Engineered dissipation using symmetry-protected superconducting circuits

Project Information

EDSP

Grant agreement ID: 832814

DOI 10.3030/832814

Project terminated on 29 January 2021

EC signature date 11 April 2019

Start date 1 May 2019

End date 3 July 2021

Funded under

• EXCELLENT SCIENCE - Marie Skłodowska-Curie Actions

Total cost € 224 933,76

EU contribution € 224 933,76

Coordinated by

THE CHANCELLOR, MASTERS AND SCHOLARS OF THE UNIVERSITY OF OXFORD
United Kingdom

Periodic Reporting for period 1 - EDSP (Engineered dissipation using symmetry-protected superconducting circuits)

Reporting period: 2019-05-01 to 2021-04-30

Summary of the context and overall objectives of the project

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.

Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far

Initial work was to design a two-mode qubit with symmetry protection, the ‘split-coaxial qubit’ (SCQ). This required the development of new simulation tools to determine parameters in a coupling regime significantly different from typical devices.

We measured two SCQ devices. We demonstrated symmetry-protection of the ‘dipole’ mode by (a) the observation of higher coherence compared to the unprotected ‘coaxial’ mode and (b) showed that the single-photon Rabi-rates (i.e. drive strength) for each mode were demonstrably different. This work was disseminated at the March Meeting 2020.

During this period we discovered an unexplored mechanism in multi-mode qubits, correlated charge-noise. We determined charge fluctuations were correlated between qubit-modes and that such drifts could be tracked. We were able to observe spatial correlations of charge drift. This opens the opportunity of a spatially sensitive charge detector using the SCQ. We developed theory and numerical simulations which can predict this behaviour. A scientific article is in preparation.

In order to show-case two-mode entanglement and coherent operations, we re-designed the SCQ to mitigate the effects of charge-noise. Measurements showed that charge noise can be effectively removed with certain design parameters.

A new measurement set-up was built to optimise for high-coherence through improved shielding, filtering and signal control. We brought coherence times up from ~5 us up to ~100 us.

Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)

We were able to generate tangible progress on multi-mode superconducting qubits beyond the state of the art. This includes:

1: First demonstration of a coaxial two-mode transmon.
Impact: Demonstrates viability of two-mode qubits in a scalable circuit design.

2: First demonstration of symmetry protection in a multi-mode coaxial qubit.
Impact: Demonstrates that out-of-plane control (a requirement for scalability) can be achieved while maintaining symmetry protection. This symmetry protection provides a hardware-efficient alternative to on-chip Purcell filtering techniques which are used widely in the field.

3: First investigation into correlated charge noise in multi-mode transmon qubits.
Impact: This work could lead to the development of a spatially sensitive charge detector which could have profound effects in the understanding of the origin and dynamics of charge fluctuations in superconducting devices. In addition, we report the limitations of multi-mode qubit design and provide a framework for predicting unwanted charge dependence in future multi-mode qubit development.

4: Characterised a novel Z-gate using the ‘cross-Stark’ effect on a multi-mode superconducting qubit.
Impact: A ‘cross-Stark’ effect which is unsuitable for typical qubit architectures allows entanglement generation between multi-mode qubits. By demonstrating a ‘cross-Stark’ effect on a single multi-mode qubit we have the know-how to transfer directly to two-qubit entanglement operations. This will be the first demonstration of entangling gates between multi-mode qubits and could have a profound effect on methods for quantum computation with superconducting qubits. Coupled multi-mode devices have been designed and are due to be fabricated and measured by a PhD student after the conclusion of this fellowship.

Schematic of the split-coaxial qubit