Abstract Macroscopic phase coherence in superconductors enables quantum interference and phase manipulation at realistic device length scales. Numerous superconducting electronic devices are based on the modulation of the supercurrent in superconducting loops. While the overall behavior of symmetric superconducting loops has been studied, the effects of asymmetries in such devices remain under-explored and poorly understood. Here we report on an experimental and theoretical study of the flux modulation of the persistent current in a doubly connected asymmetric aluminum nanowire loop. A model considering the length and electronic cross-section asymmetries in the loop provides a quantitative account of the observations. Comparison with experiments give essential parameters such as persistent and critical currents as well as the amount of asymmetry which can provide feedback into the design of superconducting quantum devices.
Abstract Macroscopic phase coherence in superconductors enables quantum interference and phase manipulation at realistic device length scales. Numerous superconducting electronic devices are based on the modulation of the supercurrent in superconducting loops. While the overall behavior of symmetric superconducting loops has been studied, the effects of asymmetries in such devices remain under-explored and poorly understood. Here we report on an experimental and theoretical study of the flux modulation of the persistent current in a doubly connected asymmetric aluminum nanowire loop. A model considering the length and electronic cross-section asymmetries in the loop provides a quantitative account of the observations. Comparison with experiments give essential parameters such as persistent and critical currents as well as the amount of asymmetry which can provide feedback into the design of superconducting quantum devices.
We have measured the switching current versus applied magnetic flux of a highly asymmetric, hysteretic dc superconducting quantum interference device (SQUID) from 30 mK to 400 mK. The SQUID loop had an inductance of 1 nH, and the capacitances of the two Al-Al2O3-Al Josephson junctions comprising the SQUID were 100 fF and 5 fF. As expected, the switching current of the SQUID displayed a small, periodic modulation with applied flux, from which the switching currents of the large and small junctions at 30 mK were determined to be 846.3 ± 0.1 nA and 0.7 ± 0.08 nA, respectively. Measurements of an identically prepared small junction exhibited a highly suppressed switching current (IS < 10 pA), consistent with phase diffusion in this device. The observed modulation in the SQUID thus demonstrates that the quantum mechanical uncertainty in the phase difference across the small junction was significantly reduced by coupling to the large junction through the loop inductance. We discuss the sensitivity of this technique for determining the gauge-invariant phase difference of small Josephson junctions, and the implications for imaging the phase of superconductors at the atomic scale using scanning tunneling microscopy.
Direct current-superconducting quantum interference devices (SQUIDs) characterized by asymmetric Josephson junctions have never been used for applications. In order to demonstrate their potential advantages, a throughout numerical analysis of different asymmetric configurations has been carried out. A damping resistance has been included in the SQUID circuit and the thermal noise associated with junction and damping resistances has been considered in the numerical model. Magnetic modulations and flux noise spectral densities have been computed as a function of many parameters (bias current, asymmetry, SQUID inductance, and damping resistance) and the performance of symmetric and asymmetric devices have been compared. The results show that, by properly optimizing the SQUID design, asymmetric SQUIDs are characterized by higher magnetic flux to voltage transfer coefficient and lower flux noise. As a result, asymmetric configurations can be very useful in all the applications where high sensitivity is required.
The development of superconducting electronics requires careful characterization of the components that makeup electronic circuits. Superconducting weak links are the building blocks of most superconducting electronic components and are characterized by highly nonlinear current-to-phase relations (CPRs), which are often not perfectly known. Recent research has found that the Josephson diode effect (JDE) can be related to the high harmonic content of the current-to-phase relation of the weak links embedded in superconducting interferometers. This makes the JDE a natural tool for exploring the harmonic content of weak links beyond single-harmonic CPR. In this study, we present the theoretical model and experimental characterization of a double-loop superconducting quantum interference device (DL-SQUID) that embeds all-metallic superconductor-normal metal-superconductor junctions. The proposed device exhibits the JDE due to the interference of the supercurrents of three weak links in parallel, and this feature can be adjusted through two magnetic fluxes, which act as experimental knobs. We carry out a theoretical study of the device in terms of the relative weight of the interferometer arms and the experimental characterization concerning flux tunability and temperature.
Macroscopic quantum tunneling is a process by which a macroscopic object, rather than a single electron, tunnels between two macroscopically distinct quantum states. In this dissertation, we will present the study of macroscopic quantum tunneling of phase-slips in superconducting devices, including one-dimensional nanowires, quasi-two-dimensional superconducting strips and doubly connected superconducting devices composed of two nanowires connected in parallel. We observe macroscopic quantum tunneling in these devices by measuring switching current distributions. It is known that the standard deviation of switching current distributions can be measured on a single nanowire to reveal temperatures at which macroscopic quantum tunneling is responsible for phase-slips. Therefore we begin by studying higher moments of the switching current distributions, namely the skewness S (a measure of the asymmetry of a distribution) and kurtosis K (a measure of its peakedness). We find that the skewness and kurtosis of the switching current distributions obtained via the Kurkijarvi process on devices composed of single nanowires do not depend on whether the switching events are initiated by quantum or thermal phase-slips. Skewness and kurtosis deviate considerably from the values associated with a Gaussian distribution (S=0 and K=3). If, in an experiment these higher moments approach Gaussian values they indicate the existence of noise. Next we study macroscopic quantum tunneling of phase-slips in quasi-two-dimensional superconducting strips, which are commercially available devices used as single-photon detectors. These devices are composed of (250 um) long, (120 nm) wide, meandering superconducting strips. Each time a photon with sufficient energy strikes the strip a voltage pulse is produced and counted. However, the accuracy of these detectors is limited by a rate of dark counts (false events). We find that at sufficiently low temperatures, the macroscopic quantum tunneling of a vortex through an edge barrier on the wire (which results in a phase-slip as the vortex crosses the wide wire) contributes a base-level dark count rate in these detectors which must be considered during operation at low enough temperatures. After studying tunneling in single one-dimensional nanowires and wide, quasi-two-dimensional strips, we move our focus to doubly connected superconducting samples composed of two nanowires connected in parallel. We characterize and model these devices, and show their applicability as nanometer-scale superconducting memory cells. We develop precise algorithms allowing us to write and read the information onto such memory cells. We also observe signatures of macroscopic quantum tunneling in these doubly connected devices by observing a saturation in the standard deviation of switching current distributions at low temperatures. We then discuss how macroscopic quantum tunneling of the memory state can lead to a dissipationless operation of the superconducting memory cell. In each of these devices, whether single one-dimensional nanowires, wide quasi-two-dimensional superconducting strips, or two nanowires in parallel, we find, quite surprisingly, that the same equations can be used to model macroscopic quantum tunneling at the high currents we apply. Therefore we argue that the physics of macroscopic quantum tunneling in wide quasi-two-dimensional strips and doubly connected nanowire devices can be well understood in the context of tunneling in a single, one-dimensional nanowire.
Abstract We propose a qubit design based on two parallel superconducting nanowires (i.e. a ‘Dayem loop qubit’). The inclusion of two nanowires instead of one leads to the Little-Parks effect, which provides an option to control the qubit frequency in a wide range, which is possible because the vorticity remains fixed, since the rate of phase slips is exponentially low at low temperatures. Our key result is that even if each individual nanowire does not have a cubic nonlinearity in its current-phase relationship (CPR), quantum interference between two condensates, induced by a magnetic field, produces a cubic nonlinearity for the entire device. Our numerical simulations, supported by analytical solutions, show that such nonlinearity is sufficient to create functional transmon qubits based on pairs of thin homogeneous superconducting nanowires. We analyze both generic, i.e. cubic CPR, as well as more realistic microscopic CPR, having higher-order nonlinearities. For higher-order CPRs, we propose a simple power-law phenomenological approximation valid at very low temperatures, at which superconducting qubits normally operate.
We present the design of a superconducting qubit that has circulating currents of opposite sign as its two states. The circuit consists of three nanoscale aluminum Josephson junctions connected in a superconducting loop and controlled by magnetic fields. The advantages of this qubit are that it can be made insensitive to background charges in the substrate, the flux in the two states can be detected with a superconducting quantum interference device, and the states can be manipulated with magnetic fields. Coupled systems of qubits are also discussed as well as sources of decoherence.