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).
In conventional ‘macroscopic’ engines like the ones that guzzle fossil fuels to power cars and motorcycles, the fuels are set ablaze to release heat, which is converted to mechanical energy and transferred to the vehicle’s moving parts. In order to perform these functions over and over in a continuous manner, the engine cycles through four repeating steps. There are different kinds of cycles depending on the engine’s design and needs. A common example is the Otto cycle, where the engine’s four steps are:
1. Adiabatic compression: The piston compresses the air-fuel mixture, increasing its pressure and temperature without exchanging heat with the surroundings
2. Constant volume heat addition: At the piston’s top position, a spark plug ignites the fuel-air mixture, rapidly increasing pressure and temperature while the volume remains constant
3. Adiabatic expansion: The high-pressure gas pushes the piston down, doing work on the piston, which powers the engine
4. Constant volume heat rejection: At the bottom of the piston stroke, heat is expelled from the gas at constant volume as the engine prepares to clear the exhaust gases
So the engine goes 1-2-3-4-1-2-3-4 and so on. This is useful. If you plot the pressure and volume of the fuel-air mixture in the engine on two axes of a graph, you’ll see that at the end of the ‘constant volume heat rejection’ step (no. 4), the mixture is in the same state as it is at the start of the adiabatic compression step (no. 1). The work that the engine does on the vehicle is equal to the difference between the work done during the expansion and compression steps. Engines are designed to meet the cyclical requirement while increasing the amount of work it does for a given fuel and vehicle design.
It’s easy to understand the value of machines like this. They’re the reason we have vehicles that we can drive in different ways using our hands, legs, and our senses and in relative comfort. As long as we refill the fuel tank once in a while, engines can repeatedly perform mechanical work using their fuel combustion cycles. It’s understandable then why scientists have been trying to build quantum engines. While conventional engines use classical physics to operate, quantum engines are machines that use the ideas of quantum physics. For now, however, these machines are futuristic because scientists have found that they don’t understand the working principles of quantum engines well enough. University of Kaiserslautern-Landau professor Artur Widera told me the following in September 2023 after he and his team published a paper reporting that they had developed a new kind of quantum engine:
Just observing the development and miniaturisation of engines from macroscopic scales to biological machines and further potentially to single- or few-atom engines, it becomes clear that for few particles close to the quantum regime, thermodynamics as we use in classical life will not be sufficient to understand processes or devices. In fact, quantum thermodynamics is just emerging, and some aspects of how to describe the thermodynamical aspects of quantum processes are even theoretically not fully understood.
This said, recent advances in ultracold atomic physics have allowed physicists to control substances called quantum gases in the so-called low-dimensional regimes, laying the ground for them to realise and study quantum engines. Two recent studies exemplify this progress: the study by Widera et al. in 2023 and a new theoretical study reported in Physical Review E. Both studies have explored engines based on ultracold quantum gases but have approached the concept of quantum energy conversion from complementary perspectives.
The Physical Review E work investigated a ‘quantum thermochemical engine’ operating with a trapped one-dimensional (1D) Bose gas in the quasicondensate regime as the working fluid — just like the fuel-air mixture in in the internal combustion engine of a petrol-powered car. A Bose gas is a quantum system that consists of subatomic particles called bosons. The ‘1D’ simply means they are limited to moving back and forth on a straight line, i.e. a single spatial dimension. This restriction dramatically changes the bosons’ physical and quantum properties.
According to the paper’s single author, University of Queensland theoretical physicist Vijit Nautiyal, the resulting engine can operate on an Otto cycle where the compression and expansion steps — which dictate the work the engine can do — are implemented by tuning how strongly the bosons interact, instead of changing the volume as in a classical engine. In order to do this, the quantum engine needs to exchange not heat with its surroundings but particles. That is, the particles flow from a hot reservoir to the working boson gas, allowing the engine to perform net work.
Energy enters and leaves the system in the A-B and C-D steps, respectively, when the engine absorbs and releases particles from the hot reservoir. The engine consumes work during adiabatic compression (D-A) and performs work during adiabatic expansion (B-C). The difference between these steps is the engine’s net work output. Credit: arXiv:2411.13041v2
Nautiyal’s study focused on the engine’s performance in two regimes: one where the strength of interaction between bosons was suddenly quenched in order to maximise the engine’s power at the cost of its efficiency, and another where the quantum engine operates at maximum efficiency but produces negligible power. Nautiyal has reported doing this using advanced numerical simulations.
The simulations showed that if the engine only used heat but didn’t absorb particles from the hot reservoir, it couldn’t really produce useful energy at a finite temperatures. This was because of complicated quantum effects and uneven density in the boson gas. But when the engine was allowed to gain or lose particles from/to the reservoir, it got the extra energy it needed to work properly. Surprisingly, this particle exchange allowed the engine operate very efficiently, even when it ran fast. Usually, engines have to choose between going fast and losing efficiency or go slow and being more efficient. The particle exchange allowed Nautiyal’s quantum thermochemical engine avoid that trade-off. Letting more particles flow in and out also made the engine produce more energy and be even more efficient.
Finally, unlike regular engines where higher temperature usually means better efficiency, increasing the temperature of the quantum thermochemical engine too much actually lowered its efficiency, speaking to the important role chemical work played in this engine design.
In contrast, the 2023 experimental study — which I wrote about in The Hindu — realised a quantum engine that, instead of relying on conventional heating and cooling with thermal reservoirs, operated by cycling a gas of particles between two quantum states, a Bose-Einstein condensate and a Fermi gas. The process was driven by adiabatic changes (i.e. changes that happen while keeping the entropy fixed) that converted the fundamental difference in total energy distribution arising from the two states into usable work. The experiment demonstrated that this energy difference, called the Pauli energy, constituted a significant resource for thermodynamic cycles.
The theoretical 2025 paper and the experimental 2023 work are intimately connected as complementary explorations of quantum engine operation using ultracold atomic gases. Both have taken advantage of the unique quantum effects accessible in such systems while focusing on distinct energy resources and operational principles.
The 2025 work emphasised the role of chemical work arising from particle exchange in a one-dimensional Bose gas, exploring the balance of efficiency and power in finite-time quantum thermochemical engines. It also provided detailed computational frameworks to understand and optimise these engines. Likewise, the 2023 experiment physically realised a related but conceptually different mechanism: the movement of lithium atoms between two states and converting their Pauli energy to work. This approach highlighted how the fundamental differences between the two states could be a direct energy source, rather than conventional heat baths, and one operating with little to no production of entropy.
Together, these studies broaden the scope of quantum engines beyond traditional heat-based cycles by demonstrating the usefulness of intrinsically quantum energy forms such as chemical work and Pauli energy. Such microscopic ‘machines’ also herald a new class of engines that harness the fundamental laws of quantum physics to convert energy between different forms more efficiently than the best conventional engines can manage with classical physics.
Physics World asked Nautiyal about the potential applications of his work:
… Nautiyal referred to “quantum steampunk”. This term, which was coined by the physicist Nicole Yunger Halpern at the US National Institute of Standards and Technology and the University of Maryland, encapsulates the idea that as quantum technologies advance, the field of quantum thermodynamics must also advance in order to make such technologies more efficient. A similar principle, Nautiyal explains, applies to smartphones: “The processor can be made more powerful, but the benefits cannot be appreciated without an efficient battery to meet the increased power demands.” Conducting research on quantum engines and quantum thermodynamics is thus a way to optimize quantum technologies.
Every once in a while, I dive into a topic in science for no reason other than that I find it interesting. This is how I learnt about Titan, laser-cooling, and random walks. This post is about the fourth topic in this series: solitons.
A soliton is a stable wave that maintains its shape and characteristics as it moves around. In 1834, a civil engineer named John Scott Russell spotted a single wave moving through the Edinburgh and Glasgow Union Canal in Scotland. He described it thus in a report to the British Association for the Advancement of Science in 1844 (pp. 319-320):
I was observing the motion of a boat which was rapidly drawn along a narrow channel by a pair of horses, when the boat suddenly stopped—not so the mass of water in the channel which it had put in motion; it accumulated round the prow of the vessel in a state of violent agitation, then suddenly leaving it behind, rolled forward with great velocity, assuming the form of a large solitary elevation, a rounded, smooth and well-defined heap of water, which continued its course along the channel apparently without change of form or diminution of speed. I followed it on horseback, and overtook it still rolling on at a rate of some eight or nine miles an hour [14 km/h], preserving its original figure some thirty feet [9 m] long and a foot to a foot and a half [30−45 cm] in height. Its height gradually diminished, and after a chase of one or two miles [2–3 km] I lost it in the windings of the channel.
Such, in the month of August 1834, was my first chance interview with that singular and beautiful phenomenon which I have called the Wave of Translation, a name which it now very generally bears; which I have since found to be an important element in almost every case of fluid resistance, and ascertained to be the type of that great moving elevation in the sea, which, with the regularity of a planet, ascends our rivers and rolls along our shores.
Russell was able to reproduce a similar wave in a water tank and study its properties. American physicists later called this wave a ‘soliton’ because of its solitary nature as well as to recall the name of particles like protons and electrons (to which waves are related by particle-wave duality).
Solitons are unusual in many ways. They are very stable, for one: Russell was able to follow his soliton for almost 3 km before it vanished completely. Solitons are able to collide with each other and still come away intact. There are types of solitons with still more peculiar properties.
These entities are not easy to find: they arise due to the confluence of unusual circumstances. For example, Russell’s “wave of translation” was born when a boat moving in a canal suddenly stopped, pushing a single wave of in front that kept going. The top speed at which a wave can move on the surface of a water body is limited by the depth of the body. This is why a tsunami generated in the middle of the ocean can travel rapidly towards the shore, but as it gets closer and the water becomes shallower, it slows down. (Since it must also conserve energy, the kinetic energy it must shed goes into increasing its amplitude, so the tsunami becomes enormous when it strikes land.)
In fluid dynamics, the ratio of the speed of a vessel to the square root of the depth of the water it is moving in is called the Froude number. If the vessel was moving at the maximum speed of a wave in the Union Canal, the Froude number would have been 1.
If the Froude number had been 0.7, the vessel would have generated V-shaped pairs of waves about its prow, reminiscent of the common sight of a ship cutting through water.
Image created with ChatGPT
Then the vessel started to speed up and its Froude number approached 1. This would have caused the waves generated off the sides to bend away from the prow and straighten at the front. This is the genesis of a soliton. Since the Union Canal has a fixed width, waves forming at the front of the vessel will have had fewer opportunities to dissipate and thus keep moving forward.
Since the boat stopped, it produced the single soliton that won Russell’s attention. If it had kept moving, it would have produced a series of solitons in the water, and at the same have acquired a gentle up and down oscillating motion of its own as the Froude number exceeded 1.
Waves occur in a wide variety of contexts in the real world — and in the right conditions, scientists expect to find solitons in almost all of them. For example they have been spotted in optical fibres that carry light waves, in materials carrying a moving wave of magnetisation, and in water currents at the bottom of the ocean.
In the wave physics used to understand these various phenomena, a soliton is said to emerge as a solution to non-linear partial differential equations.
The behaviour of some systems can be described using partial differential equations. The plucked guitar string is a classic example. The string is fixed at both ends; when it is plucked, a wave travels along its length producing the characteristic sound. The corresponding equation goes like this: ∂2u/∂t2 = c2 • ∂u2/∂x2, where u is the string’s displacement, x is where it was plucked, c is the maximum speed the wave can have, and t is of course the time lapsed.
The equation itself is not important. The point is that there’s a left-hand side and a right-hand side, and one side can equal the other for different combinations of u, x, and t. Each such combination is called a solution. One particular solution is called the soliton when the corresponding wave meets three conditions: it’s localised, preserves its shape and speed, and doesn’t lose energy when interacting with other solitons.
The “non-linear” part of “non-linear partial differential equations” means that these equations describe ways whose properties that have different properties depending on their amplitude. The guitar string equation is an example of a linear system because u, the string’s displacement, has a power of 1 (i.e. it isn’t squared or cubed) nor are the other terms of the equation multiplied with each other. Another famous example of a non-linear partial differential equation is the Schrödinger equation, which describes how the wave function of a quantum system will change over time given a set of initial conditions.
(The Austrian-Irish physicist Erwin Schrödinger postulated it in 1925, which is one of the reasons the UN has designated our current year — a century later — the International Year of Quantum Science & Technology.)
An example of a non-linear partial differential equation is the Korteweg-de Vries equation, which predicts how waves behave in shallow water: ut + 6u • ux + uxxx = 0. The second term is the problem: ux is a way to write ∂u/∂x and since it is multiplied by 6u the equation is non-linear, i.e. a change in its character induces changes in itself that may also change its character.
But for better or for worse, this is the only milieu in which a soliton will emerge.
(If you’re really interested: for example a soliton solution of the Korteweg-de Vries equation looks like this: u = A sech2 [ k (x – vt – x0) ], where A is the soliton’s amplitude or maximum height, k is a term related to its width, x0 is its initial position, and vt is its velocity over time. ‘sech’ is the hyperbolic secant function.)
Physicists are more interested in particular types of soliton than others because they closely mimic specific phenomena in the real world. Sometimes it’s a good idea to understand these phenomena as if they were solitons because the mathematics of the latter may be easier to work with. This lucid Institute of Physics video starring theoretical physicist David Tong sets out the quirky case of quarks.
I myself was more piqued by the Peregrine and breather solitons.
The Peregrine soliton isn’t a soliton that travels. Its name comes from its discoverer, a British mathematician named Howell Peregrine. In fact, one of the things that distinguish a Peregrine soliton is that it’s stuck in one place. More specifically it emerges from pre-existing waves, has a much greater amplitude than the background, and appears at and disappears from a single location in a blip.
Peregrine solitons are interesting because they have been used to explain killer waves: freakish waves in the open sea that have no discernible cause and tower over all the other waves. One famous example is the Draupner wave, which was the first killer wave to also be measured by an instrument as it happened. It occurred on January 1, 1995, near the Draupner platform, a natural-gas rig in the Norwegian part of the North Sea. This is the wave’s sounding chart:
Credit: Ingvald Straume/Wikimedia Commons, CC BY-SA
That’s one heck of a soliton.
The breather soliton is equally remarkable. It’s a regular soliton that also has an oscillating amplitude, frequency or something else as it moves around. Imagine a breather soliton to be a soliton in water: it might look like a wave with an undulating shape, its surface heaving one moment and sagging the other like the head of a strange sea monster breathing as it glides along. This is exactly the spirit in which the breathing soliton was named. Here’s an animation of a particular variety called the sine-Gordon breather soliton:
Credit: Danko Georgiev/Wikimedia Commons, CC BY-SA
The Peregrine soliton is a particular instance of a breather soliton. Breathers have also been found in an exotic state of matter called a Bose-Einstein condensate (which physicists are studying with the expectation that it will inspire technologies of the future), in plasmas in outer space, in the operational parameters of short-pulse lasers, and in fibre optics. Some researchers also think entities analogous to breather solitons could help proteins inside the cells in our bodies transport energy.
If you’re interested in jumping down this rabbit hole, you could also look up the Akhmediev and the Kuznetsov-Ma breathers.
At first blush, solitons seem like monastic wanderers of a world otherwise replete with waves travelling as if loath to be separated from another. Recall that one wave in 1834 gliding ever so placidly for over half a league, followed by a curious man on a horse galloping along the canal’s bank. But for this venerable image, solitons are the children of a world far too sophisticated to admit waves crashing into each other with little more consequence than an enlivening spray of water and the formidable mathematics they demand to be understood.
Featured image: A scan of a print of Hokusai’s ‘The Great Wave off Kanagawa’. Credit: H. O. Havemeyer Collection, 1929.
One of the most exotic phases of matter is called the Bose-Einstein condensate. As its name indicates, this type of matter is one whose constituents are bosons – which are basically all subatomic particles whose behaviour is dictated by the rules of Bose-Einstein statistics. These particles are also called force particles. The other kind are matter particles, or fermions. Their behaviour is described by the rules of Fermi-Dirac statistics. Force particles and matter particles together make up the universe as we know it.
To be a boson, a particle – which can be anything from quarks (which make up protons and neutrons) to entire atoms – needs to have a spin quantum number of certain values. (All of a particle’s properties can be described by the values of four quantum numbers.) An important difference between fermions and bosons is that Pauli’s exclusion principle doesn’t apply to bosons. The principle states that in a given quantum system, no two particles can have the same set of four quantum numbers at the same time. When two particles have the same four quantum numbers, they are said to occupy the same state. (‘States’ are not like places in a volume; instead, think of them more like a set of properties.) Pauli’s exclusion principle forbids fermions from doing this – but not bosons. So in a given quantum system, all the bosons can occupy the same quantum state if they are forced to.
For example, this typically happens when the system is cooled to nearly absolute zero – the lowest temperature possible. (The bosons also need to be confined in a ‘trap’ so that they don’t keep moving around or combine with each other to form other particles.) More and more energy being removed from the system is equivalent to more and more energy being removed from the system’s constituent particles. So as fermions and bosons possess less and less energy, they occupy lower and lower quantum states. But once all the lowest fermionic states are occupied, fermions start occupying the next lowest states, and so on. This is because of the principle. Bosons on the other hand are all able to occupy the same lowest quantum state. When this happens, they are said to have formed a Bose-Einstein condensate.
In this phase, all the bosons in the system move around like a fluid – like the molecules of flowing water. A famous example of this is superconductivity (at least of the conventional variety). When certain materials are cooled to near absolute zero, their electrons – which are fermions – overcome their mutual repulsion and pair up with each other to form composite pairs called Cooper pairs. Unlike individual electrons, Cooper pairs are bosons. They go on to form a Bose-Einstein condesate in which the Cooper pairs ‘flow’ through the material. In the material’s non-superconducting state, the electrons would have scattered by some objects in their path – like atomic nuclei or vibrations in the lattice. This scattering would have manifested as electrical resistance. But because Cooper pairs have all occupied the same quantum state, they are much harder to scatter. They flow through the material as if they don’t experience any resistance. This flow is what we know as superconductivity.
Bose-Einstein condensates are a big deal in physics because they are a macroscopic effect of microscopic causes. We can’t usually see or otherwise directly sense the effects of most quantum-physical phenomena because they happen on very small scales, and we need the help of sophisticated instruments like electron microscopes and particle accelerators. But when we cool a superconducting material to below its threshold temperature, we can readily sense the presence of a superconductor by passing an electric current through it (or using the Meissner effect). Macroscopic effects are also easier to manipulate and observe, so physicists have used Bose-Einstein condensates as a tool to probe many other quantum phenomena.
While Albert Einstein predicted the existence of Bose-Einstein condensates – based on work by Satyendra Nath Bose – in 1924, physicists had the requisite technologies and understanding of quantum mechanics to be able to create them in the lab only in the 1990s. These condensates were, and mostly still are, quite fragile and can be created only in carefully controlled conditions. But physicists have also been trying to figure out how to maintain a Bose-Einstein condensate for long periods of time, because durable condensates are expected to provide even more research insights as well as hold potential applications in particle physics, astrophysics, metrology, holography and quantum computing.
An important reason for this is wave-particle duality, which you might recall from high-school physics. Louis de Broglie postulated in 1924 that every quantum entity could be described both as a particle and as a wave. The Davisson-Germer experiment of 1923-1927 subsequently found that electrons – which were until then considered to be particles – behaved like waves in a diffraction experiment. Interference and diffraction are exhibited by waves, so the experiment proved that electrons could be understood as waves as well. Similarly, a Bose-Einstein condensate can be understood both in terms of particle physics and in terms of wave physics. Just like in the Davisson-Germer experiment, when physicists set up an experiment to look for an interference pattern from a Bose-Einstein condensate, they succeeded. They also found that the interference pattern became stronger the more bosons they added to the condensate.
Now, all the bosons in a condensate have a coherent phase. The phase of a wave measures the extent to which the wave has evolved in a fixed amount of time. When two waves have coherent phase, both of them will have progressed by the same amount in the same span of time. Phase coherence is one of the most important wave-like properties of a Bose-Einstein condensate because of the possibility of a device called an atom laser.
‘Laser’ is an acronym for ‘light amplification by stimulated emission of radiation’. The following video demonstrates its working principle better than I can in words right now:
The light emitted by an optical laser is coherent: it has a constant frequency and comes out in a narrow beam if the coherence is spatial or can be produced in extremely short pulses if the coherence is temporal. An atom laser is a laser composed of propagating atoms instead of photons. As Wolfgang Ketterle, who led the creation of the first Bose-Einstein condensate and later won a Nobel Prize for it, put it, “The atom laser emits coherent matter waves whereas the optical laser emits coherent electromagnetic waves.” Because the bosons of a Bose-Einstein condensate are already phase-coherent, condensates make excellent sources for an atom laser.
The trick, however, lies in achieving a Bose-Einstein condensate of the desired (bosonic) atoms and then extracting a few atoms into the laser while replenishing the condensate with more atoms – all without letting the condensate break down or the phase-coherence being lost. Physicists created the first such atom laser in 1996 but it did not have a continuous emission nor was very bright. Researchers have since built better atom lasers based on Bose-Einstein condensates, although they remain far from being usable in their putative applications. An important reason for this is that physicists are yet to build a condensate-based atom laser that can operate continuously. That is, as atoms from the condensate lase out, the condesate is constantly replenished, and the laser operates continuously for a long time.
On June 8, researchers from the University of Amsterdam reported that they had been able to create a long-lived, sort of self-sustaining Bose-Einstein condensate. This brings us a giant step closer to a continuously operating atom laser. Their setup consisted of multiple stages, all inside a vacuum chamber.
In the first stage, strontium atoms (which are bosons) started from an ‘oven’ maintained at 850 K and were progressively laser-cooled while they made their way into a reservoir. (Here is a primer of how laser-cooling works.) The reservoir had a dimple in the middle. In the second stage, the atoms were guided by lasers and gravity to descend into this dimple, where they had a temperature of approximately 1 µK, or one-millionth of a kelvin. As the dimple became more and more crowded, it was important for the atoms here to not heat up, which could have happened if some light had ‘leaked’ into the vacuum chamber.
To prevent this, in the third stage, the physicists used a carefully tuned laser shined only through the dimple that had the effect of rendering the strontium atoms mostly ‘transparent’ to light. According to the research team’s paper, without the ‘transparency beam’, the atoms in the dimple had a lifetime of less than 40 ms, whereas with the beam, it was more than 1.5 s – a 37x difference. At some point, when a sufficient number of atoms had accumulated in the dimple, a Bose-Einstein condensate formed. In the fourth stage, an effect called Bose stimulation kicked in. Simply put, as more bosons (strontium atoms, in this case) transitioned into the condensate, the rate of transition of additional bosons also increased. Bose stimulation thus played the role that the gain medium plays in an optical laser. The size of the condensate grew until it matched the rate of loss of atoms out of the dimple, and reached an equilibrium.
And voila! With a steady-state Bose-Einstein condensate, the continuous atom laser was almost ready. The physicists have acknowledged that their setup can be improved in many ways, including by making the laser-cooling effects more uniform, increasing the lifetime of strontium atoms inside the dimple, reducing losses due to heating and other effects, etc. At the same time, they wrote that “at all times after steady state is reached”, they found a Bose-Einstein condensate existing in their setup.
On June 24, a press release from CERN said that scientists and engineers working on upgrading the Large Hadron Collider (LHC) had “built and operated … the most powerful electrical transmission line … to date”. The transmission line consisted of four cables – two capable of transporting 20 kA of current and two, 7 kA.
The ‘A’ here stands for ‘ampere’, the SI unit of electric current. Twenty kilo-amperes is an extraordinary amount of current, nearly equal to the amount in a single lightning strike.
In the particulate sense: one ampere is the flow of one coulomb per second. One coulomb is equal to around 6.24 quintillion elementary charges, where each elementary charge is the charge of a single proton or electron (with opposite signs). So a cable capable of carrying a current of 20 kA can essentially transport 124.8 sextillion electrons per second.
The line is composed of cables made of magnesium diboride (MgB2), which is a superconductor and therefore presents no resistance to the flow of the current and can transmit much higher intensities than traditional non-superconducting cables. On this occasion, the line transmitted an intensity 25 times greater than could have been achieved with copper cables of a similar diameter. Magnesium diboride has the added benefit that it can be used at 25 kelvins (-248 °C), a higher temperature than is needed for conventional superconductors. This superconductor is more stable and requires less cryogenic power. The superconducting cables that make up the innovative line are inserted into a flexible cryostat, in which helium gas circulates.
The part in bold could have been more explicit and noted that superconductors, including magnesium diboride, can’t carry an arbitrarily higher amount of current than non-superconducting conductors. There is actually a limit for the same reason why there is a limit to the current-carrying capacity of a normal conductor.
This explanation wouldn’t change the impressiveness of this feat and could even interfere with readers’ impression of the most important details, so I can see why the person who drafted the statement left it out. Instead, I’ll take this matter up here.
An electric current is generated between two points when electrons move from one point to the other. The direction of current is opposite to the direction of the electrons’ movement. A metal that conducts electricity does so because its constituent atoms have one or more valence electrons that can flow throughout the metal. So if a voltage arises between two ends of the metal, the electrons can respond by flowing around, birthing an electric current.
This flow isn’t perfect, however. Sometimes, a valence electron can bump into atomic nuclei, impurities – atoms of other elements in the metallic lattice – or be thrown off course by vibrations in the lattice of atoms, produced by heat. Such disruptions across the metal collectively give rise to the metal’s resistance. And the more resistance there is, the less current the metal can carry.
These disruptions often heat the metal as well. This happens because electrons don’t just flow between the two points across which a voltage is applied. They’re accelerated. So as they’re speeding along and suddenly bump into an impurity, they’re scattered into random directions. Their kinetic energy then no longer contributes to the electric energy of the metal and instead manifests as thermal energy – or heat.
If the electrons bump into nuclei, they could impart some of their kinetic energy to the nuclei, causing the latter to vibrate more, which in turn means they heat up as well.
Copper and silver have high conductance because they have more valence electrons available to conduct electricity and these electrons are scattered to a lesser extent than in other metals. Therefore, these two also don’t heat up as quickly as other metals might, allowing them to transport a higher current for longer. Copper in particular has a higher mean free path: the average distance an electron travels before being scattered.
In superconductors, the picture is quite different because quantum physics assumes a more prominent role. There are different types of superconductors according to the theories used to understand how they conduct electricity with zero resistance and how they behave in different external conditions. The electrical behaviour of magnesium diboride, the material used to transport the 20 kA current, is described by Bardeen-Cooper-Schrieffer (BCS) theory.
According to this theory, when certain materials are cooled below a certain temperature, the residual vibrations of their atomic lattice encourages their valence electrons to overcome their mutual repulsion and become correlated, especially in terms of their movement. That is, the electrons pair up.
While individual electrons belong to a class of particles called fermions, these electron pairs – a.k.a. Cooper pairs – belong to another class called bosons. One difference between these two classes is that bosons don’t obey Pauli’s exclusion principle: that no two fermions in the same quantum system (like an atom) can have the same set of quantum numbers at the same time.
As a result, all the electron pairs in the material are now free to occupy the same quantum state – which they will when the material is supercooled. When they do, the pairs collectively make up an exotic state of matter called a Bose-Einstein condensate: the electron pairs now flow through the material as if they were one cohesive liquid.
In this state, even if one pair gets scattered by an impurity, the current doesn’t experience resistance because the condensate’s overall flow isn’t affected. In fact, given that breaking up one pair will cause all other pairs to break up as well, the energy required to break up one pair is roughly equal to the energy required to break up all pairs. This feature affords the condensate a measure of robustness.
But while current can keep flowing through a BCS superconductor with zero resistance, the superconducting state itself doesn’t have infinite persistence. It can break if it stops being cooled below a specific temperature, called the critical temperature; if the material is too impure, contributing to a sufficient number of collisions to ‘kick’ all electrons pairs out of their condensate reverie; or if the current density crosses a particular threshold.
At the LHC, the magnesium diboride cables will be wrapped around electromagnets. When a large current flows through the cables, the electromagnets will produce a magnetic field. The LHC uses a circular arrangement of such magnetic fields to bend the beam of protons it will accelerate into a circular path. The more powerful the magnetic field, the more accurate the bending. The current operational field strength is 8.36 tesla, about 128,000-times more powerful than Earth’s magnetic field. The cables will be insulated but they will still be exposed to a large magnetic field.
Type I superconductors completely expel an external magnetic field when they transition to their superconducting state. That is, the magnetic field can’t penetrate the material’s surface and enter the bulk. Type II superconductors are slightly more complicated. Below one critical temperature and one critical magnetic field strength, they behave like type I superconductors. Below the same temperature but a slightly stronger magnetic field, they are superconducting and allow the fields to penetrate their bulk to a certain extent. This is called the mixed state.
A hand-drawn phase diagram showing the conditions in which a mixed-state type II superconductor exists. Credit: Frederic Bouquet/Wikimedia Commons, CC BY-SA 3.0
Say a uniform magnetic field is applied over a mixed-state superconductor. The field will plunge into the material’s bulk in the form of vortices. All these vortices will have the same magnetic flux – the number of magnetic field lines per unit area – and will repel each other, settling down in a triangular pattern equidistant from each other.
An annotated image of vortices in a type II superconductor. The scale is specified at the bottom right. Source: A set of slides entitled ‘Superconductors and Vortices at Radio Frequency Magnetic Fields’ by Ernst Helmut Brandt, Max Planck Institute for Metals Research, October 2010.
When an electric current passes through this material, the vortices are slightly displaced, and also begin to experience a force proportional to how closely they’re packed together and their pattern of displacement. As a result, to quote from this technical (yet lucid) paper by Praveen Chaddah:
This force on each vortex … will cause the vortices to move. The vortex motion produces an electric field1 parallel to [the direction of the existing current], thus causing a resistance, and this is called the flux-flow resistance. The resistance is much smaller than the normal state resistance, but the material no longer [has] infinite conductivity.
1. According to Maxwell’s equations of electromagnetism, a changing magnetic field produces an electric field.
Since the vortices’ displacement depends on the current density: the greater the number of electrons being transported, the more flux-flow resistance there is. So the magnesium diboride cables can’t simply carry more and more current. At some point, setting aside other sources of resistance, the flux-flow resistance itself will damage the cable.
There are ways to minimise this resistance. For example, the material can be doped with impurities that will ‘pin’ the vortices to fixed locations and prevent them from moving around. However, optimising these solutions for a given magnetic field and other conditions involves complex calculations that we don’t need to get into.
The point is that superconductors have their limits too. And knowing these limits could improve our appreciation for the feats of physics and engineering that underlie achievements like cables being able to transport 124.8 sextillion electrons per second with zero resistance. In fact, according to the CERN press release,
The [line] that is currently being tested is the forerunner of the final version that will be installed in the accelerator. It is composed of 19 cables that supply the various magnet circuits and could transmit intensities of up to 120 kA!
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While writing this post, I was frequently tempted to quote from Lisa Randall‘s excellent book-length introduction to the LHC, Knocking on Heaven’s Door (2011). Here’s a short excerpt:
One of the most impressive objects I saw when I visited CERN was a prototype of LHC’s gigantic cylindrical dipole magnets. Event with 1,232 such magnets, each of them is an impressive 15 metres long and weighs 30 tonnes. … Each of these magnets cost EUR 700,000, making the ned cost of the LHC magnets alone more than a billion dollars.
The narrow pipes that hold the proton beams extend inside the dipoles, which are strung together end to end so that they wind through the extent of the LHC tunnel’s interior. They produce a magnetic field that can be as strong as 8.3 tesla, about a thousand times the field of the average refrigerator magnet. As the energy of the proton beams increases from 450 GeV to 7 TeV, the magnetic field increases from 0.54 to 8.3 teslas, in order to keep guiding the increasingly energetic protons around.
The field these magnets produce is so enormous that it would displace the magnets themselves if no restraints were in place. This force is alleviated through the geometry of the coils, but the magnets are ultimately kept in place through specially constructed collars made of four-centimetre thick steel.
… Each LHC dipole contains coils of niobium-titanium superconducting cables, each of which contains stranded filaments a mere six microns thick – much smaller than a human hair. The LHC contains 1,200 tonnes of these remarkable filaments. If you unwrapped them, they would be long enough to encircle the orbit of Mars.
When operating, the dipoles need to be extremely cold, since they work only when the temperature is sufficiently low. The superconducting wires are maintained at 1.9 degrees above absolute zero … This temperature is even lower than the 2.7-degree cosmic microwave background radiation in outer space. The LHC tunnel houses the coldest extended region in the universe – at least that we know of. The magnets are known as cryodipoles to take into account their special refrigerated nature.
In addition to the impressive filament technology used for the magnets, the refrigeration (cryogenic) system is also an imposing accomplishment meriting its own superlatives. The system is in fact the world’s largest. Flowing helium maintains the extremely low temperature. A casing of approximately 97 metric tonnes of liquid helium surrounds the magnets to cool the cables. It is not ordinary helium gas, but helium with the necessary pressure to keep it in a superfluid phase. Superfluid helium is not subject to the viscosity of ordinary materials, so it can dissipate any heat produced in the dipole system with great efficiency: 10,000 metric tonnes of liquid nitrogen are first cooled, and this in turn cools the 130 metric tonnes of helium that circulate in the dipoles.
Featured image: A view of the experimental MgB2 transmission line at the LHC. Credit: CERN.
The Large Hadron Collider (LHC) performs an impressive feat every time it accelerates billions of protons to nearly the speed of light – and not in terms of the energy alone. For example, you release more energy when you clap your palms together once than the energy imparted to a proton accelerated by the LHC. The impressiveness arises from the fact that the energy of your clap is distributed among billions of atoms while the latter all resides in a single particle. It’s impressive because of the energy density.
A proton like this should have a very high kinetic energy. When lots of protons with such amounts of energy come together to form a macroscopic object, the object will have a high temperature. This is the relationship between subatomic particles and the temperature of the object they make up. The outermost layer of a star is so hot because its constituent particles have a very high kinetic energy. Blue hypergiant stars, thought to be the hottest stars in the universe, like Eta Carinae have a surface temperature of 36,000 K and a surface 57,600-times larger than that of the Sun. This isn’t impressive on the temperature scale alone but also on the energy density scale: Eta Carinae ‘maintains’ a higher temperature over a larger area.
Now, the following headline and variations thereof have been doing the rounds of late, and they piqued me because I’m quite reluctant to believe they’re true:
This headline, as you may have guessed by the fonts, is from Nature News. To be sure, I’m not doubting the veracity of any of the claims. Instead, my dispute is with the “coolest lab” claim and on entirely qualitative grounds.
The feat mentioned in the headline involves physicists using lasers to cool a tightly controlled group of atoms to near-absolute-zero, causing quantum mechanical effects to become visible on the macroscopic scale – the feature that Bose-Einstein condensates are celebrated for. Most, if not all, atomic cooling techniques endeavour in different ways to extract as much of an atom’s kinetic energy as possible. The more energy they remove, the cooler the indicated temperature.
The reason the headline piqued me was that it trumpets a place in the universe called the “universe’s coolest lab”. Be that as it may (though it may not technically be so; the physicist Wolfgang Ketterle has achieved lower temperatures before), lowering the temperature of an object to a remarkable sliver of a kelvin above absolute zero is one thing but lowering the temperature over a very large area or volume must be quite another. For example, an extremely cold object inside a tight container the size of a shoebox (I presume) must be lacking much less energy than a not-so-extremely cold volume across, say, the size of a star.
This is the source of my reluctance to acknowledge that the International Space Station could be the “coolest lab in the universe”.
While we regularly equate heat with temperature without much consequence to our judgment, the latter can be described by a single number pertaining to a single object whereas the former – heat – is energy flowing from a hotter to a colder region of space (or the other way with the help of a heat pump). In essence, the amount of heat is a function of two differing temperatures. In turn it could matter, when looking for the “coolest” place, that we look not just for low temperatures but for lower temperatures within warmer surroundings. This is because it’s harder to maintain a lower temperature in such settings – for the same reason we use thermos flasks to keep liquids hot: if the liquid is exposed to the ambient atmosphere, heat will flow from the liquid to the air until the two achieve a thermal equilibrium.
An object is said to be cold if its temperature is lower than that of its surroundings. Vladivostok in Russia is cold relative to most of the world’s other cities but if Vladivostok was the sole human settlement and beyond which no one has ever ventured, the human idea of cold will have to be recalibrated from, say, 10º C to -20º C. The temperature required to achieve a Bose-Einstein condensate is the temperature required at which non-quantum-mechanical effects are so stilled that they stop interfering with the much weaker quantum-mechanical effects, given by a formula but typically lower than 1 K.
The deep nothingness of space itself has a temperature of 2.7 K (-270.45º C); when all the stars in the universe die and there are no more sources of energy, all hot objects – like neutron stars, colliding gas clouds or molten rain over an exoplanet – will eventually have to cool to 2.7 K to achieve equilibrium (notwithstanding other eschatological events).
This brings us, figuratively, to the Boomerang Nebula – in my opinion the real coolest lab in the universe because it maintains a very low temperature across a very large volume, i.e. its coolness density is significantly higher. This is a protoplanetary nebula, which is a phase in the lives of stars within a certain mass range. In this phase, the star sheds some of its mass that expands outwards in the form of a gas cloud, lit by the star’s light. The gas in the Boomerang Nebula, from a dying red giant star changing to a white dwarf at the centre, is expanding outward at a little over 160 km/s (576,000 km/hr), and has been for the last 1,500 years or so. This rapid expansion leaves the nebula with a temperature of 1 K. Astronomers discovered this cold mass in late 1995.
(“When gas expands, the decrease in pressure causes the molecules to slow down. This makes the gas cold”: source.)
The experiment to create a Bose-Einstein condensate in space – or for that matter anywhere on Earth – transpired in a well-insulated container that, apart from the atoms to be cooled, was a vacuum. So as such, to the atoms, the container was their universe, their Vladivostok. They were not at risk of the container’s coldness inviting heat from its surroundings and destroying the condensate. The Boomerang Nebula doesn’t have this luxury: as a nebula, it’s exposed to the vast emptiness, and 2.7 K, of space at all times. So even though the temperature difference between itself and space is only 1.7 K, the nebula also has to constantly contend with the equilibriating ‘pressure’ imposed by space.
Further, according to Raghavendra Sahai (as quoted by NASA), one of the nebula’s cold spots’ discoverers, it’s “even colder than most other expanding nebulae because it is losing its mass about 100-times faster than other similar dying stars and 100-billion-times faster than Earth’s Sun.” This implies there is a great mass of gas, and so atoms, whose temperature is around 1 K.
All together, the fact that the nebula has maintained a temperature of 1 K for around 1,500 years (plus a 5,000-year offset, to compensate for the distance to the nebula) and over 3.14 trillion km makes it a far cooler “coolest” place, lab, whatever.
Consider this post the latest in a loosely defined series about atomic cooling techniques that I’ve been writing since June 2018.
Atoms can’t run a temperature, but things made up of atoms, like a chair or table, can become hotter or colder. This is because what we observe as the temperature of macroscopic objects is at the smallest level the kinetic energy of the atoms it is made up of. If you were to cool such an object, you’d have to reduce the average kinetic energy of its atoms. Indeed, if you had to cool a small group of atoms trapped in a container as well, you’d simply have to make sure they – all told – slow down.
Over the years, physicists have figured out more and more ingenious ways to cool atoms and molecules this way to ultra-cold temperatures. Such states are of immense practical importance because at very low energy, these particles (an umbrella term) start displaying quantum mechanical effects, which are too subtle to show up at higher temperatures. And different quantum mechanical effects are useful to create exotic things like superconductors, topological insulators and superfluids.
One of the oldest modern cooling techniques is laser-cooling. Here, a laser beam of a certain frequency is fired at an atom moving towards the beam. Electrons in the atom absorb photons in the beam, acquire energy and jump to a higher energy level. A short amount of time later, the electrons lose the energy by emitting a photon and jump back to the lower energy level. But since the photons are absorbed in only one direction but are emitted in arbitrarily different directions, the atom constantly loses momentum in one direction but gains momentum in a variety of directions (by Newton’s third law). The latter largely cancel themselves out, leaving the atom with considerably lower kinetic energy, and therefore cooler than before.
In collisional cooling, an atom is made to lose momentum by colliding not with a laser beam but with other atoms, which are maintained at a very low temperature. This technique works better if the ratio of elastic to inelastic collisions is much greater than 50. In elastic collisions, the total kinetic energy of the system is conserved; in inelastic collisions, the total energy is conserved but not the kinetic energy alone. In effect, collisional cooling works better if almost all collisions – if not all of them – conserve kinetic energy. Since the other atoms are maintained at a low temperature, they have little kinetic energy to begin with. So collisional cooling works by bouncing warmer atoms off of colder ones such that the colder ones take away some of the warmer atoms’ kinetic energy, thus cooling them.
In a new study, a team of scientists from MIT, Harvard University and the University of Waterloo reported that they were able to cool a pool of NaLi diatoms (molecules with only two atoms) this way to a temperature of 220 nK. That’s 220-billionths of a kelvin, about 12-million-times colder than deep space. They achieved this feat by colliding the warmer NaLi diatoms with five-times as many colder Na (sodium) atoms through two cycles of cooling.
Their paper, published online on April 8 (preprint here), indicates that their feat is notable for three reasons.
First, it’s easier to cool particles (atoms, ions, etc.) in which as many electrons as possible are paired to each other. A particle in which all electrons are paired is called a singlet; ones that have one unpaired electron each are called doublets; those with two unpaired electrons – like NaLi diatoms – are called triplets. Doublets and triplets can also absorb and release more of their energy by modifying the spins of individual electrons, which messes with collisional cooling’s need to modify a particle’s kinetic energy alone. The researchers from MIT, Harvard and Waterloo overcame this barrier by applying a ‘bias’ magnetic field across their experiment’s apparatus, forcing all the particles’ spins to align along a common direction.
Second: Usually, when Na and NaLi come in contact, they react and the NaLi molecule breaks down. However, the researchers found that in the so-called spin-polarised state, the Na and NaLi didn’t react with each other, preserving the latter’s integrity.
Third, and perhaps most importantly, this is not the coldest temperature to which we have been able to cool quantum particles, but it still matters because collisional cooling offers unique advantages that makes it attractive for certain applications. Perhaps the most well-known of them is quantum computing. Simply speaking, physicists prefer ultra-cold molecules to atoms to use in quantum computers because physicists can control molecules more precisely than they can the behaviour of atoms. But molecules that have doublet or triplet states or are otherwise reactive can’t be cooled to a few billionths of a kelvin with laser-cooling or other techniques. The new study shows they can, however, be cooled to 220 nK using collisional cooling. The researchers predict that in future, they may be able to cool NaLi molecules even further with better equipment.
Note that the researchers didn’t cool the NaLi atoms from room temperature to 220 nK but from 2 µK. Nonetheless, their achievement remains impressive because there are other well-established techniques to cool atoms and molecules from room temperature to a few micro-kelvin. The lower temperatures are harder to reach.
One of the researchers involved in the current study, Wolfgang Ketterle, is celebrated for his contributions to understanding and engineering ultra-cold systems. He led an effort in 2003 to cool sodium atoms to 0.5 nK – a record. He, Eric Cornell and Carl Wieman won the Nobel Prize for physics two years before that: Cornell, Wieman and their team created the first Bose-Einstein condensate in 1995, and Ketterle created ‘better’ condensates that allowed for closer inspection of their unique properties. A Bose-Einstein condensate is a state of matter in which multiple particles called bosons are ultra-cooled in a container, at which point they occupy the same quantum state – something they don’t do in nature (even as they comply with the laws of nature) – and give rise to strange quantum effects that can be observed without a microscope.
Ketterle’s attempts make for a fascinating tale; I collected some of them plus some anecdotes together for an article in The Wire in 2015, to mark the 90th year since Albert Einstein had predicted their existence, in 1924-1925. A chest-thumper might be cross that I left Satyendra Nath Bose out of this citation. It is deliberate. Bose-Einstein condensates are named for their underlying theory, called Bose-Einstein statistics. But while Bose had the idea for the theory to explain the properties of photons, Einstein generalised it to more particles, and independently predicted the existence of the condensates based on it.
This said, if it is credit we’re hungering for: the history of atomic cooling techniques includes the brilliant but little-known S. Pancharatnam. His work in wave physics laid the foundations of many of the first cooling techniques, and was credited as such by Claude Cohen-Tannoudji in the journal Current Science in 1994. Cohen-Tannoudji would win a piece of the Nobel Prize for physics in 1997 for inventing a technique called Sisyphus cooling – a way to cool atoms by converting more and more of their kinetic energy to potential energy, and then draining the potential energy.
Indeed, the history of atomic cooling techniques is, broadly speaking, a history of physicists uncovering newer, better ways to remove just a little bit more energy from an atom or molecule that’s already lost a lot of its energy. The ultimate prize is absolute zero, the lowest temperature possible, at which the atom retains only the energy it can in its ground state. However, absolute zero is neither practically attainable nor – more importantly – the goal in and of itself in most cases. Instead, the experiments in which physicists have achieved really low temperatures are often pegged to an application, and getting below a particular temperature is the goal.
For example, niobium nitride becomes a superconductor below 16 K (-257º C), so applications using this material prepare to achieve this temperature during operation. For another, as the MIT-Harvard-Waterloo group of researchers write in their paper, “Ultra-cold molecules in the micro- and nano-kelvin regimes are expected to bring powerful capabilities to quantum emulation and quantum computing, owing to their rich internal degrees of freedom compared to atoms, and to facilitate precision measurement and the study of quantum chemistry.”
It’s a matter of some irony that forces that act across larger distances also give rise to lots of empty space – although the more you think about it, the more it makes sense. The force of gravity, for example, can act across millions of kilometres but this only means two massive objects can still influence each across this distance instead of having to get closer to do so. Thus, you have galaxies with a lot more space between stars than stars themselves.
The electromagnetic force, like the force of gravity, also follows an inverse-square law: its strength falls off as the square of the distance – but never fully reaches zero. So you can have an atom with a nucleus of protons and neutrons held tightly together but electrons located so far away that each atom is more than 90% empty space.
In fact, you can use the rules of subatomic physics to make atoms even more vacuous. Electrons orbit the nucleus in an atom at fixed distances, and when an electron gains some energy, it jumps into a higher orbit. Physicists have been able to excite electrons to such high energies that the atom itself becomes thousands of times larger than an atom of hydrogen.
This is the deceptively simple setting for the Rydberg polaron: the atom inside another atom, with some features added.
In January 2018, physicists from Austria, Brazil, Switzerland and the US reported creating the first Rydberg polaron in the lab, based on theoretical predictions that another group of researchers had advanced in October 2015. The concept, as usual, is far simpler than the execution, so exploring the latter should provide a good sense of the former.
The January 2018 group first created a Bose-Einstein condensate, a state of matter in which a dilute gas of particles called bosons is maintained in an ultra-cold container. Bosons are particles whose quantum spin takes integer values. (Other particles called fermions have half-integer spin). As the container is cooled to near absolute zero, the bosons begin to collectively display quantum mechanical phenomena at the macroscopic scale, essentially becoming a new form of matter and displaying certain properties that no other form of matter has been known to exhibit.
Atoms of strontium-84, -86 and -88 have zero spin, so the physicists used them to create the condensate. Next, they used lasers to bombard some strontium atoms with photons to impart energy to electrons in the outermost orbits (a.k.a. valence electrons), forcing them to jump to an even higher orbit. Effectively, the atom expands, becoming a so-called Rydberg atom[1]. In this state, if the distance between the nucleus and an excited electron is greater than the average distance between the other strontium atoms in the condensate, then some of the other atoms could technically fit into the Rydberg atom, forming the atom-within-an-atom.
[1] Rydberg atoms are called so because many of their properties depend on the value of the principal quantum number, which the Swedish physicist Johannes Robert Rydberg first (inadvertently) described in a formula in 1888.
Rydberg atoms are gigantic relative to other atoms; some are even bigger than a virus, and their interactions with their surroundings can be observed under a simple light microscope. They are relatively long-lived, in that the excited electron decays to its ground state slowly. Astronomers have found them in outer space. However, Rydberg atoms are also fragile: because the electron is already so far from the nucleus, any other particles in the vicinity, even a weak electromagnetic field or a slightly warmer temperature could easily knock the excited electron out of the Rydberg atom and end the Rydberg state.
Some clever physicists took advantage of this property and used Rydberg atoms as sensitive detectors of single photons of light. They won the Nobel Prize for physics for such work in 2011.
However, simply sticking one atom inside a Rydberg atom doth not a Rydberg polaron make. A polaron is a quasiparticle, which means it isn’t an actual particle by itself, as the –on suffix might suggest, but an entity that scientists study as if it were a particle. Quasiparticles are thus useful because they simplify the study of more complicated entities by allowing scientists to apply the rules of particle physics to arrive at equally correct solutions.
This said, a polaron is a quasiparticle that’s also a particle. Specifically, physicists describe the properties and behaviour of electrons inside a solid as polarons because as the electrons interact with the atomic lattice, they behave in a way that electrons usually don’t. So polarons combine the study of electrons and electrons-interacting-with-atoms into a single subject.
Similarly, a Rydberg polaron is formed when the electron inside the Rydberg atom interacts with the trapped strontium atom. While an atom within an atom is cool enough, the January 2018 group wanted to create a Rydberg polaron because it’s considered to be a new state of matter – and they succeeded. The physicists found that the excited electron did develop a loose interaction with the strontium atoms lying between itself and the Rydberg atom’s nucleus – so loose that even as they interacted, the electron could still remain part of the Rydberg atom without getting kicked out.
In effect, since the Rydberg atom and the strontium atoms inside it influence each other’s behaviour, they altogether made up one larger complicated assemblage of protons, neutrons and electrons – a.k.a. a Rydberg polaron.
I watched Spectral, the movie that released on Netflix on December 9, 2016, after Universal Studios got cold feet about releasing it on the big screen – the same place where a previous offering, Warcraft, had been gutted. Spectral is sci-fi and has a few great moments but mostly it’s bland and begging for some tabasco. The premise: an elite group of American soldiers deployed in Moldova come upon some belligerent ghost-like creatures in a city they’re fighting in. They’ve no clue how to stop them, so they fly in an engineer to consult from DARPA, the same guy who built the goggles that detected the creatures in the first place. Together, they do things. Now, I’d like to talk about the science in the film and not the plot itself, though the former feeds the latter.
A scene from the film ‘Spectral’ (2016). Source: Netflix
Towards the middle of the movie, the engineer realises that the ghost-like creatures have the same limitations as – wait for it – a Bose-Einstein condensate (BEC). They can pass through walls but not ceramic or heavy metal (not the music), they rapidly freeze objects in their path, and conventional weapons, typically projectiles of some kind, can’t stop them. Frankly, it’s fabulous that Ian Fried, the film’s writer, thought to use creatures made of BECs as villains.
A BEC is an exotic state of matter in which a group of ultra-cold particles condense into a superfluid (i.e., it flows without viscosity). Once a BEC forms, a subsection of a BEC can’t be removed from it without breaking the whole BEC state down. You’d think this makes the BEC especially fragile – because it’s susceptible to so many ‘liabilities’ – but it’s the exact opposite. In a BEC, the energy required to ‘kick’ a single particle out of its special state is equal to the energy that’s required to ‘kick’ all the particles out, making BECs as a whole that much more durable.
This property is apparently beneficial for the creatures of Spectral, and that’s where the similarity ends because BECs have other properties that are inimical to the portrayal of the creatures. Two immediately came to mind: first, BECs are attainable only at ultra-cold temperatures; and second, the creatures can’t be seen by the naked eye but are revealed by UV light. There’s a third and relevant property but which we’ll come to later: that BECs have to be composed of bosons or bosonic particles.
It’s not clear why Spectral‘s creatures are visible only when exposed to light of a certain kind. Clyne, the DARPA engineer, says in a scene, “If I can turn it inside out, by reversing the polarity of some of the components, I might be able to turn it from a camera [that, he earlier says, is one that “projects the right wavelength of UV light”] into a searchlight. We’ll [then] be able to see them with our own eyes.” However, the documented ability of BECs to slow down light to a great extent (5.7-million times more than lead can, in certain conditions) should make them appear extremely opaque. More specifically, while a BEC can be created that is transparent to a very narrow range of frequencies of electromagnetic radiation, it will stonewall all frequencies outside of this range on the flipside. That the BECs in Spectral are opaque to a single frequency and transparent to all others is weird.
Obviating the need for special filters or torches to be able to see the creatures simplifies Spectral by removing one entire layer of complexity. However, it would remove the need for the DARPA engineer also, who comes up with the hyperspectral camera and, its inside-out version, the “right wavelength of UV” searchlight. Additionally, the complexity serves another purpose. Ahead of the climax, Clyne builds an energy-discharging gun whose plasma-bullets of heat can rip through the BECs (fair enough). This tech is also slightly futuristic. If the sci-fi/futurism of the rest of Spectral leading up to that moment (when he invents the gun) was absent, then the second-half of the movie would’ve become way more sci-fi than the first-half, effectively leaving Spectral split between two genres: sci-fi and wtf. Thus the need for the “right wavelength of UV” condition?
Now, to the third property. Not all particles can be used to make BECs. Its two predictors, Satyendra Nath Bose and Albert Einstein, were working (on paper) with kinds of particles since called bosons. In nature, bosons are force-carriers, acting against matter-maker particles called fermions. A more technical distinction between them is that the behaviour of bosons is explained using Bose-Einstein statistics while the behaviour of fermions is explained using Fermi-Dirac statistics. And only Bose-Einstein statistics predicts the existence of states of matter called condensates, not Femi-Dirac statistics.
(Aside: Clyne, when explaining what BECs are in Spectral, says its predictors are “Nath Bose and Albert Einstein”. Both ‘Nath’ and ‘Bose’ are surnames in India, so “Nath Bose” is both anyone and no one at all. Ugh. Another thing is I’ve never heard anyone refer to S.N. Bose as “Nath Bose”, only ‘Satyendranath Bose’ or, simply, ‘Satyen Bose’. Why do Clyne/Fried stick to “Nath Bose”? Was “Satyendra” too hard to pronounce?)
All particles constitute a certain amount of energy, which under some circumstances can increase or decrease. However, the increments of energy in which this happens are well-defined and fixed (hence the ‘quantum’ of quantum mechanics). So, for an oversimplified example, a particle can be said to occupy energy levels constituting 2, 4 or 6 units but never of 1, 2.5 or 3 units. Now, when a very-low-density collection of bosons is cooled to an ultra-cold temperature (a few hundredths of kelvins or cooler), the bosons increasingly prefer occupying fewer and fewer energy levels. At one point, they will all occupy a single and common level – flouting a fundamental rule that there’s a maximum limit for the number of particles that can be in the same level at once. (In technical parlance, the wavefunctions of all the bosons will merge.)
When this condition is achieved, a BEC will have been formed. And in this condition, even if a new boson is added to the condensate, it will be forced into occupying the same level as every other boson in the condensate. This condition is also out of limits for all fermions – except in very special circumstances, and circumstances whose exceptionalism perhaps makes way for Spectral‘s more fantastic condensate-creatures. We known one such as superconductivity.
In a superconducting material, electrons flow without any resistance whatsoever at very low temperatures. The most widely applied theory of superconductivity interprets this flow as being that of a superfluid, and the ‘sea’ of electrons flowing as such to be a BEC. However, electrons are fermions. To overcome this barrier, Leon Cooper proposed in 1956 that the electrons didn’t form a condensate straight away but that there was an intervening state called a Cooper pair. A Cooper pair is a pair of electrons that had become bound, overcoming their like-charges repulsion because of the vibration of atoms of the superconducting metal surrounding them. The electrons in a Cooper pair also can’t easily quit their embrace because, once they become bound, the total energy they constitute as a pair is lower than the energy that would be destabilising in any other circumstances.
Could Spectral‘s creatures have represented such superconducting states of matter? It’s definitely science fiction because it’s not too far beyond the bounds of what we know about BEC today (at least in terms of a concept). And in being science fiction, Spectral assumes the liberty to make certain leaps of reasoning – one being, for example, how a BEC-creature is able to ram against an M1 Abrams and still not dissipate. Or how a BEC-creature is able to sit on an electric transformer without blowing up. I get that these in fact are the sort of liberties a sci-fi script is indeed allowed to take, so there’s little point harping on them. However, that Clyne figured the creatures ought to be BECs prompted way more disbelief than anything else because BECs are in the here and the now – and they haven’t been known to behave anything like the creatures in Spectral do.
For some, this information might even help decide if a movie is sci-fi or fantasy. To me, it’s sci-fi.
On the more imaginative side of things, Spectral also dwells for a bit on how these creatures might have been created in the first place and how they’re conscious. Any answers to these questions, I’m pretty sure, would be closer to fantasy than to sci-fi. For example, I wonder how the computing capabilities of a very large neural network seen at the end of the movie (not a spoiler, trust me) were available to the creatures wirelessly, or where the power source was that the soldiers were actually after. Spectral does try to skip the whys and hows by having Clyne declare, “I guess science doesn’t have the answer to everything” – but you’re just going “No shit, Sherlock.”
His character is, as this Verge review puts it, exemplarily shallow while the movie never suggests before the climax that science might indeed have all the answers. In fact, the movie as such, throughout its 108 minutes, wasn’t that great for me; it doesn’t ever live up to its billing as a “supernatural Black Hawk Down“. You think about BHD and you remember it being so emotional – Spectral has none of that. It was just obviously more fun to think about the implications of its antagonists being modelled after a phenomenon I’ve often read/written about but never thought about that way.
Over November 2015, physicists and commentators alike the world over marked 100 years since the conception of the theory of relativity, which gave us everything from GPS to blackholes, and described the machinations of the universe at the largest scales. Despite many struggles by the greatest scientists of our times, the theory of relativity remains incompatible with quantum mechanics, the rules that describe the universe at its smallest, to this day. Yet it persists as our best description of the grand opera of the cosmos.
Incidentally, Einstein wasn’t a fan of quantum mechanics because of its occasional tendencies to violate the principles of locality and causality. Such violations resulted in what he called “spooky action at a distance”, where particles behaved as if they could communicate with each other faster than the speed of light would have it. It was weirdness the likes of which his conception of gravitation and space-time didn’t have room for.
As it happens, 2015 also marks another milestone, also involving Einstein’s work – as well as the work of an Indian scientist: Satyendra Nath Bose. It’s been 20 years since physicists realised the first Bose-Einstein condensate, which has proved to be an exceptional as well as quirky testbed for scientists probing the strange implications of a quantum mechanical reality.
Its significance today can be understood in terms of three ‘periods’ of research that contributed to it: 1925 onward, 1975 onward, and 1995 onward.
