We review current understanding of the non-equilibrium dynamics of collective quantum systems. We describe an approach based on the Hamiltonian formulation of superfluid hydrodynamics. It is shown that, in the presence of constant energy pumping, the nonlinear coupling of fluctuations in the density and entropy strongly affects the nonequilibrium dynamics of the system. We use the results obtained to analyze the properties of out-of-equilibrium superfluid 4He and of exciton polariton Bose-Einstein condensates, both in semiconductor quantum wells and in graphene layers in presence of high magnetic field.
This article provides an overview, primarily from an experimental perspective, of recent progress and future prospects in using helium to realize a range of quantum materials of generic interest, by "top-down" and "bottom-up" nanotechnology. We can grow model systems to realise new quantum states of matter, and explore key issues in condensed matter physics. In the language of cold atomic gases, two dimensional and confined 3He and 4He provide "quantum simulators", with the potential to uncover new emergent quantum states. These include: strictly 2D Fermi system with Mott-Hubbard transition; i…
Helium is recognized as a model system for the study of phase transitions. Of particular interest is the superfluid phase in two dimensions. We report measurements on superfluid helium films adsorbed on the surface of a suspended carbon nanotube. We measure the mechanical vibrations of the nanotube to probe the adsorbed helium film. We demonstrate the formation of helium layers up to five atoms thickness. Upon increasing the vapour pressure, we observe layer-by-layer growth with discontinuities in both the number of adsorbed atoms and the speed of sound in the adsorbed film. These hitherto uno…
Electrons floating on the surface of superfluid helium have been suggested as promising mobile spin qubits. Three micron wide channels fabricated with standard silicon processing are filled with superfluid helium by capillary action. Photoemitted electrons are held by voltages applied to underlying gates. The gates are connected as a 3-phase charge-coupled device (CCD). Starting with approximately one electron per channel, no detectable transfer errors occur while clocking $10^9$ pixels. One channel with its associated gates is perpendicular to the other 120, providing a CCD which can transfer…
Superfluid $^4$He (He-II) is a widely studied model system for exploring finite-size effects in strongly confined geometries. Here, we study He-II confined in mm-scale channels of 25 and 50 nm height at high pressures using a nanofluidic Helmholtz resonator. We find that the superfluid density is measurably suppressed in the confined geometry from the transition temperature down to 0.6 K. Importantly, this suppression can be accounted for by roton-like thermal excitation with an energy gap of 5 K. We show that the surface-bound excitations lead to the previously unexplained lack of finite-size…
The quasi-one-dimensional system of surface electrons over superfluid helium with negative charge on substrate is investigated. The temperature dependence of conductivity is ladder-like which is observed at temperature lower 1.3 K. The parameters of ladder have been different in different experiments. The observed dependence isn't explained by classical models of carry with account to the usual scatters in channel. It is supposed the dependence have been connected with quantum character of electron transport in narrow channels.
We have studied the appearance of vortex arrays in a rotating helium-4 nanodroplet at zero temperature within density functional theory. Our results are compared with those for classical rotating fluid drops used to analyze the shape and vorticity in recent experiments [L.F. Gomez et al., Science 345, 906 (2014)], where vortices have been directly seen in superfluid droplets for the first time. In agreement with the experiments, we have found that the shape of the droplet changes from pseudo-spheroid, oblate-like for a small number of vortices to a peculiar "wheel-like" shape, delimited by nea…
We have studied spinning superfluid $^4$He nanodroplets at zero temperature using Density Functional theory. Due to the irrotational character of the superfluid flow, the shapes of the spinning nanodroplets are very different from those of a viscous normal fluid drop in steady rotation. We show that when vortices are nucleated inside the superfluid droplets, their morphology, which evolves from axisymmetric oblate to triaxial prolate to two-lobed shapes, is in good agreement with experiments. The presence of vortex arrays confers to the superfluid droplets the rigid-body behavior of a normal f…
We predict a new quantum electronic structure at the interface between two condensed phases of noble-gas elements: solid neon and superfluid helium. An excess electron injected onto this interface self-confines its wavefunction into a nanometric dome structure whose size varies with pressure. A collection of such electrons can form a classical Wigner crystal visualizable by mid-infrared photons. The ultralong spin-coherence time allows the electrons in this system to serve as perfect quantum bits. They can be deterministically arranged on-chip at a spacing of several microns. Their spin states…
Electrons floating on the surface of superfluid helium have been suggested as promising mobile spin quantum bits (qubits). Transferring electrons extremely efficiently in a narrow channel structure with underlying gates has been demonstrated, showing no transfer error while clocking $10^9$ pixels in a 3-phase charge coupled device (CCD). While on average, one electron per channel was clocked, it is desirable to reliably obtain a single electron per channel. We have designed an electron turnstile consisting of a narrow (0.8$μ$m) channel and narrow underlying gates (0.5$μ$m) operating across sev…
The flow of liquid helium through a single nanohole with radius smaller than 25 nm was studied for the first time. Mass flow was induced by applying a pressure difference of up to 1.4 bar across a 50 nm thick Si3N4 membrane and was measured directly by means of mass spectrometry. In liquid He I, we experimentally show that the fluid is not clamped by the short pipe with diameter-to-length ratio D/L~1, despite the small diameter of the nanohole. This viscous flow is quantitatively understood by making use of a model of flow in short pipes. In liquid He II, a two-fluid model for mass flow is use…
Recent experiments on the flow of helium-4 fluid through nanopores with tunable pore radius provide a platform for studying the quasi-one-dimensional (quasi-1D) superfluid behaviors. In the extreme 1D limit, the helium atoms are localized by disordered small variations in the substrate potential provided by the pore walls. In the limit of wide pore radius, a solid layer of helium-4 is expected to coat the pore walls smoothing out the substrate potential, and superfluidity is observed in the central region. Building on earlier quantum Monte Carlo results, we propose a scenario for this crossove…
Photonic nanocavities are a key component in many applications because of their capability of trapping and storing photons and enhancing interactions of light with various functional materials and structures. The maximal number of photons that can be stored in silicon photonic cavities is limited by the free-carrier and thermo-optic effects at room temperature. To reduce such effects, we performed the first experimental study of optical nonlinearities in ultrahigh-Q silicon disk nanocavities at cryogenic temperatures in a superfluid helium environment. At elevated input power, the cavity trans…
The quantized lateral motional states and the spin states of electrons trapped on the surface of superfluid helium have been proposed as basic building blocks of a scalable quantum computer. Circuit quantum electrodynamics (cQED) allows strong dipole coupling between electrons and a high-Q superconducting microwave resonator, enabling such sensitive detection and manipulation of electron degrees of freedom. Here we present the first realization of a hybrid circuit in which a large number of electrons are trapped on the surface of superfluid helium inside a coplanar waveguide resonator. The hig…
The size-dependent and flux-dependent effective thermal conductivity of narrow capillaries filled with superfluid helium is analyzed from a thermodynamic continuum perspective. The classical Landau evaluation of the effective thermal conductivity of quiescent superfluid, or the Gorter-Mellinck regime of turbulent superfluids, are extended to describe the transition to ballistic regime in narrow channels wherein the radius $R$ is comparable to (or smaller than) the phonon mean-free path $\ell$ in superfluid helium. To do so we start from an extended equation for the heat flux incorporating non-…
The image-charge detection provides a new direct method for the detection of the Rydberg transition in electrons trapped on the surface of liquid helium. The interest in this method is motivated by the possibility to accomplish the spin state readout for a single trapped electron, thus opening a new pathway towards using electron spins on liquid helium for quantum computing. Here, we report on the image-charge detection of the Rydberg transition in a many-electron system confined in an array of 20-um wide and 4-um deep channels filled with superfluid helium. Such detection is made possible bec…
We present an experimental study of a zero-phonon line (ZPL) in the absorption spectrum of Dy atoms solvated in superfluid He. The dopants reside in nanometer-sized spherical cavities known as atomic bubbles. We observe a temperature-dependent broadening of ZPL in the absorption spectrum. The effect is attributed to the scattering of thermal phonons on the atomic bubble that leads to the dephasing of the Dy transition dipole. The extrapolated ZPL intrinsic spectral width at zero temperature is 2300 times larger than the natural linewidth in a free atom. This can be assigned to a fast radiation…
Electrons on helium form a unique two-dimensional electron system on the interface of liquid helium and vacuum. On liquid helium, trapped electrons can arrange into strongly correlated states known as Wigner molecules, which can be used to study electron interactions in the absence of disorder, or as a highly promising resource for quantum computation. Wigner molecules have orbital frequencies in the microwave regime and can therefore be integrated with circuit quantum electrodynamics (cQED), which studies light-matter interactions using microwave photons. Here, we experimentally realize a cQE…
During the last decade, density function theory (DFT) in its static and dynamic time dependent forms, has emerged as a powerful tool to describe the structure and dynamics of doped liquid helium and droplets. In this review, we summarize the activity carried out in this field within the DFT framework since the publication of the previous review article on this subject [M. Barranco et al., J. Low Temp. Phys. 142, 1 (2006)]. Furthermore, a comprehensive presentation of the actual implementations of helium DFT is given, which have not been discussed in the individual articles or are scattered in …
