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Physicists watch the “birth, life and death of a photon”

Detecting a photon usually involves absorbing the photon – and ultimately destroying it – in a photodetector. However, it is sometimes possible to make a measurement in a much gentler manner, leaving the system in more or less the same state as was measured. Such QND measurements have become commonplace for large systems like atoms – which can be probed gently using photons. But photons are much more delicate than atoms, which makes QND very difficult.

Now, Michel Brune and colleagues at the Ecole Normal Supériore in Paris have turned the table and used atoms to make a QND measurement of the quantum state of a system containing one photon. Brune’s system is a microwave cavity that has been cooled to 0.8K. At this temperature, there is about a 5% chance that the cavity will be devoid of microwave photons and a 50% chance that the cavity will contain just one photon (that has spontaneously appeared from the vacuum, only to vanish less than one second later).

The presence of a photon is detected by passing a stream of rubidium atoms though the cavity. These are so-called Rydberg atoms, which have an electron in a highly excited state and are very sensitive to external perturbations, such as electric fields. The atoms are prepared such that they can exist in one of two quantum states (“g” and “e”). If the atoms cross an empty microwave cavity, most of them will emerge in state g, whereas if they encounter a photon the majority will emerge in state e.

The atoms are flipped between g and e by a non-resonant interaction with the cavity field. The photon cannot be absorbed without violating energy conservation, and instead it leaves its “imprint” on the atom by displacing the position of the atomic energy levels. A high resolution spectroscopy method is used to determine the state of the atoms as they emerge from the cavity. In this way, Brune and colleagues were able to make hundreds of such measurements on a single photon without destroying it.

By measuring the state of the emerging atoms, Brune and colleagues were able to watch as a single photon emerged from the vacuum, lived a brief life of less than one second, and then vanished. While this phenomenon was predicted nearly one hundred years ago, this is the first time that it has been observed directly.

Brune told Physics Web that the researchers now plan to repeat the experiment with tens of photons in the cavity. This should provide insight into the so-called “semi-classical” regime between the quantum description of light as single particles and the classical view of light as a continuous electromagnetic wave.

Beyond demonstrating the fundamentals of quantum mechanics, Brune believes that the technique could be used in quantum information systems, which try to exploit the bizarre nature of quantum systems to process information. For example, the cavity can be thought of as a logic gate that switches the quantum state of the atoms according to the presence of a single photon.

“Thermal runaway” weakens even the best crystals

Frenkel’s theory applies to perfect crystals, and has long been known to set the stress limit too high for most real materials. This is because real materials often contain defects that can move through the structure and make it easier for planes to slip. But this is not always true: some materials such as rocks in the Earth’s interior and metallic glasses have structures that act to prevent defects from moving, and so can demonstrate unusually high shear strengths.

However, even these materials do not quite reach Frenkel’s limit. 40 years after Frenkel’s proposal, scientists suggested that the discrepancy arises because most materials have an inherent viscosity that is a function of strain. Although viscosity is heavily temperature dependent, strain itself produces heat. Therefore, a local increase in strain will cause a temperature rise that decreases viscosity, which in turn enhances the strain – a process known as “thermal runaway”.

Now, another 40 years later, Simen Braeck and Yuri Podladchikov from the University of Oslo have designed a theoretical model that can predict the stress limit while taking thermal runaway into account. In their model, a wide slab of material contains a small central region at a slightly higher temperature. Stress is then applied to force the central region, and the resulting strain is calculated. The physicists could then use the model to search only for the localized failure indicative of thermal runaway.

Braeck and Podladchikov found that the width of the central region and the viscosity in their model do not have a big impact on the value of the “critical” stress that a material can tolerate. However, they did find that all materials with viscous behaviour will experience failure due to thermal runaway before Frenkel’s limit is reached. Applying their model’s formulae to real materials, they found that metallic glasses would fail at a stress three times lower than Frenkel’s limit, and rocks in the Earth’s interior would fail (at best) four times lower – predictions very close to the values observed in experimental studies.

Braeck told Physics Web that the research shows how thermal runaway could be a potential mechanism for deep earthquakes, and could be of importance to engineers who employ metallic glasses as structural materials. “Our calculations show that thermal runaway in general is unavoidable at high enough stresses, [and] therefore puts a fundamental upper limit on the maximum attainable strength of solids,” he said.

Molten sodium mimics Earth’s magnetic-field flipping

Dynamo action involves the flow of an electrically conducting fluid converting mechanical energy into magnetic energy. In the Earth, the fluid is the liquid iron of the outer core, which is in constant turbulent motion because of convection and the Earth’s overall rotation. But such dynamos are difficult to study – particularly when one wants to understand its effects on the scale of the Earth. Experiments require massive amounts of energy to sustain large volumes of swirling molten metal, and it is almost impossible to model the Earth’s inner dynamics computationally because of the huge number of variables involved.

Nicolas Mordant and colleagues from the École Normale Supérieure in Paris and other French institutions, however, have witnessed one of the peculiar effects of dynamo action for the first time. The team heated 160 litres of molten sodium to over 110 °C in a chamber, keeping the metal in highly turbulent motion using two counter-rotating propellers. Then they monitored the size and direction of the magnetic field and noticed a strange effect when they happened to set the speed of one propeller to 16 Hz and the other to 22 Hz – the magnetic field of the entire sample began to flip direction at irregular intervals ranging from 10 s to 180 s.

Similar “magnetic reversals” are thought to have occurred in the Earth’s history, but are not clearly understood. The last event is believed to have occurred some 780,000 years ago, but prior events could have been separated by anything from tens of thousands to many millions of years. Mordant’s team thinks that his experiment demonstrates this same “hierarchy” of time intervals, albeit on a much reduced scale.

However, Mordant warns that their observations should not necessarily be used to draw insights into large-scale systems. “We may have just been lucky [to see this effect],” he told Physics Web. “We should be cautious as to whether the physical origin of the flipping in the Earth is the same as in the experiment.”

Mordant’s experiment builds upon work performed in 1999 at the Riga Sodium Facility in Latvia, where a team led by Agris Gailitis first managed to create a self-excited magnetic field by keeping molten sodium in turbulent flow. (See related story: “Mini Earth created in the lab”.)

Been there…got the t-shirt

APS memorabilia: these will
be worth a fortune on
eBay someday.

I actually didn’t buy an APS t-shirt — or a bumper sticker, slinky or travel mug — but I’m still glad I came to Denver.

Here’s some miscellany I learned today.

• There are no guns allowed in the Colorado Convention Center — but nowhere to “check ’em at the door”.

• Top basketball coaches at US universities earn in excess of one million dollars a year. Not sure how much a top physicist earns…

Bumper stickers: who says
physicists have no sense
of humour?

• APS editor in chief Gene Sprouse told me that the funny symbol that appears next to articles of interest in Physical Review Letters is a colophon that appeared on the cover of the first issue of PRL.

A little bang at SLAC

What’s the best way to determine the structure of a molecule that cannot be integrated within a periodic lattice?

Blow the heck out of it using x-ray pulses from the Linac Coherent Light Source (LCLS) — at least according to SLACs Philip Bucksbaum.

The LCLS will open next year at SLAC in California and will be much brighter than existing x-ray sources and will be capable of producing very short x-ray pulses.

It takes about 200 fs for the molecule to explode and the trick is to collect the diffraction data in the first 10-20 fs — when the molecule is still intact. By blowing up 100,000s of the same molecules, Bucksbaum claims that a 3d image of the molecule can be reconstructed. This technique has already been proven at an x-ray facility in Germany.

In his talk “Ultrafast X-ray Science at SLAC and LCLS” (U19 1), Bucksbaum described several other ways of harnessing the violent reaction between the X-ray pulses and sample.

For example, the pulses could be slammed into a solid such as iron. This would heat the sample and drive structural phase transitions that would propagate through the iron as “shock waves”. Real-time images of this process could be generated from the diffracted x-rays. This could give physicists valuable information about how lattice defects and other material properties affect the dynamics of phase transitions.

The great pages debate

IOP Publishing’s reception was a big hit last night, judging by the fact that the buffet had to be replenished several times — you can work up a huge appetite running back and forth between sessions for nearly 10 hours a day.

I spent most of my time talking to IOP referees and journal board members as well as the competition — two editors from Physical Review B (not sure how they got in!).

Journal of Physics A and B
referee David Feder (left) and New Journal
of Physics board member Fernando Sols
(centre) chatting with the PhysicsWeb blogger.

It seems that paper length — or the lack thereof — is a growing concern in the journals community. Authors are apparently under lots of pressure to summarize their work in four pages (I wonder where that comes from?).

One IOP referee told me that he often asks authors to add clarifying paragraphs to their papers, but the authors are reluctant to do so because they believe that publishers favour shorter papers. The referee was concerned that highly truncated papers are of little pedagogic use to newcomers to a field, and that brief papers are so focused on results that the purpose of the research and the underlying physics is sometimes lost.

On the other hand, longer papers are much more difficult to write and referee — with some folks expressing concerns that quality could slip if papers were longer.

Gamma-ray observatory takes share of Descartes prize

HESS is a system of four gamma-ray telescopes designed to probe extreme cosmic events and put constraints on the nature of “dark matter”. In 2005, HESS detected eight new high-energy gamma ray sources – doubling the previous number on record. The EU praised HESS for having “revolutionized existing astronomical techniques and increased our knowledge and understanding of the Milky Way and beyond”.

The two other winners of the research prize were “Hydrosol”, a project that has developed a means of splitting water using sunlight to produce hydrogen, and “APOPTOSIS”, a project that has helped to understand cell death in diseases such as cancer and AIDS.

Wendy Sadler

Meanwhile, the €275,000 Descartes prize for science communication has been awarded to five candidates. One of these has gone to a team of female scientists led by physicists Wendy Sadler from Cardiff University, who instigated the “Science Made Simple” project to tackle the problem of science apathy among 11 to 18 year olds in the UK and overseas. Other winners included Eoin Gill and Sheila Donegan – directors of CALMAST (Centre for the Advancement of learning of Maths, Science and Technology) – who were involved in the primary school science magazine “Eureka”, and Città della Scienza in Naples, the first science centre founded in Italy.

Brrr…it's cold in there

Some of the most interesting condensed matter physics occurs at very low temperatures and physicists need accurate ways of knowing just how cold their samples are. Traditionally, this has meant spending hours building, calibrating and troubleshooting temperature measurement and control systems — instead of actually doing the experiments.

Those days are over in many labs — at least according to Shane Hritz of Lake Shore Cryotronics. Hritz was in town to talk to physicists about the company’s cryogenic sensors, temperature control systems and magnetic measurement systems. He believes that physicists are hesitant to commit valuable resources to building and maintaining laboratory equipment. Instead, they want off-the-shelf kit that works the first time.

Resistance bridge: Lake Shore’s
Shane Hritz shows off the company’s
latest equipment for measuring
temperatures as low as 20mK.

The company has just launched a new ruthenium oxide temperature sensor that is said to be the first commercial system calibrated down to 20mK — its last sensor was calibrated down to 50mK. “This doesn’t sound like much,” said Hritz, “but at these temperatures heat transfer through the leads becomes a big problem — any small bit of energy that gets into the sample can affect its temperature.”

Hritz say the company is now working on a sensor that is calibrated down to 10mK.

On to the exhibition

I was having so much fun with the physicists that I nearly forgot to check out the exhibition before it closed for good this afternoon.

First stop was the IOP Publishing stand where I had a chat with Sharice Collins, who is our senior marketing manager for the Americas. Sharice is based in our Philly office and has organized a proper knees-up tonight for the IOP Journals community (Sharice prefers to call it a reception). I will of course be reporting on the reception in a future entry.

Pyramid power: Matt, Sharice, Tim
and Max on the IOP Publishing stand.

It seems that my initial concerns about the remote location of the exhibition were unfounded. Sharice reported a steady stream of traffic through the IOP stand. She said that delegates were very pleased to hear about our community website strategy — and the IOP flashing badges were a big hit.

Living surfaces self-organize

Here’s a new recipe from Jeffrey Brinker of the University of New Mexico (P42 1)

Mix together water, alcohol, detergent, silica and a good dollop of single-cell organisms.

Dip in a substrate of your choice.

Remove and let dry.

While you do the washing up, the silica, detergent and cells will be busy organizing themselves on the substrate surface to create a highly ordered solid film. The amazing thing about this film is that cells survive the assembly process — and they remain alive for up to one month by eating the detergent.

Such films become even more interesting if they are made with organisms that act as biosensors — organisms that react to changes in light or the presence of certain chemicals.

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