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Physicists reveal a quantum Cheshire cat

“It’s the most curious thing I ever saw in my life!” Alice thought to herself when she saw a Cheshire cat disappear and leave only its grin behind. It is not only in Wonderland, however, that properties of objects can exist independently of the objects themselves. That is the conclusion of a group of physicists from Israel and the UK, which has shown how the strange laws of quantum mechanics permit a photon to be in one place and its circular polarization in another.

This counterintuitive result was achieved thanks to the quantum-mechanical concept of post-selection. In classical physics, the initial conditions of a set of particles and the rules governing the behaviour of those particles are in principle enough to determine the properties of the particles at any arbitrary point in the future. That is not the case in quantum mechanics, in which a particle’s evolution is inherently probabilistic. So while the results of a measurement carried out on a set of particles will have a known probability distribution, individual results cannot be predicted.

Post-selection, pioneered by Yakir Aharonov of Tel Aviv University, involves preparing a group of particles in some initial state, measuring each of the particles at a certain point in time, and then making a second set of measurements at a slightly later time. The results of the intermediate measurements will, on average, imply certain results for the later measurements but will not determine them. If the group is then split into sub-groups according to these later results, the identity of the members of those various sub-groups is information that can only be obtained after the final measurements, and not before.

Superposition of paths

In the latest work, Aharonov has teamed up with Sandu Popescu of the University of Bristol, Daniel Rohrlich of Ben Gurion University and Paul Skrzypczyk, then at Cambridge University. The group has devised an experiment, which it says can be implemented with current technology, in which individual horizontally polarized photons pass through a beamsplitter and then traverse a series of optical devices before being registered in one of three detectors. When leaving the beamsplitter, each photon is in some kind of superposition of two different paths that it can take to reach the other devices, the two paths representing the two arms of an interferometer (see figure “An optical Cheshire cat”).

Schematic of the proposed Cheshire-cat experiment
An optical Cheshire cat Schematic of the proposed Cheshire-cat experiment. Photons travelling from the bottom left enter a beamsplitter (BS1) and are sent along two different paths (L and R) before being recombined in a second beamsplitter (BS2). More measurements are then made on the photons. (Courtesy: Yakir Aharonov, New J. Phys. 15 113015)

The devices are chosen and arranged so that the first of the detectors only clicks when the photon is in a specific superposition state, and it is this state that is post-selected. The researchers then consider what happens to the photon – the Cheshire cat – and its polarization – the grin – in that post-selected state. They find that while any photon detector would reveal the photon to always travel along the left-hand arm, a polarization detector would occasionally measure angular momentum in the right-hand one. “We seem to see what Alice saw,” the researchers write, “a grin without a cat!”

The researchers point out that this analysis falls down because it relies on the two kinds of detector being used at different times, and that if they were to be used simultaneously, the detectors would always show the photon and its polarization together in the same arm. But Aharonov and colleagues argue that they can “regain the paradox” by carrying out what are known as “weak measurements”, which do not provide definitive values of particle parameters but do have the virtue of not completely destroying a particle’s quantum state, as usually happens during the measurement process.

Making weak measurements

The researchers say that weak measurements can be made of the photons’ trajectory by replacing the first detector in their hypothetical experiment with a CCD camera and by placing a sheet of glass in one of the arms. Deflection by the glass – which reveals photons to have travelled down that arm and which would be registered by the camera – is made deliberately much smaller than the width of the photon beam, with the resulting uncertainty then reduced via multiple measurements. Analogously, polarization is measured by placing a suitable optical element in one of the arms and recording a deflection at right angles to that caused by the glass sheet.

The crucial point about this revised set-up, explains the Israeli–UK team, is that it can be used to measure different parameters at the same time. As such, the researchers claim, putting both the glass and the optical element in the right arm of the interferometer would prove that the polarization could exist independently of its photon. Which would mean, the researchers write, that they had “finally found [the] Cheshire cat”.

“Beyond the mainstream”

Having had to wait for a 21 months between posting its proposal on the arXiv preprint server and seeing it published in New Journal of Physics, Popescu acknowledges that his group’s scheme was not well received by all of the referees who reviewed it. “It is beyond the mainstream,” he says. “But quantum mechanics has been around for almost 100 years and people still don’t understand it profoundly. Discovering effects like this, which expose the weirdness of quantum mechanics, may help.”

Popescu says that the Cheshire-cat effect is quite general – that there is nothing in principle to prevent the separation of, say, an electron’s spin and charge, or an atom from its internal energy. Indeed, an alternative to the current experimental proposal would involve cutting off a group of electrons from its own magnetic field. Being a group phenomenon, he points out, this would have the advantage of revealing the Cheshire cat unambiguously at a single instant in time rather than as the average of a series of repeated measurements, but would, he says, require experimental techniques beyond the realm of current technology.

Antonio Di Lorenzo of the Federal University of Uberlandia in Brazil agrees that the experiment proposed by Aharonov and co-workers could be used to find quantum Cheshire cats. But he says they are mistaken in the criterion that they use to identify their quarry. Rather than consider the outputs of the “cat detector” and “smile detector” separately, he argues, they should instead establish the product of these two outputs. A non-zero answer, he says, would reveal the cat.

  • Find out much more about weak measurement in “In praise of weakness” by Aephraim Steinberg, Amir Feizpour, Lee Rozema, Dylan Mahler and Alex Hayat

Colliding exhibits, influential researchers, edible particle-detectors and more

Collider exhibition at London's Science Museum (Courtesy: Nick Rochowski for the Science Museum)
The “Collider” exhibition at London’s Science Museum. (Courtesy: Jennie Hills / Science Museum)

 

By Matin Durrani and Tushna Commissariat

If you’re in the tiny minority of people whose job title says “particle physicist”, chances are you’ll have been to CERN at least once in your career to help build a detector, analyse some collision data or muse in the cafeteria over supersymmetry (or the apparent lack of it so far). But for the rest of the world, going to the Geneva lab is simply not on the agenda, which is one reason why the Science Museum in London has this week unveiled a big new exhibition devoted to CERN’s Large Hadron Collider. Entitled simply Collider, the exhibition “blends theatre, video and sound art with real artefacts from CERN” that will, say organizers, “recreate a visit to the famous particle-physics laboratory”.

(more…)

Four quarks for Muster Mark?

Family of four? (Courtesy: Shutterstock/paul_june)
Family of four? (Courtesy: Shutterstock/paul_june)

By Tushna Commissariat

In June we reported that physicists working on the BESIII experiment in Beijing and the Belle experiment in Tsukuba, Japan found evidence for a new “charged charmonium” called Zc(3900). A “charged charmonium” is a particle that is made of four quarks – something that had never been seen before. Since that discovery, the BESIII collaboration says it has made “a rapid string of related discoveries” of four-quark particles. “While quarks have long been known to bind together in groups of twos or threes, these new results seem to be quickly opening the door to a previously elusive type of four-quark matter,” says Frederick Harris, spokesman for the BESIII experiment. “The unique data sample collected by the BESIII collaboration has continued to yield a stream of clues about the nature of multi-quark objects.”

(more…)

Quantum state endures for 39 minutes at room temperature

Quantum states have been shown to endure in a room-temperature solid-state device for a whopping 39 minutes, shattering the previous record of 2 s. The feat was achieved by physicists in Canada, the UK and Germany, who used phosphorus atoms in silicon as their quantum bits – or qubits. The breakthrough offers hope that normally fragile quantum states could be made robust enough to be used in practical quantum computers or even in “quantum money”.

Quantum computers are designed to exploit the counterintuitive idea that tiny objects can exist in more than one state at the same time. Rather than processing bits – which are either 0 or 1 – such devices instead manipulate qubits, which can be 0 and 1 simultaneously. Vast numbers of operations could therefore, in principle, be carried out in parallel and rendering these devices far quicker than classical computers.

But anyone trying to build a working quantum computer has to deal with the fact that qubits tend to be incredibly fragile, which means the quantum information they hold is rapidly destroyed by external noise. One way of getting around this problem is to cool the qubit to near absolute zero to minimize its exposure to thermal noise. But working at such low temperatures is not particularly practical, which is why researchers are keen on find qubits that can operate at room temperature.

Record breakers

The new record-breaking system has been created by Mike Thewalt of Simon Fraser University and colleagues, by storing quantum information in the nuclear spins of phosphorous atoms in a silicon crystal. The idea of using these nuclear spins is not new and the system has already been shown to retain quantum information for long times at extremely low temperatures. But even at 10 K, this “coherence time” drops precipitously to just a few milliseconds.

To get around this problem, Thewalt and colleagues took advantage of the fact that phosphorous atoms in silicon at room temperature tend to give up their electrons and become positive ions. Removing the electrons eliminates an important link between the nuclear spins and surrounding electrical noise. Nuclear spins can therefore retain quantum information for much longer than those in neutral phosphorous.

The downside is that removing the electrons makes the nuclear spins so well isolated that they cannot be “read” or “written” to. So to get around this problem, the team first cooled its crystal to 4.2 K and used laser and radio frequency (RF) pulses to put neutral phosphorous atoms into specific quantum states. A laser pulse then ionized the atoms before the crystal was warmed up to room temperature (298 K).

Under these conditions, RF pulses were used to perform a “spin echo” measurement of the coherence time, which was found to be 39 minutes. The crystal was then cooled back down to 4.2 K and another laser pulse was used to neutralize the phosphorus ions before the quantum information was read out using a sequence of laser and RF pulses.

Walking round the lab

Although measurements reveal that the coherence time at room temperature is 39 minutes, team member John Morton from University College London says that under these conditions, it would be possible in principle to remove the crystal from the cryostat and carry it around the lab while the spins maintain their coherence. What’s more, repeating the experiment with the sample at 4.3 K revealed a coherence time of as long as three hours.

Stephanie Simmons from the University of Oxford, who is also part of the team, says that while 39 minutes “may not seem very long”, it takes just 10 microseconds to flip the nuclear spin of a phosphorus ion – the type of operation used to run quantum calculations. “In theory, over 20 million operations could be applied in the time it takes for the superposition to naturally decay by 1%,” she says.

On the money

Morton adds that it is unlikely that anyone would build a quantum computer that is cycled between 4.2 and 298 K and so qubits based on phosphorous ions would probably be operated at cold temperatures where their even longer coherence time would be an asset. However, he points out that such a system could be used to create “quantum money”, which in principle would be impossible to counterfeit.

The serial number of a “banknote” could, for example, be encoded into the nuclear spins at 4.2 K before the system is heated to room temperate and carried about until it is “spent” by cooling it down. The serial number could then be read out, but a counterfeiter trying to copy the quantum serial number would be thwarted by the “no-cloning” theorem of quantum mechanics, which prevents an unknown quantum state to be copied.

Although the team has shown that ionized phosphorous qubits can endure for very long times, there is more work to be done before the nuclear spins could be used in a quantum computer or quantum money. The measurements were made simultaneously on a collection of about 10 billion ions and physicists must now work out how to read and write information to an individual ion – and also how to get two or more ions to interact with each other to create quantum-logic devices.

Thewalt told physicsworld.com that physicists at the University of New South Wales in Australia have already worked out a way of reading and writing information to individual ions – albeit at low temperatures – and are now looking at how they could be entangled. Meanwhile, Thewalt’s team is now looking at other atoms in silicon, including arsenic, antimony and bismuth.

The research is described in Science.

In pictures: the opinions of Physics World readers

By James Dacey

Love it, or love to hate it, one thing that social media has undoubtedly achieved is to break down some of the barriers between professional journalists and their readers. Gone are the days when we had to rely almost exclusively on guesswork and intuition when it came to picking the issues that matter the most to our readers. Of course, we have always received “proper” letters in the days and weeks following the publication of Physics World to inform us when readers were pleased (or slightly less approving!) of the words they had read. But these days, the feedback starts pouring in almost as soon as our online articles are published, courtesy of our 170,000 Facebook fans and 50,000 Twitter followers. If our readers’ hackles are raised by certain articles and issues, then believe me – we know about it very quickly.

(more…)

Search for electron’s electric dipole moment narrows

Physicists in the US and Canada have put the smallest limit yet on the size of the electric dipole moment (EDM) of the electron. By making precise measurements on a slow-moving beam of thorium-oxide molecules, the team has shown that the electron EDM is at most one-12th of the previous upper limit set by a different experiment in 2011. Constraining the value of the EDM provides important information to those developing new theories of particle physics such as supersymmetry, whereas actually measuring a non-zero EDM would be a major breakthrough in physics.

The simplest take on the Standard Model prohibits the electron from having a permanent EDM. This is because the combination of an EDM with the electron’s well-known spin magnetic moment would violate time-reversal symmetry, which says that physical interactions should look the same if the direction of the flow of time is reversed (see figure “Incompatible moments”). While more-sophisticated versions of the Standard Model do allow for an EDM, they nevertheless suggest it would be much too small to measure in the lab. However, theories of physics that go beyond the Standard Model – such as those invoking supersymmetry – do predict much larger values of the EDM that could be determined experimentally.

In 2011 Jony Hudson and colleagues at Imperial College London found the EDM to be less than 10.5 × 10–28 e cm. While this is still much larger than the upper limit allowed by the Standard Model (about 10–39 e cm), it does begin to rule out certain theories that go beyond the Standard Model (see figure “Less room for new physics”).

Shrinking EDM

Now, physicists working on the Advanced Cold Molecule EDM Experiment (ACME) in the US have improved on this limit by a factor of 12, setting the upper limit on the EDM at 8.7 × 10–29 e cm. ACME is a collaboration of physicists at several universities in the US and Canada – with the main players at Yale and Harvard.

The experiment begins by creating a relatively slow-moving pulse of very cold thorium-oxide (ThO) molecules, which is sent through a region where parallel electric and magnetic fields run perpendicular to the beam. Laser pulses are used to put the molecules into specific states in which the spin magnetic moment of an excited electron (and its EDM, if it has one) is perpendicular to the applied fields. The molecules then travel about 22 cm through the parallel fields, causing the spins (and EDMs) to rotate about the field direction. This precession angle is then measured precisely using a spectroscopic technique.

Figure showing EDM predictions of several physics models
Less room for new physics The electron electric dipole moment (de) predictions of several leading theories of physics beyond the Standard Model. The ACME result makes several theories less likely than others. (Courtesy: IOP Publishing)

If the electron has an EDM, its presence will contribute to the precession angle by an amount proportional to the electric field in the region of the electron. This is where the ACME team uses a clever trick. ThO is a polar molecule that has an extremely large electric dipole moment. This creates a huge “effective electric field” in the vicinity of the electron – much larger than could be applied externally in the lab. The molecules are prepared such that the effective electric field is either parallel or antiparallel to the applied fields. These configurations shift the precession angle in opposite directions. By measuring the difference in the precession angle between both of these configurations, the team is able to determine the EDM.

“Significant step forward”

The ACME experiment measured the EDM to be zero to within very small experimental uncertainties. As a result, it was able to put the most stringent upper limit on its value so far: 8.7 × 10–29 e cm with a 90% confidence. “This is a significant step forward, the biggest improvement in EDM measurements in a decade or so, and the researchers haven’t exhausted the potential of the system,” says Chad Orzel of Union College in the US, who was not involved in the research.

Orzel points out that the measurement is the first result to come from ACME and he believes that the team will be able to improve on it. “This is basically the first real run of the experiment, and people can always find ways to improve the initial systematic uncertainties,” he says. Indeed, Orzel believes that the team should be able to improve its initial result by an order of magnitude.

On a par with the LHC

Time-reversal symmetry is related to another important symmetry of physics: charge–parity (CP) symmetry. As a result, measurements of the electron EDM also provide a constraint on CP violation, which occurs in many physics theories that go beyond the Standard Model. Testing these models is the current focus of particle-physics experiments around the world, including those at the Large Hadron Collider (LHC) at CERN. The ACME team says that its experiment has constrained CP violation at energies on a par with those accessible at the LHC.

Orzel says that some of ACME’s success can be attributed to the fact that it is a collaboration of three leading atomic-physics groups, headed by David DeMille at Yale, John Doyle at Harvard, and Gerald Gabrielse at Harvard. “ACME has really only been going for something like three to four years, and they’ve already produced a great measurement,” Orzel says. “This shows something of the power of the ‘particle physics’ sort of model they’re using, combining several high-power groups together in a bigger collaboration than you usually see in [atomic, molecular and optical] physics, so as to bring greater resources to bear on the problem.”

The research is described in a preprint on arXiv.

Politics or physics?

In late May 1944 Niels Bohr met Winston Churchill to discuss the atom bomb. Certain that the new weapon – when it arrived – would completely transform great-power politics, Bohr wanted to alert Churchill to the hazards ahead. Churchill, who resented being lectured on international affairs by a scientist, rudely told Bohr to mind his own business. “What is he talking about, politics or physics?”, Churchill asked his science adviser, Frederick Lindemann. “This new bomb is just going to be bigger than our present bombs and involves no difference in the principles of war.”

Bohr did not perceive a dividing line between politics and physics; they were simply two interlocking elements in his comprehensive world view. Churchill, however, insisted that the two could, and should, be kept separate, which explains why he was reluctant to accept political advice from a scientist, unless (as was the case with Lindemann) the scientist’s opinions mirrored his own. Leaving that aside, Churchill’s retort to Bohr seems incredibly obtuse, especially from a man noted for his acute sensitivity to the wider implications of technology. As Graham Farmelo reveals in his intriguing book Churchill’s Bomb, Britain’s wartime prime minister was uncharacteristically myopic when it came to the bomb. Furthermore, his myopia probably proved costly for Britain – both in politics and in physics.

Before the war, the US lagged far behind Europe in atomic physics. The great strides were made in Cambridge, in Heidelberg and at Bohr’s laboratory in Copenhagen. When attention turned to the possibility of an atom bomb, the British were the early leaders, in part because of the contributions of refugee scientists from Central and Eastern Europe. The Frisch–Peierls Memorandum of 1940, which was drafted by two such émigrés, was the first practical exposition of an atom bomb and, significantly, an astute exploration of its political and military implications. That progress was confirmed the following year when the MAUD Committee, commissioned to explore the possibility of a British bomb, effectively provided a blueprint for one.

Progress up to this stage was, however, confined to theory. When the problem of the bomb morphed from theoretical to practical, the British encountered a technological challenge beyond their capacity in wartime. At this stage, American strengths became predominant, especially so when the US entered the war after the Japanese attack on Pearl Harbor. America alone had the natural resources, labour force and money to turn the theoretical bomb into an actual weapon.

At this point, British science and American technology might have been brought together in perfect harmony. Unfortunately, as Farmelo shows, Churchill squandered golden opportunities to derive maximum benefit from the early lead the British had enjoyed. In particular, he waited two months to reply to US president Franklin Roosevelt’s offer of partnership, made in the autumn of 1941. By the time he responded, Pearl Harbor had occurred, the American war machine was in top gear and Roosevelt no longer worried about keeping the British sweet. What might have been an Anglo-American project became instead an exclusively American one in which British scientists were individually offered jobs, if their expertise warranted. More importantly, it was made patently clear that the end result would be an American bomb, over which the British would have no control. This meant that if the British wanted a bomb of their own, they would have to start from scratch after the war.

Churchill’s failure to exploit the opportunities offered by the bomb appears strange in a man who was so keen to preserve British power past its logical shelf life. His clumsy handling of the bomb seems especially bizarre given his fascination with atomic power before the war, as evidenced by his friendships with Lindemann and H G Wells and by his musings in the popular press. It is refreshing to read a book so critical of Churchill, given the worship he customarily receives. Churchill’s Bomb is a story of abject failure by the man widely considered to be the greatest Briton ever to have lived. While there is nothing particularly new in this book, its brilliance lies in the way the story is told, for it is a tale not just of physics or politics but also, more importantly, of people.

Farmelo exposes the abundant errors Churchill made in formulating nuclear policy. However, he is less adept at explaining those failures. Why, in other words, did the man best situated to exploit the new weapon fail to do so? It would be easy to argue that Churchill was distracted by the war: the real threat of invasion might have caused him to ignore theoretical possibilities like the bomb. Such an excuse would, however, be too charitable given that Churchill frequently allowed himself to be diverted by far less important abstractions, like bizarre suggestions for new weapons that were physically impossible. The explanation in part lies with Lindemann, a distinctly average physicist who had far too much influence over the prime minister. J Robert Oppenheimer was one of many physicists to be amazed by the limits of what Lindemann understood. Yet, in this case, the fault lies with Churchill for choosing an adviser who would echo his views rather than challenge them.

In my view, the best explanation for Churchill’s failure to exploit British atomic expertise lies in his inability to understand the modern world and the role of the Americans within it. The qualities that made him a brilliant war leader also rendered him incapable of coming to terms with the future. His romantic conceptions of British greatness were perfectly suited to the heroic struggle he presided over in the first three years of the war. They proved an impediment, however, when it came to carving out a role for Britain in the age of the atom.

The American version of this book is subtitled How the United States Overtook Britain in the First Nuclear Arms Race. The difference in subtitles speaks volumes about how the division of power in the atomic age played out. In truth, the opportunities for Britain to take a larger role – which Farmelo effectively implies – might never have existed. Roosevelt and his successor, Harry Truman, were fully aware of the implications of American might, especially now that her supremacy was punctuated with atomic weapons. Churchill thought that British prestige would trump American power. He was wrong – disastrously so – since the Americans didn’t give a fig for how great the British had once been. While Churchill undoubtedly mishandled nuclear politics, no British leader, no matter how perceptive, could have stopped the Americans from strutting on the atomic stage.

  • 2013 Faber & Faber/Basic Books £25.00/$29.99hb 576pp

Deciding with science

Activists protesting against genetically modified organisms recently destroyed a field trial of genetically modified rice in the Philippines. Participants and sympathizers claimed that the crop was poisonous, would destroy biodiversity and was a means for industry to exploit the poor.

But the rice had no known health hazards. It did not dominate other rice species and thus would not threaten biodiversity. It had been altered to create beta carotene – a precursor to vitamin A, which counteracts blindness and other illnesses associated with weakened immune systems, thereby helping hundreds of thousands of children lacking vitamin A. The rice was developed not by industry but the International Rice Research Institute, a non-profit organization.

The Philippine vandalism, however, harmed far more than the rice and its intended beneficiaries. It damaged the credibility of the entire scientific infrastructure that created the rice and determined it to be safe.

This controversy is but one of several instances – including fracking, nuclear power, climate change, vaccination and evolution – playing out in ways that marginalize expertise and make scientific evidence seem irrelevant. One may have sound non-scientific reasons for opposing things like genetic modification, such as not wanting to “instrumentalize” nature. But making good judgments requires respecting what science has discovered about the world and debating issues on their merits. Marginalizing science allows such controversies to unfold as “morality plays” about social inequalities or injustices, or only about politics or economics, rather than as complex negotiations between what we want and what is possible. Inevitably, bad decisions result.

Never in history have good judgments about issues such as energy, pollution and health depended more on science. Yet incorporating science into such debates has been beset by saboteurs, undermined by politicians and met with scepticism. Why?

Three reasons

The answer lies in three separate entangled ingredients, which I dub “manipulated acoustics”, “impure science” and “magical thinking”.

“Acoustics” refers to the way partisans of positions are ever more adept at mimicking and manipulating the voice of science itself. They do this by manufacturing facts and spreading pseudo-evidence, generating pseudo “experts”, using celebrities as spokespeople and taking advantage of the media’s penchant for granting equal time to different sides of an argument, and priority to what’s extreme and photogenic. I needn’t supply examples; you can find them yourself. These factors amplify voices in a controversy regardless of their integrity.

By “impure science”, I mean the way partisans often denounce scientific findings they don’t like. They point to discrepancies between how these findings were produced and the image of science we learned in school as emerging “from nowhere”: pure, value-free and definitive. If a particular study was partly funded by a pharmaceutical company, if a model depends on projections rather than certainties, or if someone has ties to the nuclear industry, the results must be suspect, they claim.

An anti-fracking protest in Balcombe, UK, in 2013
Just saying no Anti-fracking protest in Balcombe, UK, in August 2013. (Courtesy: iStockphoto/rsokoloff)

Finally, and surprisingly, scientific advance itself implicitly promotes a harmful “magical thinking”. This curious phenomenon was first identified by the Italian philosopher Giambattista Vico almost 300 years ago in his 1725 book New Science. Vico pointed out that the very maturation of human thought tends to foster an over-reliance on analytical rationality that encourages people to indulge themselves and view the world’s resources as at their disposal. The very success of science and technology, in other words, encourages the illusion that almost everything is within our grasp.

This magical thinking makes us feel like free agents – entitled to choose our forms of energy, nutrition and environmental conditions, without having to make severe, costly and risky trade-offs. It’s an illusion that’s amplified by powerful money and political influence. Don’t like fossil fuels but scared by nuclear? Go solar! Hate starvation but creeped out by genetic manipulation? Grow more food! And if we can’t do these things, it must be someone else’s fault, probably a conspiracy. We are accustomed to relying on our benefits but resent having to pay for the infrastructure that produced them. As one US congressman remarked – my informant heard it first-hand but insists on remaining anonymous – “Why do we need Landsat satellites when we have Google Earth?”

The critical point

How, then, can we deal with these problems?

Contending with wonky acoustics requires patiently tracking down and exposing fabrications and misrepresentations – a task for scientists and the sceptics movement. It is tedious and time-consuming for sure, but there is a silver lining: when partisans manipulate acoustics they at least presuppose that offering evidence and appealing to experts is how such debates should work.

Dealing with charges of impure science, meanwhile, is not a task for scientists but science educators. Science education needs to convey the reality that real science does not emerge “from nowhere” but from real people with passion, values and commitments. Science, we need to remind everyone, is our best tool for navigating the complex modern world filled with a fear of hazards and people who manipulate and prey on that fear.

As for countering science-induced magical thinking, that is not a task for either scientists or educators but for the humanities. It would require an improved human self-recognition; making better known the full story of how human thought and institutions evolved, and how some things are gained and others lost with each step. Realizing progressive human ambitions, we need to remind ourselves, usually comes with hidden costs.

There is, in short, no quick fix for repairing the dismal way we resolve controversies. It won’t work simply by restating the importance of science, which would only make science appear like one lobby among others. Vico thought the solution required the development of what he called a “new science”; today, it will require revamping science, science education and the humanities for the 21st century.

Thermal technique improves blood-flow measurements

A new method for imaging the flow of blood has been developed by researchers in the US. By using ultrasound to thermally tag blood, along with photoacoustics to image the resulting heat flow, the new technique is considerably more sensitive than the conventional Doppler ultrasound method that is currently used. While presently at the in vitro testing stage, this technique might have a variety of clinical applications, especially in medical diagnosis.

Being able to image the flow of blood within deep tissue would provide valuable information for the diagnosis and understanding of many diseases, with potential applications including functional brain imaging, detection of vascular diseases (such as atherosclerosis – the thickening of artery walls with calcium and fat) and the analysis of blood within tumour microenvironments (commonly having complicated vasculatures with intermittent blood flows), which could help in the early detection of cancer.

Going with the slow flow

Currently, flow imaging is undertaken through Doppler ultrasound (or sonography) in which the flowing blood causes a Doppler shift in the reflected ultrasound waves that can be measured and used to discern the flow rate. However, this technique has its limitations. Doppler ultrasound has poor sensitivity to blood, which weakly scatters in comparison with tissue. This issue is even more pronounced at the low frequencies that are commonly used to probe deeper into flesh. At slow flow rates, the small Doppler shifts from the moving blood can therefore be difficult to distinguish from shifts that are caused by the surrounding tissues. Indeed, the effect is undetectable for blood flowing slower than about 10 mm s–1.

One possible alternative to Doppler ultrasound is photoacoustic imaging, which uses short pulses of low-energy laser light to locally heat the target for analysis. This heating results in thermal expansion, creating ultrasound waves in proportion to the optical absorption of the target tissue. When recorded, this emitted ultrasound can therefore be used to create a map of the target’s absorptivity. While this method works well at shallow tissue depths (up to 1 mm deep), flow sensing in deep tissue is hindered by the high densities of blood cells.

Thermal tagging

Photoacoustic imaging is, however, very sensitive to temperature variations. Taking advantage of this, Lidai Wang and colleagues at the Washington University in St Louis have developed an alternative method of analysing blood flow by combining photoacoustic imaging with thermal ultrasound tagging that they refer to as “photoacoustic flowgraphy”. The researchers used focused ultrasound to locally heat a fixed part of a blood vessel and mapped out the resulting temperature distribution along the vessel as the blood flowed downstream.

As the laser pulse expands the material, it generates sound waves of a particular amplitude that, in turn, vary with the temperature. In a test set-up, the team could see how the heated sample moved by tagging the sample with the laser 10 times a second. Comparing subsequent images of the sound waves (taken using an array of detectors) allowed the researchers to follow the flow in a 1.5-mm tube of cow’s blood, including its faster speed at the centre of the tube.

“The spatial frequency (or period) of the temperature distribution changes with blood-flow speed,” explains Wang, with blood that flows faster generating a longer spatial period and a lower spatial frequency. This enables the flow speed to be calculated from the spatial frequency of the photoacoustic images recorded. Compared with traditional Doppler sonography, photoacoustic flowgraphy is considerably more sensitive, enabling the team to measure capillary-level blood flow at speeds as low as 0.24 mm s–1. “[This] is four times slower than Doppler sonography,” Wang adds. Furthermore, use of spectral photoacoustic imaging with this new method could provide other useful functional information, such as the blood’s oxygen saturation and even the metabolic rate of oxygen.

Assessment tool

“What I find particularly exciting is how this can be combined with functional and anatomical photoacoustic imaging to determine several tumour properties at once using one measurement,” comments Eric Strohm, a biophysicist at Ryerson University in Canada who was not involved in this study. “While in the early stages of development, this technology could eventually see clinical applications as an early assessment tool for evaluating tumours.”

The team is now looking at improving the technique by developing a reflection-mode system in which the ultrasound heating and photoacoustic transducers would be located on the same side of the subject tissue. Paving the way for in vivo testing, this set-up would ultimately allow access to a wider variety of anatomical sites for clinical applications.

The research is published in Physical Review Letters.

Locust eardrum is a tiny frequency analyser

Locusts have a highly integrated and miniaturized hearing system that bears little resemblance to either the human ear or an electronic microphone. That is the conclusion of researchers in the UK who have done a detailed study of how the insects detect and process sounds. The insect’s hearing system, which makes use of a nanostructured eardrum to discern between high- and low-frequency sounds, could provide inspiration for the development of tiny microphones or systems for processing human speech.

Locusts and other insects are too small to accommodate the kind of highly developed hearing systems that are found in some larger animals. Mammals, for example, first capture sound with an eardrum, then amplify vibrations through middle-ear bones, and finally transmit these to the cochlea, which functions as a frequency analyser.

Locusts need to distinguish between different frequencies for survival: low-frequency sound from other locusts and high-frequency sound from foes such as bats. But these insects “do not enjoy the luxury of such a complicated, large and biologically expensive-to-build apparatus”, says Rob Malkin of the University of Bristol, who was involved in the study. Instead, their ears are much simpler and all of the necessary functions are performed by the eardrum. “So far, such eardrum behaviour has been observed in locusts only,” he says.

Tiny vibrations

The Bristol team was led by Daniel Robert and it used a laser Doppler vibrometer to analyse how a locust’s eardrum membrane responded to incoming sound waves that were produced by a loudspeaker. The membrane is kidney-shaped and has two points on its inner surface, where mechanoreceptor cells – neurones that respond to mechanical stress – are attached in two different groups.

The researchers scanned the laser over the surface of the membrane, where they measured tiny picometre (10–12 m) out-of-plane vibrations induced by the sound waves. They found that for low-frequency sounds, the membrane vibrated in such a way that both groups of cells were mechanically excited. But high-frequency sounds managed to excite one group only – meaning that the eardrum effectively behaved like “a basic, but efficient, frequency analyser”, says Malkin.

The researchers then studied the membrane’s nanostructure using a focused ion-beam mill. The results reveals that waves caused by low-frequency sounds will travel completely across the membrane, where low-frequency-sensitive neurons attach to the membrane. But high-frequency waves will only travel half that distance, stopping at the attachment point of high-frequency neurons. This confirms that locusts are able to distinguish between high and low frequencies, says Malkin. The ion beam also revealed that the membrane had internal fluid-filled chambers, which could dampen sound depending on its frequency.

Energy localization

The team also found that the energy density contained in the travelling wave was amplified by as much as 56,000 times as it travelled across the eardrum. This means that the shape of the membrane is such that acoustic energy is collected by the surface of the eardrum and then focused towards the receptor cells.

Such energy localization has so far only been observed in locusts, says Malkin. However, the team’s analysis suggests that the process is remarkably simple and it is possible that the same mechanism might also exist in mammalian ears. If that is the case, it would be an exciting new function of the mammalian ear.

Indeed, James Windmill at the University of Strathclyde in the UK believes that the research could “provide insight into more complicated ears such as our own”.

Tiny but tough microphones

The research could also lead to the development of microphones and sensors that are much smaller and simpler, yet less fragile, than the existing technologies. “Insects are generally robust to mechanical shocks, much more so than larger vertebrate species,” says Robert. “We are currently looking at making detectors that encapsulate such features, and display the desirable characteristics of locust and other insect ears.”

Ron Miles of Binghamton University in the US, who was not involved in the research, believes that locust ears could also offer inspiration for creating new signal-processing technologies. “This research demonstrates that some of the frequency-dependent signal processing could be performed efficiently through careful structural-acoustical design of the pressure-sensing microphone diaphragm,” he says. “The fact that this approach has been successfully used in a fairly simple insect auditory system shows that the idea has considerable merit.”

One specific application could be in converting complex waveforms of speech signals into written text.

The study appears in the Journal of the Royal Society Interface.

  • Members of the Institute of Physics can read more about the wonders of locusts in the November 2013 issue of Physics World magazine, which shows how these creatures’ ability to avoid crashing into things could lead to collision-avoidance sensors for cars. The magazine is available online or by downloading the Physics World app onto your iPhone, iPad or Android device, available from the App Store or Google Play, respectively.
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