The readout of semiconductor spin qubits based on spin blockade is fast but suffers from a small charge signal. Previous work suggested large benefits from additional charge mapping processes; however, uncertainties remain about the underlying mechanisms and achievable fidelity. In this work, we study the single-shot fidelity and limiting mechanisms for two variations of an enhanced latching readout. We achieve average single-shot readout fidelities greater than 99.3% and 99.86% for the conventional and enhanced readout, respectively, the latter being the highest to date for spin blockade. The signal amplitude is enhanced to a full one-electron signal while preserving the readout speed. Furthermore, layout constraints are relaxed because the charge sensor signal is no longer dependent on being aligned with the conventional (2,0)-(1,1) charge dipole. Silicon donor-quantum-dot qubits are used for this study, for which the dipole insensitivity substantially relaxes donor placement requirements. One of the readout variations also benefits from a parametric lifetime enhancement by replacing the spin-relaxation process with a charge-metastable one. This provides opportunities to further increase the fidelity. The relaxation mechanisms in the different regimes are investigated. This work demonstrates a readout that is fast, has a one-electron signal, and results in higher fidelity. It further predicts that going beyond 99.9% fidelity in a few microseconds of measurement time is within reach.

Individual donors in silicon chips are used as quantum bits with extremely low error rates. However, physical realizations have been limited to one donor because their atomic size causes fabrication challenges. Quantum dot qubits, in contrast, are highly adjustable using electrical gate voltages. This adjustability could be leveraged to deterministically couple donors to quantum dots in arrays of qubits. In this work, we demonstrate the coherent interaction of a 31P donor electron with the electron of a metal-oxide-semiconductor quantum dot. We form a logical qubit encoded in the spin singlet and triplet states of the two-electron system. We show that the donor nuclear spin drives coherent rotations between the electronic qubit states through the contact hyperfine interaction. This provides every key element for compact two-electron spin qubits requiring only a single dot and no additional magnetic field gradients, as well as a means to interact with the nuclear spin qubit.

Membrane projection lithography is used to create vertically oriented split‐ring resonators which show measured electric and magnetic resonances (λ = 22, 11, and 7 μm). We then create composite structures with 5 split ring resonators per unit cell (image). This approach provides a long‐sought, manufacturable path toward the realization of 3D optical and infrared metamaterials.

By analyzing the temperature ͑T͒ and density ͑n͒ dependence of the measured conductivity ͑͒ of twodimensional ͑2D͒ electrons in the low-density ͑ϳ10 11 cm −2 ͒ and temperature ͑0.02-10 K͒ regimes of highmobility ͑1.0 and 1.5ϫ 10 4 cm 2 / Vs͒ Si metal-oxide-semiconductor field-effect transistors, we establish that the putative 2D metal-insulator transition is a density-inhomogeneity-driven percolation transition where the density-dependent conductivity vanishes as ͑n͒ ϰ ͑n − n p ͒ p , with the exponent p ϳ 1.2 being consistent with a percolation transition. The "metallic" behavior of ͑T͒ for n Ͼ n p is shown to be well described by a semiclassical Boltzmann theory, and we observe the standard weak localization-induced negative magnetoresistance behavior, as expected in a normal Fermi liquid, in the metallic phase.The so-called two-dimensional ͑2D͒ metal-insulator transition ͑MIT͒ has been a subject 1,2 of intense activity and considerable controversy ever since the pioneering experimental discovery 3 of the 2D MIT phenomenon in Si metaloxide-semiconductor field-effect transistors ͑MOSFETs͒ by Kravchenko and Pudalov some 15 years ago. The apparent MIT has now been observed in almost all existing 2D semiconductor structures, including Si MOSFETs, 3,4 electrons, 5-7 and holes [8][9][10][11] in GaAs/AlGaAs, and electrons in Si/SiGe ͑Refs. 12 and 13͒ systems. The basic phenomenon refers to the observation of a carrier density-induced qualitative change in the temperature dependence of the resistivity ͑n , T͒, where n c is a critical density separating an effective "metallic" phase ͑n Ͼ n c ͒ from an "insulating" phase ͑n Ͻ n c ͒, exhibiting d / dT Ͼ 0͑Ͻ0͒ behavior typical of a metal ͑insulator͒.The high-density metallic behavior ͑n Ͼ n c ͒ often manifests in a large ͑by 25% for electrons in GaAs/AlGaAs heterostructures to factors of 2-3 in Si MOSFETs͒ increase in resistivity with increasing temperature in the lowtemperature ͑0.05-5 K͒ regime where phonons should not play much of a role in resistive scattering. The insulating regime, at least for very low ͑n Ӷ n c ͒ densities and temperatures, seems to be the conventional activated transport regime of a strongly localized system. The 2D MIT phenomenon occurs in relatively high-mobility systems, although the mobility values range from 10 4 cm 2 / Vs ͑Si MOSFET͒ to 10 7 cm 2 / Vs͑GaAs/ AlGaAs͒ depending on the 2D system under consideration. The 2D MIT phenomenon is also considered to be a low-density phenomenon although, depending on the 2D system under consideration, the critical density n c differs by 2 orders of magnitude ͑n c ϳ 10 11 cm −2 in 2D Si and ϳ10 9 cm −2 in high-mobility GaAs/AlGaAs heterostructures͒. The universal features of the 2D MIT phenomenon are ͑1͒ the existence of a critical density n c distinguishing an effective high-density metallic ͑d / dT Ͼ 0 for n Ͼ n c ͒ phase from an effective low-density insulating ͑d / dT Ͻ 0 for n Ͻ n c ͒ phase, and ͑2͒ while the insulating phase for n Ͻ n c seems mostly to manifest the conventional activated transport be...

We demonstrate that temperature-and strain-insensitive long-period gratings can be fabricated in conventional optical fibers. The former is employed to measure strain with resolution of 20 ⑀ under thermal fluctuations in the surroundings, while the latter is used to detect temperature variations as small as 0.8°C in the presence of axial strain. © 1997 Society of Photo-Optical Instrumentation Engineers. [S0091-3286(97)00407-8]