Vide quantique

Résumé

En physique, le vide quantique décrit l'état du vide selon les principes de la mécanique quantique. Alors que l'on croyait l'univers rempli d'éther, la physique du a abandonné cette notion pour un espace littéralement vide de matière. Les principes quantiques montrent que ce vide est en réalité rempli d'énergie qui engendre de nombreux effets : on parle alors d'énergie du vide. Description Dans la théorie de l'électrodynamique quantique, les particules élémentaires échangent des photons virtuels pour interagir. Ces particules virtuelles utilisent l'énergie du vide pour apparaître, agir puis disparaître, dans les limites du principe d'incertitude. Inégalité d'Heisenberg Les inégalités d'Heisenberg, connues sous le nom de principe d'incertitude, sont une conséquence directe de la dualité onde-corpuscule. L'une d'elles, controversée, permet d'écrire : \Delta E\cdot\Delta t\geq\hbar /2, où \hbar est la constante de Planck réduite. Cette i

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With the development of quantum optics, photon correlations acquired a prominent role as a tool to test our understanding of physics, and played a key role in verifying the validity of quantum mechanics. The spatial and temporal correlations in a light field also reveal information about its origin, and allow us to probe the nature of the physical systems interacting with it. Additionally, with the advent of quantum technologies, they have acquired technological relevance, as they are expected to play an important role in quantum communication and quantum information processing.This thesis develops techniques that combine spontaneous Raman scattering with Time Correlated Single Photon Counting, and uses them to study the quantum mechanical nature of high frequency vibrations in crystals and molecules. We demonstrate photon bunching in the Stokes and anti-Stokes fields scattered from two ultrafast laser pulses, and use their cross-correlation to measure the 3.9 ps decay time of the optical phonon in diamond. We then employ this method to measure molecular vibrations in CS2, where we are able to excite the respective vibrational modes of the two isotopic species present in the sample in a coherent superposition, and observe quantum beating between the two signals. Stokes scattering, when combined with a projective measurement, leads to a well defined quantum state. We demonstrate this by measuring the second order correlation function of the anti-Stokes field conditional on detecting one or more photons in the Stokes field, which allows us to observe a phonon modeâs transition form a thermal state into the first excited Fock state, and measure its decay over the characteristic phonon lifetime. Finally, we use this technique to prepare a highly entangled photon-phonon state, which violates a Bell-type inequality. We measure S = 2.360 Â± 0.025, violating the CHSH inequality, compatible with the non-locality of the state.The techniques we developed open the door to the study of a broad range of physical systems, where spectroscopic information is obtained with the preparation of specific quantum states. They also hold potential for future technological use, and promote vibrational Raman scattering to a resource in nonlinear quantum optics -- where it used to be considered as a source of noise instead.

EPFL2021

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Optical-Microwave Pump-Probe Studies of Electronic Properties in Novel Materials

András Bojtor, Karoly Holczer, Sándor Kollarics, Bence Gábor Márkus, Ferenc Simon

Combined microwave-optical pump-probe methods are emerging to study the quantum state of spin qubit centers and the charge dynamics in semiconductors. A major hindrance is the limited bandwidth of microwave irradiation/detection circuitry which can be overcome with the use of broadband coplanar waveguides (CPWs). The development and performance characterization of two spectrometers is presented as follows: an optically detected magnetic resonance spectrometer (ODMR) and a microwave-detected photoconductivity measurement. In the first method, light serves as detection and microwaves excite the investigated medium, whereas in the second, the roles are interchanged. The performance is demonstrated by measuring ODMR maps on the nitrogen-vacancy (NV) center in diamond and time-resolved photoconductivity in p-doped silicon. The results demonstrate both an efficient coupling of the microwave irradiation to the samples as well as an excellent sensitivity for minute changes in sample conductivity.

WILEY-V C H VERLAG GMBH2020

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Quiescent H-mode (QH-mode) operation is highly desirable relative to the well known H- mode operation because it allows for high energy density in the core as well as potentially im- proved confinement of energy and particles. A feature of QH-mode is that the Edge Localized Mode (ELM) instabilities are replaced with continuous Edge Harmonic Oscillations (EHO) [1]. Control and reproduction of robust QH-mode regimes could be crucial for future fusion opera- tion. In some cases it may be useful to modify the main characteristics of QH-mode discharges while keeping constant the key elements needed to achieve QH-mode (such as applied torque, edge current density, pedestal pressure, etc.). With that in mind, the effect of global toroidal mode seeding in the vacuum magnetic field on the amplitude of EHO is investigated. Previ- ous numerical simulations have shown that free boundary equilibrium calculations are able to recover non-linear saturated 3D equilibria with edge corrugations associated with EHO in QH- mode [2, 3]. Here we use the VMEC free boundary code to do a proof of principle investigation of the coupling between EHO and global toroidal mode seeding. This is done by first producing a QH-mode plasma using the vacuum magnetic field modeled by JET-like toroidal and poloidal field coils. Then, a global toroidal mode n=1,2 is seeded numerically by perturbing the vacuum field using a set of non-axisymmetric external coils associated with the Error Field Correction Coils (EFCC’s) in JET and the plasma response is studied. Due to the EFCC’s geometry, it is crucial to account for up-down asymmetry in the VMEC code. Spectral decomposition of the 3D plasma displacement with respect to the equivalent 2D axisymmetric equilibrium is per- formed and compared with linear numerical simulations using the KINX code. A study that effectively isolates the coupling of global n=1,2 toroidal modes with current driven and pres- sure driven EHO is presented, and the impact of external coils and associated plasma response clearly determined.

European Physical Society2019

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European Physical Society2019

EPFL2021