Manipulation Of Electron Spins Research Articles

Strong electron spin-Hall effect by a coherent optical potential

We demonstrate theoretically that a coherent manipulation of electron spins in low-dimensional semiconductor structures with a spin–orbit coupling by infrared radiation is possible. The proposed approach is based on using a dipole force acting on a two-level system in a nonuniform optical field, similar to that employed in the design of the cold atoms diode. For ballistic electrons the spin-dependent force, proportional to the intensity of external radiation, leads to a spin-Hall effect and the resulting spin separation even if the spin–orbit coupling itself does not allow for these effects. Achievable spatial separation of electrons with opposite spins can be of the order of several tenths of a micron; an order of magnitude larger than that can be produced by the charged impurity scattering in the diffusive regime.

Coherent manipulation of individual electron spin in a double quantum dot integrated with a micromagnet

We report the coherent manipulation of electron spins in a double quantum dot integrated with a micro-magnet. We performed electric dipole spin resonance experiments in the continuous wave (CW) and pump-and-probe modes. We observed two resonant CW peaks and two Rabi oscillations of the quantum dot current by sweeping an external magnetic field at a fixed frequency. Two peaks and oscillations are measured at different resonant magnetic field, which reflects the fact that the local magnetic fields at each quantum dot are modulated by the stray field of a micro-magnet. As predicted with a density matrix approach, the CW current is quadratic with respect to microwave (MW) voltage while the Rabi frequency (\nu_Rabi) is linear. The difference between the \nu_Rabi values of two Rabi oscillations directly reflects the MW electric field across the two dots. These results show that the spins on each dot can be manipulated coherently at will by tuning the micro-magnet alignment and MW electric field.

Electrical manipulation of spins in the Rashba two dimensional electron gas systems

We present our theoretical and experimental studies on manipulation of electron spins based on the Rashba spin-orbit interaction (SOI) in semiconductor heterostructures. Quantum well (QW) thickness dependence of the Rashba SOI strength α is investigated in InP/InGaAs/InAlAs asymmetric QWs by analyzing weak antilocalization. Two different QW thicknesses show inverse Ns dependence of |α| in the same heterostructures. This inverse Ns dependence of |α| is explained by the k⋅p perturbation theory. We confirm that narrow wires are effective to suppress the spin relaxation. Spin interference effects due to spin precession are experimentally studied in small array of mesoscopic InGaAs rings. This is an experimental demonstration of a time reversal Aharonov–Casher effect, which shows that the spin precession angle in an InGaAs channel can be controlled by an electrostatic gate.

Ultrafast optical manipulation of single electron spins in quantum dots

Most schemes for quantum information processing require fast single qubit operations as well as initialization and read-out steps. Here we describe recent experimental realizations of schemes for time-resolved initialization, coherent control, and read-out of a single electron spin state in an individual quantum dot using all-optical techniques. The spin state is manipulated via the optical Stark effect using off-resonant, picosecond-scale optical pulses. The coherent rotation of a single electron spin state through arbitrary angles up to π radians is demonstrated, constituting a single qubit gate. This spin manipulation is directly monitored using time-resolved Kerr rotation spectroscopy allowing for the observation of the coherent evolution of a single electron spin state in time as well as revealing the spin lifetime and g -factor.

Tuning the exchange interaction by an electric field in laterally coupled quantumdots

The effect of an external electric field on the exchange interaction has been studied by anexact diagonalization method for two electrons in laterally coupled quantum dots (QDs).We have performed a systematic study of several nanodevices that contain two gate-definedQDs with different shapes and sizes located between source and drain contacts. Theconfinement potential is modeled by two potential wells with a variable range and softness.In all the considered nanodevices, the overall dependence of exchange energyJ on electric fieldF is similar, i.e. for lowfields J increases withincreasing F, while forintermediate fields J reaches a maximum and then abruptly falls to zero ifF exceeds a certain critical value. However, theJ(F) dependence also shows certain characteristic properties that depend onthe nanodevice geometry. We have found that the low- and intermediate-fieldbehavior can be accurately parameterized by a linear functionJ(F) = αF+β, whereα isindependent of the nanodevice geometry and softness of the confinement potential. We have shown thatthe linear J(F) relation appears only if the tunnel coupling between the QDs is weak, i.e. theinterdot separation is sufficiently large. This relation becomes nonlinear for thestrong interdot coupling. For specific nanodevices we have found that theJ(F) dependence exhibits a plateau in a broad electric-field regime. The properties of theexchange energy found in the present paper can be applied to all electrical manipulation ofelectron spin qubits.

All-Optical Manipulation of Electron Spins in Carbon-Nanotube Quantum Dots

We demonstrate theoretically that it is possible to manipulate electron or hole spins all optically in semiconducting carbon nanotubes. The scheme that we propose is based on the spin-orbit interaction that was recently measured experimentally; we show that this interaction, together with an external magnetic field, can be used to achieve optical electron-spin state preparation with a fidelity exceeding 99%. Our results also imply that it is possible to implement coherent spin rotation and measurement using laser fields linearly polarized along the nanotube axis, as well as to convert spin qubits into time-bin photonic qubits. We expect that our findings will open up new avenues for exploring spin physics in one-dimensional systems.

Ultrafast manipulation of electron spins in a double quantum dot device: A real-time numerical and analytical study

We consider a double single-level quantum dot system with two embedded and nonaligned spin impurities to manipulate the magnitude and polarization of the electron-spin density. The device is attached to semi-infinite one-dimensional leads which are treated exactly. We provide a real-time description of the electron-spin dynamics when a sequence of ultrafast voltage pulses acts on the device. The numerical simulations are carried out using a spin-generalized modified version of a recently proposed algorithm for the time propagation of open systems [Kurth et al., Phys. Rev. B 72, 035308 (2005)]. Time-dependent spin accumulations and spin currents are calculated during the entire operating regime, which includes spin-injection and read-out processes. The full knowledge of the electron dynamics allows us to engineer the transient responses and improve the device performance. An approximate rate equation for the electron spin is also derived and used to discuss the numerical results.

Electrical Manipulation of Spins in Nonmagnetic Semiconductors

Electrical manipulation of conduction band electron spins in nonmagnetic semiconductors is achieved by exploiting various manifestations of the spin–orbit interaction. The g -factor engineering combined with bandgap engineering leads to electrical tuning of the g -factor by changing the position of the electron wave function within parabolic quantum wells. Anisotropy of the g -tensor in these structures enables g -tensor modulation resonance through electrical control of the g -tensor. In a complementary approach, the momentum-dependent spin splitting in the conduction band manipulates spins. The strain-induced spin splitting is used to demonstrate coherent rotation of spins, even at zero magnetic field. The effective internal magnetic fields can also be used to drive spin resonance and to electrically polarize electron spins. Also originating from the spin–orbit interaction, the spin Hall effect generates a pure spin current transverse to an applied electric field even in the absence of applied magnetic ...

Theoretical study of optical excitation in quantum dots by circularly polarized light

AbstractWe study theoretically electron states in quantum dots and their excitations by circularly polarized light. First, we calculate the hole states in a quantum dot modeled by three‐dimensional harmonic confinement potential. In the bulk direct‐gap semiconductors, the k · p perturbation method around Γ point adequately yields heavy‐hole, light‐hole and split‐off bands. We adopt the effective‐mass approximation based on the k · p perturbation method and obtain quantized energy levels for holes in the quantum dot. The ground levels are two‐fold degenerate and mainly consist of |j, m 〉 = |3/2, ±3/2〉 components. It should be mentioned that a small amount of |3/2, ±1/2〉 components are coherently mixed in the states. Second, we examine an electron‐hole excitation by the irradiation of circularly polarized light σ–. The main component of the exciton is |3/2, ‐3/2〉 hole|1/2, ‐1/2〉electron. The electron state|1/2, 1/2〉electron is also excited owing to the mixture between |3/2, ±3/2〉 and |3/2, ±1/2〉 components in the hole states, which should lead to an inaccuracy in the manipulation of electron spins. The Overhauser field created by nuclear spins plays a role when it is larger than an external magnetic field. It randomizes the direction of electron spins excited by the circularly polarized light σ‐ in an ensemble of self‐assembled quantum dots. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Electron spin and nuclear spin manipulation in semiconductor nanosystems

AbstractManipulations of electron spin and nuclear spin have been studied in AlGaAs/GaAs semiconductor nanosystems. Non‐local manipulation of electron spins has been realized by using the correlation effect between localized and mobile electron spins in a quantum dot‐ quantum wire coupled system. Interaction between electron and nuclear spins was exploited to achieve a coherent control of nuclear spins in a semiconductor point contact device. Using this device, we have demonstrated a fully coherent manipulation of any two states among the four spin levels of Ga and As nuclei. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Optical manipulation of single electron spin in doped and undoped quantum dots

The optical manipulation of electron spins is of great benefit to solid-state quantum information processing. In this letter, we provide a comparative study on the ultrafast optical manipulation of single electron spin in the doped and undoped quantum dots. The study indicates that the experimental breakthrough can be preliminarily made in the undoped quantum dots, because of the relatively less demand.

Quantum logic processor: Implementation with electronic Mach-Zehnder interferometer

An approach for implementation of quantum logic in electronic Mach-Zehnder interferometer (MZI) has been described in this letter. All single qubit gates can be achieved by electron spin manipulation using Rashba spin-orbit coupling. Double qubit gates can also be implemented using the orbital degree of freedom of the electron. The MZI can be realized with intertwined ballistic nanowires. Spin injection and detection in the system can be done by a mesoscopic Stern-Gerlach apparatus. The system can be coupled in an array to form the quantum logic processor.

Study of the microwave-induced transport through a quantum dot inserted in a 35-GHz loop-gap resonator

We report on the design and performance of an experimental setup dedicated to the high-frequency manipulation of individual electron spins in quantum dots in the milliKelvin temperature range. Engel and Loss [Phys. Rev. Lett. 86 (2001) 004648] proposed how the spin-state can be read out via a charge current through the dot in the Coulomb blockade state. A major challenge is that fast manipulation compared to decoherence requires a large microwave (mw) magnetic field, which is technically difficult to produce on a nanometer scale without much power and without substantial capacitive coupling between the dot and its electromagnetic surrounding. We show that our setup overcomes these difficulties.

Bias-dependent spin relaxation in a [formula omitted]-InAs/AlSb 2DES

Manipulation of electron spin is a critical component of many proposed semiconductor spintronic devices. One promising approach utilizes the Rashba effect by which an applied electric field can be used to reduce the spin lifetime or rotate spin orientation through spin–orbit interaction. The large spin–orbit interaction needed for this technique to be effective typically leads to fast spin relaxation through precessional decay, which may severely limit device architectures and functionalities. An exception arises in [1 1 0]-oriented heterostructures where the crystal magnetic field associated with bulk inversion asymmetry lies along the growth direction and in which case spins oriented along the growth direction do not precess. These considerations have led to a recent proposal of a spin-FET that incorporates a [1 1 0]-oriented, gate-controlled InAs quantum well channel. We report measurements of the electron spin lifetime as a function of applied electric field in a [1 1 0]-InAs 2DES. Measurements made using an ultrafast, mid-IR pump-probe technique indicate that the spin lifetime can be reduced from its maximum to minimum value over a range of less than 0.2 V per quantum well at room temperature.

Experimental observation of the spin-Hall effect in a spin–orbit coupled two-dimensional hole gas

Electrically induced ordering and manipulation of electron spins in semiconductors has a number of practical advantages over the established techniques using circularly polarized light sources, external magnetic fields and spin injection from a ferromagnet. The spin-Hall effect utilizes spin–orbit coupling to induce edge spin accumulation in response to a longitudinal electric field which can be applied locally and lead to low energy consumption devices. We study spin accumulation near the edge of a weakly disordered two-dimensional hole gas (2DHG) in a GaAs/AlGaAs heterostructure where the magnitude of the transverse spin current approaches the intrinsic, disorder independent value, in contrast to the impurity dominated regime observed in 3D electron doped systems. In our experiment, the induced spin polarization is detected by the electroluminescence resulting from two p–n junctions bordering the 2DHG channel. When an electric field is applied across the 2DHG channel, a non-zero out-of-plane component of the spin is optically detected. The sign of the spin depends on the direction of the field and is opposite for the two edges, consistent with theory predictions. We also report and analyze an in-plane spin-polarization effect induced in the device by asymmetric electron–hole recombination.

Theory of ultrafast optical manipulation of electron spins in quantum wells

Based on a multiparticle-state stimulated Raman adiabatic passage approach, a comprehensive theoretical study of the ultrafast optical manipulation of electron spins in quantum wells is presented. In addition to corroborating experimental findings [Gupta et al., Science 292, 2458 (2001)], we improve the expression for the optical-pulse-induced effective magnetic field, in comparison with the one obtained via the conventional single-particle ac Stark shift. Further study of the effect of hole-spin relaxation reveals that, while the coherent optical manipulation of electron spin in undoped quantum wells would deteriorate in the presence of relatively fast hole-spin relaxation, the coherent control in doped systems can be quite robust against decoherence. The implications of the present results on quantum dots will also be discussed.

Electron spin-echo relaxation and envelope modulation of shallow phosphorus donors in silicon

Spins of single donor atoms are attractive candidates for large scale quantum information processing in silicon, since quantum computation can be realized through the manipulation of electron and/or nuclear spins. In this paper we report on two-pulse electron spin-echo experiments on phosphorus shallow donors in natural and $^{28}\mathrm{Si}$-enriched silicon epilayers doped with ${10}^{16}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$ P donors. The experiments address the spin-spin relaxation times and mechanisms and provide, through the electron spin-echo envelope modulation (ESEEM) effect, information on the donor electron wave function. Experiments performed as a function of the pulse turning angle allowed us to measure the exponential relaxation and spectral diffusion times depurated by instantaneous diffusion. According to these results, isotopically purified samples are necessary to reduce the spectral diffusion contribution and the P shallow donors concentration plays a fundamental role in determining the intrinsic phase memory time. ESEEM peaks have been assigned to hyperfine-coupled silicon-29 nuclei at specific crystallographic positions on the basis of a spectral fit procedure including instrumental distortions.

Taking the Hall Effect for a Spin

The Hall effect is a phenomenon where a current is produced in the direction perpendicular to an electric field applied to a solid. Discovered in its original form 126 years ago, new variants of the Hall effect in the absence of a magnetic field that involve the spin rather than the charge of electrons continue to be discovered. In their Perspective, Inoue and Ohno discuss recent work in which two groups have recently reported the existence of spin Hall currents. The exact mechanisms and behavior are still uncertain, but the results may have practical significance to spintronics, a new generation of electronic devices based on manipulation and control of electron spin.

Coherent Manipulation of Coupled Electron Spins in Semiconductor Quantum Dots

We demonstrated coherent control of a quantum two-level system based on two-electron spin states in a double quantum dot, allowing state preparation, coherent manipulation, and projective readout. These techniques are based on rapid electrical control of the exchange interaction. Separating and later recombining a singlet spin state provided a measurement of the spin dephasing time, T2*, of approximately 10 nanoseconds, limited by hyperfine interactions with the gallium arsenide host nuclei. Rabi oscillations of two-electron spin states were demonstrated, and spin-echo pulse sequences were used to suppress hyperfine-induced dephasing. Using these quantum control techniques, a coherence time for two-electron spin states exceeding 1 microsecond was observed.

Control and Detection of Singlet-Triplet Mixing in a Random Nuclear Field

We observed mixing between two-electron singlet and triplet states in a double quantum dot, caused by interactions with nuclear spins in the host semiconductor. This mixing was suppressed when we applied a small magnetic field or increased the interdot tunnel coupling and thereby the singlet-triplet splitting. Electron transport involving transitions between triplets and singlets in turn polarized the nuclei, resulting in marked bistabilities. We extract from the fluctuating nuclear field a limitation on the time-averaged spin coherence time T2* of 25 nanoseconds. Control of the electron-nuclear interaction will therefore be crucial for the coherent manipulation of individual electron spins.