We analyze the reading and initialization of a topological qubit encoded by Majorana fermions in one-dimensional semiconducting nanowires, weakly coupled to a single level quantum dot （QD）. It is shown that when the Majorana fermions are fused by tuning gate voltage, the topological qubit can be read out directly through the occupation of the QD in an energy window. The initialization of the qubit can also be realized via adjusting the gate voltage on the QD, with the total fermion parity conserved. As a result, both reading and initialization processes can be achieved in an all-electrical way.
We demonstrate radiation-pressure-driven mechanical oscillations from high optical quality factor silica microdisk resonators on chip. Mechanical quality factors of 3520 in air and 12540 in vacuum for the fundamental radial breathing modes are obtained from 73 μm-diarneter silica microdisks with mechanical frequencies of -51 MHz. The measured mechanical oscillation threshold powers for the input light are determined to be 62.5 μW in air and down to 26.6 μW in vacuum.
WANG GuanZhongZHAO MingMingMA JiYangLI GuanYuCHEN YuanJIANG XiaoShunXIAO Min
We experimentally demonstrate high optical quality factor silica microdisk resonators on a silicon chip with large wedge angles by reactive ion etching. For 2-μm-thick microresonators, we have achieved wedge angles of 59°, 63°,70°, and 79° with optical quality factors of 2.4 × 10~7, 8.1 × 10~6, 5.9 × 10~6, and 7.4 × 10~6, respectively, from ～80 μm diameter microresonators in the 1550 nm wavelength band. Also, for 1-μm-thick microresonators, we have obtained an optical quality factor of 7.3 × 10~6 with a wedge angle of 74°.
We fabricate high-quality A1/A1Ox/A1 junctions using improved bridge and bridge-free techniques at 30-keV e-beam voltage,in which the length of undercut and the size of junction can be well controlled by the pre-exposure technique.The dose window is 5 times as large as that used in the usual Dolan bridge technique,making this technique much more robust.Similar results,comparable with those achieved using a 100-keV e-beam writer,are obtained,which indicate that the 30-keV e-beam writer could be an economic choice for the superconducting qubit fabrication.
An in-line,all-optical fiber modulator based on a stereo graphene–microfiber structure(GMF)utilizing the lab-on-rod technique was demonstrated in this study.Owing to its unique spring-like geometry,an ultra-long GMF interaction can be achieved,and a modulation depth of,7.5 dB(,2.5 dB)and a modulation efficiency of,0.2 dB mW^(-1)(,0.07 dB mW^(-1))were demonstrated for two polarization states.The modulation depth and modulation efficiency are more than one order of magnitude larger than those of other graphene–microfiber hybrid all-optical modulators,although at the cost of a higher insertion loss.By further optimizing the transferring and cleaning process,the upper limit of the modulation depth is mainly determined by the loss from the intrinsic absorption,which depends on the light–graphene interaction.Then,the modulator can quickly switch between the on-state and the off-state with a theoretically maximized modulation depth of tens of decibels.This modulator is compatible with the current fiber-optic communication systems and may be applied in the near future to meet the impending need for ultrafast optical signal processing.
Jin-Hui ChenBi-Cai ZhengGuang-Hao ShaoShi-Jun GeFei XuYan-Qing Lu
Quantum metrology holds the promise of improving the measurement precision beyond the limit of classical ap- proaches. To achieve such enhancement in performance requires the development of quantum estimation theories as well as novel experimental techniques. In this article, we provide a brief review of some recent results in the field of quantum metrology. We emphasize that the unambiguous demonstration of the quantum-enhanced precision needs a careful analysis of the resources involved. In particular, the implementation of quantum metrology in practice requires us to take into ac- count the experimental imperfections included, for example, particle loss and dephasing noise. For a detailed introduction to the experimental demonstrations of quantum metrology, we refer the reader to another article ＇Quantum metrology＇ in the same issue.
We demonstrate ultralow-threshold thulium-doped, as well as thulium-holmium-codoped, microtoroid lasers on silicon chips, operating at the wavelength of around 2 ?m. High quality factor whispering gallery mode(WGM) microtoroid cavities with proper thulium and holmium concentrations are fabricated from the silica sol-gel films. The highly confined WGMs make the microcavity lasers operate with ultralow thresholds, approximately 2.8 ?W and 2.7 ?W for the thulium-doped and the thulium-holmium-codoped microlasers, respectively.
