arXiv subjects

T. Kuroda

Publications and source records attributed to T. Kuroda.

31 records · Page 2Linked to original sources

arXiv2013

Symmetric quantum dots as efficient sources of highly entangled photons

An ideal source of entangled photon pairs combines the perfect symmetry of an atom with the convenient electrical trigger of light sources based on semiconductor quantum dots. We create a naturally symmetric quantum dot cascade that emits highly entangled photon pairs on demand. Our source consists of strain-free GaAs dots self-assembled on a triangular symmetric (111)A surface. The emitted photons strongly violate Bell's inequality and reveal a fidelity to the Bell state as high as 86 (+-2) % without postselection. This result is an important step towards scalable quantum-communication applications with efficient sources.

quant-ph↗

arXiv2012

Magnetic field induced valence band mixing in [111] grown semiconductor quantum dots

We present a microscopic theory of the magnetic field induced mixing of heavy-hole states +/- 3/2 in GaAs droplet dots grown on (111)A substrates. The proposed theoretical model takes into account the striking dot shape with trigonal symmetry revealed in atomic force microscopy. Our calculations of the hole states are carried out within the Luttinger Hamiltonian formalism, supplemented with allowance for the triangularity of the confining potential. They are in quantitative agreement with the experimentally observed polarization selection rules, emission line intensities and energy splittings in both longitudinal and transverse magnetic fields for neutral and charged excitons in all measured single dots.

cond-mat.mes-hall↗

arXiv2012

Angle-resolved photoemission spectroscopy of Co-based boride superconductor LaCo1.73Fe0.27B2

We have performed angle-resolved photoemission spectroscopy of Co-based boride superconductor LaCo1.73Fe0.27B2 (Tc = 4.1 K), which is isostructural to the 122-type Fe-pnictide superconductor with the pnictogen atom being replaced with boron. We found that the Fermi level is located at a dip in the density of states (DOS) in contrast to Co-pnictide ferromagnets. This reduction in DOS together with the strong Co 3d-B 2p covalent bonding removes the ferromagnetic order and may cause the superconductivity. The energy bands near the Fermi level show higher three dimensionality and a weaker electron-correlation effect than those of Fe pnictides. The Fermi surface topology is considerably different from that of Fe pnictides, suggesting the difference in the superconducting mechanism between boride and pnictide superconductors.

cond-mat.supr-con↗

arXiv2011

Dark-bright mixing of interband transitions in symmetric semiconductor quantum dots

In photoluminescence spectra of symmetric [111] grown GaAs/AlGaAs quantum dots in longitudinal magnetic fields applied along the growth axis we observe in addition to the expected bright states also nominally dark transitions for both charged and neutral excitons. We uncover a strongly non-monotonous, sign changing field dependence of the bright neutral exciton splitting resulting from the interplay between exchange and Zeeman effects. Our theory shows quantitatively that these surprising experimental results are due to magnetic-field-induced \pm 3/2 heavy-hole mixing, an inherent property of systems with C_3v point-group symmetry.

cond-mat.mes-hall↗

arXiv2010

Impact of heavy hole-light hole coupling on optical selection rules in GaAs quantum dots

We report strong heavy hole-light mixing in GaAs quantum dots grown by droplet epitaxy. Using the neutral and charged exciton emission as a monitor we observe the direct consequence of quantum dot symmetry reduction in this strain free system. By fitting the polar diagram of the emission with simple analytical expressions obtained from k$\cdot$p theory we are able to extract the mixing that arises from the heavy-light hole coupling due to the geometrical asymmetry of the quantum dot.

cond-mat.mes-hall↗

arXiv2008

Bunching visibility for correlated photons from single GaAs quantum dots

We study photon bunching phenomena associated with biexciton-exciton cascade in single GaAs self-assembled quantum dots. Experiments carried out with a pulsed excitation source show that significant bunching is only detectable at very low excitation, where the typical intensity of photon streams is less than the half of their saturation value. Our findings are qualitatively understood with a model which accounts for Poissonian statistics in the number of excitons, predicting the height of a bunching peak being determined by the inverse of probability of finding more than one exciton.

cond-mat.mtrl-sci↗

arXiv2008

Photon correlation in GaAs self-assembled quantum dots

We report on photon coincidence measurement in a single GaAs self-assembled quantum dot (QD) using a pulsed excitation light source. At low excitation, when a neutral exciton line was present in the photoluminescence (PL) spectrum, we observed nearly perfect single photon emission from an isolated QD at 670 nm wavelength. For higher excitation, multiple PL lines appeared on the spectra, reflecting the formation of exciton complexes. Cross-correlation functions between these lines showed either bunching or antibunching behavior, depending on whether the relevant emission was from a biexciton cascade or a charged exciton recombination.

cond-mat.other↗

arXiv2008

Notes on the bunching peak of g(2) functions for correlated photons from single quantum dots

Second-order correlation functions for photon pulses associated with exciton-biexciton cascades are theoretically derived. A finite efficiency in photon detection and statistical distribution in exciton numbers are taken into account. It is found that the bunching peak height of photon statistics (g(2)(0)) depends on the mean number of excitons, N, and significant bunching is only detectable at very low excitation, N<<2.

cond-mat.mtrl-sci↗

arXiv2006

Final-state read-out of exciton qubits by observing resonantly excited photoluminescence in quantum dots

We report on a new approach to detect excitonic qubits in semiconductor quantum dots by observing spontaneous emissions from the relevant qubit level. The ground state of excitons is resonantly excited by picosecond optical pulses. Emissions from the same state are temporally resolved with picosecond time resolution. To capture weak emissions, we greatly suppress the elastic scattering of excitation beams, by applying obliquely incident geometry to the micro photoluminescence set-up. Rabi oscillations of the ground-state excitons appear to be involved in the dependence of emission intensity on excitation amplitude.

cond-mat.mtrl-sci↗

arXiv2006

Self-assembly of laterally aligned GaAs quantum dot pairs

We report the fabrication of self-assembled, strain-free GaAs/Al$_{0.27}$Ga$_{0.73}$As quantum dot pairs which are laterally aligned in the growth plane, utilizing the droplet epitaxy technique and the anisotropic surface potentials of the GaAs (100) surface for the migration of Ga adatoms. Photoluminescence spectra from a single quantum dot pair, consisting of a doublet, have been observed. Finite element energy level calculations of a model quantum dot pair are also presented.

cond-mat.mtrl-sci↗

arXiv2005

Optical transitions in quantum ring complexes

Making use of a droplet-epitaxial technique, we realize nanometer-sized quantum ring complexes, consisting of a well-defined inner ring and an outer ring. Electronic structure inherent in the unique quantum system is analyzed using a micro-photoluminescence technique. One advantage of our growth method is that it presents the possibility of varying the ring geometry. Two samples are prepared and studied: a single-wall ring and a concentric double-ring. For both samples, highly efficient photoluminescence emitted from a single quantum structure is detected. The spectra show discrete resonance lines, which reflect the quantized nature of the ring-type electronic states. In the concentric double--ring, the carrier confinement in the inner ring and that in the outer ring are identified distinctly as split lines. The observed spectra are interpreted on the basis of single electron effective mass calculations.

cond-mat.mtrl-sci↗

arXiv2002

Picosecond Nonlinear Relaxation of Photoinjected Carriers in a Single GaAs/AlGaAs Quantum Dot

Photoemission from a single self-organized GaAs/AlGaAs quantum dot (QD) is temporally resolved with picosecond time resolution. The emission spectra consisting of the multiexciton structures are observed to depend on the delay time and the excitation intensity. Quantitative agreement is found between the experimental data and the calculation based on a model which characterizes the successive relaxation of multiexcitons. Through the analysis we can determine the carrier relaxation time as a function of population of photoinjected carriers. Enhancement of the intra-dot carrier relaxation is demonstrated to be due to the carrier-carrier scattering inside a single QD.

cond-mat.mtrl-sci↗

arXiv1996

Asteroseismological Observations of the Central Star of the Planetary Nebula NGC 1501

We report on a global CCD time-series photometric campaign to decode the pulsations of the nucleus of the planetary nebula NGC1501. The star is hot and hydrogen-deficient, similar to the pre-white-dwarf PG 1159 stars. NGC1501 shows pulsational brightness variations of a few percent with periods ranging from 19 to 87 minutes. The variations are very complex, suggesting a pulsation spectrum that requires a long unbroken time series to resolve. Our CCD photometry of the star covers a two-week period in 1991 November, and used a global network of observatories. We obtained nearly continuous coverage over an interval of one week in the middle of the run. We have identified 10 pulsation periods, ranging from 5235 s down to 1154 s. We find strong evidence that the modes are indeed nonradial g-modes. The ratios of the frequencies of the largest-amplitude modes agree with those expected for modes that are trapped by a density discontinuity in the outer layers. We offer a model for the pulsation spectrum that includes a common period spacing of 22.3 s and a rotation period of 1.17 days; the period spacing allows us to assign a seismological mass of 0.55+/-0.03 Msun.

astro-ph↗