H. K. Cummins, J. A. Jones
We describe novel composite pulse sequences which act as general rotors and thus are suitable for nuclear magnetic resonance (NMR) quantum computation. The Resonance Offset Tailoring To Enhance Nutations (ROTTEN) approach permits perfect compensation of off-resonance errors at two selected frequencies placed symmetrically around the frequency of the RF source.
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Pursuit and Review of Magnetic Resonance Imaging (MRI) based Quantum Computing -- Qubit Generation, Spin Purification, Tailored RF Pulses and MRI Sequences for Quantum Computing
本研究提出一种基于MRI技术、利用水质子NMR生成多量子比特的新方法，结合局部反向梯度实现高均匀场单频量子比特，可室温运行并复用MRI现有软硬件，能为量子计算提供复杂信号处理能力。
Grover's quantum search algorithm, involving a large number of qubits, is highly sensitive to errors in the physical implementation of the unitary operators. This poses an intrinsic limitation to the size of the database that can be practically searched. The lack of robustness of Grover's algorithm for a large number of qubits is due to quite stringent "phase-matching" condition. To overcome this limitation, Tulsi suggested a modified search algorithm [PRA 78, 022332] which succeeds as long as the errors are reproducible and reversible while Grover's algorithm fails. Such systematic errors ari…
We present a framework that uses a continuous frequency space to describe and design solid-state NMR experiments. The approach is similar to the well established Floquet treatment for NMR, but is not restricted to periodic Hamiltonians and allows the design of experiments in a reverse fashion. The framework is based on perturbation theory on a continuous Fourier space, which leads to effective, i.e., time-independent, Hamiltonians. It allows the back calculation of the pulse scheme from the desired effective Hamiltonian as a function of spin-system parameters. We show as an example how to back…
Large-scale NMR simulation on a trapped-ion quantum computer
本研究在Quantinuum H2离子阱量子计算机上，通过硬件高效约简和误差抑制，实现了对1,2-二叔丁基二膦烷的21自旋模型大规模数字NMR模拟，结果与经典计算一致，推动了量子模拟的实用化。
Lee-Goldburg cross polarization is used in many NMR experiments to determine internuclear distances. Distances calculated from frequency peaks in the dipolar coupled dimension are subject to scale factors. Experimentally determined scale factors can differ significantly from the theoreticalyy useful value described in the literature. In this work, simulations using SPINEVOLUTION explore effects of varying experimental parameters of frequency offset and cross polarization powers on $^{15}$N-H systems. Experimental data is used to corroborate the simulation. The scale factor is shown not only to…
We present highly robust, optimal control-based shaped pulses designed to replace all 90° and 180° hard pulses in a given pulse sequence for improved performance. Special attention was devoted to ensuring that the pulses can be simply substituted in a one-to-one fashion for the original hard pulses without any additional modification of the existing sequence. The set of four pulses for each nucleus therefore consists of 90° and 180° point-to-point (PP) and universal rotation (UR) pulses of identical duration. These 1 ms pulses provide uniform performance over resonance offsets of 20 kHz (1H) a…
Many applications of magnetic resonance are limited by rapid loss of spin coherence caused by large transverse relaxation rates. In nuclear magnetic resonance (NMR) of large proteins, increased relaxation losses lead to poor sensitivity of experiments and increased measurement time. In this paper we develop broadband relaxation optimized pulse sequences (BB-CROP) which approach fundamental limits of coherence transfer efficiency in the presence of very general relaxation mechanisms that include cross-correlated relaxation. These broadband transfer schemes use new techniques of chemical shift r…
Feedback Pulses
该研究提出了名为反馈脉冲的NMR脉冲设计新范式，通过监测偏置磁化状态、调整射频相位迭代优化的方法，可便捷实现宽带反转、宽带激发及带选激发，并给出了算法、仿真与实验结果。
Risk-aware neural pulse compilation of continuously parameterized single-qubit gates with optimal-control benchmarking and NMR hardware validation
本文提出一种基于保真度的神经脉冲编译框架，用于连续单量子比特门的编译，通过端到端训练实现对任意SU(2)操作的轴角参数到射频控制序列的映射，并通过不确定性分析提升容错能力。
Composite pulses have found widespread use in both conventional Nuclear Magnetic Resonance experiments and in experimental quantum information processing to reduce the effects of systematic errors. Here we describe several families of time symmetric and antisymmetric fully compensating composite pulses, inspired by the previous Fn, Gn and BB1 families family developed by Wimperis. We describe families of composite 180 degree pulses (NOT gates) which exhibit unprecedented tolerance of pulse strength errors without unreasonable sensitivity to off-resonance errors, and related families with more …
The composite rotation approach has been used to develop a range of robust quantum logic gates, including single qubit gates and two qubit gates, which are resistant to systematic errors in their implementation. Single qubit gates based on the BB1 family of composite rotations have been experimentally demonstrated in a variety of systems, but little study has been made of their application in extended computations, and there has been no experimental study of the corresponding robust two qubit gates to date. Here we describe an application of robust gates to Nuclear Magnetic Resonance (NMR) stu…
A common way to manipulate a quantum system, for example spins or artificial atoms, is to use properly tailored control pulses. In order to accomplish quantum information tasks before coherence is lost, it is crucial to implement the control in the shortest possible time. Here we report the near time-optimal preparation of a Bell state with fidelity higher than $99\%$ in an NMR experiment, which is feasible by combining the synergistic capabilities of modelling and experiments operating in tandem. The pulses preparing the Bell state are found by experiments that are recursively assisted with a…
Decoupling Dipolar Interactions in Dense Spin Ensembles
本文结合数值模拟与固态核磁共振实验，评估了多种脉冲序列在抑制稠密自旋系综偶极耦合中的性能，揭示了局部无序等因素对解耦效率的影响，并验证了时间暂停序列保护多自旋关联的能力。
Adiabatic operations are powerful tools for robust quantum control in numerous fields of physics, chemistry and quantum information science. The inherent robustness due to adiabaticity can, however, be impaired in applications requiring short evolution times. We present a single versatile gradient-based optimization protocol that combines adiabatic control with effective Hamiltonian engineering in order to design adiabatic operations tailored to the specific imperfections and resources of an experimental setup. The practicality of the protocol is demonstrated by engineering a fast, 2.3 Rabi cy…
Probing an atomic resonance without disturbing it is an ubiquitous issue in physics. This problem is critical in high-accuracy spectroscopy or for the next generation of atomic optical clocks. Ultra-high resolution frequency metrology requires sophisticated interrogation schemes and robust protocols handling pulse length errors and residual frequency detuning offsets . This review reports recent progress and perspective in such schemes, using sequences of composite laser-pulses tailored in pulse duration, frequency and phase, inspired by NMR techniques and quantum information processing. After…
The nitrogen-vacancy (NV) centre in diamond is a leading platform for nanoscale sensing and imaging, as well as quantum information processing in the solid state. To date, individual control of two NV electronic spins at the nanoscale has been demonstrated. However, a key challenge is to scale up such control to arrays of NV spins. Here we apply nanoscale magnetic resonance frequency encoding to realize site-selective addressing and coherent control of a four-site array of NV spins. Sites in the array are separated by 100 nm, with each site containing multiple NVs separated by ~15 nm. Microcoi…
The robustness of composite pulses elucidated by classical mechanics. II. The role of initial state imperfection
本文通过经典力学框架研究复合脉冲的鲁棒性，发现初始状态不完美性对脉冲序列性能有显著影响，并提出优化方法以提升相干群体倒转效率。
The robustness of composite pulses elucidated by classical mechanics: Stability around the globe
本文利用经典力学稳定性矩阵解释复合脉冲的鲁棒性，将布洛赫球运动映射到正则坐标，揭示系综重新聚焦的方向性，并以Levitt的$90(x)180(y)90(x)$脉冲为例验证了该方法。
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