Novel Approaches to Error Detection and Protection with Superconducting Qubits

Project Information

NovaDePro

Grant agreement ID: 101077479

DOI

10.3030/101077479

EC signature date 8 February 2023

Start date 1 April 2023

End date 31 March 2028

Funded under

• European Research Council (ERC)

Total cost

€ 1 454 635,00

EU contribution

€ 1 454 635,00

Coordinated by

KOBENHAVNS UNIVERSITET
Denmark

Periodic Reporting for period 1 - NovaDePro (Novel Approaches to Error Detection and Protection with Superconducting Qubits)

Reporting period: 2023-04-01 to 2025-09-30

Summary of the context and overall objectives of the project

The NovaDePro project addresses a core challenge in quantum information science: how to reliably detect and correct errors in quantum computing systems based on superconducting qubits. While recent years have seen rapid progress in building multi-qubit quantum processors, achieving truly fault-tolerant operation remains a formidable obstacle. At the heart of this problem lies the need for improved quantum error correction (QEC), which requires not only robust qubit performance, but also new hardware-efficient strategies for detecting and correcting errors in real time. Existing methods often rely on architectures and protocols that are difficult to scale or suffer from limitations in readout fidelity, calibration stability, or protection against environmental decoherence.
NovADePro was conceived to respond to these challenges from two complementary angles. First, it explores new physical mechanisms for detecting multi-qubit parity — a central operation in many QEC codes — using simplified yet tunable coupling architectures. Second, it aims to develop a new class of quantum circuits that incorporate intrinsic error protection into the physical design of the qubit itself. These two directions, while distinct, both serve the overarching goal of building more robust and scalable quantum processors capable of supporting the stringent demands of QEC.
The project is grounded in a recognition that overcoming the limitations of current approaches requires a departure from incremental tuning of existing designs. Instead, NovADePro pursues alternative circuit topologies, novel control paradigms, and material-platform integration strategies that may offer advantages in error suppression and ‘error diagnostis’.
The expected impact of the project lies in its potential to shift the capabilities of superconducting qubit platforms closer to fault-tolerant thresholds. By demonstrating novel error detection protocols and protected qubit architectures, NovADePro contributes to enabling future reliable quantum computation. The outcomes of NovADePro are anticipated to inform future approaches to building superconducting qubits specifically for realizing quantum error corrected quantum computers.

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

During the reporting period, the project developed and validated new mechanisms for error detection and protection in superconducting quantum processors, combining novel coupling schemes with protected qubit designs. In Work Package 1, we shifted from Josephson Ring Modulators to resonator-mediated longitudinal ZZ interactions, and implemented a dry-etched Tantalum qubit platform as the basis for these couplings. We achieved a proof-of-principle demonstration of longitudinal coupling between qubits mediated by a shared resonator and are now optimizing circuit parameters for two- and four-qubit parity gates. In Work Package 2, we carried out the first systematic experimental study of SIS–SIS series junctions and demonstrated the role of higher harmonics for enhancing double-well protection in the “Doublewellmon” qubit. We also completed a comprehensive characterization of SNS-based superconducting qubits, including control protocols, design constraints, an anharmonicity-based method to extract conducting channel numbers, and the identification of a previously unrecognized decoherence mechanism. Building on these insights, we realized an improved DW-derived protected qubit via flux-controlled harmonic balancing between SIS and SNS elements, now under detailed experimental study. Finally, we discovered a new parametric interaction between a fluxonium and a transmon qubit, defining a promising route to parity gates that connects the coupling concepts of WP1 with the protected encodings of WP2.

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)

The project delivers a set of device concepts and control techniques that directly support more efficient parity-based readout and intrinsically protected qubit encodings in superconducting platforms. The resonator-mediated longitudinal coupling scheme and deeply protected DW-type qubit offer concrete building blocks for scalable parity gates and protected qubits, while the systematic SNS-qubit study and associated decoherence “budget” provide design rules for future hybrid-junction processors. The newly identified fluxonium–transmon gate opens an additional pathway to compact parity operations with potential for intellectual property and targeted applications in quantum error correction. To ensure further uptake and success, key needs include continued research and optimization of coupling strengths and coherence, experimental demonstrations in larger multi-qubit architectures, support for IPR protection where appropriate, and engagement with fabrication facilities to standardise advanced Tantalum and hybrid SIS/SNS processes.