ac Josephson effect and resonant Cooper pair tunneling emission of a single Cooper pair transistor

We measure the high-frequency emission of a single Cooper pair transistor (SCPT) in the regime where transport is only due to tunneling of Cooper pairs. This is achieved by coupling on chip the SCPT to a superconductor-insulator-superconductor junction and by measuring the photon assisted tunneling current of quasiparticles across the junction. This technique allows a direct detection of the ac Josephson effect of the SCPT and provides evidence of Landau-Zener transitions for proper gate voltage. The emission in the regime of resonant Cooper pair tunneling is also investigated. It is interpreted in terms of transitions between charge states coupled by the Josephson effect.     

Quantum transitions of a small Josephson junction array

We have experimentally studied a small Josephson junction array in the presence of microwave irradiation. The array has comparable energy scales for single-charge effects and the Josephson effect, resulting in a discrete set of macroscopic eigenenergy levels. Excitation of the array by low-power microwaves is possible at frequencies where the photon energy matches the level spacing. The microwave frequency and amplitude dependence show that the excitation mechanism involves resonant quantum coherent dynamics of the array.     

Microwave cooling of Josephson plasma oscillations

An extended Josephson junction can be described as a microwave cavity coupled to a Josephson oscillator. This is formally equivalent to a Fabry-Perot cavity with a freely vibrating mirror, where it has been shown that radiation pressure from photons in the cavity can reduce (increase) the vibrations of the mirror, effectively cooling (heating) it. We demonstrate that, similarly, the superconducting phase difference across a Josephson junction-the Josephson phase-can be "cooled" or "heated" by microwave excitation of the junction and that both these effects increase with microwave power.     

约瑟夫森结跳变电流在微波辐照下的分布

在16 mK的极低温下,我们对电流偏置的Nb/AlOx/Nb结进行了跳变电流统计分布的测量.在没有外加微波的辐照时,分布曲线只有一个峰;在有外加微波的辐照时,其跳变电流的统计分布除了基峰外,会出现谐振峰,形成双峰结构.该谐振峰的位置和结的量子化的能级相关,其高度随外加微波功率的改变而改变.研究约瑟夫森结跳变电流在微波辐照下的分布,对于研究超导量子比特的能级量子化、Rabi振荡等具有重要的意义.     

Bright side of the Coulomb blockade

We explore the photonic (bright) side of the dynamical Coulomb blockade (DCB) by measuring the radiation emitted by a dc voltage-biased Josephson junction embedded in a microwave resonator. In this regime Cooper pair tunneling is inelastic and associated with the transfer of an energy 2eV into the resonator modes. We have measured simultaneously the Cooper pair current and the photon emission rate at the resonance frequency of the resonator. Our results show two regimes, in which each tunneling Cooper pair emits either one or two photons into the resonator. The spectral properties of the emitted radiation are accounted for by an extension to DCB theory.     

Contribution of the triplet component of the supercurrent-carrying density of states to the Josephson current in a nonequilibrium superconductor/ferromagnet/superconductor junction

A weak link of two superconductors with s-type pairing through a ferromagnet has been theoretically investigated in the regime of a nonequilibrium spin-dependent distribution of electrons over energy levels in a ferromagnetic interlayer. It has been shown that, under the given conditions, the triplet component of the supercurrent-carrying density of states, which does not participate in the Josephson current transfer under equilibrium and spin-independent nonequilibrium conditions, is involved in the Josephson current transfer through the junction. In this case, the standard supercurrent transferred by the singlet component of the supercurrent-carrying density of states remains unchanged as compared to the case of the equilibrium distribution of electrons in the interlayer. An additional current transferred by the triplet component is controlled by a voltage that controls the specific shape and the degree of nonequilibrium of the electron distribution function in the interlayer. Depending on this controlling parameter, the additional current can substantially amplify or attenuate the standard supercurrent and also switch the junction between 0 and π states.     

微波场中Josephson结的微波发射与吸收研究

采用数值仿真方法,研究微波场中约瑟夫森结微波感应台阶处的功率特性。发现微波场中约瑟夫森结不止吸收辐射,也发射辐射的情况。得到微波辐照和结电阻对功率吸收和发射的影响规律。结果表明,处于发射功率的状态对应的偏置电流范围占整个台阶总电流范围的比例可以趋于50%。这一发现对约瑟夫森结理论和应用研究具有重要的意义。     

Tunneling spectroscopy of two-level systems inside a Josephson junction

We consider a two-level system (TLS) with energy level separation plankvOmega0 inside a Josephson junction. The junction is shunted by a resistor R and is voltage V biased. If the TLS modulates the Josephson energy and/or is optically active, it is Rabi driven by the Josephson oscillations in the running phase regime near the resonance 2eV=plankvOmega0. The Rabi oscillations, in turn, translate into oscillations of current and voltage that can be detected in noise measurements. This effect provides an option to fully characterize the TLS inside Josephson junction and to find the TLS's contribution to the decoherence when the junction is used as a qubit.     

Narrow-band microwave radiation from a biased single-Cooper-pair transistor

We show that a single-Cooper-pair transistor (SCPT) electrometer emits narrow-band microwave radiation when biased in its subgap region. Photoexcitation of quasiparticle tunneling in a nearby SCPT is used to spectroscopically detect this radiation in a configuration that closely mimics a qubit-electrometer integrated circuit. We identify emission lines due to Josephson radiation and radiative transport processes in the electrometer and argue that a dissipative superconducting electrometer can severely disrupt the system it attempts to measure.     

Effect of Cherenkov dissipation on vortex velocity in a waveguide-coupled Josephson junction

A motion of slow and fast vortices in a waveguide-coupled Josephson junction induced by a transport current flowing through the entire structure is studied; the coupling is not assumed to be weak. For a fast vortex, conditions are established under which current oscillations due to energy dissipation via Cherenkov radiation of Swihart waves become comparable to the current compensating for ohmic losses in the Josephson junction, waveguide, and adjacent superconductors. For a slow vortex, it is proved that intermediate and strong couplings of the Josephson junction to the waveguide shift current oscillations to the velocity range below the Swihart velocity of the Josephson junction.     

High-efficiency deterministic Josephson vortex ratchet

We investigate experimentally a Josephson vortex ratchet--a fluxon in an asymmetric periodic potential driven by a deterministic force with zero time average. The highly asymmetric periodic potential is created in an underdamped annular long Josephson junction by means of a current injector providing an efficiency of the device up to 91%. We measured the ratchet effect for driving forces with different spectral content. For monochromatic high-frequency drive the rectified voltage becomes quantized. At high driving frequencies we also observe chaos, subharmonic dynamics, and voltage reversal due to the inertial mass of a fluxon.     

Phase escape of current-biased Josephson junctions

Switching current distributions of an Nb/Al--AlO_x/Nb Josephson junction are measured in a temperature range from 25~mK to 800~mK. We analyse the phase escape properties by using the theory of Larkin and Ovchinnikov (LO) which takes discrete energy levels into account. Our results show that the phase escape can be well described by the LO approach for temperatures near and below the crossover from thermal activation to macroscopic quantum tunneling. These results are helpful for further study of macroscopic quantum phenomena in Josephson junctions where discrete energy levels need to be considered.     

嵌入Fabry-Perot谐振腔的高温超导双晶约瑟夫森结的毫米波辐照特性研究

采用嵌入Fabry-Perot谐振腔的方式,研究了Tl2Ba2CaCu2O8(Tl-2212)高温超导薄膜双晶约瑟夫森结的毫米波辐照特性.重点研究了双晶约瑟夫森结与Fabry-Perot谐振腔的耦合特性,发现放置在谐振腔中的结的位置、角度、结的图形以及Fabry-Perot谐振腔中两镜面间的距离等对耦合都有很大的影响.通过精细调节这些参数,可使双晶约瑟夫森结与外加毫米波达到最佳耦合.在最佳耦合情况下,能观察到9级明显的夏皮罗台阶.利用Fabry-Perot谐振腔技术,解决了约瑟夫森结与毫米波的耦合问题,为高温超导约瑟夫森结的毫米波及太赫兹波的辐射和检测研究奠定了基础.     

Phase-charge duality of a josephson junction in a fluctuating electromagnetic environment

We have measured the current-voltage characteristics of a single Josephson junction placed in a high impedance environment. The transfer of Cooper pairs through the junction is governed by overdamped quasicharge dynamics, leading to Coulomb blockade and Bloch oscillations. Exact duality exists to the standard overdamped phase dynamics of a Josephson junction, resulting in a dual shape of the current-voltage characteristic, with current and voltage changing roles. We demonstrate this duality with experiments which allow for a quantitative comparison with a theory that includes the effect of fluctuations due to the finite temperature of the electromagnetic environment.     

Microwave photon counter based on Josephson junctions

We describe a microwave photon counter based on the current-biased Josephson junction. The junction is tuned to absorb single microwave photons from the incident field, after which it tunnels into a classically observable voltage state. Using two such detectors, we have performed a microwave version of the Hanbury Brown-Twiss experiment at 4 GHz and demonstrated a clear signature of photon bunching for a thermal source. The design is readily scalable to tens of parallelized junctions, a configuration that would allow number-resolved counting of microwave photons.     

Influence of variable current on the plasma frequency, barrier height, and thermal activation threshold in the Josephson junction

The problem of simultaneous renormalization of the plasma frequency, barrier height, and thermal activation threshold by high-frequency current passing through a Josephson junction is solved exactly. Renormalization is accomplished by a single universal function.     

高温超导Josephson阵列体系中的磁通涡旋势能分布

本从二维Josephson弱连接网络模型出发，讨论了单个磁通涡旋的势能分布，并利用数值分析，得出势能分布表达式，其分布呈余弦曲线，在此基础上，进一步研究了外磁场下的磁通涡旋势能分布，从分布图可以看出，在磁场作用下，势能增加，并随着最小网格单元内的磁通量子数f的不同，能量变化也不一样，但也呈现周期性。     

利用超导结的直流约瑟夫逊效应探测β粒子的初步实验结果

利用超导结的直流约瑟夫逊效应探测粒子,具有能量分辨率高和时间响应快的优点。我们首次进行了用Sr⁹⁰-Y⁹⁰β源辐照Nb-NbO_x-Pb超导结的实验。结果表明:在β粒子的辐照下,不仅超导结的临界电流和能隙电压都变小,而且在超导结的两端产生电压脉冲讯号。本文报道了利用直流约瑟夫逊效应探测β粒子所观察到的一些现象。 关键词：     

Fast Generation of 2N-Photon Fock States using Shortcuts to Adiabaticity and Ultrastrong Light–Matter Coupling

By using shortcuts to adiabatic (STA) method, the proposal is to implement a fast multi-photon down-conversion, which can rapidly create 2N photons from the quantum vacuum based on the counter-rotating effect of an ultrastrong light–matter coupling. The energy for the produced photons is given by a high-frequency pump field. The STA method is used to design the driving fields to induce a rapid population transfer. Such an accelerated evolution can restrain the influence of decoherence during the evolution, so as to generate Fock states from vacuum with high fidelities.     

Mode selection in a neuron driven by Josephson junction current in presence of magnetic field

Josephson junction is an active electric component and its channel current can be adjusted by external magnetic field, which can trigger additive phase error along the junction. From physical viewpoint, the Josephson junction can capture and release field energy when it is exposed to a magnetic field, and this time-varying current can be used to excite neural circuit for generating target firing patterns. In this paper, a Josephson junction is connected to a simple neural circuit, which is made of a capacitor, induction coil, a nonlinear resistor, two linear resistors and one constant voltage source in the branch, and the improved neural circuit is adjusted to percept external magnetic field and estimate the potential application of Josephson junction in nonlinear circuits. Bifurcation analysis is applied to explore the mode selection and dynamics dependence on parameters setting in the biophysical neural circuits. Furthermore, the neural circuit is exposed to external magnetic field and its physical effect is estimated by applying scale transformation on the variables and parameters in the neural circuit. It is confirmed that the neural circuit can be controlled and the neural activity shows distinct mode transition by taming the intensity (or angular frequency in periodic field) of external magnetic field. This kind of neural circuit can be further used as smart sensor for detecting weak magnetic field.