Direct detection of Majorana surface states in superfluid 3He-B
In plain words
Theory says the B phase of superfluid helium-3 is topological, so its surfaces should carry Majorana states (particles that are their own antiparticles). Experiments see surface states, but none has shown the unusual magnetic response that would identify them as Majorana.
Precise statement
3He-B is a time-reversal-invariant topological superfluid (class DIII, winding number 1) with a gapless Majorana cone of surface Andreev bound states. Measure a Majorana-specific property, namely the predicted Ising-like spin response in which a magnetic field normal to the surface gaps the cone while an in-plane field does not, e.g. by surface NMR or transverse acoustic impedance in a slab with specular walls. An answer is observation or exclusion of the anisotropic surface spin response.
What would settle it
Field-orientation-dependent measurement of the surface density of states showing gap opening only for the surface-normal field.
Status in the literature
Unverified note
Ion-mobility experiments below the free surface were interpreted as Majorana surface bound states (Ikegami and Kono, review 2019), but the field-orientation-dependent Ising spin response that would identify them as Majorana had not been demonstrated as of 2025.