Coherent Spin Rotation Research Articles

Complete quantum control of a single quantum dot spin using ultrafast optical pulses

A basic requirement for quantum information processing systems is the ability to completely control the state of a single qubit. For qubits based on electron spin, a universal single-qubit gate is realized by a rotation of the spin by any angle about an arbitrary axis. Driven, coherent Rabi oscillations between two spin states can be used to demonstrate control of the rotation angle. Ramsey interference, produced by two coherent spin rotations separated by a variable time delay, demonstrates control over the axis of rotation. Full quantum control of an electron spin in a quantum dot has previously been demonstrated using resonant radio-frequency pulses that require many spin precession periods. However, optical manipulation of the spin allows quantum control on a picosecond or femtosecond timescale, permitting an arbitrary rotation to be completed within one spin precession period. Recent work in optical single-spin control has demonstrated the initialization of a spin state in a quantum dot, as well as the ultrafast manipulation of coherence in a largely unpolarized single-spin state. Here we demonstrate complete coherent control over an initialized electron spin state in a quantum dot using picosecond optical pulses. First we vary the intensity of a single optical pulse to observe over six Rabi oscillations between the two spin states; then we apply two sequential pulses to observe high-contrast Ramsey interference. Such a two-pulse sequence realizes an arbitrary single-qubit gate completed on a picosecond timescale. Along with the spin initialization and final projective measurement of the spin state, these results demonstrate a complete set of all-optical single-qubit operations.

Tuning of single to multi-domain behavior for monodispersed ferromagnetic cobalt nanoparticles

Self-assembled monodispersed ferromagnetic fcc cobalt nanoparticles with sizes varying from 6 to 24 nm were synthesized by reductive thermal decomposition of cobalt–oleyl amine complex in the presence of various surfactants such as oleic acid, polyvinyl pyrollidone and trioctylphosphine. The magnetic measurements at 5 and 300 K established the single to multi-domain behavior. The highest coercivity for the nanoparticles was found to be of the values 622 and 210 Oe at 5 and 300 K respectively. This is due to the coherent rotation of spins.

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.

Coherent spin rotations in open driven double quantum dots

We analyze coherent spin rotations in a dc biased double quantum dot driven by crossed dc and ac magnetic fields. In this configuration, spatial delocalization due to interdot tunneling competes with intradot spin rotations induced by the time dependent magnetic field, giving rise to a complicated time dependent behavior of the tunneling current. When the Zeeman splitting has the same value in both dots and spin flip is negligible, the electrons remain in the triplet subspace performing coherent spin rotations and current does not flow. This electronic trapping is removed either by finite spin relaxation or when the Zeeman splitting is different in each quantum dot. In the last case, we will show that by applying a resonant bichromatic magnetic field, the electrons become trapped in a coherent superposition of states and electronic transport is blocked. Then, manipulating ac magnetic fields allows one to drive electrons to perform coherent spin rotations which can be unambiguously detected by direct measurement of the tunneling current.

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 ...

Dual Nature of Improper Ferroelectricity in a Magnetoelectric Multiferroic

Using first-principles calculations, we study the microscopic origin of ferroelectricity (FE) induced by magnetic order in the orthorhombic HoMnO3. We obtain the largest ferroelectric polarization observed in the whole class of improper magnetic ferroelectrics to date. We find that the two proposed mechanisms for FE in multiferroics, lattice and electronic based, are simultaneously active in this compound: a large portion of the ferroelectric polarization arises due to quantum-mechanical effects of electron orbital polarization, in addition to the conventional polar atomic displacements. An interesting mechanism for switching the magnetoelectric domains by an electric field via a 180 degrees coherent rotation of Mn spins is also proposed.

Electron spin resonance in double quantum dots

We analyze the charge and spin dynamics in a double quantum dot driven by crossed DC and AC magnetic fields. The AC field produces coherent electron spin rotations within each quantum dot. In a double quantum dot, these oscillations compete with coherent inter-dot tunneling, giving rise to a complicated time dependent behavior. We present some simple cases which can be treated analytically and describe the electron dynamics as a function of the AC frequency in resonant and non-resonant conditions. In fact, for different Zeeman splittings, just one electron is in resonance with the AC magnetic field. Our results show that the electron dynamics depends also on the off resonant electron.

Driven coherent oscillations of a single electron spin in a quantum dot

The ability to control the quantum state of a single electron spin in a quantum dot is at the heart of recent developments towards a scalable spin-based quantum computer. In combination with the recently demonstrated controlled exchange gate between two neighbouring spins, driven coherent single spin rotations would permit universal quantum operations. Here, we report the experimental realization of single electron spin rotations in a double quantum dot. First, we apply a continuous-wave oscillating magnetic field, generated on-chip, and observe electron spin resonance in spin-dependent transport measurements through the two dots. Next, we coherently control the quantum state of the electron spin by applying short bursts of the oscillating magnetic field and observe about eight oscillations of the spin state (so-called Rabi oscillations) during a microsecond burst. These results demonstrate the feasibility of operating single-electron spins in a quantum dot as quantum bits.

Perpendicular magnetization reversal, magnetic anisotropy, multistep spin switching, and domain nucleation and expansion in Ga1−xMnxAs films

We present a comprehensive study of the reversal process of perpendicular magnetization in thin layers of the ferromagnetic semiconductor Ga1−xMnxAs. For this investigation we have purposely chosen Ga1−xMnxAs with a low Mn concentration (x≈0.02), since in such specimens contributions of cubic and uniaxial anisotropy parameters are comparable, allowing us to identify the role of both types of anisotropy in the magnetic reversal process. As a first step we have systematically mapped out the angular dependence of ferromagnetic resonance in thin Ga1−xMnxAs layers, which is a highly effective tool for obtaining the magnetic anisotropy parameters of the material. The process of perpendicular magnetization reversal was then studied by magnetotransport (i.e., Hall effect and planar Hall-effect measurements). These measurements enable us to observe coherent spin rotation and noncoherent spin switching between the (100) and (010) planes. A model is proposed to explain the observed multistep spin switching. The agreement of the model with experiment indicates that it can be reliably used for determining magnetic anisotropy parameters from magnetotransport data. An interesting characteristic of perpendicular magnetization reversal in Ga1−xMnxAs with low x is the appearance of double hysteresis loops in the magnetization data. This double-loop behavior can be understood by generalizing the proposed model to include the processes of domain nucleation and expansion.

In-situ Lorentz microscopy and Lorentz phase imaging of artificially structured Co arrays

We combined in-situ magnetizing experiments with Lorentz Fresnel microscopy and Lorentz phase microscopy to analyse both qualitatively and quantitatively the in-situ magnetization process of artificially structured two dimensional arrays of Co islands. A newly developed Lorentz phase microscopy allows detailed mapping of in-plane magnetization on a nanometre scale following the hysteresis loop. For polycrystalline Co films, 20–40 nm thick, we observed four major steps in the magnetization process: coherent spin rotation in adjacent nanodomains, nucleation of reverse domains, motion of the domain walls and additional spin rotation in magnetic domains and, finally, an expulsion of energetically non-favourable domains followed by the domain wall annihilation at the edges of Co islands. We offer a clear physical explanation of the magnetization ripple contrast.

Switching behavior of single nanoscale permalloy wires studied with magnetoresistance

Magnetoresistance measurements are performed at 2, 4.2 and 300 K on single nanoscale permalloy (Py) wires of widths ( w ) ranging from 0.47 to 10.4 μm . The magnetization reversal process in transverse magnetic field is discussed in terms of coherent spin rotation and in longitudinal magnetic field the switching field depends linearly on the reciprocal width indicating an incoherent magnetization reversal.

Switching behavior of single nanoscale permalloy wires studied with magnetoresistance

Abstract Magnetoresistance measurements are performed at 2, 4.2 and 300 K on single nanoscale permalloy (Py) wires of widths ( w ) ranging from 0.47 to 10.4 μm . The magnetization reversal process in transverse magnetic field is discussed in terms of coherent spin rotation and in longitudinal magnetic field the switching field depends linearly on the reciprocal width indicating an incoherent magnetization reversal.

Magnetization process of a single chain of nanomagnets

We have investigated the magnetization reversal process of a single chain of identical nanomagnetic dots fabricated from 30 nm thick Ni80Fe20. The structures consist of two 5 μm wide support wires bridged with a single chain of identical dots of diameter δ in the range 100–250 nm. For fields applied perpendicular to the single chain, we observed an unusual size dependent hysteretic behavior in the magnetoresistance curve at high field. This is due to the magnetostatic interaction arising from the proximity of the magnetic charges. We are able to deduce from a simple micromagnetic simulation that the reversal process in the chain of dots with δ=100 nm is mediated by a collective process of nearly coherent spin rotation. The magnetotransport measurements along the chain reveal a complex magnetization reversal process in the chain of nanomagnets.

Electron and hole spin relaxation in modulation-doped CdMnTe quantum wells

We report on the electron and hole spin relaxation times in n-type and p-type CdMnTe quantum wells with nonmagnetic barriers measured via the time-resolved magneto-optical Kerr effect in a pump-probe experiment. In addition to the decay of the Kerr effect at short pump-probe delays, we show that the hole spin relaxation time may be also estimated from a phase shift induced during the coherent rotation of the Mn spins. The variations of the spin relaxation times are studied as a function of Mn concentration, doping level, and magnetic field applied parallel to the quantum well plane. The results show that the electron spin relaxation is dominated by the exchange scattering with the Mn ions in the quantum well, and good agreement with theory is obtained provided exciton effects are included. Relatively slow hole spin relaxation is observed in the p-type samples (hole spin lifetime up to 32 ps) as compared to published values for similar diluted magnetic II-VI heterostructures.

Anomalous hysteresis behavior in a cobalt ferrite ferrofluid

The magnetization of cobalt ferrite ferrofluid has been studied at low temperatures in the presence of the carrier fluid. A prominent difference between slow and fast cooled samples was observed. In particular the slow cooled samples display an anomalous increase in magnetization on field reversal. The data is analyzed in terms of the possibilities of both coherent and incoherent spin rotations.

Rotational loss separation in grain-oriented Fe–Si

Rotational and alternating magnetic losses have been investigated in high permeability grain-oriented laminations as a function of frequency (2.5 Hz⩽f⩽150 Hz) and peak magnetization (0.15 T⩽Ip⩽1.7 T). The results have been analyzed according to the concept of loss separation. The rotational hysteresis Whrot and excess Wexcrot energy loss components exhibit a similar increasing behavior with the magnetization Ip and reach a maximum value around 1.5–1.6 T. Wexcrot shows, in particular, a definite f1/2 dependence. The ratios between rotational and alternating hysteresis and excess losses (Rh=Whrot/Whalt, Rexc=Wexcrot/Wexcalt) follow similar trends with Ip, with their values depending, however, on the testing configuration adopted for the alternating loss measurements (single strip versus X-stacked strips). The 90° domain wall transitions between the in-plane [001] and the out-of-plane [100] and [01̄0] directions are the basic loss producing mechanism under rotational fields, but, on approaching the saturation, the magnetization can eventually rotate only through coherent spin rotations and the classical loss term Wclrot alone survives.

Exchange coupling of Co/Cr(0 0 1) superlattices for in-plane and perpendicular anisotropy

We have investigated the exchange coupling of Co/Cr(0 0 1) superlattices by polar and longitudinal magneto-optical Kerr effect measurements, by varying both the Co and Cr film thicknesses. At a Co thickness of ≈10 Å a nearly perpendicular anisotropy is found with antiferromagnetic order in the range of 5–15 Å of Cr thickness. For these superlattices the magnetization curve starting from remanence to saturation is characterized by a surface spin-flip transition at low field, followed by domain wall nucleation and motion, and finally by a coherent spin rotation with increasing field. Antiferromagnetic coupling is also observed for superlattices with thicker Co layers and with in-plane magnetic anisotropy.

One- and two-dimensional magnetization processes in amorphous alloys

The hysteresis loop and loss properties of near-zero-magnetostrictive amorphous ribbons have been investigated as a function of the induced macroscopic magnetic anisotropy Ku from dc to 10 kHz. It is found that, following the evolution of Ku, imposed by means of sequential field annealings along the longitudinal (LD) and transverse (TD) directions, the hysteresis loop shape and area and the frequency dependence of energy losses suffer dramatic changes, dictated by the value of Ku and the local distributed anisotropies K. On passing from longitudinal to transverse macroscopic easy axis, coherent spin rotations come into play and, in association with the domain wall displacements, eventually provide a net magnetization reversal along both LD and TD. The hysteresis loss component Wh correspondingly increases and the energy loss at high frequencies decreases. It is concluded that: (1) optimal quasi-static magnetic behavior requires relatively high and positive Ku values (>50–100 J/m3), with longitudinal plane rigid walls and (2) the best high frequency properties are attained for Ku∼0, where flexible walls and complex domain patterns are created.

Energy losses and magnetization curve in field annealed amorphous ribbons

Energy losses and magnetization curves have been investigated from dc to 20 kHz in transversally field annealed Co-based amorphous ribbons. The envisaged loss classical limit is never approached, even though sharp transverse domain structures can be obtained, with coherent spin rotations accounting for most of the magnetization reversal. A rationale for this effect is found in a persistent wall motion, driven by the evolution with spin rotation of the magnetostatic free charge at the ribbon edges.

Reversible magnetization processes in amorphous alloys

The role of coherent moment rotations on the magnetization process of soft amorphous alloys has been investigated in theory and experiment. In particular, a theoretical formulation for the reversible magnetization curve has been worked out by modeling the spatial distribution of the magnetization in ribbon-shaped samples and its evolution under an applied field. Such a distribution is assumed to derive from the combined effects of frozen-in random anisotropies and the macroscopic in-plane anisotropy. Experiments are reported concerning Fe-based alloy ribbons, where a range of macroscopic anisotropy values are achieved through the application of a variable tensile/compressive stress. In order to determine the reversible component of the magnetization curve I rev( H) a method is devised that exploits the susceptibility after-effect, which permits one to discriminate between contributions from coherent spin rotations and domain wall displacements. The so obtained I rev( H) curve and its dependence on stress are found to agree with the theoretical prediction.