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In the recent work of Kiss et al. [Phys. Rev. Lett. 107 (2011) 100501], the evolvement of two-qubit quantum states in a measurement-based purification process is studied. As they pointed out, the purification results manifest sensitivity to the applied initial states. The convergence regions to different stable circles are depicted on a complex plane. Because of the result patterns' likeness to typical fractals, we make further study on the interesting patterns' connection to fractals. Finally, through a numerical method we conclude that the boundaries of different islands of the patterns are fractals, which possess a non-integral fractal dimension. Also, we show that the fractal dimension would vary with the change of the portion of the noise added to the initial states. [ABSTRACT FROM AUTHOR]

In order to investigate the carrier redistribution mechanisms in InAs/GaAs self-assembled quantum dots, the photoluminescent energy peak shift is studied with increasing excitation power. Unusually for samples of relatively low density, it is shown that the energy peak position could recover slowly after a fast redshift, associated with the increasing excitation power. A theoretical model is presented, which involves the Auger effect assisting carrier recapture as important mechanisms during the relaxation process. [ABSTRACT FROM AUTHOR]

The recently proposed compressed sensing (CS) method sheds light on quantum state tomography of multi-qubit systems with low rank, which greatly reduces the complexity of measurement and computation. However, the restricted isometry property requirement of CS is difficult to be promised or verified in practice, which makes this method probably assign unreasonable results. In regard to this problem, we adopt a two-step procedure and implement an adaptive strategy to update measurement operators based on the measurement results of the first step for CS, which not only serves as a way to verify the estimate but also improves the accuracy of tomography. Our numerical simulations manifest that our adaptive protocol can reduce about half of the infidelity of non-adaptive protocol and is still efficient even when the rank of the state is slightly high, which would greatly benefit multi-qubit state tomography in future experiments. [ABSTRACT FROM AUTHOR]

As a widely used reconstruction algorithm in quantum state tomography, maximum likelihood estimation tends to assign a rank-deficient matrix, which decreases estimation accuracy for certain quantum states. Fortunately, hedged maximum likelihood estimation (HMLE) [Phys. Rev. Lett. 105 (2010) 200504] was proposed to avoid this problem. Here we study more details about this proposal in the two-qubit case and further improve its performance. We ameliorate the HMLE method by updating the hedging function based on the purity of the estimated state. Both performances of HMLE and ameliorated HMLE are demonstrated by numerical simulation and experimental implementation on the Werner states of polarization-entangled photons. [ABSTRACT FROM AUTHOR]

We investigate the violation of two types of Leggett-Garg (LG) inequalities in self-assembled quantum dots under the stationarity assumption. By comparing the two types of LG inequalities, we find the better one that is easier to be tested in an experiment. In addition, we show that the fine-structure splitting, background noise and temperature of quantum dots greatly affect the violation of LG inequalities. [ABSTRACT FROM AUTHOR]

The peaks of exciton and biexciton from the single quantum dot (QD) micro-photoluminescence spectra are identified by observing the intensity of those peaks in relation to increasing excitation power. In order to further verify the properties of the exciton and biexciton, we perform auto- and cross correlation measurements. Using the former, we confirm the antibunching property of the signal light emitted from the single QD. Using cross correlation measurement, we verify that the exciton and biexciton emissions originate from the same QD and they are strongly correlated with each other. Lastly, we analyze the behavior of the cross correlation function in both the cases of saturated and unsaturated excitation of the QD. [ABSTRACT FROM AUTHOR]

We demonstrate a fiber Fabry–Pérot cavity in the ultraviolet range, which covers the florescence wavelength for the 2P1/2 to 2S1/2 transition of Yb+ and is designed in the bad cavity limit for florescence collection. Benefiting from both the small cavity mode volume and the large atom dipole, a cavity with moderate finesse and high transmission still supports a good cooperativity, which is made and tested in experiment. Based on the measured experimental parameters, simulation performed on the cavity and ion shows a Purcell factor better than 2.5 and a single-mode fiber collection efficiency over 10%. This technology can support ultra-bright single photon sources based on trapped ions and can provide the possibility to link remote atoms as a quantum network. [ABSTRACT FROM AUTHOR]