Controlling Spin Squeezing and Quantum Fisher Information with a Strong External Field

We investigate the mean spin direction, length of mean spin and spin squeezing in a general superposition of a GHZ state and a W state. It is shown that the mean spin direction and spin squeezing parameter are determined by the coefficients of superposition and the relative phase between the GHZ state and the W state; while the length of mean spin is only determined by coefficients of superposition.     

Spin Squeezing of Superposition

We investigate the mean spin direction, length of mean spin and spin squeezing in a general superposition of a GHZ state and a W state. It is shown that the mean spin direction and spin squeezing parameter are determined by the coefficients of superposition and the relative phase between the GHZ state and the W state; while the length of mean spin is only determined by coefficients of superposition.     

Non-Markovian dynamics in a spin star system: the failure of thermalisation

In most cases, a small system weakly interacting with a thermal bath will eventually reach a thermal state with the temperature of the bath. We show that this intuitive picture is not always true, by using a spin star model where the non-Markov effect dominates the dynamical process. The spin star system consists of a central spin homogeneously interacting with an ensemble of identical noninteracting spins. We find that the correlation time of the bath is infinite, which implies that the bath has a perfect memory, and that the dynamical evolution of the central spin must be non-Markovian. A direct consequence of this is that the final state of the central spin is not the thermal state in equilibrium with the bath, but a steady state which depends on the initial state of the spin.     

Spin Squeezing of a General 3-Qubit State

We investigate the spin squeezing of a general 3-qubit state, which is superposed by a GHZ state and two W states. Numerical solutions for the length of mean spin, mean spin direction and spin squeezing were given. It is shown that the mean spin direction, the length of mean spin and the spin squeezing parameter are determined by the superposition coefficients and the relative phases between the GHZ state and the two W states.     

Quantum statistical properties of photon-added spin coherent states

The photon-added spin coherent state as a new kind of coherent state has been defined by iterated actions of the proper raising operator on the ordinary spin coherent state. In this paper, the quantum statistical properties of photon-added spin coherent states such as photon number distribution, second-order correlation function and Wigner function are studied. It is found that the Wigner function shows the negativity in some regions and the second-order correlation function is less than unity. Therefore, the photon-added spin coherent state is a nonclassical state.     

利用信息流方法优化多激发自旋链中的量子态传输

研究了量子态在一条均匀耦合的反铁磁自旋链中传输时, 信道中自旋激发数变化对其传输性质的影响. 利用信息流方法分析输出端粒子的算符演化动力学, 获得了量子态传输的平均保真度与信道自旋初态之间的关系. 结果表明, 平均保真度的大小只依赖于信道中自旋激发数的奇偶性. 通过比较在奇偶激发信道中获得的最大平均保真度, 构建了优化信道来提升量子态在自旋链中的传输质量. 进一步分析了纠缠在激发信道中的传输情况, 发现纠缠的传输质量不仅和信道中自旋激发的具体个数有关, 还取决于激发自旋的初始排列. 特别地, 当信道中自旋无激发或全部激发时, 纠缠传输的大小和持续时间都优于其他的激发信道. 上述研究结果有助于在实际系统中搭建适合量子态和纠缠传输的量子信道.     

Non-Abelian spin liquid in a spin-one quantum magnet

We study a time-reversal invariant non-Abelian spin-liquid state in an SU(2) symmetric spin S=1 quantum magnet on a triangular lattice. The spin liquid is obtained by quantum disordering a noncollinear nematic state. We show that such a spin liquid cannot be obtained by the standard projective construction for spin liquids. We also study the phase transition between the spin liquid and the noncollinear nematic state and show that it cannot be described within the Landau-Ginzburg-Wilson paradigm.     

Spin Squeezing of Superposition of Multi-Qubit GHZ State and W State

We investigate the properties of spin squeezing in a general superposition of multi-qubit GHZ state and W state. It is shown that the mean spin direction is determined by the coefficients of superposition and the relative phase between the GHZ state and the W state; while the length of mean spin and spin squeezing parameter are only determined by coefficient of superposition.     

Surface bound states and spin currents in noncentrosymmetric superconductors

We investigate the ground state properties of a noncentrosymmetric superconductor near a surface. We determine the spectrum of Andreev bound states due to surface-induced mixing of bands with opposite spin helicities for a Rashba-type spin-orbit coupling. We find that the order parameter suppression qualitatively changes the bound state spectrum. The spin structure of Andreev states leads to a spin supercurrent along the interface, which is strongly enhanced compared to the normal state spin current. Particle and hole coherence amplitudes show Faraday-like rotations of the spin along quasiparticle trajectories.     

Characterizing a high spin magnetic impurity via Andreev reflection spectroscopy

The ground state properties of a high spin magnetic impurity and its interaction with an electronic spin are probed via Andreev reflection. We see that through the charge and spin conductance one can effectively estimate the interaction strength, the ground state spin and magnetic moment of any high spin magnetic impurity. We show how a high spin magnetic impurity at the junction between a normal metal and superconductor can contribute to superconducting spintronics applications. Particularly, while spin conductance is absent below the gap for Ferromagnet-Insulator-Superconductor junctions we show that in the case of a Normal metal-High spin magnetic impurity-Normal Metal-Insulator-Superconductor (NMNIS) junction it is present. Further, it is seen that pure spin conduction can exist without any accompanying charge conduction in the NMNIS junction.     

Disordered and ordered states in a frustrated Heisenberg Hamiltonian with spatial anisotropy

We use a recently proposed perturbative numerical renormalization group algorithm to investigate ground-state properties of a frustrated three-dimensional Heisenberg model on an anisotropic lattice. We analyze the ground-state energy, the finite size spin gap and the static magnetic structure factor. We find in two dimensions a frustration-induced gapless spin liquid state which separates two magnetically ordered phases. In the spin liquid state, the magnetic structure factor shows evidence that this state is made of nearly disconnected chains reminiscent of a sliding Luttinger liquid. This spin liquid state is unstable against unfrustrated interplane couplings.     

Equilibrium and Nonequilibrium Spin Polarization near Filling Factor 3/2

The spin polarization features of an electron system and the relaxation of nonequilibrium spin excitations near an even-denominator fractional state of 3/2 in a two-dimensional electron system based on the GaAs/AlGaAs heterostructure are experimentally investigated. It is shown that the 3/2 state is a singular point in the filling factor dependence of the spin ordering of the two-dimensional electron system, at which the spin subsystem is rearranged. A giant slowing down of the relaxation of spin excitations to the ground state is revealed in a certain range of filling factors near filling factor 3/2.     

Laser-induced orbital and spin excitations in ferromagnets: insights from a two-level system

A recent time-resolved measurement showed that laser-induced orbital and spin excitations proceed in unison and the spin-orbit ratio is held constant during demagnetization. Here a two-level model shows that these orbital and spin excitations originate from state population and state interference effects. For an addressed state, spin and orbital dynamics are solely from the state interference, where the spin and orbital momenta oscillate with the laser frequency and match the dipole moment exactly, an unambiguous test case for the time-resolved magneto-optical Kerr effect. For an undressed state, the interference effect introduces a rapid beating in orbital momentum, which is observed in the first-principles calculation in fcc Ni. The state population change leads to a constant spin-orbit ratio, which explains the linear dependence between spin and orbital momentum changes within 2 ps upon the arrival of a pump pulse in Fe.     

Quantum effects of nonlinear spin waves in ferromagnets

We investigate a squeezed thermal spin state of nonlinear spin waves in Heisenberg ferromagnets. In this state, the magnon system possesses a new kind of quasiparticle, the dressed magnon, whose mass is a monotonically decreasing function of temperature. The noise of one spin component in the squeezed thermal spin state can be below the noise level in the vacuum state. The magnon system undergoes a first-order phase transition from the normal state to the squeezed thermal spin state. The critical temperature is much lower than the Curie temperature. A possible detection scheme based on a polarized neutron-scattering technique is suggested.     

Stimulated and spontaneous optical generation of electron spin coherence in charged GaAs quantum dots

We report on the coherent optical excitation of electron spin polarization in the ground state of charged GaAs quantum dots via an intermediate charged exciton (trion) state. Coherent optical fields are used for the creation and detection of the Raman spin coherence between the spin ground states of the charged quantum dot. The measured spin decoherence time, which is likely limited by the nature of the spin ensemble, approaches 10 ns at zero field. We also show that the Raman spin coherence in the quantum beats is caused not only by the usual stimulated Raman interaction but also by simultaneous spontaneous radiative decay of either excited trion state to a coherent combination of the two spin states.     

Antiferromagnetism of nuclear matter in the model with effective Gogny interaction

The possibility of ferromagnetic (FM) and antiferromagnetic (AFM) phase transitions in symmetric nuclear matter is analyzed within the framework of a Fermi liquid theory with effective Gogny interaction. It is shown that, at some critical density, nuclear matter with the D1S effective force undergoes a phase transition to the AFM spin state (opposite directions of neutron and proton spins). The self-consistent equations of spin-polarized nuclear matter with the D1S force have no solutions corresponding to FM spin ordering (the same direction of neutron and proton spins) and, hence, the FM transition does not appear. The AFM spin polarization parameter is found for zero and finite temperature. It is shown that the AFM spin polarization parameter of partially polarized nuclear matter at low enough temperatures increases with temperature. The entropy of the AFM spin state for some temperature range is larger than the entropy of the normal state. Nevertheless, the free energy of the AFM spin state is always less than the free energy of the normal state, and the AFM spin-polarized state is preferable for all temperatures below the critical temperature. The text was submitted by the authors in English.     

Coherent optical writing and reading of the exciton spin state in single quantum dots

We demonstrate a one-to-one correspondence between the polarization state of a light pulse tuned to neutral exciton resonances of single semiconductor quantum dots and the spin state of the exciton that it photogenerates. This is accomplished using two variably polarized and independently tuned picosecond laser pulses. The first "writes" the spin state of the resonantly excited exciton. The second is tuned to biexcitonic resonances, and its absorption is used to "read" the exciton spin state. The absorption of the second pulse depends on its polarization relative to the exciton spin direction. Changes in the exciton spin result in corresponding changes in the intensity of the photoluminescence from the biexciton lines which we monitor, obtaining thus a one-to-one mapping between any point on the Poincaré sphere of the light polarization to a point on the Bloch sphere of the exciton spin.     

Spin filter,spin amplifier and spin diode in graphene nanodisk

Graphene nanodisk is a graphene derivative with a closed edge. The trigonal zigzag nanodisk with size N has N-fold degenerated zero-energy states. It can be interpreted as a quantum dot with an internal degree of freedom. The ground state of nanodisk is a quasi-ferromagnet, which is a ferromagnetic-like state with a finite but very long life time. We investigate spin-filter effects in the system made of nanodisks and leads. A novel feature of the nanodisk spin filter is that its spin can be controlled by the spin current. We propose some applications for spintronics, such as spin memory, spin amplifier and spin diode. It is argued that a spin current is reinforced (rectified) by feeding it into a nanodisk spin amplifier (diode). Graphene nanodisk would be a promising candidate of future electronic and spintronic nanodevices.     

Excited-state spectroscopy using single spin manipulation in diamond

We use single-spin resonant spectroscopy to study the spin structure in the orbital excited state of a diamond nitrogen-vacancy (N-V) center at room temperature. The data show that the excited-state spin levels have a zero-field splitting that is approximately half of the value of the ground state levels, a g factor similar to the ground state value, and a hyperfine splitting approximately 20x larger than in the ground state. In addition, the width of the resonances reflects the electronic lifetime in the excited state. We also show that the spin level splitting can significantly differ between N-V centers, likely due to the effects of local strain, which provides a pathway to control over the spin Hamiltonian and may be useful for quantum-information processing.     

Observation of coherent oscillations in a single electron spin

Rabi nutations and Hahn echo modulation of a single electron spin in a single defect center have been observed. The coherent evolution of the spin quantum state is followed via optical detection of the spin state. Coherence times up to several microseconds at room temperature have been measured. Optical excitation of the spin states leads to decoherence. Quantum beats between electron spin transitions in a single spin Hahn echo experiment are observed. A closer analysis reveals that beats also result from the hyperfine coupling of the electron spin to a single 14N nuclear spin. The results are analyzed in terms of a density matrix approach of an electron spin interacting with two oscillating fields.