Coherent control of the spin current through a quantum dot

Within the weak-coupling regime the spin current through a quantum dot system is calculated using a quantum master equation approach which includes a sum over Matsubara terms. To be able to efficiently calculate, also at low temperatures, the time evolution of the reduced density matrix a high-temperature approximation was derived which proves to be rather accurate in comparison to the exact results. In the present model it is assumed that the energy levels of the dot are split by a constant magnetic field. An additional external (laser) field is used to control the currents of the two spin polarizations. This is either done using the phenomenon of coherent destruction of tunneling or optimal control theory. Scenarios are studied in which the spin current is reversed while the charge current is kept constant.     

Coherent transfer of light polarization to electron spins in a semiconductor

We demonstrate that the superposition of light polarization states is coherently transferred to electron spins in a semiconductor quantum well. By using time-resolved Kerr rotation, we observe the initial phase of Larmor precession of electron spins whose coherence is transferred from light. To break the electron-hole spin entanglement, we utilized the big discrepancy between the transverse g factors of electrons and light-holes. The result encourages us to make a quantum media converter between flying photon qubits and stationary electron-spin qubits in semiconductors.     

Coherent spin transport through a 350 micron thick silicon wafer

We use all-electrical methods to inject, transport, and detect spin-polarized electrons vertically through a 350-micron-thick undoped single-crystal silicon wafer. Spin precession measurements in a perpendicular magnetic field at different accelerating electric fields reveal high spin coherence with at least 13pi precession angles. The magnetic-field spacing of precession extrema are used to determine the injector-to-detector electron transit time. These transit time values are associated with output magnetocurrent changes (from in-plane spin-valve measurements), which are proportional to final spin polarization. Fitting the results to a simple exponential spin-decay model yields a conduction electron spin lifetime (T1) lower bound in silicon of over 500 ns at 60 K.     

Coherent Hall effect in a semiconductor superlattice

The coherent Hall effect denotes the transient Hall response of impulsively excited coherent charge-carrier wave packets in a solid. We report the first experimental study of this phenomenon (i) using a semiconductor superlattice in crossed electric and magnetic fields as a model for three-dimensional materials and (ii) employing a contactless optoelectronic technique to probe the transient currents. Two field regimes with distinctly different oscillatory wave packet dynamics are found, separated from each other by a transition region where all oscillations are suppressed.     

Nonequilibrium spin transport through a diluted magnetic semiconductor quantum dot system with noncollinear magnetization

The spin-dependent transport through a diluted magnetic semiconductor quantum dot (QD) which is coupled via magnetic tunnel junctions to two ferromagnetic leads is studied theoretically. A noncollinear system is considered, where the QD is magnetized at an arbitrary angle with respect to the leads’ magnetization. The tunneling current is calculated in the coherent regime via the Keldysh nonequilibrium Green’s function (NEGF) formalism, incorporating the electron–electron interaction in the QD. We provide the first analytical solution for the Green’s function of the noncollinear DMS quantum dot system, solved via the equation of motion method under Hartree–Fock approximation. The transport characteristics (charge and spin currents, and tunnel magnetoresistance (TMR)) are evaluated for different voltage regimes. The interplay between spin-dependent tunneling and single-charge effects results in three distinct voltage regimes in the spin and charge current characteristics. The voltage range in which the QD is singly occupied corresponds to the maximum spin current and greatest sensitivity of the spin current to the QD magnetization orientation. The QD device also shows transport features suitable for sensor applications, i.e., a large charge current coupled with a high TMR ratio.     

Demonstration of electrical spin injection into a semiconductor using a semimagnetic spin aligner

Spin injection into semiconductors has been a field of growing interest during recent years, because of the large possibilities in basic physics and for device applications that a controlled manipulation of the electrons spin would enable. However, it has proven very difficult to realize such a spin injector experimentally. Here we demonstrate electrical spin injection and detection in a GaAs/AlGaAs p-i-n diode using a semimagnetic II–VI semiconductor (Zn_{1 − x − y}Be_xMn_ySe) as a spin aligner. The degree of circular polarization of the electroluminescence from the diode is related to the spin polarization of the conduction electrons. Thus, it may be used as a detector for injected spin-polarized carriers. Our experimental results indicate a spin polarization of the injected electrons of up to 90% and are reproduced for several samples. The degree of optical polarization depends strongly on the Mn concentration and the thickness of the spin aligner. Electroluminescence from a reference sample without spin aligner as well as photoluminescence after unpolarized excitation in the spin aligner sample show only the intrinsic polarization in an external magnetic field due to the GaAs bandstructure. We can thus exclude side effects from Faraday effect or magnetic circular dichroism in the semimagnetic layer as the origin of the observed circularly polarized electroluminescence.     

Coherent switching of semiconductor resonator solitons

We demonstrate the switching on and off of spatial solitons in a semiconductor microresonator by injection of light coherent with the background illumination. The results give evidence that the formation of the solitons and their switching are not limited by thermal processes. Received: 11 February 2002 / Revised version: 25 April 2002 / Published online: 8 August 2002     

Coherent spectroscopy of semiconductor quantum dots

Coherent spectroscopy of semiconductor quantum dots, such as photon echo, accumulated photon echo, interferometric coherency measurement, quantum beat and coherent phonon measurement, are reviewed in relation to the historical progress in the coherent spectroscopy of semiconductors.     

Coherent state transfer through a multi-channel quantum network: Natural versus controlled evolution passage

Quantum state transfer (QST) is an important task in quantum information processing. In this study, we describe two approaches for the high-fidelity transfer of a quantum state between two opposite quantum dots attached to a multi-channel quantum network. First, we demonstrate that a high-efficiency QST can be achieved with the coherent time evolution of a quantum system without any external control. Second, we present an approach that uses an alternative mechanism for a high-fidelity QST. By adiabatically varying tunnel couplings, it is possible to implement the complete transmission of a quantum state based on this quantum mechanical mechanism.     

Anisotropic spin diffusion in a semiconductor quantum well

We show that spin diffusion of an inhomogeneous spin-density distribution in an asymmetric zinc-blende semiconductor quantum well is anisotropic in coordinate space, if the D’yakonov–Perel’spin-relaxation mechanism is dominant. This anisotropy depends on the relation between the Dresselhaus and Rashba contributions to the spin splitting and reaches its maximum when both contributions are equal in magnitude. Under this condition, the temporal behavior of spin density strongly depends on the relation between the initial spatial extent of the spin packet and spin diffusion length.     

Coherent photonic coupling of semiconductor quantum dots

We report a new type of coupling between quantum dot excitons mediated by the strong single-photon field in a high-finesse micropillar cavity. Coherent exciton coupling is observed for two dots with energy differences of the order of the exciton-photon coupling. The coherent coupling mode is characterized by an anticrossing with a particularly large line splitting of 250 microeV. Because of the different dispersion relations with temperature, the simultaneous photonic coupling of quantum dot excitons can be easily distinguished from cases of sequential strong coupling of two quantum dots.     

Coherent dynamics of magnetoexcitons in semiconductor nanorings

The coherent dynamics of magnetoexcitons in semiconductor nanorings following pulsed optical excitation is studied. The calculated temporal evolution of the excitonic dipole moment may be understood as a superposition of the relative motion of electrons and holes and a global circular motion associated with the magnetic-field splitting of these states. This dynamics of the electron-hole pairs can be generated either by local optical excitation of an ordered ring or, alternatively, by homogeneous excitation of rings with broken rotational symmetry due to disorder or band tilting. Received 27 September 2000     

Coherent single electron spin control in a slanting Zeeman field

We consider a single electron in a 1D quantum dot with a static slanting Zeeman field. By combining the spin and orbital degrees of freedom of the electron, an effective quantum two-level (qubit) system is defined. This pseudospin can be coherently manipulated by the voltage applied to the gate electrodes, without the need for an external time-dependent magnetic field or spin-orbit coupling. Single-qubit rotations and the controlled-NOT operation can be realized. We estimated the relaxation (T1) and coherence (T2) times and the (tunable) quality factor. This scheme implies important experimental advantages for single electron spin control.     

Josephson current through a ferromagnetic semiconductor/semiconductor/ferromagnetic semiconductor structure

We theoretically calculate the Josephson current for two superconductor/ferromagnetic semiconductor (SC/FS) bilayers separated by a semiconductor (SM) layer. It is found that the critical Josephson current I_C in the junction is strongly determined by not only the relative orientations of the effective exchange field of the two bilayers and scattering potential strengths at the interfaces but also the kinds of holes (the heavy or light) in the two FS layers. Furthermore, a robust approach to measuring the spin polarization P for the heavy and light holes is presented.     

Coherent response of a semiconductor microcavity in the strong coupling regime

We have studied the coherent dynamics of a semiconductor microcavity by means of interferometric correlation measurements with subpicosecond time resolution in a backscattering geometry. Evidence is brought of the resolution of a homogeneous polariton line in an inhomogeneously broadened exciton system. Surprisingly, photon-like polaritons exhibit an inhomogeneous dephasing. Moreover, we observe an unexpected stationary coherence up to 8 ps for the lower polariton branch close to resonance. All these experimental results are well reproduced within the framework of a linear dispersion theory assuming a coherent superposition of the reflectivity and resonant Rayleigh scattering signals with a well-defined relative phase.     

Coherent collisional spin dynamics in optical lattices

We report on the observation of coherent, purely collisionally driven spin dynamics of neutral atoms in an optical lattice. For high lattice depths, atom pairs confined to the same lattice site show weakly damped Rabi-type oscillations between two-particle Zeeman states of equal magnetization, induced by spin-changing collisions. Moreover, measurement of the oscillation frequency allows for precise determination of the spin-changing collisional coupling strengths, which are directly related to fundamental scattering lengths describing interatomic collisions at ultracold temperatures.