Shan Williams Jolin
Friday 10 June
09:30 - 12:30
This thesis investigates frequency-domain correlations, both classical and quantum, in nonlinear microwave circuits. The goal is to establish a compact method for generating quantum correlations between harmonic oscillators (modes) at different frequencies in the microwave domain, thus realizing quantum entanglement in a continuous variable system. En route to this goal we need to develop methods to generate and measure microwave frequency combs. Hence this work also includes a study of undesirable frequency mixing effects in analog IQ mixers, commonly known as mixer imbalance. We use mitigating methods originally developed for telecommunications applications, based on Kalman filters, and demonstrate their suitability for microwave experiments with superconducting circuits. We use the experimental setup based on analog IQ mixers to study classical correlations in frequency combs generated by driving a nonlinear Josephson junction circuit.
Similarly to the telecommunication industry, the advent of fully digital microwave methods present a significant advance in experiments with superconducting circuits. This thesis uses a new all-digital microwave platform, which is capable of synthesizing and measuring response at multiple comb frequencies with all frequencies being phase-coherent to a single reference and enable us to dispense with analog frequency converters such as IQ mixers. We use this digital microwave system to detect photonic and phononic Gaussian multipartite entanglement. The power of the platform is demonstrated by measuring a covariance matrix of 64 modes, or frequencies. Using multipartite entanglement criteria we present evidence of seven fully inseparable itinerant photon modes and four fully inseparable phononic modes. While it is possible to generate entanglement between many more modes, it becomes increasingly difficult to perform entanglement tests as the number of modes become larger. Another impediment is the added noise of the microwave amplifiers used in this thesis.
Our work nevertheless demonstrates the possibility to generate resources for continuous variable quantum information processing in very a compact device with a fully programmable digital control system. This thesis therefore represents a step toward the use of microwave frequency combs for one-way quantum information processing. Other applications include nonlinear characterization, quantum simulations and reservoir computing. We can therefore conclude that microwave frequency combs are a promising tool for quantum engineering with superconducting circuits.