Fibre-Coupled GaSb Quantum Dot Tunable Single-Photon Sources for Field Deployed Quantum Key Distribution

Security of data transmission in classical cryptography is based on mathematical complexity required to hack passwords and cryptographic keys. Alternative method of encryption is quantum cryptography. In this case the security is ensured by fundamental laws of physics driving world in the microscale and described by quantum mechanics: it is not possible to copy unknown physical state (one can only make another object to be in this state – realize quantum teleportation, but in this case the state of the original object is modified), every measurement influences the state of the measured object – every measurement leaves a fingerprint. It is also important that matter and energy are quantized, thus there are minimal amount of them called quanta which cannot be further divided.

Such a quanta of electromagnetic radiation, is called a photon. Single photons can be used to encrypt cryptographic key. To realize this goal efficient single-photon sources are required. They should feature low probability of multiphoton emission and high generation rate of single photons. For such a source to be practical, it should emit photons with wavelengths in the III telecommunication window around 1550 nm, which correspond to minimum loss in commonly used silica fibres. This ensured the longest distance of data transmission. Additionally, single-photon sources should exhibit compact designs and be fibre coupled for compatibility with existing fibre infrastructure.

The most promising physical system enabling generation of single photons are epitaxial quantum dots. They offer outstanding quality of emitted single photons and possibility to simultaneously achieve high generation rates and low probability of multiphoton events, as well as compatibility with semiconductor technology. Quantum dots emitting in the visible and near infrared spectral range reached the level of technological maturity allowing for realization of almost ideal single-photon sources. Despite 20 years of research, it has not yet been possible to transfer these achievements to quantum dots emitting in the telecommunication spectral range.

Within the ‘Fibre-Coupled GaSb Quantum Dot Tuneable Single-Photon Sources for Field Deployed Quantum Key Distribution’ (FiGAnti) project we propose alternative material system, not yet explored in this regard, namely (In)GaSb/AlGaSb quantum dots. It will be necessary to optimize their growth to achieve efficient emission in the 1550 nm spectral range and description of their electronic structure and optical properties. Partner responsible for epitaxial growth of structures with quantum dots is University of Tampere (Finland) and the feedback about quality of investigated structures will be provided by Wrocław University of Science and Technology (Poland). Optimised structures will be hand over to partners from Germany (University of Würzburg) and France (The French Alternative Energies and Atomic Energy Commission in Grenoble), who will design and fabricate optical cavities, enabling higher generation rates of single photons. Additional functionalities of developed single-photon sources will be stabilisation of emission, possibility to tune emission wavelength and robust connection to the fibre. The most promising sources will be used for implementation of BB84 quantum key distribution protocol. It will not be a proof-of-principle demonstration but using existing deployed fibre link between KTH Royal Institute of Technology in Stockholm and Ericsson Labs 17 km away. This allows verification of application potential of developed source of single photons.

CONSORTIUM

• Coordinator:  Anna Musial (Wroclaw University of Science and Technology, Department of Experimental Physics, PL)
  
• Tobias Huber (University of Würzburg, Technische Physik, DE)
• Mircea Guina (Tampere University, Optoelectronics Research Centre, FI)
• Jean-Michel Gérard (Photonique Electronique et Ingénierie Quantiques, FR)
• Valery Zwiller (KTH Royal Institute of Technology, Quantum Nanophotonic, SE)