Irreversible Dephasing Research Articles

Towards detection of inflammation in adipose tissue: Microscopic field simulations to estimate water signal properties

Inflammation of adipose tissue, particularly visceral adipose tissue, is assumed to be a causal factor for the development of type 2 diabetes, non-alcoholic fatty liver disease, and cardiovascular diseases. Invasive biopsy is currently mandatory for assessment and grading of adipose tissue inflammation. Magnetic resonance detection of the increased water content of inflamed adipose tissue is considered to be a non-invasive alternative. Additional water is mainly originating from macrophages clustering in small regions between adipocytes. This article addresses the characteristics of water signals from areas between adipocytes in terms of line width, line shape, and relaxation properties. Since water and lipids inside adipose tissue have different magnetic susceptibilities, microscopic field inhomogeneities arise depending on the geometry and orientation of the water containing confinements. Relatively pronounced microscopic field inhomogeneities in the water compartments cause a broad spectral distribution of water signals. As a consequence the water signal of adipose tissue shows special characteristics different to common parenchyma tissues, in which cell content and intercellular space consist of water. The broad and non-Lorentzian field distribution of signals emanating from the water compartments in adipose tissue results in a fast non-exponential signal decay. Therefore, short echo times are recommended for sensitive gradient-echo based imaging. A non-exponential irregular signal decay potentially leads to problems in fat/water separation using Dixon techniques. Marked microscopic field inhomogeneities in combination with diffusion related displacement of water molecules cause irreversible dephasing and therefore accelerated signal decay even for spin-echo sequences. A volume localized spectrum of porcine fat recorded at 3T by a STEAM sequence with an echo time of 5.4ms shows a broad water signal with a line width of 70Hz±4Hz, in contrast to the CH2-peak of lipids in the same spectrum with a line width of only 14.7Hz±0.7Hz. This finding is qualitatively consistent with the results of finite element modelling of the magnetic field in geometric models and experiments in phantoms with oil-filled balloons surrounded by water.

Coherence properties of nanofiber-trapped cesium atoms.

We experimentally study the ground state coherence properties of cesium atoms in a nanofiber-based two-color dipole trap, localized ∼ 200 nm away from the fiber surface. Using microwave radiation to coherently drive the clock transition, we record Ramsey fringes as well as spin echo signals and infer a reversible dephasing time of T(2)(*) = 0.6 ms and an irreversible dephasing time of T(2)(') = 3.7 ms. By modeling the signals, we find that, for our experimental parameters, T(2)(*) and T(2)(') are limited by the finite initial temperature of the atomic ensemble and the heating rate, respectively. Our results represent a fundamental step towards establishing nanofiber-based traps for cold atoms as a building block in an optical fiber quantum network.

Improved Interpretation of Nuclear Magnetic Resonance T1 and T2 Distributions for Permeability Prediction: Simulation of Diffusion Coupling for a Fractal Cluster of Pores

NMR relaxometry is a powerful tool for inferring porosity and permeability data. In practice, measured magnetization decay curves are inverted for relaxation time distributions. Subsequently, one presumes a linear relationship between the pore radius distribution and the T1 and T2 distribution, for longitudinal and transverse magnetization, respectively. The fundamental equations used are based on a pore model, in which pores are assumed to be isolated from each other with respect to the NMR process and have smooth walls. The present study is based on a geometrical pore space model with connected pores and structured pore walls. The physical processes of surface relaxation, irreversible dephasing of magnetic spins and diffusive proton exchange between pores, are described by a system of differential equations. The solution yields a set of exponential functions representing the relaxation time distribution. We describe the difference between the distributions obtained for diffusion coupling and for isolated pores. With diffusion coupling on, the spectral width of the T1 distribution is strongly reduced, which indicates that the influence of large and small radii according to the T1-pore radius relationship is mixed to some extent. For a fractal pore space structure, where large pores are surrounded by adjacent minor pores, the T1 distribution does not resolve these substructures. Nevertheless, permeability values calculated from the logarithmic mean relaxation time T1,LM are quite the same for diffusion coupling and for isolated pores. The T2 distribution for diffusion coupling is little constricted and gives a better resolution of the pore wall structures than the corresponding T1 distribution. The permeability values from T2 distributions agree with the values from longitudinal magnetization, provided that we use a corrected relaxation time T2,corr, accounting for the dependence of the surface relaxivity ρ2 on pore radius. The study shows that radius distributions calculated from a T1 and from a T2 distribution differ from one another and both present an altered image of the true pore radius distribution. In practice, this has no serious influence on estimating permeability of medium- to high-permeability sandstones with the currently applied methods. The presented methodology of calculating the NMR response of pore space models with diffusion coupling may facilitate understanding porosity-permeability relationships of different rock types such as carbonate rocks with micro-porosity.

Extracting irreversible dephasing rates from electric dipole echoes in Rydberg Stark wave packets

The precession of an electric dipole moment in an external electric field can be reversed by reversing the field direction. This time-reversal operation allows study of coherence in the time evolution of Rydberg Stark wave packets by measuring the resulting echoes. Different sources of reversible dephasing and irreversible dephasing, i.e., decoherence, are discussed in detail. Stochastic interactions with the environment are simulated in a controlled manner by using artificially synthesized noise. The rate of irreversible dephasing is determined from the reduction of the echo amplitude under multiple field reversals.

Electric dipole echoes and noise-induced decoherence

The generation of echoes in the electric dipole moment of a Rydberg wavepacket precessing in an external electric field by reversal of the field is described. When the wavepacket experiences reversible dephasing, large echoes are observed pointing to strong refocusing of the wavepacket. The presence of irreversible dephasing leads to a reduction in the size of the echoes. The effect of irreversible dynamics on echoes is investigated using artificially synthesized noise. Methods to determine the decoherence rate are discussed.

Fast quantum state control of a single trapped neutral atom

We demonstrate the initialization, readout, and high-speed manipulation of a qubit stored in a single $^{87}\mathrm{Rb}$ atom trapped in a submicrometer-sized optical tweezer. Single-qubit rotations are performed on a time scale below $100\phantom{\rule{0.3em}{0ex}}\mathrm{ns}$ using two-photon Raman transitions. Using the spin-echo technique, we measure an irreversible dephasing time of $34\phantom{\rule{0.3em}{0ex}}\mathrm{ms}$. The readout of the single atom qubit is at the quantum projection noise limit when averaging up to 1000 individual events.

Dephasing of Stark wave packets induced by colored noise

The time evolution of Stark wave packets in Rydberg atoms under the influence of colored noise is analyzed. The Stark wave packets are generated by exposing $K(350p)$ Rydberg atoms to an electric field step. Their time evolution is monitored using a half-cycle probe pulse that is applied after a variable time delay. The noise is generated by randomly modulating the amplitude of the static field. We show that noise results in an enhanced and irreversible dephasing of Stark quantum beats, as monitored by the damping of the beat amplitude. This source of decoherence is most effective when the characteristic frequency of the noise power spectrum matches the orbital frequency of the Rydberg electron. Noise driven resonant $\ensuremath{\Delta}n=1$ transitions broaden the energy distribution thereby accelerating the dephasing.

Collisional decoherence in very-high- n Rydberg atoms

Collisional interactions represent a major source of decoherence for a gas of Rydberg atoms. We investigate the irreversible dephasing of coherently excited Rydberg wavepackets due to decoherence in collisions with ambient rare gas atoms. We show that the rate of decoherence provides a sensitive tool for measuring quasi-elastic electron-atom (or molecule) collisions at energies extending down to micro electron volts. We present proof of principle calculations for n ≃ 388 potassium Rydberg atoms in an ambient gas of xenon atoms.

Coherent population transfer in molecules coupled with a dissipative environment by intense ultrashort chirped pulse. II. A simple model

We have developed a simple and physically clear picture of adiabatic rapid passage (ARP) in molecules in solution by careful examination of all the conditions needed for ARP. The relaxation effects were considered in the framework of the Landau-Zener model for random crossing of levels. The model enables us to include into consideration non-Markovian Gaussian-correlated noise. It explains all the numerical results obtained in the first paper of the series [B. D. Fainberg and V. A. Gorbunov, J. Chem. Phys. 117, 7222 (2002)], in particular, that for positive chirp pulse excitation relaxation favors more efficient population transfer with respect to the relaxation-free system with frozen nuclear motion. We also relate parameters of non-Markovian Gaussian-correlated noise with irreversible dephasing time of an optical transition by calculating the photon echo signal attenuation.

Coherence properties and quantum state transportation in an optical conveyor belt.

We have prepared and detected quantum coherences of trapped cesium atoms with long dephasing times. Controlled transport by an "optical conveyor belt" over macroscopic distances preserves the atomic coherence with slight reduction of coherence time. The limiting dephasing effects are experimentally identified, and we present an analytical model of the reversible and irreversible dephasing mechanisms. Our experimental methods are applicable at the single-atom level. Coherent quantum bit operations along with quantum state transport open the route towards a "quantum shift register" of individual neutral atoms.

On the entropy and the density matrix of cosmological perturbations

We look at the transition to semiclassical behaviour and the decoherence process for the inhomogeneous perturbations in the inflationary universe. Two different decoherence mechanisms appear: one dynamical, accompanied with a negligible, if present at all, entropy gain, and the other, effectively irreversible dephasing, due to a rapid variation in time of the off-diagonal density matrix elements in the post-inflationary epoch. We thus settle the discrepancies in the entropy content of perturbations evaluated by different authors.