Pushing the boundaries of superfluid vacuum and coherence

Macroscopic quantum systems such as superfluids, superconductors, and atomic gas condensates bring quantum physics to scales observable by the naked eye. These quantum-coherent phenomena originated in the laboratory but are now either already used for commercial applications (for example superconductors) or being actively developed with technological applications in mind (for example Bose condensates). At the same time the edges of our knowledge about these systems keep being extended, revealing unprecedented phenomena: a good example is the recent discovery of time crystals that bend the categorical impossibility of perpetual motion machines.

The proposed research programme will explore the edges of macroscopic quantum order in superfluid 3He. Superfluid 3He is a macroscopic quantum system with extremely rich phenomenology, touching seemingly distant fields such as high-energy physics and cosmology. The most famous example is the Higgs mechanism, which was originally discovered in a superconductor (-fluid) system and later become a part of the Standard Model of particle physics. Another example is the Kibble-Zurek mechanism, originally a cosmological speculation, which was discovered in superfluid 3He and now forms a cornerstone of modern laboratory physics.

The right way to understand low-temperature superfluid 3He from a mechanical perspective is to think about a vacuum where a rod can be moved around as if the superfluid is not there in the first place. Only if the probing exceeds an intrinsic threshold of the vacuum, such as a minimum size set by the Cooper pair size or a maximum velocity set by the superfluid energy gap, will the quantum nature of the vacuum be revealed. This means that the vacuum ceases to be a background and starts interacting with the probe. For example, a probe that is small enough will reveal the intrinsic structure of the vacuum which is hidden from large probes. An unexpected corollary of the intrinsic structure is that the surfaces of the superfluid form a two-dimensional system nearly detached from the three-dimensional bulk: move a rod near the surface and any energy released will be stuck to the surface.

The magnetic properties of the superfluid are largely determined by the dynamics of magnetic particles emerging from the bulk vacuum. These particles can form a time crystal, a dynamic phase of matter in permanent repeating motion. Other "time phases" such as disordered time liquids can be created with a similar approach and explained by harnessing the toolbox of equilibrium physics to explore dynamic systems.

This fellowship will explore the quantum vacuum mechanically and magnetically:

1. I will lead the exploration of the surface-bound fermions by carrying out a series of transport experiments in the few hundred nanometre thick surface layer of superfluid 3He. In practice this means heating the surface layer at one point and observing how the heat flows along the surface by measuring temperature at another point on the surface. The technology commissioned for this project will also allow revealing the superfluid vacuum's intrinsic structure by moving a tiny rod in the bulk of the superfluid where the interaction with the vacuum dramatically changes at scales smaller than Cooper pair radius.

1. My team will create a new bosonic phase of matter which spontaneously becomes incoherent - a time liquid - by melting a quantum time crystal. The melting process is initiated by increasing the particle density. Mapping the phase diagram of the "time phases" in the superfluid vacuum will cement this new field of study.

This project is backed by leading technical, experimental and theoretical collaborators in the Host Institution and internationally. Discoveries delivered by this fellowship will lead new fields of research with academic and technological implications spanning from two-dimensional physics of bound fermions to magnon-based room temperature quantum devices.