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## 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.