73% Bell-State Fidelity Reached Across 30 MHz Frequency Range

A Bell-state fidelity of 73 percent has been achieved in quantum communication across a 30 MHz frequency range, a relatively high number given the difficulties of maintaining entanglement between distant qubits. Researchers led by Takeaki Miyamura at The University of Tokyo and RIKEN Center for Quantum Computing demonstrated deterministic quantum state transfer and remote entanglement generation between fixed-frequency superconducting qubits on separate chips. This advance bypasses a key limitation of previous demonstrations by utilizing broadband transfer resonators, composed of two coupled coplanar-waveguide resonators, to avoid complex tuning of circuit elements. “Our approach avoids the complexity of control lines and noise channels, providing a flexible pathway toward scalable quantum networks,” the researchers state, suggesting a step toward practical and more easily expanded quantum networks.

This innovation circumvents a major obstacle to scalability by eliminating the need for intricate tuning procedures, enabling more robust and interconnected quantum systems. These resonators, constructed from two coupled coplanar-waveguide resonators, expanded the available bandwidth for quantum communication, offering a significant improvement over existing methods. Takeaki Miyamura, the contact author of the study, explained, “To enhance the frequency tunability, we implement broadband transfer resonators.” The approach relies on a frequency-tunable photon-generation technique to address sender-receiver mismatch, allowing for photon frequency adjustment without modifying the underlying circuit parameters. Quantum process tomography confirmed state transfer fidelities averaging around 79 percent across the tested frequency range, validating the efficacy of this new architecture.

Researchers are increasingly focused on transmitting quantum information between physically separated qubits, a necessity for building practical quantum networks, yet maintaining the delicate state of entanglement over distance remains a significant hurdle. This advancement bypasses a common limitation of prior demonstrations, which required complex and adjustable circuit elements to account for variations in fabrication. Instead, the team employed a frequency-tunable photon-generation technique, allowing them to adjust the photon frequency without altering the qubits themselves. This expanded bandwidth is crucial, as it allows for reliable quantum communication even with slight discrepancies between the sender and receiver devices. Quantum process tomography confirmed that Bell-state fidelities averaged around 73 percent across the entire 30 MHz range, indicating a robust and reliable transfer of quantum information.

The ability to perform quantum communication without relying on extensive tuning of individual qubits represents a substantial step toward scalability, reducing the complexity of control lines and potential noise channels. This approach offers a more flexible pathway for constructing larger, more practical quantum networks by minimizing the need for precise calibration of each component. The researchers suggest that this method could be instrumental in realizing future quantum communication systems capable of transmitting information securely and efficiently over long distances.