Received ZZZ, revised ZZZ, accepted ZZZ Published online ZZZ (Dates will be provided by the publisher.)Keywords (quantum dot, single photon, g (2) (0), photon extraction efficiency) We introduce novel metal-embedded semiconductor pillar structure including quantum dots as highly efficient single-photon sources for quantum information applications. We have developed related processes for fabricating the proposed structure, and have demonstrated strongly suppressed multi-photon generation and improved photon extraction efficiency. As a result, under non-resonant excitation, which is highly preferable for the practical use, single-photon purity with g (2) (0) as low as 0.015 has been measured. Extremely small semiconductor volume in the metal embedded structure is a crucial factor for this observation. Moreover, single-photon generation rate from a single quantum dot up to 6.4 million per second has been achieved.Copyright line will be provided by the publisher IntroductionFor quantum information applications, realization of efficient single-photon sources is one of the key issues. Semiconductor quantum dots (QDs) are a good candidate for realizing single-photon sources, entangled-photon-pair sources [1][2][3], and quantum gates [4] etc. Especially, in view of the single-photon sources based on QDs, high-bitrate on-demand generation is expected.However, for the practical use, two main issues are remaining. First, multi-photon generation evaluated by a second-order photon correlation function at zero time delay g (2) (0) should be minimized. For long-distance quantum key distribution, lower g (2) (0) is crucial for practical applications with high bit rate operation [5]. So far, many groups have reported single photon emission from QDs [6][7][8][9], and relatively low g (2) (0) values of 0.02-0.03 were reported in ref. 6, 7. However, these values have been obtained under p-shell resonant excitation which requires strict energy tuning. For practical devices driven by electrical pumping, further suppression of multi-photon generation without strict energy tuning is much preferable. Another issue is to approach the on-demand operation. Generally, photon extraction efficiency from QDs is less than 1% in the case of a planar surface due to large difference
Single-photon as well as polarization-correlated photon pair emission from a single semiconductor quantum dots is demonstrated. Single photon generation and single photon-pair generation with little uncorrelated multiphoton emission and the feasibility of media conversion of the quantum states between photon polarization and electron spin are fundamental functions for quantum information applications. Mutual media conversion for the angular momentum between photon polarization and electron spin is also achieved with high fidelity via positively charged exciton state without external magnetic field. This is a clear indication that the coupling of photon polarizations and electron spins keeps secured during whole processes before photon emission. Possibility of a metal-embedded structure is demonstrated with the observation of drastic enhancement of excitation and/or collection efficiency of luminescence as well as clear antibunching of photons generated from a quantum dot.
Abstract:We experimentally prepare bi-photon mixed states in polarization employing an entangled-classical hybrid photon emitter which can properly model solid-state entangled photon sources with uncorrelated background photons. Polarization-uncorrelated photon pairs in totally mixed (TM) states are embodied with classical thermal radiation, while the polarization-entangled ones in a Bell state are generated by conventional parametric down conversion. The bi-photon states generated from the hybrid photon emitter are characterized in terms of a linear entropy-tangle plane, which reveals the formation of two-qubit Werner states. We also propose a direct way for evaluating the Werner states by means of minimal coincidence counts measurements. This simple method can be widely applicable in examining the bi-photon states from solid-state entangled photon sources, in which the polarization-entangled photon pairs have temporal correlation while the background photons in the TM states do not.
High degree of excitonic spin-state preservation during energy relaxation is demonstrated in a single quantum dot. Optical-phonon resonances and corresponding suppression of spin relaxation are clearly identified as dip structures in photoluminescence excitation spectra probed by the positive trion emission. Consequently, under the longitudinal optical-phonon resonant excitation, a distinguishably high degree of circular polarization up to ϳ0.85 is achieved. Extended spin-relaxation time in the positive trion ground state compared to that in neutral exciton ground state is also revealed. DOI: 10.1103/PhysRevB.78.081306 PACS number͑s͒: 78.67.Hc, 71.35.Ϫy, 72.25.Fe, 72.25.Rb Spin states in a single quantum dot ͑QD͒ can be one of the most fundamental physical platform for quantum bits ͑qu-bits͒ since the spin states in a QD can be much more stabilized than those in higher dimensional systems.1 They are also promising from the viewpoint of realizing practical solid-state devices and their integrations based on highly developed semiconductor technologies. Another essential feature of electronic spin states is the ability to mutually convert into photon polarization states or vise versa via dipole interactions. This enables us to prepare QD-based nonclassical Fock-state photons with well-defined polarizations, which are required in quantum key distribution. 2In order to realize efficient state conversions between exciton spins in a QD and the photon polarizations in number states, exciton spins should be highly preserved in each process of the state conversions. Toward this direction, spinrelaxation mechanisms in a QD have been widely investigated in terms of the electron-hole ͑e-h͒ exchange interaction, 3,4 the spin-orbit interaction, 5-7 as well as the hyperfine interaction [8][9][10][11] for both neutral excitons and singly charged excitons ͑trions͒. Among them, trions are free from the e-h exchange interaction 4 and this makes them more attractive than neutral excitons ͑X 0 ͒. Depolarization of photoexcited electron spins during energy relaxation is generally much slower than hole spins, 12 which makes positively charged trions ͑X + ͒ with spin-singlet hole pairs more attractive than negatively charged trions ͑X − ͒. Furthermore, since the initial and final states of the X + emission are half spin systems, both states are spin degenerate due to Kramer's theorem in the absence of a magnetic field. Thus, the X + state couples to the degenerate two kinds of photons with orthogonal circular polarizations.13 Therefore, the X + state is an excellent spin state to examine the state conversions between the exciton spins in a QD and the photon polarizations.Experimentally, the relaxation of the spin states can be directly examined by measuring the degree of circular polarization ͑DCP͒ of photons emitted from a QD under circularly polarized photoexcitation. In actual experimental setups, measurements under the resonant excitation of exciton ground states are rather difficult due to the leakage photons from the exciting ...
High degree of preservation of spin states mediated by optical phonons is demonstrated with a positive trion state in a single quantum dot. At the optical phonon resonance, suppression of spin relaxation is clearly identified as dip structures in photoluminescence excitation spectra under cross-circularly polarize detection with respect to a pumping. As a result, distinguishably high degree of circular polarization up to ∼0.85 is achieved in the absence of external magnetic field, which is not the case for neutral excitons with finite fine structure splitting. The absence of continuum states plays a crucial role for this observation. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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