Quantum vacuum state | EPFL Graph Search

In quantum field theory, the quantum vacuum state (also called the quantum vacuum or vacuum state) is the quantum state with the lowest possible energy. Generally, it contains no physical particles. The term zero-point field is sometimes used as a synonym for the vacuum state of a quantized field which is completely individual. According to present-day understanding of what is called the vacuum state or the quantum vacuum, it is "by no means a simple empty space". According to quantum mechanics, the vacuum state is not truly empty but instead contains fleeting electromagnetic waves and particles that pop into and out of the quantum field. The QED vacuum of quantum electrodynamics (or QED) was the first vacuum of quantum field theory to be developed. QED originated in the 1930s, and in the late 1940s and early 1950s it was reformulated by Feynman, Tomonaga, and Schwinger, who jointly received the Nobel prize for this work in 1965. Today the electromagnetic interactions and the weak

Non-classical photon-phonon correlations at room temperature

Santiago Tarrago Velez

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

Modelling of non-linear edge harmonic oscillations and the effect of non-axisymmetric magnetic coils

Daniele Brunetti, Guillermo Bustos Ramirez, Wilfred Anthony Cooper, Jonathan Graves, Andreas Kleiner

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