"Quantum matter is everywhere, from the interiors of neutron stars to the electrons in everyday metals. Like ordinary, classical matter, it is made up of many interacting particles. In classical matter, however, it is possible to think of each particle as an individual entity, whereas in quantum matter Heisenberg’s uncertainty principle forbids us from telling individual particles apart: their behaviour can only be described collectively. In spite of this, many types of quantum matter are quite well understood from a theoretical point of view. For example, the “electron liquid” that is responsible for the flow of electricity through ordinary metals, the magnetic properties of many insulating materials and the normal and superfluid phases of helium at very low temperatures have all succumbed to the probing of theorists.
But the behaviour of some forms of quantum matter has proved a much harder nut to crack. High-temperature superconductors, for example, are not really understood despite more than two decades of research since they were first discovered. Also mysterious are various exotic types of magnet; while the electrical resistance of most metals increases with the square of their temperature, T, for some magnetic metals like manganese silicon the resistance is proportional to T1.5. And then there is the quark—gluon plasma, which occurs when neutrons are pressed together so tightly that their quarks lose their identity and form a single homogenous liquid. Such a plasma is believed to have formed during the first few microseconds after the Big Bang, but has also recently been recreated in the Relativistic Heavy Ion Collider (RHIC) at the Brookhaven National Laboratory in the US, with further experiments planned at CERN’s Large Hadron Collider.
All these forms of quantum matter have one thing in common: very strong — rather than weak — correlations between the particles from which they are composed. Materials with weak correlations are relatively easy to understand: as the component particles barely interact with each other, one can extrapolate the behaviour of non-interacting particles (like those in an ideal gas) to get a good insight into how they behave en masse. Strong correlations, however, lead to qualitatively new behaviour. High-temperature superconductors, for example, display not only an unconventional superconducting phase but also mysterious “bad metal” and “pseudogap” behaviours."