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Algorithms for wavefront sensing and error correction from intensity attract great concern in many fields. Here we propose Bayesian optimization to retrieve phase and demonstrate its performance in simulation and experiment. For small aberration, this method demonstrates a convergence process with high accuracy of phase sensing, which is also verified experimentally. For large aberration, Bayesian optimization is shown to be insensitive to the initial phase while maintaining high accuracy. The approach’s merits of high accuracy and robustness make it promising in being applied in optical systems with static aberration such as AMO experiments, optical testing shops, and electron or optical microscopes.

Multi-mode quantum memory is a basic element required for long-distance quantum communication, as well as scalable quantum computation. For on-demand readout and long storage times, control pulses are crucial in order to transfer atomic excitations back and forth into spin excitations. Here, we introduce noise-robust composite pulse sequences for high-fidelity excitation transfer in multi-mode quantum memory. These pulses are robust to the deviations in amplitude and the detuning parameters of realistic conditions. We show the efficiency of these composite pulses with a typical rare-earth ion-doped system. This approach could be applied to a variety of quantum memory schemes.

It is of great importance to determine an unknown quantum state for fundamental studies of quantum mechanics, yet it is still difficult to characterize systems of large dimensions in practice. Although the scan-free direct measurement approach based on a weak measurement scheme was proposed to measure a high-dimensional photonic state, how weak the interaction should be to give a correct estimation remains unclear. Here we propose and experimentally demonstrate a technique that measures a high-dimensional quantum state with the combination of scan-free measurement and direct strong measurement. The procedure involves sequential strong measurement, in which case no approximation is made similarly to the conventional direct weak measurement. We use this method to measure a transverse state of a photon with effective dimensionality of 65000 without the time-consumed scanning process. Furthermore, the high fidelity of the result and the simplicity of the experimental apparatus show that our approach can be readily used to measure the complex field of a beam in diverse applications such as wavefront sensing and quantitative phase imaging.

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.

We demonstrate a fiber Fabry–Perot 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.

Following a suggestion by Wheeler, several delayed choice experiments have been performed. However, in those experiments one fact has always been ignored: that due to the fact that the single photon is nonlocal in time, the outcome will change if one changes the time to switch the experimental configuration within the photon's wavepacket. This study obtains some insights into this aspect by proposing an experimental scheme based on the delayed choice experiment and giving some related analysis. The result shows a transitional behavior from a particle to a wave, leading to a conclusion that the measuring operator could be time dependent and be changed in a single measurement.

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.

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.

In the original time-domain Bell's inequalities (Leggett-Garg-type inequalities), the physical objective is measured at three time points. When more time points are chosen, several methods can be used to extend these inequalities. We experimentally demonstrate the violation of these extended inequalities using single photons from a self-assembled quantum dot. In general, for each extension, the quantity by which the quantum-mechanics value exceeds the classical limit becomes larger as the number of measurement time points increases. This quantity has a maximum value for the extensions that have the same number of measurement time points. Furthermore, we evaluate the noise tolerance for these extensions with a quantity that is related to the number of standard deviations by which the experimental result surpasses the classical limit.