arXiv subjects

Lingran Kong

Publications and source records attributed to Lingran Kong.

7 recordsLinked to original sources

arXiv2024

Macroscopic superposition of vortex states in a matter wave

Generating the vortex-state superposition in a matter wave is demanded in many quantum processes such as quantum memory and quantum metrology. Here we report the experimental generation of macroscopic superposition of vortex states in ultracold quantum gases. By transferring an optical vortex-state superposition to the center-of-mass rotational state of ultracold atoms using the Raman coupling technique, we realize two-vortex and three-vortex superposition states in quantum gases, demonstrating the high dimensionality of the vortex state. We show the controllability of the superposition states on the Bloch sphere. The lifetime of the vortex superposition state in quantum gases is as large as 25 ms, about two orders of magnitude longer than the storage time in atomic ensembles. This work paves the way for high dimensional quantum processing in matter waves.

cond-mat.quant-gas↗

arXiv2022

Characterizing the temporal rotation and radial twist of the interference pattern of vortex beam

We experimentally and theoretically characterize the temporal rotation and radial twist of the interference pattern of vortex beam with its conjugate copy. To quantitatively study the temporal rotation and radial twist, we controllably modify the conjugate beam with a frequency or wavefront curvature difference using a movable Mach-Zehnder interferometer. The effects of the physical parameters (i.e., the topological charge, frequency difference and wavefront curvature difference of the vortex beams) on the temporal rotation as well as radial twist are systematically explored. We further measure two parameters, the rotation velocity $Ω$ and twist coefficient $α$, respectively, to characterize the degree of the temporal rotation and radial twist of the interference pattern. The theory of the interference model on vortex beams have good agreements with the experimental results. This work is favorite to study the detailed structure of the interference pattern and manipulate matter with superimposed vortex beams.

physics.optics↗

arXiv2022

Phase-locking matter-wave interferometer of vortex states

Matter-wave interferometer of ultracold atoms with different linear momenta has been extensively studied in theory and experiment. The vortex matter-wave interferometer with different angular momenta is applicable as a quantum sensor for measuring the rotation, interatomic interaction, geometric phase, etc. Here we report the first experimental realization of a vortex matter-wave interferometer by coherently transferring the optical angular momentum to an ultracold Bose condensate. After producing a lossless interferometer with atoms only populating the two spin states, we demonstrate that the phase difference between the interferences in the two spin states is locked on $π$. We also demonstrate the robustness of this out-of-phase relation, which is independent of the angular-momentum difference between the two interfering vortex states, constituent of Raman optical fields and expansion of the condensate. The experimental results agree well with the calculation from the unitary evolution of wave packet in quantum mechanics. This work opens a new way to build a quantum sensor and measure the atomic correlation in quantum gases.

cond-mat.quant-gas↗

arXiv2018

Expansion dynamics of a spherical Bose-Einstein condensate

We experimentally and theoretically observe the expansion behaviors of a spherical Bose-Einstein condensate. A rubidium condensate is produced in an isotropic optical dipole trap with an asphericity of 0.037. We measure the variation of the condensate size during the expansion process. The free expansion of the condensate is isotropic, which is different from that of the condensate usually produced in the anisotropic trap. The expansion in the short time is speeding and then after a long time the expansion velocity asymptotically approaches a constant value. We derive an analytic solution of the expansion behavior based on the spherical symmetry, allowing a quantitative comparison with the experimental measurement. The interaction energy of the condensate is gradually converted into the kinetic energy at the beginning of the expansion and the kinetic energy dominates after a long-time expansion. We obtain the interaction energy of the condensate in the trap by probing the expansion velocity, which is consistent with the theoretical prediction.

cond-mat.quant-gas↗

arXiv2018

Observation of phonon parametric down-conversion in a spherical Bose-Einstein condensate

We report the observation of parametric down-conversion of phonons in a spherical Bose-Einstein condensate. The spherical symmetry, which is crucial for observing this phenomenon, is experimentally demonstrated by measuring the collective mode and expansion behavior of the condensate. The low-energy monopole mode is excited by coupling with a high-energy mode with a nearly twice eigen-frequency. The population of the low-energy mode becomes maximum only when the high-energy mode is resonantly excited. Furthermore, we directly observe the parametric down-conversion process in the driving process, through simultaneously probing the two coupling modes. The experimental observation is consistent with the perturbation theory including the gravity effect. This work opens the challenge in related study of the condensate beyond mean-field theory and has potential applications in quantum information.

cond-mat.quant-gas↗

arXiv2018

Observation of quantum phase transition in spin-orbital-angular-momentum coupled Bose-Einstein condensate

Orbital angular momentum (OAM) of light represents a fundamental optical freedom that can be exploited to manipulate quantum state of atoms. In particular, it can be used to realize spin-orbital-angular-momentum (SOAM) coupling in cold atoms by inducing an atomic Raman transition using two laser beams with differing OAM. Rich quantum phases are predicted to exist in many-body systems with SOAM coupling. Their observations in laboratory, however, are often hampered by the limited control of the system parameters. In this work we report, for the first time, the experimental observation of the ground-state quantum phase diagram of the SOAM coupled Bose-Einstein condensate (BEC). The discontinuous variation of the spin polarization as well as the vorticity of the atomic wave function across the phase boundaries provides clear evidence of first-order phase transitions. Our results open up a new way to the study of phase transitions and exotic quantum phases in quantum gases.

cond-mat.quant-gas↗

arXiv2017

Optimizing the optical imaging system by \emph{in-situ} imaging the plugged hole in the ultracold atoms

Optical absorption imaging has become a common technique for detecting the density distribution of ultracold atoms. The defocus effect generally produces artificial spatial structures in the obtained images, which confuses our understanding of the quantum systems. Here we experimentally demonstrate one method to optimize the optical imaging system by \emph{in-situ} imaging the plugged hole in the cold atoms. The atoms confined in a magnetic trap are cooled to tens of or several microkelvin by the radio-frequency evaporation cooling, and then are plugged using a blue-detuned laser beam, forming a hole in the center of the atomic cloud. We image the hole with a charge-coupled device (CCD) and quantitatively analyze the artificial spatial structure due to the defocus effect. Through minimizing the artificial structures by precisely adjusting the CCD position, we can optimize the imaging system with an accuracy of 0.1 mm. We also demonstrate the necessity of this method in probing rubidium BEC with a time of flight (TOF) of 5 ms. Compared to other methods in focusing the imaging system, the proposal demonstrated in this paper is simple and efficient, particularly for experimentally extracting large-scale parameters like atomic density, atomic number and the size of the atomic cloud.

physics.atom-ph↗