A team of international researchers has made significant progress in controlling the photon number coherence (PNC) of solid-state quantum light sources, a crucial development for quantum cryptographic protocols. The team used semiconductor quantum dots as emitters, known for generating high-quality single photons. By exploiting the two-photon excitation of a quantum dot combined with a stimulation pulse, they demonstrated the generation of single photons with a controllable degree of PNC. This advancement could significantly enhance the security of single-photon quantum cryptography schemes and broaden the range of quantum cryptographic protocols.
Quantum Light Sources for Quantum Cryptography
A team of researchers, including Yusuf Karli, Daniel A Vajner, Florian Kappe, Paul C A Hagen, Lena M Hansen, René Schwarz, Thomas K Bracht, Christian Schimpf, Saimon F Covre da Silva, Philip Walther, Armando Rastelli, Vollrath Martin Axt, Juan C Loredo, Vikas Remesh, Tobias Heindel, Doris E Reiter, and Gregor Weihs, have made significant strides in controlling the photon number coherence (PNC) of solid-state quantum light sources. This development is crucial for quantum cryptographic protocols, including quantum key distribution (QKD), which relies on single photons.
Quantum Dots as Emitters
The team used semiconductor quantum dots as emitters, known for generating on-demand single photons with high purity and indistinguishability. By exploiting two-photon excitation of a quantum dot combined with a stimulation pulse, they demonstrated the generation of high-quality single photons with a controllable degree of PNC. The main tuning knob is the pulse area, providing full control from minimal to maximal PNC.
Photon Number Coherence and Quantum Cryptography
Photon number coherence (PNC) is a crucial quantity relevant to the security of single-photon quantum cryptography schemes. The PNC gives information on the influence of the vacuum state as it is defined as the phase relation between vacuum and one-photon Fock state. This deviation from the one-photon Fock state can compromise security but might also be a resource for advanced QKD protocols.
Quantum Cryptographic Protocols
The excitation scheme presented by the team covers the requirements of a broad range of quantum cryptographic protocols. For instance, established protocols like BB84, decoy BB84, 6-state protocol, SARG04, LM05, and primitives like strong quantum coin flipping, unforgeable quantum tokens, quantum bit commitment, or quantum oblivious transfer require the absence of PNC to ensure security in QKD or fairness in coin flipping protocols.
Quantum Dot Photon Source
The team achieved tailored degrees of PNC from a quantum dot photon source on-demand, assuring high purity and indistinguishability. Their optical excitation protocol helps generate single-photon states in a well-defined polarization basis, and the photon counts are almost twice as large as achieved by the resonant excitation. This sets the stage for the quantum dot platform for its use in advanced cryptographic implementations.
Two-photon Excitation of Quantum Dots
The researchers chose to work with semiconductor quantum dots, excellent photon sources. The quantum dot can be modeled as a four-level system with the ground state, two linearly polarized exciton states, and a biexciton state. They chose the excitation protocols based on the resonant two-photon excitation (TPE) of the quantum dot from the ground state into the biexciton state. The TPE excitation leads to Rabi rotations between the ground and the biexciton state.
The article titled “Controlling the photon number coherence of solid-state quantum light sources for quantum cryptography” was published on January 27, 2024, in the npj Quantum Information journal. The authors of the study include Yusuf Karli, Daniel A. Vajner, Florian Kappe, P. Hagen, Lena M. Hansen, Roman Schwarz, Thomas K. Bracht, Christian Schimpf, Saimon Filipe Covre da Silva, Philip Walther, Armando Rastelli, Vollrath M. Axt, J. C. Loredo, Vikas Remesh, Tobias Heindel, Doris E. Reiter, and Gregor Weihs. The article can be accessed through its DOI reference https://doi.org/10.1038/s41534-024-00811-2.
