Coulomb-driven organization and enhancement of spin-orbit fields in collective spin excitations

Spin-orbit (SO) fields in a spin-polarized electron gas are studied by angle-resolved inelastic light scattering on a CdMnTe quantum well. We demonstrate a striking organization and enhancement of SO fields acting on the collective spin excitation (spin-flip wave). While individual electronic SO fields have a broadly distributed momentum dependence, giving rise to D'yakonov-Perel' dephasing, the collective spin dynamics is governed by a single collective SO field which is drastically enhanced due to many-body effects. The enhancement factor is experimentally determined. These results provide a powerful indication that these constructive phenomena are universal to collective spin excitations of conducting systems.Comment: 5 pages, 4 figure

Giant Collective Spin-Orbit Field in a Quantum Well: Fine Structure of Spin Plasmons

We employ inelastic light scattering with magnetic fields to study intersubband spin plasmons in a quantum well. We demonstrate the existence of a giant collective spin-orbit (SO) field that splits the spin-plasmon spectrum into a triplet. The effect is remarkable as each individual electron would be expected to precess in its own momentum-dependent SO field, leading to D'yakonov-Perel' dephasing. Instead, many-body effects lead to a striking organization of the SO fields at the collective level. The macroscopic spin moment is quantized by a uniform collective SO field, five times higher than the individual SO field. We provide a momentum-space cartography of this field.Comment: 5 pages, 4 figures. Supplemental material available here as an ancillary fil

Fundamental limitations of time measurement precision in Hong-Ou-Mandel interferometry

In quantum mechanics, the precision achieved in parameter estimation using a quantum state as a probe is determined by the measurement strategy employed. The ultimate quantum limit of precision is bounded by a value set by the state and its dynamics. Theoretical results have revealed that in interference measurements with two possible outcomes, this limit can be reached under ideal conditions of perfect visibility and zero losses. However, in practice, this cannot be achieved, so precision {\it never} reaches the quantum limit. But how do experimental setups approach precision limits under realistic circumstances? In this work we provide a general model for precision limits in two-photon Hong-Ou-Mandel interferometry for non-perfect visibility. We show that the scaling of precision with visibility depends on the effective area in time-frequency phase space occupied by the state used as a probe, and we find that an optimal scaling exists. We demonstrate our results experimentally for different states in a set-up where the visibility can be controlled and reaches up to $99.5\%$. In the optimal scenario, a ratio of $0.97$ is observed between the experimental precision and the quantum limit, establishing a new benchmark in the field

Effets spin-orbite géants sur les modes collectifs de spin de puits quantiques

We have studied the effects of spin-orbit coupling in doped semiconductor quantum wells (GaAs and CdMnTe) with electronic Raman scattering. In these structures exist intrinsic magnetic fields (Dresselhaus and Rashba). These fields offer attractive means to manipulate the electron spin, but contribute also to spin relaxation, through their dependence on the electronic wavevector (D'yakonov-Perel' mechanism). We show that for the collective spin modes of quantum wells, the destructive D'yakonov-Perel' scenario is transformed into a constructive scenario: Coulombic interactions lead to the emergence of a collective spin-orbit field, proportional to the excitation wavector, and several times enhanced with respect to the single-particle spin-orbit fields. We first demonstrate these giant spin-orbit effects on the intersubband spin plasmon, in GaAs quantum wells. The collective spin-orbit field, which produces a fine structure splitting of the plasmon spectrum, is superposed to an external magnetic field and mapped in momentum space. Then, we study the intrasubband spin wave of the spin-polarized electron gas, in diluted magnetic quantum wells of CdMnTe. Here the collective spin-orbit field adds to the giant Zeeman field of the compound. We measure the enhancement factor of the collective spin-orbit field. Finally, we determine the dependence of the enhancement factor on the electronic density, and demonstrate the ability to control the amplitude of the collective spin-orbit field through above-barrier illumination.Cette thèse est consacrée à l'étude des effets du couplage spin-orbite dans des puits quantiques semi-conducteurs dopés (GaAs et CdMnTe), par spectroscopie Raman électronique. Dans ces structures existent des champs magnétiques intrinsèques (Dresselhaus et Rashba). Ces champs offrent des moyens attractifs pour manipuler le spin des électrons, mais contribuent aussi à la relaxation de spin via leur dépendance avec le vecteur d'onde de l'électron (mécanisme D'yakonov-Perel'). Nous montrons que pour les modes collectifs de spin de puits quantiques, le scénario destructif D'yakonov-Perel' est transformé en un scénario constructif : les interactions Coulombiennes font émerger un champ spin-orbite collectif, proportionnel au vecteur d'onde de l'excitation, et renforcé d'un facteur de plusieurs unités par rapport aux champs spin-orbite individuels. Nous mettons d'abord en évidence ces effets spin-orbite géants sur le plasmon de spin inter-sous-bande, dans des puits quantiques de GaAs. Le champ spin-orbite collectif, qui conduit à un éclatement de structure fine du spectre plasmon, est superposé à un champ magnétique extérieur et cartographié dans l'espace réciproque. Nous étudions ensuite l'onde de spin intra-sous-bande du gaz d'électrons polarisé en spin, dans des puits quantiques magnétiques dilués de CdMnTe. Le champ spin-orbite collectif se superpose ici au champ Zeeman géant du composé. Nous mesurons le facteur de renforcement du champ spin-orbite collectif. Enfin, nous déterminons la dépendance du facteur de renforcement avec la densité électronique, et démontrons la possibilité de contrôler l'amplitude du champ spin-orbite collectif à l'aide d'une grille optique

Les sources intégrées de photons intriqués au coeur des technologies quantiques

L’intrication est un des aspects les plus fascinants et contre-intuitifs des systèmes quantiques, devenue une ressource clé dans tous les domaines de l’information quantique, des communications à la métrologie, des simulations au calcul. La photonique quantique intégrée est en train de jouer un rôle moteur pour le développement de sources de photons intriqués de plus en plus performantes et leur insertion dans des circuits quantiques complexes

Electron density magnification of the collective spin-orbit field in quantum wells

International audienceThe spin-orbit field acting on the spin waves of a spin-polarized electron gas is studied by in-elastic light scattering on a CdMnTe quantum well. Above-barrier illumination allows us to vary the electronic density and control the collective Rashba and Dresselhaus coupling constants. We demonstrate that the enhancement between the single-particle and the collective spin-orbit field increases with increasing electronic density. This result is reproduced by a first-principles calculation. This behavior, which is opposite to usual Coulombic spin enhancements, reveals a novel aspect of the interplay of spin-orbit and Coulomb interactions in collective spin modes

The Hong-Ou-Mandel experiment: from photon indistinguishability to continuous variables quantum computing

We extensively discuss the Hong-Ou-Mandel experiment taking an original phase-space-based perspective. For this, we analyze time and frequency variables as quantum continuous variables in perfect analogy with position and momentum of massive particles or with the electromagnetic field's quadratures. We discuss how this experiment can be used to directly measure the time-frequency Wigner function and implement logical gates in these variables. We also briefly discuss the quantum/classical aspects of this experiment providing a general expression for intensity correlations that explicit the differences between a classical Hong-Ou-Mandel like dip and a quantum one. Throughout the manuscript, we will often focus and refer to a particular system based on AlGaAs waveguides emitting photon pairs via spontaneous parametric down-conversion, but our results can be extended to other analogous experimental systems and to different degrees of freedom

The Hong-Ou-Mandel experiment: from photon indistinguishability to continuous variables quantum computing

We extensively discuss the Hong-Ou-Mandel experiment taking an original phase-space-based perspective. For this, we analyze time and frequency variables as quantum continuous variables in perfect analogy with position and momentum of massive particles or with the electromagnetic field's quadratures. We discuss how this experiment can be used to directly measure the time-frequency Wigner function and implement logical gates in these variables. We also briefly discuss the quantum/classical aspects of this experiment providing a general expression for intensity correlations that explicit the differences between a classical Hong-Ou-Mandel like dip and a quantum one. Throughout the manuscript, we will often focus and refer to a particular system based on AlGaAs waveguides emitting photon pairs via spontaneous parametric down-conversion, but our results can be extended to other analogous experimental systems and to different degrees of freedom

Reconstructing the full modal structure of photonic states by stimulated emission tomography

Stimulated emission tomography is a powerful and successful technique to both improve the resolution and experimentally simplify the task of determining the modal properties of biphotons. In the present manuscript we provide a theoretical description of SET valid for any quadratic coupling regime between a non-linear medium and pump fields generating photons by pairs. We use our results to obtain not only information about the associated modal function modulus but also its phase, for any mode, and we discuss the specific case of time-frequency variables as well as the quantities and limitations involved in the measurement resolution