Cosφ-coupling Suppresses Unwanted State Transitions, Boosting Quantum Readout Fidelity in Transmon Circuits

Unwanted state transitions currently limit the performance of superconducting quantum bits, hindering both their readout and manipulation, and researchers are actively seeking ways to overcome these challenges. Cyril Mori, Francesca D’Esposito, and Alexandru Petrescu, alongside their colleagues at the Université Grenoble Alpes and other institutions, investigate a novel approach to this problem by employing a unique ‘cosφ-coupling’ scheme for connecting a quantum bit to its readout system. This new coupling, built using a transmon molecule circuit, possesses inherent symmetry properties that effectively suppress measurement-induced state transitions, a common source of errors in quantum measurements. The team successfully demonstrates single-shot readout of the quantum bit up to its fifth excited state, and importantly, finds no evidence of these unwanted transitions even at high readout powers, levels exceeding 300 photons, demonstrating a significant improvement in readout fidelity and stability. Furthermore, they show the ability to controllably introduce these transitions, confirming the underlying mechanism and highlighting the potential of this cosφ-coupling scheme for advanced quantum control.

Measurement-induced state transitions arise from intrinsic resonances described by the readout process. They can occur even at moderate power levels, limiting the readout signal-to-noise ratio and quantum non-demolition readout fidelity. This work investigates the high-power readout regime using a different transmon readout scheme.

Superconducting Qubit Foundations and Control Techniques

This extensive collection of references details research into superconducting qubits, quantum computing, and related experimental techniques. Research focuses on transmon qubits, exploring inductive shunting and alternative designs, with a major theme being qubit anharmonicity, crucial for addressing and controlling qubits. Studies address lifetime renormalization, strong drive effects, and Kerr reversal to maintain qubit coherence and fidelity. The references also explore the impact of strong drives and investigate high-fidelity quantum gate operations through parametric gates and cross-resonance techniques.

Significant attention is given to decoherence, relaxation, and environmental factors affecting qubit coherence. Readout is a key focus, with research dedicated to improving fidelity and speed through resonator-based methods, parametric amplification, and minimizing backaction. Studies explore rapid driven reset and high-power readout to optimize speed and efficiency, and investigate nonlinear coupling for improved readout. Precise fabrication techniques, such as shadow evaporation, are also highlighted. The collection covers quantum computing architectures and scalability, focusing on coupling qubits and designing architectures for quantum information processing.

Resonator-based architectures are a recurring theme, alongside challenges in scaling up quantum systems. Research also addresses quantum measurement and state preparation, including quantum non-demolition measurement, efficient state preparation, and trajectory analysis of qubit dynamics. Theoretical and computational methods are well-represented, with studies on chaos theory, resonance processes, and Hamiltonian dynamics to understand driven qubit behavior. Numerical simulations are employed to model qubit behavior and optimize designs. Finally, the references cover experimental techniques and materials, including thin film fabrication and Josephson junction fabrication.

Key themes emerge, including understanding and mitigating the effects of strong driving on qubit coherence and fidelity, improving qubit readout speed and efficiency, and exploring different architectures to build larger quantum computers. A strong emphasis on theoretical modeling and simulation underpins the research, alongside investigations into the complex dynamics of driven qubits and potential unwanted behavior. In conclusion, this bibliography provides a comprehensive overview of research in superconducting qubits and quantum computing, highlighting key challenges and opportunities in this rapidly evolving field.

Cos φ-Coupling Suppresses Qubit Measurement Errors

Scientists have developed a novel transmon readout scheme, termed cos φ-coupling, that suppresses measurement-induced state transitions, a significant limitation in superconducting qubit performance. This breakthrough addresses a key obstacle to achieving both fast and high-fidelity qubit readout, as conventional methods suffer from unwanted state transitions at higher power levels. The team implemented a unique circuit design, a transmon molecule capacitively coupled to a readout resonator, leveraging its inherent symmetry properties to minimize these transitions. Experiments reveal an absence of these transitions up to a power level of 300 photons in the readout mode, a substantial improvement over standard transverse coupling schemes.

The cos φ-coupling achieves this robustness through a combination of parity symmetry and detuning, effectively suppressing multi-excitation transitions that drive unwanted state changes. Unlike conventional readout schemes, the cos φ-coupling relies on suppressing transitions involving an exchange of even numbers of qubit excitations and readout photons. This parity symmetry, combined with detuning, significantly reduces the probability of unwanted state transitions, maintaining the qubit’s quantum information. Researchers were also able to controllably induce these transitions by intentionally breaking the parity symmetry using a flux bias, demonstrating precise control over the system.

Detailed analysis confirms the theoretical predictions and quantitatively demonstrates the superiority of the cos φ-coupling. The team successfully performed multi-state single-shot readout up to the fifth excited state of the transmon, enabling identification of leakage pathways and confirming the stability of the qubit manifold. These findings pave the way for significantly improved readout fidelity and speed, crucial for advancing the development of practical quantum computing technologies and enabling more complex quantum operations.

Cosφ-Coupling Enables High-Fidelity Qubit Readout

This research demonstrates a new approach to reading the state of superconducting qubits, employing a nonlinear coupling called the cosφ-coupling, which reduces unwanted state transitions during measurement. The team successfully performed multi-state single-shot readout, accurately identifying the state of the qubit up to its fifth excited state, and found the system remained free from measurement-induced state transitions at high power levels, up to 300 photons in the readout mode. This contrasts with standard readout methods where such transitions limit accuracy and fidelity. The key to this improved performance lies in the cosφ-coupling’s inherent parity symmetry, which effectively suppresses transitions between certain quantum states.

By carefully controlling the symmetry using an external magnetic flux, the researchers could even induce these transitions when desired, confirming the underlying mechanism. Simulations and experiments align, demonstrating the cosφ-coupling’s robustness to high readout powers and its potential to overcome limitations found in conventional transverse coupling schemes. The authors acknowledge that the quantum nature of the readout is likely interrupted beyond approximately 231 photons due to cavity bistability or inelastic scattering. Future work could explore scaling this technique and integrating it into more complex quantum circuits, potentially paving the way for more reliable and high-fidelity quantum computation.