QUIE2T Quantum Information Entanglement-Enabled Technologies

Quantum communication, the transfer of quantum superposition states over long distances, is presently limited to about 200km (both in optical fibre and free space) due to unavoidable photon absorption losses. For this reason, theoretical schemes to extend this distance using “entanglement swapping” and “teleportation” have been established. By concatenating short entanglement swapping sub-sections it is in principle possible to generate entangled (correlated) bits over very long distances with bit rate only limited by the losses in one short section. If realised this would extend quantum communication applications such as quantum cryptography and quantum teleportation out to distances of thousands of kilometres.

In this consortium we propose to work towards such a deterministic quantum network based on semiconductor quantum dot-micropillar cavity systems. We will generate entangled photon sources from the biexciton-exciton cascade of a quantum dot (QD), with a potential fidelity of >90%. Moreover, we will develop a QD-spin micropillar cavity system, which acts as an all-in-one spin-photon-interface and a Bell-state analyser. This component eliminates the need for synchronous arrival of the two photons, and allows a wait-until-success protocol over the timescale of the spin coherence time (microseconds to milliseconds). Further subcomponents will include electro-optically tuneable single photon sources and recently proposed sequentially entangled sources.

With this suite of subcomponents we will be able to realise all the functions required for a scalable quantum network including the final entanglement purification steps. This is in contrast to previous experimental demonstrations of entanglement swapping (and teleportation) which were probabilistic and thus unscalable. The project involves collaboration between four partners. We will bring together two world-class groups, LPN and Würzburg (UWUERZ), working on micropillar cavities producing highly efficient entangled pair sources (LPN), and strongly-coupled QD-spin-cavity systems (UWUERZ), with the aim of addressing the challenging issues of entangled-pair sources and spin-cavity systems. Theoretical support for novel and practical entanglement schemes will be provided by Imperial College (IMP), and the experimental implementation will be performed by Bristol (BRIS) and LPN, who have world-class expertise in quantum optical communication, QD spins and semiconductor microcavity quantum electro-dynamics.

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 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.

The first part of the project is devoted to photonic components, i.e. the development of entangled 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. Several materials, including cold and ultra-cold atomic ensembles, trapped-ion strings and rare-earth ion doped crystals will be studied.

The third part 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.

At present quantum repeaters constitute a well-identified milestone on the quantum technology road maps, so the proposed project is a high-risk but also high-pay-off one.

Coherent optics has been known since the 1960’s to be, in principle, the best tool to achieve very high bandwidths and bit rates in optical communication. While the development of fiber optical amplifiers in the 1980’s has reduced the need for developing such a technology, the advent of quantum information sciences has triggered a renewed interest in using coherent optics to realize high-rate quantum communication systems. The present project is focused on coherent quantum communication as a way to combine the intrinsically very high rates achievable by homodyne or heterodyne detection with the fundamental benefits of using quantum mechanics such as unconditional security. Unlike with classical communication systems, an optical amplifier cannot be used as a repeater in a quantum communication setup because it is inherently limited by quantum noise. The thrust of this project is to explore different techniques aiming at circumventing this problem and improving the range of coherent (also called continuous-variable) quantum communication systems, with a special emphasis on today’s most developed platform towards practical applications, namely continuous-variable quantum key distribution.

Different strategies will be followed in order to attain this goal, ranging from the use of classical coding and other post-processing algorithms, which is the most directly applicable solution in the short term, to more elaborate longer-term techniques relying on specific quantum optical schemes and ultimately on the use of quantum coding. In particular, the potential solutions offered by the heralded noiseless linear amplifier, or other non-Gaussian heralded operations, will be investigated in detail.

The specificity of our consortium is to combine the strength of 5 academic groups having an outstanding track record in the area of coherent (continuous-variable) quantum information science, including 2 theory (ULB, UY) and 3 experimental (IO, MPL, TPT) groups, together with 1 industrial partner (SQN) who will naturally orient the research towards the needs of the information society. We envision that this synergy between applied and fundamental – both theoretical and experimental – teams will be highly stimulating and productive, and will reinforce European competitiveness in information technologies.

Device-Independent Quantum Information Processing represents a new paradigm for quantum information processing: the goal is to design protocols to solve relevant information tasks without relying on any assumption on the devices used in the protocol. For instance, protocols for device-independent key distribution aim at establishing a secret key between two honest users whose security is independent of the devices used in the distribution. Contrary to standard quantum information protocols, which are based on entanglement, the main resource for device-independent quantum information processing is quantum non-locality. Apart from the conceptual interest, device-independent protocols offer important advantages from an implementation point of view: being device-independent, the realizations of these protocols, though technologically challenging, are more robust against device imperfections. Current and near-future technology offer promising perspectives for the implementation of device-independent protocols.

This project explores all these fascinating possibilities. Its main objectives are (i) obtaining a better characterization of non-local quantum correlations from an information perspective, (ii) improve existing and derive new application of this resource for device-independent quantum information processing and (iii) design feasible implementations of device-independent protocols. We plan to tackle these questions with an inter-disciplinary approach combining concepts and tools from Theoretical and Experimental Physics, Computer Science and Information Theory.

The power of information theory – classical as well as quantum – originates in the abstraction of information from its physical carrier. On this level of discussion, every process, every time evolution and every operation is described by a quantum channel – an input-output relation abstracting from the microscopic origin of the physical dynamics. Quantum channels are therefore central objects and basic building blocks in quantum information theory. The composition of quantum channels is a very natural operation arising in most physical situations. Sequential composition arises, for instance, when two quantum processes are carried out one after the other. It is therefore surprising that a systematic study is still missing that analyses the effect of composition on basic properties of quantum channels, such as the ability to reliably transmit quantum information.

With this project we propose to fill this gap and provide a first in-depth analysis of fundamental properties of quantum channels, with a particular emphasis on the behaviour under sequential and parallel composition. We will, furthermore, initiate the study of complexity-theoretic properties of quantum channels, thereby providing a novel computer science perspective on quantum channels.

We expect the results from this project to have a profound impact to the study of quantum spin chains, quantum complexity theory and quantum cryptography. The project as well as the consortium is of interdisciplinary nature and will use modern tools from operator space theory, signal processing, convex geometry and complexity theory.