Universality of Uhrig dynamical decoupling for suppressing qubit pure dephasing and relaxation

The optimal N-pulse dynamical decoupling discovered by Uhrig for a spin-boson model [Phys. Rev. Lett. 98, 100504 (2007)10.1103/PhysRevLett.98.100504] is proved to be universal in suppressing to O(T;{N+1}) the pure dephasing or the longitudinal relaxation of a qubit (or spin 1/2) coupled to a generic bath in a short-time evolution of duration T. For suppressing the longitudinal relaxation, a Uhrig pi-pulse sequence can be generalized to be a superposition of the ideal Uhrig pi-pulse sequence as the core and an arbitrarily shaped pulse sequence satisfying certain symmetry requirements. The generalized Uhrig dynamical decoupling offers the possibility of manipulating the qubit while simultaneously combating the longitudinal relaxation.     

Simple two-pulse time-reversal sequence for dipolar and quadrupolar-coupled spin systems

We demonstrate, both theoretically and experimentally, that two-pulse sequence (2n+1) X 90 degrees(Y) - 90 degrees(X) -Acq(t) without delay between pulses, yields the reverse of time evolution of spin system with dipolar and quadrupole interactions. This process results in refocusing of the spin magnetization into magic echo at te = t1/2 after the second pulse, where t1 is the length of the first pulse.     

Photon echo without a free induction decay in a double-Λ system

We have characterized a novel photon-echo pulse sequence for a double-Λ-type energy level system where the input and rephasing transitions are different from the applied π pulses. We show that, despite having imperfect π-pulses associated with large coherent emission due to free induction decay (FID), the noise added in the echo mode is only 0.2 ± 0.1 photons per shot, compared to 4 × 10? photons in the FID modes. Using this echo pulse sequence in the "rephased amplified spontaneous emission" (RASE) scheme [Phys. Rev. A 81, 012301 (2010)] will allow for generation of entangled photon pairs that are in different frequency, temporal, and potentially spatial modes to any bright driving fields. The coherence and efficiency properties of this sequence were characterized in a Pr(3+):Y?SiO? crystal.     

Total dephasing-rephasing balancing in Stark-pulse-modulated photon echoes

Using Stark-pulse-modulated photon echoes, we observe a novel type of rephasing phenomenon in a Eu(3+) - and Pr(3+) -codoped Y(2)SiO(5) crystal. By adjusting the Stark pulse during the rephasing and dephasing periods one can observe a full recovery of the photon echo, corresponding to perfect dephasing-rephasing balancing of the perturbations. We propose to use this effect as a spectroscopic technique to distinguish between reversible and irreversible Stark interactions.     

Gradient BIRDR: a method to select uncoupled magnetization

The gradient-BIRD-method is an effective way to select 13C-bound protons. By changing the phase of the proton pi-pulse, the same sequence can be used to select protons attached to 12C atoms, i.e., for filtering out the 13C-bound protons. These filters, consisting of single or double gradient-BIRDR clusters (BIRD inversion for remote protons) have considerable filtering bandwidth. Because of the efficient suppression of the coupled magnetization, we implemented the gradient-BIRDR filter in HMBC to replace the conventional low-pass filter.     

In Vivo3D LocalizedC Spectroscopy Using Modified INEPT and DEPT

The 3D localized¹³C spectroscopy methods LINEPT and LODEPT, which are modifications of INEPT and DEPT, are proposed. As long as a¹³C inversion pulse (180-degree pulse) is applied at 1/(4J) before the proton echo time in LINEPT and a¹³C excitation pulse (90-degree pulse) is applied at 1/(2J) before the proton echo time in LODEPT, the proton echo time can be set to any value longer than 1/(2J) in LINEPT and longer than 1/Jin LODEPT. As a result, the proton and the¹³C pulses can be applied separately and these proton pulses can be made slice-selective pulses. These localization features of LINEPT and LODEPT were evaluated using a phantom consisting of a cylinder filled with ethanol placed inside another cylinder filled with oil, and localized ethanol spectra could be obtained.In vivo3D localized¹³C spectra from the brain of a monkey could be obtained using decoupled LINEPT, and glutamate C-4 appeared directly after the administration of glucose C-1, followed by the appearance of glutamate C-2, C-3 and glutamine C-2, C-3, C-4.     

The edge echo

An experimental and theoretical study of the features of the edge echo is presented. This echo may be stimulated by applying a single rf pulse to the nuclear spin system of a liquid sample in thermal equilibrium in a slightly inhomogeneous magnetic field. It follows the trailing edge of the pulse at a time shorter than the pulse duration and shows the refocusing of isochromats which were dephased during precession about the effective field of the pulse. The refocusing takes the form of a rapid frequency scan through the nuclear magnetic resonance spectrum. The edge echo is associated with a virtual echo and with a real and virtual stimulated echo pair.     

An improvement in the range resolution of ultrasonic pulse echo systems by deconvolution

Ultrasonic pulse echo systems are often limited in range resolution by the bandwidth of the piezoelectric transducer. Significant improvements in the range resolution of such systems can be obtained by minimizing the effects of the transducer's dynamic response on the overall pulse echo process. An approach to minimize the effects of the transducer is developed from linear system and impulse response techniques. In essence, the pulse echo voltage of interest is deconvolved with a pulse echo reference voltage which is obtained from an air/water interface in the nearfield of the transducer . A computer study of the pulse echo process and the deconvolution process is presented to illustrate the nature of the improvement in range resolution for several cases of interest. Finally, experimental results are presented to illustrate the improvement using commercially available transducers.     

脉冲激光测距回波信号时间选通处理技术

为了降低高重频脉冲激光测距回波信号的误识别率,提高测距性能,对高重频脉冲激光测距回波信号调理技术进行深入研究。采用FPGA作为主控芯片,产生激光调制脉冲,并根据被测距离3.33 μs～33.33 μs时间选通方波信号,驱动开关芯片产生与量程关联的时间波门,有效滤除脉冲回波中的干扰脉冲,该方法改进了常规脉冲激光测距信号处理系统的自动增益控制环节。测试实验结果表明:在脉冲回波信号60 dB的动态范围内,可有效滤除回波信号中引入的干扰脉冲,极大地降低了干扰脉冲误识别造成粗大误差的可能性。该方法可推广应用于脉冲激光测距信号处理系统,使系统测距精度提高12.6%。     

Density Matrix Simulations of the Effects of J Coupling in Spin Echo and Fast Spin Echo Imaging

A computer simulation has been used to calculate the effects of J coupling on the amplitudes of echoes produced by CPMG sequences. The program computes the evolution of the density matrix for different pulse intervals and can predict the signals obtainable from spin systems of any size and complexity. Results from the simulation confirm the prediction that a decrease in the effects of J coupling is largely responsible for the bright fat signal seen in fast spin echo imaging at high pulse rates. The effects of J coupling on CPMG echotrains are examined for A3B2 and A3B2C2 spin systems over a wide range of J coupling and chemical shift values and pulse spacings. The effects of J coupling on the point spread function obtained with fast spin echo imaging are also discussed.     

Nuclear quadrupole resonance echoes from hexamethylenetetramine

We investigated the echo phenomenon of nuclear quadrupole resonance (NQR) from hexamethylenetetramine (HMT). We detected the pure NQR echo signal of HMT with a short pulse interval. The intensity of the echo signal increased as the pulse interval time was decreased. We observed that a clean echo signal was generated even when the pulse interval was shorter than the decay time constant T₂^*. Since the short interval time gives a strong echo, our result insists that shorter interval time is preferred for the NQR detection.     

Double spin-echo sequence for rapid spectroscopic imaging of hyperpolarized 13C

Dynamic nuclear polarization of metabolically active compounds labeled with (13)C has been introduced as a means for imaging metabolic processes in vivo. To differentiate between the injected compound and the various metabolic products, an imaging technique capable of separating the different chemical-shift species must be used. In this paper, the design and testing of a pulse sequence for rapid magnetic resonance spectroscopic imaging (MRSI) of hyperpolarized (13)C is presented. The pulse sequence consists of a small-tip excitation followed by a double spin echo using adiabatic refocusing pulses and a "flyback" echo-planar readout gradient. Key elements of the sequence are insensitivity to calibration of the transmit gain, the formation of a spin echo giving high-quality spectral information, and a small effective tip angle that preserves the magnetization for a sufficient duration. Experiments in vivo showed three-dimensional coverage with excellent spectral quality and SNR.     

The nuclear spin-echo signals under combined action of magnetic field and RF pulses

The effect of a pulse magnetic field on formation of the single-pulse echo signal in some magnetic materials has been studied. The signals of nuclear-spin echo formed by the combined action of radio frequency and magnetic pulses have been observed. The duration of the magnetic pulse was only limited by the length of the radio frequency pulse. The echo signal formed by the two magnetic pulses, the edges of a long magnetic pulse, and other situations that allowed us to reproduce the analogue of multipulse action, were observed as well.     

NMR spin echoes for molecular rotators with C(3) symmetry

NMR spin echoes are calculated for a rotator consisting of three spin-(1/2) nuclei whose three-particle wave function obeys C(3) symmetry. On decomposing the nuclear dipole-dipole interactions in terms of irreducible operators of the representations of the group C(3) it is found that for spins belonging to the A-representation, the echo amplitude is maximized for a RF pulse sequence. The dependences of the echo formation on the orientation of the rotor and on the strengths of the dipolar interactions and the magnetic field inhomogeneities are discussed. Copyright 2000 Academic Press.     

Nuclear quadrupole resonance echoes from hexamethylenetetramine

We investigated the echo phenomenon of nuclear quadrupole resonance (NQR) from hexamethylenetetramine (HMT). We detected the pure NQR echo signal of HMT with a short pulse interval. The intensity of the echo signal increased as the pulse interval time was decreased. We observed that a clean echo signal was generated even when the pulse interval was shorter than the decay time constant T₂^*. Since the short interval time gives a strong echo, our result insists that shorter interval time is preferred for the NQR detection.     

机载多脉冲激光雷达目标信号模拟器的研究

研究多脉冲激光雷达目标信号模拟器,用以评估回波数字信号处理算法及其实现平台的性能。结合信号能量与波形关系指出由激光雷达方程建立回波信号波形模型是不充分的。对空中目标回波脉冲展宽进行建模及数值仿真,得到不同倾角、目标距离及目标尺寸下的波形展宽时间数据。根据展宽前后能量保持不变的性质,利用高斯脉冲函数特征参数求解建立回波波形数学模型。基于目标距离误差及目标回波噪声统计特性建立回波仿真模型,提出了依信噪比发生回波观测信号的方法。对比两种模拟器的信噪比误差、最小模拟信噪比、连续工作时间等性能指标,总结了新型模拟器的优势。     

Excitation of photon echo by noise pulses

The excitation of photon echo by noise pulses that are formed by modulation of the carrying frequency with Gauss noise is modeled. The modeling is based on optical Bloch equations the solution of which for noise pulse realizations is constructed by their stepwise approximation. In terms of the formalism of state transfer matrices, the two- and three-pulse excitation modes are analyzed. The complex envelopes of the primary and stimulated echo responses are determined. In the linear (low-level-signal) mode, the shape of the two-pulse echo corresponds to that of the time delayed and inverted noise pulse. The boundary of the linear mode, upon exceeding of which distortions of the shape of the noise pulse become noticeable, is determined. The shape of the stimulated (three-pulse) echo in the linear mode corresponds to that of the autocorrelation function of the noise pulse realization. Upon passage beyond the boundary of the linear mode, the shape of the three-pulse echo corresponds either to the cross-correlation function of distorted noise pulses (with different intensities) or to the autocorrelation function of distorted pulses (with the same intensities). The modeled photon echo excitation modes can be used in photon echo processors to process signals in the light range.     

Hyperfine field and electric quadrupole interaction of51V in YFe2 and HoFe2

Spin echo measurements were made on Y(Fe_0.99V_0.01)₂, (Y_0.97Ho_0.03) (Fe_0.99V_0.01)₂ and Ho(Fe_0.96 V_0.04)₂ compounds. The addition of Ho to YFe₂ causes a spin reorientation from <111> to <100> but this is not reflected in the⁸⁹Y or⁵¹V hyperfine fields. A well resolved⁵¹V NMR line was observed at 51.5 MHz and 60.0 MHz respectively for Y and Ho Laves compounds but with no evidence of quadrupole satellites. Small quadrupole splittings ¦2P_t¦ = 171 ±5 kHz and 178 ±5 kHz respectively for Y and Ho compounds were deduced from the oscillatory decay of the⁵¹V spin echo after a two pulse sequence.On leave from Department of Physics, University College, The University of New South Wales, Australian Defence Force Academy, Campbell, ACT, Australia.     

Modified stimulated photon echo with hyperfine structure of resonant levels

The intensity and polarization of a modified stimulated photon echo generated on resonant levels with hyperfine structure are obtained. It is proposed that the polarization properties of this kind of a photon echo can be used for the identification of the structure of the resonant transitions on which the echo is formed as well as for the separation of the modified stimulated photon echo pulse from the third exciting pulse.     

High resolution 13C DOSY: the DEPTSE experiment

High Resolution Diffusion-ordered Spectroscopy (HR-DOSY) is a valuable tool for mixture analysis by NMR. It separates the signals from different components according to their diffusion behavior, and can provide exquisite diffusion resolution when there is no signal overlap. In HR-DOSY experiments on (1)H (by far the most common nucleus used for DOSY) there is frequent signal overlap that confuses interpretation. In contrast, a (13)C spectrum usually has little overlap, and is in this respect a much better option for a DOSY experiment. The low signal-to-noise ratio is a critical limiting factor, but with recent technical advances such as cryogenic probes this problem is now less acute. The most widely-used pulse sequences for (13)C DOSY perform diffusion encoding with (1)H, using a stimulated echo in which half of the signal is lost. This signal loss can be avoided by encoding diffusion with (13)C in a spin echo experiment such as the DEPTSE pulse sequence described here.