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Quantum hydrodynamics

Quantum hydrodynamics (QHD) is a theoretical framework that extends classical fluid dynamics to describe the collective behavior of quantum many-body systems, such as fluids and plasmas, by incorporating quantum mechanical effects like wave-particle duality, degeneracy pressure, and the Bohm potential.^[1] Originating from Erwin Madelung's 1927 reformulation of the Schrödinger equation in hydrodynamic form, QHD treats quantum particles as a fluid with density and velocity fields, enabling the study of phenomena where quantum coherence plays a dominant role.^[2] Key concepts in QHD include the quantum degeneracy parameter, which quantifies the importance of quantum statistics when the thermal de Broglie wavelength approaches the mean interparticle distance, and the quantum coupling parameter, which measures electron-electron interactions relative to the Bohr radius.^[3] The framework derives fluid equations—such as continuity and momentum conservation—from moments of the quantum kinetic equation or the Madelung equations, often augmented with Fermi-Dirac pressure for degenerate electrons and a non-local quantum potential to account for diffraction.^[4] While valid primarily for weakly coupled, non-relativistic systems at low temperatures and length scales larger than the Thomas-Fermi screening length, QHD has limitations in capturing strong correlations, finite-temperature effects, and nonlinear regimes without additional corrections from density functional theory or kinetic approaches.^[3] Applications of QHD span condensed matter physics, astrophysics, and nanotechnology, including the modeling of superfluid helium and Bose-Einstein condensates where quantized vortices and quantum turbulence emerge.^[5] In dense quantum plasmas, it addresses electron dynamics in metals, semiconductors, and plasmonic nanostructures, as well as phenomena in white dwarfs and inertial confinement fusion plasmas with densities exceeding 10^{26} cm^{-3}.^[3] Recent extensions, such as quantum generalized hydrodynamics for integrable one-dimensional models, further refine its use in predicting transport and relaxation in isolated quantum systems.^[6]

Fundamentals

Definition and scope

Quantum hydrodynamics (QHD) is a theoretical framework that describes the collective behavior of quantum many-body systems using hydrodynamic principles, incorporating quantum mechanical effects such as wave-particle duality, coherence, and superposition.^[2] It treats quantum systems as continuous fluids where microscopic quantum phenomena manifest macroscopically, enabling the analysis of fluid-like dynamics at the quantum scale.^[7] This approach reformulates quantum mechanics in terms of fluid variables like density and velocity fields, providing an alternative perspective to traditional wavefunction or operator-based methods.^[8] The scope of QHD primarily encompasses zero-temperature quantum fluids, where thermal fluctuations are negligible, and the system exhibits coherent, collective excitations akin to inviscid flows.^[5] Extensions to finite temperatures incorporate statistical mechanics to account for thermal effects, such as partial coherence or mixed states, while maintaining the hydrodynamic structure.^[9] QHD distinguishes itself from semiclassical approximations by fully retaining quantum corrections, avoiding the classical trajectory assumptions that limit the latter's accuracy in strongly quantum regimes.^[8] At its core, QHD relies on the emergence of macroscopic wavefunctions that encode the probability density and phase information, from which fluid variables—such as mass density and velocity—are derived to describe the system's evolution.^[2] This formulation originates from a hydrodynamic reinterpretation of the Schrödinger equation, bridging quantum probability currents with classical fluid conservation laws.^[7] In the classical limit, quantum effects diminish, recovering standard hydrodynamics without the additional quantum terms that capture phenomena like tunneling or interference in fluid motion.^[5]

Relation to classical hydrodynamics

Quantum hydrodynamics (QHD) maintains a structural analogy to classical fluid dynamics through its use of a continuity equation and a momentum balance equation that parallel the classical forms. The continuity equation in QHD is identical to its classical counterpart, \partial_t \rho + \nabla \cdot (\rho \mathbf{v}) = 0, where \rho is the probability density and \mathbf{v} is the velocity field derived from the phase of the wavefunction. The momentum equation takes the form of the Euler equation with quantum corrections, \partial_t \mathbf{v} + (\mathbf{v} \cdot \nabla) \mathbf{v} = -\frac{1}{m} \nabla (V + Q), where m is the particle mass, V is the classical potential, and Q is the quantum potential; for interacting many-body systems, additional terms such as a pressure gradient -\frac{1}{\rho} \nabla p from an equation of state may be included, often framed via the Madelung transformation of the Schrödinger equation.^[9] A primary divergence arises from the inclusion of a quantum pressure term, which stems from the curvature of the wavefunction amplitude and has no classical analog. This term, typically expressed as -\frac{\hbar^2}{2m} \frac{\nabla^2 \sqrt{\rho}}{\sqrt{\rho}} in the effective potential, accounts for quantum dispersion effects and influences the dynamics of density fluctuations in ways that classical pressure alone cannot. In ideal quantum fluids at absolute zero temperature, QHD equations lack a viscosity term, reflecting the dissipationless nature of superfluid flow, unlike classical Navier-Stokes equations that include viscous stresses for real fluids.^[9] In limiting regimes, QHD recovers classical hydrodynamics. As the reduced Planck constant \hbar \to 0, the quantum pressure term vanishes, reducing the equations to the classical Euler form without quantum corrections. Similarly, in high-temperature limits where thermal de Broglie wavelengths become negligible compared to system scales, quantum effects diminish, yielding classical behavior. These correspondences highlight QHD as a quantum extension of classical fluid dynamics, bridging microscopic wave mechanics with macroscopic fluid descriptions.^[10]^[11]

Historical development

Early theoretical foundations

The early theoretical foundations of quantum hydrodynamics emerged from the nascent field of quantum mechanics in the mid-1920s, as physicists sought to reconcile wave-particle duality with intuitive classical descriptions. In 1926, Erwin Schrödinger formulated wave mechanics through a series of papers, introducing the Schrödinger equation as a fundamental wave equation governing the behavior of quantum systems. This framework described particles not as point-like entities but as distributed waves, providing a continuous, field-like representation that echoed classical wave phenomena in electromagnetism and acoustics. Initially, Schrödinger interpreted the square of the wave function's amplitude as a measure of charge density, aiming for a deterministic, classical-analogous picture rather than the probabilistic view that would soon dominate.^[12] These developments were motivated by the need for more accessible interpretations of counterintuitive quantum effects, such as tunneling—where particles seemingly penetrate energy barriers—and interference patterns observed in double-slit experiments, particularly when extending to multi-particle systems where statistical behaviors complicated traditional particle models. Wave mechanics offered a pathway to visualize these phenomena through propagating waves, akin to fluid waves in classical hydrodynamics, thereby bridging the abstract matrix mechanics of Werner Heisenberg with more tangible, visualizable dynamics. This quest for classical-like intuition laid the groundwork for hydrodynamic analogies, emphasizing collective flow and density variations over isolated particle trajectories. A pivotal early contribution came in 1927 with Louis de Broglie's pilot-wave theory, presented at the Solvay Conference, which explicitly introduced a hydrodynamic-like analogy in the pre-superfluid era. De Broglie proposed that quantum particles are guided by an accompanying pilot wave, much like a boat steered by ocean currents, where the wave's phase dictates the particle's velocity in a deterministic manner. This double-solution approach envisioned the quantum entity as both a singular particle and a surrounding physical wave field, providing an intuitive framework for interference and tunneling without invoking inherent randomness, and foreshadowing fluid-dynamic interpretations of quantum motion in multi-particle contexts.^[13] Contemporaneously in 1927, Erwin Madelung advanced this line of thinking by deriving a hydrodynamic formulation of the Schrödinger equation, transforming the wave function into fluid variables of density and velocity potential, thus establishing a direct mathematical link to classical fluid equations.

Key advancements in quantum fluids

A pivotal advancement in quantum hydrodynamics came with Lev Landau's formulation of the two-fluid model for superfluid helium in 1941, which described the fluid as a mixture of a normal component carrying entropy and viscosity, and a superfluid component exhibiting frictionless flow.^[14] This model provided a hydrodynamic framework to explain phenomena like the abrupt drop in viscosity below the lambda point and the propagation of second sound, tying quantum effects directly to macroscopic fluid behavior in helium II.^[14] In 1952, David Bohm revived and expanded de Broglie's pilot-wave theory through his causal interpretation of quantum mechanics, reinterpreting the Madelung equations in terms of definite particle trajectories guided by a quantum potential. This formulation emphasized the hydrodynamic aspects of quantum flow, influencing subsequent developments in quantum fluids by providing a deterministic picture of collective quantum behavior.^[15] In the 1950s and 1960s, the development of the Gross-Pitaevskii equation marked a significant step toward hydrodynamic descriptions of weakly interacting Bose gases, building on mean-field approximations to capture the condensate wave function's evolution.^[16] Eugene Gross's 1957 work introduced a unified theory for interacting bosons, enabling the derivation of nonlinear equations that describe vortex structures and superfluid dynamics in dilute systems.^[16] Lev Pitaevskii's 1961 contribution extended this to imperfect Bose gases, formalizing the equation's role in predicting quantized vortices and collective excitations, thus bridging microscopic quantum interactions with hydrodynamic flow.^[17] The 1970s saw quantum hydrodynamics applied to extreme astrophysical contexts, particularly neutron stars, where superfluidity in neutron matter influences rotational dynamics and frictional heating. George Greenstein's 1975 analysis modeled the steady-state hydrodynamics of superfluid neutron star interiors, accounting for interactions between superfluid and normal components under the torque from pulsar slowdown, which generates internal heat through mutual friction.^[18] Concurrently, studies of quantum turbulence in helium advanced, with experimental probes revealing tangled vortex arrays and their decay, highlighting similarities to classical turbulence while emphasizing quantized circulation.^[19] The 1990s brought a revival of quantum hydrodynamics through Bose-Einstein condensate (BEC) experiments, which realized superfluidity in dilute atomic gases and validated hydrodynamic predictions on laboratory scales.^[20] The 1995 achievement by Eric Cornell and Carl Wieman, who produced a BEC in rubidium-87 vapor via evaporative cooling, earned the 2001 Nobel Prize and solidified the Gross-Pitaevskii framework for describing collective modes and vortex formation in these systems.^[20] These experiments demonstrated irrotational flow and quantized circulation in ultracold gases, extending quantum fluid hydrodynamics beyond cryogenic liquids to tunable atomic ensembles.

Mathematical formulation

Madelung equations

The Madelung equations provide a hydrodynamic reformulation of the single-particle time-dependent Schrödinger equation, transforming the quantum mechanical description of a wave function into equations resembling those of classical fluid dynamics. Introduced by Erwin Madelung in 1927, this approach expresses the wave function in polar form, interpreting its modulus squared as a probability density and its phase as related to a velocity potential.^[21] This transformation highlights quantum effects through an additional potential term, while maintaining the probabilistic interpretation of quantum mechanics.^[22] The derivation begins with the single-particle time-dependent Schrödinger equation for a wave function \psi(\mathbf{r}, t) in the presence of an external potential V(\mathbf{r}, t):

i \hbar \frac{\partial \psi}{\partial t} = -\frac{\hbar^2}{2m} \nabla^2 \psi + V \psi,

where \hbar is the reduced Planck's constant and m is the particle mass. Madelung's ansatz decomposes \psi into a real-valued amplitude and phase: \psi = \sqrt{\rho} \exp(i S / \hbar), where \rho(\mathbf{r}, t) = |\psi|^2 represents the probability density and S(\mathbf{r}, t) is the real-valued phase function.^[21]^[22] Substituting this form into the Schrödinger equation and separating the real and imaginary parts yields two coupled equations. The imaginary part leads to the continuity equation, which describes the conservation of probability density:

\frac{\partial \rho}{\partial t} + \nabla \cdot (\rho \mathbf{v}) = 0,

where the velocity field is defined as \mathbf{v} = \nabla S / m. This equation mirrors the mass conservation law in classical hydrodynamics, treating \rho as a fluid density and \rho \mathbf{v} as the probability current.^[22] The real part produces the momentum equation:

\frac{\partial \mathbf{v}}{\partial t} + (\mathbf{v} \cdot \nabla) \mathbf{v} = -\frac{1}{\rho} \nabla (V + Q),

with the quantum potential Q given by

Q = -\frac{\hbar^2}{2m} \frac{\nabla^2 \sqrt{\rho}}{\sqrt{\rho}}.

This equation resembles the Euler equation for an irrotational fluid, but includes the quantum potential Q, which introduces non-local quantum effects such as wave-like interference and tunneling. Without Q, the equations reduce to the classical Euler equations for a barotropic fluid.^[22] The ansatz assumes \rho > 0 and smooth functions for validity, and the formulation is particularly suited to coherent quantum states where the phase S defines a well-behaved velocity field.^[21] While originally derived for single-particle systems, the Madelung equations have been extended to many-body quantum systems through mean-field approximations, such as those replacing the full many-body wave function with a product of single-particle orbitals.^[23]

Quantum potential and stress tensor

In quantum hydrodynamics (QHD), the quantum potential serves as a key correction term that encapsulates non-classical effects in the fluid equations, arising from the transformation of the wave function into density and velocity fields. This potential, often denoted as Q = -\frac{\hbar^2}{2m} \frac{\nabla^2 \sqrt{\rho}}{\sqrt{\rho}}, where \rho is the probability density, m is the particle mass, and \hbar is the reduced Planck constant, originates from the inherent quantum delocalization of particles and their zero-point motion. These features lead to phenomena such as enhanced tunneling in quantum fluids, where particles can permeate potential barriers that would be insurmountable in classical descriptions, influencing collective behaviors in systems like liquid helium.^[24] From the Bohmian perspective, the quantum potential functions as a guiding field that directs particle trajectories along deterministic paths within the hydrodynamic flow, with the velocity field derived from the phase of the wave function influencing the overall fluid motion. This interpretation contrasts with purely statistical views by emphasizing an ontological role for the potential in shaping individual particle dynamics amid the ensemble.^[25] In many-body extensions of QHD, the scalar quantum potential generalizes to a stress tensor \sigma_{ij} = -\frac{\hbar^2}{4m} \rho \frac{\partial^2 \ln \rho}{\partial x_i \partial x_j}, which introduces anisotropic quantum pressure effects beyond isotropic assumptions in single-particle models. This tensor modifies the momentum balance equation by contributing a divergence term \partial_j \sigma_{ij}, accounting for spatial variations in density that reflect quantum dispersion in interacting systems.^[9] A sketch of the derivation for this stress tensor begins with the Wigner phase-space distribution function, which evolves according to a quantum Liouville-like equation; taking moments yields hydrodynamic equations where higher-order quantum corrections close into the stress tensor form under gradient expansions or entropy maximization principles. Alternatively, in density functional theory frameworks for finite-temperature systems, the tensor emerges from variational minimization of the free energy functional, incorporating exchange-correlation gradients.^[26]^[9] Unlike the single-particle quantum potential, which assumes indistinguishable bosons or non-interacting particles, the many-body stress tensor in quantum plasmas integrates Fermi-Dirac statistics to capture degeneracy pressures at finite temperatures, parameterized by the ratio \theta = T / T_F where T_F is the Fermi temperature; this leads to wavelength-dependent coefficients in the tensor, reducing to classical limits as \theta \to \infty.^[9]

Applications in quantum systems

Superfluids and helium

Superfluid helium-4 exhibits quantum hydrodynamic behavior below the lambda transition temperature of 2.17 K, where it transitions from a normal fluid (helium I) to a superfluid state (helium II) characterized by zero viscosity and macroscopic quantum coherence.^[27] In this regime, the superfluid component flows without dissipation, enabling phenomena such as persistent currents and the ability to climb container walls due to the absence of frictional losses. Quantum hydrodynamics provides a framework to model these properties by treating the superfluid as a coherent wavefunction, with density and velocity fields derived from the phase and amplitude of the order parameter. The velocity field in superfluid helium-4 is irrotational, expressed as \mathbf{v}_s = \frac{\hbar}{m} \nabla \phi, where \hbar is the reduced Planck's constant, m is the mass of a helium-4 atom, and \phi is the phase of the wavefunction.^[28] Rotational motion is accommodated through quantized vortices, line singularities where the circulation \kappa around a closed path enclosing the vortex core is quantized as \kappa = n \frac{h}{m}, with h the Planck's constant and n an integer.^[29] These vortices form due to the single-valuedness of the wavefunction, leading to a topological constraint that prevents arbitrary vorticity, unlike in classical fluids. The underlying Madelung equations describe this velocity field as part of a hydrodynamic transformation of the Schrödinger equation. The two-fluid model, developed to describe helium II at finite temperatures, integrates seamlessly with quantum hydrodynamics by separating the fluid into a viscous normal component and an inviscid superfluid component. The superfluid velocity arises from the phase gradient as in the irrotational flow, while the normal fluid carries entropy and viscosity; quantum hydrodynamics extends this by incorporating quantum pressure terms to model interactions between the components, particularly in vortex dynamics. This integration is crucial for simulating quantum turbulence, where tangles of quantized vortices evolve through reconnections and stretching, mimicking classical turbulence at large scales but with discrete quantum circulation.^[30] Key experimental observations underpin these models, including the 1938 discovery of superfluidity by Pyotr Kapitza, who observed anomalously high thermal conductivity in liquid helium below 2.17 K, and independently by John F. Allen and Don Misener, who measured zero viscosity in narrow channels. Landau's theory further elucidated the excitation spectrum, introducing rotons as gapped quasiparticles with a minimum energy around \Delta \approx 8.65 K at finite momentum, explaining the temperature dependence of superfluid properties through phonon-roton interactions. In quantum hydrodynamics, simulations of quantized vortex dynamics in superfluid helium-4 often employ the Gross-Pitaevskii equation in the hydrodynamic limit, where the healing length is much smaller than system scales, approximating the Madelung fluid equations. These simulations reveal vortex reconnection events, where two anti-parallel vortices approach, develop sound emission, and topologically reconnect, dissipating energy quantum-mechanically and driving turbulence cascades.^[30] Such numerical approaches validate experimental observations of vortex tangles in counterflow setups, highlighting the role of quantum effects in macroscopic flow instabilities. Recent advances include proposals for using superfluid ^4He Josephson junction gyrometers to detect gravitational frame-dragging at 0.2% precision in one second of measurement at 10 mK, leveraging QHD for quantum sensing applications.^[31] Additionally, a 2025 nonlinear Schrödinger equation model describes solitary and periodic quantum waves in helium films, reducing to classical limits for long waves and validating phonon-roton excitations.^[32]

Bose-Einstein condensates

Bose-Einstein condensates (BECs) represent a cornerstone for applying quantum hydrodynamics (QHD) to weakly interacting dilute quantum gases, where the macroscopic wave function captures coherent matter-wave phenomena under mean-field approximations.^[33] In these systems, atoms cooled to near-absolute zero occupy the ground state, enabling superfluid-like behavior tunable via external potentials and interactions. QHD provides a fluid-dynamic interpretation of BEC dynamics, revealing analogies to classical hydrodynamics while incorporating quantum effects like zero-point pressure.^[34] The dynamics of BECs are governed by the Gross-Pitaevskii equation (GPE), a nonlinear Schrödinger equation describing the evolution of the condensate wave function \psi(\mathbf{r}, t):

i \hbar \frac{\partial \psi}{\partial t} = -\frac{\hbar^2}{2m} \nabla^2 \psi + V(\mathbf{r}) \psi + g |\psi|^2 \psi,

where \hbar is the reduced Planck's constant, m is the atomic mass, V(\mathbf{r}) is the external potential, and g = 4\pi \hbar^2 a_s / m is the interaction strength with s-wave scattering length a_s.^[33] This equation admits a hydrodynamic formulation through the Madelung transformation, \psi = \sqrt{n} e^{i S / \hbar}, yielding continuity and Euler-like equations for density n = |\psi|^2 and velocity \mathbf{v} = \nabla S / m, augmented by a quantum potential term Q = -\hbar^2 \nabla^2 \sqrt{n} / (2m \sqrt{n}).^[33] This transformation bridges wave mechanics and fluid dynamics, facilitating analysis of irrotational flows and quantum stresses in dilute gases.^[35] Experimental realization of BECs occurred in 1995 using evaporative cooling of alkali metal vapors, such as rubidium-87, marking the first gaseous atomic condensate and enabling direct tests of QHD predictions.^[36] Superfluidity in these systems was subsequently observed in optical lattices around 2002, where BECs loaded into periodic potentials exhibited phase coherence and interference indicative of superfluid flow across lattice sites. QHD excels in modeling applications within trapped BECs, such as sound propagation, where Bogoliubov modes emerge as linear excitations with dispersion \omega(k) = c k \sqrt{1 + (\hbar k / 2mc)^2} and speed c = \sqrt{g n / m}, observed experimentally via Bragg spectroscopy.^[37] Matter-wave solitons, stable dark or bright density dips sustained by nonlinear balancing, propagate without dispersion in one-dimensional geometries, as demonstrated in elongated condensates.^[38] Vortex lattices, quantized circulatory flows arranged in Abrikosov-like patterns under rotation, form stable structures in harmonic traps, with healing lengths \xi = \hbar / \sqrt{2 m g n} setting core sizes.^[39] A key advantage of QHD lies in elucidating collective excitations and instabilities, such as modulational instability, where plane waves break into filamentary structures due to interplay between dispersion and nonlinearity, leading to soliton trains in repulsive BECs.^[40] This framework highlights how quantum pressure stabilizes or destabilizes flows, contrasting with denser systems like superfluid helium while emphasizing tunability in dilute gases.^[34] As of November 2025, experiments on ^39K BECs in optical boxes revealed a universal coarsening speed for coherence length growth, D = 3.4(3) ħ/m, independent of interactions, linked to quantum vortex circulation in QHD via the Gross-Pitaevskii equation.^[41] A contemporary theory, published November 2025, contradicts traditional views by rejecting ground-state condensation and irrotational flow, proposing new thermodynamic principles for superfluidity that may update QHD frameworks.^[42]

Extensions to other domains

Quantum plasmas

Quantum hydrodynamics (QHD) extends to quantum plasmas, which are charged systems where quantum degeneracy and wave-particle duality play crucial roles, particularly in high-density environments like dense electron gases. In this framework, the continuity and momentum equations from neutral quantum fluids are modified to include electromagnetic interactions. The momentum equation incorporates the Lorentz force, \frac{\rho}{m} (\mathbf{E} + \frac{\mathbf{v} \times \mathbf{B}}{c}), where \rho is the charge density, \mathbf{v} the fluid velocity, \mathbf{E} the electric field, and \mathbf{B} the magnetic field, enabling the description of collective plasma oscillations coupled to fields. For fully degenerate electrons obeying Fermi-Dirac statistics, the pressure term is dominated by the Fermi pressure, P_F = \frac{(3\pi^2)^{2/3} \hbar^2}{5m_e} n_e^{5/3}, where n_e is the electron density, m_e the electron mass, and \hbar the reduced Planck's constant, reflecting the Pauli exclusion principle's influence on the equation of state. A hallmark of quantum plasma dynamics in QHD is the dispersion relation for Langmuir waves, modified by both thermal-like Fermi motion and quantum diffraction. The Bohm-Gross dispersion relation in this context reads:

\omega^2 = \omega_p^2 + \frac{3}{5} v_F^2 k^2 + \frac{\hbar^2 k^4}{4 m_e^2},

where \omega_p = \sqrt{4\pi n_e e^2 / m_e} is the plasma frequency, v_F = \hbar (3\pi^2 n_e)^{1/3} / m_e the Fermi velocity, and k the wave number; the cubic term arises from the quantum Bohm potential, while the quadratic term stems from Fermi pressure, leading to enhanced wave propagation at short wavelengths compared to classical cases. This relation captures the transition from classical plasma behavior at long wavelengths to quantum-dominated regimes at nanoscale or high-density scales. Applications of QHD to quantum plasmas span high-energy physics and materials science. In inertial confinement fusion, QHD models the quantum effects in compressed warm dense matter, such as electron degeneracy influencing shock propagation and energy transport during laser-driven implosions.^[43] Astrophysically, it describes the structure of white dwarfs, where degenerate electron plasmas provide the pressure balancing gravitational collapse, with quantum tunneling corrections affecting stability in dense cores. In semiconductor devices, QHD simulates nanoscale electron hydrodynamics, incorporating quantum confinement and degeneracy to predict transport properties in quantum dots and wires under applied fields.^[44] Developments in the 2010s advanced QHD through magneto-quantum hydrodynamics (MQHD), which integrates spin dynamics for fermions in magnetic fields, deriving moment equations from the many-particle Schrödinger equation to describe spin-orbit coupling and magnetization effects in quantum plasmas.^[45] This approach has enabled studies of spin-polarized waves and instabilities in systems like magnetized semiconductors and astrophysical plasmas.

Hydrodynamic quantum analogs

Hydrodynamic quantum analogs refer to classical fluid systems designed to replicate key features of quantum hydrodynamic phenomena, offering a macroscopic platform for visualizing and studying effects typically confined to microscopic scales. These analogs leverage wave-particle interactions in accessible experimental setups to mimic behaviors such as interference, tunneling, and quantized orbits, providing insights into the underlying physics without requiring quantum technologies.^[46] A prominent example is the pilot-wave hydrodynamic system developed in experiments by John W. M. Bush and colleagues starting in the mid-2000s. In this setup, a millimetric droplet of oil "walks" across the surface of a vertically vibrated bath of the same fluid, just above the Faraday instability threshold, where the droplet bounces periodically and generates a coherent pilot wave beneath it. The resonance between the droplet's bouncing motion (typically 80 Hz) and the wave's wavelength (around 8 mm) creates a self-propelling feedback loop, guiding the droplet along paths reminiscent of de Broglie's pilot-wave interpretation of quantum mechanics. Over repeated trials, ensembles of such walkers produce statistical distributions that closely match quantum predictions for various confined geometries.^[46] Key phenomena in these walking-droplet experiments include the emergence of an effective quantum potential that governs the droplet's motion, arising from the spatial gradient of the self-generated pilot wave field. This classical analog of the Bohm quantum potential deflects the droplet toward wave maxima, leading to stable orbits and chaotic transitions between them, with orbital radii quantized as r_n \approx n \lambda_F / 2 (where \lambda_F is the Faraday wavelength and n is an integer). Additionally, statistical equivalence to quantum tunneling is observed when walkers encounter submerged barriers taller than their kinetic energy; the probability of crossing decreases exponentially with barrier height, mirroring quantum barrier penetration, as demonstrated in single- and double-slit configurations where walkers exhibit path memory and interference-like patterns over long times.^[46] These analogs offer significant advantages, including experimental accessibility in standard fluid dynamics laboratories without cryogenic or vacuum requirements, enabling real-time visualization of wave-mediated particle guidance. They provide a testing ground for exploring interpretations of quantum mechanics, particularly Bohmian mechanics, where the non-local pilot-wave influence is replicated classically, and have inspired theoretical models bridging deterministic chaos and quantum statistics. Recent advances as of 2025 include hydrodynamic quantum analogs of spin effects and studies of megastable quantization in low-dissipation pilot-wave models.^[47]^[48]

Computational methods

Numerical simulations

Numerical simulations play a crucial role in quantum hydrodynamics (QHD) by enabling the solution of the Madelung equations for time-dependent dynamics in complex quantum systems, such as superfluids and Bose-Einstein condensates (BECs). These methods address the nonlinear and dispersive nature of QHD equations, which combine classical fluid equations with quantum corrections like the Bohm potential. High-fidelity simulations require balancing accuracy, stability, and computational efficiency, often leveraging spectral or finite-difference techniques to capture wave propagation and vortex dynamics. Spectral methods, particularly Fourier pseudospectral approaches, are widely used for solving the Gross-Pitaevskii equation (GPE) in periodic domains, which underpins QHD descriptions of BECs. These methods discretize the spatial domain using Fourier transforms to achieve exponential convergence for smooth solutions, making them suitable for simulating soliton interactions and condensate oscillations. An extended Fourier pseudospectral scheme has been developed to handle low-regularity potentials in the GPE, demonstrating high accuracy in long-time integrations of BEC ground states and dynamics. For superfluids, vortex filament models approximate quantized vortices as thin line singularities, evolving them via the Biot-Savart law to simulate tangle formation and reconnection in helium-4. This approach has visualized vortex motion in turbulent superfluids, revealing scaling laws in vortex density decay consistent with experimental observations. Finite-difference time-domain (FDTD) methods are employed for simulating quantum plasma waves incorporating the Bohm potential, which accounts for quantum tunneling and diffraction effects in electron fluids. In plasmonics, an improved FDTD scheme integrates the Bohm potential into the hydrodynamic model to capture nonlocal responses, enabling accurate prediction of surface plasmon polaritons with subwavelength resolution. These simulations highlight dispersion relations modified by quantum pressure, essential for understanding high-frequency plasma instabilities. Hybrid approaches combine techniques to handle the stiffness in nonlinear QHD equations. Time-splitting methods decompose the GPE into linear kinetic and nonlinear interaction steps, often paired with pseudospectral spatial discretization for efficient propagation in rotating BECs. For quantum plasmas, particle-in-cell (PIC) methods track individual particles while solving self-consistent fields, extended to include quantum corrections like Fermi-Dirac statistics and Bohm potential for degenerate electron gases. Recent PIC implementations simulate quantum hydrodynamic effects in condensed matter, such as plasmon oscillations at solid densities. Dedicated software facilitates these simulations for BEC dynamics. GPUE, a GPU-accelerated solver, computes time-dependent GPE solutions for rapidly rotating condensates, achieving speedups over CPU-based codes for large-scale vortex studies. Similarly, GPELab provides a MATLAB toolbox for solving multidimensional GPEs, supporting both stationary states and dynamical evolutions with user-friendly interfaces for parameter sweeps in QHD applications.

Challenges and recent developments

One major challenge in quantum hydrodynamics (QHD) lies in incorporating beyond-mean-field effects, such as quantum fluctuations in Bose-Einstein condensates (BECs), which introduce corrections to the standard Gross-Pitaevskii equation and affect phenomena like vortex dynamics and superfluid turbulence.^[49] These fluctuations, arising from interparticle correlations, complicate the mean-field approximation by altering the equation of state and leading to phenomena like the quantum depletion of the condensate, yet fully capturing them requires advanced many-body techniques that increase computational demands. High-dimensional simulations pose another hurdle, as QHD models in three or more spatial dimensions suffer from the curse of dimensionality, where the exponential growth in degrees of freedom limits accurate numerical resolution of quantum effects like tunneling and coherence over large scales.^[50] Relativistic extensions of QHD, aimed at describing high-speed quantum fluids or particles near light speed, remain underdeveloped, with efforts to generalize the Madelung equations to Lorentz-invariant forms encountering issues in preserving causality and handling the quantum potential in curved spacetime.^[51] Recent advances have addressed some of these issues through adaptations of classical hydrodynamic frameworks to quantum regimes. In 2025, researchers from the University of Warsaw derived Navier-Stokes equations for nearly integrable one-dimensional quantum gases, demonstrating how dissipative effects emerge from microscopic dynamics and enabling predictions of transport coefficients in low-dimensional quantum liquids.^[52] This work bridges classical and quantum hydrodynamics by incorporating weak integrability-breaking interactions, offering a pathway to simulate viscous quantum flows without full many-body diagonalization. At the frontiers, QHD is being integrated with optomechanics to probe quantum fluids via light-matter interactions, as seen in proposals to detect superfluid drag in hybrid cavity systems where mechanical resonators couple to two-dimensional quantum fluids for enhanced sensitivity to topological defects.^[53] In topological quantum fluids, such as those exhibiting fractional quantum Hall effects, QHD formulations reveal robust edge currents and anyonic statistics through effective BF theories, opening avenues for fault-tolerant quantum information processing.^[54] Connections to quantum computing simulations are strengthening, with algorithms based on the hydrodynamic Schrödinger equation enabling efficient modeling of nonlinear quantum flows on near-term devices, potentially scaling to three-dimensional turbulence beyond classical limits.^[55] A key gap persists in the incomplete unification of QHD with relativistic quantum field theory, particularly for high-energy applications like quark-gluon plasmas, where current extensions fail to fully reconcile hydrodynamic approximations with field-theoretic renormalization and ultraviolet divergences.^[51] This limitation hinders applications in heavy-ion collisions and cosmology, necessitating further development of covariant formulations that preserve quantum coherence at relativistic scales.^[56]

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QUANTIZED VORTICES AND TURBULENCE IN HELIUM II
They showed experimentally that when ions are drawn through an electric field at low temperatures the ions become attached to vorticity in the liquid and this ...
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Observation of Bose-Einstein Condensation in a Dilute Atomic Vapor
A Bose-Einstein condensate was produced in a vapor of rubidium-87 atoms that was confined by magnetic fields and evaporatively cooled.
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https://www.science.org/doi/10.1126/science.269.5221.198
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Towards a mathematical Theory of the Madelung Equations
Aug 1, 2022 · The above equations can be generalized to the many-body case (cf. e.g. Sec. 7.1.2 in. Ref. [69]) and there also exist analogue equations ...
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Nuclear Quantum Effects in Water and Aqueous Systems
This review highlights the recent significant developments in the experiment, theory, and simulation of nuclear quantum effects in water.
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[PDF] Bohmian mechanics versus Madelung quantum hydrodynamics - arXiv
At the time of inception of quantum mechanics Madelung [2] has demonstrated that the Schrödinger equa- tion can be transformed in hydrodynamic form. This so- ...<|control11|><|separator|>
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Derivation of New Quantum Hydrodynamic Equations Using Entropy ...
New quantum hydrodynamic equations are derived from a Wigner–Boltzmann model, using the quantum entropy minimization method recently developed by Degond and ...
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Transition from phase slips to the Josephson effect in a superfluid ...
Dec 18, 2005 · When superfluid 4He, well below its transition temperature Tλ=2.17 K ... superfluid 4He near the lambda transition. Phys. Rev. Lett. 90 ...
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Visualization of two-fluid flows of superfluid helium-4 - PNAS
Mar 24, 2014 · In superfluid helium-4, vorticity is concentrated along the filamentary cores of quantized vortex lines, and the velocity circulation around any ...
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Direct visualization of the quantum vortex lattice structure ...
Jul 28, 2023 · Quantum mechanics imposes that the circulation of the velocity field around these vortex lines is quantized: Γ ≡ ∮ vs · dl = nκ, where κ ≡ h/m ≃ ...
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Quantum vortex reconnections | Physics of Fluids - AIP Publishing
We study reconnections of quantum vortices by numerically solving the governing Gross-Pitaevskii equation. We find that the minimum distance between ...
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[PDF] Gross–Pitaevskii dynamics of Bose–Einstein condensates and ...
Feb 18, 2023 · The Gross-Pitaevskii equation, also called the nonlinear Schrödinger equation (NLSE), describes the dynamics of low-temperature superflows ...
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Quantum hydrodynamic theory of quantum fluctuations in dipolar ...
Feb 11, 2021 · They are the continuity equation and the Euler equation. The Gross–Pitaevskii equation can be derived from the quantum mechanics by different ...
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[PDF] geometric quantum hydrodynamics and bose-einstein condensates ...
Geometric quantum hydrodynamics merges geometric hydrodynamics with quantum hydrodynamics to study the geometric properties of vortex structures in ...
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June 5, 1995: First Bose Einstein Condensate
Jun 1, 2004 · The world's first BEC was achieved at 10:54 AM on June 5, 1995 in a laboratory at JILA, a joint institute of University of Colorado, Boulder, and NIST.
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Superfluidity of Bose–Einstein condensate in an optical lattice
Jul 31, 2003 · Other approaches to superfluidity and experimental observation and general implications of dynamical instability are discussed. Finally in ...
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[PDF] arXiv:0805.0761v2 [cond-mat.other] 21 May 2008
May 21, 2008 · These, so-called, matter-wave solitons and vortices can be viewed as fundamental nonlinear excitations of BECs, and as such have attracted ...
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Vortices and vortex lattices in quantum ferrofluids - IOPscience
Feb 1, 2017 · Here we give a review of the theory of vortices in dipolar Bose–Einstein condensates, exploring the interplay of magnetism with vorticity.
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Modulational instability in Bose-Einstein condensates in optical lattices
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[2109.09081] Shock Physics in Warm Dense Matter - arXiv
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[1305.0541] Quantum Hydrodynamics Approach to The Research of ...
May 2, 2013 · In this paper we explicate a method of magneto quantum hydrodynamics (MQHD) for the study of the quantum evolution of a system of spinning fermions in an ...
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Pilot-Wave Hydrodynamics - Annual Reviews
Jan 3, 2015 · This article reviews experimental evidence indicating that the walking droplets exhibit certain features previously thought to be exclusive to the microscopic, ...
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Hydrodynamics of the atomic Bose–Einstein condensate beyond the ...
Several hydrodynamic models of the atomic Bose–Einstein condensate (BEC) obtained beyond the mean-field approximation are discussed together from a single ...
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Quantum effects beyond mean-field treatment in quantum optics
Nov 29, 2021 · Our work clearly reveals the attendant quantum effects under mean-field treatment and provides a more precise theoretical framework to describe quantum optics ...
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Enabling Large-Scale and High-Precision Fluid Simulations on Near ...
Jun 10, 2024 · This paper introduces a comprehensive QCFD method, including an iterative method "Iterative-QLS" that suppresses error in quantum linear solver.
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Direct Relativistic Extension of the Madelung-de-Broglie-Bohm ...
Abstract. The basic equations of the non-relativistic quantum mechanics with trajectories and quantum hydrodynamics are extended to the relativistic domain.
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Signature of Andreev-Bashkin superfluid drag from cavity ...
Apr 14, 2025 · Our proposal brings the powerful techniques of cavity optomechanics, which enabled the detection of gravitational waves, to the study of ...
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[1408.5417] Topological BF theory of the quantum hydrodynamics of ...
Aug 22, 2014 · The two degrees of freedom of the BF theory are associated to the mass (charge) flows of the fluid and its polarization vorticities.
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Quantum computing of fluid dynamics using the hydrodynamic ...
Feb 20, 2023 · We propose a framework for quantum computing of fluid dynamics based on the hydrodynamic Schrödinger equation (HSE), which can be promising in simulating three ...
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[physics/0504062] Relativistic Quantum Dynamics: A non-traditional ...
Apr 8, 2005 · This book is an attempt to build a consistent relativistic quantum theory of interacting particles.Missing: hydrodynamics | Show results with:hydrodynamics