Quantum Optics Laboratory

We present an experimental study of the coherence properties of amplified slow light by four-wave mixing (FWM) in a three-level electromagnetically induced transparency (EIT) system driven by one additional pump field. High energy gain (up to 19) is obtained with a weak pump field (a few mW∕cm2) using optically dense cold atomic gases. A large fraction of the amplified light is found to be phase incoherent to the input signal field. The dependence of the incoherent fraction on pump field intensity and detuning and the control field intensity is systematically studied. With the classical input pulses, our results support a recent theoretical study by Lauk et al. [Phys. Rev. A 88, 013823 (2013)], showing that the noise resulting from the atomic dipole fluctuations associated with spontaneous decay is significant in the high gain regime. This effect has to be taken into consideration in EIT-based applications in the presence of FWM.

We present an experimental study to achieve ultrahigh optical depths for cold atomic media with a two dimensional magneto-optical trap (MOT) of cesium. By combining large atom number, a temporally dark and compressed MOT, and Zeeman-state optical pumping, we achieve an optical depth of up to 1306 for the open transition of the cesium D1 line. Our work demonstrates that it is feasible to push the optical depth up to the 1000 level with a convenient MOT setup. This development paves the way to many important proposals in quantum optics and many-body physics.

The efficiency of a nonlinear optical process is proportional to the interaction time. We report a scheme of all-optical switching based on two motionless light pulses via the effect of electromagnetically induced transparency. One pulse was stopped as the stationary light pulse (SLP) and the other was stopped as stored light. The time of their interaction via the medium can be prolonged and, hence, the optical nonlinearity is greatly enhanced. Using a large optical density (OD) of 190, we achieved a very long interaction time of 6.9 μs. This can be analogous to the scheme of trapping light pulses by an optical cavity with a Q factor of 8×109. With the approach of using moving light pulses in the best situation, a switch can only be activated at 2 photons per atomic absorption cross section. With the approach of employing a SLP and a stored light pulse, a switch at only 0.56 photons was achieved and the efficiency is significantly improved. Moreover, the simulation results are in good agreement with the experimental data and show that the efficiency can be further improved by increasing the OD of the medium. Our work advances the technology in quantum information manipulation utilizing photons.

We demonstrate that the long-lived bound states (supermolecules) can exist in the dilute limit when we tune the shape of the effective potential between polar molecules by an external microwave field. Binding energies, average sizes, and phase diagrams for both s-orbital (bosons) and p-orbital (fermions) dimers are studied, together with bosonic trimer states. We explicitly show that the nonadiabatic transition rate can be easily tuned small for such ground-state supermolecules, so that the system can be stable from collapse even near the associated potential resonance. Our results, therefore, suggest a feasible cold molecule system to investigate novel few-body and many-body physics (for example, the p-wave BCS-Bose-Einstein-condensate crossover for fermions and the paired condensate for bosons) that cannot be easily accessed in single species atomic gases.

We use a double-passed acousto-optic modulator (AOM), driven by an arbitrary waveform generator to produce multiple frequency components for a laser with arbitrary frequency spacings. A programmed sequence containing various sections of radio-frequency sinusoidal signal at different frequency is applied to drive the AOM. The diffracted light is used to injection-lock a diode laser. The combined techniques allow us to generate the multi-line spectra for the diode laser with arbitrary frequency spacings in the range of 100 MHz at a relatively high output power of 80 mW and a small power variation of 2%. Such a light source can be used in the application for laser cooling of molecules. (C) 2011 American Institute of Physics. [doi:10.1063/1.3626903]

We have developed a continuous SrF radical beam for the loading of helium buffer gas cooling. The SrF molecules are efficiently generated by high-temperature chemical reaction of the solid precursor SrF(2) with boron in a graphite oven. The beam properties are characterized with laser-induced fluorescence spectroscopic method. We obtain a molecular flux of up to 2.1 x 10(15) sr(-1) s(-1) at the detection region for all rotational states. The dependence of the flux on oven temperature suggests that even higher flux is possible if a higher temperature in the oven is achieved. (C) 2009 American Institute of Physics. [doi:10.1063/1.3262631]

We have systematically studied a simple technique that accurately determines number of atoms in a magneto-optical trap, Absorption energy of a laser field that interacts with cold atoms is a direct measurement of atom number. The measured energy neither depends on the detuning, intensity, and polarization of the laser field nor is affected by other system parameters. Our work also demonstrates that such technique can be applied to study the phenomenon of coherent population trapping.

We investigate systematically pump-probe spectroscopy of cold Rb-87 atoms produced by a magneto-optical trap. The pump-probe spectra are measured without the presence of the trapping beams or any optical molasses. Various polarization configurations of the probe and pump fields result in very different spectra of probe absorption. The observed spectra exhibit a dispersive profile, a dispersionlike profile, a Lorentzian profile, or a dispersive profile plus a Lorentzian profile. The widths of all the spectral profiles are narrower than the natural linewidth of the excited state. Our work clarifies the mechanisms behind these different spectral profiles and provides essential information for the pump-probe spectroscopy of cold atoms.

We have experimentally studied the effect of the trapping laser linewidth on the number of capped atoms in a magneto-optical trap (MOT). Our data show that a significant number of the atoms can still be trapped in the MOT, even when the trapping laser linewidth is larger than the natural linewidth of the excited state of the driving transition.

We capture Rb-87 atoms from room-temperature background vapor with a magneto-optical trap (MOT). The temperature of the atoms in the MOT is 320 mu K as the result of Doppler cooling. We further employ polarization gradient cooling to lower atom temperature. The factors that can affect the performance of polarization gradient cooling have been systematically studied. An atom temperature of 75 mu K has been reached with the optimized conditions. Temperatures are measured by the release and recapture method and the time of flight method. Such cold atoms are ready for the evaporative cooling which will finally realize the Bose-Einstein condensation.

We conducted a detailed study of the structure and magnetic properties of (R1-xPrx)Ba2Cu3O7 sintered samples, where R = Lu, Yb, Tm, Er, Y, Ho, Dy, Gd, Eu, Sm, and Nd for x = 0.5-1.0. We found that the temperature dependence of the dc susceptibility follows the Curie-Weiss law in the temperature range 20-300 K and the paramagnetism of the Pr and R sublattices exist independently of one another. The antiferromagnetic ordering temperature T(N) of Pr ions decreases monotonically with increasing R concentration (1-x). At a given x, T(N) is R-ion-size dependent. The slope in the T(N) vs x curve is steeper for ions with smaller ionic radii. The observed results are interpreted in terms of the hybridization between the local states of the Pr ion and the valence-band states of the CuO2 planes.