An artist’s impression of one of the most distant and brightest quasars ever seen. (Courtesy: NASA)
This week marks 50 years since astronomer Maarten Schmidt’s discovery of the quasar, using the giant Palomar Observatory telescope. Quasars or quasi-stellar objects are a kind of active galactic nucleus that astronomers believe are powered by supermassive black holes and are scattered throughout the universe. They have always fascinated me, being some of the brightest, most distant and highly red-shifted astronomical objects in our universe. Over the years, thousands of quasars have been identified and they have dramatically influenced our ideas about the scale of the observable universe and have helped astronomers shed some light on the early universe.
In fact, just this week an international team of researchers announced the discovery of an extremely rare triple quasar system – only the second one observed to date. These systems are considered to be extremely rare and are difficult to spot. By combining multiple telescope observations and advanced modelling, the team – led by Emanuele Farina of the University of Insubria in Como, Italy – was able to discover the triplet quasar, called QQQ J1519+0627. The researchers say that light from the quasars has travelled nine billion light-years to reach us, meaning that it was emitted when the universe was only a third of its current age. Advanced analysis confirmed that what the team found was indeed three distinct sources of quasar energy and that the phenomenon is extremely rare.
So in light of these exciting findings, in this week’s Facebook poll we are asking you to pick your favourite astronomical objects.
It seems like only yesterday that the particle-physics blogosphere was on fire with rumours, speculation and even a bit of real information about the hunt for Higgs boson at the Large Hadron Collider (LHC).
How things have changed since a Higgs-like particle was identified in July last year. Since then, further analysis has revealed that the particle is even more Higgs-like – and today CERN has officially said that the particle is “a Higgs boson”.
Physicists in the US and Canada say that they have done the best job yet of realizing Richard Feynman’s famous thought experiment about how single electrons pass through two slits. Although the researchers are not the first to recreate the experiment in the lab, they say that their incarnation best captures the essence of the original exercise.
Feynman originally outlined his thought experiment in volume three of his famous series The Feynman Lectures on Physics as a way of illustrating wave–particle duality in quantum mechanics. In the book, he invites the reader to imagine firing individual electrons through two slits and then marking the position where each electron strikes a screen behind the slits.
After many electrons have passed through the slits, the marks on the screen will comprise a diffraction pattern – illustrating the wave-like behaviour of each electron. But if one were to cover up one of the slits so that each electron could only pass through the other slit, the diffraction pattern would not appear – showing that each electron does indeed travel through both slits.
Potted history
When the third volume of The Feynman Lectures on Physics was published in 1965, physicists already knew that firing a beam of electrons at a double slit results in a diffraction pattern because the experiment had been performed in 1961 by Claus Jönsson at the University of Tübingen in Germany. But while Jönsson’s work clearly illustrated that a beam of electrons can behave as a wave, it did not establish a crucial point of Feynman’s experiment – that an individual electron itself can behave like a wave.
Single-electron double-slit diffraction was first demonstrated in 1974 by Giulio Pozzi and colleagues at the University of Bologna in Italy, who passed single electrons through a biprism – an electron optical device that serves the same function as a double slit – and observed the build-up of a diffraction pattern. A similar experiment was also carried out in 1989 by Akira Tonomura and colleagues at Hitachi’s research lab in Japan.
The first single-electron experiment to use an actual double slit was reported in 2008 by Pozzi and colleagues. The Italian team also conducted the experiment with one slit plugged, which – as expected – did not lead to the creation of a double-slit diffraction pattern. The team also performed another experiment in 2012, in which the arrivals of individual electrons from a double slit were recorded one at a time.
True to Feynman’s methodology
Herman Batelaan of the University of Nebraska-Lincoln, together with colleagues there and at the Perimeter Institute for Theoretical Physics in Waterloo, Canada, now say that they have created a double-slit experiment that follows the precise methodology of Feynman’s thought experiment.
The work originally began as an undergraduate research project at Nebraska and gained momentum when the Perimeter’s Damian Pope found out that Batelaan and colleagues were working on a realization of the experiment. Pope, who is involved in outreach work at Perimeter, had been keen on making a film about the thought experiment.
The team created a double slit in a gold-coated silicon membrane, in which each slit is 62 nm wide and 4 μm long with a slit separation of 272 nm. To block one slit at a time, a tiny mask controlled by a piezoelectric actuator was slide back and forth across the double slits.
The electrons were created at a tungsten filament and accelerated across 600 V and collimated into a beam. After passing through the double slit, they were detected using a multichannel plate.
One electron per second
The intensity of the electron source was set so low that only about one electron per second was detected – which ensured that only one electron at a time would ever pass through the slits. At this rate it took about two hours for a pattern to build up on the detector – a process that was recorded in real time (see video below). Measurements were repeated with the mask in a series of positions: first blocking both slits, then one slit, then none and then the opposite slit. As expected, the double-slit pattern was seen when the electrons had access to both slits, but not seen when one slit was blocked.
Batelaan told physicsworld.com that the experiment is particularly important from an outreach perspective because unlike the biprism experiments of the past, it actually uses a physical double slit and is therefore more accessible to the public. Young’s double-slit experiment with single electrons was voted the “most beautiful experiment in physics” by Physics World readers in 2002.
On the outskirts of Cambridge, next door to the Lyndsey McDermott hair salon on Castle Street, is a pub called the Sir Isaac Newton. Ask those inside why it’s so named and drinkers are likely to stare at you, muttering something about British greatness, history or the small fact that Newton was educated at the university down the road. But the pub’s name reminds us that Newton not only is still a highly influential scientist, but remains a popular icon too. Indeed, his name has also been given to Cambridge University Library’s online course catalogue, to an orbiting X-ray observatory and a unit of force, as well as a computer operating system.
But the use of Newton’s name as a recognizable “brand” is only the most trivial way in which his work has influenced culture. His greatest legacy – Newtonian mechanics – has affected all human life by deepening our knowledge of the world, by expanding our ability to control it, and by reshaping how scientists and non-scientists alike experience it. The arrival of the Newtonian universe was attractive, liberating and even comforting to many of those in the 17th and 18th centuries; its promise was that the world was not the chaotic, confusing and threatening place it seemed to be – ruled by occult powers and full of enigmatic events – but was simple, elegant and intelligible. Newton’s work helped human beings to understand in a new way the basic issues that human beings seek: what they could know, how they should act and what they might hope for.
The Newtonian moment
The Earth and the heavens, according to Newtonian mechanics, were not separate places made of different stuff but part of a “uni-verse” in which space and time – and the laws that govern them – are single, uniform and the same across all scales. This universe is also homogeneous. It is not ruled by ghosts or phantoms that pop up and disappear unpredictably. Everything has a distinct identity and is located at a specific place at a specific time. The Newtonian world is like a cosmic stage or billiard table, where things change only when pushed by forces. All space is alike and continuous, all directions comparable, all events caused.
This picture strongly influenced philosophers, theologians, writers, artists and even political thinkers. Indeed, the philosopher Richard Rorty once referred to “Newtonian political scientist[s]”, who centre social reforms around “what human beings are like – not knowledge of what Greeks or Frenchmen or Chinese are like, but of humanity as such”. Meanwhile, in 2003–2004, the New York Public Library staged an exhibition entitled “The Newtonian Moment” to showcase Newton’s cultural impact and illustrate the revolution in worldview his work brought about. Writing in the exhibition’s catalogue, the historian of science Mordechai Feingold declared that the name was chosen because the Enlightenment and Revolution comprised “the epoch and the manner in which Newtonian thought came to permeate European culture in all its forms”.
Feingold was using the word “moment” in the way historians do, referring to special turning points in which a radically new idea recasts past conflicts and tensions to open up new possibilities for the future. These turning points are cultural paradigm shifts that change what human beings know and do, and how they interpret their experiences. Features of the Newtonian Moment include the assumption of universal continuity, certainty, predictability, sameness across scales, and the ability of scientists to “take themselves out” of measurements to see nature as it is apart from human existence.
The quantum ambush
The Newtonian Moment lasted for some 250 years until the start of the 20th century, when it was ambushed by the quantum. Many scientists initially hoped that they could find a comfortable place for the quantum on the Newtonian stage, but by 1927 it had become clear that the quantum undermined many features of the Newtonian world, raising unprecedented philosophical as well as scientific issues. “Never in the history of science,” wrote the science historian Max Jammer, “has there been a theory which has had such a profound impact on human thinking as quantum mechanics”.
Has the cultural impact of quantum mechanics been simply to supply us with a storehouse of unusual, vivid and sometimes pretentious or even loopy images?
Some scientists tried to explain what was happening by spreading word of quantum physics into ever-widening social spheres that lay beyond science itself. These popularizations encountered an enthusiastic audience. Artists, novelists, poets and journalists were fascinated by the non-Newtonian features of quantum mechanics, including discontinuity, uncertainty, unpredictability, and differences across scales and areas where scientists could not take themselves out of measurements. Quantum terms and concepts – including quantum leap, uncertainty principle, complementarity, Schrödinger’s cat and parallel worlds – eventually appeared in everyday language in sparkling prose and flamboyant metaphors.
But has the cultural impact of quantum mechanics been simply to supply us with a storehouse of unusual, vivid and sometimes pretentious or even loopy images? Or has the cumulative effect been more serious, and reshaped how even non-scientists view the world?
To some extent, the quantum’s impact on artists, writers and philosophers was that it helped free themselves from their own Newtonian-inspired misconceptions. A year or two after the discovery of the uncertainty principle in 1927, for instance, the writer D H Lawrence penned the following poem fragment.
I like relativity and quantum theories Because I don’t understand them. And they make me feel as if space shifted About like a swan that can’t settle, Refusing to sit still and be measured; And as if the atom were an impulsive thing Always changing its mind.
Lawrence’s playfully negative remarks may suggest that his attraction is superficial: he likes relativity and quantum theories because they connect better with his experiences of the world as quixotic and immeasurable. A similar sentiment was expressed by the Austrian-Mexican artist Wolfgang Paalen in 1942 when he wrote excitedly that quantum mechanics heralds “a new order in which science will no longer pretend to a truth more absolute than that of poetry”. The outcome, he continued, will be to legitimize the value of the humanities, and “science will understand the value of art as complementary to her own”.
Meanwhile, in 1958 when the New York University philosopher William Barrett reviewed 20th century scientific developments, including quantum mechanics, he concluded that they paint an image of man “that bears a new, stark, more nearly naked, and more questionable aspect”. We have been forced to confront our “solitary and groundless condition” not only through existentialist philosophy but also via science itself, which has triggered “a denudation, a stripping down, of this being who has now to confront himself at the centre of all his horizons”.
Such remarks suggest that humanists embraced quantum mechanics because they experienced the Newtonian universe as a cold and constricting place in which they felt defensive and marginalized – with the news of the strangeness of the quantum domain coming almost as a relief. But if this is the only reason humanists found developments of the quantum world liberating, it was surely their own doing, for they were relying far too seriously on science to begin with in understanding their own experience.
A new humanism
In 1967 the critic and novelist John Updike wrote a brief reflection on the photographs and amateur films taken in Dealey Plaza in Dallas, Texas on 22 November 1963, in the few momentous seconds when President John F Kennedy’s motorcade drove through and he was hit by an assassin’s bullets. The more closely and carefully the frames were examined, Updike noted, the less sense the things in them made. Who was the “umbrella man” sporting an open umbrella despite it being a sunny day? Who was the “tan-coated man” who first runs away, then is seen in “a gray Rambler driven by a Negro?” What about the blurry figure in the window nextto the one from which the shots were fired? Were these innocent bystanders or part of a conspiracy?
Popular physics A couple walks past “Quantum Field-X3”, an installation by Japanese artist Hiro Yamagata, outside the Guggenheim Museum in Bilbao, Spain. (Courtesy: Reuters/Vincent West)
“We wonder,” Updike wrote, “whether a genuine mystery is being concealed here or whether any similar scrutiny of a minute section of time and space would yield similar strangenesses – gaps, inconsistencies, warps and bubbles in the surface of circumstance. Perhaps, as with the elements of matter, investigation passes a threshold of common sense and enters a subatomic realm where laws are mocked, where persons have the life-span of beta particles and the transparency of neutrinos, and where a rough kind of averaging out must substitute for absolute truth.”
Years later, many frames turned out to have rational explanations. The “umbrella man” was identified – to the satisfaction of all but diehard conspiracy theorists. Testifying before a Congressional committee, the man in question said he had been simply protesting against the Kennedy family’s dealings with Hitler’s Germany, with the black umbrella – Neville Chamberlain’s trademark fashion accessory – being a symbol for Nazi appeasers. Far from heralding a breach in the rationality of the world, the umbrella man was just a heckler.
Barrett, being a philosopher, had proposed that the cultural effect of quantum mechanics was to strip us of illusions. Updike, a novelist with a keen interest in science who followed contemporary developments in physics with care, reached a different conclusion. His words above indicate that he saw the impact of quantum mechanics on culture to be deeper and more positive than Barrett had. Indeed, Updike often has his fictional characters refer to physics terms in a metaphorical way that allows them to voice their experiences more articulately.
The novelist was fully aware that when scientists look at the subatomic world frame by frame, so to speak, what they find is discontinuous and strange – its happenings random except when collectively considered. Updike also knew that most of us tend to find our lives following a similar crazy logic. Our world does not always feel smooth, continuous, reliable, law-governed, stable and substantive; close up, its palpable sensuousness is often jittery, discontinuous, chaotic, irrational, unstable and ephemeral. Reality today does not seem to have the gentle, universal continuities of the Newtonian world, but is more like that of the surface of a boiling pot of water. Using quantum language to describe everyday conditions may therefore be technically incorrect but is metaphorically apt.
In another essay, Updike wrote that “our century’s revelations of unthinkable largeness and unimaginable smallness, of abysmal stretches of geological time when we were nothing, of supernumerary galaxies and indeterminate subatomic behaviour, of a kind of mad mathematical violence at the heart of matter have scorched us deeper than we know”. The scorching brought about by such scientific discoveries, Updike proposed, had given birth to a “new humanism” whose “feeble, hopeless voice” is provided by the “minimal monologuists” of the Irish playwright Samuel Beckett – and which is also evident in the instantly recognizable “wire-thin, eroded figures” of the Swiss sculptor Alberto Giacometti.
The critical point
If only all human voices were as articulate as Beckett and Giacometti! Too frequently, the use of quantum language and concepts in popular culture amounts to what the physicist John Polkinghorne calls “quantum hype”, or the invocation of quantum mechanics as “sufficient licence for lazy indulgence in playing with paradox in other disciplines”. This is how it principally appears in things like TV programmes, cartoons, T-shirts and coffee cups.
Updike’s remarks, however, suggest that quantum mechanics – a theory of awesome comprehensiveness that has yet to make an unconfirmed prediction – has done more than help to deepen our knowledge of the world and to expand our ability to manipulate it. The novelist’s remarks suggest that quantum mechanics – though a modification, not a replacement, of Newtonian mechanics – has provided us with a range of novel and helpful images to interpret our experiences of the world in a new way, on a scale equal to or possibly even greater than Newtonian mechanics. Quantum physics is metaphorically appealing because it reflects the difficulty we face in describing our own experiences; quantum mechanics is strange and so are we.
Someday, indeed, the era after the Newtonian Moment may come to be known as the Quantum Moment.
Massachusetts General Hospital (MGH) is considered to be one of the leading hospitals in the US – in terms of both patient care and its cutting-edge medical research. In this short film, Physics World visits the MGH to meet the chief of the hospital’s physics division, Thomas Bortfeld, who describes the hospital’s pioneering cancer treatments at its proton-therapy centre. Bortfeld also shares his thoughts on what it is like to move from an academic research setting to the dynamic environment of a working hospital.
Just as you don’t expect snow while hiking through a hot desert, astronomers never thought they would find hydrogen molecules on stars that have surfaces twice as hot as the Sun. But that’s just what researchers in the US and Germany have spotted on the surfaces of several white dwarf stars. The surprising discovery should help astronomers gain a better understanding of the extreme conditions on these stars – and may even lead to the first detection of the isotope deuterium in planetary systems beyond our solar system.
Although hydrogen is the most abundant molecule in space, it typically forms at frigid temperatures, in dark interstellar clouds just a few degrees above absolute zero. Now, however, astronomers have accidentally detected molecular hydrogen on three white dwarfs, which are dense stars with huge gravitational forces. The team believes that these extreme forces are causing the molecules to form – despite the heat, which would otherwise prevent their formation.
A white dwarf is a dying star that’s roughly as massive as the Sun but as small as the Earth, so the gravity on its surface is immense – something weighing an ounce on our planet would weigh tonnes on a white dwarf. Exotic though they are, white dwarfs are common, making up 5% of all stars, and the nearest is just 8.6 light-years away, orbiting the bright star Sirius. Our Sun will become a white dwarf in 7.8 billion years.
Polluted stars
In a typical white dwarf, the extreme gravity drags heavy elements beneath the surface where they can’t be seen. As a result, astronomers detect only hydrogen or helium atoms on the surface of a white dwarf by observing their signature absorption lines in the star’s spectrum. In some cases, though, asteroids have recently struck the star, temporarily polluting its surface with heavier elements that can be observed.
Siyi Xu, a graduate student at the University of California, Los Angeles, was using the Hubble Space Telescope’s high-resolution Cosmic Origins Spectrograph to obtain the ultraviolet spectra of six polluted white dwarfs. “Two stars were really weird,” she says. “After we got the data, we were like, ‘What are these lines?’ We were just completely lost.”
For about six months, the astronomers struggled to explain the odd spectral lines. The team first suspected trouble with Hubble, but other stars didn’t show the same strange lines. Nor did the lines’ wavelengths match those from any atoms.
Hotter than the Sun
Then Xu’s adviser, Michael Jura, suggested she examine the Sun’s ultraviolet spectrum, where she found several lines from molecular hydrogen that matched those in the stars’ spectra. Says Xu, “I’m like, ‘Really? Molecular hydrogen? At that high a temperature?'” The two white dwarfs – Giclas 29-38 in the constellation Pisces and GD 133 in Leo – have surface temperatures of 11,820 K and 12,120 K, respectively, much hotter than the Sun, which has a surface of a mild 5780 K.
The team – which also includes an astronomer at the University of Kiel – then noticed molecular hydrogen in a white dwarf called GD 31 that others had observed. Located in the constellation Cetus, it’s even hotter: 13,700 K.
“This surprises the hell out of me,” says white-dwarf expert Jay Holberg of the University of Arizona in Tucson, who wrote a book about Sirius and its white dwarf companion but was not involved with the new work. “Hydrogen dissociates – that is, the molecule comes apart – quite readily. I would have thought that you wouldn’t get an observable amount of molecular hydrogen.” Holberg calls its discovery “impressive”.
Lurking in sunspots
Prior to this find, the hottest star known to have molecular hydrogen was our own, where sunspots, which are cooler than the solar surface, harbour the molecule. But the great gravity and density on white dwarfs produce intense pressure that forces some of the hydrogen atoms to join. From the strength of the spectral lines, the astronomers estimate there is about one hydrogen molecule for every 100,000 atoms.
The discovery may lead to another: the first detection of deuterium in other solar systems. “We could have an additional tool for understanding the history and evolution and formation of extrasolar planets,” says Jura. Unlike ordinary hydrogen, the nucleus of which has a proton and nothing more, deuterium has both a proton and a neutron, so it weighs twice as much. The Big Bang produced the isotope, but nuclear reactions in stars annihilate it; therefore, the white dwarfs have destroyed all their original deuterium.
However, asteroids crashing into these stars probably still bear deuterium. Astronomers can’t detect atomic deuterium on white dwarfs, because the hydrogen lines overwhelm them, but if white dwarfs also have molecules that include deuterium atoms, observers may see them, because the spectral lines lie away from those of molecules comprising ordinary hydrogen.
Terrestrial water is 10 times richer in deuterium relative to hydrogen than the interstellar clouds giving birth to solar systems. “It would be quite interesting to know if other planetary systems have the same enhancement of deuterium,” says Jura.
The astronomers will report their discovery in the 1 April issue of Astrophysical Journal Letters and a preprint is available on arXiv.
Finding the fossilized remains of extraterrestrial life in a meteorite would surely be the biggest scientific discovery of the century. That’s what appears to be reported in a paper published in the Journal of Cosmology and available on the arXiv preprint server.
A new method of digital holography with the ability to see clearly through both smoke and flames has been developed by researchers in Italy. The technology, which operates at a far-infrared wavelength, has potential for use in fire rescue situations where existing infrared cameras – while able to see through smoke – are blinded by the radiation given off by flames.
According to the researchers, this problem is caused by the need for a zoom lens within current cameras. It is this that focuses incoming light onto the digital sensor, forming an image. The lens, however, has the undesired effect of also focusing the infrared light emitted from flames, causing the sensor pixels to become saturated. The new imaging technique avoids this problem by switching to a lens-free, holographic system.
Holographic view
Holograms are created by splitting laser beams into two. One part of the light – the object beam – is reflected off of the object being visualized. When this ray is recombined with the other – the reference beam – the ensuing interference pattern encodes a 3D image of the target object.
Using this principle, the new imaging system operates by dispersing a wide object beam of continuous-wave infrared laser light. Infrared light – unlike visible light – passes freely through both smoke and flames. The laser reflects off of the objects it meets, and is received back by an imaging sensor. At the sensor the object beam meets the reference beam, forming the hologram, which is then decoded into a live, 3D view of the objects being filmed – regardless of any smoke or flames in the way. The long wavelength employed by this method enables not only large-scale objects and scenes to be visualized, but also reduces the hologram’s sensitivity to vibrations.
Clear imaging
Perhaps most importantly, we demonstrated for the first time that a holographic recording of a live person can be achieved even while the [person] is moving
Pietro Ferraro, Consiglio Nazionale delle Ricerche, Italy
“It became clear to us that we had in our hands a technology that could be exploited by emergency responders and firefighters at a fire scene to see through smoke without being blinded by flames,” says Pietro Ferraro, from the Consiglio Nazionale delle Ricerche (CNR) Istituto Nazionale di Ottica in Italy. “Perhaps most importantly, we demonstrated for the first time that a holographic recording of a live person can be achieved even while the [person] is moving.”
The key of this holographic technology lies in the numerical processing of the images received at the sensor. Such analysis not only removes the need for a zoom lens, but can also be used to improve the quality of the visualization by comparing subsequent captures, eliminating the minor interference caused by smoke particles and speckle noise – a type of interference that occurs whenever laser light is fired at a disordered material.
On-site analysis
Ferraro explains that the analysis can easily be carried out at the same time as the actual holographic recording is being done, and could be conducted at the site of a fire – possibly using a remote connection from the recording device to an external, mobile processing station – such as a standard laptop or even a smaller device.
All laboratory tests of the imaging method are being carried out in conditions that simulate outdoor usage. “No anti-vibration systems have been used and no darkrooms have been employed for the scope,” Ferraro told Physics World. “For these reasons, we are strongly confident about the possibility to bring this technology out of the lab. We foresee the possibility to realize a portable tripod system in the future, but it is too early to evaluate the costs for such [a] kind of holographic camera.”
A number of other applications are proposed for this new holographic technology. In industry, this technique could be used to detect problems in operating boilers and furnaces, which if left unchecked could result in poor-quality goods or damage to the production line itself. “The potential to record dynamic scenes of a human body could have a variety of other biomedical uses including studying or monitoring breathing, cardiac-beat detection and analysis, or measurement of body deformation due to various stresses during exercise,” Ferraro adds. “We are excited to further develop this technology and realize its application for saving and improving human life.”
Life-saving methods
“This technique could well have potential to be a valuable tool in fire fighting, but some further development is needed,” comments Philip Wilksch, a holography specialist at the Royal Melbourne Institute of Technology. “I suspect [the] ability to see through flames is due mainly to the fact that holography discriminates between the coherent light backscattered from the infrared laser, and the incoherent light from the flames. I imagine that imaging through a solid wall of flame would be a problem. Use of a narrow-band filter centred on the laser line would help reject the unwanted infrared light,” he explains.
The researchers are continuing to refine this technology towards possible commercial applications. “We think that in [the near] future this system could be applied for fixed installations, for example in hospitals, schools, tunnels or even highways,” says Ferraro. “This will be the next step in exploiting this new technology.”
The latest quantum fridge: NIST’s prototype solid-state refrigerator uses quantum physics in the square chip mounted on the green circuit board to cool the much larger copper platform (in the middle of the photograph) below standard cryogenic temperatures. Other objects can also be attached to the platform for cooling. (Courtesy: PML/NIST)
Physicists in the US have built a new solid-state refrigerator that provides continuous cooling of objects to temperatures below 300 mK. The device has no moving parts and uses 48 tiny quantum-tunnelling junctions to cool a copper plate a million times heavier than the refrigerating elements themselves. The team believes that the device could be further optimized and could find use in refrigeration applications where conventional cryogenics are difficult to implement – such as cooling detectors on space missions.
Temperatures below about 300 mK are integral to many areas of modern physics research from quantum computers to dark-matter detection. However, temperatures below about 300 mK cannot be reached by simply cooling with liquid helium. Conventional cooling to millikelvin temperatures is done using a dilution refrigerator, which involves pumping helium isotopes. While extremely effective, dilution refrigerators can be difficult to implement in specialized applications such as cooling detectors in space. An alternative technique is adiabatic demagnetization, which is also unwieldy because it involves placing the sample in a powerful magnetic field that is repeatedly turned on and off.
Researchers have for years been seeking a viable solid-state cryogenic refrigerator – ideally a system that can cool objects the size of a computer chip to below 300 mK simply by running electric current through it. Various proposals have been advanced and success has been achieved in cooling small objects less than 1 mm in size. In 2005 Joel Ullom’s team at the National Institute for Standards and Technology (NIST) in Boulder, Colorado, unveiled a cooler based on quantum tunnelling that can chill objects much larger than the refrigerating elements – which were extremely small. However, the object to be cooled had to be integrated onto a cold membrane at the time of manufacture, so the refrigerator was not reusable.
A ‘true’ refrigerator
Now Ullom and colleagues have created a new solid-state refrigerator and used it to cool a removable copper stage sized 2.5 cm from 290 mK to 256 mK over the course of 18 hours. The stage was thermally connected to a membrane, while remaining electrically isolated.
Ullom explains that the electrical isolation is important: “This is why we consider the device to be a true refrigerator,” he says. “Just as it would be very awkward if your household refrigerator had current running through whatever you put in it, so too it would be awkward if there was current running through your payload from the refrigeration process.”
Thermal isolation
The membrane, conversely, was electrically (but not thermally) connected to the power supply using thin superconducting wires. This isolation means that the device can be connected to a standard power supply or even a 9 V battery.
Attached to the membrane are a series of junctions, each comprising a 30-nm-thick layer of normal conductor and a 300-nm-thick layer of superconductor, separated by a very thin 1 nm layer of insulator. The circulating current creates a potential difference across these junctions. The hotter, more energetic electrons are more likely to tunnel across the insulating gap from the normal conductor to the superconductor on the outside. As a result, these hotter electrons are preferentially removed from the system, cooling the stage.
“It’s almost like the way you cool a cup of coffee by blowing on it,” says NIST researcher Peter Lowell. “You remove some of the hottest particles, which cools down the cup of coffee.”
‘Very impressive’
Hervé Courtois at the Néel Institute in Grenoble, France, describes the device as “very impressive”, although he stresses that “in terms of physics, there is nothing new”. He explains, “The point is that they can couple the electronic cooler and this copper stage, which could be anything. It could be a detector for astronomy or any kind of thermometer.”
Courtois cautions, however, that the device will need to get to colder temperatures before it can replace more complex refrigerators in devices such as the Planck Cosmic Microwave Background detectors. “I think the next step is to start from 300 mK and go down to 100 mk,” he says. “For me this 10% temperature reduction is a very nice demonstration but it’s not really useful. I think they can manage to get down further, but the demonstration is still to be made.”
The researchers’ thoughts lie in the same direction. “We would like to cool from 300 mK down to 100 mK,” says Ullom, “and we are also looking at a refrigerator that could start at 100 mK and maybe reach the low tens of millikelvin. That’s a temperature range that is very difficult to access presently.”
Leonid Kuzmin from Chalmers University, Sweden, is more sceptical that they will be able to get down much further, as he points out that the thermal conductance will decrease as the fourth power as the temperature goes down. As a result, Kuzmin thinks it is unrealistic to expect cooling to near 100 mK after only incremental improvements to the NIST device.
Physicists in Canada and the US claim to be the first to make a direct measurement of the polarization quantum state of light – a feat that at first glance appears to defy Heisenberg’s uncertainty principle. The technique, which relies on a process known as weak measurement, could help in fundamental studies on quantum mechanics or in the development of quantum computing.
In quantum mechanics, it is normally considered impossible to know everything about a system at one time. Measure the position of a particle accurately, for instance, and the particle’s momentum will suddenly become very ill defined. Physicists call pairs of variables such as position and momentum “conjugate”: they are innately connected, such that the measurement of one essentially destroys information about the other.
On the face of it, this phenomenon – which is enshrined in Heisenberg’s famous uncertainty principle – restricts the information that physicists can gain from studying quantum systems. But in the last 20 years, new techniques have been developed to get a better handle on uncertainty and the exact limit it presents. Known as weak measurements, they involve taking tiny “peeks” at quantum systems, so that information can be gained bit by bit, without greatly affecting the system itself.
Reconstructing the wavefunction
In 2011 physicists at the National Research Council (NRC) in Ottawa, Canada, claimed that they could use weak measurement to directly reconstruct a system’s wavefunction, which describes how a quantum system evolves over time. Before weak measurement, wavefunctions had only been measured indirectly in a technique known as quantum tomography. This latter technique involves making many different conventional measurements on equivalent quantum systems – single photons emerging from the same source, for example. This information is then processed to create a map of the quantum state.
The NRC group’s technique involved making a “weak” measurement of the position of a photon followed by a “strong” conventional measurement of its momentum. By repeating the process many times, the information gained from the weak measurements ramped up until the researchers had effectively got the same amount of information from doing two strong measurements. As a result, they were able to reconstruct the wavefunction in a single sitting.
“Not just a fluke”
Robert Boyd and colleagues at the University of Ottawa and University of Rochester have built on the NRC work by applying weak measurement to the polarization states of light. “It demonstrates a second example of a situation in which direct measurement can be used to determine a quantum wavefunction,” says Boyd. “It shows that the earlier result was not just a fluke.”
The polarization of light can be described using different orthogonal bases, including horizontal–vertical and diagonal–antidiagonal. As with position and momentum, these bases are conjugate – a measurement of the horizontal–vertical polarization, for example, should destroy information about the diagonal–antidiagonal polarization and vice versa.
The team used birefingent crystals to make its measurements. When a beam of light passes through such a crystal, the light is deflected according to its polarization – with the amount of deflection depending on the thickness of the crystal. Boyd and colleagues used a thin birefingent crystal to perform a weak measurement of the horizontal–vertical polarization, and then a thick crystal to perform a strong measurement of the diagonal–antidiagonal polarization. By performing this process many times for identical photons in a stream, they could gradually build up a complete knowledge of both polarizations. Then, having inferred the value of the third, left-or-right, polarization, they could reconstruct the polarization state in full.
Jeff Lundeen, a member of the NRC group that measured the spatial wavefunction in 2011, believes the latest study is important because polarization is a discrete, “two-level” variable – as opposed to position and momentum, which are continuous. It is the two-level nature of polarization – equivalent to a binary one and zero – that lends it to quantum computing, where it forms the basis of quantum bits of information, or qubits. The techniques of Boyd and colleagues will therefore be of great interest to those developing quantum computing, Lundeen says.
Bona fide quantum state
Howard Wiseman at Griffith University in Brisbane, Australia, calls the result a “generalization” of the 2011 result. However, he points out that by forcing a system to divulge its wavefunction directly, the researchers may be losing some of the “mathematical requirements” that a bona fide quantum state should exhibit. “Given that experiments are prone to error, it remains to be seen how useful this technique will be,” he says.
Boyd’s group is now building on its technique. The researchers would like to understand how a quantum wavefunction becomes distorted, such as when a photon passes through turbulence in the Earth’s atmosphere. “Such measurements are needed for the implementation of quantum communications,” Boyd says.
