July 2016

Abstracts of the QSIT Lunch Seminar, Thursday, July 7, 2016

Imaging transport of neutral atoms using a scanning gate microscope

Samuel Häusler, Quantum Optics Group, ETH Zurich

Scanning gate microscopy is a powerful technique to image transport, routinely applied to semiconductor devices. A repulsive “tip” depletes the carriers underneath and hence locally hinders their flow. By scanning its position and monitoring the subsequent variations of conductance, a spatial map of the structure is obtained.

Recently, we implemented the technique for ultracold atoms using a tightly focused, repulsive laser beam, displaced and corrected for aberrations using a Digital Mirror Device. It allows operation close to the Fermi wavelength, contrary to its solid state counterpart, making it sensitive to quantum tunnelling. We applied the technique to a single mode conductor and achieved a spatial resolution close to the transverse wavefunction inside the conductor. Furthermore, we validated the technique by comparing the scanning gate pictures to exact simulations for non-interacting particles. This technique is readily applied to interacting, as well as disordered systems.

Optical control of a levitated nanoparticle

Vijay Jain, Photonics Laboratory, ETH Zurich

The momentum transfer between a photon and an object defines a fundamental limit for the precision with which the object’s position can be continuously measured. If the object oscillates at a frequency Ω₀, measurement backaction adds quanta ħΩ₀ to the oscillator’s energy at a rate Γ_recoil, a process called photon recoil heating. Here, we use an optically levitated nanoparticle in ultrahigh vacuum to directly measure Γ_recoil. Using a phaseisensitive feedback scheme, we cool the harmonic motion of the nanoparticle from ambient to microkelvin temperatures and measure its reheating rate under the influence of the radiation field. The recoil heating rate is measured for different particle sizes and for different excitation powers, without the need for cavity optics or cryogenic environments.

The measurements are in quantitative agreement with theoretical predictions and provide valuable guidance for the realization of quantum groundistate cooling protocols. Improving measurement sensitivity enhances the stability of feedback control and reduces the nanoparticle’s natural fluctuations to 30 pm, or less than one Bohr radius. With such optomechanical control, we estimate a force sensitivity on the order of zN/Hz^1/2, which could be used for the measurement of weak, nearifield forces.
V. Jain, et al., Phys. Rev. Lett. (2016) (In press)