Quantum Communication Theory
Research overview
Quantum communication studies how the properties of quantum mechanics can be used to transmit information securely and connect quantum systems in different locations. The theory team uses quantum information theory to model communication schemes and networks mathematically, examining their security, communication rates, and performance in the presence of loss and noise.
The Quantum Communication Theory (QCT) team studies quantum key distribution (QKD), quantum repeaters, and networks that use entanglement. Alongside new communication protocols, we study fundamental limits: how much performance can be improved and under what conditions security can be guaranteed.
Distributing entanglement over long distances
Quantum networks require entanglement to be shared between distant locations. Photons can be lost as they travel through communication channels, making entanglement harder to generate over longer distances. We study entanglement-swapping and quantum-repeater schemes that can distribute entanglement efficiently even through channels with high loss.
Examples of our research include polarization-entanglement distribution using single-click entanglement swapping, and loss-tolerant protocols for distributing GHZ states.
Sharing secure keys without fully trusting the devices
Quantum key distribution uses the properties of quantum mechanics to detect eavesdropping and share cryptographic keys securely. Device-independent approaches allow security to be assessed without detailed assumptions about how the devices work internally. We theoretically study the effects of communication distance, noise, and detection efficiency on quantum key distribution using Bell inequalities and on schemes in which multiple users share the same key.
Examples of our research include long-distance device-independent quantum key distribution and device-independent conference key agreement.
Using entanglement across multiple sites for communication and sensing
Quantum networks can use entanglement shared across multiple sites as well as between two locations. We study how to distribute multipartite entanglement, such as GHZ and W states, efficiently and apply it to communication and distributed quantum sensing. We also investigate schemes that protect information while performing measurements, even when communication channels are affected by loss or noise.
Examples of our research include privacy-preserving quantum network sensing using loss-tolerant GHZ-state distribution, and quantum networks based on W states.
How we conduct research
The Quantum Communication Theory team uses quantum mechanics and information theory to build mathematical models of communication protocols and networks. We analyze their security, communication rates, and performance under loss and noise, using numerical simulations where needed. Some projects involve collaborating with our Quantum Communication Experiment team or experimental groups in Japan and abroad to demonstrate the schemes proposed in theory.