Superconducting qubit in waveguide cavity with coherence time approaching 0.1ms (1202.5533v1)

Abstract: We report a superconducting artificial atom with an observed quantum coherence time of T2*=95us and energy relaxation time T1=70us. The system consists of a single Josephson junction transmon qubit embedded in an otherwise empty copper waveguide cavity whose lowest eigenmode is dispersively coupled to the qubit transition. We attribute the factor of four increase in the coherence quality factor relative to previous reports to device modifications aimed at reducing qubit dephasing from residual cavity photons. This simple device holds great promise as a robust and easily produced artificial quantum system whose intrinsic coherence properties are sufficient to allow tests of quantum error correction.

• The paper demonstrates a transmon qubit design achieving T₂* of 95 μs and T₁ of 70 μs through effective photon dephasing suppression.
• The device employs a copper waveguide cavity with conventional lithography to create a robust thermal sink and minimize electromagnetic interference.
• The reported coherence quality factor of 2.5 million advances scalable and fault-tolerant quantum computing.

Superconducting Qubit in Waveguide Cavity with Enhanced Coherence Time

Overview

The paper presents a significant advancement in the field of superconducting qubits by demonstrating a transmon qubit with unprecedented coherence times. The authors achieve a quantum coherence time, $T_{2}^{\ast}$, of 95 $\mu$s and an energy relaxation time, $T_{1}$, of 70 $\mu$s with this system. This improvement is attributed to a carefully engineered device that suppresses dephasing induced by residual cavity photons. The paper is grounded in the framework of three-dimensional circuit quantum electrodynamics (3D cQED), which provides a robust platform for achieving high coherence.

Key Technical Contributions

1. Device Design and Fabrication:
   • The device employs a transmon qubit embedded in an empty copper waveguide cavity. The copper cavity serves as a robust thermal sink that helps in managing cavity photon number fluctuations.
   • The transmon qubit is manufactured using standard lithographic processes, while the cavity is crafted from bulk copper, ensuring efficient thermalization and minimal electromagnetic interference.
2. Photon Dephasing Suppression:
   • The system is engineered to minimize dephasing from residual photons by optimizing qubit and cavity parameters, thus reducing qubit dephasing rate per residual photon.
   • The cavity design ensures a large detuning from higher-order modes, diminishing their influence on qubit coherence.
3. Measurement and Results:
   • The reported coherence and relaxation times position this device within a desirable performance range necessary for error correction and reliable quantum computation.
   • The coherence quality factor $Q_{2}$ achieved is 2.5 times 10$^{6}$, significantly higher than earlier designs.

Implications and Future Directions

The coherence times demonstrated in this paper are critical for advancing error correction and fault-tolerant quantum computing techniques, potentially accelerating the development of scalable quantum processors. The successful suppression of photon-induced dephasing indicates that the proposed methods could be generalized to other systems within and beyond 3D cQED.

Moving forward, exploring the use of hybrid cavities, such as those with partial aluminum coatings, could yield even higher coherence properties. Furthermore, achieving sufficient qubit coherence with alternative manufacturing processes and materials offers intriguing possibilities for scalability and integration with current quantum computing architectures.

Conclusion

This research underscores the potential of 3D cQED systems in elevating qubit performance to levels suitable for large-scale quantum computing. By addressing a key limitation—qubit dephasing due to residual photons—with a sophisticated but practical device design, the authors provide valuable insights and methods applicable to future quantum technologies. The work stands as a testament to the feasibility of building coherent quantum systems using conventional fabrication technologies with strategic enhancements.