If cosmic strings have an irreducible core, then dark matter may consist of entities that are core-sized string remnants. As such they may interact solely by gravity. These talons may be many orders of magnitude more massive than any known fundamental particle. Due to interacting solely via gravity it would seem almost impossible to detect talons in any laboratory search.

Two reservoirs of superfluid helium can be weakly coupled together to form a neutral matter analog of superconducting Josephson junctions. In this chapter, we present an overview of the development of the field of Josephson effects in superfluid helium, reviewing both the experimental and theoretical milestones that led to our current understanding. We discuss the physics of these systems with emphasis on the quantum oscillations that result from varying the coupling strength. We review the principles of superfluid helium quantum interference devices (SHeQUID) and some novel fundamental applications of this new technology.

By drawing an analogy with superfluid 4He vortices we suggest that dark matter may consist of irreducibly small remnants of cosmic strings.

We describe recent progress in developing a superfluid helium analog of the superconducting dc-SQUID. The devices tested thus far are sensitive detectors of rotation as well as useful probes for studies of superfluidity. The key ingredients of the superfluid helium quantum interference device (SHeQUID) involve commercially available technology and modest cryogenic facilities.

We report the characteristics of a flux locked, superfluid 4He interferometer that can continuously measure time-varying rotation rates. We describe the principles underlying the interferometer, including the dynamics of a superfluid chemical potential battery used to obtain continuous operation. We also discuss noise and drift issues and their possible amelioration.

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.

Emerging devices for measuring quantum phase offer a possible new window into phenomena far outside condensed-matter physics.

S uperfluid helium is a macroscopic quantum system, as are superconductors, gaseous Bose–Einstein condensates, and the interiors of neutron stars. The common feature of those disparate systems is a complex “order parameter” having magnitude and phase. The order parameter can be a Schrödinger-like wavefunction, or it can be some other function that reflects the system’s physical state. A macroscopic quantum state emerges in a sample of matter when the particles’ thermal de Broglie wavelength approaches the interparticle spacing. The particles then lose their individual identities and merge into a smoothed cloud that behaves as a single correlated quantum state. Matter in such a state differs markedly from a classical collection of distinguishable pointlike particles. For example, the unique quantum properties of superfluid He include zero viscosity, which allows it to flow around a torus indefinitely. Although superfluid He has been known and studied since 1938, there is still no complete microscopic theory that predicts the transition between normal liquid 4He at 2.18 K and the superfluid state at lower temperatures. However, two phenomenological theories explain almost all experiments to date.1 The first model, due to Lev Landau, is essentially thermodynamic. It describes the superfluid as a mixture of two interpenetrating components, one normal and one super, each with its own density (ρn and ρs for the normal and super components, respectively) and its own velocity field (vn and vs). The fluid’s total density is the sum of the densities of the two components, and each velocity field is determined independently by its own hydrodynamics. (The motion of the normal component is governed by the Navier–Stokes equation for viscous flow, and the super component in the absence of a temperature gradient is described by the Euler equation for an ideal inviscid fluid.) A complementary view, provided by Fritz London, Lars Onsager, and Richard Feynman, treats the superfluid as a macroscopic quantum state

We show through numerical simulations that absolute quantum mechanical phase differences could be observed using an asymmetric superfluid quantum interference grating. By balancing the dynamic range and the degree of change required per period in an interference pattern, a device could be optimized and used to probe heretofore inaccessible quantum subtleties. We make connections to experimental results and discuss possible applications for such systems.

We present an overview of recent developments related to superfluid helium quantum interference devices (SHeQUIDs). We discuss the physics of two reservoirs of superfluid helium coupled together and describe the quantum oscillations that result from varying the coupling strength. We explain the principles behind SHeQUIDs that can be built based on these oscillations and review some techniques and applications.

A new displacement sensor that uses a rare-earth magnet attached to a flexible diaphragm is demonstrated for superfluid experiments. Its construction, calibration, and performance are described.

We describe an experiment in which we induce a heat-driven superfluid flow in a straight tube and monitor the phase difference across the tube's ends with a superfluid 4He quantum interference device (SHeQUID). We quantitatively verify the relation ?s = (/m) ? . We also demonstrate the linearization of a SHeQUID using the heat injection method.

We describe an experiment in which we induce a heat-driven superfluid flow in a straight tube and monitor the phase difference across the tube's ends with a superfluid 4He quantum interference device (SHeQUID). We quantitatively verify the relation ?s = (/m) ? . We also demonstrate the linearization of a SHeQUID using the heat injection method.

We describe an experiment in which we induce a heat-driven superfluid flow in a straight tube and monitor the phase difference across the tube's ends with a superfluid He-4 quantum interference device (SHeQUID). We quantitatively verify the relation upsilon(s) = ((h) over bar /m) del phi. We also demonstrate the linearization of a SHeQUID using the heat injection method

It has been predicted that, in the presence of combined radial electric field and axial magnetic field, superfluid 4He in a torus will have a persistent current in its ground state. This surprising result arises from non-cancellation of the Aharonov-Bohm phase shifts associated with the opposite charges in the induced electric dipole moment of the neutral 4He atoms. We briefly review this prediction and describe our proposed experiment. In this feasibility study we show that by applying laboratory accessible electric and magnetic fields, a superfluid 4He interferometer (SHeQUID) will have sufficient sensitivity to conclusively determine whether or not the predicted physical phenomenon exists.