arXiv subjects

G. Tastevin

Publications and source records attributed to G. Tastevin.

6 recordsLinked to original sources

arXiv2025

The outcomes of measurements in the de Broglie-Bohm theory-II

Part I of this article discussed the quantum measurement process within the de Broglie-Bohm theory. In the experiment considered, the outcome of the measurements was primarily determined by the initial Bohmian positions within the measurement apparatus. This is nevertheless not always the case. Part II is a short addendum with a more general discussion: cases where the measurement results reveal the values of the positions attached to the measured system, the measurement apparatus, or both, are considered.

quant-ph↗

arXiv2021

The outcomes of measurements in the de Broglie-Bohm theory

Within the de Broglie-Bohm (dBB) theory, the measurement process and the determination of its outcome are usually discussed in terms of the effect of the Bohmian positions of the measured system S. %} This article shows that the Bohmian positions associated with the measurement apparatus M can actually play a crucial role in the determination of the result of measurement. Indeed, in many cases, the result is practically independent of the initial value of a Bohmian position associated with S, and determined only by those of M. The measurement then does not reveal the value of any pre-existing variable attached to S, but just the initial state of the measurement apparatus. Quantum contextuality then appears with particular clarity as a consequence of the dBB dynamics for entangled systems.

quant-ph↗

arXiv2018

Nuclear hyperpolarization of 3He by magnetized plasmas

We describe a method to hyperpolarize 3He nuclear spins at high magnetic fields (4.7 Tesla) solely by a magnetized plasma. The conditions for such a magnetized plasma are fulfilled when the mean free path of the free electrons is much larger than their gyration radius in the rf gas discharge. Investigations are carried out in the 1-15 mbar pressure range with rf excitation at ~100 MHz. Quantitative NMR measurements show that for different cell sizes and 3He densities nuclear polarizations in the range 1% to 9% are observed. We explain this phenomenon by an alignment-to-orientation conversion mechanism in the excited 2 3P state of 3He which is most efficient when the Zeeman and the spin-orbit energies are comparable. The method appears as a very attractive alternative to established laser polarization techniques (spin exchange or metastability exchange optical pumping). Application to 3He nuclear magnetometry with a relative precision of 10-12 is demonstrated.

physics.atom-ph↗

arXiv2018

Surrealistic Bohmian trajectories do not occur with macroscopic pointers

We discuss whether position measurements in quantum mechanics can be contradictory with Bohmian trajectories, leading to what has been called \textquotedblleft surrealistic trajectories\textquotedblright\ in the literature. Previous work has considered that a single Bohmian position can be ascribed to the pointer. Nevertheless, a correct treatment of a macroscopic pointer requires that many particle positions should be included in the dynamics of the system, and that statistical averages should be made over their random initial values. Using numerical as well as analytical calculations, we show that these surrealistic trajectories exist only if the pointer contains a small number of particles; they completely disappear with macroscopic pointers. With microscopic pointers, non-local effects of quantum entanglement can indeed take place and introduce unexpected trajectories, as in Bell experiments; moreover, the initial values of the Bohmian positions associated with the measurement apparatus may influence the trajectory of the test particle, and determine the result of measurement. Nevertheless, a detailed observation of the trajectories of the particles of the pointer can still reveal the nature of the trajectory of the test particle; nothing looks surrealistic if all trajectories are properly interpreted.

quant-ph↗

arXiv2004

MRI of the lung using hyperpolarized He-3 at very low magnetic field (3 mT)

Optical pumping of He-3 produces large (hyper) nuclear-spin polarizations independent of the magnetic resonance imaging (MRI) field strength. This allows lung MRI to be performed at reduced fields with many associated benefits, such as lower tissue susceptibility gradients and decreased power absorption rates. Here we present results of 2D imaging as well as accurate 1D gas diffusion mapping of the human lung using He-3 at very low field (3 mT). Furthermore, measurements of transverse relaxation in zero applied gradient are shown to accurately track pulmonary oxygen partial pressure, opening the way for novel imaging sequences.

physics.med-ph↗

arXiv2002

Magnetic Field Effects on the 1083 nm Atomic Line of Helium. Optical Pumping of Helium and Optical Polarisation Measurement in High Magnetic Field

The structure of the excited $2^{3}$S and $2^{3}$P triplet states of $^{3}$He and $^{4}$He in an applied magnetic field B is studied using different approximations of the atomic Hamiltonian. All optical transitions (line positions and intensities) of the 1083 nm $2^{3}$S-$2^{3}$P transition are computed as a function of B. The effect of metastability exchange collisions between atoms in the ground state and in the $2^{3}$S metastable state is studied, and rate equations are derived, for the populations these states in the general case of an isotopic mixture in an arbitrary field B. It is shown that the usual spin-temperature description remains valid. A simple optical pumping model based on these rate equations is used to study the B-dependence of the population couplings which result from the exchange collisions. Simple spectroscopy measurements are performed using a single-frequency laser diode on the 1083 nm transition. The accuracy of frequency scans and of measurements of transition intensities is studied. Systematic experimental verifications are made for B=0 to 1.5 T. Optical pumping effects resulting from hyperfine decoupling in high field are observed to be in good agreement with the predictions of the simple model. Based on adequately chosen absorption measurements at 1083 nm, a general optical method to measure the nuclear polarisation of the atoms in the ground state in an arbitrary field is described. It is demonstrated at $B\sim$0.1 T, a field for which the usual optical methods could not operate.

physics.atom-ph↗