Tag: Wolfgang Ketterle

A giant leap closer to the continuous atom laser

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.

Where is the coolest lab in the universe?

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.

When cooling down really means slowing down

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.”

Relativity’s kin, the Bose-Einstein condensate, is 90 now

Excerpt:

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.

Read the full piece here.