Quantum computing is a new form of computing under intense development that has the potential to dramatically outperform classical computing for a range of computationally intensive tasks. Quantum computing requires the precise control, measurement, and initialisation of physical quantum bits (qubits). In superconducting qubits this is typically achieved with significant support hardware including couplers, filters, and readout resonators. In this work, we investigate strongly-coupled superconducting qubits that exhibit a multi-mode behaviour to minimise the required hardware resources. This work was split into three objectives:
1. Achieving symmetry protection in a multi-mode superconducting quantum bit.
Context: In order to achieve a multi-purpose superconducting qubit, we must create large differences in decay rates between a storage and a dissipative transition. By designing the qubit with two modes, one which produces in-plane electric polarization and the other with out-of-plane polarization directly above the substrate, we aim to achieve dramatic differences in dissipation. In addition, by employing a longitudinal coupling instead of a typical, transverse coupling, mode hybridization will not cause the standard ‘Purcell’ limitation found in other superconducting qubit architectures.
Conclusion: In this investigation, we demonstrated Purcell filtering using symmetry protection in a multi-mode superconducting transmon qubit. This was shown through energy relaxation characterisation and drive-strength comparisons between a symmetry ‘protected’ mode and ‘unprotected’ mode. Pivotally, this symmetry protection was demonstrated with out-of-plane control, a necessary requirement for scalable quantum computation. This first demonstration showed a factor ~10 improvement. Future work includes increasing this further by improved fabrication tolerances.
2. Demonstrating applications of a multi-purpose superconducting qubit for quantum measurement and state initialization.
Context: When symmetrical protection is achieved, single photon dissipation is prevented, and only non-linear multi-photon processes allowed. We set out to explore this by demonstrating multi-photon decay and fast qubit state initialization. We aim to demonstrate a readout mechanism known as ‘resonance fluorescence’, to perform improved qubit measurement.
Conclusion: After an initial demonstration of symmetry protection, we found that the ratio of ‘single-photon’ and ‘multi-photon’ processes we achieved would not achieve high-fidelity readout. To improve this we formed a new external collaboration (led by the fellow) to obtain high-performance quantum amplifiers for our readout chain. In addition, a new design led by a PhD student was generated to further suppress single-photon processes using waveguide structures. This new design and amplifier will be used in follow-up work.
3. Demonstration of entanglement generation using engineered dissipation in multi-mode superconducting qubits.
Context: In order to fulfill all requirements for quantum computation using multi-mode circuits, we must be able to show a universal gate set between qubits. Using longitudinal coupling, we aim to investigate the extensibility of this architecture by building a multi-qubit processor each with symmetry protection. We hope to demonstrate entanglement generation between multi-mode qubits using novel energy transitions unavailable to typical qubits. By developing protected quantum ‘nodes’, we aim to demonstrate a key building block for a modular superconducting quantum computer with a hardware-efficient platform
Conclusion: We determined that we could achieve entanglement between multi-mode qubits using a ‘cross Stark’ effect for full ZZ control. Towards this, we demonstrated single multi-mode qubit cross-Stark Z-gate control at rates equivalent to other single qubit operations. Devices with coupled qubits have been designed and will be used in follow-up work.
