Despite superfluids being irrotational, they allow for the existence of vortices. However, each vortex line can only have a discrete quantised value of circulation. Thus, all vortices in the superfluid are the same. The quantum vortices can tangle around each other to form a complex fluctuating chaotic flow field known as “quantum turbulence”. Of particular interest is what governs the nucleation and evolution of vortices and how quantum turbulence can decay in the absence of viscosity.
Turbulence plays a large role in our everyday experiences and it can be observed over a massive range of length scales from the molecular to the cosmological. The study of quantum turbulence should eventually allow us to better understand turbulence in general. This is a great challenge, and any success in this will have far-reaching implications for science and technology since turbulence plays a central role in a very wide range of systems. Its effects are easily observable in the atmosphere, in the oceans, and in water running from a tap.
Despite the substantial research effort on this topic, turbulence is notoriously difficult to characterise. Richard Feynman once said that “Turbulence is the last great unsolved problem of classical physics”. The main difficulties in understanding turbulence arise from its enormous complexity. This complexity was expressed perfectly in 1932 when, during a speech to the British Association for the Advancement of Science, Horace Lamb quipped, “I am an old man now, and when I die and go to heaven there are two matters on which I hope for enlightenment. One is quantum electrodynamics and the other is the turbulent motion of fluids. And about the former, I am rather optimistic.”
