Engineering random spin models with atoms in a high-finesse cavity.

All-to-all interacting, disordered quantum many-body models have a wide range of applications across disciplines, from spin glasses in condensed-matter physics over holographic duality in high-energy physics to annealing algorithms in quantum computing. Typically, these models are abstractions that do not find unambiguous physical realizations in nature. Here we realize an all-to-all interacting, disordered spin system by subjecting an atomic cloud in a cavity to a controllable light shift. Adjusting the detuning between atom resonance and cavity mode, we can tune between disordered versions of a central-mode model and a Lipkin-Meshkov-Glick model. By spectroscopically probing the low-energy excitations of the system, we explore the competition of interactions with disorder across a broad parameter range. We show how disorder in the central-mode model breaks the strong collective coupling, making the dark-state manifold cross over to a random distribution of weakly mixed light-matter, 'grey', states. In the Lipkin-Meshkov-Glick model, the ferromagnetic finite-sized ground state evolves towards a paramagnet as disorder is increased. In that regime, semi-localized eigenstates emerge, as we observe by extracting bounds on the participation ratio. These results present substantial steps towards freely programmable cavity-mediated interactions for the design of arbitrary spin Hamiltonians.

Automated wide-ranged finely tunable microwave cavity for narrowband phase noise filtering.

Narrowband microwave filters have wide ranging applications, including the reduction in phase noise of microwave sources within a given frequency band. The prospect of developing an automated filter that tunes itself to an arbitrary desired frequency at maximum extinction promises many experimental advantages such as an enhanced efficiency in performing fine frequency detuning scans and saving time and effort as compared to manual tuning. We design, construct, and program such an automated system and present its hardware and software for reproducibility. It consists of a cylindrical cavity filter and two motors, which change the cavity length and the coupling strength of the microwave field into the cavity, respectively. By measuring the cavity response, an algorithm implemented in Python optimizes these two parameters to achieve the tuning of the filter cavity to the desired frequency with a precision of around 20 kHz, which is significantly better than the cavity linewidth (∼1 MHz). We also demonstrate the suppression of phase noise at the desired frequency by more than 10 dB.

Deformable mirror for wavefront shaping of infrared radiation.

We present proof-of-principle results on terahertz wavefront shaping by means of a deformable mirror (DM). The DM is based on a reflective gold-coated steel membrane pushed by 35 powerful stepper actuators to enable a surface deformation of up to 1 mm out of equilibrium. The maximum excursion is equivalent to 10 wavelengths of the terahertz source centered at 3 THz and, thus, offers excellent opportunities for shaping the terahertz wavefront and beam intensity profile. As a proof of principle, we demonstrate terahertz focal spot optimization towards the diffraction limit, focal depth shifting, and terahertz imaging application. The large aperture DM offers new opportunities for the wavefront manipulation demanded by high-field terahertz science. The extreme excursion range of the DM will be beneficial for beam shaping at other wavelengths, such as visible and UV.