Observing quantum gases in real time: Single-atom detection on a chip

Single magnetically trapped ${}^{87}\text{R}\text{b}$ atoms on an atom chip are detected with $67\pm 12\%$ efficiency by three photon ionization and subsequent ion detection in a channel electron multiplier. State selective detection of single atoms and high-resolution optical spectroscopy on trapped atom clouds are demonstrated. The temperature and particle density of a trapped atomic gas is monitored in situ and in real time with negligible atom loss due to ionization below 5%.

Figure 1

(Color online) Sketch of the experimental situation (not to scale). A cloud of rubidium atoms is trapped in a magnetic microtrap. Two laser beams with wavelength of 778 nm and 1080 nm are focused through a $200\text{−}\mu \text{m}$-wide hole in the chip ionizing atoms in the cloud. The ions are collected and guided by an ion optics to the channel electron multiplier (CEM) detector.

Figure 2

(Color online) Ion counting rate for different activation schemes of the lasers. If the fiber laser is activated synchronously with the diode laser, the counting rate decays exponentially on a time scale of $\tau =771\text{ms}$ [blue curve (i)]. Activating the fiber laser prior to the diode laser (1 s linear ramp), the counting rate shows a double exponential decay [red curve (ii)]: The atoms in the adiabatically loaded dipole trap are first ionized on a time scale of ${\tau }_1=27\text{ms}$, whereas the remaining part of the cloud is ionized on a time scale of ${\tau }_2=724\text{ms}$.

Figure 3

(Color online) Scanning the two-photon frequency over the hyperfine splitting of the $5D_{5/2}$ state (a), we observe resonance lines in the CEM signal, corresponding to an increased ionization rate for resonant two-photon transitions (b) and (c). The origin of the frequency detuning is chosen arbitrarily. The measured ion counting rate is fitted by a superposition of Gauss profiles, one for each resonance line. At higher power in the fiber laser, the spectral lines are shifted to higher frequencies (dashed vertical lines) and power broadened. The power broadening is illustrated by the green dashed curve in (c), showing the shifted spectrum of (b).

Figure 4

(Color online) (a) The Zeeman-splitted $5S_{1/2}$ hyperfine states are coupled by a microwave near 6.8 GHz. Atoms in the $F=2,m_F=2$ state are coupled to the $F=1,m_F=1$ state from where they are ionized. (b) and (c) By scanning the microwave frequency we record the spectrum of the ion counting rate. It directly reveals the energy distribution of the atoms in the trap from which we deduce the clouds temperature [(b) $9.6\mu \text{K}$; (c) $29.9\mu \text{K}$] which corresponds to time of flight measurements. The origin of the frequency axis is arbitrarily set to the $F=1,m_F=0\to F=2,m_F=0$ transition. Additional structures indicated by the vertical arrows are due to residual population in the $F=2,m_F=1$ state.