Consider a bunch of molecules that have been trapped between two mirrors facing each other very closely. In this ‘box’ the light can’t move around freely; it can only exist in certain fixed patterns, somewhat like sound inside a finely tuned musical instrument. Even when all light has been removed from this box, quantum physics causes small flickers of light to pop in and out of existence in fractions of a second. These immutable flickers of the electromagnetic field are sometimes collectively called the vacuum field.
Now, what would happen if the natural motion of matter inside molecules and these confined light patterns become strongly linked?
This isn’t an esoteric thought experiment. Over the last decade, physicists have shown in experiments that when this link is strong, new hybrid states appear that are part light and part matter — and these states can spread over many molecules at once. And in this condition, physicists have found that the materials in which these states occur suddenly have very different properties, all without changing the chemical formulae of their constituent molecules.
A new paper by PhD scholar Subha Biswas and assistant professor Anoop Thomas — both at the Department of Inorganic and Physical Chemistry of the Indian Institute of Science — brought the many findings in this area of research together and argued that the confined vacuum field reshapes the weak forces between molecules, which is the same force that determine how the molecules arrange themselves and how easily energy and charge can move between them. And the duo has shown that this explanation can account for a wide range of surprising findings. Their findings were published in ACS Applied Optical Materials on November 11.
The paper isn’t a single experiment; instead the authors read and compared many published papers about experiments in which other scientists placed molecules inside small mirror cavities and forced them to interact strongly with the trapped light.
Thus, the duo reported, energy could travel quite differently in these cavities. Normally, when energy passes from one molecule to the next, it does so through a series of short hops if the material is disordered. A simple example is an ordinary piece of glass. In a crystal, atoms or molecules sit on a tidy 3D grid, like seats in a stadium. In glass however they’re more like people milling around in a crowd: there’s some short-range order but no long-range one.
When motion inside the molecules is strongly coupled to the trapped light, however, the new light–matter states can extend over many molecules, creating a sort of highway that energy travel on faster, more efficiently, and across much larger regions than physicists generally expect in disordered materials.
For another, the duo reported that some organic materials, especially plastics that usually conduct electricity very poorly, are suddenly able to conduct a lot better when certain internal motions of their molecules are strongly coupled to the trapped light. In some experiments scientists had found that protons also moved faster through water, suggesting the cavity could in some way alter the network of hydrogen bonds in water.
Based on analysing these and other examples, the duo arrived at a central conclusion: that by modifying how light is confined around a material, scientists can influence how its molecules ‘feel’ each other and thus alter its bulk behaviour, even when it’s in its lowest-energy state. (Usually materials become capable of unusual things when they’re imparted more energy.) Put another way the cavity isn’t just a passive container but an active design tool that can reshapes the background electromagnetic field, and with that the landscape of intermolecular forces.
This is interesting because it’s a new way to control molecules in chemistry. Usually when scientists need to change the way a chemical reaction happens, they change the molecules that are in play. For instance they alter the molecules’ structure, add side groups, and/or ‘mix in’ new components. But the new study has found what seems to be an additional handle: scientists may be able to leave the molecules as they are but just place them in a carefully designed optical environment. And by carefully choosing the spacing between the mirrors, the materials that make up the apparatus, and how the cavity resonates with specific molecular motions, they can adjust the reaction rate, favour one crystal form over another, stabilise certain structures, shift the balance between different reaction products, and guide how large molecules assemble into various shapes, etc.
The implications are similarly broad for materials science, where physicists can use these cavities to improve energy transport in thin films, enhance conduction in soft or flexible materials, guide how polymers and other large molecules arrange themselves in devices, and tune how materials crystallise as they’re fabricated. Because the effect comes from light and matter coupling together over a longer range, it might be especially useful in systems that are otherwise disordered and where conventional design tools struggle.
For instance, the molecules of polystyrene are arranged in a messy, haphazard way. This disorder makes it very hard for scientists to ‘engineer’ clean paths within the material through which electrons can flow, so polystyrene is usually an excellent insulator. However, when scientists place thin films of such plastics in a carefully tuned optical cavity, experiments have shown that their ability to conduct electricity increases drastically even though the material’s chemical makeup hasn’t changed.
A simple salt solution, like of table salt dissolved in water, offers another good example. Here the water molecules and the salt ions move around in an incessantly shifting and disordered fashion, and chemists typically control them by changing how much salt or what additives there are. But when the solution is strongly coupled to the confined light, the way water molecules surround and move around the ions changes and the ions travel more easily through the liquid.
The authors have stressed that these effects are selective — which is to say not every material will change dramatically. Instead scientists will have to select details such as how molecules pack together and which internal motion is coupled to the light in the cavity all matter. This in turn means this new vacuum-field engineering won’t altogether replace chemical design but could in fact complement it. In fact the authors have sketched a future in which chemists and materials scientists routinely think together about what molecules to build and the kind of electromagnetic environment to place them in to coax new or improved properties out.
Scientists have uncovered a remarkable self-healing property in a strange class of materials known as quasicrystals, revealing their ability to grow into a perfect, single structure even when faced with obstacles. The discovery challenges a long-held understanding of crystal formation and opens the door to creating large, strong materials free of defects for a new generation of applications.
Imagine you’re tiling a massive bathroom floor. You could use identical square tiles, laying them in a simple, repeating grid. This is analogous to a regular crystal. The pattern is predictable and repeats perfectly. When the arrangement of atoms repeats at fixed intervals, called periodicity, you have a conventional crystal.
But as you’re laying these tiles, you come across a pipe sticking out of the floor. To get around it, you would have to cut tiles into awkward shapes, breaking your perfect pattern. The lines where the mismatched tiles meet would also create a permanent scar on your floor. In the world of materials, these ‘scars’ are called defects or grain boundaries.
These defects are often the weakest points in a material, making it more prone to breaking or corrosion. Say you’ve a chair that’s starting to crack: the odds are the crack would’ve originated at a grain boundary. For many decades, a major goal for materials scientists has been to create large single crystals: materials with no grain boundaries and thus maximum strength and performance.
Now, what if instead of squares, you were tiling with a special set of tiles, say a mix of two different diamond shapes (see below). You could cover the entire floor without gaps, creating a pattern that looks ordered and intricate. But then you notice the catch: the pattern never exactly repeats. This is the essence of a quasicrystal. It has long-range order—its atoms are arranged in a pattern you can predict—but that pattern doesn’t repeat.
A Penrose tiling using thick and thin rhombi (blue and green). Note the aperiodic structure, shared by all Penrose tilings. This particular Penrose tiling exhibits five-fold symmetry. Credit: Public domain
This fundamental difference led scientists to ask a critical question: what happens when a growing quasicrystal encounters an obstacle like that pipe in the floor? Does it also form messy, weakening defects or does its unique, more flexible structure give it a way to grow around the disruption and ‘fix’ itself on the other side?
In a new study in Physical Review Letters, researchers from the University of Michigan, Ann Arbor, hypothesised that quasicrystals possess a unique “self-healing” ability. They’ve written that this ability stems from special atomic rearrangements, known as phasons, that are only possible in quasicrystals. To test this, they decided to watch a specific type of quasicrystal, a decagonal quasicrystal, grow around a common, bubble-shaped obstacle found in metal alloys called shrinkage pores.
To watch this microscopic drama unfold, the researchers used a two-part strategy in which they combined a real-world experiment with a sophisticated computer simulation.
For the experiment, they used a technique called synchrotron X-ray microtomography. Think of it as a super-powered CT scan for materials. They took a small cylinder of an aluminium-cobalt-nickel alloy, melted it, and then carefully cooled it down. As the alloy solidified, quasicrystals began to form and grow, eventually running into the shrinkage pores that were already present in the material. The X-rays allowed the scientists to capture rapid, 3D “movies” of this process in real-time, tracking the crystal’s growth as it navigated the porous landscape.
To understand the atom-level mechanics behind what they were seeing, the team ran molecular dynamics simulations. This is like creating a virtual universe in a computer, where they could build a model quasicrystal atom by atom. They then programmed a virtual pore in its path and let the simulation run to watch exactly how the individual atoms rearranged themselves as the crystal grew and enveloped the obstacle.
The results were astonishing: both the live-action X-ray movies and the computer simulations showed the quasicrystal growing around the pores without forming any permanent defects.
As the growing front of the quasicrystal met a pore, it momentarily distorted its shape to flow around the void. Then when the two fronts met on the far side of the pore, they merged together perfectly. The crystal continued to grow as if the pore had never been there, resulting in a single, flawless quasicrystal that had simply swallowed the obstacle.
The key evidence from the experiments was the smoothness of the final quasicrystal’s surface. In a normal crystal, if a defect like a grain boundary had formed where the two fronts met, it would have created a permanent groove on the surface. The fact that the quasicrystal’s surface healed into a smooth, convex shape confirmed that no such defect was present.
The simulated growth of a quasicrystal around an obstacle (the white circle at the centre). The dotted red box identifies the axis along which two fronts meet as the quasicrystal grows around the obstacle. Credit: DOI: 10.1103/bsbs-rryl
The computer simulations revealed the secret to this ability: the phasons. A phason flip is one pattern of tiles changing into another without disrupting the overall order. When the growing crystal collided with the pore, it created localised stress. But through a cascade of these phason flips, the quasicrystal was able to distribute this stress and seamlessly stitch its structure back together. The scientists described this as an err-and-repair mechanism because the quasicrystal makes a temporary “mistake” at the collision point, then quickly corrects it. The result: perfect structure.
The finding has profound implications for materials science. First, it suggests that creating large, defect-free single crystals—currently a notoriously difficult and expensive process for conventional materials—could be dramatically easier with quasicrystals. Their built-in fault tolerance means they might be grown perfectly using simpler and cheaper methods.
The ability to easily create large, single-grain quasicrystals could also better unlock their incredible potential. Lacking the weak points of grain boundaries, quasicrystals could be used to create exceptionally strong, hard, and lightweight alloys for aerospace and industrial applications. Their unique structure also gives them interesting properties like low friction and low adhesion, making them ideal as durable nonstick coating.
The err-and-repair mechanism is also a form of self-healing at the most fundamental level. Understanding this process could inspire the design of a new class of “smart” materials. For instance, imagine a composite that when cracked or damaged can automatically absorb the defect and ‘repair’ its own atomic structure, maintaining its integrity and performance. The principles revealed in this study could be applied to other fields, such as creating more durable catalysts for the chemical industry that can resist degradation from internal pores.
Overall, this research reshapes our understanding of how ordered matter can form and provides a blueprint for a new generation of resilient materials built from the atom up.
When light, sound or any kind of wave travels through a complex medium like fog, murky water, or biological tissue, it scatters in many directions. Each particle or irregularity in the medium changes the path of the waves, scrambling them and blurring the resulting image. This is why doctors struggle to image deep inside tissue using ultrasound, why optical microscopes can’t see through thick samples, and why radar and sonar sometimes miss objects hidden behind clutter.
Scientists have long looked for ways to focus waves through such disordered environments — and while many have tried to compensate for scattering, their success has been limited when the medium becomes very opaque.
A team led by Alexandre Aubry at ESPCI Paris and collaborators from Vienna and Aix-en-Provence wanted to turn this problem around. Instead of correcting or undoing the scattering, they wondered if something in the wave patterns remains stable even in the middle of all that complexity. That is, could they identify and locate a target based on the part of the signal that still carries its unique ‘fingerprint’?
Their new study, published in Nature Physics, introduces a mathematical tool called the fingerprint operator that allows exactly this. This operator can detect, locate, and even characterise an object hidden inside a strongly scattering medium by comparing the reflected light to a reference pattern recorded in simpler conditions. The method can work for sound, light, radar, and other kinds of waves.
At the heart of the technique is the reflection matrix, a large dataset recording how each source in an array of sources sends a wave into the medium and how every receiver picks up the returning echoes. Each element of this matrix contains information about how waves bounce off of different points, so together they capture the complete response of the system.
To find a target within this sea of signals, the researchers introduced the fingerprint operator, written as Γ = R × R₀†, where R is the measured reflection matrix from the complex medium and R₀ is a reference matrix measured for the same target in clear, homogeneous conditions. The dagger (†) indicates a mathematical conjugate that makes the comparison sensitive to how well the two patterns match. By calculating how strongly the two matrices correlate, the team obtained a likelihood index, which indicates how likely it is that a target with certain properties — e.g. position, size or shape — is present at a given spot.
Effectively the team has developed a way to image hidden objects using scattered light.
The researchers tested this concept with ultrasound. They used arrays containing up to 1,024 transducers (devices that convert energy from one form to another) to send and receive sound waves. First, they embedded small metal spheres inside a suspension of glass beads mixed with water, making for a strongly scattering environment.
In the granular suspension, conventional ultrasound couldn’t see the buried metal spheres at all. The multiple scattering caused an exponential loss of contrast with depth, making the target signals roughly a 100x weaker than the background noise. Yet when the fingerprint operator was applied, the two spheres appeared sharply on the reconstructed likelihood map, each represented by a bright peak at its correct location. The contrast improvement reached factors of several hundred, strong enough to rule out false positive signals with a probability of error smaller than 1 in a hundred million.
This success came from the fingerprint operator’s ability to filter out diffuse, randomly scattered waves and isolate those faint waves that behave as if the medium were transparent. In simple terms, the operator is a mathematical tool that can use the complexity of the target’s own echo to cancel the complexity of the medium.
The same approach worked inside a foam that mimicked human tissue. A normal ultrasound image was dominated by speckle (random bright and dark spots caused by small scattering events), rendering a small pre-inserted marker nearly invisible. But when the fingerprint operator was applied to the data, the marker was revealed clearly and precisely.
To its credit, the fingerprint operator doesn’t require scientists to fully known the medium, only the ability to record a reflection matrix and a reference response. It can then use these resources to find patterns that survive scattering and extract meaningful information.
For medicine, this could improve ultrasound detection of small implants, needles, and markers that currently get lost in tissue noise. It could also help map the internal fibre structure of muscles or hearts, providing new diagnostic insights into diseases like cardiomyopathy and fibrosis. In materials science, it could reveal the orientation of grains in metals or composites. In military settings, it could locate targets hidden behind foliage or turbulent water.
The approach is also computationally efficient: according to the researchers’ paper, generating the likelihood map takes about the same time as developing a standard ultrasound image and can be adapted for moving targets by incorporating motion parameters into the fingerprint.
Finally, the idea animating the study also challenges a long-standing view that multiple scattering is purely a nuisance, incapable of being useful. The study overturns this view by extracting information from the multiple scattering data, using the fingerprint operator to account for how a target’s own echoes evolve through scattering, and leveraging those distortions to detect it more confidently.
Rubidium isn’t respectable. It isn’t iron, whose strength built railways and bridges and it isn’t silicon, whose valley became a dubious shrine to progress. Rubidium explodes in water. It tarnishes in air. It’s awkward, soft, and unfit for the neat categories by which schoolteachers tell their students how the world is made. And yet, precisely because of this unruly character, it insinuates itself into the deepest places of science, where precision, control, and prediction are supposed to reign.
For centuries astronomers counted the stars, then engineers counted pendulums and springs — all good and respectable. But when humankind’s machines demanded nanosecond accuracy, it was rubidium, a soft metal that no practical mind would have chosen, that became the metronome of the world. In its hyperfine transitions, coaxed by lasers and microwave cavities, the second is carved more finely than human senses can comprehend. Without rubidium’s unstable grace, GPS collapses, financial markets fall into confusion, trains and planes drift out of sync. The fragile and the explosive have become the custodians of order.
What does this say about the hierarchies of knowledge? Textbooks present a suspiciously orderly picture: noble gases are inert, alkali metals are reactive, and their properties can be arranged neatly in columns of the periodic table, they say. Thus rubidium is placed there like a botanical specimen. But in practice, scientists turned to it not because of its box in a table but because of accidents, conveniences, and contingencies. Its resonance lines happen to fall where lasers can reach them easily. Its isotopes are abundant enough to trap, cool, and measure. The entire edifice of atomic clocks and exotic Bose-Einstein condensates rests not on an inevitable logic of discovery but on this convenient accident. Had rubidium’s levels been slightly different, perhaps caesium or potassium would have played the starring role. Rational reconstruction will never admit this. It prefers tidy sequences and noble inevitabilities. Rubidium, however, laughs at such tidiness.
Take condensed matter. In the 1990s and 2000s, solar researchers sought efficiency in perovskite crystals. These crystals were fragile, prone to decomposition, but again rubidium slipped in: a small ion among larger ones, it stabilised the lattice. A substitution here, a tweak there, and suddenly the efficiency curve rose. Was this progress inevitable? No; it was bricolage: chemists trying one ion after another until the thing worked. And the journals now describe rubidium as if it were always destined to “enhance stability”. But destiny is hindsight dressed as foresight. What actually happened was messy. Rubidium’s success was contingent, not planned.
Then there’s the theatre of optics. Rubidium’s spectral lines at 780 nm and 795 nm became the experimentalist’s playground. When lasers cooled atoms to microkelvin temperatures and clouds of rubidium atoms became motionless, they merged into collective wavefunctions and formed the first Bose-Einstein condensates. The textbooks now call this a triumph of theory, the “inevitable” confirmation of quantum statistics. Nonsense! The condensates weren’t predicted as practical realities — they were curiosities, dismissed by many as impossible in the laboratory. What made them possible was a melange of techniques: magnetic traps, optical molasses, sympathetic cooling. And rubidium, again, happened to be convenient, its transitions accessible, its abundance generous, its behaviour forgiving. Out of this messiness came a Nobel Prize and an entire field. Rubidium teaches us that progress comes not from the logical unfolding of ideas but from playing with elements that allegedly don’t belong.
Rubidium rebukes dogma. It’s neither grand nor noble, yet it controls time, stabilises matter, and demonstrates the strangest predictions of quantum theory. It shows science doesn’t march forward by method alone. It stumbles, it improvises, it tries what happens to be at hand. Philosophers of science prefer to speak of method and rigour yet their laboratories tell a story of messy rooms where equipment is tuned until something works, where grad students swap parts until the resonance reveals itself, where fragile metals are pressed into service because they happen to fit the laser’s reach.
Rubidium teaches us that knowledge is anarchic. It isn’t carved from the heavens by pure reason but coaxed from matter through accidents, failures, and improvised victories. Explosive in one setting, stabilising in another; useless in industry, indispensable in physics — the properties of rubidium are contradictory and it’s precisely this contradiction that makes it valuable. To force it into the straitjacket of predictable science is to rewrite history as propaganda. The truth is less comfortable: rubidium has triumphed where theory has faltered.
And yet, here we are. Our planes and phones rely on rubidium clocks. Our visions of renewable futures lean on rubidium’s quiet strengthening of perovskite cells. Our quantum dreams — of condensates, simulations, computers, and entanglement — are staged with rubidium atoms as actors. An element kings never counted and merchants never valued has become the silent arbiter of our age. Science itself couldn’t have planned it better; indeed, it didn’t plan at all.
Rubidium is the fragment in the mosaic that refuses to fit yet holds the pattern together. It’s the soft yet explosive, fragile yet enduring accident that becomes indispensable. Its lesson is simple: science also needs disorder, risk, and the unruliness of matter to thrive.
Featured image: A sample of rubidium metal. Credit: Dnn87 (CC BY).
Quantum technologies and the prospect of advanced, next-generation electronic devices have been maturing at an increasingly rapid pace. Both research groups and governments around the world are investing more attention in this domain.
India for example mooted its National Quantum Mission in 2023 with a decade-long outlay of Rs 6,000 crore. One of the Mission’s goals, in the words of IISER Pune physics professor Umakant Rapol, is “to engineer and utilise the delicate quantum features of photons and subatomic particles to build advanced sensors” for applications in “healthcare, security, and environmental monitoring”.
On the science front, as these technologies become better understood, scientists have been paying increasingly more attention to managing and controlling heat in them. These technologies often rely on quantum physical phenomena that appear only at extremely low temperatures and are so fragile that even a small amount of stray heat can destabilise them. In these settings, scientists have found that traditional methods of handling heat — mainly by controlling the vibrations of atoms in the devices’ materials — become ineffective.
Instead, scientists have identified a promising alternative: energy transfer through photons, the particles of light. And in this paradigm, instead of simply moving heat from one place to another, scientists have been trying to control and amplify it, much like how transistors and amplifiers handle electrical signals in everyday electronics.
Playing with fire
Central to this effort is the concept of a thermal transistor. This device resembles an electrical transistor but works with heat instead of electrical current. Electrical transistors amplify or switch currents, allowing the complex logic and computation required to power modern computers. Creating similar thermal devices would represent a major advance, especially for technologies that require very precise temperature control. This is particularly true in the sub-kelvin temperature range where many quantum processors and sensors operate.
This circuit diagram depicts an NPN bipolar transistor. When a small voltage is applied between the base and emitter, electrons are injected from the emitter into the base, most of which then sweep across into the collector. The end result is a large current flowing through the collector, controlled by the much smaller current flowing through the base. Credit: Michael9422 (CC BY-SA)
Energy transport at such cryogenic temperatures differs significantly from normal conditions. Below roughly 1 kelvin, atomic vibrations no longer carry most of the heat. Instead, electromagnetic fluctuations — ripples of energy carried by photons — dominate the conduction of heat. Scientists channel these photons through specially designed, lossless wires made of superconducting materials. They keep these wires below their superconducting critical temperatures, allowing only photons to transfer energy between the reservoirs. This arrangement enables careful and precise control of heat flow.
One crucial phenomenon that allows scientists to manipulate heat in this way is negative differential thermal conductance (NDTC). NDTC defies common intuition. Normally, decreasing the temperature difference between two bodies reduces the amount of heat they exchange. This is why a glass of water at 50º C in a room at 25º C will cool faster than a glass of water at 30º C. In NDTC, however, reducing the temperature difference between two connected reservoirs can actually increase the heat flow between them.
NDTC arises from a detailed relationship between temperature and the properties of the material that makes up the reservoirs. When physicists harness NDTC, they can amplify heat signals in a manner similar to how negative electrical resistance powers electrical amplifiers.
A ‘circuit’ for heat
In a new study, researchers from Italy have designed and theoretically modelled a new kind of ‘thermal transistor’ that they have said can actively control and amplify how heat flows at extremely low temperatures for quantum technology applications. Their findings were published recently in the journal Physical Review Applied.
To explore NDTC experimentally, the researchers studied reservoirs made of a disordered semiconductor material that exhibited a transport mechanism called variable range hopping (VRH). An example is neutron-transmutation-doped germanium. In VRH materials, the electrical resistance at low temperatures depends very strongly, sometimes exponentially, on temperature.
This attribute makes them ideal to tune their impedance, a property that controls the material’s resistance to energy flow, simply by adjusting temperature. That is, how well two reservoirs made of VRH materials exchange heat can be controlled by tuning the impedance of the materials, which in turn can be controlled by tuning their temperature.
In the new study, the researchers reported that impedance matching played a key role. When the reservoirs’ impedances matched perfectly (when their temperatures became equal), the efficiency with which they transferred photonic heat reached a peak. As the materials’ temperatures diverged, heat flow dropped. In fact, the researchers wrote that there was a temperature range, especially as the colder reservoir’s temperature rose to approach that of the warmer one, within which the heat flow increased even as the temperature difference shrank. This effect forms the core of NDTC.
The research team, associated with the NEST initiative at the Istituto Nanoscienze-CNR and Scuola Normale Superiore, both in Pisa in Italy, have proposed a device they call the photonic heat amplifier. They built it using two VRH reservoirs connected by superconducting, lossless wires. One reservoir was kept at a higher temperature and served as the source of heat energy. The other reservoir, called the central island, received heat by exchanging photons with the warmer reservoir.
The proposed device features a central island at temperature T1 that transfers heat currents to various terminals. The tunnel contacts to the drain and gate are positioned at heavily doped regions of the yellow central island, highlighted by a grey etched pattern. Each arrow indicates the positive direction of the heat flux. The substrate is (shown as and) maintained at temperature Tb, the gate at Tg, and the drain at Td. Credit: arXiv:2502.04250v3
The central island was also connected to two additional metallic reservoirs named the “gate” and the “drain”. These points operated with the same purpose as the control and output terminals in an electrical transistor. The drain stayed cold, allowing the amplified heat signal to exit the system from this point. By adjusting the gate temperature, the team could modulate and even amplify the flow of heat between the source and the drain (see image below).
To understand and predict the amplifier’s behaviour, the researchers developed mathematical models for all forms of heat transfer within the device. These included photonic currents between VRH reservoirs, electron tunnelling through the gate and drain contacts, and energy lost as vibrations through the device’s substrate.
(Tunnelling is a quantum mechanical phenomenon where an electron has a small chance of floating through a thin barrier instead of going around it.)
Raring to go
By carefully selecting the device parameters — including the characteristic temperature of the VRH material, the source temperature, resistances at the gate and drain contacts, the volume of the central island, and geometric factors — the researchers said they could tailor the device for different amplification purposes.
They reported two main operating modes. The first was called ‘current modulation amplifier’. In this configuration, the device amplified small variations in thermal input at the gate. In this mode, small oscillations in the gate heat current produced much larger oscillations, up to 15-times greater, in the photon current between the source and the central island and in the drain current, according to the paper. This amplification was efficient down to 20 millikelvin, matching the ultracold conditions required in quantum technologies. The output range of heat current was similarly broad, showing the device’s suitability to amplify heat signals.
The second mode was called ‘temperature modulation amplifier’. Here, slight changes of only a few millikelvin in the gate temperature, the team wrote, caused the output temperature in the central island to swing by as large as 3.3 times the changes in the input. The device could also handle input temperature ranges over 100 millikelvin. This performance reportedly matched or surpassed other temperature amplifiers already reported in the scientific literature. The researchers also noted that this mode could be used to pre-amplify signals in bolometric detectors used in astronomy telescopes.
An important ability relevant for practical use is the relaxation time, i.e. how soon after operating once the device returned to its original state, ready for the next run. The amplifier in both configurations showed relaxation times between microseconds and milliseconds. According to the researchers, this speed resulted from the device’s low thermal mass and efficient heat channels. Such a fast response could make it suitable to detect and amplify thermal signals in real time.
The researchers wrote that the amplifier also maintained good linearity and low distortion across various inputs. In other words, the output heat signal changed proportionally to the input heat signal and the device didn’t add unwanted changes, noise or artifacts to the input signal. Its noise-equivalent power values were also found to rival the best available solid-state thermometers, indicating low noise levels.
Approaching the limits
For these promising results, realising this device involves some significant practical challenges. For instance, NDTC depends heavily on precise impedance matching. Real materials inevitably have imperfections, including those due to imperfect fabrication and environmental fluctuations. Such deviations could lower the device’s heat transfer efficiency and reduce the operational range of NDTC.
The system also banked on lossless superconducting wires being kept well below their critical temperatures. Achieving and maintaining these ultralow temperatures requires sophisticated and expensive refrigeration infrastructure, which adds to the experimental complexity.
Fabrication also demands very precise doping and finely tuned resistances for the gate and drain terminals. Scaling production to create many devices or arrays poses major technical difficulties. Integrating numerous photonic heat amplifiers into larger thermal circuits risks unwanted thermal crosstalk and signal degradation, a risk compounded by the extremely small heat currents involved.
Furthermore, the fully photonic design offers benefits such as electrical isolation and long-distance thermal connections. However, it also approaches fundamental physical limits. Thermal conductance caps the maximum possible heat flow through photonic channels. This limitation could restrict how much power the device is able to handle in some applications.
Then again, many of these challenges are typical of cutting-edge research in quantum devices, and highlight the need for detailed experimental work to realise and integrate photonic heat amplifiers into operational quantum systems.
If they are successfully realised for practical applications, photonic heat amplifiers could transform how scientists manage heat in quantum computing and nanotechnologies that operate near absolute zero. They could pave the way for on-chip heat control, computers to autonomously stabilise the temperature, and perform thermal logic operations. Redirecting or harvesting waste heat could also improve the efficiency and significantly reduce noise — a critical barrier in ultra-sensitive quantum devices like quantum computers.
In the last two or three years, groups of scientists from around the world have made several claims that they had discovered a room-temperature superconductor. Many of these claims concerned high-pressure superconductors — materials that superconduct electricity at room temperature but only if they are placed under extreme pressure (a million atmospheres’ worth). Yet other scientists had challenged these claims on many grounds, but one in particular was whether these materials really exhibited the Meissner effect.
Room-temperature superconductors are often called the ‘holy grail’ of materials science. I abhor clichés but in this case the idiom fits perfectly. If such a material is invented or discovered, it could revolutionise many industries. To quote at length from an article by electrical engineer Massoud Pedram in The Conversation:
Room-temperature superconductors would enable ultra high-speed digital interconnects for next-generation computers and low-latency broadband wireless communications. They would also enable high-resolution imaging techniques and emerging sensors for biomedical and security applications, materials and structure analyses, and deep-space radio astrophysics.
Room-temperature superconductors would mean MRIs could become much less expensive to operate because they would not require liquid helium coolant, which is expensive and in short supply. Electrical power grids would be at least 20% more power efficient than today’s grids, resulting in billions of dollars saved per year, according to my estimates. Maglev trains could operate over longer distances at lower costs. Computers would run faster with orders of magnitude lower power consumption. And quantum computers could be built with many more qubits, enabling them to solve problems that are far beyond the reach of today’s most powerful supercomputers.
However, this surfeit of economic opportunities could also lure scientists into not thoroughly double-checking their results, cherry-picking from their data or jumping to conclusions if they believe they have found a room-temperature superconductor. Many papers written by scientists claiming they had found a room-temperature superconductor have in fact been published in and subsequently retracted from peer-reviewed journals with prestigious reputations, including Nature and Science, after independent experts found the papers to contain flawed data. Whatever the reasons for these mistakes, independent scrutiny of such reports has become very important.
If a material is a superconductor, it needs to meet two conditions*. The first of course is that it needs conduct a direct electric current with zero resistance. Second, the material should display the Meissner effect. Place a magnet over a superconducting material. Then, gradually cool the material to lower and lower temperatures, until you cross the critical temperature. Just as you cross this threshold, the magnet will start to float above the material. You’ve just physically observed the Meissner effect. It happens because when the material transitions to its superconducting state, it will expel all magnetic fields within its bulk to its surface. This results in any magnets already sitting nearby to be pushed away. In fact, the Meissner effect is considered to be the hallmark sign of a superconductor because it’s difficult to fake.
An illustration of the Meissner effect. B denotes the magnetic field, T is the temperature, and Tc is the critical temperature. Credit: Piotr Jaworski
Wait for the 1:03 mark.
The problem with acquiring evidence of the Meissner effect is the setup in which many of these materials become superconductors. In order to apply the tens to hundreds of gigapascals (GPa) of pressure, a small sample of the material — a few grams or less — is placed between a pair of high-quality diamond crystals and squeezed. This diamond anvil cell apparatus leaves no room for a conventional magnetic field sensor to be placed inside the cell. Measuring the magnetic properties of the sample is also complicated because of the fields from other sources in the apparatus, which will have to be accurately measured and then subtracted from the final data.
To tackle this problem, some scientists have of late suggested measuring the sample’s magnetic properties using the only entity that can still enter and leave the diamond anvil cell: light.
In technical terms, such a technique is called optical magnetometry. Magnetometry in general is any technique that converts some physical signal into data about a magnetic field. In this case the signal is in the form of light, thus the ‘optical’ prefix. To deploy optical magnetometry in the context of verifying whether a material is a high-pressure superconductor, scientists have suggested using nitrogen vacancy (NV) centres.
Say you have a good crystal of diamond with you. The crystal consists of carbon atoms bound to each other in sets of four in the shape of a pyramid. Millions of copies of such pyramids together make up the diamond. Now, say you substitute one of the carbon atoms in the gem with a nitrogen atom and also knock out an adjacent carbon atom. Physicists have found that this vacancy in the lattice, called an NV centre, has interesting, useful properties. For example, an NV centre can fluoresce, i.e. absorb light of a higher frequency and emit light of a lower frequency.
An illustration of a nitrogen vacancy centre in diamond. Carbon atoms are shown in green. Credit: Public domain
Because each NV centre is surrounded by three carbon atoms and one nitrogen atom, the vacancy hosts six electrons, two of which are unpaired. All electrons have a property called quantum spin. The quantum spin is the constitutive entity of magnetism the same way the electric charge is the constitutive entity of electricity. For example, if a block of iron is to be turned into a magnet, the spins of all the electrons inside have to be made point in the same direction. Each spin can point in one of two directions, which for a magnet are called ‘north’ and ‘south’. Planet earth has a magnetic north and a magnetic south because the spins of the trillions upon trillions of electrons in its core have come to point in roughly the same direction.
The alignment of the spins of different electrons also affects what energy they have. For example, in the right conditions, an atom with two electrons will have more energy if the electrons’ spins are aligned (↑↑) than when the electrons’ spins are anti-aligned (↑↓). This fundamental attribute of the electrons in the NV centres allows the centres to operate as a super-sensitive detector of magnetic fields — and which is what scientists from institutions around France have reported doing in a June 30 paper in Physical Review Applied.
The scientists implanted a layer of 10,000 to 100,000 NV centres a few nanometres under the surface of one of the diamond anvils. These centres had electrons with energies precisely 2.87 GHz apart.** When the centres were then exposed to microwave laser of some frequency, every NV centre could absorb green laser light and re-emit red light.
The experimental setup. DAC stands for ‘diamond anvil cell’. PL stands for ‘photoluminescence’, i.e. the red light emission. Credit: arXiv:2501.14504v1
As the diamond anvils squeezed the sample past 4 GPa, the pressure at which it would have become a superconductor, the sample displayed the Meissner effect, expelling magnetic fields from within its bulk to the surface. As a result, the NV centres were exposed to a magnetic field in their midst that wasn’t there before. This field affected the electrons’ collective spin and thus their energy levels, which in turn caused the red light being emitted from the centres to dim.
The researchers could easily track the levels and patterns of dimming in the NV centres with a microscopy, and based on that were able to ascertain whether the sample had displayed the Meissner effect. As Physical Review Letters associate editor Martin Rodriguez-Vega wrote in Physics magazine: “A statistical analysis of the [optical] dataset revealed information about the magnetic-field strength and orientation across the sample. Mapping these quantities produced a visualisation of the Meissner effect and revealed the existence of defects in the superconductor.”
In (a), the dotted lines show the parts of the sample that the diamond anvils were in contact with. (b) shows the parts of the sample associated with the red-light emissions from the NV centres, meaning these parts of the sample exhibited the Meissner effect in the experiment. (c) shows the normalised red-light emission along the y-axis and the frequency of microwave light shined along the x-axis. Red lines show the emission in normal conditions and blue lines show the emissions in the presence of the Meissner effect. Credit: arXiv:2501.14504v1
Because the NV centres were less than 1 micrometre away from the sample, they were extremely sensitive to changes in the magnetic field. In fact the researchers reported that the various centres were able to reveal the critical temperature for different parts of the sample separately than for the sample as a whole — a resolution not possible with conventional techniques. The pristine diamond matrix also conferred the electrons’ spins inside the NV centres with a long lifetime. And because there were so many NV centres, the researchers were able to ‘scan’ them with the microwave laser en masse instead of having to maintain focus on a single point on the diamond anvil, when looking for evidence of changes in the sample’s magnetic field. Finally, while the sample in the study became superconducting at a critical temperature of around 140 K, the centres were stable to under 4 K.
Another major advantage of the technique is that it can be used with type II superconductors as well. Type I superconductors are materials that transition to their superconducting state in a single step, under the critical temperature. Type II superconductors transition to their superconducting states in more than one step and display a combination of flux-pinning and the Meissner effect. From my piece in The Hindu in August 2023: “When a flux-pinned superconductor is taken away from a particular part of the magnetic field and put back in, it will snap back to its original relative position.” This happens because type II materials, while they don’t expel magnetic fields from within their bulk, also prevent the fields from moving around inside. Thus the magnetic field lines are pinned in place.
Because of the spatial distribution of the NV centres and their sensitivity, they can reveal flux-pinning in the sample by ‘sensing’ the magnetic fields at different distances.
* The material can make a stronger case for itself if it displays two more properties. (i) The heat energy required to raise the material’s electrons by 1º C has to change drastically at the critical temperature, which is the temperature below which the material becomes a superconductor. (ii) The material’s electrons shouldn’t be able to have certain energy readings. (That is, a map of the energies of all the electrons should show some gaps.) These properties are however considered optional.
** While 2.87 GHz is a frequency figure, recall Planck’s equation from high school: E = hv. Energy is equal to frequency times Planck’s constant, h. Since h is a constant (6.62 × 10-34 m2kg/s), energy figures are frequently denoted in terms of frequency in physics. An interested party can calculate the energy by themselves.
These are some remarks that have been fermenting in my mind and for which I don’t have the time or the inclination to supply a beginning-middle-end structure to publish as individual posts. I’m just packing them into this one post so I can say what I’d like to say, clear some headspace and move on.
1. MOM end of mission
The Mars Orbiter Mission (MOM) of the Indian Space Research Organisation (ISRO) reached end of life on October 3, 2022, a healthy seven years beyond its design lifespan of six months. While the confirmation from ISRO was muted, to the accompaniment of a characteristically verbose PTI copy, the occasion was nothing short of the end of an era. MOM was ISRO’s last fully successful major mission and the last time ISRO undertook an outreach campaign of any sort that was as candid and as effective as many of us ISRO enthusiasts have wished all of their campaigns to be. ISRO’s last partly successful major mission was Chandrayaan 2; the way it responded to the lander’s failure was regrettable. And there hasn’t been a publicity campaign since that wasn’t also closely orchestrated by the office of the Supreme Leader et al. So the end of MOM was symbolically the end of a time in which things other than total narrative control were possible.
2. An IIT Mandi press release
IIT Mandi recently emailed me a press release about a newly published paper (which I couldn’t find) describing a study led by a researcher and his team at the institute – in which they recovered polymer composites from used wind-turbine blades in what the release claimed was a “green” procedure. The two chemical compounds required in this procedure are hydrogen peroxide and acetic acid. Dear readers, hydrogen peroxide is not “green”. Nothing, really, is green unless it’s green throughout its lifecycle. Hydrogen peroxide manufacturing is currently not a green process. You can’t just say “hydrogen peroxide is the water molecule plus one more oxygen atom, so it’s green”. That’s like saying “ozone is dioxygen with one more oxygen atom, so it’s okay to inhale.” Diluted hydrogen peroxide is okay but at higher concentrations (typically >40%), it is highly toxic to living things. It’s also very reactive chemically and is hard to store, transport and use. So without knowing where the hydrogen peroxide in their experiment came from, without knowing the volume of hydrogen peroxide required to make the research team’s solution commercially feasible, and without knowing the concentration at which it must be used, let’s not make any claims about greenness.
Addendum: Also according to the press release (emphasis added), “The recovered fibres retained nearly 99% of the strength and greater than 90% of other mechanical properties as compared to the virgin fibres.” Do we really need to use terms like “virgin” to describe pre-utilisation objects? I doubt anyone’s going to tell the IIT Mandi press office this but both universe press offices and scientists need to put some thought into their language instead of playing it safe from within their lanes. Other English words rooted in objectionable sexual notions include ‘seminal’ (from semen) and ‘hysterical’ (from the Latin for ‘suffering in the womb’). The lingua franca is what we consider okay to say, okay to think, eventually okay to believe, so it’s important we tend to it.
3. “Top 3 wishes”
The The Science Talk blog published a post discussing the results of a call it’d put out earlier, to materials scientists, asking them to list their top three wishes. The question received a hundred responses and, according to the post, the most common three wishes were: More funding and longer contracts; “resources – unlimited microscopes, open access and less bureaucracy”; and “informal networking, comfortable lab shoes and outreach”. Let’s set a part of our common sense aside for a moment and assume that these hundred materials scientists are speaking for the millions of scientists working on thousands of topics worldwide in a variety of contexts. Doing this allows us to consider their wishes as a monolithic set of requests so that they can do science better – and leaves us to think about which wishes we can and can’t allow, and to what extents, so that science can fulfill its purpose in our lives, in our countries, in our politics without at the same time exacting too high a cost. Take “longer contracts”, for example: obviously that will allow scientists to work with larger questions, build towards bigger ideas and so forth – but the gains for those funding that scientific work, the government and by extension the people, will also manifest over longer time-periods and come with a greater risk of sunk costs. That in turn should make us think about what sort of nation, with the attendant economic and sociopolitical features, can afford longer contracts for scientists. (In my view, richer, more economically developed and more powerful countries, where there is little social or political expectation for science to contribute to the betterment of society.)
I didn’t have a point to make here as much as express the hope that more people who read the The Science Talk post will be interested in asking such questions, and thereon become interested in the government of science, the place of science in your country and, ultimately, the politics of rooting for science.
There are many types of superconductors. Some of them can be explained by an early theory of superconductivity called Bardeen-Cooper-Schrieffer (BCS) theory.
In these materials, vibrations in the atomic lattice force the electrons in the material to overcome their mutual repulsion and team up in pairs, if the material’s temperature is below a particular threshold (very low). These pairs of electrons, called Cooper pairs, have some properties that individual electrons can’t have. One of them is that all Cooper pairs together form an exotic state of matter called a Bose-Einstein condensate, which can flow through the material with much less resistance than individuals electrons experience. This is the gist of BCS theory.
When the Cooper pairs are involved in the transmission of an electric current through the material, the material is an electrical superconductor.
Some of the properties of the two electrons in each Cooper pair can influence the overall superconductivity itself. One of them is the orbital angular momentum, which is an intrinsic property of all particles. If both electrons have equal orbital angular momentum but are pointing in different directions, the relative orbital angular momentum is 0. Such materials are called s-wave superconductors.
Sometimes, in s-wave superconductors, some of the electric current – or supercurrent – starts flowing in a vortex within the material. If these vortices can be coupled with a magnetic structure called a skyrmion, physicists believe they can give rise to some new behaviour previously not seen in materials, some of them with important applications in quantum computing. Coupling here implies that a change in the properties of the vortex should induce changes in the skyrmion, and vice versa.
However, physicists have had a tough time creating a vortex-skyrmion coupling that they can control. As Gustav Bihlmayer, a staff scientist at the Jülich Research Centre, Germany, wrote for APS Physics, “experimental studies of these systems are still rare. Both parts” of the structures bearing these features “must stay within specific ranges of temperature and magnetic-field strength to realise the desired … phase, and the length scales of skyrmions and vortices must be similar in order to study their coupling.”
In a new paper, a research team from Nanyang Technical University, Singapore, has reported that they have achieved just such a coupling: they created a skyrmion in a chiral magnet and used it to induce the formation of a supercurrent vortex in an s-wave superconductor. In their observations, they found this coupling to be stable and controllable – important attributes to have if the setup is to find practical application.
A chiral magnet is a material whose internal magnetic field “typically” has a spiral or swirling pattern. A supercurrent vortex in an electrical superconductor is analogous to a skyrmion in a chiral magnet; a skyrmion is a “knot of twisting magnetic field lines” (source).
The researchers sandwiched an s-wave superconductor and a chiral magnet together. When the magnetic field of a skyrmion in the chiral magnet interacted with the superconductor at the interface, it induced a spin-polarised supercurrent (i.e. the participating electrons’ spin are aligned along a certain direction). This phenomenon is called the Rashba-Edelstein effect, and it essentially converts electric charge to electron spin and vice versa. To do so, the effect requires the two materials to be in contact and depends among other things on properties of the skyrmion’s magnetic field.
There’s another mechanism of interaction in which the chiral magnet and the superconductor don’t have to be in touch, and which the researchers successfully attempted to recreate. They preferred this mechanism, called stray-field coupling, to demonstrate a skyrmion-vortex system for a variety of practical reasons. For example, the chiral magnet is placed in an external magnetic field during the experiment. Taking the Rashba-Edelstein route means to achieve “stable skyrmions at low temperatures in thin films”, the field needs to be stronger than 1 T. (Earth’s magnetic field measures 25-65 µT.) Such a field could damage the s-wave superconductor.
For the stray-field coupling mechanism, the researchers inserted an insulator between the chiral magnet and the superconductor. Then, when they applied a small magnetic field, Bihlmayer wrote, the field “nucleated” skyrmions in the structure. “Stray magnetic fields from the skyrmions [then] induced vortices in the [superconducting] film, which were observed with scanning tunnelling spectroscopy.”
Experiments like this one reside at the cutting edge of modern condensed-matter physics. A lot of their complexity resides in scientists being able to closely control the conditions in which different quantum effects play out, using similarly advanced tools and techniques to understand what could be going on inside the materials, and to pick the right combination of materials to use.
For example, the heterostructure the physicists used to manifest the stray-field coupling mechanism had the following composition, from top to bottom:
Platinum, 2 nm (layer thickness)
Niobium, 25 nm
Magnesium oxide, 5 nm
Platinum, 2 nm
The next four layers are repeated 10 times in this order:
Platinum, 1 nm
Cobalt, 0.5 nm
Iron, 0.5 nm
Iridium, 1 nm
Back to the overall stack:
Platinum, 10 nm
Tantalum, 2 nm
Silicon dioxide (substrate)
The first three make up the superconductor, the magnesium oxide is the insulator, and the rest (except the substrate) make up the chiral magnet.
It’s possible to erect a stack like this through trial and error, with no deeper understanding dictating the choice of materials. But when the universe of possibilities – of elements, compounds and alloys, their shapes and dimensions, and ambient conditions in which they interact – is so vast, the exercise could take many decades. But here we are, at a time when scientists have explored various properties of materials and their interactions, and are able to engineer novel behaviours into existence, blurring the line between discovery and invention. Even in the absence of applications, such observations are nothing short of fascinating.
Applications aren’t wanting, however.
A quasiparticle is a packet of energy that behaves like a particle in a specific context even though it isn’t actually one. For example, the proton is a quasiparticle because it’s really a clump of smaller particles (quarks and gluons) that together behave in a fixed, predictable way. A phonon is a quasiparticle that represents some vibrational (or sound) energy being transmitted through a material. A magnon is a quasiparticle that represents some magnetic energy being transmitted through a material.
On the other hand, an electron is said to be a particle, not a quasiparticle – as are neutrinos, photons, Higgs bosons, etc.
Now and then physicists abstract packets of energy as particles in order to simplify their calculations.
(Aside: I’m aware of the blurred line between particles and quasiparticles. For a technical but – if you’re prepared to Google a few things – fascinating interview with condensed-matter physicist Vijay Shenoy on this topic, see here.)
We understand how these quasiparticles behave in three-dimensional space – the space we ourselves occupy. Their properties are likely to change if we study them in lower or higher dimensions. (Even if directly studying them in such conditions is hard, we know their behaviour will change because the theory describing their behaviour predicts it.) But there is one quasiparticle that exists in two dimensions, and is quite different in a strange way from the others. They are called anyons.
Say you have two electrons in an atom orbiting the nucleus. If you exchanged their positions with each other, the measurable properties of the atom will stay the same. If you swapped the electrons once more to bring them back to their original positions, the properties will still remain unchanged. However, if you switched the positions of two anyons in a quantum system, something about the system will change. More broadly, if you started with a bunch of anyons in a system and successively exchanged their positions until they had a specific final arrangement, the system’s properties will have changed differently depending on the sequence of exchanges.
This is called path dependency, and anyons are unique in possessing this property. In technical language, anyons are non-Abelian quasiparticles. They’re interesting for many reasons, but one application stands out. Quantum computers are devices that use the quantum mechanical properties of particles, or quasiparticles, to execute logical decisions (the same way ‘classical’ computers use semiconductors). Anyons’ path dependency is useful here. Arranging anyons in one sequence to achieve a final arrangement can be mapped to one piece of information (e.g. 1), and arranging anyons by a different sequence to achieve the same final arrangement can be mapped to different information (e.g. 0). This way, what information can be encoded depends on the availability of different paths to a common final state.
In addition, an important issue with existing quantum computers is that they are too fragile: even a slight interaction with the environment can cause the devices to malfunction. Using anyons for the qubits could overcome this problem because the information stored doesn’t depend on the qubits’ existing states but the paths that they have taken there. So as long as the paths have been executed properly, environmental interactions that may disturb the anyons’ final states won’t matter.
However, creating such anyons isn’t easy.
Now, recall that s-wave superconductors are characterised by the relative orbital angular momentum of electrons in the Cooper pairs being 0 (i.e. equal but in opposite directions). In some other materials, it’s possible that the relative value is 1. These are the p-wave superconductors. And at the centre of a supercurrent vortex in a p-wave superconductor, physicists expect to find non-Abelian anyons.
So the ability to create and manipulate these vortices in superconductors, as well as, more broadly, explore and understand how magnet-superconductor heterostructures work, is bound to be handy.
The Nanyang team’s paper calls the vortices and skyrmions “topological excitations”. An ‘excitation’ here is an accumulation of energy in a system over and above what the system has in its ground state. Ergo, it’s excited. A topological excitation refers to energy manifested in changes to the system’s topology.
On this subject, one of my favourite bits of science is topological phase transitions.
I usually don’t quote from Wikipedia but communicating condensed-matter physics is exacting. According to Wikipedia, “topology is concerned with the properties of a geometric object that are preserved under continuous deformations, such as stretching, twisting, crumpling and bending”. For example, no matter how much you squeeze or stretch a donut (without breaking it), it’s going to be a ring with one hole. Going one step further, your coffee mug and a donut are topologically similar: they’re both objects with one hole.
I also don’t like the Nobel Prizes but some of the research that they spotlight is nonetheless awe-inspiring. In 2016, the prize was awarded to Duncan Haldane, John Kosterlitz and David Thouless for “theoretical discoveries of topological phase transitions and topological phases of matter”.
David Thouless in 1995. Credit: Mary Levin/University of Washington
There are four popularly known phases of matter: plasma, gas, liquid and solid. If you cooled plasma, its phase would transit to that of a gas; if you cooled gases, you’d get a liquid; if you cooled liquids, you’d get a solid. If you kept cooling a solid until you were almost at absolute zero, you’d find substances behaving strangely because, suddenly, quantum mechanical effects show up. These phases of matter are broadly called quantum phases. And their phase transitions are different from when plasma becomes a gas, a gas becomes a liquid, and so on.
A Kosterlitz-Thouless transition describes a type of quantum phase transition. A substance in the quantum phase, like all substances, tries to possess as low energy as possible. When it gains some extra energy, it sheds it. And how it sheds it depends on what the laws of physics allow. Kosterlitz and Thouless found that, at times, the surface of a flat quantum phase – like the surface of liquid helium – develops vortices, akin to a flattened tornado. These vortices always formed in pairs, so the surface always had an even number of vortices. And at very low temperatures, the vortices were always tightly coupled: they remained close to each other even when they moved across the surface.
The bigger discovery came next. When Kosterlitz and Thouless raised the temperature of the surface, the vortices moved apart and moved around freely, as if they no longer belonged to each other. In terms of thermodynamics alone, the vortices being alone or together wouldn’t depend on the temperature, so something else was at play. The duo had found a kind of phase transition – because it did involve a change in temperature – that didn’t change the substance itself but only a topological shift in how it behaved. In other words, the substance was able to shed energy by coupling the vortices.
Reality is so wonderfully weird. It’s also curious that some concepts that seemed significant when I was learning science in school (like invention versus discovery) and in college (like particle versus quasiparticle) – concepts that seemed meaningful and necessary to understand what was really going on – don’t really matter in the larger scheme of things.
