Superconducting Circuit Generates High-Fidelity Four-Qubit Frequency-Bin Dual-Rail Cluster States

Entangled states are fundamental to quantum technologies, offering the potential for revolutionary advances in computation and communication, and researchers are continually seeking ways to create and maintain these fragile states with greater reliability. Zhiling Wang, Takeaki Miyamura, and colleagues at RIKEN Center for Quantum Computing have now demonstrated a new method for generating complex entangled states known as cluster states, using microwave photons encoded with information in their frequency. This innovative approach employs a technique called time-frequency multiplexing, sequentially emitting photons of different frequencies to create robust entanglement, and crucially, incorporates a dual-rail encoding scheme that allows for the detection and correction of photon loss. The team’s implementation achieves high-fidelity entanglement across chains of up to eleven logical qubits, representing a significant step towards scalable quantum information processing in the microwave domain and offering improved resilience against a major source of error in quantum systems.

Utilizing continuous-variable quantum information encoded in microwave photonic qubits offers advantages in coherence and manipulation. This research focuses on a method for generating multi-qubit cluster states with high fidelity and entanglement by precisely controlling the timing and characteristics of microwave photonic qubits. Frequency-bin encoding enhances robustness against decoherence and allows for efficient manipulation of qubit states, while dual-rail encoding improves resilience against errors.

Time-frequency multiplexing creates multiple qubit channels within a single microwave frequency band, increasing qubit density and improving scalability. Researchers achieve this by controlling the interactions between microwave photonic qubits in the time-frequency domain, precisely shaping microwave pulses to induce controlled-NOT (CNOT) gates and create the desired entanglement structure. Successful demonstration of this technique will pave the way for larger and more robust quantum processors based on microwave photonic qubits, ultimately advancing quantum computation.

Photonic Cluster States for Quantum Computation

This research program focuses on building a photonic quantum computer using circuit QED as a platform. The core idea is to create and manipulate entangled photons, specifically cluster states, to perform quantum computations. Circuit QED is used to generate and control the photons, with superconducting qubits acting as transducers to create, manipulate, and measure photonic qubits. The research progresses from fundamental theoretical concepts to experimental demonstrations of increasingly complex photonic cluster states. Initial work focused on the fundamentals of cluster state generation and demonstrating entanglement between itinerant microwave photons and superconducting qubits. The team recently achieved states with over twenty qubits and also focuses on characterizing and verifying the generated states using theoretical tools for characterizing entanglement and quantum states. The research also investigates fault tolerance and quantum error correction, essential for building a practical quantum computer.

Frequency-Bin Encoding Boosts Entanglement Persistence

This research demonstrates a successful method for generating multi-qubit entangled states, known as cluster states, using a superconducting circuit and frequency-bin encoding of microwave photons. The team created cluster states with up to four logical qubits, achieving fidelities exceeding 50%. Importantly, the use of frequency-bin encoding, which allows for the detection of photon loss, improved the robustness of these states, maintaining fidelity even with errors detected and discarded. The researchers also quantified the persistence of entanglement across longer chains of qubits, finding that entanglement extended across up to eleven logical qubits using this approach. The results highlight the potential of frequency-bin encoding for building more resilient quantum systems for information processing. Future work will likely focus on improving the quality of generated single photons and the coherence of the superconducting qubit to scale up the size and complexity of the generated cluster states and explore their applications in quantum computation and communication.