Partial Dephasing Research Articles

Balanced Steady-State Free Precession Cine MR Imaging in the Presence of Cardiac Devices: Value of Interleaved Radial Linear Combination Acquisition With Partial Dephasing.

Balanced steady-state free precession (bSSFP) is important in cardiac MRI but suffers from off-resonance artifacts. The interpretation-limiting artifacts in patients with cardiac implants remain an unsolved issue. To develop an interleaved radial linear combination bSSFP (lcSSFP) method with partial dephasing (PD) for improved cardiac cine imaging when implanted cardiovascular devices are present. Prospective. Flow phantom adjacent to a pacemaker and 10 healthy volunteers (mean age ± standard deviation: 31.9 ± 2.9 years, 4 females) with a cardioverter-defibrillator (ICD) positioned extracorporeally at the left chest in the prepectoral region. A 3-T, 1) Cartesian bSSFP, 2) Cartesian gradient echo (GRE), 3) Cartesian lcSSFP, and 4) radial lcSSFP cine sequences. Flow artifacts mitigation using PD was validated with phantom experiments. Undersampled radial lcSSFP with interleaving across phase-cyclings and cardiac phases (RLC-SSFP), combined with PD, was then employed for achieving improved quality of cine images from left ventricular short-axis view. The image quality in the presence of cardiac devices was qualitatively assessed by three independent raters (1=worst, 5=best), regarding five criteria (banding artifacts, streak artifacts, flow artifacts, cavity visibility, and overall image quality). Wilcoxon rank-sum test for the five criteria between Cartesian bSSFP cine and RLC-SSFP with PD. Fleiss kappa test for inter-reader agreement. A P value < 0.05 was considered statistically significant. Based on simulations and phantom experiments, 60 projections per phase cycling and 1/6 PD were chosen. The in vivo experiments demonstrated significantly reduced banding artifacts (4.8 ± 0.4 vs. 2.7 ± 0.7), fewer streak artifacts (3.7 ± 0.6 vs. 2.6 ± 0.7) and flow artifacts (4.4 ± 0.4 vs. 3.7 ± 0.6), therefore improved cavity visibility (4.1 ± 0.4 vs. 2.9 ± 0.9) and overall quality (4.0 ± 0.4 vs. 2.7 ± 0.7). RLC-SSFP method with PD may improve cine image quality in subjects with cardiac devices. 2. Stage 1.

Inversion of coherent backscattering with interacting Bose-Einstein condensates in two-dimensional disorder: A truncated Wigner approach

We theoretically study the propagation of an interacting Bose-Einstein condensate in a two-dimensional disorder potential, following the principle of an atom laser. The constructive interference between time-reversed scattering paths gives rise to coherent backscattering, which may be observed under the form of a sharp cone in the disorder-averaged angular backscattered current. As is found by the numerical integration of the Gross-Pitaevskii equation, this coherent backscattering cone is inversed when a non-vanishing interaction strength is present, indicating a crossover from constructive to destructive interferences. Numerical simulations based on the Truncated Wigner method allow one to go beyond the mean-field approach and show that dephasing renders this signature of antilocalisation hidden behind a structureless and dominant incoherent contribution as the interaction strength is increased and the injected density decreased, in a regime of parameters far away from the mean-field limit. However, despite a partial dephasing, we observe that this weak antilocalisation scenario prevails for finite interaction strengths, opening the way for an experimental observation with $^{87}$Rb atoms.

Cooperative emission spectra as an efficient key probe of qubits pair entanglement along with field state tomography: an effective response to nonlinearity and classical drive power

Cooperative atomic emission spectrum as a key probe of entanglement dynamics of an independent and a classically-pumped qubits pair is showed and investigated. The qubits are initially maximally-entangled while they penetrate a detuned, single-photon and coherent cavity embedded in nonlinear media. The system Hamiltonian; that is a nonlinear and a time-dependent system, is diagonalized and the time evolution unitary operator in an exact form is derived. Various characteristics are monitored while observations are theoretically documented—via tracing the field state tomography—and experimentally calibrated. Cooperation and decoherence as well as the statistics of the emitted radiation are discussed and analyzed in terms of the other phenomena. We show that, a measurement of the spectrum outputs represent a highly effective key calibration for entanglement attributes, where complete time dynamics may be predicted with a high considerable accuracy. The most striking feature realized is that, on high detuned interaction, a full matching of these phenomena outputs can be achieved regardless of the absence or presence or pumping. Once the pumping starts, a partial dephasing is encouraged, thus, the spectrum is inversely but slightly affected, nevertheless, matching remains highly closed. Within various surroundings of Kerr materials of different Kerr constants, that is an additional source of an effective dephasing, spectrum rapidly oscillates and so also entanglement that shows rapid Rabi oscillations during its fast degradation depending on the detuning width. However, on measuring the intracavity field state tomography, measurements confirm such a matching, that is, calibration is well documented.

Mitigation of near-band balanced steady-state free precession through-plane flow artifacts using partial dephasing.

To mitigate artifacts from through-plane flow at the locations of steady-state stopbands in balanced steady-state free precession (SSFP) using partial dephasing. A 60° range in the phase accrual during a TR was created over the voxel by slightly unbalancing the slice-select dephaser. The spectral profiles of SSFP with partial dephasing for various constant flow rates and during pulsatile flow were simulated to determine if partial dephasing decreases through-plane flow artifacts originating near SSFP dark bands while maintaining on-resonant signal. Simulations were then validated in a flow phantom. Lastly, phase-cycled SSFP cardiac cine images were acquired with and without partial dephasing in six subjects. Partial dephasing decreased the strength and non-linearity of the dependence of the signal at the stopbands on the through-plane flow rate. It thus mitigated hyper-enhancement from out-of-slice signal contributions and transient-related artifacts caused by variable flow both in the phantom and in vivo. In six volunteers, partial dephasing noticeably decreased artifacts in all of the phase-cycled cardiac cine datasets. Partial dephasing can mitigate the flow artifacts seen at the stopbands in balanced SSFP while maintaining the sequence's desired signal. By mitigating hyper-enhancement and transient-related artifacts originating from the stopbands, partial dephasing facilitates robust multiple-acquisition phase-cycled SSFP in the heart. Magn Reson Med 79:2944-2953, 2018. © 2017 International Society for Magnetic Resonance in Medicine.

Coherent-state discrimination via nonheralded probabilistic amplification

A scheme for the detection of low-intensity optical coherent signals was studied which uses a probabilistic amplifier operated in the non-heralded version, as the underlying non-linear operation to improve the detection efficiency. This approach allows us to improve the statistics by keeping track of all possible outcomes of the amplification stage (including failures). When compared with an optimized Kennedy receiver, the resulting discrimination success probability we obtain presents a gain up to ~1.85% and it approaches the Helstrom bound appreciably faster than the Dolinar receiver, when employed in an adaptive strategy. We also notice that the advantages obtained can be ultimately associated with the fact that, in the high gain limit, the non-heralded version of the probabilistic amplifier induces a partial dephasing which preserves quantum coherence among low energy eigenvectors while removing it elsewhere. A proposal to realize such transformation based on an optical cavity implementation is presented.

Self-force gravitational waveforms for extreme and intermediate mass ratio inspirals. III. Spin-orbit coupling revisited

The first- and second-order dissipative self force and the first order conservative self force are applied together with spin-orbit coupling to the quasi-circular motion of a test mass in the spacetime of a Schwarzschild black hole, for extreme or intermediate mass ratios. The partial dephasing of the gravitational waveform (at the order that is independent of the system's mass ratio) due to the self force is compared with that of spin-orbit coupling. We find that accurate waveforms for parameter estimation need to include both effects. Specifically, we find a particular value for the spin parameter such that the spin-orbit effect cancels out the self-force effect on the waveform. Exclusion of dephasing effects that are independent of the mass ratio therefore might lead to a non-perturbative error in the estimation of the system's parameters.

Self-force gravitational waveforms for extreme and intermediate mass ratio inspirals. II. Importance of the second-order dissipative effect

We consider the importance of the second-order dissipative self-force for gravitational wave dephasing for an extreme or intermediate mass ratio system moving along a quasicircular Schwarzschild orbit. For the first-order self-force we use the fully relativistic force in the Lorenz gauge for eternally circular geodesics. The second-order self-force is modeled by its 3.5 post-Newtonian counterpart. We evolve the system using the osculating orbits method, and obtain the gravitational waveforms, whose phase includes all the terms---within our approximation (and using the self-force along circular geodesics)---that are independent of the system's mass ratio. The partial dephasing due to the second-order dissipative self-force is substantially smaller than that of the first-order conservative self-force, although they are both at the same order in the mass ratio.

Controlled dephasing of electrons by non-gaussian shot noise

In a 'controlled dephasing' experiment [1-3], an interferometer loses its coherence due to entanglement with a controlled quantum system ('which path' detector). In experiments that were conducted thus far in mesoscopic systems only partial dephasing was achieved. This was due to weak interactions between many detector electrons and the interfering electron, resulting in a Gaussian phase randomizing process [4-10]. Here, we report the opposite extreme: a complete destruction of the interference via strong phase randomization only by a few electrons in the detector. The realization was based on interfering edge channels (in the integer quantum Hall effect regime, filling factor 2) in a Mach-Zehnder electronic interferometer, with an inner edge channel serving as a detector. Unexpectedly, the visibility quenched in a periodic lobe-type form as the detector current increased; namely, it periodically decreased as the detector current, and thus the detector's efficiency, increased. Moreover, the visibility had a V-shape dependence on the partitioning of the detector current, and not the expected dependence on the second moment of the shot noise, T(1-T), with T the partitioning. We ascribe these unexpected features to the strong detector-interferometer coupling, allowing only 1-3 electrons in the detector to fully dephase the interfering electron. Consequently, in this work we explored the non-Gaussian nature of noise [11], namely, the direct effect of the shot noise full counting statistics [12-15].

Decay of Entanglement Due to Pure Dephasing: the Role of Geometry of Entangled States

The study of the effect that pure dephasing has on the entanglement of a pair of two-level systems shows that complete disentanglement may be caused by partial pure dephasing for some initial entangled states. We examine the behaviour of the entanglement of two confined charge states under the influence of partial pure dephasing introduced by a super-Ohmic reservoir (typical for semiconductors). We classify the set of completely dephasible states and provide an explanation of this effect.

Change of Decoherence Scenario and Appearance of Localization due to Reservoir Anharmonicity

Although coupling to a super-Ohmic bosonic reservoir leads only to partial dephasing on short time scales, exponential decay of coherence appears in the Markovian limit (for long times) if anharmonicity of the reservoir is taken into account. This effect not only qualitatively changes the decoherence scenario but also leads to localization processes in which superpositions of spatially separated states dephase with a rate that depends on the distance between the localized states. As an example of the latter process, we study the decay of coherence of an electron state delocalized over two semiconductor quantum dots due to anharmonicity of phonon modes.

Complete disentanglement by partial pure dephasing

We study the effect of pure dephasing on the entanglement of a pair of two-level subsystems (qubits). We show that partial dephasing induced by a super-Ohmic reservoir, corresponding to well-established properties of confined charge states and phonons in semiconductors, may lead to complete disentanglement. We show also that the disentanglement effect increases with growing distance between the two subsystems.

Analysis and compensation of eddy currents in balanced SSFP

Balanced steady-state free precession (SSFP) completely compensates for all gradients within each repetition time (TR), and is thus very sensitive to any magnetic field imperfection that disturbs the perfectly balanced acquisition scheme. It is demonstrated that balanced SSFP is especially sensitive to changing eddy currents that are induced by stepwise changing phase-encoding (PE) gradients. In contrast to the linear k-space trajectory, which has small variations between consecutive encoding steps, other encoding schemes (e.g., centric, random, or segmented orderings) exhibit significant jumps in k-space between adjacent PE steps, and consequently induce rapidly changing eddy currents. The resulting disturbances induce significant image artifacts, such that compensation strategies are essential when nonlinear PE schemes are applied. Although direct annihilation of the induced eddy currents by additional, opposing magnetic fields has been investigated, it is limited by uncertainty regarding the time evolution of induced eddy currents. A generic (and thus system-unrelated) compensation strategy is proposed that consists of "pairing" of consecutive PE steps. Another approach is based on partial dephasing along the slice direction that annihilates eddy-current-induced signal oscillations. Both pairing of the PE steps and "through-slice equilibration" are easy to implement and allow the use of arbitrary k-space trajectories for balanced SSFP.

Selective observation of nitrogen-bonded carbons in solid-state NMR by saturation-pulse induced dipolar exchange with recoupling

A solid-state NMR experiment for identifying nitrogen-bonded carbons with good efficiency at standard magic-angle-spinning (MAS) frequencies is presented. In the center of a sequence of 180° pulses recoupling the 14 N – 13 C dipolar interaction, the 14 N coherences are saturated by a train of small-angle 14 N pulses for 1–2 ms. This saturation-pulse induced dipolar exchange with recoupling (SPIDER) provides efficient partial dephasing of 13 C bonded to 14 N at 5-kHz MAS and low radio-frequency strengths where REAPDOR and TRAPDOR are inefficient. SPIDER is demonstrated on an amide group with a 14 N quadrupole-coupling constant of −3.1 MHz, on a benzamide, and on chitin.

Dephasing in semiconductor–superconductor structures by coupling to a voltage probe

We study dephasing in semiconductor–superconductor structures caused by coupling to a voltage probe. We consider structures where the semiconductor consists of two scattering regions between which partial dephasing is possible. As a particular example we consider a situation with a double barrier junction in the normal region. For a single-mode system we study the conductance both as a function of the position of the Fermi level and as a function of the barrier transparency. At resonance, where the double barrier is fully transparent, we study the suppression of the ideal factor-of-two enhancement of the conductance when a finite coupling to the voltage probe is taken into account.

Neutrino oscillations as two-slit experiments in momentum space

The passage of a neutrino from source to detector is described as a multipath experiment where knowing the path destroys coherence. The concept of partial coherence and partial dephasing is developed in a toy model for recent mesoscopic `which-path' experiments and shown to require considering the beam and the detector as a correlated quantum system. The application to Bragg scattering by X-rays gives the Debye–Waller factor. Neutrino amplitudes with the same energy and different masses are shown to be detected coherently and can produce oscillations at a detector localized for long times in space. Amplitudes with different energies are incoherent. Quantum mechanics alone shows the existence of a neutrino mass difference. to be required to explain the observed Super-Kamiokande data.

Spectroscopy from Proton Spin Magnetization Evolution in Their Rotating Frame: A Study of Small Tunneling Splitting

The time evolution of proton Zeeman magnetization in the rotating frame at exact resonance, ω = ω0, is evaluated for an isolated tunneling methyl group CH3. The Fourier transform of this evolution in time is calculated and both its real and imaginary components are presented. It is shown that the real component does not depend significantly on the strength of the preparation pulse when the tunneling splitting of the methyl rotator ground state is less than 100 kHz. It is also found that the imaginary component of the transform is inversely proportional to the strength of the preparation RF pulse. This is a consequence of the partial dephasing of proton spins during the preparation pulse. The results of the calculation compare well with the experimental spectra of CH3CD2I.

Two interacting particles in a disordered chain III: Dynamical aspects of the interplay disorder-interaction

The interplay between the quantum interferences responsible for one particle localization over a length L_1, and the partial dephasing induced by a local interaction of strength U with another particle leading to partial delocalization over a length L_2 > L_1, is illustrated by a study of the motion of two particles put close to each other at the initial time. Localization is reached in two steps. First, before the time t_1 necessary to propagate over L_1, the interaction slows down the ballistic motion. On the contrary, after t_1 the interaction favors a very slow delocalization, characterized by a $\log(t)$ spreading of the center of mass, until L_2 is reached. This slow motion is related to the absence of quantum chaos in this one dimensional model, the interaction being only able to induce weaker chaos with critical spectral statistics. Under appropriate initial conditions, the motion remains invariant under the duality transformation mapping the behavior at small U onto the behavior at large U.

Partial dephasing in interacting many-particle systems and current echo

Current echos as a response to a sequence of two ultrashort voltage pulses and their delay dependence are studied for interacting particles. The echo amplitude is reduced compared to its noninteracting counterpart, and decays even without an external bath coupling. However, it ends up with a nonzero value in the large-delay limit. This partial dephasing can be traced back to correlated energy differences in the many-body system. It is expected for all echo phenomena in small isolated systems. The time-dependent Hartree-Fock approximation fails to describe the current echo's dephasing even qualitatively.