Efficient device features to improve quantum algorithms with superconducting qubits (experiment)

Over the last years, several features have been implemented on dedicated superconducting qubit control electronics to decrease the round-trip time of the experimental apparatus. Some examples are the encoding of quantum computing control sequence in an assembly-code manner, as opposed to a pre-compiled, data-heavy waveform file defining the experimental sequence. This approach allows the use of FPGAs to implement real-time phase advances and parameter control, drastically reducing the data volume of the experimental sequence. To benchmark the improvements, systematic characterization of the processing time are required to quantify the benefit of the features used as a function of number of measurements, iteration of the experiments, and the number of repetitions. These efforts aim to allow for fast and reliable re-calibration of the experimental quantum processor, including heavy optimization tasks for quantum optimal control applications. The latter are particularly dependent on these measures, as runtimes of optimization algorithms approach days of measurement, making the outcome sensitive to long term drifts of the experiment.

In the scope of the Master project, you would investigate the efficiency of the experimental setup as a function of the features used by the control stack, to optimize the round-trip time of heavy-duty quantum computing algorithms. If possible, the project would include the implementation of FPGA features that allow for the measurement of quantum benchmarking algorithms such as shadow tomography and quantum process tomography in drastically reduced experimental runtimes. These techniques are crucial to improve the uptime and accuracy of quantum processors, and could be used to improve the performance of control pulses found by extensive optimization algorithms.

Contact: Max Werninghaus (max.werninghaus@wmi.badw.de)  [Prof. Filipp group]

Improved measurement routines and system characterization (experiment)

The quantum-computing group is currently collaborating with partners in the European flagship project to establish a digital twin of the quantum processor running in parallel to the experimental system, to perform live learning of system parameters.

In this project, the aim is to establish an interface with the numerical algorithm simulating the digital twin, which will be mainly developed by an external partner.  The experimental aspect of this work will investigate the accurate measurement of system parameters, employing measurements over time, which are then analyzed in Fourier space to characterize the frequency spectra of system parameter fluctuations.

The project includes the estimation of frequency ranges of different measurement approaches to combine multiple measurements and analysis routines to characterize a wide range of the noise spectrum. The characterized noise spectra will finally be included in the digital model system, which is then used to simulate and verify experimental results. The resulting digital twin is meant to be used in search of optimal control routines to improve the performance of quantum processors, and have so far not been established in this detail. Doing so would have significant impact on the field, as modelling quantum processors has been significantly difficult for superconducting qubits.

Contact: Max Werninghaus (max.werninghaus@wmi.badw.de)  [Prof. Filipp group]

Single qubit gate optimization for robust control (experiment)

In this project, the quantum computing group is looking to optimize single qubit pulses and find control shapes that are robust against particular drifts and fluctuations of qubit parameters. This effort is pursued at the WMI in close collaboration with the control theory group of Prof. Steffen Glaser. At the current stage, simulation and experiment yield matching results for limited parameter spaces.

The goal of this project includes the expansion of the existing efforts to more complex pulses which promise to achieve a robustness of the qubit against parameter fluctuations. The complexity of these pulses will be handled with our in-house optimization algorithm, which should be able to close the gap between numerical simulations and experimental data. Once the numerically found pulse shapes are successfully implemented and calibrated on the experimental setup, a sensitivity study has to be conducted to confirm the robustness of the control shape.

Such robust control pulses are a crucial step to shield quantum processors against drifts and unwanted interactions within the processor, to ultimately make advances for the operation of large quantum systems.

Contact: Max Werninghaus (max.werninghaus@wmi.badw.de)  [Prof. Filipp group]

Towards practical flux-pumped Josephson traveling wave parametric amplifier (experiment)

Quantum-limited amplifiers are one the critical elements required for efficient readout of superconducting qubits in quantum computers.  The ongoing challenge is to find a reliable way to combine large spectral bandwidths with reasonable gain and noise values in such devices. Flux-pumped Josephson Traveling Wave Parametric Amplifiers (JTWPAs) represent one of the promising approaches in this context. They rely on usage of a spatially separated parametric pump, thus, avoiding  detrimental self-Kerr effects which limit performance of other JTWPA designs. Here, particular goals are to develop a dispersion gap design blocking propagation of the second pump harmonic through the flux-pumped JTWPA and consider a flip-chip geometry of devices in order to improve the overall pump efficiency. This master project focuses on a design, fabrication, and characterization of superconducting circuits operated in the microwave quantum regime. Particular tasks include electromagnetic simulation & optimization  of superconducting microwave designs with nonlinear Josephson elements, realizing the JTWPs, their fabrication at WMI cleanroom facilities, and microwave measurements in the cryogenic environment. Prospective JTWPA samples are to be used further in quantum computing and quantum communication experiments.

Contact: Stefan Filipp (sfilipp@wmi.badw.de)

An up-to-the-minute list of bachelor's theses offered at WMI/TUPHE23 can be found on TUM Moodle.