Investigation into the dynamics and variations in the magnetic structure of an ensemble of ferromagnetic particles

The nonlinear dynamics of an ensemble of single-domain interacting particles upon excitation by radio-frequency pulses of a magnetic field is investigated. Reorientation of magnetic moments of ensemble particles under the effect of the pulses of the radio-frequency field is observed. The effect of varying the magnetic structure on the frequency dependences of susceptibility is studied. Temporal characteristics of precession in separate particles under the conditions of an orientation transition are considered.     

Dynamical Suppression of Decoherence in Two-Qubit Quantum Memory

In this paper, we have detailedly studied the dynamical suppression of the phase damping for the two-qubit quantum memory of Ising model by the quantum “bang-bang” technique. We find the sequence of periodic radio-frequency pulses repetitively to flip the state of the two-qubit system and quantitatively find that these pulses can be used to effectively suppress the phase damping decoherence of the quantum memory and freeze the system state into its initial state. The general sequence of periodic radio-frequency pulses to suppress the phase damping of multi-qubit of Ising model is also given.     

Pattern pulses: design of arbitrary excitation profiles as a function of pulse amplitude and offset

A novel class of pulses is presented which can be regarded as a generalization of both frequency-selective pulses and B1-selective pulses. The excitation profile of these pulses forms a pre-defined pattern in two dimensions, which are spanned by pulse offset and radio-frequency (RF) amplitude. The presented pulses were designed numerically based on principles of optimal control theory. For simple test patterns, we demonstrate the flexibility of this approach by simulations and experiments. This previously unknown flexibility may trigger novel applications in NMR spectroscopy and imaging. As a first practical application, we demonstrate a direct approach for calibrating RF pulses.     

Control of atomic ionization by two-color few-cycle pulses

We have studied the ionization of Rydberg atoms by few-cycle radio-frequency pulses and used two-color fields to control the ionization dynamics. We show that the number of times that electrons are emitted during a pulse can be limited and that the duration of the electron emission can be shortened. These results, once they are transposed to the optical domain, may inspire new strategies for the production of single attosecond pulses.     

Asymmetry in the strong-field ionization of Rydberg atoms by few-cycle pulses

We present measurements of the electron ejection direction in the ionization of high (n=90) Rydberg states of rubidium subjected to few-cycle radio-frequency (RF) pulses. For weak pulses we find a strong asymmetry for even (cosine) pulses and no asymmetry for odd (sine) pulses. This asymmetry disappears for pulses longer than four RF cycles. For strong pulses, very large asymmetry is found for both sine and cosine pulses that persists up to eight RF cycles and is attributed to initial state depletion effects within a cycle.     

Probeheads and instrumentation for pulse EPR and ENDOR spectroscopy with chirped radio frequency pulses and magnetic field steps

Probeheads and instrumentation for modern X-band pulse EPR and ENDOR experiments with chirped radio-frequency pulses and rapidB ₀-field pulses are described. The resonant frequency, the quality factor and, for the first time, the response of a pulse ENDOR resonator structure to a microwave pulse in the subnanosecond time scale have been calculated. The performance of the probeheads for time-domain chirp ENDOR and electron Zeeman-resolved EPR is demonstrated.     

Using quaternions to design composite pulses for spin-1 NQR

The fictitious spin-1/2 operators are well known to describe the evolution of a pure nuclear quadrupole resonance (NQR) system; particularly, the application of a radio-frequency pulse at one of the NQR transition frequencies is equivalent to a three-dimensional rotation in a space defined by the corresponding fictitious spin-1/2 operators. We demonstrate, theoretically and experimentally, that consecutive noncommuting rotations applied at the same transition frequency are well described by a single rotation given by quaternion parameterization of the rotations in ficitious spin-1/2 operator space. This new route could greatly save computing time and efforts. We extend this approach to design composite pulses that compensate for the effects of the radio-frequency field inhomogeneity for a powder sample of spin-1 nuclei.     

Selective inversion in inhomogeneously broadened powder spectra

It is shown how amplitude-modulated radio-frequency pulses optimized for selective inversion of magnetization can be applied to static powder samples with inhomogeneously broadened spectra. Selective one-dimensional methods can in principle provide information similar to two-dimensional exchange spectroscopy, and they are often more efficient for determining the rates of dynamic processes. However, in systems where transverse magnetization is subject to significant homogeneous decay during the rf pulses, selective one-dimensional methods do not necessarily offer the expected advantages over two-dimensional spectroscopy.     

Carrier phase dependence in the ionization of Rydberg atoms by short radio-frequency pulses: a model system for high order harmonic generation

We report time-resolved electron emission in experiments on ionization of rubidium Rydberg atoms (n=90) by few-cycle radio-frequency (RF) (1-10 MHz) pulses. The electron emission occurs in multiple bursts and strongly depends on the carrier-envelope phase as well as the duration and amplitude of the RF pulses. Remarkably, ionization is observed during a series of cycles with the same amplitude. Even at the low RF frequencies, ionization is not completed in a single cycle. Remixing of the states at the zero crossing of the field is believed to play an essential role. Similarities with the ionization process leading to high order harmonic generation are discussed.     

Radio-frequency superradiance at the rheological explosion of a paramagnetic polymer composite containing manganese complexes

It has been revealed that the rheological explosion of a paramagnetic composite (manganese (III) polystyrene acetylacetonate—spatially complicated phenol) is accompanied by the generation of radio-frequency superradiance owing to the annihilation of triplet manganese complexes, as well as by the generation of current pulses.     

Rectangular pulse formation in a laser harmonic generation

In a harmonic generation process the temporal profile of the up-converted pulse undergoes significative changes depending on the input profile and crystal length. A simple theoretical treatment and the corresponding physical view are presented. The matter and the properties of two shaping systems are investigated in view of producing rectangular up-converted pulses, as required by laser driven radio-frequency electron sources. PACS 42.65.Re; 42.65.k; 42.79.Hp     

Surface coil depth measurement from steady-state free induction decays of partially relaxed spins

We propose improved protocols for depth measurement of underground liquids with a planar coil on the ground surface. The existing Earth’s field nuclear magnetic resonance (NMR) prospecting method uses radio-frequency pulses to accumulate free induction decays from fully recovered magnetization as a function of radio-frequency power per pulse, that is, the pulse moment. Our proposal is to accumulate free induction decays from partially relaxed spins, resulting in enhanced signal-to-noise ratio per unit time of measurement. First, we consider the consequences of simply shortening the interval between pulses in the traditional method. The maximum signal-to-noise ratio per unit time increases and, in addition, is reached at a smaller value of pulse moment to shorten the sequence. We also consider an alternative sequence wherein the pulse moment is kept fixed, while the pulse intervals are varied. Both methods require an increase in the average transmitter power but neither method requires any other modification in the basic NMR hardware. A time saving by a factor of at least 5 is possible with less than an order of magnitude increase in average power required. An added benefit of these approaches is its potential to measureT ₁ at the same time as the depth.     

Pulse-to-pulse coherent beat note generated by a passively Q-switched two-frequency laser

We demonstrate experimentally the pulse-to-pulse coherence of the beat note produced by a dual-polarization passively Q-switched Nd:YAG laser subjected to a frequency-shifted, polarization-rotated, optical feedback. The reinjection of one laser eigenstate into the other eigenstate ensures the phase-locking of the beat note against an external acoustic reference wave at the onset of each pulse, circumventing the intrinsic memory loss of the optical phase between successive pulses. It opens the possibility to generate optically a coherent pulsed beat note in the radio-frequency range with a subhertz linewidth, i.e., over thousands of pulses. An application to lidar radar is discussed.     

Selective saturation in strongly inhomogeneous magnetic fields

The possibility to produce selective saturation by nuclear magnetic resonance (NMR) sequences of low-power radio-frequency pulses in strongly inhomogeneous magnetic fields is explored. The saturation of parts of the sensitive volume is produced by a particular pulse sequence with reduced amplitude distribution and the spectrum of the recorded signal is compared with the simulated spectrum. The spectra of the recorded free induction decays and echo signals are in good agreement with the simulated spectra of the pulse sequence, which demonstrates the effect of the selective saturation. The results obtained are an important step towards the development of new mobile and lowpower NMR equipments operating with inhomogeneous magnetic fields.     

T1 and T2 effects during radio-frequency pulses in spoiled gradient echo sequences

Finite pulse durations in diverse pulse schemes lead to the reduction of the magnitude of the magnetization vector due to T₁ and T₂ effects during the radio-frequency pulses. This paper presents an analysis of the steady state signal in the presence of relaxation effects during radio-frequency pulses in MRI spoiled gradient echo sequences. It is shown that minor attenuations of the magnetization vector can have dramatic consequences on the measured signal, and may thus entail a loss in SNR benefits at high static magnetic fields if a careful analysis is not performed. It is emphasized that it is the time-integrated magnetization vector trajectory that matters for these effects and not only the pulse duration. Some experimental results obtained on a phantom at 3 T verify this analysis.     

Universal quantum logic gates in a scalable Ising spin quantum computer

We consider the model of quantum computer, which is represented as a Ising spin lattice, where qubits (spin-half systems) are separated by the isolators (two spin-half systems). In the idle mode or at the single bit operations the total spin of isolators is 0. There are no need of complicated protocols for correcting the phase and probability errors due to permanent interaction between the qubits. We present protocols for implementation of universal quantum gates with the rectangular radio-frequency pulses.     

Localized cross-polarization in solids

Single quantum heteronuclear cross-polarization in solids is strongly sensitive to resonance offsets. In the presence of main field- or radio-frequency field gradients, the cross-polarization efficiency, therefore, shows a strong spatial dependence, which represents a new principle for localized NMR in solids. Since slices-selective excitation is achieved simultaneously to cross-polarization, the suggested pulse sequences avoid the use of shaped pulses, the application of which is problematic with solid. The dependence of the localization efficiency on experimental and sample parameters is analyzed theoretically for a spin-1/2 system in the presence of a static or a radio-frequency magnetic field gradient. The resulting slice profiles and the calculated dependence of the slice thickness on the parameters of the basic cross-polarization procedures are discussed and confirmed experimentally on the example of¹H-³C spin systems.     

Switching speed effect of phase shift keying in SLED for generating high power microwaves

SLAC energy doubler(SLED) type radio-frequency pulse compressors are widely used in large-scale particle accelerators for converting long-duration moderate-power input pulses into short-duration high-power output pulses. Phase shift keying(PSK) is one of the key components in SLED pulse compression systems. Performance of the PSK will influence the output characteristics of the SLED, such as the rise-time of the output pulse, maximal peak power gain, and energy efficiency. In this paper, a high power microwave source based on power combining and pulse compression of conventional klystrons is introduced. The effects of nonideal PSK with slow switching speed and PSK without power output during the switching process are investigated, and the experimental results with nonideal PSK agree well with the analytical results.     

A comparative analysis of two methods of realizing elementary logic operators for a quantum computer on qutrits

In the present work, elementary logic operators (including selective rotation, Fourier transform, controlled phase shift, and controlled NOT operators) for a quantum computer on tristable systems (qutrits) are examined. Computer modeling of realization of these operators based on a system of two nuclear spins I = 1 is carried out by the methods of nuclear magnetic resonance. Two different methods of realizing the controlled NOT operator are presented: with the help of weak pulses of radio-frequency magnetic field selective in spin-spin interaction and with the help of strong pulses selective in quadrupole interaction. A dependence of realization errors on the interaction parameters, variable field amplitude, and pulse duration is calculated. Advantages and disadvantages of each method in the process of realization are indicated. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 6, pp. 41–47, June, 2007.     

Spectral characterization of diffusion with chemical shift resolution: Highly concentrated water-in-oil emulsion

We present a modulated gradient spin-echo method, which uses a train of sinusoidally shaped gradient pulses separated by 180° radio-frequency (RF) pulses. The RF pulses efficiently refocus chemical shifts and de-phasing due to susceptibility differences, resulting in undistorted, high-resolution diffusion weighted spectra. This allows for the simultaneous spectral characterization of the diffusion of several molecular species with different chemical shifts. The technique is robust against susceptibility artifacts, field inhomogeneity and imperfections in the gradient generating equipment. The feasibility of the technique is demonstrated by measuring the diffusion of water, oil, and water-soluble salt in a highly concentrated water-in-oil emulsion. The diffusion of water and salt reveal precise information about the droplet size distribution below the μm-range. Common droplet size distribution explains both the data for water with finite long-range diffusion and the data for salt with negligible long-range diffusion. The results of water diffusion show that the technique is efficient in deconvolving the effects of molecular exchange between droplets and restricted diffusion within droplets. The effects of water exchange suggest that droplets of different sizes are uniformly distributed within the sample.