arXiv subjects

Nils A. Krause

Publications and source records attributed to Nils A. Krause.

5 recordsLinked to original sources

arXiv2026

Evaporative damping in open system theory of Bose-Einstein Condensates

We derive a new damping mechanism in the open quantum systems description of Bose-Einstein condensates. It stems from previously neglected terms in the derivation of the stochastic projective Gross-Pitaevskii equation (SPGPE), accounting for a nonlinear evaporation of particles from the coherent into the incoherent region. We demonstrate that the mechanism, while so far assumed to be of minor importance, is comparable in strength to the widely employed number damping. We also provide a simplified (pseudo)-local and a dimensionally reduced form of this evaporative damping. The process completes the SPGPE description of ultracold Bose gases giving a full first-principles picture of their evolution at finite temperature.

cond-mat.quant-gas↗

arXiv2025

Velocity correlations of vortices and rarefaction pulses in compressible planar quantum fluids

We present a quantitative analytical framework for calculating two-point velocity correlations in compressible quantum fluids, focusing on two key classes of superfluid excitations: vortices and rarefaction pulses. We employ two complementary approaches. First, we introduce a new ansatz for vortex cores in planar quantum fluids that enhances analytic integrability. This ansatz yields closed-form expressions for power spectra and velocity correlations for general vortex distributions. Using it, we identify distinct signatures of short- and long-range velocity correlations corresponding to vortex dipoles and vortex pairs, respectively. Second, we analyze the fast rarefaction pulse regime of the Jones-Roberts soliton. By applying the asymptotic high-velocity wavefunction, we derive analytic expressions for the velocity power spectrum and correlation function, capturing the soliton characteristic length scale. We validate our analytical results for the homogeneous system against numerical treatment of a large trapped system, finding close quantitative agreement. Our findings provide the first analytic treatment of velocity correlations for Jones-Roberts solitons in quantum fluids of light [M. Baker-Rasooli et al., Physical Review Letters, 134, 233401 (2025)], and establish a foundation for characterizing vortices and solitons in compressible quantum fluids.

cond-mat.quant-gas↗

arXiv2025

Equilibrium, Relaxation and Fluctuations in homogeneous Bose-Einstein Condensates: Linearized Classical Field Analysis

Open quantum systems theory is central to describing the dynamics and equilibration of dilute-gas Bose-Einstein condensates (BECs). We present an analysis of the linearized stochastic projected Gross-Pitaevskii equation (SPGPE) describing finite-temperature BECs. Our treatment provides an optimal choice for the cut-off that divides the Bose gas into the low-energy coherent region forming a classical wave, and the high-energy thermal cloud treated as a reservoir. Moreover, it highlights the relevance of energy damping, the number-conserving scattering between thermal and coherent atoms. We analyze the equilibrium properties and near-equilibrium relaxation of a homogeneous BEC in one, two and three dimensions at high phase-space density, and calculate the autocorrelation function and power spectrum of the density and phase fluctuations. Simulations of the full non-linear SPGPE are in close agreement, and extend our arguments beyond the linear regime. Our work suggests the need for a re-examination of decay processes in BECs studied under the neglect of energy damping.

cond-mat.quant-gas↗

arXiv2025

Observation of Jones-Roberts solitons in a paraxial quantum fluid of light

We investigate the formation and dynamics of Jones-Roberts solitons in a smoothly inhomogeneous quantum fluid. To do so, we create a superfluid of light using paraxial, near-resonant laser beam propagating through a hot rubidium vapor. We excite a bounded vortex-antivortex dipole in the superfluid and observe its transition to a rarefaction pulse and back, in agreement with the seminal predictions of Jones and Roberts. Employing an analogy with ray optics, we calculate the trajectory of the interacting vortices, deriving an effective refractive index from the inhomogeneous fluid density. Finally, we examine analytically and experimentally the superfluid velocity correlations, observing a transfer of coherence from incompressible to compressible velocity of the quantum fluid, a direct signature of the dynamical conversion between vortices and rarefaction pulse.

cond-mat.quant-gas↗

arXiv2024

Thermal Decay of Planar Jones-Roberts Solitons

Homogeneous planar superfluids exhibit a range of low-energy excitations that also appear in highly excited states like superfluid turbulence. In dilute gas Bose-Einstein condensates, the Jones- Roberts soliton family includes vortex dipoles and rarefaction pulses in the low and high velocity regimes, respectively. These excitations carry both energy and linear momentum, making their decay characteristics crucial for understanding superfluid dynamics. In this work, we develop the theory of planar soliton decay due to thermal effects, as described by the stochastic projected Gross-Pitaevskii theory of reservoir interactions. We analyze two distinct damping terms involving transfer between the condensate and the non-condensate reservoir: particle transfer that also involves energy and usually drives condensate growth, and number-conserving energy transfer. We provide analytical treatments for both the low and high velocity regimes and identify conditions under which either mechanism dominates. Our findings indicate that energy damping prevails at high phase space density. These theoretical results are supported by numerical studies covering the entire velocity range from vortex dipole to rarefaction pulse. We use interaction energy to characterize rarefaction pulses, analogous to the distance between vortices in vortex dipoles, offering an experimentally accessible test for finite temperature theory in Bose-Einstein condensates.

cond-mat.quant-gas↗