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arXiv:1211.2093 · Physical Review A

Reducing number fluctuations of ultracold atomic gases via dispersive interrogation

Bianca J. Sawyer, Amita B. Deb, Thomas McKellar, Niels Kjærgaard

Published Dec 26, 2012• 11 citations
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Abstract

We have used dispersive laser probing to follow the central density evolution of a trapped atomic cloud during forced evaporative cooling. This was achieved in a heterodyne detection scheme. We propose to use this as a nondestructive precursor measurement for predicting the atom number subsequent to evaporation and provide a simple experimental demonstration of the principle leading to a conditional reduction of classical number fluctuations.

DOI
10.1103/physreva.86.065401

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We have used dispersive laser probing to follow the central density evolution of a trapped atomic cloud during forced evaporative cooling. This was achieved in a heterodyne detection scheme. We propose to use this as a nondestructive precursor measurement for predicting the atom number subsequent to evaporation and provide a simple experimental demonstration of the principle leading to a conditional reduction of classical number fluctuations.
This thesis describes the use of microwave frequency sweeps across the resonance of a magnetic dipole transition to induce adiabatic rapid passage of an ultracold (<1 μK) atomic ensemble between Zeeman sub-levels of the 87Rb ground state. Atoms begin in the |F = 2, mF = 2> quantum state, and efficient preparation of the sample in the |F = 1, mF = 1> state is demonstrated.\nA dispersive probing system is developed using the technique of frequency modulation spectroscopy, which involves a heterodyne detection scheme. Dispersive probing is implemented in parallel to the state preparation, such that the real-time evolution of coherent transfer processes can be monitored.\nThe dispersive probe is used as a tool to quickly and accurately determine the microwave resonance frequency to within a 30 kHz range. Resonant coherent manipulations (Rabi oscillations) between the |F = 1, mF = 1> and |F = 2, mF = 2> states are then demonstrated and probed dispersively.\nA proof of concept application of the dispersive probing system is also presented. It is used to monitor an RF evaporative cooling sequence, providing a precursor measurement on a trapped atomic cloud. This leads to the ability to terminate the experimental sequence in the case of an unfavourable starting point, and thus a reduction of classical number fluctuations in the final atom number.
Abstract We demonstrate how dispersive atom number measurements during evaporative cooling can be used for enhanced determination of the parameter dependence of the transition to a Bose–Einstein condensate (BEC). In this way shot-to-shot fluctuations in initial conditions are detected and the information extracted per experimental realization is increased. We furthermore calibrate in situ images from dispersive probing of a BEC with corresponding absorption images in time-of-flight. This allows for the determination of the transition point in a single experimental realization by applying multiple dispersive measurements. Finally, we explore the continuous probing of several consecutive phase transition crossings using the periodic addition of a focused ‘dimple’ potential.
We describe a scheme for probing a gas of ultracold atoms trapped in an optical lattice and moving in the presence of an external potential. The probe is nondestructive and uses the existing lattice fields as the measurement device. Two counterpropagating cavity fields simultaneously set up a conservative lattice potential and a weak quantum probe of the atomic motion. Balanced heterodyne detection of the probe field at the cavity output along with integration in time and across the atomic cloud yield information about the atomic dynamics in a single run. The scheme is applied to a measurement of the Bloch oscillation frequency for atoms moving in the presence of the local gravitational potential. Signal-to-noise ratios are estimated to be as high as 104{10}^{4}104.
We demonstrate how dispersive atom number measurements during evaporative cooling can be used for enhanced determination of the non-linear parameter dependence of the transition to a Bose-Einstein condensate (BEC). Our analysis demonstrates that conventional averaging of shot-to-shot fluctuations introduces systematic errors and reduces precision in comparison with our method. We furthermore compare in-situ images from dispersive probing of a BEC with corresponding absorption images in time-of-flight. This allows for the determination of the transition point in a single experimental realization by applying multiple dispersive measurements. Finally, we explore the continuous probing of several consecutive phase transition crossings using the periodic addition of a focused dimple potential.
The paper reports on the optical detection and spectroscopy of ultracold atoms near a gold surface.A probe light field is used to excite surface plasmon polaritons.The refractive index of the atomic gas shifts the plasmon resonance and changes the reflected light power.Thus, the sensitivity of the detection is plasmonically enhanced.Absorption of photons from the evanescent wave is avoided by detuning the laser from atomic resonance which makes the detection scheme potentially nondestructive.The spectrum of the signal is determined by a Fano resonance.We show that atoms can be detected nondestructively with single atom resolution for typical parameters in cold atom experiments.Thus, the method is suitable for quantum nondemolition measurements of matter wave amplitudes.Experimentally, we measure a technically limited sensitivity of 30 atoms and extend the detection scheme to dispersively image the atom cloud near the surface.
We demonstrate a new method for nondestructive imaging of laser-cooled atoms. This spatial heterodyne technique forms a phase image by interfering a strong reference laser beam with a weak probe beam that passes through the cold atom cloud. The figure of merit equals or exceeds that of phase-contrast imaging, and the technique can be used over a wider range of spatial scales. We show images of a dark-spot magneto-optic trap taken with imaging fluences as low as 61 pJ/cm(2) at a detuning of 11? , resulting in 0.0004 photons scattered per atom.