Effect of external magnetic field on the shift of resonant frequency in photoassociation of ultracold Cs atoms

We study the influence of external magnetic field on the shift of the resonant frequency in the photoassociation of ultracold Cs atoms, which are captured in a magnetically levitated optical crossed dipole trap. With the increase of the photoassociation laser intensity, the linear variation of the frequency shift is measured by recording the photoassociation spectra of the long-range 0_u^+ state of Cs molecule below the 6S_(1/2)+ 6P_(1/2) dissociation limit at different magnetic fields.The slope of the frequency shift to the intensity of the photoassociation laser exhibits a strong dependence on the external magnetic field. The experimental data is simulated with an analytic theory model, in which a single channel rectangular potential with the tunable well depth is introduced to acquire the influence of the magnetic field on the atomic behavior in the effective range where photoassociation occurs.

Absorption of laser radiation in ultracold plasma

In this paper, we analyze the spectral behavior(optical thickness, shape and linewidth) of laser radiation absorption under the correlation heating of ions in an ultracold plasma. The Voigt formula is used to find the absorption coefficient.The spectral line width is shown to grow with time while the optical thickness reduces. Our modeling results are in good agreement with the experimental findings reported in the literature.

Systematically investigating the polarization gradient cooling in an optical molasses of ultracold cesium atoms

We systematically investigate the polarization gradient cooling （PGC） process in an optical molasses of ultracold cesium atoms. The SR mode for changing the cooling laser, which means that the cooling laser frequency is stepped to the setting value while its intensity is ramped, is found to be the best for the PGC, compared with other modes studied. We verify that the heating effect of the cold atoms, which appears when the cooling laser intensity is lower than the saturation intensity, arises from insufficient polarization gradient cooling. Finally, an exponential decay function with a statistical explanation is introduced to explain the dependence of the cold atom temperature on the PGC interaction time.

Measurement of quantum defects of nS and nD states using field ionization spectroscopy in ultracold cesium atoms

This paper reports that ultracold atoms are populated into different nS and nD Rydberg states （n=25-52） by two-photon excitation. The ionization spectrum of an ultracold Rydberg atom is acquired in a cesium magneto-optical trap by using the method of pulse field ionization. This denotes nS and nD states in the ionization spectrum and fits the data of energy levels of different Rydberg states to obtain quantum defects of nS and nD states.

Rotational Population Measurement of Ultracold 85Rb^133Cs Molecules in the Lowest Vibrational Ground State

We measure the rotational populations of ultracold SS Rbla3 Cs molecules in the lowest vibrational ground state by a depletion spectroscopy and quantify the molecular production rate based on the measurement of single ion signal area. The SSRb133Cs molecules in the X1∑＋（v = 0） are formed from the short-range （2）^3П0＋（V = 10, J = 0） molecular state. A home-made external-cavity diode laser is used as the depletion laser to measure the rotational populations of the formed molecules. Based on the determination of single ion signal, the production rates of molecules in the J=0 and J = 2 rotational levels are derived to be 4800mole/s and 7200mole/s, respectively. The resolution and quantification of molecules in rotational states are facilitative for the manipulation of rotational quantum state of ultracold molecules.

Analytical Solution for an Ultracold Trapped Ion at the Node of the Standing Wave Laser Without Rotating Wave Approximation

A method of solving an ultracold trapped ion at the node of the standing wave laser without rotating wave approximation is proposed and the analytical forms of the eigenfunctions and eigenenergies of the system are presented.

Long Lifetime Ultracold Plasma Produced by a Nanosecond Laser in a Supersonic Nitric Oxide Molecular Beam Environment

Ultracold plasma provides a possible route to approach the strongly-coupled regime under laboratory conditions. Normally, the lifetime of ultracold plasma is very limited due to plasma heating and expansion mechanisms. We present a new method to generate long lifetime ultracold plasmas consisting mainly of cations and anions. This plasma demonstrates a capability of traversing a DC barrier of up to 5 （or -3） V. The lifetime of this plasma is expected to be more than 250us. Finally, molecular dynamics （MD） simulation is used to explain how anions slow the expansion rate and prolong the lifetime of this plasma.

Highly sensitive photoassociation spectroscopy of ultracold 23Na133Cs molecular long-range states below the 3S1/2＋6P3/2 limit

We present high resolution photoassociation spectroscopy of ultracold 23Na133Cs molecules in a long-range c3∑＋ state below the （3S1/2 ＋ 6P3/2） asymptote. We perform photoassociation spectroscopy in a dual-species magneto-optical trap （MOT） and detect the photoassociation resonances using trap-loss spectroscopy. By fitting the experimental data with the semi-classical LeRoy-Bernstein formula, we deduce the long-range molecular coefficient C6 and derive the empirical potential energy curve in the long-range region.

High resolution photoassociation spectra of an ultracold Cs_2 long-range 0_u^+ (6S_(1/2)+6P_(1/2) ) state

In this paper, ultracold cesium molecules are formed through photoassociation technology, which is carried out in a magneto-optical trap. High resolution photoassociaion spectra with the rotational progressions up to J = 7 are obtained. Three rovibrational levels of the long-range 0＋ state of Cs2 below the （6S1/2 ＋ 6P1/2） dissociation limit are specifically investigated. By fitting their binding energy intervals to the non-rigid rotational model, the rotational constant of the long- range 0u＋ state is determined. A proportional dependence of the value of the rotational constant on the vibrational quantum number is demonstrated.

Enhancement of signal-to-noise ratio of ultracold polar NaCs molecular spectra by phase locking detection

We report a method of high-sensitively detecting the weak signal in photoassociation （PA） spectra of ultracold NaCs molecules by phase sensitive-demodulated trap-loss spectra of Na atoms from a photomultiplier tube. We find that the signal-to-noise ratio （SNR） of the PA spectra is strongly dependent on the integration time and the sensitivity of the lock-in amplifier, and our results show reasonable agreement with the theoretical analyses of the SNR with the demodulation parameters. Meanwhile, we investigate the effect of the interaction time of the PA laser with the colliding Na-Cs atom pairs on the SNR of the PA spectra. The atom loss rate is dependent on both the PA-induced atom loss and the loading of the MOT. The high-sensitive detection of the excited ultracold NaCs molecules lays a solid foundation for further study of the formation and application of ultracold NaCs molecules.

Superfluidity of Bose Einstein condensates in ultracold atomic gases

Liquid helium 4 had been the only bosonic superfluid available in experiments for a long time. This situation was changed in 1995, when a new superfluid was born with the realization of the Bose-Einstein condensation in ultracold atomic gases. The liquid helium 4 is strongly interacting and has no spin; there is almost no way to change its parameters, such as interaction strength and density. The new superfluid, Bose-Einstein condensate （BEC）, offers various advantages over liquid helium. On the one hand, BEC is weakly interacting and has spin degrees of freedom. On the other hand, it is convenient to tune almost all the parameters of a BEC, for example, the kinetic energy by spin--orbit coupling, the density by the external potential, and the interaction by Feshbach resonance. Great efforts have been devoted to studying these new aspects, and the results have greatly enriched our understanding of superfluidity. Here we review these developments by focusing on the stability and critical velocity of various superfluids. The BEC systems considered include a uniform superfluid in free space, a superfluid with its density periodically modulated, a superfluid with artificially engineered spinorbit coupling, and a superfluid of pure spin current. Due to the weak interaction, these BEC systems can be well described by the mean-field Gross-Pitaevskii theory and their superfluidity, in particular critical velocities, can be examined with the aid of Bogoliubov excitations. Experimental proposals to observe these new aspects of superfluidity are discussed.

Recombination during expansion of ultracold plasma

Signals of ultracold plasma are observed by two-photon ionization of laser-cooled caesium atoms in a magnetooptical trap. Recombination of ions and electrons into Rydberg atoms during the expansion of ultracold plasma is investigated by using state-selective field ionization spectroscopy. The dependences of recombination on initial electron temperature （1 70 K） and initial ion density （-10^10 cm-3） are investigated. The measured dependence on initial ion density is N^1.547±0.004 at a delay time of 5μs. The recombination rate rapidly declines as initial electron temperature increases when delay time is increased. The distributions of Rydberg atoms on different values of principal quantum number n, i.e. n = 30-60, at an initial electron temperature of 3.3 K are also investigated. The main experimental results are approximately explained by the three-body recombination theory.

Energy spectrum and superfluidity of spin-2 ultracold bosons in optical lattices

This paper studies the superfluidity of ultracold spin-2 Bose atoms with weak interactions in optical lattices by calculating the excitation energy spectrum using the Bogoliubov approach. The energy spectra exhibit the characteristics of the superfluid-phase explicitly and it finds the nonvanishing critical speeds of superfiuid. The obtained results display that the critical speeds of superfiuid are different for five spin components and can be controlled by adjusting the lattice parameters in experiments. Finally it discusses the feasibilities of implementing and measuring superfluid.

Photoassociation spectra of ultracold(85)Rb2 molecule in 0u+long range state near the 5S(1/2)+5P(1/2)asymptote

We investigate the high resolution photoassociation spectra of ^（85）Rb_2 molecules in 0~＋_u long range state below the（5S_（1/2）＋ 5P_（1/2）） asymptote. The ^（85）Rb atomic samples are trapped in a dark magneto–optical trap（MOT） and prepared in the dark state. With the help of trap loss technique, we obtain considerable photoassociation spectroscopy with rovibrational resolution, some of which have never been observed before. The observed spectrum is fitted by a rigid rotation model, and the rotational constants of ultracold ^（85）Rb_2 molecule in long range 0~＋_u are obtained for different vibrational states. By applying the Le Roy–Bernstein method, we assign the vibrational quantum numbers and derive C_3 coefficient, which is used to obtain the potential energy curve.

Production and detection of ultracold Cs_2 molecules via four-photon adiabatic passage

We study theoretically how to produce and detect the ultracold ground-state Cs2 molecule from Feshbach state. Nu- merical calculations are performed by solving the quantum Liouville equation based on multilevel Bloch model. The producing efficiency reaches 55% and the detecting efficiency is 31%. The producing and detecting efficiencies are closely related to the Rabi frequencies of laser pulses. The decay of relevant electronic and vibrational states obviously reduces the producing and detecting efficiencies.

Photoassociation of ultracold RbCs molecules into the (2)0^- state (v = 189, 190) below the 5S_(1/2) + 6P_(1/2) dissociation limit

Ultracold polar RbCs molecules are produced via photoassociation in a laser-cooled mixture of ^85Rb and ^133Cs atoms. The a3∑＋ state molecules which decay from electronically excited （2）0- state RbCs molecules are detected by resonance- enhanced two-photon ionization. The new rovibrational levels （v = 189, 190） in the （2）0- state are also observed, which exist in theory and have not been observed in experiments yet. The corresponding rotational constants are measured by photoassociation spectroscopy, which are consistent with theoretical calculations using a nonrigid rotor model.

The effect of the direct current electric field on the dynamics of the ultracold plasma

We created an ultracold plasma by photoionizing the laser-cooled and trapped rubidium atoms in a magneto-optical trap. In the externally applied direct current（DC） electric field environment,the electrons which escape from the potential well of the ultracold plasma were detected for different numbers of the ions and initial kinetic energies of the electrons. The results are in good agreement with the calculations, based on the Coulomb potential well model, indicating that the external DC field is an effective tool to adjust the depth of potential well of the plasma, and it is possible to create an ultracold plasma in a controlled manner.

Sonic horizon dynamics of ultracold Fermi system under elongated harmonic potential

We study the phenomena of the sonic horizon in an ultracold atomic Fermi system in an elongated harmonic trap.Based on the one-dimensional Gross–Pitaevskii equation model and variational method combined with exact derivation approach, we derive an analytical formula which describes the occurrence of the sonic horizon and the associated Hawking radiation temperature. Using a pictorial demonstration of the key physical quantities we identify the features reported in prior numerical studies of a three-dimensional（3 D） ultracold atomic system, proving the applicability of the theoretical model presented here.

Magnetic-Field Dependence of Raman Coupling Strength in Ultracold ^(40)K Atomic Fermi Gas

We experimentally demonstrate the relation of Raman coupling strength with the external bias magnetic field in degenerate Fermi gas of 40K atoms. Two Raman lasers couple two Zeeman energy levels, whose energy splitting depends on the external bias magnetic field. The Raman coupling strength is determined by measuring the Rabi oscillation frequency. The characteristics of the Rabi oscillation is to be damped after several periods due to Fermi atoms in different momentum states oscillating with different Rabi frequencies. The experimental results show that the Raman coupling strength will decrease as the external bias magnetic field increases, which is in good agreement with the theoretical prediction.

High-resolution photoassociation spectroscopy of ultracold Cs_2 long-range 0_g~- state: The external well potential depth

High-resolution photoassociation spectroscopy is reported using a modulation spectroscopy technology in a cesium atomic magneto-optical trap. The two lowest vibrational levels have been experimentally observed which have been theo- retically predicted in [Phys. Rev. A 75 052501 （2007）]. A new potential curve is obtained by using the Rydberg-Klein-Ress method with a well depth of -82,384 ＋ 0,026 cm-1, which is deeper than the result of previous experiment （-77.909 cm-1） and the theoretical prediction （-81.6445 cm- l ）.