As the world of quantum computing moves toward building scalable and reliable systems, one of the biggest challenges researchers face is managing errors in quantum computations. Unlike classical computers, which rely on bits that are either in a state of 0 or 1, quantum computers use quantum bits, or qubits, which can exist in multiple states simultaneously. This unique property of qubits opens up the potential for unprecedented computational power, but it also introduces new complications in terms of error correction and system performance.
A promising strategy for improving the reliability and scalability of quantum computers lies in the use of dual-type qubits. These qubits can encode quantum information using two different types of quantum states, providing increased flexibility and reducing undesirable interactions, such as crosstalk between qubits. In simple terms, dual-type qubits may enable more stable and error-resistant quantum computations without requiring the addition of complex hardware components.
Recently, a team of researchers from Tsinghua University and other institutes in China achieved a significant breakthrough by realizing an entangling gate between dual-type qubits in an experimental setting. Their work, published in Physical Review Letters, introduces an effective approach to realize these gates, helping to push quantum computing towards more efficient and robust systems.
The Importance of Dual-Type Qubits
In quantum computing, entangling gates are operations that link multiple qubits together in such a way that the state of one qubit depends on the state of another, regardless of the distance between them. This entanglement is fundamental for many quantum algorithms and computational tasks, including error correction. Traditionally, for quantum systems to utilize multiple types of qubits, such as those encoded in different quantum states, one approach is to convert all qubits into the same type before performing entanglement operations. However, this introduces a significant overhead in terms of computation, as the system must go back and forth between the two different qubit types, leading to inefficiencies and higher error rates.
A more optimal solution is to directly entangle dual-type qubits, thereby preserving the benefits of using different quantum states without the need for state conversion. This dual-type encoding could potentially improve the operational fidelity of quantum systems, making them more reliable while keeping their design simpler and less resource-heavy.
According to Luming Duan, a senior author of the study and a leading researcher in quantum computation, encoding two types of qubits in the same ion species is an excellent strategy. One type can carry quantum information, while the other can be used for auxiliary operations. “A crucial gadget in this scheme is to entangle the qubits encoded in these two types,” Duan said. The breakthrough came when they succeeded in developing a method to directly entangle dual-type qubits without needing the traditional and resource-demanding qubit type conversions.
How Dual-Type Qubits Work in Practice
To entangle dual-type qubits in their experimental setting, Duan and his team used a 532 nm laser system to drive Raman transitions, which are interactions between light and matter that can affect the quantum state of ions. They meticulously designed the laser’s frequency components so that it could couple two types of qubits—each encoded in the hyperfine levels of two different ions—while also working with the ions’ collective spatial oscillation. This collective oscillation was used as a quantum bus, meaning it allowed quantum information to be exchanged between the ions.
The team’s careful engineering allowed the creation of entanglement between two ions, each containing qubits encoded in different quantum states, while maintaining operational simplicity. No additional hardware components were required, which is a critical development since reducing the complexity and resource demands of quantum systems is a primary goal of the quantum computing community.
Results and Performance
After running a series of tests, the team achieved entanglement between dual-type qubits encoded in different manifolds—specifically the S and D manifolds of ions. They also found that their experimental setup generated a Bell state fidelity of 96.3%, a performance that rivaled that of same-type entangling gates (such as S-S or D-D gates).
Bell state fidelity is a measure of how effectively quantum entanglement has been achieved, with higher fidelities indicating a more reliable entanglement. In the context of quantum computing, high Bell state fidelity is essential for creating trustworthy quantum states that can be used in computational operations and quantum algorithms.
Duan and his team found that the performance of their dual-type entangling gates was virtually the same as that of conventional same-type gates. This outcome suggests that there are no fundamental obstacles to scaling this method, making dual-type entangling gates a practical solution for enhancing quantum computational systems. According to Duan, “using a single setup, we achieve dual-type and same-type entangling gates with similar gate performance, suggesting that there is no additional fundamental limitation in realizing a dual-type entangling gate, such that it can be applied in practical quantum circuits to help reduce the overhead of back-and-forth qubit type conversions.”
Significance for Quantum Computing
The implications of this research are far-reaching. By demonstrating that dual-type qubits can be entangled with the same high fidelity as traditional qubits, the study removes a significant roadblock in the development of scalable quantum systems. Specifically, the dual-type system can reduce computational overhead and complexity, making it easier to create more sophisticated quantum algorithms that can perform complex tasks while maintaining high error rates and reliable performance.
This study’s approach could help in the minimization of quantum errors—a persistent challenge in the field. Quantum error correction is essential for reliable quantum computing, as qubits are extremely sensitive to environmental interference. As the number of qubits in a system increases, errors propagate and magnify, which necessitates the application of error-correcting codes. By reducing overhead and error propagation through dual-type qubits, the implementation of error correction might become more feasible in practical scenarios.
Moreover, Duan and his colleagues aim to take this work further. Plans for future upgrades to their experimental system include better stabilization of optical paths and improved frequency locking of the ion trap. Such advancements are crucial in reducing experimental noise and ensuring that the dual-type entangling gate can perform under real-world conditions over extended periods.
Another promising direction for future research involves utilizing the dual-type entangling gates for mid-circuit quantum state detection, a key component for implementing quantum error correction techniques in real-time during a quantum computation. Additionally, the researchers aim to apply their results to trapped-ion-based quantum networks, creating nodes that can exchange information efficiently and without the overhead associated with traditional quantum communication schemes.
Looking Ahead: Broader Applications and Next Steps
The research conducted at Tsinghua University represents a critical milestone toward the realization of more efficient and practical quantum computers. Quantum systems that use dual-type qubits could improve error tolerance and operational stability without relying on overly complex hardware. Moreover, with minimal resource requirements, dual-type qubits could serve as the foundation for highly scalable quantum circuits that are capable of tackling problems far beyond the reach of today’s classical computers.
As the quantum computing community continues to push the boundaries of what’s possible, Duan and his team’s research may inspire a new wave of experimentation, further cementing dual-type qubits as a key tool for driving the quantum revolution forward.
The team’s vision for the future includes developing quantum network nodes, implementing real-time error correction, and enhancing quantum systems’ general performance, ensuring that these sophisticated devices can one day fulfill their promise for practical use in industries ranging from cryptography and optimization to machine learning and complex scientific simulations.
Reference: Chenxi Wang et al, Experimental Realization of Direct Entangling Gates between Dual-Type Qubits, Physical Review Letters (2025). DOI: 10.1103/PhysRevLett.134.010601. On arXiv: DOI: 10.48550/arxiv.2410.05659