Efficient multi-step distillation of quantum states counteracting Gaussian decoherence without the need for quantum memories

• Applicant: Professor Dr. Roman Schnabel
• Subject Area: Optics, Quantum Optics and Physics of Atoms, Molecules and Plasmas
• Term: from 2018 to 2021
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 388405666

An essential component to quantum communication protocols is the distribution of quantum states between distant parties. Light is the most suitable carrier, due to its low sensitivity to decoherence effects. When bridging long distances, however, in particular Gaussian decoherence effects, such as optical loss, become critical issues. Decoherence generally degrades quantum communication speed, which quickly reaches zero in practice. Decoherence is indeed one of the main obstacles at the dawn of new quantum technologies. Multi-step (iterative) distillation protocols have long been proposed to overcome decoherence, but their probabilistic nature makes them inefficient since the success probability decays exponentially with the number of steps. To overcome this and to make multistep distillation efficient, quantum memories have been contemplated. But suitable quantum memories are not fully realised to date. The aim of this project is the experimental demonstration of efficient multi-step distillation of Gaussian states that suffered from Gaussian decoherence, without using quantum memories. We particularly focus on Gaussian entangled (two-mode-squeezed) states, and Gaussian decoherence in terms of optical loss. To work around the well-known no-go theorem for distillation within the Gaussian regime, we trigger our protocol on successful photon subtraction. The specific and new feature of our project is that no quantum memories are required. Instead, we follow the recently proposed approach of measuring the Husimi Q-function combined with appropriate data post-processing. This approach is able to emulate multi-step distillation using data taken at different times. The distilled data are indistinguishable from those an efficient distillation scheme using quantum memories would produce. Since the approach includes the final measurement it is particularly promising for enhancing continuous-variable quantum key distribution in real-world environments.

• DFG Programme: Research Grants
• International Connection: Czech Republic
• Cooperation Partner: Professor Dr. Jaromir Fiurásek