Quantum Optics Laboratory

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 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 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.

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 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 study equilibration of strongly coupled ions in an ultracold neutral plasma produced by photoionizing laser-cooled and trapped atoms. By varying the electron temperature, we show that electron screening modifies the equilibrium ion temperature. Even with few electrons in a Debye sphere, the screening is well described by a model using a Yukawa ion-ion potential. We also observe damped oscillations of the ion kinetic energy that are a unique feature of equilibration of a strongly coupled plasma.

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 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.

A high-storage efficiency and long-lived quantum memory for photons is an essential component in long-distance quantum communication and optical quantum computation. Here, we report a 78% storage efficiency of light pulses in a cold atomic medium based on the effect of electromagnetically induced transparency. At 50% storage efficiency, we obtain a fractional delay of 74, which is the best up-to-date record. The classical fidelity of the recalled pulse is better than 90% and nearly independent of the storage time, as confirmed by the direct measurement of phase evolution of the output light pulse with a beat-note interferometer. Such excellent phase coherence between the stored and recalled light pulses suggests that the current result may be readily applied to single photon wave packets. Our work significantly advances the technology of electromagnetically induced transparency-based optical memory and may find practical applications in long-distance quantum communication and optical quantum computation. DOI: 10.1103/PhysRevLett.110.083601

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.