Nobuhiko Yokoshi

Fano effect in a Josephson junction with a quantum dot

We theoretically investigate the Fano effect in dc Josephson current at the absolute zero of temperature. The system under consideration is a double-path Josephson junction in which one path is through an insulating barrier and the other one is through a quantum dot (QD). Here the Kondo temperature is assumed to be much smaller than the superconducting gap, and the Coulomb interaction inside the QD is treated by the Hartree-Fock approximation. It is shown that the Josephson critical current exhibits an asymmetric resonance against the QD energy level. This behavior is caused by the interference between the two tunneling processes between the superconductors: the direct tunneling across the insulating barrier and the resonant one through the QD. Moreover, we find that the Josephson critical current changes its sign around the resonance when the Coulomb interaction is sufficiently strong. Our results suggest that

Enhanced up-conversion of entangled photons and quantum interference under a localized field in nanostructures.

0\text{\ensuremath{-}}\ensuremath{\pi}

Synchronization Dynamics in a Designed Open System

transition is induced by the cooperation of the Fano effect and the Coulomb interaction inside the QD.

Upconverted photoluminescence induced by radiative coupling between excitons

We theoretically investigate the up-conversion process of two entangled photons on a molecule, which is coupled by a cavity or nanoscale metallic structure. Within one-dimensional input-output theory, the propagators of the photons are derived analytically and the up-conversion probability is calculated numerically. It is shown that the coupling with the nanostructure clearly enhances the process. We also find that the enhancement becomes further pronounced for some balanced system parameters, such as the quantum correlation between photons, radiation decay, and coupling between the nanostructure and molecule. The nonmonotonic dependencies are reasonably explained in view of quantum interference between the coupled modes of the whole system. This result indicates that controlling quantum interference and correlation is crucial for few-photon nonlinearity, and provides a new guidance to wide variety of fields, e.g., quantum electronics and photochemistry.

Electrical measurement of a two-electron spin state in a double quantum dot.

We theoretically propose a unifying expression for synchronization dynamics between two-level constituents. Although synchronization phenomena require some substantial mediators, the distinct repercussions of their propagation delays remain obscure, especially in open systems. Our scheme directly incorporates the details of the constituents and mediators in an arbitrary environment. As one example, we demonstrate the synchronization dynamics of optical emitters on a dielectric microsphere. We reveal that the whispering gallery modes (WGMs) bridge the well-separated emitters and accelerate the synchronized fluorescence, known as superfluorescence. The emitters are found to overcome the significant and nonuniform retardation, and to build up their pronounced coherence by the WGMs, striking a balance between the roles of resonator and intermediary. Our work directly illustrates the dynamical aspects of many-body synchronizations and contributes to the exploration of research paradigms that consider designed open systems.

Two-photon up-conversion affected by inter-molecule correlations near metallic nanostructure

We propose an unconventional scheme of photoluminescence in a semiconductor thin film, where the nonlocal correlation between an excitonic wave and light wave prominently enhances the interaction between different excitonic states via radiation beyond the long-wavelength approximation (the so-called excitonic superradiance regime). On the basis of the developed method extending input-output theory, we elucidate atypical photoluminescence effects due to the strong wave-wave correlation. In particular, the upconverted photoluminescence based on the coherent quantum superposition of excitons is found to be highly efficient, i.e., it can be realized by weak pumping without auxiliary systems such as cavities or photonic antennas.

Up-Converted Luminescence of a Two-Level Molecule with Population Inversion

We propose a scheme for electrical measurement of two-electron spin states in a semiconductor double quantum dot. We calculated the adiabatic charge transfer when surface gates are modulated in time. Because of spin-orbit coupling in the semiconductor, spatial displacement of the electrons causes a total spin rotation. It follows that the expectation value of the transferred charge reflects the relative phase as well as the total spin population of a prepared singlet-triplet superposition state. The precise detection of the charge transfer serves to identify the quantum superposition.

Magnetic spin modulation by optical vortex-induced spin-spin interaction

We investigate an efficient two-photon up-conversion process in more than one molecule coupled to an optical antenna. In the previous work [Y. Osaka et al., PRL 112, 133601 (2014)], we considered the two-photon up-conversion process in a single molecule within one-dimensional input-output theory, and revealed that controlling the antenna-molecule coupling enables the efficient up-conversion with radiative loss in the antenna suppressed. In this work, aiming to propose a way to enhance the total probability of antenna-photon scattering, we extend the model to the case of multiple molecules. In general, the presence of more than one molecule decreases the up-conversion probability because they equally share the energy of the two photons. However, it is shown that we can overcome the difficulty by controlling the inter-molecule coupling. Our result implies that, without increasing the incident photon number (light power), we can enlarge the net probability of the two-photon up-conversion.

Weak-Light Nonlinearity Using a Dark State in Coupled Quantum Dots

We theoretically study a novel up-converted luminescence from a two-level molecule coupled with an auxiliary system such as an anharmonic LC resonator and a metallic nano-antenna. The auxiliary system is assumed to be strongly driven by a continuous wave laser so that a Mollow triplet appears in the luminescent spectrum. When the peak energy of the upper Mollow triplet sideband is tuned to the molecular excited level, the intensity of the up-converted luminescence is considerably enhanced. At the same time, the population of the two-level molecule is inverted. We also discuss the conditions for high photoemission efficiency in the up-converted luminescence under a highly inverted population. This result indicates the potential applications in future single-molecule lasers.

Remarkable nonlinear optical effect in plasmon-assisted radiation force

We investigate how an optical vortex radiation modulates magnetic spin order of a metallic chiral magnet. The optical vortex carries its intrinsic orbital angular momentum and has a toroidal field intensity, hence such a helical beam is expected to couple to angular momentum of electrons. Here we theoretically construct microscopic interactions between an optical vortex and electrons in the chiral magnet. As a result, we derive a spin-spin interaction which is induced by the optical vortex radiation in the one-dimensional tight-binding model, and find that this interaction can modulate the magnetic order. The optical vortex should be one of the plausible candidates for spin control source, and open a new door to future spintronics devices.