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We have investigated the optical anisotropy in individual self-assembled quantum dots. The linear polarization analysis of the positive trion photoluminescence reveals the effect of the strain-induced valence band mixing since the positive trion has the spin-paired holes and therefore exchange interaction has no influence. Meanwhile, the neutral exciton indicates the complex polarization states due to both the in-plain asymmetries of the dot shape and the strain distributions. The experimental and theoretical polarization analysis has been performed for tens of InAlAs quantum dots and the correlation between the important parameters was investigated. © 2011 American Institute of Physics. ͓doi:10.1063/1.3583453͔Studies on localized spins in semiconductor quantum dots ͑QDs͒ have been attracting considerable interest. This is because the discrete electronic levels involved in the optical transitions serve the fascinating applications in which QDs are used as emitters of single, indistinguishable, and entangled photons.1 For these applications, it is crucial to study the polarization of the emitted photons associated with exciton annihilation. For ideal QDs as artificial atoms, the relevant eigenstates are bright excitons with the angular momentum of Ϯ1, and the circularly polarized photons ͑ + or − ͒ are to be absorbed ͑emitted͒ to ͑from͒ the eigenstates. However, actual QDs have the anisotropic distributions of shape and strain, and as a result, the confinement potential symmetry is reduced from D 2d to C 2v or lower. It is well known that the shape anisotropy induces the change in the emission polarization as well as the level splitting as an exciton fine structure.2-5 Also, QDs formed by self-assembly in the Stransky-Krastanov ͑SK͒ growth mode is believed to have a large strain originating from the QD formation process, and the strain with the anisotropic distribution more or less remains inside a QD even after QD formation is complete. Consequently, the emission polarization is affected by the anisotropic exchange interaction ͑AEI͒ and the straininduced valence-band mixing ͑SI-VBM͒. The former originates from the QD shape asymmetry and the latter comes mainly from the in-plane anisotropic relaxation of strains. Accordingly, the polarization of the QD emissions is one of valuable probes for the origin of the QD symmetry lowering. Since the strain anisotropy may be different largely from QD to QD, the investigation of the polarization is necessary for the individual QDs.In this work, we investigated the polarization of the photoluminescence ͑PL͒ in single InAlAs QDs in order to probe the interactions that reduce the QD potential symmetry. By comparing the positive trion PL with the neutral exciton PL in the same single QD, the intrinsic parameters for AEI and SI-VBM were deduced. Since III-V semiconductor QDs generally have a smaller strain anisotropy than that in II-VI QDs, 6,7 the precise estimation of the composite of AEI and SI-VBM in III-V semiconductor QDs have not been reported so far. The simple method pr...

We report the hysteresis of optically-pumped nuclear spin polarization and the degree of circular polarization of photoluminescence on the excitation power and electron spin polarization in single InAlAs quantum dots. By increasing (or decreasing) the excitation power at a particular excitation polarization, an abrupt rise (or drop) and a clear hysteretic behavior were observed in the Overhauser shift of the photoluminescence of the exciton and exciton complexes from the same single quantum dot under an external magnetic field of 5 T. However, the degree of circular polarization shows different behaviors between a positively charged exciton and a neutral exciton or biexciton; further, only positively charged exciton exhibits the precisely synchronized change and hysteretic behavior. It is suggested that the electron spin distribution is affected by the flip-flop of electronnuclear spins. Further, the hysteresis is observed as a function of the degree of circular polarization of the excitation light and its dependence on the excitation power is studied. The saturation of the Overhauser shift after the abrupt rise indicates the almost complete cancellation of the external magnetic field by the nuclear field created within the width that is decided by the correlation time between the electron and the nuclei spin system. PACS numbers: 73.21.La, 78.67.Hc, 71.35.Pq,71.70.Jp Recently, research on electron-nuclear spin interaction has been revived in view of its applications. This is because semiconductor quantum dots (QDs) enhance the electron-nuclear spin interaction (hyperfine interaction) due to their strong 3D confinement of the electron wavefunction, and the enhanced interaction gives the possibility of aligning nuclear spins in one direction up to several tens % in a single QD through the optical pumping. In fact, a large rate of nuclear spin polarization and the resultant large effective nuclear field up to several tesla were observed recently in interface GaAs QDs 1,2 , selfassembled InAlAs QDs 3,4 and InGaAs QDs 5,6,7 . Because of the ultralong coherence, nuclear spin is expected to contribute to applications such as a long-lived quantum memory at the nuclear level 8 and qubit conversion by using the nuclear field 9 . Beyond such potential applications for quantum information processing, nuclear magnetic ordering and optically induced ferromagnetic ordering of spin systems are of surpassing interest in fundamental physics. Therefore, the control of nuclear spins in nanostructures has both fundamental as well as practical significance.In this study, we investigated the optical pumping of nuclear spin polarizations in a single self-assembled InAlAs QD. An abrupt rise in and the hysteresis of the Overhauser shift in addition to the degree of circular polarization (DCP) in the photoluminescence (PL) of positively charged excitons were clearly observed in the excitation power and excitation polarization dependences. Additionally, with the aid of this abrupt change, the sign of the electron g-factors in the z dire...

We demonstrated the cancellation of the external magnetic field by the nuclear field at one edge of the nuclear polarization bistability in single InAlAs quantum dots. The cancellation for the electron Zeeman splitting gives the precise value of the hole g-factor. By combining with the exciton g-factor that is obtained from the Zeeman splitting for linearly polarized excitation, the magnitude and sign of the electron and hole g-factors in the growth direction are evaluated.Semiconductor quantum dots (QDs) exhibit a variety of confinement-related optical and electronic properties useful for opto-electronic device applications such as QD lasers and detectors. In particular, broad efforts are currently underway to develop new techniques for controlling spin degrees of freedom in QDs for quantum information processing. A key quantity for the spin manipulation is a g-factor, which is a coefficient connecting magnetic dipole moments with the spin degrees of freedom. Therefore, the knowledge of electron and hole g-factors and their control are important. For example, zero electron g-factor is required to convert the photon qubit into the electron spin qubit 1 while the system with a large gfactor is preferable for controlling spin qubit in terms of the energy selectivity. The g-factors of self-assembled QDs have been obtained by optical measurements and transport measurements. Generally, the electron g-factor is deduced from transport measurements while the exciton g-factor, which is the sum of an electron and a hole g-factors, is deduced from optical measurements. In the optical measurements, since the photoluminescence (PL) is generated by the annihilation of an electron and a hole, it is usually difficult to independently obtain an electron or a hole g-factor of QDs. In addition, sensitivity of the g-factors to the spatial confinement has been predicted by theoretical studies and partly confirmed in the experiments 2 . The obtained values of the g-factors are much different from bulk ones possibly due to size quantization, strain, and other effects, however those effects are difficult to evaluate nondestructively and noncontactly for individual QD. Therefore, the direct probing method of the electron or hole g-factor is required for individual QD target.In this study, we demonstrate the precise measurements of electron and hole g-factors in single InAlAs QDs by using the optically induced nuclear field. The measurement principle is based on the fact that nuclear field is effective only on electrons and can compensate the external magnetic field. We first show that the nuclear field exactly cancels the external magnetic field at one edge of nuclear bistability. Recently, we proposed to use the cancellation of external and nuclear field for photon-spin qubit conversion to dispense with the zero gfactor engineering 3 . The cancellation at the bistability indicates that the condition for the qubit conversion is automatically realized there. Then, the measured Zeeman splitting corresponds to that of not an exciton but a ...

We demonstrate fiber-optic quantum key distribution (QKD) at 1550 nm using single-photon detectors operating at 5 MHz. Such high speed single-photon detectors are essential to the realization of efficient QKD. However, after-pulses increase bit errors. In the demonstration, we discard after-pulses by measuring time intervals of detection events. For a fiber length of 10.5 km, we have achieved a key rate of 17 kHz with an error of 2%.