The QScale project focuses on the development of advanced quantum communication technologies, specifically of quantum repeater architectures, which represent a major and timely challenge for the field of quantum information science and technology. Quantum repeaters are indeed needed in order to overcome losses and errors in the transmission of quantum data. It allows the distribution of entanglement at arbitrary large distances, which is a universal resource for quantum information applications, including quantum cryptography and quantum teleportation. QScale holds the promise for bringing major advances in this field of research.
To reach these objectives, the consortium is composed of five academic and one industrial partners. Each team has a leading experience in quantum optics, atomic physics and quantum information processing. The consortium is spanning a variety of physical systems and encoding techniques: cold and ultracold atoms (Laboratoire Kastler Brossel in Paris and Istituto Nazionale di Ottica in Firenze), solid-‐state (Institute of Photonic Sciences in Barcelona), trapped ions (Universitat des Saarlandes in Saarbrucken), continuous-‐ variables and discrete variables. It also has a strong theoretical component with the group of Applied Physics in Geneva, who was a leading force in the last years to clearly identify physical requirements for scalable repeaters and proposed innovative solutions.
The first part of the project is devoted to photonic components, i.e. the development of photonic sources compatible with quantum memories, and of continuous-‐variable quantum light pulses, including non-‐ Gaussian fields for hybrid quantum repeater architectures. In the second part, efficient coupling between light and material systems will be implemented. It will allow the reversible mapping of quantum photonic information into and out of the memory device or the synchronized emission of single-‐photons from remote systems. The third part of the project will integrate these outcomes. It will address effective storage of entanglement in the devices developed previously, assessing their ability to operate as nodes of quantum repeaters. It will also pave the way towards deterministic entanglement swapping. The various photonic carriers and material memory systems investigated above will be compared. Finally, procedures and architectures for quantum repeater systems based on the previous elements will be examined and investigated, including novel hybrid schemes and new deterministic operations. Their implementation with the devices developed in the project will be assessed.
A great variety of results have already been demonstrated during the first 18 months of the project and 20 papers have been published in international journals. Here are some of these achievements:
- ICFO developed a solid-state quantum memory based on a Praseodymium doped Y2SiO5 crystal. For the first time, the team demonstrated the quantum storage of polarization photonic qubits in such a system. In this experiment, the storage was performed in the excited state of the ions, leading to short and pre-determined storage times. In order to achieve on demand read-out and longer storage times, the optical excitations need to be transferred to a long-lived ground state level, such that the photons are converted to collective spin excitations as a spin wave. ICFO took also this step recently and demonstrated the most efficient full atomic frequency comb scheme to date.
- LKB demonstrated for the first time the storage and readout of twisted photons in the single- photon regime, with a large ensemble of cold atoms. The demonstrated capability opens the possibility to the storage of qubits encoded as superpositions of orbital angular momentum states and to multi-dimensional light-matter interfacing.
- LKB demonstrated a very high-fidelity source of single-photons based on an optical parametric oscillator. The reported fidelity is the highest to date. In collaboration with GAP, they also demonstrated a novel operational and trustworthy witness suited for single-photon entanglement, a central resource of quantum repeater architectures. This tool constitutes the groundwork for insuring that future networks will perform well.
- INO developed an adaptive method to realize the mode-selective detection of quantum light states. They put this approach to a first stringent experimental test by analyzing the spectrotemporal mode of ultrashort single photons with a novel combination of techniques from the fields of ultrafast coherent control and quantum optics. Besides demonstrating the capability to detect and characterize states in unknown and arbitrarily shaped modes, they also showed that this scheme can be an important tool for novel quantum information protocols based on the encoding of qubits and qudits onto the spatiotemporal degrees of freedom of light.
- UdS used a single trapped ion as a resonant, polarization-sensitive absorber to detect and characterize the entanglement of tunable narrowband photon pairs. Single-photon absorption is marked by a quantum jump in the ion and heralded by coincident detection of the partner photon. In a second time, they also established heralded interaction between two remotely trapped single ions through the exchange of single photons. In the sender ion, they released single photons with controlled temporal shape and transmit them to the distant receiver ion, 1 meter apart. These works constitute very significant steps towards the use of ions trapped in remote strings for quantum communication schemes.