Nicolas Spethmann

Dynamics of single neutral impurity atoms immersed in an ultracold gas.

We report on controlled doping of an ultracold Rb gas with single neutral Cs impurity atoms. Elastic two-body collisions lead to a rapid thermalization of the impurity inside the Rb gas, representing the first realization of an ultracold gas doped with a precisely known number of impurity atoms interacting via s-wave collisions. Inelastic interactions are restricted to a single three-body recombination channel in a highly controlled and pure setting, which allows us to determine the Rb-Rb-Cs three-body loss rate with unprecedented precision. Our results pave the way for a coherently interacting hybrid system of individually controllable impurities in a quantum many-body system.

Optically measuring force near the standard quantum limit

Measuring tiny forces with atomic clouds For projects such as detecting gravity waves, physicists need to measure tiny forces precisely. Schreppler et al. developed an extremely sensitive method for force measurement. They applied a known force on a cloud of ultracold rubidium atoms in an optical cavity. The force caused the atoms to oscillate, and the researchers used optical measurements to monitor the motion. Under optimal conditions, the authors could measure forces with a level of sensitivity only four times worse than the fundamental limit imposed by the Heisenberg uncertainty principle. Science, this issue p. 1486 A very sensitive force-measuring technique uses ultracold rubidium atoms in an optical cavity as a mechanical oscillator. The Heisenberg uncertainty principle sets a lower bound on the noise in a force measurement based on continuously detecting a mechanical oscillator’s position. This bound, the standard quantum limit, can be reached when the oscillator subjected to the force is unperturbed by its environment and when measurement imprecision from photon shot noise is balanced against disturbance from measurement back-action. We applied an external force to the center-of-mass motion of an ultracold atom cloud in a high-finesse optical cavity and measured the resulting motion optically. When the driving force is resonant with the cloud’s oscillation frequency, we achieve a sensitivity that is a factor of 4 above the standard quantum limit and consistent with theoretical predictions given the atoms’ residual thermal disturbance and the photodetection quantum efficiency.

Cavity-mediated coupling of mechanical oscillators limited by quantum back-action

Coupling two mechanical objects becomes tricky when they are quantum and can interact only through photons. An experiment now demonstrates such an optomechanical system with two separate atomic ensembles in the same optical cavity.

Single Cs atoms as collisional probes in a large Rb magneto-optical trap

We study cold interspecies collisions of cesium and rubidium in a strongly imbalanced system with single and few Cs atoms. Observation of the single-atom fluorescence dynamics yields insight into light-induced loss mechanisms, while both subsystems can remain in steady state. This significantly simplifies the analysis of the dynamics, as Cs-Cs collisions are effectively absent and the majority component remains unaffected, allowing us to extract a precise value of the Rb-Cs collision parameter. Extending our results to ground-state collisions would allow to use single neutral atoms as coherent probes for larger quantum systems.

Complex Squeezing and Force Measurement Beyond the Standard Quantum Limit.

A continuous quantum field, such as a propagating beam of light, may be characterized by a squeezing spectrum that is inhomogeneous in frequency. We point out that homodyne detectors, which are commonly employed to detect quantum squeezing, are blind to squeezing spectra in which the correlation between amplitude and phase fluctuations is complex. We find theoretically that such complex squeezing is a component of ponderomotive squeezing of light through cavity optomechanics. We propose a detection scheme called synodyne detection, which reveals complex squeezing and allows the accounting of measurement backaction. Even with the optomechanical system subject to continuous measurement, such detection allows the measurement of one component of an external force with sensitivity only limited by the mechanical oscillators thermal occupation.

Cavity-Assisted Measurement and Coherent Control of Collective Atomic Spin Oscillators

We demonstrate continuous measurement and coherent control of the collective spin of an atomic ensemble undergoing Larmor precession in a high-finesse optical cavity. The coupling of the precessing spin to the cavity field yields phenomena similar to those observed in cavity optomechanics, including cavity amplification, damping, and optical spring shifts. These effects arise from autonomous optical feedback onto the atomic spin dynamics, conditioned by the cavity spectrum. We use this feedback to stabilize the spin in either its high- or low-energy state, where, in equilibrium with measurement backaction heating, it achieves a steady-state temperature, indicated by an asymmetry between the Stokes and the anti-Stokes scattering rates. For sufficiently large Larmor frequency, such feedback stabilizes the spin ensemble in a nearly pure quantum state, in spite of continuous measurement by the cavity field.

Note: Reliable low-vibration piezo-mechanical shutter

We present a mechanical shutter based on a bending piezo-actuator. The shutter features an active aperture of about 2 mm, allowing for full extinction and lossless transmission of a beam. Acoustic noise and mechanical vibrations produced are very low and the shutter is outstandingly long-lived; a test device has undergone 20 × 10(6) cycles without breaking. A reflector makes the shutter capable of reliably interrupting a beam with at least 2 W of cw power at 780 nm. The shutter is well suited to create pulses as short as 16 ms, while pulse lengths down to 1 ms are possible. The rise and fall times are approximately 120 µs, with a delay of 2 ms. Jitter stays below 10 µs, while long-term drifts stay well below 500 µs.

Interspecies interaction in a strongly imbalanced Bose-Bose mixture

In our experiment, we study and intend to manipulate the interaction between the two species viz. Rubidium (Rb) and Caesium (Cs) at ultracold temperatures. We propose to explore the world of many body physics and small particle physics by inserting a few Cs atoms into a Rb condensate.