Well-Tempered Collective
Magnetic oscillations in a solid already form a quantum mechanical unit at room temperature. Experts are not yet in complete agreement as to whether it is a Bose-Einstein condensate or a special form of a laser. A good eighty years ago, Satyendra Nath Bose and Albert Einstein predicted an extremely unusual state of special matter. According to their calculations, certain particles, which belong to the class of bosons named after Bose, should lose their fear of each other under certain – usually very extreme – circumstances and coalesce into an indistinguishable clump. This could then be described as a single object according to the rules of quantum mechanics, which were still very new at the time. Almost 70 years later, in 1995, two working groups led by the physicists Eric Cornell, Wolfgang Ketterle and Carl Wieman succeeded almost simultaneously in producing such a Bose-Einstein condensate for the first time from a gas of rubidium or sodium atoms, for which they received the Nobel Prize in Physics in 2001.
The bosons are also one of two universal types of particles that make up all matter - and according to Albert Einstein, all energy as well. If they are not at rest, they rotate like a spinning top, but always according to a very special pattern: their angular momentum – or spin – is always exactly an integer multiple of the quantum of action introduced into modern physics by Max Planck in 1900, the discovery of which led to the development of quantum physics. The other class of particles, on the other hand, are called fermions – named after the Italian physicist Enrico Fermi. These always seem to rotate with a half-integer multiple of the natural constant found by Planck, although they can form pairs and then also act like bosons.
Why these two fundamentally different species exist in the universe is still one of the greatest mysteries in the world. The fact is, the particles behave very differently in many respects. So fermions behave more like billiard balls or oranges: where one is, there is no other place. Bosons, on the other hand, are more like the droplets of a liquid: if you put several together, they usually get along and form a larger puddle. Something similar happens with the Bose-Einstein conglomerates, which is why the wording of the condensate makes sense.
Until now, experimenters have only been able to produce this novel state of matter under extreme conditions. They had to cool down their samples, which usually consist of several hundred thousand atoms, to an extremely high degree. Temperatures a few thousandths of a Kelvin above absolute zero were usually a must. A working group led by Sergej Demokritov from the University of Münster has now shown that there is another way.
The starting point for their considerations was the realization that such a state of matter could also be achieved by increasing the density of a gas made up of bosons. Instead of experimenting with atoms or other real particles, Demokritov's team investigated so-called magnons - quasiparticles which, according to Albert Einstein's concept of the equivalence of mass and energy, carry the energy of magnetic oscillations in quantized form. They therefore occur in magnetic substances.
For their experiments, Demokritov and his colleagues used a wafer-thin, (ferri)magnetic compound made of yttrium, iron and oxygen with the chemical name YIG. It is quite sensitive to microwave radiation. They placed this connection on a metal strip through which they sent the microwaves. In this way, together with an externally applied magnetic field, the scientists stimulated magnetic oscillations in their material and thus generated additional magnons. So they increased their density in the YIG, or to put it another way: They "pumped" magnons into their sample. The scientists measured the success of their efforts by scattering a laser beam at the YIG, which the working group aimed at the material and whose characteristically scattered light they finally analyzed.
Up to a certain pump power of the microwave resonator, the results of the scattering tests resembled the expected, natural behavior. With higher pump power, the researchers could only explain their results by the fact that some magnons in their sample had formed the desired Bose-Einstein condensate [1].
This result is particularly important because the researchers operated their experimental setup at room temperature. There was no cooling down to ultra-cold temperatures at all. According to the physicists, this is possible because the quasiparticles considered here have a relatively low mass, which drastically increases their tendency to be complicit.
Even if it is still disputed in the professional world whether one can rightly call this proven state a Bose-Einstein condensate, since, strictly speaking, for example, the number of magnons is not conserved due to the subsequent supply of quasiparticles, this seems to open up new experimental methods. For example, David Snoke from the University of Pittsburgh thinks it would be more appropriate to compare the state found by Demokritov's working group with laser light, whose quasiparticles - the photons - also assume a so-called coherent - i.e. uniform - state. In this respect, however, Snoke also believes that there are some interesting research approaches. He is thinking of new types of lasers that, because of the lightness of the quasi-particles, also save a lot of energy and do not work with light, but with these different kinds of vibrations.
A similar view is held by BenoÎt Deveaud-Plédran from the Technical University of Lausanne, who together with his working group now also wants to have produced a Bose-Einstein condensate in a solid [2]. He used so-called polaritons, the quanta of lattice vibrations in a crystal made of ions. However, he still had to cool his sample – a semiconductor compound made up of the chemical elements cadmium, tellurium and magnesium – to 19 Kelvin.
The Swiss physicist can imagine, for example, that Bose-Einstein condensates held in solids will one day take over information processing in so-called quantum computers. In the future, these will not only calculate in binary with the classic zeros or ones, but also with intermediate values, which will make them significantly more powerful. "In the middle of the 20th century, transistors replaced vacuum tubes," says Deveaud-Plédran. "Now most of the most useful devices are made of solids." And in fifty years it might be as easy to deal with polaritons or even magnons as it is with electrons today.
