arXiv subjects

Q. Guan

Publications and source records attributed to Q. Guan.

At least 19 recordsLinked to original sources

arXiv2026

Dynamics of spinor Bose-Einstein condensates close to spin-spatial resonances

We develop a coupled-channel framework to describe the dynamics of spinor Bose-Einstein condensates (BECs), with particular emphasis on the behavior near resonances between spin dynamics and spatial excitations. Taking advantage of the disparity between the spin-dependent and spin-independent scattering lengths in typical spinor BECs, the Bogoliubov modes of the spin-independent part of the full system Hamiltonian provide an efficient set of basis functions for describing the system dynamics in a coupled-channel framework. For quadratic Zeeman shifts far from any resonance, the system can be described by a single spatial wavefunction during the spin dynamics, i.e., the so-called single-mode approximation holds. By tuning the quadratic Zeeman shift, we find resonant excitations of the Bogoliubov modes, which can be classified into two categories: those with particle-hole correlations and those without particle-hole correlations. We show that the beyond-quadratic-order terms that are neglected in standard Bogoliubov theories become increasingly important for capturing the long-time dynamics of the system near resonances. The coupled-channel framework is benchmarked against results from 1D Gross-Pitaevskii equation simulations. The framework developed in this work not only provides a numerically efficient tool for describing spinor BEC dynamics governed by different length scales, but also provides a clean physical interpretation of resonance phenomena in spinor BECs. Applications of this approach to other systems and extensions to the beyond-mean-field regime are also discussed.

cond-mat.quant-gas↗

arXiv2025

Controlling the dynamical phase diagram of a spinor BEC using time-dependent potentials

We theoretically investigate the spin-mixing dynamics of a spinor BEC subject to a time-dependent confining potential. Our study provides a theory framework for the experimental results reported in Phys. Rev. A 109, 043309 (2024). We exploit the disparity of energy scales associated with the spatial and internal (spin) degrees of freedom under typical experimental conditions to develop an effective few-mode description of the spin dynamics. Our model demonstrates how the details of the potential, such as driving frequency and amplitude, can be used to independently control spin-changing and spin-preserving collision processes as well as the effective Zeeman energy of the internal states. We obtain the dynamical phase diagram of the effective model and discuss how its structure is altered relative to a spinor BEC with frozen spatial degrees of freedom. The applicability of our effective model is verified through Gross-Pitaevskii simulations that capture the interplay of spin and spatial degrees of freedom, and we identify parameter regimes that can be feasibly explored by future experiments. Our findings highlight the utility of dynamical confining potentials for the control of non-equilibrium spin-mixing dynamics in spinor BECs.

cond-mat.quant-gas↗

arXiv2025

Modeling the laser-pulse induced helium trimer dynamics

Motivated by ongoing pump-probe spectroscopy experiments, this work develops a theoretical framework for describing the rovibrational wave packet dynamics that ensues when a single weakly-bound van der Waals trimer is exposed to a short, sub-picosecond linearly polarized pump laser pulse. The intensity I of the pump laser is chosen such that excitation and ionization of the electronic degrees of freedom are negligible while excitation of the wavepacket in the nuclear degrees of freedom is non-negligible. The numerical treatment, which takes advantage of the fact that the laser pulse is very short compared to typical molecular time scales, is based on a wave packet decomposition that utilizes hyperspherical coordinates. The framework is applied to the extremely floppy bosonic helium trimer. A convergence analysis of the partial wave decomposition is conducted. The kinetic energy release and orientation dynamics are presented. While the dynamics of more strongly-bound van der Waals trimers such as, e.g., the argon trimer display negligible coupling between vibrational and rotational degrees of freedom, rendering a description within a rigid-body picture appropriate, those of weakly-bound trimers display non-negligible coupling between vibrational and rotational degrees of freedom, rendering a description within a rigid-body picture inappropriate. It is shown that a model that constructs the helium trimer dynamics from the dynamics of the helium dimer captures a number of key characteristics of the alignment signal, including the interference between different angular momentum wave packet components.

quant-ph↗

arXiv2024

Ro-vibrational Dynamics of the Neon Dimer

Short intense laser pulses are routinely used to induce rotational wave packet dynamics of molecules. Ro-vibrational wave packet dynamics has been explored comparatively infrequently, focusing predominantly on extremely light and rigid molecules such as H$_2^+$, H$_2$, and D$_2$. This work presents quantum mechanical calculations that account for the rotational {\em{and}} the vibrational degrees of freedom for a heavier and rather floppy diatomic molecule, namely the neon dimer. For pumping by a strong and short non-resonant pump pulse, we identify several phenomena that depend critically on the vibrational (i.e., radial) degree of freedom. Our calculations show (i) fingerprints of the radial dynamics in the alignment signal; (ii) laser-kick induced dissociative dynamics on very short time scales (ejection of highly structured "jets"); and (iii) tunneling dynamics that signifies the existence of resonance states, which are supported by the effective potential curves for selected finite relative angular momenta. Our theory predictions can be explored by existing state-of-the-art experiments.

cond-mat.quant-gas↗

arXiv2024

Floquet dynamics of Rabi model beyond the counterrotating hybridized rotating wave method

Monochromatically driven two-level systems (i.e., Rabi models) are ubiquitous in various fields of physics. Though they have been exactly solved, the physical pictures in these exact solutions are not clear. Recently, approximate analytical solutions with neat physics have been obtained by using the counterrotating hybridized rotating wave (CHRW) method, which has been proven to be effective over a wider range of parameters than the previous analytical solutions. However, the CHRW depends on a parameter {\xi}, which has no solution in some regimes. Here we combine the double-unitary-transformation approach with the generalized Van Vleck nearly degenerate perturbation theory, and present approximate analytical results with clear physics for almost all parameter regimes, which agree well with the numerical solutions and the previous experimental results. Moreover, the dynamic frequencies of the Rabi model are regular, and the frequency with the highest Fourier amplitude changes from the Rabi frequency to 2n{\omega} with driving frequency {\omega} and integer n, as the driving intensity increases from weak to deep-strong. In addition, we further explore the Floquet dynamics of the dissipative open Rabi model. Remarkably, the dissipations are tunable in the rotating frame, and the approximate analytical results obtained by our method are in good agreement with the numerical results in the strong driving regime. These results pave the way to quantum control using strong and deep-strong driving with applications in quantum technologies.

quant-ph↗

arXiv2023

Nonlinear multi-state tunneling dynamics in a spinor Bose-Einstein condensate

We present an experimental realization of dynamic self-trapping and non-exponential tunneling in a multi-state system consisting of ultracold sodium spinor gases confined in moving optical lattices. Taking advantage of the fact that the tunneling process in the sodium spinor system is resolvable over a broader dynamic energy scale than previously observed in rubidium scalar gases, we demonstrate that the tunneling dynamics in the multi-state system strongly depends on an interaction induced nonlinearity and is influenced by the spin degree of freedom under certain conditions. We develop a rigorous multi-state tunneling model to describe the observed dynamics. Combined with our recent observation of spatially-manipulated spin dynamics, these results open up prospects for alternative multi-state ramps and state transfer protocols.

cond-mat.quant-gas↗

arXiv2023

Dynamical mean-field driven spinor condensate physics beyond the single-mode approximation

$^{23}$Na spin-1 Bose-Einstein condensates are used to experimentally demonstrate that mean-field physics beyond the single-mode approximation can be relevant during the non-equilibrium dynamics. The experimentally observed spin oscillation dynamics and associated dynamical spatial structure formation confirm theoretical predictions that are derived by solving a set of coupled mean-field Gross-Pitaevskii equations [J. Jie et al., Phys. Rev. A 102, 023324 (2020)]. The experiments rely on microwave dressing of the $f=1$ hyperfine states, where $f$ denotes the total angular momentum of the $^{23}$Na atom. The fact that beyond single-mode approximation physics at the mean-field level, i.e., spatial mean-field dynamics that distinguishes the spatial density profiles associated with different Zeeman levels, can -- in certain parameter regimes -- have a pronounced effect on the dynamics when the spin healing length is comparable to or larger than the size of the Bose-Einstein condensate has implications for using Bose-Einstein condensates as models for quantum phase transitions and spin squeezing studies as well as for non-linear SU(1,1) interferometers.

cond-mat.quant-gas↗

arXiv2021

Tailored generation of quantum states in an entangled spinor interferometer to overcome detection noise

We theoretically investigate how entangled atomic states generated via spin-changing collisions in a spinor Bose-Einstein condensate can be designed and controllably prepared for atom interferometry that is robust against common technical issues, such as limited detector resolution. We use analytic and numerical treatments of the spin-changing collision process to demonstrate that triggering the entangling collisions with a small classical seed rather than vacuum fluctuations leads to a more robust and superior sensitivity when technical noise is accounted for, despite the generated atomic state ideally featuring less metrologically useful entanglement. Our results are relevant for understanding how entangled atomic states are best designed and generated for use in quantum-enhanced matter-wave interferometry.

cond-mat.quant-gas↗

arXiv2021

Roaming pathways and survival probability in real-time collisional dynamics of cold and controlled bialkali molecules

Perfectly controlled molecules are at the forefront of the quest to explore chemical reactivity at ultra low temperatures. Here, we investigate for the first time the formation of the long-lived intermediates in the time-dependent scattering of cold bialkali $^{23}$Na$^{87}$Rb molecules with and without the presence of infrared trapping light. During the nearly 50 nanoseconds mean collision time of the intermediate complex, we observe unconventional roaming when for a few tens of picoseconds either NaRb or Na$_2$ and Rb$_2$ molecules with large relative separation are formed before returning to the four-atom complex. We also determine the likelihood of molecular loss when the trapping laser is present during the collision. We find that at a wavelength of 1064 nm the Na$_2$Rb$_2$ complex is quickly destroyed and thus that the $^{23}$Na$^{87}$Rb molecules are rapidly lost.

physics.chem-ph↗

arXiv2021

Magic Conditions for Multiple Rotational States of Bialkali Molecules in Optical Lattices

We investigate magic-wavelength trapping of ultracold bialkali molecules in the vicinity of weak optical transitions from the vibrational ground state of the X$^1Σ^+$ potential to low-lying rovibrational states of the b$^3Π_0$ potential, focussing our discussion on the $^{87}$Rb$^{133}$Cs molecule in a magnetic field of $B=181\,$G. We show that a frequency window exists between two nearest neighbor vibrational poles in the dynamic polarizability where the trapping potential is "near magic" for multiple rotational states simultaneously. We show that the addition of a modest DC electric field of $E=0.13\,\text{kV}/\text{cm}$ leads to an exact magic-wavelength trap for the lowest three rotational states at a angular-frequency detuning of $Δ_{v'=0} = 2π\times 218.22$\,GHz from the X$^1Σ^+ (v=0, J=0)\rightarrow$ b$^3Π_0 (v'=0, J=1)$ transition. We derive a set of analytical criteria that must be fulfilled to ensure the existence of such magic frequency windows and present an analytic expression for the position of the frequency window in terms of a set of experimentally measurable parameters. These results should inform future experiments requiring long coherence times on multiple rotational transitions in ultracold polar molecules.

quant-ph↗

arXiv2020

Mean-field spin-oscillation dynamics beyond the single-mode approximation for a harmonically trapped spin-1 Bose-Einstein condensate

Compared to single-component Bose-Einstein condensates, spinor Bose-Einstein condensates display much richer dynamics. In addition to density oscillations, spinor Bose-Einstein condensates exhibit intriguing spin dynamics that is associated with population transfer between different hyperfine components. This work analyzes the validity of the widely employed single-mode approximation when describing the spin dynamics in response to a quench of the system Hamiltonian. The single-mode approximation assumes that the different hyperfine states all share the same time-independent spatial mode. This implies that the resulting spin Hamiltonian only depends on the spin interaction strength and not on the density interaction strength. Taking the spinor sodium Bose-Einstein condensate in the $f=1$ hyperfine manifold as an example and working within the mean-field theory framework, it is found numerically that the single-mode approximation misses, in some parameter regimes, intricate details of the spin and spatial dynamics. We develop a physical picture that explains the observed phenomenon. Moreover, using that the population oscillations described by the single-mode approximation enter into the effective potential felt by the mean-field spinor, we derive a semi-quantitative condition for when dynamical mean-field induced corrections to the single-mode approximation are relevant. Our mean-field results have implications for a variety of published and planned experimental studies.

cond-mat.quant-gas↗

arXiv2020

Non-exponential tunneling due to mean-field induced swallowtails

Typically, energy levels change without bifurcating in response to a change of a control parameter. Bifurcations can lead to loops or swallowtails in the energy spectrum. The simplest quantum Hamiltonian that supports swallowtails is a non-linear $2 \times 2$ Hamiltonian with non-zero off-diagonal elements and diagonal elements that depend on the population difference of the two states. This work implements such a Hamiltonian experimentally using ultracold atoms in a moving one-dimensional optical lattice. Self-trapping and non-exponential tunneling probabilities, a hallmark signature of band structures that support swallowtails, are observed. The good agreement between theory and experiment validates the optical lattice system as a powerful platform to study, e.g., Josephson junction physics and superfluidity in ring-shaped geometries.

cond-mat.quant-gas↗

arXiv2020

Rabi oscillations and Ramsey-type pulses in ultracold bosons: Role of interactions

Double-well systems loaded with one, two, or many quantum particles give rise to intriguing dynamics, ranging from Josephson oscillation to self-trapping. This work presents theoretical and experimental results for two distinct double-well systems, both created using dilute rubidium Bose-Einstein condensates with particular emphasis placed on the role of interaction in the systems. The first is realized by creating an effective two-level system through Raman coupling of hyperfine states. The second is an effective two-level system in momentum space generated through the coupling by an optical lattice. Even though the non-interacting systems can, for a wide parameter range, be described by the same model Hamiltonian, the dynamics for these two realizations differ in the presence of interactions. The difference is attributed to scattering diagrams that contribute in the lattice coupled system but vanish in the Raman coupled system. The internal dynamics of the Bose-Einstein condensates for both coupling scenarios is probed through a Ramsey-type interference pulse sequence, which constitutes a key building block of atom interferometers. These results have important implications in a variety of contexts including lattice calibration experiments and momentum space lattices used for quantum analog simulations.

cond-mat.quant-gas↗

arXiv2019

Spinor Bose-Einstein Condensate Interferometer within the Undepleted Pump Approximation: Role of the Initial State

Most interferometers operate with photons or dilute, non-condensed cold atom clouds in which collisions are strongly suppressed. Spinor Bose-Einstein condensates (BECs) provide an alternative route toward realizing three-mode interferometers; in this realization, spin-changing collisions provide a resource that generates mode entanglement. Working in the regime where the pump mode, i.e., the m=0 hyperfine state, has a much larger population than the side or probe modes (m=+1 and m=-1 hyperfine states), f=1 spinor BECs approximate SU(1,1) interferometers. We derive analytical expressions within the undepleted pump approximation for the phase sensitivity of such an SU(1,1) interferometer for two classes of initial states: pure Fock states and coherent spin states. The interferometer performance is analyzed for initial states without seeding, with single-sided seeding, and with double-sided seeding. The validity regime of the undepleted pump approximation is assessed by performing quantum calculations for the full spin Hamiltonian. Our analytical results and the associated dynamics are expected to guide experiments as well as numerical studies that explore regimes where the undepleted pump approximation makes quantitatively or qualitatively incorrect predictions.

cond-mat.quant-gas↗

arXiv2019

Density Oscillations Induced by Individual Ultracold Two-Body Collisions

Access to single-particle momenta provides new means of studying the dynamics of a few interacting particles. In a joint theoretical and experimental effort, we observe and analyze the effects of a finite number of ultracold two-body collisions on the relative and single-particle densities by quenching two ultracold atoms with an initial narrow wave packet into a wide trap with an inverted aspect ratio. The experimentally observed spatial oscillations of the relative density are reproduced by a parameter-free zero-range theory and interpreted in terms of cross-dimensional flux. We theoretically study the long-time dynamics and find that the system does not approach its thermodynamic limit. The setup can be viewed as an advanced particle collider that allows one to watch the collision process itself.

cond-mat.quant-gas↗

arXiv2018

K-matrix formulation of two-particle scattering in a wave guide in the presence of one-dimensional spin-orbit coupling

The creation of artificial gauge fields in neutral ultracold atom systems has opened the possibility to study the effects of spin-orbit coupling terms in clean environments. This work considers the multi-channel scattering properties of two atoms confined by a wave guide in the presence of spin-orbit coupling terms within a K-matrix scattering framework. The tunability of resonances, induced by the interplay of the external wave guide geometry, the interactions, and the spin-orbit coupling terms, is demonstrated. Our results for the K-matrix elements as well as partial and total reflection coefficients for two identical fermions interacting through a finite-range interaction potential in the singlet channel only are compared with those obtained for a strictly one-dimensional effective low- energy Hamiltonian, which uses the effective coupling constant derived in Zhang et al. [Scientific Reports 4, 1 (2014)] and Zhang et al. [Phys. Rev. A 88, 053605 (2013)] as input. In the regime where the effective Hamiltonian is applicable, good agreement is obtained, provided the energy- dependence of the coupling constant is accounted for. Our approach naturally describes the energy regime in which the bands associated with excited transverse modes lie below a subset of the bands associated with the lowest transverse modes. The threshold behavior is discussed and scattering observables are linked to bound state properties.

physics.atom-ph↗

arXiv2018

Three-boson spectrum in the presence of 1D spin-orbit coupling: Efimov's generalized radial scaling law

Spin-orbit coupled cold atom systems, governed by Hamiltonians that contain quadratic kinetic energy terms typical for a particle's motion in the usual Schrödinger equation and linear kinetic energy terms typical for a particle's motion in the usual Dirac equation, have attracted a great deal of attention recently since they provide an alternative route for realizing fractional quantum Hall physics, topological insulators, and spintronics physics. The present work focuses on the three-boson system in the presence of 1D spin-orbit coupling, which is most relevant to ongoing cold atom experiments. In the absence of spin-orbit coupling terms, the three-boson system exibits the Efimov effect: the entire energy spectrum is uniquely determined by the $s$-wave scattering length and a single three-body parameter, i.e., using one of the energy levels as input, the other energy levels can be obtained via Efimov's radial scaling law, which is intimately tied to a discrete scaling symmetry. It is demonstrated that the discrete scaling symmetry persists in the presence of 1D spin-orbit coupling, implying the validity of a generalized radial scaling law in five-dimensional space. The dependence of the energy levels on the scattering length, spin-orbit coupling parameters, and center-of-mass momentum is discussed. It is conjectured that three-body systems with other types of spin-orbit coupling terms are also governed by generalized radial scaling laws, provided the system exhibits the Efimov effect in the absence of spin-orbit coupling.

cond-mat.quant-gas↗

arXiv2017

Analytical coupled-channels treatment of two-body scattering in the presence of three-dimensional isotropic spin-orbit coupling

It is shown that the single-particle spin-orbit coupling terms, which---in the cold atom context---are associated with synthetic gauge fields, can significantly and non-trivially modify the phase accumulation at small interparticle distances even if the length scale $(k_{\text{so}})^{-1}$ associated with the spin-orbit coupling term is significantly larger than the van der Waals length $r_{\text{vdW}}$ that characterizes the two-body interaction potential. A theoretical framework, which utilizes a generalized local frame transformation and accounts for the phase accumulation analytically, is developed. Comparison with numerical coupled-channels calculations demonstrates that the phase accumulation can, to a very good approximation, be described over a wide range of energies by the free-space scattering phase shifts---evaluated at a scattering energy that depends on $k_{\text{so}}$---and the spin-orbit coupling strength $k_{\text{so}}$.

cond-mat.quant-gas↗