Detection of Magnetic Field Gradient and Single Spin Using Optically Levitated Nano-Particle in Vacuum

Optically levitated nano-particle with spins is a promising system for high-precision measurement and quantum information processing. We theoretically analyze the ratio between the fluctuation of particle's displacement caused by spins in magnetic field and caused by molecular collisions of the residual air. When the ratio is larger than unity, the displacement fluctuation of spins flipping can be remarkably detected. By theoretical analysis and numerical simulation, we propose and validate a scheme for the detection of gradient of the magnetic field by levitating ferromagnetic nano-particle, and also put forward a realizable detection scheme of the single spin by levitating nano-diamond particle with single nitrogen-vacancy(NV) centers.     

Detection and manipulation of statistical polarization in small spin ensembles

We report the detection of the square root of N statistical polarization in a small ensemble of electron spin centers in SiO2 by magnetic resonance force microscopy. A novel detection technique was employed that captures the statistical polarization and cycles it between states that are either locked or antilocked to the effective field in the rotating frame. Using field gradients as high as 5 G/nm, we achieved a detection sensitivity equivalent to roughly two electron spins, and observed ultralong spin-lock lifetimes, as long as 20 s. Given a sufficient signal-to-noise ratio, this scheme should be extendable to single electron spin detection.     

Spectral characterization of diffusion in porous media by the modulated gradient spin echo with CPMG sequence

Carr-Purcell-Meiboom-Gill train of radiofrequency pulses applied to spins in the constant magnetic field gradient is an efficient variant of the modulated magnetic field gradient spin echo method, which provides information about molecular diffusion in the frequency-domain instead in the time-domain as with the two-pulse gradient spin echo. The frequency range of novel technique is broad enough to sample the power spectrum of displacement fluctuation in water-saturated pulverized silica (SiO(2)) and provides comprehensive information about the molecular restricted motion as well as about the structure of medium.     

Velocity autocorrelation spectra of fluid in porous media measured by the CPMG sequence and constant magnetic field gradient

Carr-Purcell-Meiboom-Gill (CPMG) train of radiofrequency pulses applied to spins in the constant magnetic field gradient is an efficient variant of the modulated magnetic field gradient spin echo method, which provides information about molecular diffusion in the frequency domain instead of in the time domain as with the two-pulse gradient spin echo. The frequency range of this novel technique is broad enough to sample the power spectrum of displacement fluctuation in water-saturated pulverized silica (SiO2) and provides comprehensive information about the molecular restricted motion as well as about the structure of the medium.     

Levitating states of superconducting rings in the field of a fixed ring with constant current

We consider the possibility of designing a plasma trap with a magnetic system formed by super-conducting rings and coils levitating in the field of a fixed coaxial coil carrying constant current. An analytic dependence of the potential energy of such a system with one or two levitating superconducting rings having trapped preset magnetic fluxes on their coordinates in the uniform gravitational field is obtained in the thin ring approximation. Calculations performed in the Mathcad system show that equilibrium states of such a system exist for certain values of parameters. Levitating states of a single superconducting ring and two superconducting rings in the field of the coil with constant current are observed experimentally in positions corresponding to calculated values.     

Entanglement in a two-spin system with long-range interactions

The quantum entanglement between two spins in the Ising model with an added Dzyaloshinsky–Moriya(DM) interaction and in the presence of the transverse magnetic field is studied. The exchange interaction is considered as a function of the distance between spins. The negativity as a function of magnetic field, exchange and DM interaction is calculated.The effect of the distance between spins is studied based on the negativity. In addition, the effect of the thermal fluctuation on the negativity is also investigated.     

NMR at earth's magnetic field using para-hydrogen induced polarization

A method to achieve NMR of dilute samples in the earth's magnetic field by applying para-hydrogen induced polarization is presented. Maximum achievable polarization enhancements were calculated by numerically simulating the experiment and compared to the experimental results and to the thermal equilibrium in the earth's magnetic field. Simultaneous 19F and 1H NMR detection on a sub-milliliter sample of a fluorinated alkyne at millimolar concentration (～10(18) nuclear spins) was realized with just one single scan. A highly resolved spectrum with a signal/noise ratio higher than 50:1 was obtained without using an auxiliary magnet or any form of radio frequency shielding.     

Photon counts modulation in optical time domain reflectometry

The quantum fluctuation of photon counting limits the field application of optical time domain reflection. A method of photon counts modulation optics time domain reflection with single photon detection at 1.55 μm is presented. The influence of quantum fluctuation can be effectively controlled by demodulation technology since quantum fluctuation shows a uniform distribution in the frequency domain. Combined with the changing of the integration time of the lock-in amplifier, the signal to noise ratio is significantly enhanced. Accordingly the signal to noise improvement ratio reaches 31.7 dB compared with the direct photon counting measurement.     

Playing with quantum Hall effects and single-electron-tunneling effects

The usefulness of quantum Hall effect (QHE) conductors and quantum dot (QD) devices is revealed by reviewing five remarkable effects. The first is the sensitive detection of terahertz (THz) radiation by QHE conductors. The second is the imaging of THz emission from non-equilibrium carriers in QHE conductors, by using scanning THz microscopes. The third is the single-photon detection of THz radiation in strong magnetic fields, which is carried out by incorporating a QHE electron system into a QD. Individual events of single-THz-photon absorption within the QD via cyclotron resonance cause the QD to electrically polarize, which, in turn, is detected as switches of the tunnel conductance through the QD. The fourth is the single-photon detection of THz radiation by using double QDs in the absence of a magnetic field. Both of the photon detectors are implemented in gate-voltage-induced lateral GaAs/AlGaAs QDs, and exploiting the extraordinary sensitivity of single-electron transistors to the charge. The fifth is the coherent control of nuclear spins in QHE conductors. Nuclear spins are (i) electrically polarized by unequally populating spin-split QHE edge channels via the hyperfine interaction, (ii) coherently controlled via pulsed nuclear magnetic resonance induced by local RF magnetic fields, and (iii) finally detected by the edge channels through resistance change of the Hall device. The controlled nuclear spins are limited to those along the edge channels, on the order of 10⁹.     

Beating standard quantum limit via two-axis magnetic susceptibility measurement

We report a metrology scheme which measures the magnetic susceptibility of an atomic spin ensemble along the $x$ and $z$ directions and produces parameter estimation with precision beating the standard quantum limit. The atomic ensemble is initialized via one-axis spin squeezing with optimized squeezing time and parameter $\phi$ (to be estimated) assumed as uniformly distributed between 0 and $2\pi$ while fixed in each estimation. One estimation of $\phi$ can be produced with every two magnetic susceptibility data measured along the two axes respectively, which has an imprecision scaling $({1.43\pm0.02})/N^{0.687\pm0.003}$ with respect to the number $N$ of the atomic spins. The measurement scheme is easy to implement and is robust against the measurement fluctuation caused by environment noise and measurement defects.     

Muon spin relaxation in diluted triangular‐lattice antiferromagnet

Zero and longitudinal field μSR measurements on diluted triangular‐lattice antiferromagnet LuFeMgO₄ have revealed gradual and highly anisotropic slowing‐down of spin fluctuation. Relaxation rate \lambda in the condition with initial muon spins perpendicular to the hexagonal c‐plane shows divergent behavior while that in the parallel configuration remains finite. At certain temperature range the fluctuation of spins is suppressed by a small external magnetic field. Monte Carlo calculation suggests that the correlation time for z(c)‐component of spins diverges first on cooling. This revised version was published online in July 2006 with corrections to the Cover Date.     

Enhancement of electron spin coherence by optical preparation of nuclear spins

We study a large ensemble of nuclear spins interacting with a single electron spin in a quantum dot under optical excitation and photon detection. At the two-photon resonance between the two electron-spin states, the detection of light scattering from the intermediate exciton state acts as a weak quantum measurement of the effective magnetic (Overhauser) field due to the nuclear spins. In a coherent population trapping state without light scattering, the nuclear state is projected into an eigenstate of the Overhauser field operator, and electron decoherence due to nuclear spins is suppressed: We show that this limit can be approached by adapting the driving frequencies when a photon is detected. We use a Lindblad equation to describe the driven system under photon emission and detection. Numerically, we find an increase of the electron coherence time from 5 to 500 ns after a preparation time of 10 micros.     

Nanoscale detection of faint machinery vibration using the NV center in diamond

We propose a novel method for detection of the faint machinery vibration at the nanometer resolution based on the spin magnetic resonant effect. A suspension magnet acts as a vibration sensor to transfer the vibration signals as the magnetic field fluctuation to excite the spin magnetic resonance. Due to the high sensitivity of the magnetic field of the nitrogen-vacancy center in diamond, the theoretical detection limit of mechanical vibration is as high as 5.7 nm, and the actual measurement resolution reached 12.8 nm, and proved the potential for further improvement to ∼pm resolution. The feasibility of this method is verified by dynamic tests. This method provides a novel approach for the detection of micro-mechanical vibrations.     

基于磁通门和时域数字鉴频的磁场锁定系统

永磁型磁共振仪器的磁体易受温度和其他环境磁场干扰,造成主磁场波动,进而影响仪器测量的重复性和准确性．本文讨论了两种解决磁场波动的锁定方法：一方面,通过磁通门传感器对环境波动引起的瞬态磁场进行高灵敏探测,然后采用现场可编程门阵列进行实时处理并计算磁场补偿量;另一方面,针对环境温度变化引起的缓慢磁场偏移,则采用时域数字鉴频锁场方法,在对锁样品进行射频激发后,将磁共振信号通过混频变换到较低的频率范围,再转换为方波,然后直接送入现场可编程门阵列进行周期测量,并计算磁场补偿量．将两种方法获得的磁场补偿量叠加后,再转换为电流信号驱动安装在磁体上的B₀补偿线圈,并研制了一套磁场锁定系统,以实现对磁场的锁定.在0.5 T食品快检磁共振分析仪上进行测试验证,结果显示当受到瞬态干扰时,可将磁场稳定在±4 Hz(对应磁场为±0.093 9μT)范围内,同时也可以精准测量温度造成的磁场偏移,该结果验证了本文磁场锁定方法的可行性．     

Quantum gates using electronic and nuclear spins of Yb+ in a magnetic field gradient

An efficient scheme is proposed to carry out gate operations on an array of trapped Yb⁺ ions, based on a previous proposal using both electronic and nuclear degrees of freedom in a magnetic field gradient. For this purpose we consider the Paschen-Back regime (strong magnetic field) and employ a high-field approximation in this treatment. We show the possibility to suppress the unwanted coupling between the electron spins by appropriately swapping states between electronic and nuclear spins. The feasibility of generating the required high magnetic field is discussed.     

Spin orientation and excitation of magnetic nanocluster on metal surface

The spin orientation and excitation of the ferromagnetic nanocluster on the magnetic metal surface are studied numerically. We show that localized magnetic excitation modes are generated by the spin fluctuation of the cluster, when the ferromagnetic interaction J′ between the cluster and the metal surface is small and the spins in the cluster are oriented in the opposite direction with those of the metal surface by the external field. This magnetic structure is similar to the domain wall (DW) structure of a ferromagnetic wire, both sides of which connect with metal surfaces. As the interaction J′ increases, the sign of the thermal average of the spins in the cluster changes, i.e., the spin-flip takes place. In this time, the magnetic fluctuation of the cluster becomes large and the magnetic excitation energies, except for that of one excitation mode, overlap with the excitation spectrum of the spin wave. We also show that, by the overlap, sharp peaks and dips occur in the excitation spectrum of the spin wave.     

High-field measurements of electron decoherence time in metallic nanowires: switching off magnetic impurity spins

We report low-temperature measurements of electron decoherence time in a series of pure gold wires, 18 nm thick and 30 nm wide. At fields up to 15 T, large enough to polarize any concentration of magnetic impurity spins, conductance fluctuation measurements show almost no temperature dependence of the decoherence time below 300 mK, both in the correlation field for interference and the root-mean-square value of the fluctuations. Combined with previous low-field weak localization measurements on samples from similar material, our experiment suggests that the ubiquitous saturation of decoherence time in these samples is not due to any mechanism based on magnetic impurity spins.     

Quantum Homodyne Detection Based on Polarization Diversity Technique

A polarization diversity receiver scheme is presented for improving efficiency of balance homodyne detection. The proposed scheme may mitigate polarization fluctuation between signal and local oscillator field. With simple linear optical component and electronic processing circuit, the noise caused by differential phase and polarization mode between signed and local oscillators may be significantly decreased. To track the polarization fluctuation, a novel algorithm based on polarization diversity receiver which can achieve better performance in terms of linear quantum optics principle is proposed.     

Implementing 2→<Emphasis Type="Italic">M</Emphasis> Phase-covariant Cloning in Spin Networks

We propose a scheme to implement 2→M phase-covariant quantum cloning machine by using a M+2 spin star network in which the two central spins interact with the M outer spins respectively. The effect of the magnetic field on the fidelity of the cloning is also investigated. By applying an external magnetic field on the spin system, the fidelity of the cloning machine can be largely improved.     

脉冲激光探测方位角磁电检测技术

针对常规弹药脉冲激光周向探测系统无法精确获取目标方位信息的问题,设计了基于磁电检测的单光束激光引信全向方位角探测方案.对磁电检测系统进行建模,建立圆柱形永磁体转动磁场模型,推导出磁阻传感器所测位置磁场的解析式,验证所测磁场为一正弦磁场信号.依据此正弦信号,设计了上升沿阈值周期检测算法,并运用FPGA与TDC-GP21对激光回波出现时间与电机转速信号周期进行高准确度时间间隔测量实现方位角的解算.依据方案设计原理样机并编写上位机程序,进行方位角探测实验.实验结果表明:磁电检测系统采用多重屏蔽方法,能有效抑制电磁干扰;并能实时监测电机转速,实现方位角解算,方位角解算误差在±2°以内.满足激光引信方位角测量的高准确度、抗干扰能力强等要求.