Quantum superposition of localized and delocalized phases of photons

Based on a variant of 2-site Jaynes–Cummings–Hubbard model constructed using superconducting circuits, we propose a method to coherently superpose the localized and delocalized phases of microwave photons, which makes it possible to engineer the collective features of multiple photons in the quantum way using an individual two-level system. Our proposed architecture is also a promising candidate for implementing distributed quantum computation since it is capable of coupling remote qubits in separate resonators in a controllable way.     

一种机械型超导开关的设计与初步实验

文中所述机械型超导开关是一种新型超导开关,其主要用做超导储能磁体的持续电流开关(persistentcurrent switch,简称PCS)。文章简要介绍机械型超导开关的原理并计算PCS的损耗,然后详述机械型超导开关的设计,最后描述初步实验结果。     

极低温微波半导体开关探索研究

针对开关滤波器组中超导滤波器的应用,文中探讨了极低温微波半导体开关的研制,制作了一种微波PIN管SPDT开关.开关在20K温度下的插损≤1.1dB,隔离度≥56dB,开关速度14ns.对低温对开关性能的影响进行了分析,为极低温微波开关的设计提供了依据.     

An investigation into density-wave propagation within high-temperature superconducting plasmas for use in creating traveling-wave devices

This paper theoretically investigates a novel application of high-temperature superconductors where the superconductor serves as the active component in a microwave or millimeter traveling-wave amplifier. A guided electromagnetic wave interacts with a dc superconducting electron current to set up charge-density gradients within the superconducting electron "plasma." The electromagnetic wave gradually extracts energy from the superconducting electrons by traveling in phase synchronism with these charge gradients. The interaction mechanism is similar to that of a conventional traveling-wave tube amplifier or oscillator. We have modeled the wave behavior of superconducting electrons using the London equations and a two-fluid approach. Our model includes dissipation within the superconductor, and it shows that traveling-wave devices may be possible using high-quality thin-film superconductors in which dissipation is kept low.     

Measurement of the reflector antenna absorption at terahertz frequencies by the method of a balanced radiometer

The inherent noise of a prototype panel of the millimeter-wave telescope main reflector is measured at room temperature. A modulation radiometer based on a heterodyne detector with an input superconducting mixer and a balanced antenna switch is used. The noise temperature of the detector in the double-sideband mode equals 570 K at a local oscillator frequency of 842 GHz. The absorption factor of the aluminum panel calculated from the thermal radiation intensity is 0.31 ± 0.07%. This value is checked on samples made of phosphor bronze and stainless steel. The respective absorption factors are 0.41 ± 0.07 and 1.30 ± 0.07%, which are close to those obtained earlier for these materials using radiometers with direct-reading detectors. The wide dynamic range of the heterodyne detector simplifies calibration of the radiometer: there is no need to cool down the sensitive element below liquid helium temperature. A balanced switch of a new design makes it possible to radically suppress the influence of the background and take precision room-temperature measurements.     

Manipulation of superconducting qubit with direct digital synthesis

We investigate the XY control and manipulation of the superconducting qubit state using direct digital synthesis(DDS)for the microwave pulse signal generation.The decoherence time, gate fidelity, and other qubit properties are measured and carefully characterized, and compared with the results obtained by using the traditional mixing technique for the microwave pulse generation.In particular, the qubit performance in the state manipulation with respect to the sampling rate of DDS is studied.Our results demonstrate that the present technique provides a simple and effective method for the XY control and manipulation of the superconducting qubit state.Realistic applications of the technique for the possible future scalable superconducting quantum computation are discussed.     

Current-induced suppression of superconductivity in a three-dimensional lattice of weakly linked indium grains in opal

The current-voltage characteristics of the metal-dielectric composite have been investigated in the range of the resistive state near the superconducting transition temperature T _c . The composite structure can be represented as a face-centered cubic lattice, which involves a large number of weakly linked indium nanograins and is stabilized in structural cavities of opal. The response to microwave radiation is used to characterize the resistive state of the composite. The comparative investigation into the current-voltage characteristics and the response of the composite to microwave radiation makes it possible to conclude that the weak links are superconducting in the region of critical current (I _c ) of the composite as a whole. The transition of weak links to the resistive state occurs at currents immediately preceding the transition of the composite from the resistive state to the ohmic state. The model of resistivity of the indium-opal composite is proposed on the basis of morphological examinations. According to this model, the energy dissipation in the resistive state is brought about by the quasi-discrete (due to the quantization of the magnetic flux in circuits of a three-dimensional lattice comprised of multiply connected grains) redistribution of transport current over the cross-section of composite.     

Continuous measurement of the energy eigenstates of a nanomechanical resonator without a nondemolition probe

We show that it is possible to perform a continuous measurement that continually projects a nanoresonator into its energy eigenstates by employing a linear coupling with a two-state system. This technique makes it possible to perform a measurement that exposes the quantum nature of the resonator by coupling it to a Cooper-pair box and a superconducting transmission line resonator.     

Fabrication of microresonators by using photoresist developer as etchant

In superconducting circuit, microwave resonators and capacitors are crucial components, and their quality has a strong impact on circuit performance. Here we develop a novel wet etching process to define these two components using common photoresist developer as etchant. This method reduces subsequent steps and can be completed immediately after development. By measuring the internal quality factor of resonators, we show that it is possible to achieve similar or better performance when compared with samples made by standard etching processes. This easy-to-implement method may boost the yield hence providing an alternative fabrication process for microwave resonators and capacitors.     

高温超导薄膜微波非线性研究的进展

文章介绍了高温超导薄膜微波非线性的主要特征，阐述了高温超导薄膜微波非线性产生的原因和相关的研究现状，指出了高温超导薄膜非线性研究中遇到的困难和尚未解决的问题．     

极低反射率光控高温超导开关的研究

提出了一种能够实现极低反射率的光控高温超导开关的新结构-微带直线结构,并提出实现此结构的新方法-辐照均匀法。理论分析、仿真数据和可行性验证实验都表明,光控高温超导开关采用辐照均匀法和微带直线结构,可以有效地扩大超导开关光辐照通断点的受辐射面积;因光束光强分布更加均匀,使通断控制性能更高;微带直线结构使得特性阻抗变化完全消失,具有极低的反射率;利用光学元件能够有效控制激光光束直径的特点,使微带线宽指标设计更加灵活,光控超导开关的适应面更广。     

Unconventional Geometric Phase Gate with Superconducting Quantum Interference Device Qubits in Cavity QED

In the system with superconducting quantum interference devices （SQUID） in cavity, a scheme for constructing two-qubit quantum phase gate via a conventional geometric phase-shift is proposed by using a quantized cavity field and classical microwave pulses. In this scheme, the gate operation is realized in the subspace spanned by the two lower flux states of the SQUID system mud the population operator of the excited state has no effect on it. Thus the effect of decoherence caused from the levels of the SQUID system is possible to minimize. Under cavity decay, our strictly numerical simulation shows that it is also possible to realize the unconventional geometric phase gate. The experimental feasibility is discussed in detail.     

A microwave photon detector

The possibility of the implementation of a single-photon detector in the microwave range has been discussed. It has been shown that, for these purposes, it is possible to use an unshunted Josephson junction, which is switched from the superconducting state into a finite-voltage state at the presence of an external signal. The sensitivity of this detector is determined by the distribution of switchings in the absence of an external signal. It has been demonstrated that there is a Josephson junction with a noise temperature below 60 mK at a nominal external temperature of 10 mK. A specific detector intended for the measurement of external microwave signals has been proposed, designed, and fabricated. The first experimental results have been presented.     

一种双模压缩微波制备的相位锁定方案

相位锁定是双模压缩微波制备的关键问题之一.针对基于超导180°混合环的制备方案相位稳定度不高且信息处理复杂等问题,提出一种相位锁定方案.对约瑟夫森参量放大器的信号输入进行相位调制,输出的单模压缩微波与另一未调制的同频单模压缩微波在超导180°混合环内干涉,实现双模压缩微波的制备与路径分离.将未调制的单模压缩微波与一路双模压缩微波混频,解调出相位调制信号可得到两路单模压缩微波的相对相位及误差,将相位误差反馈于约瑟夫森参量放大器的抽运实现相对相位的锁定,获得稳定的双模压缩输出.本研究对高性能纠缠微波源的设计提供了理论参考.     

Possible high-frequency cavity and waveguide applications of high temperature superconductors

Conventional superconducting materials used at liquid helium temperatures have been employed to greatly reduce ohmic power loss in microwave cavities. If suitable high temperature superconductor surfaces can be developed, it could be possible to alleviate constraints due to relatively low energy gaps and limits due to critical fields, and operate at more convenient temperatures with larger thermal margins. These features could be used to improve performance of present superconducting microwave cavity devices. They may also facilitate new applications and device designs at millimeter-wave and far infrared frequencies. Possible cavity and waveguide applications are described. Sensitivity of RF surface resistance to boundaries and nonaligned grains in the material and to small amounts of nonsuperconducting material may be an obstacle to development. Moreover, critical RF fields can be limiting for high power applications. An additional problem is the adverse effect of D.C. magnetic fields.Supported by Department of Energy No. ET-51013-227. Reproduction, translation, publication, use and disposal in whole or in part, by or for the United States government is permitted.     

Grid control over high currents in a cesium discharge with a cathode spot

We report on the results of investigation of a plasma switch with complete grid control in a discharge with a cathode spot on the liquid-metal cesium cathode without grid diaphragming. The retention of the working area of the grid relative to the anode area leads to an order-of-magnitude increase in the switching anode current (up to 20 A/cm² over the anode area) and a substantial (up to 100 V and higher) increase in the switching voltages. The use of the cathode jet makes it possible to reduce the working pressures of cesium vapor (down to 10^–3 Torr). We discuss the results of analysis of peculiarities of grid discharge quenching in such a switch, which make it possible to determine possible reasons for limitation of the working parameters of the switch and the ways of their further increase.     

Use of the Phenomenon of High-Temperature Superconductivity for the Creation of Integrated Devices of Microwave Electronics

Physical fundamentals justifying the use of integrated microwave circuits based on films of high-temperature superconductors in microwave electronics are presented. The peculiarities of calculations of high-temperature superconducting film microwave elements are shown. The technological conditions of the creation of integrated microwave devices based high-temperature superconducting on films are analyzed.     

Theory of superconductivity for Dirac electrons in graphene

Phonon-exchange-induced superconducting pairing of effectively ultrarelativistic electrons in graphene is investigated. The Eliashberg equation obtained for describing pairing in the Cooper channel with allowance for delayed interaction are matrix equations with indices corresponding to the valence and conduction bands. The equations are solved in the high doping limit, in which pairing is effectively a single-band process, and in the vicinity of a critical quantum point of underdoped graphene for a value of the coupling constant for which pairing is an essentially multiband process. For such cases, analytic estimates are obtained for the superconducting transition temperature of the system. It is shown that the inclusion of dynamic effects makes it possible to determine the superconducting transition temperature, as well as the critical coupling constant for underdoped graphene, more accurately than in the static approximation of the BCS type. Estimates of the constants of electron interaction with the scalar optical phonon mode in graphene indicate that an appreciable superconducting transition temperature can be attained under a high chemical doping level of graphene.     

Analysis of a teleportation scheme involving cavity field states in a linear superposition of Fock states

We analyse a teleportation scheme of cavity field states. The experimental sketch discussed makes use of cavity quantum electrodynamics involving the interaction of Rydberg atoms with superconducting (micromaser) cavities as well as with classical microwave (Ramsey) cavities. In our scheme the Ramsey cavities and the atoms play the role of auxiliary systems used to teleport a field state, which is formed by a linear superposition of vacuum |∅〉 and the one-photon state |1〉, from a micromaser cavity to another.