arXiv subjects

Takashi Nakajima

Publications and source records attributed to Takashi Nakajima.

51 records · Page 3Linked to original sources

arXiv2015

Single-electron Spin Resonance in a Quadruple Quantum Dot

Electron spins in semiconductor quantum dots are good candidates of quantum bits for quantum information processing. Basic operations of the qubit have been realized in recent years: initialization, manipulation of single spins, two qubit entanglement operations, and readout. Now it becomes crucial to demonstrate scalability of this architecture by conducting spin operations on a scaled up system. Here, we demonstrate single-electron spin resonance in a quadruple quantum dot. A few-electron quadruple quantum dot is formed within a magnetic field gradient created by a micro-magnet. We oscillate the wave functions of the electrons in the quantum dots by applying microwave voltages and this induces electron spin resonance. The resonance energies of the four quantum dots are slightly different because of the stray field created by the micro-magnet and therefore frequency-resolved addressable control of the electron spin resonance is possible.

cond-mat.mes-hall↗

arXiv2015

Singular-value decomposition using quantum annealing

In the present study, we demonstrate how to perform, using quantum annealing, the singular value decomposition and the principal component analysis. Quantum annealing gives a way to find a ground state of a system, while the singular value decomposition requires the maximum eigenstate. The key idea is to transform the sign of the final Hamiltonian, and the maximum eigenstate is obtained by quantum annealing. Furthermore, the adiabatic time scale is obtained by the approximation focusing on the maximum eigenvalue.

quant-ph↗

arXiv2015

Robust micro-magnet design for fast electrical manipulations of single spins in quantum dots

Tailoring spin coupling to electric fields is central to spintronics and spin-based quantum information processing. We present an optimal micromagnet design that produces appropriate stray magnetic fields to mediate fast electrical spin manipulations in nanodevices. We quantify the practical requirements for spatial field inhomogeneity and tolerance for misalignment with spins, and propose a design scheme to improve the spin-rotation frequency (to exceed 50MHz in GaAs nanostructures). We then validate our design by experiments in separate devices. Our results will open a route to rapidly control solid-state electron spins with limited lifetimes and to study coherent spin dynamics in solids.

cond-mat.mes-hall↗

arXiv2015

Spin-dependent current through a quantum dot from spin-polarized non-equilibrium quantum Hall edge channels

We report selective injection of both spin-up and spin-down single electrons into a quantum dot (QD) from spin-polarized non-equilibrium quantum Hall edge channels (ECs) generated by selective transmission of spin-resolved ECs using a surface gate placed at a distance from the QD. We change the spin polarization of non-equilibrium ECs by changing the bias voltages applied to different source Ohmic contacts. The efficiency of spin-up electron injection reaches 0.5, which is approximately 0.2 higher than that induced by spin-dependent tunnel coupling between QD and ECs. On the other hand, the efficiency of spin-down electron injection reaches 0.4. In addition, we rectify the underestimation of the efficiency of spin filtering for equilibrium ECs by numerically subtracting the contribution of the excited states in the QD. The obtained spin-filtering efficiency is higher than that evaluated from the raw experimental data and increases with magnetic field as expected with the increase in the spatial separation between ECs.

cond-mat.mes-hall↗

arXiv2014

Propagation of two short laser pulse trains in a $Λ$-type three-level medium under conditions of electromagnetically induced transparency

We investigate the dynamics of a pair of short laser pulse trains propagating in a medium consisting of three-level $Λ$-type atoms by numerically solving the Maxwell-Schrödinger equations for atoms and fields. By performing propagation calculations with different parameters, under conditions of electromagnetically induced transparency, we compare the propagation dynamics by a single pair of probe and coupling laser pulses and by probe and coupling laser pulse trains. We discuss the influence of the coupling pulse area, number of pulses, and detunings on the probe laser propagation and realization of electromagnetically induced transparency conditions, as well on the formation of a dark state.

physics.atom-ph↗

arXiv2014

Negative refraction of inhomogeneous waves in lossy isotropic media

We theoretically study negative refraction of inhomogeneous waves at the interface of lossy isotropic media. We obtain explicit (up to the sign) expressions for the parameters of a wave transmitted through the interface between two lossy media characterized by complex permittivity and permeability. We show that the criterion of negative refraction that requires negative permittivity and permeability can be used only in the case of a homogeneous incident wave at the interface between a lossless and lossy media. In a more general situation, when the incident wave is inhomogeneous, or both media are lossy, the criterion of negative refraction becomes dependent on an incident angle. Most interestingly, we show that negative refraction can be realized in conventional lossy materials (such as metals) if their interfaces are properly oriented.

physics.optics↗

arXiv2014

Measurement of energy relaxation in quantum Hall edge states utilizing quantum point contacts

We demonstrated a method to probe local electronic states and energy relaxation in quantum Hall edge states utilizing quantum point contacts. We evaluated relaxation lengths in two cases; first, with electron tunneling, and second, only with energy exchange without tunneling between edge states. The results were consistent with previous experiments and validated our method with quantum point contacts. We applied this method to measure the energy relaxation length around a hotspot in quantum Hall regimes and revealed the possible short-distance relaxation process to be the relaxation due to energy exchange between edge states without electron tunneling.

cond-mat.mes-hall↗

arXiv2013

All-angle collimation of incident light in $\mathbfμ$-near-zero metamaterials

We use the theory of inhomogeneous waves to study the transmission of light in $μ$-near-zero metamaterials. We find the effect of all-angle collimation of incident light, which means that the vector of energy flow in a wave transmitted to a $μ$-near-zero metamaterial is perpendicular to the interface for any incident angles if an incident wave is s-polarized. This effect is similar to the all-angle collimation of incident light recently found through a different theoretical framework in $\varepsilon$-near-zero metamaterials for a p-polarized incident wave [S. Feng, Phys. Rev. Lett. 108, 193904 (2012)]. To provide a specific example, we consider the transmission of light in a negative-index metamaterial in the spectral region with a permeability resonance, and show that all-angle collimation indeed takes place at the wavelength for which the real part of permeability is vanishingly small.

physics.optics↗

arXiv2013

Damage threshold and focusability of mid-infrared free-electron laser pulses gated by a plasma mirror with nanosecond switching pulses

The presence of a pulse train structure of an oscillator-type free-electron laser (FEL) results in the immediate damage of a solid target upon focusing. We demonstrate that the laser-induced damage threshold can be significantly improved by gating the mid-infrared (MIR) FEL pulses with a plasma mirror. Although the switching pulses we employ have a nanosecond duration which does not guarantee the clean wavefront of the gated FEL pulses, the high focusablity is experimentally confirmed through the observation of spectral broadening by a factor of 2.1 when we tightly focus the gated FEL pulses onto the Ge plate.

physics.optics↗

arXiv2013

Diagnosis of the Wavelength Stability of a Mid-Infrared Free-Electron Laser

Wavelength stability of free-electron lasers (FELs) is one of the important parameters for various applications. In this paper we describe two different methods to diagnose the wavelength stability of a mid-infrared (MIR) FEL. The first one is based on autocorrelation which is usually used to measure the pulse duration, and the second one is based on frequency upconversion through sum-frequency mixing (SFM).

physics.optics↗

arXiv2013

Two-, three-, and four-photon ionization of Mg in the circularly and linearly polarized laser fields: Comparative study using the Hartree-Fock and model potentials

We theoretically study multiphoton ionization of Mg in the circularly as well as the linearly polarized laser fields. Specifically two-, three-, and four-photon ionization cross sections from the ground and first excited states are calculated as a function of photon energy. Calculations are performed using the frozen-core Hartree-Fock and also the model potential approaches and the results are compared. We find that the model potential approach provide results as good as or even slightly better than those by the frozen-core Hartree-Fock approach. We also report the relative ratios of the ionization cross sections by the circularly and linearly polarized laser fields as a function of photon energy, which exhibit clear effects of electron correlations.

physics.atom-ph↗

arXiv2013

Effects of laser polarization on photoelectron angular distribution through laser-induced continuum structure

We theoretically investigate the effects of laser polarization on photoelectron angular distribution through laser-induced continuum structure. We focus on a polarization geometry where the probe and dressing lasers are both linearly polarized, and change the relative polarization angle between them. We find that the total ionization yield and the branching ratio into different ionization channels change as a function of the relative polarization angle, and accordingly the photoelectron angular distribution is altered. We present specific results for the $4p_{1/2}$-$6p_{1/2}$ and $4p_{3/2}$-$6p_{3/2}$ systems of the K atom, and show that the change of the polarization angle leads to the significant modification of photoelectron angular distribution.

physics.atom-ph↗

arXiv2013

Multiphoton ionization through the triplet states of Mg by linearly and circularly polarized laser pulses

We theoretically study multiphoton ionization through the triplet states of Mg by linearly polarized (LP) and circularly polarized (CP) fs laser pulses. After the construction of the atomic basis using the frozen-core Hartree-Fock potential (FCHFP) as well as the model potential (MP) approaches for both singlet and triplet series which show rather good agreements with the existing data in terms of state energies and dipole matrix elements, we solve time-dependent Schrödinger equations with $3s3p$ $^{3}P_{1}$ as an initial state, and calculate the total ionization yield and photoelectron energy spectra (PES).

physics.atom-ph↗

arXiv2013

Multiphoton ionization of the calcium atom by linearly and circularly polarized laser fields

We theoretically study multiphoton ionization of the Ca atom irradiated by the second (photon energy 3.1 eV) and third (photon energy 4.65 eV) harmonics of Ti:sapphire laser pulses (photon energy 1.55 eV). Because of the dense energy level structure the second and third harmonics of a Ti:sapphire laser are nearly single-photon resonant with the ${4s4p}$ $^1P^o$ and ${4s5p}$ $^1P^o$ states, respectively. Although two-photon ionization takes place through the near-resonant intermediate states with the same symmetry in both cases, it turns out that there are significant differences between them. The photoelectron energy spectra exhibit the absence/presence of substructures. More interestingly, the photoelectron angular distributions clearly show that the main contribution to the ionization processes by the third harmonic arises from the far off-resonant ${4s4p}$ $^1P^o$ state rather than the near-resonant ${4s5p}$ $^1P^o$ state. These findings can be attributed to the fact that the dipole moment for the ${4s^2}$ $^1S^e$ - ${4s5p}$ $^1P^o$ transition is much smaller than that for the ${4s^2}$ $^1S^e$ - ${4s4p}$ $^1P^o$ transition.

physics.atom-ph↗

arXiv2012

Single-shot spectra of temporally selected micropulses from a mid-infrared free-electron laser by upconversion

We demonstrate the measurement of single-shot spectra of temporally selected micropulses from a mid-infrared (MIR) free-electron laser (FEL) by upconversion. We achieve the upconversion of FEL pulses at 11 μm using externally synchronized Nd:YAG or microchip laser pulses at 1064 nm to produce sum-frequency mixing (SFM) signals at 970 nm, which are detected by a compact CCD spectrometer without an intensifier. Our experimental system is very cost-effective, and allows us to obtain the laser spectra of selected micropulses at any temporal position within a single macropulse from an oscillator-type FEL.

physics.optics↗