Low-frequency noise in a hysteric rf superconducting quantum interference device (SQUID) originating from fluctuations of critical current of a weak link is considered. Such noise can be due to trapping-detrapping processes within the tunnel barrier or to the presence of structural defects with internal degrees of freedom in the surrounding of the weak link. The defects can switch between two (or more) metastable states due to interaction with the thermal bath. On the other hand, elastic (and electric) fields created by defects act upon the critical current of the weak link inducing its fluctuations in time. Such noise is most important for high-Tc SQUIDs at high enough temperatures when defects are effectively interacting with the thermal bath. The dependencies of the intensity and statistics of flicker noise on the SQUID’s parameters and operating regime are analyzed. It is shown that higher correlation functions of output voltage at a given driving current can provide some information on the mechanism of 1/f noise in rf SQUIDs.
Low-frequency noise in a hysteric rf superconducting quantum interference device (SQUID) originating from fluctuations of critical current of a weak link is considered. Such noise can be due to trapping-detrapping processes within the tunnel barrier or to the presence of structural defects with internal degrees of freedom in the surrounding of the weak link. The defects can switch between two (or more) metastable states due to interaction with the thermal bath. On the other hand, elastic (and electric) fields created by defects act upon the critical current of the weak link inducing its fluctuations in time. Such noise is most important for high-Tc SQUIDs at high enough temperatures when defects are effectively interacting with the thermal bath. The dependencies of the intensity and statistics of flicker noise on the SQUID’s parameters and operating regime are analyzed. It is shown that higher correlation functions of output voltage at a given driving current can provide some information on the mechanism of 1/f noise in rf SQUIDs.
A detailed study has been made of the low-frequency excess noise of rf superconducting quantum interference devices (SQUIDs), fabricated from thin niobium films and operated at 4.2 K, with rf bias frequencies of 0.15, 1.7, and 3 GHz. When the SQUIDs were operated in an open-loop configuration in the absence of low-frequency flux modulation, the demodulated rf voltage exhibited a substantial level 1/f noise, which was essentially independent of the rf bias frequency. As the rf bias frequency was increased, the crossover frequency at which the 1/f noise power was equal to the white noise power moved to higher frequencies, because of the reduction in white noise. On the other hand, when the SQUID was flux modulated at 50 kHz and operated in a flux locked loop, no 1/f noise was observed at frequencies above 0.5 Hz. A detailed description of how the combination of rf bias and flux modulation removes 1/f noise due to critical current fluctuations is given. Thus, the results demonstrate that the 1/f noise observed in these SQUIDs is generated by critical current fluctuations, rather than by the hopping of flux vortices in the niobium films.
We measured the amplitude–frequency characteristics of radio frequency superconducting quantum interference devices (rf SQUIDs) over a temperature range between 65 and 79 K. Using the expressions derived from the recently developed rf SQUID theory, valid also at large thermal fluctuations, we determined from these data the basic parameters of high-transition-temperature superconductor (HTS) rf SQUIDs. These parameters were: (a) the high-frequency coupling coefficient between the rf SQUID and the tank circuit resonator, k, (b) the SQUIDs hysteretic parameter, β, (c) the critical current of the Josephson junction, Ic, (d) its normal resistance, Rn, and (e) its noise parameter, Γ. We found a good agreement with the values of β(Ic) and Rn determined directly after destructively opening the SQUID loop. In accordance with the theoretical predictions, our experimental results show that at large thermal fluctuation levels (T≅77 K), rf SQUIDs with large loop inductance operate in nonhysteretic mode up to β values exceeding 3. Furthermore, we have shown that the optimal energy sensitivity is attained in the nonhysteretic mode at a value of β distinctly higher than 1. A quantitative comparison of white noise predicted by the theory with that obtained from the experiment showed a reasonable agreement. We also discussed the contribution of the phase information to the SQUID’s signal and noise at optimum operation conditions, when a mixer was used as a signal detector.
We have measured the low-frequency noise in the critical current Ic of six dc superconducting quantum interference devices (SQUIDs) with resistively shunted Nb–NbOx–PbIn Josephson junctions in the temperature range T=0.09–4.2K. Each device is voltage biased, the applied flux is an integer number of flux quanta, and the current fluctuations are measured with a second dc SQUID. At low frequencies f, there is a component of the power spectrum of the critical current fluctuations given approximately by SIc(f)=CIc2T2∕Af, where A is the area of both junctions, and C≈(3.9±0.4)×10−23m2∕K2. For quantum bits based on Josephson junctions, the scaling of SIc(f) with T2 implies that the dephasing time limited by critical current l∕f noise should scale as 1∕T for temperatures down to at least 0.09K.
At low temperature and low damping, the magnetic flux of a rf Superconducting QUantum Interference Device (SQUID) acts as a macroscopic quantum variable whose dynamic is described by a double well potential. A rf SQUID consists of a superconducting loop interrupted by a thin insulating barrier, known as a Josephson junction. The modified rf SQUID design discussed in this thesis has independent, in situ, controls that allow varying the barrier height between the potential wells and their relative tilt. This high level of control of the potential together with the capability to implement an experimental setup optimized to reduce coupling to external sources of noise, made this system ideal not only for tests of quantum transition at a macroscopic level, but also as a scalable solid state device for quantum computation. A variety of measurements of quantum phenomena are reported in this thesis, including microwave level spectroscopy, coherent oscillations, and macroscopic resonant tunneling (MRT). These measurements are compared with theoretical models to study the possible sources of decoherence in a rf SQUID qubit. The analysis confirmed that the main source of decoherence is not due to coupling to the external environment, but it can be linked to intrinsic low frequency flux noise. Work from many groups over the last several years has shown that the performance of a superconductive qubit is limited by coupling to spurious defects in the materials used for the device fabrication. In this work we analyze the possibility of using the MRT as a tool for future studies of effects of materials and fabrication process in the realization of improved qubits, and we also explore the feasibility for measurements of quantum phenomena that involve fast pulsing of the potential barrier height.
The authors investigated low-frequency noise in RF and DC superconducting quantum interference devices (SQUIDs) with step-edge Josephson junctions (SEJs). The noise properties have been measured as a function of frequency, temperature, bias current, and magnetic field. The low-frequency noise has a telegraph-like character in certain conditions and depends quasi-periodically on flux and temperature. This telegraph noise was also observed in single step-edge junctions. The experimental results were explained by a model of such a junction in which an array of parallel and series connected weak links acts like a two-level fluctuator. This fluctuator arises from the quantum states of a multijunction SQUID with spatially inhomogeneous SEJs resembling a multijunction array.>
The subject of electronic noise in high- superconducting Josephson devices and in their applications is considered. Several types of grain boundary junctions, prepared in different ways by four separate international laboratories, are fully characterized in terms of their electrical and noise properties at a range of temperatures, frequencies, magnetic fields and in the presence of microwaves. Similar characterization is carried out for multijunction Josephson flux-flow arrays, and bi-epitaxial SQUIDs. The theory of Likharev and Semenov for thermal noise in low- junctions is adopted as a reference against which excess noise can be identified in high- junctions. It is combined with theoretical models of excess noise based on fluctuations of critical current and resistance to provide accurate fitting of the observed noise curves. It is shown that the normalized levels of critical current and normal resistance fluctuations for all of the junction types examined are remarkably close (typically around and at 100 Hz) and are nearly independent of temperature in the range T = 30 - 80 K. The frequency dependence is close to the universal 1/f law with some deviations due to trapping of charge carriers at discrete states in the tunnelling barrier. Measurements of voltage noise levels and critical current fluctuations in multijunction flux-flow amplifiers, reported for the first time, are consistent with levels in single junctions of the same type. Excess noise is studied as a function of external magnetic field, in a way that has not been previously described. A new interpretation of the magnetic field-dependent noise data indicates that absolute levels of critical current fluctuations are nearly independent of applied magnetic field. This observation does not appear to have been reported elsewhere. It has important implications for the operation of grain boundary Josephson devices in real applications. Voltage noise in the presence of microwave irradiation, at operating points between Shapiro steps, is consistent with a modified form of the Likharev - Semenov equation which takes into account the known level of critical current fluctuations as measured in field-free conditions. The implications of these results for further improvements in device performance are discussed, and directions for further work are suggested.