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The spectacular properties of liquid helium at low temperature are generally accepted as the signature of the bosonic nature of this system. Particularly the superfluid phase is identified with a Bose–Einstein condensed fluid. However, the relationship between the superfluidity and the Bose–Einstein condensation is still largely unknown. Studying a perturbed liquid ⁴He system would provide information on the relationship between the two phenomena. Liquid ⁴He confined in porous media provides an excellent example of a boson system submitted to disorder and finite-size effects.

Much care should be paid to the sample preparation, particularly the confining condition should be defined quantitatively. To achieve homogeneous confinement conditions, firstly a suitable porous sample should be selected, the experiments should then be conducted at a lower pressure than the saturated vapor pressure of bulk helium.

Several interesting effects have been shown in confined ⁴He samples prepared as described above. Particularly we report the observation of the separation of the superfluid-normal fluid transition temperature, T_c, from the temperature at which the Bose–Einstein condensation is believed to start, T_BEC, the existence of metastable densities for the confined liquid accessible to the bulk system as a short-lived metastable state only and strong clues for a finite lifetime of the elementary excitations at temperatures as low as 0.4 K.

We show that the suppression of light scattering off a Bose Einstein Condensate is equivalent to the Landau argument for superfluidity and thus is a consequence of the Principle of Superfluidity. The superfluid ground state of a BEC contains nonseparable, nontrivial correlations between the bosons that make up the system, i.e., it is entangled. The correlations in the ground state entangle the bosons into a coherent state for the lowest energy state. The entanglement is so extreme that the bosons that make up the system cannot be excited at long wavenumbers. Their existence at low energies is impossible. Only quantum sound can be excited, i.e. the excitations are Bogolyubov quasiparticles which do not resemble free bosons whatsoever at low energies. This means that the system is superfluid by the Landau argument and the superfluidity is ultimately the reason for suppressed scattering at low wavelengths.

We review a natural universal theoretical framework of superfluidity and its analogues, superconductivity and supersolidity. This framework has the potential to describe all physical properties of superfluids to a high level of satisfaction. A unified description of both the "intrinsic" and "extrinsic" critical velocity of liquid ⁴He is suggested. A recent clarification of the so-call pseudogap phase of a superconductor based on this framework is introduced. This paper reflects the current author's views of superfluidity, rather than being an attempt to review on a vast literature on this subject.

The emergence of quantum consciousness stems from dynamic flows of hydrogen ions in brain liquid. This liquid contains vast areas of the fourth phase of water with hexagonal packing of its molecules, the so-called exclusion zone (EZ) of water. The hydrogen ion motion on such hexagonal lattices shows as the hopping of the ions forward and the holes (vacant places) backward, caused by the Grotthuss mechanism. By supporting this motion using external infrasound sources, one may achieve the appearance of the superfluid state of the EZ water. Flows of the hydrogen ions are described by the modified Navier–Stokes equation. It, along with the continuity equation, yields the nonlinear Schrödinger equation, which describes the quantum effects of these flows, such as the tunneling at long distances or the interference on gap junctions.

Recently, there are several experiments demonstrating the possibility to tune the interaction constants using biexcitonic Feshbach resonance in resonantly created polariton condensate and single quantum well. Motivated by these experiments, we theoretically study the stationary state of a polariton condensate whose interatomic scattering length is periodically modulated with optical Feshbach resonance, which represents a novel kind of non-equilibrium superfluidity. In more detail, the spontaneous symmetry breaking of the spin degree of freedom induced by different loss rates of the linear polarizations are investigated based on driven-dissipative Gross–Pitaevskii equations coupled to the rate equation of a reservoir.