How does 2D quantum superfluid feel to touch?

Nobody has been able to answer this question during the 100-year history of quantum physics.

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The bulk superfluid 3He comprises triplet-paired Cooper pairs and exists in the B phase at the lowest temperatures. The pure superfluid regime, where the thermal quasiparticle density is minimal, can be reached by cooling the B phase of superfluid 3He. 

At the container’s edges, a quantum well encloses the bulk superfluid, keeping a sea of quasiparticles with energies lower than those of the bulk inside. These states can be produced in a non-equilibrium distribution inside the quantum well, allowing us to witness the dynamics of their mobility indirectly.

A new study by the Lancaster University scientists shows how the superfluid helium 3He would feel if you could put your hand into it. They show that the induced quasiparticle currents flow diffusively in the two-dimensional system.

Dr Samuli Autti, the lead author of the research, said, “Nobody has been able to answer this question during the 100-year history of quantum physics. We now show that, at least in superfluid 3He, this question can be answered.”

The studies used a mechanical resonator the size of a finger to probe the extremely cold superfluid while being conducted in a customized refrigerator at a temperature roughly 10000th of a degree above absolute zero.

Superfluid 3He transfers the heat it generates along the container’s surfaces when agitated with a rod. The superfluid’s bulk exhibits vacuum-like behavior and complete passivity.

Dr Autti said: “This liquid would feel two-dimensional if you could stick your finger into it. The bulk of the superfluid feels empty, while heat flows in a two-dimensional subsystem along the edges of the bulk – in other words, along your finger.”

The bulk of superfluid 3He, the scientists conclude, is encircled by an independent two-dimensional superfluid that interacts with mechanical probes rather than the bulk superfluid itself; the bulk superfluid is only accessible to the independent superfluid in the event of an abrupt energy burst.

In other words, superfluid 3He is thermo-mechanically two-dimensional at the lowest temperatures and applied energies.

Dr Autti said“This also redefines our understanding of superfluid 3He. For the scientist, that may be even more influential than hands-in quantum physics.”

“Our work shows that this two-dimensional quantum condensate and the dynamics of the surface bound states are experimentally accessible, opening the possibility of engineering two-dimensional quantum condensates of arbitrary topology.”

Journal Reference:

  1. Autti, S., Haley, R.P., Jennings, A. et al. Transport of bound quasiparticle states in a two-dimensional boundary superfluid. Nat Commun 14, 6819 (2023). DOI: 10.1038/s41467-023-42520-y