Page 1528 – Physics World

Home » Page 1528

Seeing quantum effects on a big scale

Trapped crystal Laser trap could take quantum mechanics to the macroscale

A scheme to couple the motion of a single atom with a crystal membrane could enable quantum-mechanical effects to be seen on a larger scale than ever before.

The idea, which has been proposed by researchers in Austria, Germany and the US, could help to solve the mystery of why quantum effects only seem to appear at tiny dimensions while the everyday world is governed by classical physics.

“The real drive is to understand how far we can drive quantum mechanics into the macroscopic realm,” says Klemens Hammerer, a theorist at the University of Innsbruck and lead author of the paper. “It’s still not clear where the boundaries of quantum mechanics lie. How large can objects be and still behave quantum mechanically?”

From small to big

In seeking to test the boundaries of quantum mechanics, physicists have gone down two main routes. One involves “entangling” several quantum particles into a single large state, sometimes known as a “Schrödinger cat” state after the famous thought experiment devised by Erwin Schrödinger. The other is to couple a single quantum particle to something macroscopic, typically some form of mechanical oscillator.

The trouble with this second option is that a macroscopoic object can be up to 10¹³ times heavier than, say, a single atom. To achieve a strong coupling – that is, one that can survive the other noise present in a system – the mechanism or “spring” of the coupling is crucial.

Hammerer’s group, which includes researchers at the Ludwig–Maximilians University of Munich, the National Institute of Standards and Technology (NIST) in Boulder, Colorado, and the California Institute of Technology (Caltech), suggest that the spring could take the form of an optical trap.

In this set-up, two criss-crossing laser beams reflected between a pair of cavity mirrors would create a potential well, into which a single atom would be placed. A thin membrane made from a crystal with a high refractive index, such as silicon nitride, would then be placed next to the atom to act as the macroscopic object in the set-up.

Sprung movement

The effectiveness of the optical trap as a spring results from a process of amplification. Any movement of the atom that reflects its quantum state should shift the neighbouring crystal membrane. However, any shift in the membrane should change the resonance of the cavity, which in turn would force the membrane – and the atom – to move more. This amplified movement could be detected either by shining another laser on the atom, or by monitoring the amount of light leaving the cavity, which would reveal the state of the membrane, say the researchers.

Hammerer told physicsworld.com that he is collaborating with several experimentalists to implement the proposal, including Jeff Kimble of Caltech and Jun Ye at NIST, who are both co-authors of the paper. Already, says Hammerer, Kimble has managed to get atoms into traps in the cavities and is now working on adding a membrane.

Markus Aspelmayer, a quantum physicist at the University of Vienna, considers the researchers’ proposal “a very important contribution”. “Loosely speaking, atomic quantum physics could be transferred to the macroscopic domain of massive mechanical resonators,” he says. “Hammerer and his collaborators show that this regime should be feasible even with present day experimental parameters. This poses a fascinating challenge to experimentalists and opens up the field of quantum-optomechanics to the atom physics community.”

Making graphene in a flash

Researchers in the US have found that firing a camera flash at graphite oxide is enough to make graphene – atom-thick sheets of carbon first discovered in 2004 that possess unique mechanical and electrical properties. The new process could also be used to make complex patterns of graphene that could be integrated into fast and flexible carbon-based electronic circuits.

Chemists have previously sought to turn graphite oxide into graphene, but the reduction reactions involved in removing the oxygen typically take days and require strong reducing agents such as hydrazine. What is more, these chemical processes prevent other compounds from being added to the resulting graphene.

Nano-explosion

Now, via a surge of inspiration, Jiaxing Huang, a materials scientist at Northwestern University in Illinois, US, realized that a brief burst of light could perform the same reaction in a single millisecond. Key to the process is the photothermal effect: the camera flash delivers a pulse of energy that is converted to heat in the graphite oxide. “The pulse of energy from the camera flash induces a ‘nano-explosion’ in the graphite-oxide film,” explains Huang. “Reduction takes place so rapidly that the film puffs out and expands by two orders of magnitude.”

In a video released by the group, brown, transparent sheets of graphite oxide can be seen to blacken and expand, accompanied by a loud popping sound. Huang’s team describes the resulting black material as “fluffy” – it is porous and only a fraction of the density of graphite. Further analysis showed that the material is made up of disordered graphene sheets and flakes that are well spaced from one another.

The team also showed that it could add plastic nanoparticles to the graphite oxide, so that when the mixture is exposed to the flash, its particles fuse together like liquid droplets, locking the graphene into a tough composite material.

Speedy process

Because the manufacturing process is clean, fast and simple, the team is confident that industrial-scale production of graphene through this process is possible. Peter Blake, who works with graphene suppliers Graphene Industries, based at Manchester University in the UK, agrees. “This is an intriguing proof-of-concept that will benefit greatly from other work that is going on in the same area,” he told physicsworld.com.

Blake added that printable electronics would also benefit from this technology. One existing challenge is mounting graphene onto silicon or glass surfaces to design circuitry. It may be possible to use insulating graphite oxide to make the circuit then convert it to conducting graphene with a flash of light, he believes. Alternatively, light-resistant masks could be used to generate complex patterns of graphene.

To develop this research, Huang’s team is now planning to use the process to make a nano-scale circuit, but the process is not quite as simple when it involves small amounts of material because heat generated by the light pulse may dissipate too quickly to trigger a reaction.

This research first appeared in the Journal of the American Chemical Society.

Bolt out of the blue

Usain Bolt may have crossed the line in 9.55 s at last year’s Beijing Olympics if he had kept his speed

By Michael Banks

Few would doubt that Jamaican sprinter Usain Bolt is now the fastest man on the planet and will probably hold that title for a long time to come.

Yesterday, he won the 100 m sprint in the record-breaking time of 9.58 s at the World Championships held in Berlin.

But, of course, we all knew that he could run that fast.

After his previous record-setting time of 9.69 s at the Beijing Olympics last year, astrophysicists at the University of Oslo in Norway worked out that Bolt could have run even faster if he had gone flat out rather than slowing down in the last 20 m of the race to celebrate his win.

And they got it pretty much spot on.

The physicists calculated that Bolt could have covered the 100 m in 9.55 s (plus or minus 0.04 s) if he had maintained his pre-celebration acceleration.

So maybe there is still some room for improvement for Bolt to beat his newest record.

Neutrino trigger could reveal gravitational waves

Existing neutrino and gravitational-wave detectors can be used in concert to observe gravitational waves given off during a nearby supernova — say physicists in Italy.

Gravitational waves are vibrations of space–time predicted by the general theory of relativity. A number of experiments are trying to detect gravitational waves by measuring tiny changes in the separation of two masses that are expected to occur when the waves traverse a detector. However, none have been successful so far and the most convincing evidence yet for gravitational waves is that the orbital period of the Hulse–Taylor binary star system is shrinking at the precise rate associated with the emission of gravitational waves.

With a little bit of luck, however, the first direct detection of the waves could happen if a supernova occurs in our own galaxy. Such a massive stellar explosion produces a vast amount of light and other radiation, which could help physicists narrow down their search to the precise moment that the gravitational waves reach Earth. This would be a great help in boosting the sensitivity of gravitational wave detectors.

Neutrino pulse

When radiation from supernova SN 1987A was detected here on Earth (in 1987), physicists realized that it included a pulse of neutrinos — or more precisely, electron antineutrinos. Now, Francesco Vissani and colleagues at the Gran Sasso Laboratory and INFN Torino have done calculations suggesting that the detection of neutrinos from a nearby supernova could allow existing gravitational wave detectors to make their first discovery.

At the heart of their theory is the idea that gravitational waves large enough to be detected are created when the supernova reaches the “bounce” phase. This occurs when the star’s iron core collapses rapidly to the density of a nucleus. Incoming material rebounds from the nuclear core, which is incompressible, the result being a sharp outward acceleration of vast amounts of matter.

For a typical supernova, Vissani and colleagues calculate that this bounce should last about 30 ms — creating a burst of gravitational waves of about the same duration. Most astrophysicists agree that the neutrino pulse from a supernova precedes the bounce by about 3 ms. Using a model developed from analysis of neutrino and other data from SN 1987A, the team calculated the time delay between the detection of the neutrinos in detectors on Earth and the arrival of gravitational waves.

The physicists conclude that neutrino detection could be used to narrow down the arrival time of gravitation waves to an uncertainty of about 10 ms.

Welcome reduction

According to Edward Daw at the University of Sheffield, existing gravitational wave detectors operate on the basis that the arrival time of waves can be pinpointed to within 100 ms. He cautions that while the new results do not necessarily mean this can be reduced by a full factor of ten, any significant reduction is welcome.

“This is a useful improvement, especially if you are requiring coincident signals between multiple gravitational wave interferometers within this time window”, he told physicsworld.com. If three detectors are used, a factor of 10 reduction means that the probability of a false detection due to noise in all detectors is reduced by a factor of 1000. This means that the noise threshold of the detectors can be reduced, making them more sensitive to gravitational waves.

Daw, who works on the LIGO gravitational-wave detectors, points out that Vissani’s calculations suggest that LIGO would not be able to detect gravitational waves from a supernova like SN 1987A — even using the neutrino trigger. This is because the supernova occurred outside the Milky Way. Instead, scientists will have to wait until the next supernova is spotted in our galaxy — something that is expected to occur only a few times in a century.

Let’s hope it happens when all the neutrino and gravitational–wave detectors are switched on.

The calculations are reported in Physical Review Letters.

Canadian isotope reactor will restart in 2010

By Hamish Johnston

The sorry saga of the NRU reactor and its owner Atomic Energy of Canada (AECL) continues.

The idle reactor, which normally produces about one third of the world’s supply of medical isotopes, will not be restarted until next year.

The reactor stopped running unexpectedly in May and has been down ever since — leaving the global medical community very worried.

And to make matters worse for AECL, a few weeks ago the Ontario government rejected a previously accepted bid from the firm to build two large power reactors outside Toronto. If AECL cannot win back the project, many worry that its days could be numbered.

Another threat to the Canadian reactor industry is the possibility of creating medical isotopes using an accelerator rather than a reactor. There was a nice article in the Globe and Mail earlier this week warning that the country’s reactor expertise could soon ebb away.

Theorist versus experimentalist, round one

Hooke’s masterpiece

By Hamish Johnston

Most physicists are either theorists, who solve problems using mathematics, or experimentalists who make measurements. While the two disciplines are intertwined (except perhaps in fields such as cosmology, where measurements are difficult to make) the two tend to operate in very different ways — which can sometimes lead to tension.

When did this distinction (and occasional animosity) arise in modern science, you might wonder?

One early example is the considerable friction between the greatest theorist and experimentalist of the English Enlightenment — Isaac Newton and Robert Hooke respectively.

Newton remains a celebrity to this day. However, Hooke’s considerable contributions to science (and architecture) remain mostly unsung — with the possible exception of his spring law.

On Thursday evening BBC 4 aired a programme called Robert Hooke: Victim of Genius, which tries to set the record straight. For some reason, the BBC has not made it available for viewing online, so you will have to wait for a repeat.

I came to the conclusion that many of Hooke’s problems were related to his humble beginnings — or more precisely, the fact that Hooke began as an apprentice painter, paid his way through university working as a servant to fellow students, and then earned his living by building scientific equipment for the Royal Society.

When this lowly chap informed the Lucasian Professor of Mathematics that he had formulated the inverse square law of gravitation years before the publication of Principia, Newton is said to have flown into a rage. The two had already sparred over their optical theories, and when Newton took over as president of the Royal Society in 1703 (the year of Hooke’s death), he began erasing all traces of Hooke. Famously, he tossed the only contemporary portrait of Hooke onto a fire.

It would be disingenuous to describe Hooke as a man of modest means — he made a fortune surveying London after the Great Fire — and he was a colleague of many great scientists of the day including Robert Boyle, Edmund Halley and John Flamsteed. Who apparently made liberal use of Hooke’s intellect and experimental skills, sometimes without giving due credit.

However, Hooke was a man who got his hands dirty building wonderful machines such as vacuum pumps and telescopes. He was also a skilled artist — consider the sketches in his masterpiece Micrographia (above).

In other words he was an experimentalist, and history of physics tends to remember the theorists.

The BBC programme was presented by Oxford’s Allan Chapman, and you can read his essay
England’s Leonardo: Robert Hooke (1635-1703) and the art of experiment in Restoration England on a website dedicated to Robert Hooke that has been set up by Westminster School.

Galaxy Zoo: The Hunt for Supernovae

The first of many?

By Hamish Johnston

Yesterday the folks that brought us Galaxy Zoo — which harnesses people power to sort through astronomical images and classify galaxies — launched The Hunt for Supernovae.

Overnight, they may have just found their first supernova. You can follow all the excitement on their blog.

And if you decide to join in…happy hunting!

Friction keeps molecular motors on track

When you are trying to get from A to B, friction is both friend and foe. Friction between tyres and the road allows a car to be steered, for example, whereas axle friction reduces the vehicle’s fuel efficiency. Now, however, biophysicists in Germany have discovered that friction also plays an important role in how molecular motors move within living cells.

The discovery could lead to a better understanding of cell biology because molecular motors are involved in a wide range of processes, such as DNA replication and sperm mobility.

The new study has been carried out by Joe Howard and colleagues at the Max Planck Institute of Molecular Cell Biology and Genetics, along with Erik Shaeffer at the Technical University of Dresden, who looked at “kinesin” molecules. These comprise a large family of molecular motors that transport cargoes (such as biological molecules) along microtubular strands.

Molecular motors get legless

Biophysicists believe that kinesin molecules have two “legs” and walk along the microtubule much like a human on a tightrope — gripping the microtubule with one leg, while releasing the other and pulling it forward. But because the molecules are only a few nanometres in size, thermal fluctuations make them more like drunks walking along a rapidly shaken tightrope.

To understand the forces involved in keeping the motors on track, the team attached kinesin to a tiny bead about 1 μm in diameter. They then released the bead into the focal point of a laser beam, with the optical pressure on the bead holding it firmly in place in “optical tweezers”. A microtubule mounted on a translation stage was then positioned next to the bead and pulled along at a range of speeds of about 1 μm/s, which is roughly the natural speed of the molecular motor.

The team then determined the frictional force on the ball by measuring the deflection of the ball from the centre of the tweezers. At speeds below 1 μm/s, the friction was proportional to the relative speed of the kinesin and the microtubule. This was confirmed in a different experiment by observing the motion of kinesin that had run out of fuel (ATP). These molecules diffuse along microtubules at a rate defined by Einstein’s diffusion equation, which can also be used to calculate friction.

Taking it in their stride

When the relative speed was above 1 μm/s, friction still increased — but the rate of increase began to fall. According to Howard, this is exactly what should happen if kinesin’s legs are grabbing onto specific sites on the microtubule and then letting go. By analysing the shape of the force/speed curve, the team concluded that the “stride” of the motors is about 8 nm, which also happens to be the length of a certain dimer that repeats along the microtubule. This came as a surprise to the team, because previous attempts at measuring this stride using other techniques suggested 4 nm.

Howard told physicsworld.com that the results “give us a new way of thinking about how molecule motors operate — that two elements are involved, force generation and friction”. This could help biophysicists understand how large numbers of motors work together to propel a sperm cell, for example, because, says Howard, such collective motion is likely to involve friction.

The studies could also help scientists understand motion on a much larger scale. According to Shaeffer, the forces that stick the kinesin legs to the microtubule are the same Van der Waals forces that allow some geckos scurry up a smooth wall. Geckos rely on tiny strands on their feet that are about 100 nm in size, whereas kinesin legs use structures about 25 nm in size.

The work is reported in are reported in the 14 August edition of Science.

Don’t fret over the phase problem

The unfulfilled desire of crystallographers to measure the phase of diffracted beams produced by X–ray diffraction tools is actually rather pointless, according to Emil Wolf, a theoretical physicist at the University of Rochester in the US. Instead, Wolf believes that physicists should shift their attention to measuring other properties of a diffracted X-ray beam — which hold the same elusive structural information lurking in the inaccessible phase.

Wolf, who is famous for his co-authorship of the classic treatise Principles of Optics, says that many researchers erroneously believe that knowing the phase of the diffracted beams would lead to unambiguous determination of the atomic structure of solids.

“Such beams do not occur in nature, nor can they be produced in the laboratory” Emil Wolf, University of Rochester

But this mistaken view employs an unjustifiable assumption that beams of X–rays are monochromatic. “Such beams do not occur in nature, nor can they be produced in the laboratory,” says Wolf. His technique — which has not been confirmed experimentally — uses spatially coherent X–rays that can be produced in the lab.

Intensity only

When a beam of X-rays is fired at a crystalline material, the resulting diffraction pattern provides some insight into the locations of atoms within the material. The pattern is determined by measuring the amplitude of the diffracted X–rays as a function of position. The very act of measurement destroys important crystallographic information carried by the phase of the X–rays — the phase problem. Instead, the phase of every diffraction beam has to be estimated via approximations, leading to incomplete, unreliable information.

Wolf’s calculations suggest that it would be more fruitful to measure the amplitude and phase of another physical property of the X–rays — the degree of spectral coherence. This defines how X–rays of different wavelengths interfere with each other. Armed with this information, it is possible to determine the electron density throughout the crystal, and ultimately the physical properties of the solid, he claims.

His approach is similar to that taken about six years ago in the optical domain. “I did not appreciate until very recently the relevance of the work to the X–ray reconstruction problem”, he told physicsworld.com

Maintaining coherence

In his calculations, Wolf considers spatially coherent X–ray sources, in which two different points on a wave front are correlated, leading to interference. These are routinely generated at optical wavelengths and they have been produced in the X–ray domain of the electromagnetic spectrum in recent years. Such X–ray sources are not monochromatic.

“The essential point is that the fluctuations at the two points should be in unison,” says Wolf. He cites the example of light from a star. “Such light originates in billions of independent radiating atoms; yet by the time light reaches the earth it becomes highly coherent over large areas.” This high degree of coherence produces telescopic images with sharp diffraction minima and maxima.

Obtaining the phase of the spectral degree of coherence in X–ray diffraction measurements is possible with interferometer-based techniques, explains Wolf. “I am now writing a longer paper explaining how my theoretical results can be implemented in practice.”

“I am [also] in touch with several experimental groups about the possibility of applying the theory to some reconstruction problems.”

Pawel Korecki of the Jagiellonian University in Poland believes that it is “very likely” that Wolf’s proposal will be verified in the lab, describing the measurement as “possible but not trivial”. However, the X–ray diffraction expert is more cautious about the possibility of practical applications.

Wolf’s calculations are reported in the 14 August edition of Physical Review Letters.

Hawking hits the headlines in the US…

Two agents of change

By Hamish Johnston

Cosmologist Stephen Hawking has been in the news recently for two very different reasons.

Yesterday he was awarded the US’s highest civilian honour — the Presidential Medal of Freedom — by Barack Obama.

Obama described Hawking as “an agent of change”, and someone who “saw an imperfect world and set about improving it, often overcoming great obstacles along the way”.

I believe that Hawking is only the second physicist to receive the award, the first being Edward Teller — ‘Father of the H-Bomb’ and nemesis of Robert Oppenheimer.

Teller received his award from the previous president George W Bush, so perhaps physics is enjoying a period of grace in the White House?

Although Hawking received his award at the White House, he is British born and lives in the UK…facts that seem to have escaped a commentator in the US who has written:

“People such as scientist Stephen Hawking wouldn’t have a chance in the UK, where the National Health Service would say the life of this brilliant man, because of his physical handicaps, is essentially worthless.”

I don’t want to say that this is typical of the level of debate surrounding Obama’s healthcare reforms…but you can read a corrected version of the editorial here.

Browse articles by content type