We report our progress in developing a Superfluid Helium Quantum Interference Device (SHeQuID) for the research of weak superfluidity phenomena in superfluid 4He.
Similar to more widely used superconducting SQUID, helium SQUIDs are devices that function based on the Josephson effect – relations between the phase gradient of the wave function, describing the system and macroscopic quantities such as (mass) current and chemical potential difference [1]. For a helium SQUID to work, one must have two containers of weakly coupled superfluid helium. Practically, the weak link between these two superfluid bodies is realised by a partition with small holes of the size of the coherence length of superfluid helium. A specialized nanofabrication recipe for creating such junctions on a SiN chip was developed during the project.
Detection of the Josephson oscillations in our system is realised by the so-called Paik-type detector [2]. For this, a Nb-coated kapton diaphragm is positioned near the junctions. Deflection of the membrane due to the oscillatory flow then modulates the induction of a pancake coil positioned in its proximity. A superconducting circuit is then used to transmit this change of inductance in the form of induced superconducting current to another coil coupled to a dc SQUID. A SQUID-based readout scheme like this is well-suited for extremely sensitive reading of the diaphragm deflection. It must be noted that using a SQUID also creates the need for proper magnetic shielding.
In order to reach the sensitivity needed, it is also crucial to shield the device from external disturbances. Therefore, a specialised platform was built consisting of a vibration suppression system in the form of a spring suspension to be used in a stainless steel cryostat. Other precautions were made to achieve precise temperature stabilization inside the cryostat, which was needed due to the strong temperature dependence of the coherence length.
 After perfecting all technical aspects, this apparatus will enable studying not only the Josephson oscillations, but also a wide range of other effects, such as phase slips [3], occurring on the transition from a weakly to strongly coupled superfluid system.     

[1] Y. Sato, and R. E. Packard; Superfluid helium quantum interference devices: physics and applications, Rep. Prog. Phys. 75 (2012) 016401
[2] H. J. Paik; Superconducting tunable?diaphragm transducer for sensitive acceleration measurements, J. Appl. Phys. 47, 1168–1178 (1976)
[3] E. Varoquaux, O. Avenel; Phase slip phenomena in superfluid helium, Physica B 197 (1994) 306-314
 

Keywords
Superfluid Helium|Josephson effect|SQUID|weak link