D. Cho

A Bose-Einstein condensate in an optical lattice

We have performed a number of experiments with a Bose-Einstein condensate (BEC) in a one-dimensional optical lattice. Making use of the small momentum spread of a BEC and standard atom optics techniques, a high level of coherent control over an artificial solid-state system is demonstrated. We are able to load the BEC into the lattice ground state with a very high efficiency by adiabatically turning on the optical lattice. We coherently transfer population between lattice states and observe their evolution. Methods are developed and used to perform band spectroscopy. We use these techniques to build a BEC accelerator and a novel, coherent, large-momentum-transfer beam-splitter.

Optoelectronic oscillator stabilized to an intra-loop Fabry-Perot cavity by a dual servo system

We report construction and characterization of an optoelectronic oscillator (OEO), which is stabilized to an intra-loop Fabry-Perot cavity by a dual servo system. In addition to providing strong mode selection and increasing the Q factor by adding significant loop length, the cavity serves as a stable frequency reference. In order to fully exploit the stability we employ a dual servo system. Carrier frequency is locked to the cavity mode by using Pound-Drever-Hall technique. The OEO loop length is adjusted by comparing the phase difference between the carrier-sideband beat signals at upstream and downstream sides of the cavity so that the OEO mode spacing is commensurate with the free spectral range of the cavity. This dual servo system and additional stabilizations of the seed laser frequency to a cesium transition and the laser power result in an order of magnitude improvement in OEO frequency stability over a previous work using a free-running Fabry-Perot cavity. Long term Allan deviation of the OEO is 6 x 10(-8). It represents 4 x 10(-4)of the cavity resonance linewidth. We also discuss possibility of relating the OEO frequency to an atomic transition as an absolute frequency reference.

Polarization orthogonalizer for a pair of laser beams with nearly equal frequencies

We have demonstrated a device to produce an overlapping pair of orthogonally polarized laser beams with a 3 GHz frequency offset out of a single laser beam containing the two frequency components with the same linear polarization. Our design is based on a Michelson interferometer formed by a polarizing beam splitter and two quarter-waveplates. Such a device can be used to make the polarization states of a carrier and a sideband produced through modulation mutually orthogonal. An orthogonally polarized pair of coherent laser beams can be used for an interferometric measurement of a small displacement in a heterodyne scheme or to produce a large-contrast coherent population trapping signal from alkali metal atoms. As a demonstration we used the device to achieve 40% contrast for a coherent population trapping signal from a rubidium vapor cell.

Optical dipole trap using a Fabry?Perot interferometer as a power buildup cavity

We construct an optical dipole trap using a Fabry?Perot interferometer as a power buildup cavity. Large power enhancement allows us to produce 9 mK deep potential wells with 48 ?m spot size and 100 nm detuning from the rubidium D1 resonance. The optical trap is loaded from a dark-spot magneto-optical trap which, in turn, is loaded from a low-velocity intense source of 85Rb atoms. Under typical experimental conditions, there are 1.4 ? 106 atoms in 2000 antinodes of the optical trap. Average atom density is 1.1 ? 1012 cm-3. The number of trapped atoms is limited by the atom density, or trapping volume. The standing-wave configuration with tight longitudinal confinement has much smaller trapping volume compared with the equivalent travelling wave. A method for converting the trap to a travelling wave-like configuration using phase modulation is proposed.

Magic polarization for optical trapping of atoms without stark-induced dephasing

We demonstrate that the differential ac-Stark shift of a Zeeman-sensitive ground hyperfine transition in an optical trap can be eliminated by using properly polarized trapping light. We use the vector polarizability of an alkali-metal atom to produce a polarization-dependent ac-Stark shift that resembles a Zeeman shift. We study a transition from the |2S1/2,F=2,mF=-2> to the |2S1/2,F=1,mF=-1> state of 7Li to observe 0.59±0.02  Hz linewidth with interrogation time of 2 s and 0.82±0.06  s coherence time of a superposition state. Implications of the narrow linewidth and the long coherence time for precision spectroscopy and quantum information processing using atoms in an optical lattice are discussed.

Elimination of inhomogeneous broadening for a ground-state hyperfine transition in an optical trap

We propose a way to eliminate the inhomogeneous broadening for a ground-state hyperfine transition of an alkali metal atom in an optical trap by using a properly polarized trapping field. The ac Stark shift contribution from the vector polarizability has opposite sign for a pair of ground hyperfine levels. It can be used to eliminate the inhomogeneous broadening from the difference in the scalar polarizbilities due to the hyperfine splitting. The size of the vector term is determined by the polarization state of the trapping field, and by controlling the polarization tightly one can achieve a very narrow linewidth. We estimate required tolerance in the polarization control to achieve 1-Hz linewidth for a specific case of a cesium atom. This proposal has significant implications for an electric dipole moment measurement using cesium atoms and quantum information processing using an optical lattice.

Automatic system to relock a laser frequency to a Fabry-Perot cavity

We developed a system that allowed us to unlock and at a later time automatically relock a laser frequency to a medium-finesse Fabry–Perot cavity. The system can work as a chopping wheel or a shutter for a laser field built inside a cavity. The heart of our system is the flywheel circuit for the slower-acting servo loop and the automatic gain control circuit for the faster-acting servo loop. The flywheel circuit stores a correction for a slow frequency drift so that during the unlocked period the slow loop can be safely turned off. The automatic gain control lowers the fast loop gain during a relock process and increases the gain after the lock is secured. The system works reliably at a chopping frequency up to 70Hz, and relocks well after being unlocked for up to 10s.

Production of a coherent pair of light beams with a microwave frequency difference from a single extended-cavity diode laser

We produced a pair of coherent laser beams with a 3-GHz frequency difference by optically phase locking two modes from a single, multimode extended-cavity diode laser. This method is complementary to either a direct modulation or an optical phase locking of two independent lasers. A large differential frequency shift between the two modes of the laser allows efficient phase locking. We developed a simple theory to account for the large differential frequency shift. Allan deviation of the beat frequency when the two modes are phase-locked drops as an inverse of the measurement time and it reaches 10(-14) when the time is 1 h. Coherent population trapping spectroscopy of Rb atoms using the phase-locked beams resulted in a spectrum as narrow as that of the case using direct modulation by a stable frequency synthesizer.

Two-frequency interferometer for a displacement measurement

We report on the construction of a two-frequency Michelson interferometer to measure small displacements based on the heterodyne principle. Unlike the common single-frequency interferometer, where relative displacements produce changes in output power, in the two-frequency device, displacements lead to phase shifts of the beating signal. The short- and long-term performance of the single- and two-frequency methods are compared. The heterodyne apparatus was also used to calibrate a piezoelectric transducer.

Oscillator-free atomic clock using a multimode laser

We developed an atomic clock using two modes from a single extended-cavity diode laser in multimode operation. The two modes are phase locked with reference to a dispersion signal from a coherent population trapping (CPT) resonance of R85b at 3.036 GHz. The design is in principle free from an oscillator and a modulator and it is a significant simplification over a conventional CPT-based atomic clock. Allan deviation of the beat frequency is 1×10−10 at 200 s integration time.