Research

Quantum Gravity

Combining quantum mechanics and gravity into one unified theory remains the holy grail of modern physics. Since popular concepts such as string theory and loop quantum gravity do not provide predictions falsifiable with current technology, we approach this problem phenomenologically by pushing gravity measurements to smaller and smaller scales. In our current experiment, our goal is to break the current record (Westphal et al., 2021 ) of gravity measured between the smallest masses by two orders of magnitude using a novel optomechanical measurement technique: We track the movement of gravitationally interacting gold spheres via the resonance frequency of an encapsulating microwave cavity. This will be a first measurement of gravity in a true quantum system, paving the way for future experiments of gravitational interactions in quantum mechanically entangled systems (Liu et al., 2021 ).

Render of a gold sphere sitting at the center of a square structure on a gray chip

Digital render of a gold sphere sitting in an optomechanical setup designed to measure gravity.

Complex Quantum Materials

Essential for the development of quantum devices is a thorough understanding of the materials involved. Examples for such materials are highly magnetic lanthanide ions (e.g. holmium, erbium) dispersed in noble metals (e.g. silver, gold). The latter act as a diluting medium, preventing 'simple' magnetic ordering, which makes the resulting alloy magnetically highly complex. I am working both on the experimental investigation of such materials (Herbst et al., 2021 ) and their theoretical description (Herbst et al., 2022 ), in an effort to improve the performance of cryogenic X-ray detectors.

Render of a lattice of gold and erbium atoms, with arrows indicating interactions.

Artist's rendition of erbium ions (purple) interacting in a face-centered-cubic gold lattice.

Cryogenic X-ray Detectors

High resolution X-ray detectors are essential tools for fundamental research in particle physics, nuclear physics, and astronomy. Magnetic micro-calorimeters operated in dilution refrigerators at ~20mK meet this demand with an excellent resolving power, a wide energy bandwidth, and only a small, well-understood non-linearity. In my research, I am attempting to expand the application range of this technology, by developing detectors with reduced cooling requirements (Herbst, 2023). As an example, Quasy-maXs is a detector operated at 85mK in a small, compact adiabatic demagnetization refrigerator. It is the first magnetic micro-calorimeter to be used in the field of PIXE, and is currently being installed in the HRHE-PIXE facility in Lisbon (Reis et al., 2023 ).

Render of the experimental platform used to support a cryogenic particle detector. A octagonal, gold circuit board surrounds two micro-fabricated chips.

Digital render of Quasy-maXs (central chip) on its experimental platform designed for PIXE.

Noise in Superconducting Devices

Noise is usually the limiting factor in the performance of superconducting devices such as qubits, SQUIDs, or particle detectors. It is often challenging to disentangle the overall noise into its different noise components, which is necessary in order to quantify and eventually reduce or remove the various noise sources. I am working on a novel device called the noise-o-meter (Herbst et al., 2023 ), which has the ability to read out noise via three different measurement modes, all sensitive to different combinations of noise sources. Via comparison, it is thus possible to identify the origin of the noise.

Two renders of an experiment used to measure noise. Silver and niobium cylinders support a red circuit board and microstructured chips.

Digital render of the noise-o-meter, used to disentangle noise sources.