Entanglement Swapping with Semiconductor-Generated Photons Violates Bell’s Inequality

Transferring entangled states between photon pairs is essential in quantum communication. Semiconductor quantum dots are the leading candidate for generating polarization-entangled photons deterministically. Here we show for the first time swapping of entangled states between two pairs of photons emitted by a single dot. A joint Bell measurement heralds the successful generation of the Bell state Ψ+, yielding a fidelity of 0.81±0.04 and violating the CHSH and Bell inequalities. Our photon source matches atomic quantum memory frequencies, facilitating implementation of hybrid quantum repeaters.BMBF/Q.comERC/QD-NOMSIFW Excellence Progra

Gene selection for optimal prediction of cell position in tissues from single-cell transcriptomics data.

Single-cell RNA-sequencing (scRNAseq) technologies are rapidly evolving. Although very informative, in standard scRNAseq experiments, the spatial organization of the cells in the tissue of origin is lost. Conversely, spatial RNA-seq technologies designed to maintain cell localization have limited throughput and gene coverage. Mapping scRNAseq to genes with spatial information increases coverage while providing spatial location. However, methods to perform such mapping have not yet been benchmarked. To fill this gap, we organized the DREAM Single-Cell Transcriptomics challenge focused on the spatial reconstruction of cells from the Drosophila embryo from scRNAseq data, leveraging as silver standard, genes with in situ hybridization data from the Berkeley Drosophila Transcription Network Project reference atlas. The 34 participating teams used diverse algorithms for gene selection and location prediction, while being able to correctly localize clusters of cells. Selection of predictor genes was essential for this task. Predictor genes showed a relatively high expression entropy, high spatial clustering and included prominent developmental genes such as gap and pair-rule genes and tissue markers. Application of the top 10 methods to a zebra fish embryo dataset yielded similar performance and statistical properties of the selected genes than in the Drosophila data. This suggests that methods developed in this challenge are able to extract generalizable properties of genes that are useful to accurately reconstruct the spatial arrangement of cells in tissues

Resonant Excitation Spectroscopy and Photon Statistics of Self-assembled Semiconductor Quantum Dots

Light-matter interactions in semiconductor nanostructures have attracted significant research interest because of both fundamental physics questions and practical concerns. Epitaxially grown quantum dots (QDs), with their narrow emission linewidths and atom-like density of states in a solid state system, are archetypical elements of study and are potentially useful for many applications, such as on-demand single photon emitters [1,2], efficient entangled photon-pair sources [3,4], and cavity quantum electrodynamics (QED) research [5–11]. Most experiments employ resonant or near-resonant excitation to directly interact with the bound states, which enables high excitation efficiency, precise control of quantum states, and minimal disturbance of the local environment. However, the inevitable doping of the material in growth introduces an intrinsic free charge carrier reservoir that enables charge fluctuations of the QD and defects in its surrounding local environment. The direct consequences are fluorescent intermittency (or blinking) in a QD’s emission, and spectral diffusion of the QD’s energy level. Both effects pose challenges for using these photons as flying qubits to realize a quantum network or linear optical quantum computing. Blinking compromises the properties of these QDs useful for generating on-demand single photons. Characterizing these charge dynamics and understanding the underlying physics is critical for hunting potential methods to suppress them. In this dissertation, by examining the excitation spectra of the QD and the photon statistics of its emission, we are able to determine the possible trap locations and the time scale of these charge dynamics. In fact, the temporal correlation measurement captures both the non-classical nature of these quantum emitters and the charge dynamics of both the QD and nearby defects. This information helps identify the nature of these charge traps, and provides the clues for suppressing these electric fluctuations; for example, by modifying the sample growth parameters or fabricating additional nano-structures on the sample to deplete the free charge carriers. One solution to these problems is to use a better sample with less intrinsic doping. It has been demonstrated that the photons emitted from the same QD in rapid succession can have very high indistinguishability when the QD is in an optical cavity [12,13] or is excited resonantly [14]. However, photons spaced widely in time and those from separate QDs do not show the same degree of indistinguishability [15–17] due to the inhomogeneous distribution of photon energies emitted by one QD state at different times. Considering that the state-of-art growth technique cannot achieve zero-doping growth, nor realize zero defect production, other methods are preferred. One alternative is to use coherent scattering. In this dissertation, we propose a new single photon source, a 3-level V-system, which is potentially a better single photon source than a single 2-level system in terms of generating single photons with sub-natural line-width [18]. Our calculation implies that 3-level system can output coherent scattering with a purity as high as 90% while keeping the coherent scattering intensity at the maximum value that a single 2-level system can generate in practice. Our calculation predicts an unconventional excitation line shape from 3-level V-system, which is confirmed experimentally here. The analysis indicates that the interference between the coherent scatterings from two dipoles is the cause for this phenomenon. Any systems with a V-shaped energy structure and orthogonal dipole moments are expected to observe this phenomenon if two transitions are non-degenerate and with a splitting on the order of a single line width

Chiral single-photon generators

Chiral photons have the potential to advance information technologies due to their robustness in carrying binary data against noisy backgrounds as well as their capacity for constructing single-photon isolators and circulators through nonreciprocal photon propagation. In this Perspective, we highlight recent efforts to generate chiral single photons using circularly polarized light sources. We delve into possible future technologies that integrate these light sources with other active optical elements as a versatile platform for information processing.Agency for Science, Technology and Research (A*STAR)Ministry of Education (MOE)National Research Foundation (NRF)Accepted versionWe acknowledge the financial support from the Singapore National Research Foundation through its Competitive Research Program (CRP Award Nos. NRF-CRP21-2018- 0007, NRF-CRP22-2019-0004, and NRF-CRP23-2019-0002), Singapore Ministry of Education (MOE2016-T2-2-077, MOE2016-T2-1-163, and MOE2016-T3-1-006 (S)), and the A*Star QTE programme

A Brief Analysis of Traditional Chinese Medical Elongated Needle Therapy on Acute Spinal Cord Injury and Its Mechanism

Acute spinal cord injury is one of the most common and complicated diseases among human spinal injury. We aimed to explore the effect of point-through-point acupuncture therapy with elongated needles on acute spinal cord injury in rabbits and its possible mechanism. Adult rabbits were randomly divided into a model group, elongated needle therapy group, and blank group. Immunohistochemical staining showed that the protein levels of Fas and caspase-3 in the model group were significantly higher than those in the blank group at each time point (P<0.05) and significantly lower than those in the elongated needle therapy group on the 3rd and 5th days after operation (P<0.05). RT-PCR showed that Fas and caspase-3 mRNA levels in the model group and elongated needle therapy group were significantly higher than those in the blank group (P<0.05, 0.01). The mRNA levels of Fas and caspase-3 in the elongated needle therapy group were significantly lower than those in model group on the 3rd day (P<0.05, 0.01). Therefore, we confirmed that elongated needle therapy has an obvious effect on acute spinal cord injury in rabbits. Its mechanism is made possible by inhibiting the expression of the Fas→caspase-3 cascade, thereby inhibiting cell apoptosis after spinal cord injury

Optically driven giant superbunching from a single perovskite quantum dot

Photon superbunching is a signature of a strong correlation between photons, which is a crucial resource needed in quantum communication and computation. As such, a superbunched photon source based on a material with high quantum efficiency, like cesium lead halide perovskite, is highly desirable. Utilizing the large dark–bright exciton splitting in CsPbBr3 quantum dot (QD), the authors achieve a superbunching with a large g(2)(0) ≈ 30 from an optically driven single CsPbBr3 QD emission at cryogenic temperature. The cascaded emission is identified as the cause of this superbunching by utilizing second-order cross-correlation measurement and exploring the excitation power and temperature dependence of the bunching level. The findings have immediate implications on the basic understanding of a single perovskite QD emission and its application as a quantum light source.Ministry of Education (MOE)National Research Foundation (NRF)Submitted/Accepted versionThe authors acknowledge the financial support from the Singapore National Research Foundation through its Competitive Research Program (CRP Award No. NRF-CRP21-2018-0007, NRF-CRP22-2019-0004, and NRF-CRP23-2019-0002) and Quantum Engineering Program (QEP) and from Singapore Ministry of Education (MOE2016-T3-1-006 (S))