3d Cavity Integration Advances Graphene Superconducting Quantum Circuits with 17.63ns Fidelity

Superconducting quantum circuits represent a promising pathway toward building powerful quantum computers, and researchers continually seek ways to improve their performance and scalability. Kuei-Lin Chiu, Avishma J. Lasrado, and colleagues at National Sun Yat-Sen University, along with Yen-Hsiang Lin from National Tsing-Hua University, have now demonstrated a significant advance in this field by constructing graphene-based superconducting quantum circuits within three-dimensional cavity structures. This innovative approach allows for flexible coupling between two-dimensional materials and 3D cavities, achieving fast qubit transitions and clear evidence of two-qubit coupling, ultimately paving the way for the creation of more complex, multi-qubit devices based on advanced materials. The team’s work establishes a crucial building block for future quantum technologies, offering a new platform for exploring and controlling quantum phenomena.

The research team demonstrates a flux-tunable qubit transition in a single-qubit device, achieving an energy relaxation time of approximately 48 nanoseconds and a lower bound estimate for the dephasing time of 17.63 nanoseconds. Coupling this device to cavities with varying resonant frequencies allows access to multiple qubit-cavity coupling regimes, which facilitates the observation of vacuum Rabi splitting and flux-dependent spectral linewidths, confirming strong interactions between the qubit and the cavity’s electromagnetic field.

Hybrid Superconducting Qubit Device Fabrication

The study focuses on devices incorporating both fixed-frequency and flux-tunable qubits on the same chip. Fabrication involves Josephson junctions with dimensions crucial for determining qubit frequencies. Analysis of one device reveals a decrease in qubit frequency across multiple cooldowns, suggesting potential degradation mechanisms like oxidation or changes in the dielectric environment. Researchers use dispersive shifts, changes in cavity resonance frequency due to qubit coupling, to infer qubit frequencies and coupling strengths. Power-dependence measurements on a second device distinguish between the fixed-frequency and flux-tunable qubits based on their dispersive shifts.

The tunable qubit requires more power to enter an ionized regime, further confirming its identification. Observed frequencies and shifts relate to the Josephson and charging energies of the qubits, validating the design and performance. Room temperature cavity mode mapping identifies potential qubit transitions. Comparison of the two devices reveals differences in Josephson junction widths and charging energies, influencing qubit frequency scaling. The primary goal of this work is to demonstrate the feasibility of integrating both fixed-frequency and flux-tunable qubits on a single chip, while also understanding and mitigating qubit performance degradation. Dispersive shift analysis proves to be a powerful technique for characterizing qubit properties and identifying different qubit types.

Fast Qubit Control in 3D Cavities

Scientists have achieved progress in superconducting quantum circuit design by integrating circuits into three-dimensional cavities, enabling flexible coupling between devices and 3D architectures. Experiments with a single-qubit device reveal a flux-tunable qubit transition, demonstrating rapid operational speeds. Utilizing cavities with varying resonant frequencies allows observation of vacuum Rabi splitting and flux-dependent spectral linewidths, confirming strong coupling between the qubit and the cavity’s electromagnetic field. A two-qubit device, incorporating a superconducting quantum interference device (SQUID) and a single junction, exhibits a two-stage dispersive shift in power-dependent measurements, indicating successful qubit coupling.

Precise tuning of the cavity frequency at different readout powers attributes the initial shift to the single-junction qubit and the subsequent shift to the SQUID qubit, demonstrating the ability to address and control individual qubits within a single cavity mode. Measurements confirm a coupling strength of 100.5MHz and an estimated maximum qubit frequency of 6.438GHz. These results highlight the potential of graphene-based superconducting circuits for realizing multi-qubit 3D transmon architectures, paving the way for advanced quantum computing systems with joint readout capabilities and detailed study of interactions between superconducting circuits. This work represents a crucial step toward building complex, scalable quantum processors from two-dimensional materials.

Graphene Qubits in 3D Cavity Structures

This research presents a significant advance in superconducting quantum circuit design, successfully integrating graphene-based qubits with three-dimensional cavity structures. Scientists have constructed both single- and two-qubit devices, demonstrating tunable qubit transitions with a measured coherence time of approximately 48 nanoseconds and a lower bound estimate of 17.63 nanoseconds for the single-qubit device. Accessing multiple qubit-cavity coupling regimes allows observation of vacuum Rabi splitting, confirming strong interactions between the qubits and the surrounding cavity environment.

The two-qubit device reveals a distinct two-stage dispersive shift in power-dependent measurements, clearly indicating successful coupling between a fixed qubit, a SQUID-based qubit, and a single cavity mode. This achievement highlights the flexibility possible when combining two-dimensional materials with three-dimensional cavity designs, opening avenues for building more complex, multi-qubit systems. Variations in graphene junction quality currently lead to device-to-device differences in coherence, and future work will focus on improving these properties. Researchers anticipate that improvements in coherence will enable the readout of multiple qubits and the investigation of qubit-qubit interactions within the system, ultimately paving the way for advanced quantum technologies based on two-dimensional materials.