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Light-Matter Interfaces for Quantum Enhanced Technology

Periodic Reporting for period 2 - LIMQUET (Light-Matter Interfaces for Quantum Enhanced Technology)

Okres sprawozdawczy: 2020-01-01 do 2022-06-30

The field of Quantum Technologies aims at designing applications of the concept of Quantum Superpositions (such as entanglement), known as the second quantum revolution. By manipulating single quantum systems such as atoms, ions or photons, one will be able to benefit from their quantum behavior to achieve information processing and communication well beyond the limits of what is possible with classical physics. In the long term, this field is expected to provide disruptive technologies with a strong societal impact, such as a secure worldwide communication network and the development of quantum internet, computers and simulators that will outperform classical computers.

The LIMQUET project, which stands for Light-Matter Interfaces for Quantum Enhanced Technologies, is composed of a network of seven academic and three industrial partner organisations from five different countries in Europe. The LIMQUET project focusses on the interaction of light with atoms, ions and nanostructures and light with light as central tools in the development of new quantum technologies. Examples are quantum memories and quantum networks, which rely on the transfer of optical information, via single photons as its smallest constituents, into and out of atomic or optical memories. Another elementary constituent in quantum information technology is quantum logical gates.

Confinement of light strongly coupled to atoms can be achieved between highly reflective mirrors forming a cavity (corresponding to the field of cavity quantum electrodynamics, cQED). An alternative approach uses optically very dense samples, e.g. cold atoms prepared in a micro-trap. This enables strong non-linear couplings to store information contained in light pulses, and hence to provide an optical memory or to induce effective interactions between single photons for producing optical quantum gate and single photon quantum filter. The third approach uses metallic nano-structures and nano-optics, merging integrated optics and cQED principles via quantum plasmonics (representing quantized collective oscillations of the electrons of the metal in interaction with the electromagnetic field).

Via the training of 18 Early-Stage-Researchers, the LIMQUET project promoted collaborations between teams of quantum and nano-optics to combine the approaches towards the realisation of strong coupling between single quantum emitters and single photons in nanostructures. To achieve this goal, processes and techniques originating from atoms and ions research with optical cavities have been adapted to light-matter interactions with nanostructures, providing a critical step towards the development of powerful quantum devices.
Different approaches to provide the highly efficient light-matter interfaces have been successfully implemented: strong coupling between light and matter, controlled production of single photons, efficient memories, and miniaturization of the processes at the nanoscale.
The LIMQUET Network investigates various platforms for implementing quantum technologies including (i) quantum network using single photons, (ii) quantum computer featuring quantum logical gates and (iii) optical tools.

Workpackage (WP) 1 aimed at developing robust control methods for quantum technologies and related photonics. Highlights are the development of concepts and models for
- Deterministic generation of single-photons from (i) atomic ensembles or (ii) optical cavity;
- Broadband robust photonic operation (frequency conversion, polarisation rotators);
- High fidelity, robust and optimal quantum gates.

WP 2 aimed (i) at interfacing ions and atoms with photons, exploiting their strong interaction in optical cavities of high-finesse and (ii) at developing quantum engineering over long distances.
Using a single ion trapped in an endcap style ion trap with integrated fibre cavity, we demonstrated the generation of two consecutive single photons with controlled polarisation. This is an important step towards building an efficient ion-photon interface for quantum networks.
Following the first promising steps exploring a specific photonic CNOT gate, we have shown that this approach is universally applicable in any multi-mode interferometric setup, and that it can be used for spreading quantum information across larger networks.
We have built proof of concept experiments and sources operating at telecommunication wavelengths allowing long distance (>50km) control and generation of high dimensional frequency qubit compliant with telecommunication standards and photonics.

WP 3 dealt with techniques to manipulate light (few-photon wavepackets) by light (control laser pulse), aiming at novel techniques to store, shape and process photons as carriers of optical information. It also involves developments of optical tools for quantum technology.
We implemented the first biphoton source using cold atoms interfaced with a hollow-core fibre and demonstrated a record spectral brightness.
We have proved the concept of time-resolved microscopy with single photons.
The modulation frequency of the first prototype of our universal electro-optical modulator is significantly higher than frequencies previously investigated in the literature.

The main objectives of WP 4 were the integration of single photon sources on a photonic chip in adapting the atom/ion-cavity schemes from WP 2 to nanostructures. Single-photon nanophotonic and plasmonic circuitry open the way towards complex, miniature, and reconfigurable quantum circuits.
We have set up a nanoscale optical waveguide strongly coupled to an emitter, connected to an optical fibre as the first prototype. As complementary approaches, we have demonstrated an antenna-based single-photon source coupling to photonic circuits and developed a single photonic source from a quantum dot within a nanowire grown by epitaxy.
The robust and optimal control methods and platforms we develop are crucial elements beyond the state of the art to make quantum and photonic technologies reliable and scalable.
We have achieved the currently highest ion-cavity coupling to date and show the production of two consecutive photons. This is a crucial development of scalable quantum information processing systems and networks based on trapped ions, one of the two leading platforms for implementing quantum computation.
We have explored various systems antennas/emitters/waveguides made of different of materials beyond the state of the art in order to move forward for the efficient light-matter interaction at nanometer dimension. The main objective with a major societal impact is the integration of the quantum technologies down to such nanoscale. One important result is a demonstrator consisting in setting up an optical waveguide strongly coupled to an emitter and connected to an optical fibre.

Potential impacts are:
- Design of quantum key distribution methods overcoming the Security-Distance barrier
- Scalable quantum computing
- Design of integrated photonics building blocks (ring resonator, modulator, filter…) to stimulate the industrial production of such new components.
- Commercialization of (i) the time-resolved microscopy with single photons device and (ii) a universal electro-optic modulator.
- Implementation of quantum technologies integrated at the nanoscale.
LIMQUET project