking them

By James Dacey in Mexico

From pre-Hispanic archaeological treasures to the Modernist paintings of Frida Kahlo and Diego Rivera, Mexico is brimming with cultural artefacts. Yesterday I visited a centre at the National Autonomous University of Mexico (UNAM) that has developed techniques for investigating precious objects without damaging them.

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