Quantum Optics Laboratory

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]

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.

The magnetic ordering temperatures T(m) of Pr ions in (R1-xPrx)Ba2Cu3O7 systems (R = Yb, Tm, Er, Ho, Dy, Gd, Eu, Sm, Nd and Y) with x = 0.5 - 1.0 were measured. We observe that T(m) decreases monotonically with increasing R concentration. At constant x, T(m) is R ion-size dependent. The slope in the T(m) vs. x is steeper for ion with smaller ionic radius. In comparison with the ion-size effect on the superconducting transition temperatures T(c) in these systems, the observed results can be qualitatively interpreted in terms of the hybridization between the local states of Pr ion and the conduction band states of the CuO2 planes.

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.

A study of ion equilibration in annular regions of ultracold strontium plasmas is reported. Plasmas are formed by photoionizing laser-cooled atoms with a pulsed dye laser. The experimental probe is spatially-resolved absorption spectroscopy using the S-2(1/2)-P-2(1/2) transition of the Sr+ ion. The kinetic energy of the ions is calculated from the Doppler broadening of the spectrum, and it displays clear oscillations during the first microsecond after plasma formation. The oscillations, which are a characteristic of strong coulomb coupling, are fit with a simple phenomenological model incorporating damping and density variation in the plasma.

We report on the first experimental demonstration of low-light-level cross-phase modulation (XPM) with double slow light pulses based on the double electromagnetically induced transparency (EIT) in cold cesium atoms. The double EIT is implemented with two control fields and two weak fields that drive populations prepared in the two doubly spin-polarized states. Group velocity matching can be obtained by tuning the intensity of either of the control fields. The XPM is based on the asymmetric M-type five-level system formed by the two sets of EIT. Enhancement in the XPM by group velocity matching is observed. Our work advances studies of low-light-level nonlinear optics based on double slow light pulses.

We observed electromagnetically induced transparency-based four-wave mixing (FWM) in the pulsed regime at low light levels. The FWM conversion efficiency of 3.8(9)% was observed in a four-level system of cold 87Rb atoms using a driving laser pulse with a peak intensity of ≈80 μW/cm2, corresponding to an energy of ≈60 photons per atomic cross section. Comparison between the experimental data and the theoretical predictions proposed by Harris and Hau [Phys. Rev. Lett. 82, 4611 (1999)] showed good agreement. Additionally, a high conversion efficiency of 46(2)% was demonstrated when applying this scheme using a driving laser intensity of ≈1.8 mW/cm2. According to our theoretical predictions, this FWM scheme can achieve a conversion efficiency of nearly 100% when using a dense medium with an optical depth of 500.

We report an experimental observation of narrow and high-contrast spectra. which are induced by interacting dark resonances and have been predicted in Phys. Rev. A 60, 3225 (1999). Spectra are measured with cold (87)Rb atoms produced by a magneto-optical trap. In this experimental system, a coupling laser and a weak probe laser form a three-level Lambda -type configuration of electromagnetically induced transparency (EIT); a microwave drives a magnetic-dipole transition between the fourth level and the ground state that is coupled with the excited state by the coupling laser. The observed spectral profile of probe absorption exhibits a very sharp peak emerging inside a narrow EIT dip. Such spectral feature provides more opportunities in manipulating atomic-optical response.

We report photoassociative spectroscopy of Sr-88(2) in a magneto-optical trap operating on the S-1(0)-->P-3(1) intercombination line at 689 nm. Photoassociative transitions are driven with a laser red detuned by 600-2400 MHz from the S-1(0)-->P-1(1) atomic resonance at 461 nm. Photoassociation takes place at extremely large internuclear separation, and the photoassociative spectrum is strongly affected by relativistic retardation. A fit of the transition frequencies determines the P-1(1) atomic lifetime (tau=5.22+/-0.03 ns) and resolves a discrepancy between experiment and recent theoretical calculations.

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.

Coherent two-field spectroscopy calculations with different choices of the quantization (z) axis are compared. In a system driven by a strong coupling field and a weak probe field, alignment of the z axis along either the polarization direction of a linearly polarized coupling field or the propagation direction of a circularly polarized coupling field is shown to simplify the calculations and facilitate interpretation of the results. The advantages of a suitable choice of the z axis are highlighted with an example of a degenerate three-level system of electromagnetically induced transparency. (C) 2002 Optical Society of America.

We have observed various Lambda-type electromagnetically induced transparency (EIT) spectra in laser-cooled Rb-87 atoms of different laser polarization configurations. Unexpected profiles occur in the EIT spectra. We have found the degenerate Zeeman sublevels are responsible for these profiles. The experimental data are in good agreement with the results from the theoretical calculation which takes into account all the 13 Zeeman levels in the Lambda system. Our study demonstrates that Zeeman sublevels play important roles in quantum interference phenomena such as EIT and amplification without population inversion (AWI), and should be taken into account in the analysis of these phenomena.

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 report the use of photoassociative spectroscopy to determine the ground-state s-wave scattering lengths for the main bosonic isotopes of strontium, Sr-86 and Sr-88. Photoassociative transitions are driven with a laser red detuned by up to 1400 GHz from the S-1(0)-P-1(1) atomic resonance at 461 nm. A minimum in the transition amplitude for Sr-86 at -494 +/- 5 GHz allows us to determine the scattering lengths 610a(0)< a(86)< 2300a(0) for Sr-86 and a much smaller value of -1a(0)< a(88)< 13a(0) for Sr-88.