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.

Trapped cold ions are among the most advanced systems to implement quantum information processing. In current experiments entanglement of the qubits, represented by long lived internal atomic states, is achieved via quantum control of the (collective) motion of the ion crystal. Instead, we propose an unprecedented experimental program supported by theory, where the huge dipole moments associated with Rydberg excited ions are the basis of extremely strong spin-dependent long range interactions, and thus exceptionally fast entangling operations as basic building blocks for quantum computing and quantum simulation. While in the short term the fundamental questions to be explored are the understanding of Rydberg excitation and dynamics of single and multiple ions stored in linear Paul traps, and the various ways of manipulating this dynamics with external electromagnetic fields, the long term promise of this project is a potentially scalable very fast ion trap quantum processor, and in particular also a novel efficient quantum simulator of spin models, for Heisenberg type interactions to exotic matter with topological phases. A main experimental challenge is the requirement of a coherent light source near 122nm for the ion Rydberg excitation. Our consortium is in the remarkable and unique situation where in a single laboratory both these coherent light sources as well as advanced ion quantum computing setups are available, thus allowing us to explore this extremely promising new frontier of Rydberg ion quantum information processing on a comparatively short time scale. The planned experiments will be based on the well established techniques of ion trapping, quantum state detection and manipulation with laser fields. An adapted quantum shelving method is proposed to detect transitions to Rydberg states with unity detection efficiency on individual ions even in large crystals. Initially we will accurately determine energy levels and atomic properties of ion Rydberg states, and then we aim for mutual Rydberg state interactions of adjacent ions. Such gate interactions, Rydberg induced quantum phase transitions and a full tomography of the resulting quantum state benefit from the highly developed schemes in quantum information processing. In the future, beyond the experimental horizon of the three-year project, fast Rydberg ion quantum logic operations could possibly be combined with the conventional gate schemes and modern ion trap technology.

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.

The aim of QINVC is to exploit the superior quantum coherence of the spins of the negatively charged Nitrogen-Vacancy (NV) colour centres in diamond, at both room and low temperature, for quantum information processing (QIP). This system is indeed among the best solid-state quantum systems in terms of coherence, ease of manipulation by ESR, and addressability down to the single spin using optical microscopy. Our project first focuses on two hybrid strategies for QIP (WP1). The first one consists in fabricating regular arrays of single NV centres, with gate operation performed using a movable NV spin placed at the apex of a cantilever, coming close enough to get entangled with a NV spin on the lattice using magnetic dipolar interaction. The fabrication method is based on pulsed ion beam implantation through a tiny hole threading the tip of an AFM cantilever, or through the voids of a mask deposited at the surface of the diamond sample. The second strategy implements a hybrid architecture for QIP based on circuit QED and ensembles of NV spins: transmon qubits are coupled to ensembles of NV spins through the resonator in which they are embedded. The NV spins will be used as a long-coherence time quantum memory for the transmon qubits which will be used to process quantum information.

QINVC will investigate in WP2 the optical properties of NV centres at low temperature. The first goal of WP2 is to transfer the optical techniques used at room-temperature to control electron and nuclear spins to low-temperatures, in the purpose of applying them to hybrid quantum circuits (WP1). A second goal is to investigate the potential of NV centres ensembles to build a quantum memory for optical photons.

These ambitious goals will request the optimisation of NV centre production and their spin properties in synthetic diamond (WP3) using state-of-the-art methods and beyond. This will be done in collaboration with the world industrial leader on the production of synthetic diamond Element6 Ltd. Engineered implantation of single N impurities with nanometer resolution will be performed for WP1, and suitable concentrations will be prepared for both WP1 and WP2. Sample processing will be developed for these two WPs in order to minimize the unwanted defects that cause decoherence. Innovative fabrication techniques will be also developed, such as the preferential alignment of NV centres under uniaxial stress, an appealing possibility.