arXiv subjects

Su Yi

Publications and source records attributed to Su Yi.

At least 55 records · Page 3Linked to original sources

arXiv2020

Theory for Self-Bound States of Dipolar Bose-Einstein Condensates

We investigate the self-bound states of dipolar Dy condensates with the Gaussian-state ansatz which improves the conventional coherent-state ansatz with multimode squeezed coherent states. We show that the self-bound states consist of the experimentally observed self-bound liquid phase and the unobserved self-bound gas phase. The numerically obtained gas-liquid boundary is in good agreement with experimental data. Our theory also allows one to extract the real part of the three-body coupling constant of the Dy atoms from the particle number distribution of the condensates. In particular, we results show that the self-bound states are stabilized by the short-range three-body repulsion. Our study shed a different light to understand the self-bound droplets of Bose-Einstein condensates.

cond-mat.quant-gas↗

arXiv2019

Trapped Bose-Einstein Condensates with Attractive s-wave Interaction

Within the framework of the Gaussian-state theory, we show that the quantum many-body ground state of a trapped condensate with weakly attractive interaction is a single-mode squeezed vacuum state, as oppose to the coherent state under repulsive interaction. The spatial mode of the squeezed-state condensates satisfies a Gross-Pitaevskii like equation in which the interaction strength is augmented by a factor $3$ due to the large particle fluctuation of the squeezed state. We also study the collective excitations of the condensates by the tangential space projection, which leads to new two-particle excitations and confirms the phase transition from coherent-state to squeezed-state condensates. Our investigation clarifies the quantum states of the attractive condensates and will shed new light on research of the droplet phases in dipolar and multicomponent condensates.

quant-ph↗

arXiv2019

Efficient generation of many-body entangled states by multilevel oscillations

We generate high-fidelity massively entangled states in an antiferromagnetic spin-1 Bose-Einstein condensate (BEC) by utilizing multilevel oscillations. Combining the multilevel oscillations with additional adiabatic drives, we greatly shorten the necessary evolution time and relax the requirement on the control accuracy of quadratic Zeeman splitting, from micro-Gauss to milli-Gauss, for a $^{23}$Na spinor BEC. The achieved high fidelities over $96\%$ show that two kinds of massively entangled states, the many-body singlet state and the twin-Fock state, are almost perfectly generated. The generalized spin squeezing parameter drops to a value far below the standard quantum limit even with the presence of atom number fluctuations and stray magnetic fields, illustrating the robustness of our protocol under real experimental conditions. The generated many-body entangled states can be employed to achieve the Heisenberg-limit quantum precision measurement and to attack nonclassical problems in quantum information science.

quant-ph↗

arXiv2019

Parity-Enhanced Quantum Optimal Measurements

We find a large class of pure and mixed input states with which the phase estimation precision saturates the Cramer-Rao bound under the compound measurements of parity and particle number. We further propose a quantum-phase-estimation protocol for arbitrary input states, through which the precision achieved is always higher than or equal to that obtained via the original input state. We also demonstrate the implementation of the proposed scheme using a nonlinear interferometry and the realization of the nondemolition parity measurement in atomic condensates.

quant-ph↗

arXiv2018

Geometric energy transfer in a Stückelberg interferometer of two parametrically coupled mechanical modes

Geometric phase, which is acquired after a system undergoing cyclic evolution in the Hilbert space, is believed to be noise-resilient because it depends only on the global properties of the evolution path. Here, we report geometric control of energy transfer between two parametrically coupled mechanical modes in an optomechanical system. The parametric pump is controlled along a closed loop to implement geometric Stückelberg interferometry of mechanical motion, in which the dynamical phase is eliminated using the Hahn-echo technique. We demonstrate that the interference based solely on the geometric phase is robust against certain noises. And, more remarkably, we show that the all-geometric approach can achieve a high energy-transfer rate that is comparable to those using conventional dynamical protocols.

physics.optics↗

arXiv2017

Classical dynamical localization in a strongly driven two-mode mechanical system

We report the realization of dynamical localization in a strongly driven two-mode optomechanical system consisting of two coupled cantilevers. Due to the coupling, mechanical oscillations can transport between the cantilevers. However, by placing one of the cantilevers inside a harmonically oscillating optical trap, we demonstrate that mechanical oscillations become tightly bounded to the isolated cantilevers rather than propagate away at specific driving parameters. The effect of dynamical localization is employed to induce a conductor-to-insulator-like transition in the two-mode mechanical system, which opens up new possibilities for further coherent control of transport phenomena in coupled-mechanical-resonator based lattice.

physics.optics↗

arXiv2017

Topological Fulde-Ferrell Superfluids in Triangular Lattices

Fulde-Ferrell (FF) Larkin-Ovchinnikov (LO) phases were proposed for superconductors or superfluids in strong magnetic field. With the experimental progresses in ultracold atomic systems, topological FFLO phases has also been put forward, since it is a natural consequence of realizable spin-orbital coupling (SOC).In this work, we theoretically investigate a triangular lattice model with SOC and in-plane field. By constructing the phase diagram, we show that it can produce topological FF states with Chern numbers, $C=\pm1$ and $C=-2$. We get the phase boundaries by the change of the sign of Pfaffian. The chiral edge states for different topological FF phases are also elucidated.

cond-mat.quant-gas↗

arXiv2016

Dynamics of a coupled spin vortex pair in dipolar spinor Bose-Einstein condensates

The collisional and magnetic field quench dynamics of a coupled spin-vortex pair in dipolar spinor Bose-Einstein condensates in a double well potential are numerically investigated in the mean field theory. Upon a sudden release of the potential barrier the two layers of condensates collide with each other in the trap center with the chirality of the vortex pair exchanged after each collision, showing the typical signature of in-phase collision for the parallel spin vortex phase, and out-of-phase collision for the antiparallel phase. When quenching the transverse magnetic field, the vortex center in the single-layered condensate starts to make a helical motion with oval-shaped trajectories and the displacement of the center position is found to exhibit a damped simple harmonic oscillation with an intrinsic frequency and damping rate. The oscillation mode of the spin vortex pair may be tuned by the initial magnetic field and the height of the Gaussian barrier, e.g. the gyrotropic motions for parallel spin vortex pair are out of sync with each other in the two layers, while those for the antiparallel pair exhibit a double-helix-structure with the vortex centers moving opposite to each other with the same amplitude.

cond-mat.quant-gas↗

arXiv2016

Magnetic field induced dynamical instabilities in an anti-ferromagnetic spin-1 Bose-Einstein condensate

We theoretically investigate four types of dynamical instability, in particular the periodic and oscillatory type $I_O$, in an anti-ferromagnetic spin-1 Bose-Einstein condensate in a nonzero magnetic field, by employing the coupled-mode theory and numerical method. This is in sharp contrast to the dynamical stability of the same system in zero field. Remarkably, a pattern transition from a periodic dynamical instability $I_O$ to a uniform one $III_O$ occurs at a critical magnetic field. All the four types of dynamical instability and the pattern transition are ready to be detected in $^{23}$Na condensates within the availability of the current experimental techniques.

cond-mat.quant-gas↗

arXiv2016

Classical analog of Stückelberg interferometry with two coupled mechanical resonators

Coupled nanomechanical resonators have recently attracted much attention for both fundamental studies in physics and broad applications in high-precession detection or sensing. By studying the Landau-Zener transitions and Rabi oscillation of two coupled resonators, it has been shown that such a two-mode system acts as a classical two-level system bearing the analog to a quantum mechanical two-level one. Here we construct a Stückelberg interferometer with two coupled cantilevers by driving the system through the avoided crossing twice. By measuring the non-adiabatic phase acquired at the Landau-Zener transition, we unveil an in-depth analog between the two-mode and quantum two-level systems. Our study opens up new opportunities for constructing interferometers with classical devices.

cond-mat.mes-hall↗

arXiv2015

Fermi polaron in a one-dimensional quasi-periodic optical lattice: the simplest many-body localization challenge

We theoretically investigate the behavior of a moving impurity immersed in a sea of fermionic atoms that are confined in a quasi-periodic (bichromatic) optical lattice, within a standard variational approach. We consider both repulsive and attractive contact interactions for such a simplest many-body localization problem of Fermi polarons. The variational approach enables us to access relatively large systems and therefore may be used to understand many-body localization in the thermodynamic limit. The energy and wave-function of the polaron states are found to be strongly affected by the quasi-random lattice potential and their experimental measurements (i.e., via radio-frequency spectroscopy or quantum gas microscope) therefore provide a sensitive way to underpin the localization transition. We determine a phase diagram by calculating two critical quasi-random disorder strengths, which correspond to the onset of the localization of the ground-state polaron state and the many-body localization of all polaron states, respectively. Our predicted phase diagram could be straightforwardly examined in current cold-atom experiments.

cond-mat.quant-gas↗

arXiv2015

Coupled spin-vortex pair in dipolar spinor Bose-Einstein condensates

We investigate the ground-state and magnetic properties of a dipolar spin-1 Bose-Einstein condensate trapped in a symmetric double-well potential. In particular, we focus on the spin-vortex states by assuming that each potential well is highly pancake shaped. We show that the presence of the double-well potential gives rise to two different spin configurations for the spin-vortex pair states. We also study the response of the coupled spin-vortex pair to static transverse magnetic fields.

cond-mat.quant-gas↗

arXiv2014

Gapless topological Fulde-Ferrell superfluidity in spin-orbit coupled Fermi gases

Topological superfluids usually refer to a superfluid state which is gapped in the bulk but metallic at the boundary. Here we report that a gapless, topologically non-trivial superfluid with inhomogeneous Fulde-Ferrell pairing order parameter can emerge in a two-dimensional spin-orbit coupled Fermi gas, in the presence of both in-plane and out-of-plane Zeeman fields. The Fulde-Ferrell pairing - induced by the spin-orbit coupling and in-plane Zeeman field - is responsible for this gapless feature. This exotic superfluid has a significant Berezinskii-Kosterlitz-Thouless (BKT) transition temperature and has robust Majorana edge modes against disorder owing to its topological nature.

cond-mat.quant-gas↗

arXiv2013

Observing dipolar confinement-induced resonances in waveguides

We develop a theoretical framework for the quasi-low-dimensional confinement-induced resonances (CIRs) of particles with the arbitrary three-dimensional two-body interactions, based on the Huang-Yang pseudopotential and the treatment of Feshbach resonances. Using this new approach, we analytically obtain some universal properties of dipolar CIRs in quasi-one-dimensional (1D) waveguides. We also show that the dipolar CIRs can be induced by tuning the angle between the dipole moments and the waveguide, which is experimentally observable in quasi-1D Cr and Dy atomic gases. We expect that these tilting angle induced CIRs will open up a new simpler way to control the resonant scatterings in quasi-low-dimensional systems.

cond-mat.quant-gas↗

arXiv2012

Macroscopic quantum coherence in spinor condensates confined in an anisotropic potential

We investigate the macroscopic quantum coherence of a spin-1 Rb condensate confined in an anisotropic potential. Under the single-mode approximation, we show that the system can be modeled as a biaxial quantum magnet due to the interplay between the magnetic dipole-dipole interaction and the anisotropic potential. By applying a magnetic field along the hard-axis, we show that the tunneling splitting oscillates as a function of the field strength. We also propose an experimental scheme to detect the oscillatory behavior of the tunneling splitting by employing the Landau-Zener tunneling.

cond-mat.quant-gas↗

arXiv2012

Model anisotropic quantum Hall states

Model quantum Hall states including Laughlin, Moore-Read and Read-Rezayi states are generalized into appropriate anisotropic form. The generalized states are exact zero-energy eigenstates of corresponding anisotropic two- or multi-body Hamiltonians, and explicitly illustrate the existence of geometric degrees of in the fractional quantum Hall effect. These generalized model quantum Hall states can provide a good description of the quantum Hall system with anisotropic interactions. Some numeric results of these anisotropic quantum Hall states are also presented.

cond-mat.str-el↗

arXiv2010

Quantum anti-Zeno effect without wave function reduction

We study the measurement-induced enhancement of the spontaneous decay (called quantum anti-Zeno effect) for a two-level subsystem, where measurements are treated as couplings between the excited state and an auxiliary state rather than the von Neumann's wave function reduction. The photon radiated in a fast decay of the atom, from the auxiliary state to the excited state, triggers a quasi-measurement, as opposed to a projection measurement. Our use of the term "quasi-measurement" refers to a "coupling-based measurement". Such frequent quasi-measurements result in an exponential decay of the survival probability of atomic initial state with a photon emission following each quasi-measurement. Our calculations show that the effective decay rate is of the same form as the one based on projection measurements. What is more important, the survival probability of the atomic initial state which is obtained by tracing over all the photon states is equivalent to the survival probability of the atomic initial state with a photon emission following each quasi-measurement to the order under consideration. That is because the contributions from those states with photon number less than the number of quasi-measurements originate from higher-order processes.

quant-ph↗

arXiv2009

Magnetic properties of a spin-3 Chromium condensate

We study the ground state properties of a spin-3 Cr condensate subject to an external magnetic field by numerically solving the Gross-Piteavskii equations. We show that the widely adopted single-mode approximation is invalid under a finite magnetic field. In particular, a phase separation like behavior may be induced by the magnetic field. We also point out the possible origin of the phase separation phenomenon.

cond-mat.other↗