No abstract

The absorption of traveling photons resonant with electric dipole transitions of an atomic gas naturally leads to electric dipole spin wave excitations. For a number of applications, it would be highly desirable to shape and coherently control the spatial waveform of the spin waves before spontaneous emission can occur. This work details a recently developed optical control technique to achieve this goal, where counter-propagating, shaped sub-nanosecond pulses impart sub-wavelength geometric phases to the spin waves by cyclically driving an auxiliary transition. In particular, we apply this technique to reversibly shift the wave vector of a spin wave on the D2 line of laser-cooled 87 Rb atoms, by driving an auxiliary D1 transition with shape-optimized pulses, so as to shut off and recall superradiance on demand. We investigate a spin-dependent momentum transfer during the spin-wave control process, which leads to a transient optical force as large as ∼ 1 k/ns, and study the limitations to the achieved 70 ∼ 75% spin wave control efficiency by jointly characterizing the spin-wave control and matterwave acceleration. Aided by numerical modeling, we project potential future improvements of the control fidelity to 99% level when the atomic states are better prepared and by equipping a faster and more powerful pulse shaper. Our technique also enables a backgroundfree measurement of the superradiant emission to unveil the precise scaling of the emission intensity and decay rate with optical depth for the first time to our knowledge.

Advances of quantum control technology have led to nearly perfect single-qubit control of nuclear spins and atomic hyperfine ground states. In contrast, quantum control of strong optical transitions, even for free atoms, are far from being perfect. Developments of such quantum control appears to be limited by available laser technology for generating isolated, sub-nanosecond optical waveforms with 10's of GHz programming bandwidth. Here we propose a simple and robust method for the desired pulse shaping, based on precisely stacking multiple delayed picosecond pulses. Our proof-of-principal demonstration leads to arbitrarily shapeable optical waveforms with 30 GHz bandwidth and 100 ps duration. We confirm the stability of the waveforms by interfacing the pulses with laser-cooled atoms, resulting in “super-resolved” spectroscopic signals. This pulse shaping method may open exciting perspectives in quantum optics, and for fast laser cooling and atom interferometry with mode-locked lasers.

We demonstrate a method to create dynamic optical lattices with lattice constant tunable down to the optical wavelength limit. The periodicity of 1D lattice is to be adjusted by rotating the incoming direction of one of the two interfering laser beams with its fiber port. The relative phase between the stationary and rotating lasers are stabilized with a heterodyne phase-lock loop (Ma et al, Opt. Lett. 19, 1777, 1994), by reflecting part of the rotating laser beam back from a cylindrical mirror near the experiment. Our preliminary demonstration shows tuning of lattice constant λ2sinθ/2, limited by our imaging resolution, between θ = 3° and 20°, with stable and tunable phase. The results can be extended to achieve lattice constant tuning range from ∼ 10λ down to λ/2. We discuss extension of the demonstrated scheme for improved vibration suppression, and for lattice utilizing broadband lasers. Finally we propose a 2D accordion lattice design for quantum gas experiments.

The propagation of light in moving media is dragged by atomic motion. The light-drag effect can be dramatically enhanced by reducing the group velocity with electro-magnetically induced transparency. We demonstrate a systematic procedure to estimate the velocity field of the moving atoms, by holographically reconstructing the complex wavefront of the slow light and to simultaneously retrieve the absorption and phase shift. This large-NA, photon-shot-noise-limited inline coherent imaging technique may assist a wide range of cold atom experiments to access phase space information with in situ and minimally destructive measurements. By faithfully expanding the imaging data from real to complex numbers, the holographic technique also paves a way toward single shot spectroscopic imaging of atomic ensembles, even in presence of atomic density fluctuations.