Quantum Cryptography

We design practical protocols for the photonic implementation of secure delegated computation, including quantum homomorphic encryption and blind quantum computation primitives.
A milestone achievement of our group was the demonstration of a privacy-preserving quantum computation that combines the power of quantum computing with the security of quantum cryptography in a client-server network. By using the concept of one-way quantum computing, a blind computation could be achieved where the user’s data input, processing and output remained unknown to the quantum computer. This enabled new methods and tools that allowed us to perform the first verification of a quantum computation by using limited resources.

We investigate how the laws of quantum theory can boost the security of modern communication networks. By carefully designing quantum-cryptographic primitives and securely mapping them onto state-of-the-art photonic platforms, we experimentally demonstrate new applications, ranging from quantum-digital payments to quantum leader election and quantum-secured one-time programs.

In a milestone achievement, our group showed how quantum light can provide security advantages over classical methods in everyday digital payments. This combined concepts of quantum-digital signatures with quantum money to generate unforgeable cryptograms, where the customers’ sensitive information is concealed by an information-theoretically secure cryptographic function, and the commitment to the purchase is guaranteed by the laws of quantum mechanics. Thanks to the practicality of the scheme, which requires no quantum storage or stringent spacetime-constraints, a spin-off company was founded in 2024: https://www.qubo.technology/

Quantum cryptography beyond key distribution: Theory and experiment
M. Bozzio, C. Crépeau, P. Wallden, P. Walther, Reviews of Modern Physics 97, 045006 (2025).

Quantum cryptography integrating an optical quantum memory
H. Mamann et al., Science Advances 11, (2025).

Demonstration of quantum-digital payments
P. Schiansky et al., Nature Communications 14, 3849 (2023).

Enhancing quantum cryptography with quantum dot single-photon sources
 M. Bozzio et al., npj Quantum Information 8, 104 (2022).

Experimental quantum homomorphic encryption
J. Zeuner, I. Pitsios, S.-H. Tan, A. Sharma, J. Fizsimons, R. Osellame, P. Walther, npj Quantum Information 7, 25 (2021).

Quantum advantage for probabilistic one-time programs
 M.-C. Roehsner, J. A. Kettlewell, T. B. Batalhão, J. F. Fitzsimons, P. Walther, Nature Communications 9, 5225 (2018).

Demonstration of blind quantum computing 
S. Barz, E. Kashefi, A. Broadbent, J. Fitzsimons, A. Zeilinger, P. Walther, Science 335, 303 (2012).