Yasuyoshi Mitsumori

Coherent transfer of light polarization to electron spins in a semiconductor.

We demonstrate that the superposition of light polarization states is coherently transferred to electron spins in a semiconductor quantum well. By using time-resolved Kerr rotation, we observe the initial phase of Larmor precession of electron spins whose coherence is transferred from light. To break the electron-hole spin entanglement, we utilized the big discrepancy between the transverse g factors of electrons and light-holes. The result encourages us to make a quantum media converter between flying photon qubits and stationary electron-spin qubits in semiconductors.

Spin state tomography of optically injected electrons in a semiconductor

Spin is a fundamental property of electrons, with an important role in information storage. For spin-based quantum information technology, preparation and read-out of the electron spin state are essential functions. Coherence of the spin state is a manifestation of its quantum nature, so both the preparation and read-out should be spin-coherent. However, the traditional spin measurement technique based on Kerr rotation, which measures spin population using the rotation of the reflected light polarization that is due to the magneto-optical Kerr effect, requires an extra step of spin manipulation or precession to infer the spin coherence. Here we describe a technique that generalizes the traditional Kerr rotation approach to enable us to measure the electron spin coherence directly without needing to manipulate the spin dynamics, which allows for a spin projection measurement on an arbitrary set of basis states. Because this technique enables spin state tomography, we call it tomographic Kerr rotation. We demonstrate that the polarization coherence of light is transferred to the spin coherence of electrons, and confirm this by applying the tomographic Kerr rotation method to semiconductor quantum wells with precessing and non-precessing electrons. Spin state transfer and tomography offers a tool for performing basis-independent preparation and read-out of a spin quantum state in a solid.

All-optical phase modulations in a silicon wire waveguide at ultralow light levels

Cross-phase modulation(XPM) in a silicon wire waveguide at 1.55 μ m telecom band was studied down to ultralow light levels. In the low-power regime, we found that free-carrier dispersion as well as the optical Kerr effect contributes to the XPM. Possible mechanisms of the low-power XPM are discussed.

High-visibility nonclassical interference between pure heralded single photons and weak coherent photons

We present an experiment of nonclassical interference between an intrinsically pure heralded single-photon state and a weak coherent state. Our experiment demonstrates that, without the use of bandpass filters, spectrally pure single photons can have high-visibility (89.4{+-}0.5%) interference with photons from a weak coherent field. Our scheme lays the groundwork for future experiments requiring quantum interference between photons in nonclassical states and those in coherent states.

Four-photon quantum interferometry at a telecom wavelength

Center for Frontier Science and Engineering, University of Electro-Communications, Tokyo 182-8585, Japan(Dated: December 12, 2011)We report the experimental demonstration of four-photon quantum interference using telecom-wavelength photons. Realization of multi-photon quantum interference is essential to linear opticsquantum information processing and measurement-based quantum computing. We have developeda source that efficiently emits photon pairs in a pure spectrotemporal mode at a telecom wavelengthregion, and have demonstrated the quantum interference exhibiting the reduced fringe intervals thatcorrespond to the reduced de Broglie wavelength of up to the four photon ‘NOON’ state. Our resultshould open a path to practical quantum information processing using telecom-wavelength photons.

Theory of multiwave mixing and decoherence control in a qubit array system

We develop a theory to analyze the decoherence effect in a charged qubit array system with photon-echo signals in the multiwave mixing configuration. We present how the decoherence-suppression effect by the bang-bang control with the

Experimental demonstration of quantum source coding

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Polarization transfer from photon to electron spin in g factor engineered quantum wells

pulses can be demonstrated in the laboratory by using a bulk ensemble of exciton qubits and optical pulses with a pulse area that is even smaller than

Entangled-state generation with an intrinsically pure single-photon source and a weak coherent source

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Lossless all-optical phase gate using a polarization-division Sagnac interferometer applicable to a waveguide-type Kerr medium

. Analysis is made on the time-integated multiwave mixing signals diffracted into certain phase-matching directions from a bulk ensemble. Depending on the pulse-interval conditions, the crossover from the decoherence-acceleration regime to the decoherence-suppression regime, which is a peculiar feature of the coherent interaction between a qubit and the reservoir bosons, may be observed in the time-integated multiwave mixing signals in the realistic case, including the inhomogeneous broadening effect. Our analysis will be successfully applied to the precise estimation of the reservoir parameters from the experimental data of the direction-resolved signal intensities obtained in the multiwave mixing technique.