With the development of quantum chips, quantum communication is becoming essential for quantum computing and quantum networks. Flying Qubits, quantum information carried by photons, play a vital role in transferring data between nodes. Prof. Guofeng ZHANG, Professor of Department of Applied Mathematics of The Hong Kong Polytechnic University is dedicated to developing precise control methods for flying qubits, aiming to significantly improve the reliability and fidelity of quantum information transfer in future technologies.
Quantum technology is rapidly transitioning from theoretical marvel to practical tool, promising revolutionary advances in computing, communication and sensing. At the heart of this transition lies the quantum network—a system where distant quantum processors, or stationary qubits, are connected to share information. For these networks to function, information must be transmitted between nodes. This is where the flying qubit comes in.
Imagine a flying qubit as a quantum parcel. Its contents are fragile quantum states—like the "0" and "1" of a classical bit, but existing in superposition. This parcel is not carried by a truck, but by a single particle of light (a photon) travelling down a waveguide, akin to a fibre-optic cable for quantum data. For the recipient to successfully open the parcel and retrieve its pristine quantum information, the package must arrive not only with its contents intact but also in a very specific shape and form. The "shape" here refers to the photon's temporal profile—how its probability of being detected is distributed in the time domain. Mismatched shapes lead to lost or corrupted quantum information, crippling network efficiency.
Prof. ZHANG and his research team explore a groundbreaking approach to this critical shaping problem. The research introduces Quantum Optimal Control Theory (QOCT) to the domain of flying qubits. By treating the shaping process as an optimal control problem, the authors demonstrate how to design control pulses that mitigate the imperfections of real-world hardware, paving the way for more reliable and high-fidelity quantum networks. The study, titled “Quantum optimal control theory for the shaping of flying qubits” was published in Physical Review Applied.
The study represents a significant advance in the control of quantum light-matter interfaces, with the following key achievements:
1. Pioneering Application of QOCT: This research successfully adapts Quantum Optimal Control Theory—a powerful tool in the manipulation of stationary qubits—to the distinct domain of flying qubits, establishing a novel design paradigm for quantum photonics.
2. Holistic Handling of Real-World Imperfections: The framework simultaneously addresses major nonidealities prevalent in superconducting quantum platforms: the anharmonicity of transmon emitters (level leakage) and the restricted tuning range of practical couplers (photon leakage).
3. Clarified Control Roles: The study provides a definitive analysis of the separate and joint capabilities of coherent (u(t)) and incoherent (γ(t)) controls. It conclusively shows that while a tunable coupler is fundamental for shaping, coherent control is a critical complementary tool for mitigating tunability limits.
4. Provision of a Flexible and Practical Framework: The methodology is not limited to specific hardware. The gradient-based optimisation, complete with derived formulas (Appendix A), offers a systematic and adaptable approach that can be extended to other emitter types and multiple waveguides, and integrated with advanced optimisation or robust control techniques.
The introduced framework opens several exciting avenues for future research. Immediate next steps include extending the control design to more complex tasks, such as the generation of entangled pairs of flying qubits for distributed quantum protocols or the capture and conversion of flying qubits at a receiving node. Furthermore, the ultimate goal is to design flying-qubit-mediated remote quantum gates, enabling direct quantum logic operations between two distant stationary qubits without prior entanglement distribution.
In conclusion, the efficient control of flying qubits is a cornerstone for the realisation of functional quantum networks. By translating the shaping problem into an optimal control challenge, this work provides a powerful and systematic engineering toolkit. It moves beyond idealised models to deliver solutions for today’s imperfect devices, marking a crucial step from laboratory experiments toward scalable and reliable quantum information technology. The synergy between intelligent control design and advancing hardware will ultimately shape the future of quantum connectivity.
Source: Innovation Digest
