Exploring Long-Range Coupling in Superconducting Spin Qubits: A Breakthrough powered by OPX

Researchers at Prof. Leo Kouwenhoven and Prof. Christian K. Andersen’s labs at TU Delft University have achieved a significant milestone in the field of quantum computing, as documented in a recent publication in Nature Physics [1]. Their work demonstrates the strong, tunable coupling between two distant superconducting spin qubits, marking an important step towards scalable quantum information processing. This breakthrough leverages the unique properties of Andreev spin qubits (ASQs), showcasing their potential to integrate semiconductor and superconducting technologies for advanced quantum applications. 

The Significance of Andreev Spin Qubits 

Superconducting spin qubits, specifically ASQs, represent a hybrid approach that combines the long coherence times of semiconductor spin qubits with the robust control offered by superconducting circuits. These qubits exploit the spin degree of freedom of electrons or localized holes, which is coupled to the supercurrent across a Josephson junction via spin-orbit interaction. This coupling enables fast and high-fidelity spin readout, a crucial feature for practical quantum computing applications. 

In the context of quantum information processing, superconducting spin qubits offer several advantages. They have long coherence times due to the spin degree of freedom, which is less susceptible to environmental noise compared to charge degrees of freedom. This makes them suitable for quantum computing, where maintaining quantum coherence is critical. Additionally, their small size is beneficial for scaling up to large numbers of qubits. 

Key Achievements of the Research: Leveraging Intrinsic Spin-Supercurrent Coupling 

The research team at TU Delft successfully demonstrated a strong, supercurrent-mediated coupling between two distant (approx. 25 μm) ASQs. This qubit-qubit interaction is of the longitudinal type and is both gate- and flux-tunable, achieving a coupling strength of up to 178 MHz. The coupling can be switched off in situ using a magnetic flux, highlighting the versatility and control of this system. Such characteristics are essential for implementing fast, two-qubit gates between remote spins, a significant challenge in current quantum computing architectures. 

The research leverages the intrinsic spin-supercurrent coupling in ASQs, facilitating inductive multi-qubit coupling. The ASQs are embedded in quantum dot Josephson junctions, implemented in Al/InAs nanowires. The spin states in these qubits are intrinsically linked to the supercurrent via spin-orbit interaction, which allows for rapid and accurate readout using circuit quantum electrodynamics (cQED) techniques. 

Figure 1: two coupled Andreev spin qubits connected to a coupling junction with a tunable Josephson inductance.  

Device and Experimental Setup: Magnetic Flux Control 

The experimental setup involved a device formed by two ASQs connected in parallel to a third Josephson junction with a gate-tunable Josephson inductance. This configuration defined two superconducting loops. The qubits, embedded in Al/InAs nanowires, were controlled by electrostatic gates and a magnetic field. The team utilized cQED techniques for readout, ensuring precise measurement and control of the qubit states. 

Key components of the device include: 

• Superconducting Loops: The two qubits are connected via a shared Josephson junction, forming two superconducting loops with gate-tunable Josephson inductance. 

• Electrostatic Gates: Three gates beneath each nanowire control the qubit states, while an additional gate adjusts the coupling junction. 

• Magnetic Flux Control: Magnetic flux through the loops is controlled to tune the qubit frequencies and coupling strength. This flux control allows dynamic adjustment of the coupling, crucial for implementing quantum gates. 

The ability to tune the qubit-qubit coupling strength by varying the magnetic flux through the superconducting loops and the gate voltage controlling the Josephson inductance allows for dynamic coupling adjustments. This feature is essential for scalable and adaptable quantum circuits. This is where the OPX, QM’s processor-based state-of-the-art quantum control system, plays a key role. 

Integration with Quantum Machines’ OPX: Feed-Forward and Scalability 

The processor-based OPX controller provides a comprehensive control solution tailored for quantum technologies, offering real-time control and feedback capabilities essential for manipulating semiconducting-superconducting qubits. In this specific case, the OPX system helped the researchers perform Real-Time Flux Control. OPX dynamically adjusts the magnetic flux through the superconducting loops on-the-fly, ensuring optimal coupling strength between the qubits. This real-time control is crucial for implementing fast, high-fidelity two-qubit gates and for switching the coupling on and off as needed. Moreover, it provides precise control over the pulse sequences applied to the electrostatic gates and magnetic flux lines, essential for accurate qubit manipulation. OPX allows for fine-tuned pulse shaping and timing, minimizing errors and enhancing gate fidelities. 

1. Ultra-Fast Feedback Mechanisms: The OPX system’s real-time feedback capabilities enable continuous monitoring and adjustment of qubit states, improving overall system stability and performance. This is particularly valuable for maintaining coherence and reducing dephasing during qubit operations. 
2. Complex Sequencing: Implementing advanced quantum algorithms requires complex pulse sequences and interactions between multiple qubits. The OPX system can handle these requirements efficiently, providing the necessary infrastructure for executing sophisticated quantum operations. 
3. Ease of Programming:  Using QUA, our extraordinarily intuitive pulse-level language, you can seamlessly code programs, just like writing pseudo-code.
4. Scalability: As quantum systems grow in size and complexity, managing the interactions and control signals becomes increasingly challenging. The OPX system’s scalability ensures it can support larger arrays of qubits with accurate synchronization and any-to-any data sharing (including measurements and real-time calculated results, not only Boolean decisions). Moreover, intuitive programming is maintained without requiring software redesign as the system scales.
   

Quantum Machines’ processor-based OPX is designed to handle the complexities of quantum experiments, providing the necessary infrastructure to implement dynamic and adaptive control protocols. This integration ensures that experimental setups, like the one demonstrated by the research team, can be scaled and adapted for more complex quantum computing tasks. The OPX system’s ultra-fast feedback and control capabilities are particularly valuable in tuning the qubit-qubit interactions and optimizing the performance of quantum gates.

Figure 2: Simplified measurement setup, (processor-based OPX at the top). The full setup can be found at [1-2] 

Implications for Quantum Computing 

This research addresses one of the critical challenges in quantum computing: The implementation of fast, long-range two-qubit gates. Traditional photon-mediated spin-spin coupling methods offer limited interaction strengths, typically around 10 MHz. The demonstrated coupling strength of 178 MHz in the TU Delft experiment significantly surpasses this, enabling faster and more efficient quantum gate operations. 

The longitudinal nature of the coupling in this setup offers advantages over transverse coupling, which imposes constraints on qubit frequencies. The strong and tunable coupling achieved in this study can facilitate the implementation of fast, high-fidelity two-qubit gates, essential for quantum error correction and complex quantum algorithms. 

The tunable nature of the coupling also opens up possibilities for more complex quantum algorithms and error correction methods, essential for the development of robust quantum computers. Integrating ASQs into existing quantum architectures could enhance their performance, providing a pathway to high-fidelity, scalable quantum systems. 

Future Directions 

The research conducted by TU Delft University of Technology represents a significant advancement in the quest for scalable quantum computing. By demonstrating strong, tunable coupling between distant superconducting spin qubits, the team has paved the way for more efficient and adaptable quantum circuits. The integration of advanced control systems, such as Quantum Machines’ processor-based OPX, further enhances these capabilities, bringing us closer to the realization of practical quantum computers.

Are you ready to join in on the excitement? Don’t miss out on the chance to explore this truly mind-blowing research, and learn how the OPX can enable your research. Get in touch with us to discover more about OPX+ and his new big brother, the OPX1000. 

References

[1] Pita-Vidal, M., Wesdorp, J.J., Splitthoff, L.J. et al. Strong tunable coupling between two distant superconducting spin qubits. Nat. Phys. (2024). 

[2] Pita-Vidal, M., Wesdorp, J., Splitthoff, L., Bargerbos, A., Liu, Y., Kouwenhoven, L., & Andersen, C. (2023). Strong tunable coupling between two distant superconducting spin qubits. arXiv 2307.15654