MULTIMEM · Multiple approaches for multimode quantum memories

Quantum communication aims to transmit a quantum state (e.g. entangled states) between two distant locations.Unfortunately, the losses during the transmission in optical fiber or in free space are detrimental to the entanglement and make direct practical implementation of long distances communication unrealistic.In classical communication this problem can be solved using amplifiers but this approach is forbidden by quantum mechanics in most of the cases.However the concept of quantum repeaters offers the possibility to solve this problem based on the idea that entanglement over a given distance can be created by entanglement swapping starting from two entangled pairs, covering only half the distance.This method implies the ability to store a quantum state and thus requires the availability of long-lived quantum memory.The long-term goal in the field is therefore to improve over the direct transmission rate using a quantum repeater approach.Indeed, quantum memories have been demonstrated in several materials using various protocols, but the practical implementation is still a far reaching goal.During this project, I will study two very promising protocols : the gradient-echo-memory (outgoing phase) and the atomic-frequency-comb memory (returning phase) with three different materials : hot atomic vapor, cold atoms, rare-earth crystals.Time and space channels multiplexing are key features to improve over the direct transmission rate,therefore this project focuses on the temporal and spatial multimode behavior of these memories.During the outgoing phase, I will build a gradient-echo-memory to store quantum correlated images.This is expected to be the first demonstration of multispatial mode quantum memory.During the returning phase depending of the experimental progress of the Geneva's group, I will work on temporal multiplexing and practical implementation of a quantum repeater, which fits perfectly within the European project leaded by the group : QuRep.