Coherent electronic coupling versus localization in individual molecular dimers

We have investigated electronic excitation transfer in individual molecular dimers by time and spectrally resolved confocal fluorescence microscopy. The single molecule measurements allow for directly probing the distribution of the electronic coupling strengths due to static disorder in the polymer host. We find dimers where the excitation is delocalized (superradiant emission) while for others emission originates from a localized state. Transitions between delocalized and localized states as observed for a given dimer are attributed to structural fluctuations of the guest-host system.     

Coherent states in optics

An approach has been developed with the use of coherent states for constructing wavelets based on spiral light beams.     

Two-component dimers of ultracold atoms with center-of-mass-momentum dependent interactions

In a previous paper [Phys. Rev. A 95, 060 701（R）(2017)], we demonstrated that a new type of two-body interaction, which depends on the center of mass（CoM） momentum, can be realized for ultracold atoms via laser-modulated magnetic Feshbach resonance（MFR）. Further studies(e.g. L He et al, Phys. Rev. Lett. 120, 045 302（2018）) show that various interesting phenomena, such as Fulde–Ferrell superfluids, can be induced by scattering between ultracold atoms with this interaction. In this work we investigate the shallow bound states of two ultracold atoms with this type of interaction. We show that when the magnetic field B is below the MFR point B₀, two shallow bound states can appear in this system. Namely, a ‘two-component dimer’ or a dimer with pseudo-spin 1/2 can be formed by two atoms. Furthermore, the dispersion curve of the dimer may have either single or double minimums in the CoM momentum space. The latter case can be explained as a result from significant pseudo-spin-orbital coupling（SOC） effects. Our results show that the ultracold gases with CoM momentum dependent interaction may be a candidate for quantum simulations with ultracold two-component molecules, especially the molecule gases with SOC.     

Coherent beam combination of an optical array using adaptive fiber optics collimators

A novel adaptive-fiber-optics-collimator (AFOC) compensating both piston-type and tip/tilt-type phase errors of output beam is introduced, and has been employed in experiments of coherent beam combination (CBC) of a delta distributed fiber array. Feedback control is realized using stochastic parallel gradient descent (SPGD) algorithm. Excellent CBC effect has been achieved when piston and tip/tilt errors among beamlets corrected. The necessity of wavefront tip/tilt control in CBC is verified. Experimental results exhibit great potential applications of this kind of AFOC in fiber amplifier arrays.     

Coherent transients in the femtosecond photoassociation of ultracold molecules

We demonstrate the photoassociation of ultracold rubidium dimers using coherent femtosecond pulses. Starting from a cloud of ultracold rubidium atoms, electronically excited rubidium molecules are formed with shaped photoassociation pump pulses. The excited state molecules are projected with a time-delayed probe pulse onto molecular ion states which are detected in a mass spectrometer. Coherent transient oscillations of the excited state population are observed in the wings of the pump pulse, in agreement with the time-dependent solution of the Schr?dinger equation of the excitation process.     

Coherent excitation of Rydberg atoms in an ultracold gas

We demonstrate two schemes for the coherent excitation of Rydberg atoms in an ultracold gas of rubidium atoms employing the three-level ladder system 5S_1/2-5P_3/2-n?_j. In the first approach rapid adiabatic passage with pulsed laser fields yields Rydberg excitation probabilities of 90% in the center of the laser focus. In a second experiment two-photon Rydberg excitation with continuous-wave fields is applied which results in Rabi oscillations between the ground and Rydberg state. The experiments represent a prerequisite for the control of interactions in ultracold Rydberg gases and the application of ultracold Rydberg gases for quantum information processing.     

Giant helium dimers produced by photoassociation of ultracold metastable atoms

We produce giant, purely long-range helium dimers by photoassociation of metastable helium atoms in a magnetically trapped, ultracold cloud. The photoassociation laser is detuned close to the atomic 2(3)S1-2(3)P0 line and produces strong heating of the sample when resonant with molecular bound states. The temperature of the cloud serves as an indicator of the molecular spectrum. We report good agreement between our spectroscopic measurements and our calculations of the five bound states belonging to a 0(+)(u) purely long-range potential well. These previously unobserved states have classical inner turning points of about 150a(0) and outer turning points as large as 1150a(0).     

Dissociation and decay of ultracold sodium molecules

The dissociation of ultracold molecules was studied by ramping an external magnetic field through a Feshbach resonance. The observed dissociation energies directly yielded the strength of the atom-molecule coupling. They showed nonlinear dependence on the ramp speed. This was explained by a Wigner threshold law which predicts that the decay rate of the molecules above threshold increases with the density of states. In addition, inelastic molecule-molecule and molecule-atom collisions were characterized.     

Production of ultracold trapped molecular hydrogen ions

We have cooled ensembles of the molecular hydrogen ions H2+, H3+, and all their deuterated variants to temperatures of a few mK in a radio frequency trap, by sympathetic cooling with laser-cooled beryllium ions. The molecular ions are embedded in the central regions of Coulomb crystals. Mass spectroscopy and molecular dynamics simulations were used to accurately characterize the properties of the ultracold multispecies crystals. We demonstrate species-selective purification of multispecies ensembles. These molecules are of fundamental importance as the simplest of all molecules, and have the potential to be used for precision tests of molecular structure theory, tests of Lorentz invariance, and measurements of electron to nuclear mass ratios and their time variation.     

Rotational Feshbach resonances in ultracold molecular collisions

In collisions at ultralow temperatures, molecules will possess Feshbach resonances, foreign to ultracold atoms, whose virtual excited states consist of rotations of the molecules. We estimate the mean spacing and mean widths of these resonant states, exploiting the fact the molecular collisions at low energy display chaotic motion. As examples, we consider the experimentally relevant molecules O2, OH, and PbO. Especially for polar species, the density of s-wave resonant states is quite high, implying potentially disastrous consequences for trapped molecules.     

Quantum gates driven by microwave pulses in hyperfine levels of ultracold heteronuclear dimers

We theoretically investigated the implementation of universal quantum gates in hyperfine levels of ultracold heteronuclear polar molecules in their lowest rotational manifolds. Quantum bits are manipulated by microwave pulses, taking advantage of the strong state mixing generated by the hyperfine interactions. Gate operations are either driven by a sequence of Gaussian pulses or by a pulse shaped by optimal control theory. Alkaline molecules of experimental interest are considered. We show that high fidelity gates can be driven by microsecond pulses. The richness of the energy structure and the state mixing offer promising perspectives for the manipulation of a large number of qubits.     

Coherent and non-coherent components of two-photon Rydberg excitation of ultracold Li7 atoms

A study of the optical spectra of ultracold highly excited Li7 atoms has revealed coherent and non-coherent components of two-photon excitation. The high sensitivity of the method makes it possible to record pure coherent resonances at a detuning of 803.5 MHz from the intermediate 2P_3/2 resonance.     

Coherent states and path integrals for model hamiltonians in quantum optics

A brief review of applications of the coherent states (CSes) in quantum optics is given. The CS representation of path integrals in the semi-classical approximation leads to the classical dynamics of CS parameters. Calculations of squeezing in the model of spontaneous parametric scattering are performed and the chaos in the collective dynamics of three-level atoms interacting with resonant photon modes is studied.     

Doppler-free two-photon modulation transfer spectroscopy in sodium dimers

We performed an experiment to investigate modulation transfer spectroscopy in two-photon transition of Na₂ using a CW dye laser. Comparing with UV fluorescence detection, the signals obtained by modulation transfer spectroscopy have a very high S/N without the Doppler background problem and its line shape is in agreement with theoretical calculations.Project supported by National Natural Science Fund of ChinaPermanent address: Shanghai University of Technology, Shanghai 200072, People's Republic of China     

Suppression of molecular decay in ultracold gases without Fermi statistics

We study inelastic processes for ultracold three-body systems in which only one interaction is resonant. We show that at ultracold temperatures three-body recombination in such systems leads mainly to the formation of weakly bound molecules. In addition, and perhaps more importantly, we have found that the decay rates for weakly bound molecules due to collisions with other atoms can be suppressed not only without fermionic statistics but also when bosonic statistics applies. These results indicate that recombination in three-component atomic gases can be used as an efficient mechanism for molecular formation, allowing the achievement of high molecular densities.     

Semiempirical molecular orbital calculations for ketene dimers

The semiempirical molecular orbital method known as symmetrically orthogonalised intermediate neglect of differential overlap (sindo) has been employed to determine the geometry, bonding, binding energy, ionisation potential, dipole moment and net charges of diketene and cyclobutane-1,3-dione molecules. Results obtained have been compared with available experimental data.