Observation of high coherence in Josephson junction qubits measured in a three-dimensional circuit QED architecture (1105.4652v4)

Abstract: Superconducting quantum circuits based on Josephson junctions have made rapid progress in demonstrating quantum behavior and scalability. However, the future prospects ultimately depend upon the intrinsic coherence of Josephson junctions, and whether superconducting qubits can be adequately isolated from their environment. We introduce a new architecture for superconducting quantum circuits employing a three dimensional resonator that suppresses qubit decoherence while maintaining sufficient coupling to the control signal. With the new architecture, we demonstrate that Josephson junction qubits are highly coherent, with $T_2 \sim 10 \mu$s to $20 \mu$s without the use of spin echo, and highly stable, showing no evidence for $1/f$ critical current noise. These results suggest that the overall quality of Josephson junctions in these qubits will allow error rates of a few $10^{-4}$, approaching the error correction threshold.

• The paper demonstrates a breakthrough in superconducting circuits by achieving T2 times of 10–20 µs and T1 times up to 60 µs using a 3D resonator design.
• The paper employs a three-dimensional aluminum cavity architecture to reduce dielectric losses and environmental noise in Josephson junction qubits.
• The paper’s analysis reveals error rates of approximately 5×10⁻⁴, positioning the architecture near quantum error correction thresholds for scalable computing.

Observation of High Coherence in Josephson Junction Qubits Measured in a Three-Dimensional Circuit QED Architecture

This paper delineates a significant advancement in the field of superconducting quantum circuits, specifically focusing on a novel three-dimensional architecture that notably enhances the coherence of Josephson junction qubits. The exploration of superconducting circuits has been of considerable interest for quantum information processing, primarily due to their potential scalability and operability within solid-state frameworks. However, the crux of such advancements hinges on the intrinsic coherence of the Josephson junctions and the degree to which superconducting qubits can be effectively isolated from environmental interference.

The authors introduce a three-dimensional resonator architecture designed to mitigate qubit decoherence while maintaining robust control signal coupling. The work reveals Josephson junction qubits exhibiting remarkable coherence with $T_2$ times ranging from 10 $\mu$s to 20 $\mu$s, achieved without employing spin echo techniques. Furthermore, the qubits demonstrated high stability, evidenced by a lack of $1/f$ noise in critical current measurements.

Key Findings and Results

• Coherence and Stability: The research demonstrates notably high performance in qubit coherence, achieving $T_1$ relaxation times up to 60 $\mu$s and $T_2$ coherence times between 10 $\mu$s and 20 $\mu$s. These values suggest qubit quality factors ($Q_1$ and $Q_2$) approaching one million, indicating an instability at a sub-ppm level.
• Architectural Innovation: By employing a three-dimensional cavity constructed from superconducting aluminum, the authors exploited the larger mode volume and reduced sensitivity to surface dielectric losses—a significant issue with previous designs using transmission-line resonators.
• Decoherence Mechanisms: The paper identifies spontaneous emission through the cavity, potential dielectric losses, and the presence of thermally induced quasiparticles as primary decoherence mechanisms. The coherence times independent of temperature data align well with theoretical predictions involving quasiparticles, corroborating the architecture’s efficacy.
• Numerical Analysis: Error rates were estimated to be approximately $5 \times 10^{-4}$, nearing thresholds compatible with quantum error correction, supported by the capability of fast gate operations (~10 ns) in superconducting qubits.

Implications

The findings from this paper have profound implications both practically and theoretically:

1. Practical Implications: The demonstrated qubit lifetimes and high-quality factors pave the way for implementing scalable quantum computing architectures in solid-state devices. The ability to achieve such coherence levels makes it feasible to consider larger entangled states and more complex quantum algorithms.
2. Theoretical Insights: The data provided stringent boundaries on parameters such as critical current noise, providing insight into Josephson junction behavior and interaction with environmental factors. This knowledge is imperative for building a more profound theoretical understanding of superconducting qubit systems.
3. Future Prospects: Although the current implementation showcases significant coherence improvements, opportunities remain for refining qubit-cavity interactions and further reducing undesired coupling. Ultimate scalability may involve leveraging these coherence advancements in multi-qubit systems within the three-dimensional architecture, which does not prohibit scaling.

In conclusion, this paper exemplifies a thoughtful modification to superconducting qubit architecture, markedly advancing coherence metrics. While challenges remain, the results indicate a promising horizon for this technology in constructing viable quantum computational systems. Further research will be essential to capitalize on these findings, particularly in assessing the architecture’s scalability and integration into complex quantum networks. It illuminates a pathway toward addressing critical challenges in quantum coherence and stability, moving closer to the ultimate objectives of quantum error correction and fault-tolerant quantum computation.