The research in our group focusses on Quantum Technologies in the broadest sense. In particular, we make chips using state-of-the-art nanofabrication techniques to study quantum effects in a variety of systems. For example, we look at high-frequency nanomechanical resonators at millikelvin temperatures, where these are in their quantum groundstate. Yet their tiny zero-point motion can be measured using ultra-sensitive optomechanical techniques. Another important line of research is integrated quantum optics, where photonic chips with functionality to generate, manipulate, and detect single photons are designed, made, and measured. This approach enables scalable quantum optics experiments.
The field of optomechanics is rapidly developing and a wide variety of systems is currently being studied around the globe. Instead of using macroscopic resonators, our approach is to integrate both the mechanical resonator and its optical readout on a single chip. This approach takes advantage of the quickly advancing integrated-photonics technology, and enables flexible designs for the mechanical resonator: These can range from devices with length of a few hundred micrometer to nanometer-sized vibrating structures. In general, the smaller the resonator, the higher the resonance frequency and the larger their zero-point motion. Nanomechanical devices can operate at gigahertz frequencies, and this means that when such devices are cryogenically cooled in a dilution refrigerator, they will be in their groundstate. The resonator is then a true quantum mechanical object. Alternatively, one can use lower frequencies and/or higher temperatures and apply cooling techniques to bring the resonator into the quantum regime. For this, we are developing a toolbox of feedback-assisted techniques. The beauty of mechanical systems is that they couple to almost anything, for example to charge, magnetic flux, temperature, and perhaps most importantly, to light. A mechanical resonator is therefore ideally suited to act as a quantum interface between different quantum systems such as superconducting qubits and single optical photons.
Quantum optics has a great potential for the transition from quantum science to quantum technology. In particular, photons are ideal as carriers of quantum information since they can be transferred over large distances with low loss and small decoherence. Amazing progress have been made in the past years, but it is often challenging to scale these experiments up to larger quantum systems. One issue is number of components needed and the required space; soon one will need many optical tables to perform one experiment. In our approach, all the required functionality for performing quantum optics experiments are integrated on a single chip. This includes generation of non-classical light, routing of single photons through programmable photonic circuits that implement quantum operations, as well as sensitive detection. After detection, the results can be analyzed or be fed-forward to later parts of the circuit. For this, low loss photonic components with tight tolerances are essential. Getting the best nanofabrication results is therefore an important aspect of our research. Moreover, the detection of single photons should happen with an efficiency as large as possible. For this purpose, superconducting single photon detectors are monolithically integrated on the same chip. Finally, our optomechanical structures serve as optical phase shifters with extremely low dissipation that allow for programmable circuitry, and current research focusses on implementing feed-back and feed-forward schemes using optimized devices.