Superfluid helium quantum interference devices: present status and future prospects

The development of superfluid weak links has led both to the discovery of new physical phenomena and also to the development of superfluid helium quantum interference devices (SHeQUIDs). We describe the physics underlying the SHeQUID and present a brief overview of the current state of this promising technology. Superfluid 4He and 3He are described by the Landau two fluid model [1] augmented by a macroscopic wavefunction Ψ = √ ρse iφ, where ρs is the superfluid density and φ is the quantum mechanical phase. When two such reservoirs are coupled weakly, various fascinating phenomena emerge. The superfluid mass current I across the weak link is observed to be proportional to the sine of the quantum phase difference across the junction [2, 3, 4]: