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Spin effects for quantum optoelectronics

Final Report Summary - SPIN-OPTRONICS (Spin effects for quantum optoelectronics)

This project join the forces of ten leading European teams which allows to achieve a critical mass in the new research field of Spin-Optronics, a vast novel research area at the crossroads of fundamental physics of quantum-mechanical spin, optoelectronics and nanotechnology. Spinoptronics aims to achieve spin and light polarisation effects in nanostructures, utilizing confinement of not only charges and spins, but also photons. In this field, the information is ultimately carried by the spin of photons, : it can be encoded in the confined spin state and manipulated on the nano-scale and redelivered in a form of polarized photons. This network achieved several breakthroughs which are detailed below and establishes the European leadership in this area on a world-wide scale. From the training point of view, we have delivered a top international level multidisciplinary training to 13 early stage researchers and 5 experienced researchers. We have offered an extended program of multinational exchanges and secondments and a strongly developed choice of complementary skills training. We have organized 4 project meetings, 3 international schools and one international conference devoted to spinoptronics and nanophotonics. The trained young researchers have acquired all the necessary background to become the next leaders of the European science and industry. All the 26 project milestones have been achieved. 160 research papers have been published, including 13 papers of the nature ground and 26 Physical Review Letters.

The main breakthrough achieved along the project objectives are detailed below:

1°) Coherence of individual spin, storage of quantum information.
In this domain the network successfully addressed the main challenges related to the single spin control in QD devices. One can cite the development of a new technique of radio frequency spectroscopy allowing for measuring the nuclear spin motion of individual atomic species forming the QD, the full coherent control of nuclear spins in an optically pumped single QD, or the demonstration of QD exciton and biexciton Generation via Adiabatic Passage with Frequency-Swept Optical Pulses. We demonstrated coherent optical control of a single hole spin confined to an InAs/GaAs quantum dot. A superposition of hole-spin states is created by fast (10–100 ps) dissociation of a spin-polarized electron-hole pair. Full control of the hole spin was achieved by combining coherent rotations about two axes: Larmor precession of the hole spin about an external Voigt geometry magnetic field, and rotation about the optical axis due to the geometric phase shift induced by a picosecond laser pulse resonant with the hole-trion transition. Magneto-optics investigations on new individual GaAs/AlGas dots grown by droplet epitaxy on (111)A GaAs substrates lead us to the demonstration of magnetic-field-induced valence band mixing under longitudinal magnetic field, and the full determination of the bidimensional gyromagnetic tensor of heavy hole. Concerning the control of nuclear spins in an optically pumped QD, we have demonstrated, in InAs/GaAs strained QDs, the control by an external electric field of the electron-nuclear-hyperfine-interaction correlation time in single quantum dots, which manifests through the modification or even the disappearance of the nuclear spin polarisation bistability loops, when varying the external magnetic field, both in Faraday and in Hanle configurations. In the latter case, a strong nuclear field component builds-up, which is orthogonal to the injected electron spin under optimal bias field, reaching values as high as 1T.

2°) Semiconductor entangled light sources.
Highly symmetrical (111)A GaAs/AlGas quantum dots, which present vanishingly small electron-hole exchange splitting between bright exciton states, proved quite efficient polarisation-entangled two photon sources : they allow us to demonstrate the violation of Bell's inequalities whithout any spectral nor temporal post-selection, as is usually done. These highly symmetrical dots, which present vanishingly small electron-hole exchange splitting between bright exciton states, proved quite efficient polarisation-entangled sources : they allowed us to demonstrate the violation of Bell's inequalities without any spectral nor temporal post-selection as usually done
Entangled-light-emitting diodes (ELEDs) were developed with extended photon coherence times, allowing indistinguishable entangled photon pairs to be produced. By setting the polarisation of a photon emitted by our device, and interfering it with one photon of an entangled pair emitted at a later time, we were able to prove, for the first time, quantum teleportation of a polarisation-encoded qubit using an electrically driven entangled light source. By generating input photons with a laser instead of the ELED device, we went on to show quantum teleportation with even higher fidelity, using linear optics and dissimilar light sources. Interfacing between qubits encoded on photons with different optical properties will be important for quantum networks and distributed quantum computing. Finally, we employed our ELED in a quantum key distribution system and successfully shared quantum keys using electrically generated entangled photons.

3°) Interaction of free and localised spins in diluted magnetic semiconductors and hybrid structures.
Interaction of free carrier spins and localized Mn-spins resulting in giant magneto-optical effects were investigated in nanostructures based on diluted magnetic semiconductors (Cd,Mn)Te and (Zn,Mn)Se. Optical initialization, read out and relaxation dynamics of a Mn spin in a single quantum dot was demonstrated. Spin diffusion in Mn spin system was measured experimentally and described theoretically for heteromagnetic nanostructures with various profiles of Mn distribution. A new class of hybrid spinoptronic heterostructures was proposed and fabricated. These structures combine III-V and II-VI semiconductors, both non-magnetic and diluted magnetic. A perfect heterovalent interface between them allows full optical control over tunneling and localization of carriers with spin conservation, which is confirmed experimentally. We also developed a theory of semiconductor quantum rings embedded in a magnetic material matrix. The ring-like geometry allows fine tuning of electronic spectra and optical selection rules by experimentally-attainable electric fields. When this system is embedded in a microcavity a transition from the strong- to the weak-coupling regime can be achieved by merely changing the angle between the light polarization plane and external electric field.

4°) Spinoptronic devices based on cavity exciton polaritons.
This objective has been achieved beyond any expectations. During the two first years low threshold optical switches based on the multistability of polariton have been demonstrated. The observation and manipulation of vortices in polariton superfluids has been reported. Extended spatial coherence of polariton condensates has been observed. In the second part of the project, coherent propagation of spin polarized flow of exciton-polaritons over macroscopic distances has been observed. The creation of a superfluid spin current has also been reported, and of magnetic monopole analogues able to carry informations with velocities comparable to the speed of light. A large family of polariton circuits with extremely reduced power consumption have been fabricated and studied such as polariton transistors, Double-Barrier Resonant Tunneling Diode, or a polaritonic Mach Zehnder interferometers acting as a polarization switch.

Societal implications
We have offered a wide and fruitful transnational experience to 18 young researchers and their families. These young researchers will be the leaders of tomorrow in the field of research and industry. They certainly became true European citizens, knowing people, collaborators in many different European countries. They are the cement of the future European construction. Also the transnational mixing of culture goes well beyond the 18 ESRs/ERs, because it is also strongly impacting the whole hosting groups, laboratories, and even the administrations which have to take care of the project management in their way of working and in their relation with the European reality. A special mention can be made about the implication for our Russian partner. This project was indeed one of the first big collaborative EU project involving a full Russian partner, with two ESRs attributed to the node. The employment of these two ESRs, their registration as PhD required a real revolution of the all administrative Russian rules and of the mentalities in these academic Russian institutions.
From another point of view, we can notice that several ESRs immediately found a permanent position in industrial European companies (Phillips, Attocube…), so the training received by the young researchers will positively impact the European technologic industry.
From the scientific point of view, our activity is essentially basic research, with only midterm applications. We can however foresee a few promising directions, in which the results achieved by the network could really imply large technological breakthroughs in the important field of communication technologies. For example, the fabrication of hybrid III-V/Si semiconductor devices, or the realization of low threshold polariton integrated circuits, could lead in the future to cheaper lasers for telecommunications and LEDs for lighting, and also to the integration of light emitters with CMOS logic elements, potentially enabling development of the new generation of fast data interconnects and more powerful computers with lower energy consumption. Another example is the realization of an electrically pumped, solid state based Quantum Key Distribution which is an important step for the development of secured quantum communication networks.