Secure Quantum Communication Shared Between Three Players Is Now Proven Viable

Researchers at Technical University of Munich in collaboration with Walther Meissner Institute, University of St., Munich Center for Quantum Science and Technology (MCQST) have experimentally demonstrated a novel method for distributing quantum information that necessitates collaboration between at least two parties, marking a crucial advancement towards establishing secure quantum communication networks. W. K. Yam and colleagues utilise superconducting microwave quantum networks and squeezed states of light to achieve this, implementing a quantum secret sharing (QSS) protocol. The system embodies a method where a minimum of two participants must collaborate to reveal the original data, surpassing a key security benchmark and providing a foundation for unconditionally secure communication, while also revealing links to other vital quantum technologies such as quantum dense coding and error correction. Successful implementation of this tripartite protocol signifies progress towards building a robust and secure quantum internet.

Secure quantum secret sharing realised with high-fidelity microwave networks

Reconstructed-state fidelities now surpass 2/3, a threshold previously unattainable in microwave quantum networks and critical for demonstrating unconditionally secure quantum secret sharing. This achievement is significant because it directly relates to the security of the protocol; a fidelity below this value would allow an eavesdropper to potentially gain information about the shared secret. Using a tripartite network of superconducting circuits, the team confirmed a secure communication regime where collaboration between at least two of three parties is required to reconstruct the original quantum state. This experiment extends quantum secret sharing (QSS) protocols, a method for distributing quantum information securely, from the optical domain into the microwave frequency range, opening avenues for hybrid quantum technologies and potentially allowing integration with existing microwave-based quantum processors. The microwave frequency range offers advantages in terms of compatibility with superconducting circuits, which are a leading platform for building quantum computers.

Fidelities exceeding 0.66 were achieved for reconstructed states, validating the no-cloning theorem and confirming that any attempt to perfectly copy the unknown quantum state would fail. The no-cloning theorem is a fundamental principle of quantum mechanics, and its verification is essential for establishing the security of QSS. The tripartite network utilised two Josephson parametric amplifiers, or JPAs, to generate the entangled two-mode squeezed states crucial for the protocol; these devices amplify weak quantum signals while preserving entanglement. JPAs operate by parametrically down-converting a strong microwave tone into pairs of entangled photons, effectively creating a squeezed state. These squeezed states exhibit reduced noise in one quadrature of the electromagnetic field, enhancing the sensitivity of the quantum network. A ((2,3)) threshold scheme was also successfully implemented, meaning any two of the three parties could reliably reconstruct the original secret, while a single party gained no information, a result verified across all three possible collaborative pairings. This specific threshold scheme ensures that no single entity can access the secret independently, reinforcing the collaborative security aspect of the protocol.

This advancement establishes a foundation for exploring connections between QSS and other quantum information tasks, including quantum dense coding and error correction of channel erasures. Quantum dense coding allows the transmission of two classical bits of information using a single qubit, potentially increasing the capacity of quantum communication channels. Error correction, particularly for channel erasures, is vital for mitigating the effects of noise and loss in quantum networks, ensuring reliable communication over longer distances. However, these high fidelities were achieved with a limited set of coherent states, and scaling to more complex quantum information or longer distances remains a significant engineering challenge. Coherent states, while relatively easy to generate, are susceptible to noise. Future work will likely involve exploring more robust quantum states and developing techniques to mitigate decoherence. The authors intend to focus future work on improving the durability of the network against noise and imperfections in the superconducting components, potentially through improved shielding, materials, and control electronics.

Three-party entanglement validates secure communication but highlights scaling limitations

As quantum networks mature, establishing secure quantum communication is vital, yet scaling these systems presents formidable challenges. The inherent fragility of quantum states and the difficulty of maintaining entanglement over long distances are major obstacles. Extending the successful demonstration of QSS between three parties to larger networks remains a significant hurdle. Maintaining the high fidelity required for secure communication becomes exponentially more difficult with each additional participant; the current approach relies on precise control of entangled microwave photons, a task that intensifies with network complexity. Each additional node introduces potential sources of noise and loss, degrading the overall fidelity of the network. Furthermore, generating and distributing entanglement across a larger network requires increasingly sophisticated control and calibration procedures.

Superconducting microwave circuits establish a new benchmark for secure quantum communication, paving the way for further research to address these scaling issues and realise practical, large-scale quantum communication networks. The use of superconducting qubits offers advantages in terms of coherence times and controllability, but also presents challenges related to cryogenic cooling and signal routing. This experiment broadens compatibility with existing and emerging quantum technologies, unlike previous demonstrations reliant on optical frequencies, representing a strong step towards building flexible hybrid quantum systems. Integrating microwave-based quantum networks with optical fibre networks, for example, could enable long-distance quantum communication while leveraging the strengths of both platforms. Achieving fidelities exceeding a critical threshold opens possibilities for linking QSS with other important quantum information tasks, such as quantum dense coding and error correction, potentially enhancing the durability of quantum communication channels. Combining QSS with error correction could provide a more robust and reliable means of distributing quantum information, even in the presence of significant noise and loss. The ((2,3)) scheme implemented in this work provides a basic level of security, but more sophisticated schemes with higher thresholds could be explored to further enhance resilience against attacks.

The researchers successfully implemented a quantum secret sharing protocol with three players, demonstrating that a minimum of two must collaborate to reconstruct the original quantum state. This achievement is significant because it establishes a new method for secure communication using microwave two-mode squeezed states, surpassing a fidelity threshold of 2/3 necessary for unconditional security. The experiment also highlights connections between quantum secret sharing and other quantum information processing tasks, including quantum dense coding and error correction. The authors suggest further work could explore extensions of this protocol and its relationship to blind quantum computing.