Quantum dynamics via a hidden Liouville space

Quantum dynamics for arbitrary system are traditionally realized by time evolutions of wave functions in Hilbert space and/or density operators in Liouville space. However, the traditional simulations may occasionally turn out to be challenging for the quantum dynamics, particularly those governed by the nonlinear Hamiltonians. In this letter, we introduce a nonstandard iterative technique where time interval is divided into a large number of discrete subintervals with an ultrashort duration; and the Liouville space is briefly expanded with an additional (virtual) space only within these subintervals. We choose two-state spin raising and lowering operators for virtual space operators because of their simple algebra. This tremendously reduces the cost of time-consuming calculations. We implement our technique for an example of a charged particle in both harmonic and anharmonic potentials. The temporal evolutions of the probability for the particle being in the ground state are obtained numerically and compared to the analytical solutions. We further discuss the physics insight of this technique based on a thought-experiment. Successive processes intrinsically 'hitchhiking' via virtual space in discrete ultrashort time duration, are the hallmark of our simple iterative technique. We believe that this novel technique has potential for solving numerous problems which often pose a challenge when using the traditional approach based on time-ordered exponentials.Comment: 7 pages, 3 figure

Tracking molecular wave packets in cesium dimers by coherent Raman scattering

We explore wave-packet dynamics in the ground X 1Σ+g and excited B 1Πu states of cesium dimers (Cs2). In particular, we study the dependence of the wave- packet dynamics on the relative timing between femtosecond pump, Stokes, and probe pulses in a nondegenerate BOXCARS beam geometry, which are commonly used for coherent anti-Stokes Raman scattering (CARS) spectroscopy. The experimental results are elucidated by theoretical calculations, which are based on the Liouville equations for the density matrix for the molecular states. We observe oscillations in CARS signals as functions of both Stokes and probe pulse delays with respect to the pump pulse. The oscillation period relates to the wave-packet motion cycle in either the ground or excited state of Cs2 molecules, depending on the sequence of the input laser pulses in time. The performed analysis can be applied to study and/or manipulate wave-packet dynamics in a variety of molecules. It also provides an excellent test platform for theoretical models of molecular systems

Observation of picosecond superfluorescent pulses in rubidium atomic vapor pumped by 100-fs laser pulses

Journals published by the American Physical Society can be found at http://publish.aps.org/We study the superfluorescence (SF) from a gas of rubidium atoms. The atoms of a dense vapor are excited to the 5D state from the 5S state by a two-photon process driven by 100-fs laser pulses. The atoms decay to the 6P state and then to the 5S state. The SF emission at 420 nm on the 6P-5S transition is recorded by a streak camera with picosecond time resolution. The time duration of the generated SF is tens of picoseconds, which is much shorter than the time scale of the usual relaxation processes, including spontaneous emission and atomic coherence dephasing. The dependence of the time delay between the reference input pulse and SF is measured as a function of laser power. The experimental data are described quantitatively by a simulation based on the semiclassical atom-field interaction theory. The observed change in scaling laws for the peak intensity and delay time can be elucidated by an SF theory in which the sample length is larger than the cooperation length

Third and fifth harmonics generation by tightly focused femtosecond pulses at 2.2 {\mu}m wavelength in air

We report experiments on the generation of third and fifth harmonics of millijoule-level, tightly focused, femtosecond laser pulses at 2.2 {\mu}m wavelength in air. The measured ratio of yields of the third and fifth harmonics in our setup is about 2 \cdot 10-4. This result contradicts the recent suggestion that the Kerr effect in air saturates and changes sign in ultra-intense optical fields.Comment: 3 pages, 2 figure

Intensity correlations in resonance nonlinear magneto-optical rotation

We have studied the intensity correlations between two orthogonally linearly polarized components of a laser field propagating through a resonant atomic medium. These experiments have been performed in a Rubidium atomic vapor. We observe that the correlations between the orthogonally polarized components of the laser beam are maximal in the absence of a magnetic field. The magnitude of the correlations depends on the applied magnetic field, and the magnitude first decreases and then increases with increasing magnetic field. Minimal correlations and maximal rotation angles are observed at the same magnetic fields. The width of the correlation function is directly proportional to the excited state lifetime and inversely proportional to the Rabi frequency of laser field. These results can be useful for improving optical magnetometers and for optical field or atomic spin squeezing.Comment: 8 pages, 4 figure