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Hybrid apparatus for Bose-Einstein condensation and cavity quantum electrodynamics: Single atom detection in quantum degenerate gases
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Image of FIG. 1.
FIG. 1.

(Color online) Schematic sketch of the experimental setup illustrating the nested vacuum chambers, the short magnetic transport, and the “science platform” bearing the ultrahigh finesse optical cavity on top of the vibration isolation system. The atomic cloud captured in the magneto-optical trap (MOT) is transferred through a differential pumping tube into the ultrahigh vacuum region and evaporatively cooled towards quantum degeneracy. We output couple a continuous atom laser from the BEC and direct it to the cavity mode where single atoms are detected.

Image of FIG. 2.
FIG. 2.

Overview of the complete vacuum system showing the pumping sections for the two nested vacuum regions, high vacuum (HV) and ultrahigh vacuum (UHV), respectively. The overall length is close to . The main tank offers multiple optical and electrical access and is sealed off by two CF 200 cluster flanges called “BEC production rig” and “science platform.”

Image of FIG. 3.
FIG. 3.

(Color online) Section through the UHV system illustrating the realization of the nested chambers design and revealing the details and objectives of the divers optical axes. The position of the BEC and cavity are marked by (●) and (◆), respectively. The high vacuum MOT chamber is suspended from the “BEC production rig” and sealed by a tight fit bushing against the UHV main tank. The “science platform” provides space for additional components such as the ultrahigh finesse optical cavity. [Note: For clarity in the illustration the magnet coil configuration (Fig. 5) and the optical cavity assembly (Fig. 8) are omitted in this figure.]

Image of FIG. 4.
FIG. 4.

(Color online) (a) Top view of the arrangement of coils for the magnetic transport. The line denotes the trajectory of the atomic cloud from the MOT (filled circle) into the QUIC trap. (b) Temporal sequence of currents through the different coils to realize the compression of the cold atomic cloud (negative times) and the magnetic transport.

Image of FIG. 5.
FIG. 5.

(Color online) Section through the complete assembly inside the main vacuum chamber. It illustrates the arrangement of magnet coils, the inner chamber, and the cavity with respect to each other. Functional units of the magnet coil configuration are the two transport brackets that sandwich the inner chamber and the laterally mounted Ioffe frame (elements between dashed lines). These parts, including the top gradient coil, are fixed to each other and mounted from the top flange. The optical cavity on top of the vibration isolation system, the surrounding coils, and the bottom gradient coil are mounted on the science platform.

Image of FIG. 6.
FIG. 6.

Photograph of the preassembled mu-metal hull before it is mounted around the main vacuum tank. It consists of seven large and several small individual pieces.

Image of FIG. 7.
FIG. 7.

(Color online) Photograph of the mounted science platform. The support bears the vibration isolation system (VIS) and the magnetic coil structure which surrounds the optical cavity.

Image of FIG. 8.
FIG. 8.

(Color online) Elements of the optical cavity implementation. (a) Plane cut through the assembled cavity design where the arrows indicate optical access. (b) Photograph of the cavity assembly. The electrical leads for the piezotube are pinched in a slotted Viton piece to efficiently decouple the cavity from the environment. (c) The cavity assembly resting on top of the vibration isolation stack which is positioned on the science platform. (d) Modeled frequency response of the vibration isolation stack.

Image of FIG. 9.
FIG. 9.

Absorption images of cold atom clouds. (a) Thermal cloud at a temperature above the critical temperature . (b) Bimodal distribution for . (c) “Pure” Bose-Einstein condensate at . The images were taken after time of flight with a detuning of to avoid saturation.

Image of FIG. 10.
FIG. 10.

Coherent microwave output coupling of a continuous atom laser. (a) Helix antenna built for mounted on a vacuum feedthrough. (b) Resonant absorption image of the atom laser after a propagation of .

Image of FIG. 11.
FIG. 11.

Cavity detection recording of an atom laser. The atom flux is about four orders of magnitude lower compared to Fig. 10(b). Single atom transits are clearly identified by their reduction of the shot noise limited empty cavity transmission.

Image of FIG. 12.
FIG. 12.

(Color online) Characteristics of detected single atom events. (a) The transit of a single atom significantly reduces the probe light transmission through the cavity. We integrate the signal with a sliding average and set the detection threshold to of the photon shot noise. (b) Distribution of measured coupling times (FWHM) (red) compared to the distribution of simulated events (gray). (c) Distribution of measured transmission reduction magnitudes. An evaluation with a threshold (red) is compared to a threshold [gray] revealing the discrimination of the events from the photon shot noise. (d) Dependency of the transmission reduction on the coupling time due to the non-Gaussian shape of the dips.

Image of FIG. 13.
FIG. 13.

The ultrahigh finesse optical cavity functions as a linear detector on the output coupling rate, i.e., atom flux over three orders of magnitude. Saturation occurs at a count rate of about .

Image of FIG. 14.
FIG. 14.

(Color online) Dependence of the single atom detection efficiency on the probe laser and cavity detunings. The vertical dashed line represents the cycling transition which is Zeeman shifted by from the zero field atomic transition. Best single atom detection is performed with a probe laser red detuned by about from the cycling transition and a cavity detuning of about , corresponding to the maximum dipole potential created by the probe laser. The second local detection maximum corresponds to a blue-detuned probe laser. Therefore the dipole potential is repulsive and the atom count rate reduced.

Image of FIG. 15.
FIG. 15.

(Color online) (a) The detected atom count rate for a constant atom flux is shown with respect to the inclination of the optical table along the two axes. The rectangle represents the active area of the cavity mode and the ellipse is the reconstructed size ( diameter) of the atom laser at the position of the cavity. [(b) and (c)] Fit (red) to the measured data (black) by the convolution of the active size of the cavity mode with a Gaussian beam profile along the Ioffe (b) and cavity (c) axes. It is compared to the expected shape from numerical simulations of the Gross-Pitaevskii equation (gray). (d) Visualization of the extracted two-dimensional atom laser beam profile clipped by the active area of the cavity mode.

Image of FIG. 16.
FIG. 16.

The Fourier spectrum of the detected atom laser flux exhibiting the trapping frequencies and their harmonics. A fast and precise tool to measure frequencies of collective oscillation in the trap.

Image of FIG. 17.
FIG. 17.

(Color online) Investigation of atom count rates for thermal beams. The count rate is proportional to for temperatures above the critical temperature and sharply increases when cooling across the phase transition. Just above the density and momentum distributions of the thermal cloud are governed by a Bose distribution and obey a different scaling law as expected for a Gaussian distribution.

Image of FIG. 18.
FIG. 18.

(Color online) Investigation of detected atom count rates for pure quantum degenerate samples. The scaling with the atom number in a pure BEC exhibits three different regimes. The expected behavior is only valid for intermediate particle numbers. Very small and very large condensates obey different scaling laws due to an increased Heisenberg limited momentum spread and the mean field repulsion of the remaining condensate, respectively.

Image of FIG. 19.
FIG. 19.

(Color online) Analysis of the density distribution of the trapped ultracold atom gas by output coupling at different vertical positions relative to the center of the BEC and measuring the resulting atom count rate with the cavity. The profiles for three different temperatures around are shown in comparison with the absorption images. The high sensitivity of the cavity detector to quantum degenerate atoms allows for precise observation of the onset of Bose-Einstein condensation and the deviations from a Gaussian profile (gray curve).


Generic image for table
Table I.

Electromagnetic properties of the magnet coils.


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752b84549af89a08dbdd7fdb8b9568b5 journal.articlezxybnytfddd
Scitation: Hybrid apparatus for Bose-Einstein condensation and cavity quantum electrodynamics: Single atom detection in quantum degenerate gases