While single-shot detection of silicon spin qubits is now a laboratory routine, the need for quantum error correction in a large-scale quantum computing device demands a quantum non-demolition (QND) ...implementation. Unlike conventional counterparts, the QND spin readout imposes minimal disturbance to the probed spin polarization and can therefore be repeated to extinguish measurement errors. Here, we show that an electron spin qubit in silicon can be measured in a highly non-demolition manner by probing another electron spin in a neighboring dot Ising-coupled to the qubit spin. The high non-demolition fidelity (99% on average) enables over 20 readout repetitions of a single spin state, yielding an overall average measurement fidelity of up to 95% within 1.2 ms. We further demonstrate that our repetitive QND readout protocol can realize heralded high-fidelity (>99.6%) ground-state preparation. Our QND-based measurement and preparation, mediated by a second qubit of the same kind, will allow for a wide class of quantum information protocols with electron spins in silicon without compromising the architectural homogeneity.
A uniform ferrite-loaded open waveguide structure with composite right/left-handed (CRLH) response and its application to a novel backfire-to-endfire leaky-wave antenna are presented. The structure ...consists of a ferrite-filled rectangular waveguide open to free space at one of its small sides and biased perpendicularly to its large sides. Based on the fact that the leakage from the open side represents only a small perturbation in terms of guidance, an analytical dispersion relation for the approximately equivalent perfect magnetic conductor closed waveguide structure is derived. The unique dispersive properties of this structure, including its inherent CRLH balanced response (gapless nonzero group velocity transition between the left- and right-handed bands) and low-loss characteristics due to off-resonance operation, are extensively described, parametrically studied, and concretized by design guidelines. This theory is validated by both finite-element method and finite integration technique full-wave results and demonstrated experimentally. The leakage of the structure is exploited to build a full-space backfire-to-endfire scanning leaky-wave antenna, which is capable of both fixed-bias frequency scanning and fixed-frequency bias scanning, while being a perfectly uniform structure not requiring any chip tuning components. This feature, and its subsequent design simplicity, represents a fundamental advantage over previous CRLH metmaterial implementations. To the best of our knowledge, the proposed ferrite waveguide is the first and unique uniform structure exhibiting a CRLH response.
This paper reports on the extraction of the equivalent circuit model parameters of a physically defined silicon quantum dot at a cryogenic temperature and design of the impedance matching circuits to ...improve the performance of a charge sensor for radio-frequency (RF) reflectometry. The I-V characteristics and the S-parameters of the quantum dot device are measured around a Coulomb peak at 4.2 K. The measured results are modeled by an RC parallel circuit, and the model parameters for the quantum dot device were obtained. We consider three impedance matching circuits for RF reflectometry of a quantum dot: shunt capacitor-series inductor type, shunt inductor-series capacitor type, and shunt inductor-series inductor-type. We formulate and compare the sensitivity and bandwidth of RF reflectometry for the three types of circuits. The analysis should be useful for selecting the optimal matching circuit and the circuit parameters for given equivalent circuit parameters and working frequency. This procedure is demonstrated for a quantum dot with the characterized model circuit along with simulated performance. This design technique of matching circuit for RF reflectometry can be applied to any device that can be represented by an RC parallel circuit. These results will facilitate to realize fast semiconductor qubit readout in various quantum dot platforms.
Abstract
Electron spins confined in quantum dots are an attractive system to realize high-fidelity qubits owing to their long coherence time. With the prolonged spin coherence time, however, the ...control fidelity can be limited by systematic errors rather than decoherence, making characterization and suppression of their influence crucial for further improvement. Here we report that the control fidelity of Si/SiGe spin qubits can be limited by the microwave-induced frequency shift of electric dipole spin resonance and it can be improved by optimization of control pulses. As we increase the control microwave amplitude, we observe a shift of the qubit resonance frequency, in addition to the increasing Rabi frequency. We reveal that this limits control fidelity with a conventional amplitude-modulated microwave pulse below 99.8%. In order to achieve a gate fidelity >99.9%, we introduce a quadrature control method, and validate this approach experimentally by randomized benchmarking. Our finding facilitates realization of an ultra-high-fidelity qubit with electron spins in quantum dots.
A dual-band full-space scanning leaky-wave antenna based on a ferrite-loaded open waveguide structure is proposed. The lowest three bands of this structure are: the composite right/left-handed (CRLH) ...mode with positive group velocity first reported recently, the traditional isolator edge-guided mode, and a mixed forward/backward perturbed waveguide mode with a negative group velocity in its leaky-wave region. The leaky-wave regions of the CRLH and of perturbed waveguide modes are exploited to provide dual-band radiation. Due to their opposite group velocities, the two modes have their ports placed at both ends of the structure, which allows independent matching sections and also avoids the necessity of a diplexer. The antenna was demonstrated by both full-wave simulation and by experiment.
The Lau Basin and Havre Trough are back-arc basins related to Pacific–Australian plate convergence. Seafloor spreading occurs in the Lau Basin whereas the Havre Trough is in a rifting stage. At ...present, the spreading propagator's tip lies at the southern end of the Valu Fa Ridge (VFR) at 22°40′S. Studying this propagation process provides an opportunity to characterize the evolution of rifting to the initiation of seafloor spreading which is fundamental to back-arc basin development.
New geophysical data of the southern Lau Basin reveals that as spreading propagates south, it evolves in a discrete style south of 22°40′S. The propagation axis lies along the eastern margin of the basin, where the well defined, linear VFR loses its identifying morphology. Topography in this eastern zone is characterized by grabens separated by short narrow ridges. High backscatter intensity indicates tectonic and magmatic activity in this eastern area. Mantle Bouguer anomalies (MBA) increase southwards from the VFR to form an elevated MBA area extending west from the currently active area. This indicates eastward migration of active rifting, during which the arc crust was extremely thinned. High magnetization is observed in a left-stepping pattern south of the VFR. We interpret this pattern as discrete segments that characterize the initiation of the spreading stage. There is no evidence of a single, continuous spreading axis like that which characterizes the central and northern Lau Basin. The magnetization highs are discrete and are observed in areas where deformation and magmatism are focused. They are offset relative to the VFR, though they generally follow the same north–south trend as the VFR.
► “Rifting to Spreading in the southern Lau Basin: Variations within the transition zone”. ► The results of the study are based on newly obtained geophysical data from the southern Lau Basin. ► The study describes the details of an area of nascent seafloor spreading in a back-arc basin setting. ► The distribution of elevated magnetization is especially focused on. ► Discrete patches of elevated magnetization intensity form a stepping pattern along the propagation axis of the spreading in the basin. ► Integratedly with other geophysical data, namely bathymetry and gravity, the patterns of elevated magnetizations are described as segmented areas of magmatism of an initial spreading stage.
Recently introduced magnetless nonreciprocal metamaterials (MNMs) may represent a flexible, low-cost and integrated alternative to ferrites for nonreciprocal electromagnetic structures and microwave ...components. In those MNMs, the direction of gyrotropy is fixed by the fixed unilaterality of the FETs used as loads of the MNM ring particles. This letter makes the MNM's gyrotropy direction switchable, using switchable anti-parallel FET circuit loads. This switchability property is demonstrated experimentally via reversal of the direction of the Faraday angle induced by a switchable MNM metasurface.
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