Stochastic resonance in an RF SQUID with shunted ScS junction (original) (raw)
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Zero-dispersion stochastic resonance in a model for a superconducting quantum interference device
Physical Review E, 1998
It is demonstrated that the signal-to-noise ratio for a weak periodic signal in a superconductive loop with a Josephson junction ͑a superconducting quantum interference device, or SQUID͒ can be substantially enhanced, over a wide range of frequencies, by the addition of noise. This manifestation of zero-dispersion stochastic resonance ͑ZDSR͒ is shown to occur for a wide variety of loop parameters and signal frequencies. Unlike most earlier examples of stochastic resonance, ZDSR does not depend on fluctuational transitions between coexisting stable states. Rather, it exploits the noise-enhanced susceptibility that arises in underdamped nonlinear oscillators for which the oscillation eigenfrequency possesses one or more extrema as a function of energy. The phenomenon is investigated theoretically, and by means of analog and digital simulations. It is suggested that ZDSR could be used to enhance the sensitivity of radio-frequency SQUIDs and other SQUID-based devices. In the course of the work, two additional useful results were obtained: ͑a͒ an asymptotic expression describing ZDSR for the general case in the limit of weak dissipation; ͑b͒ a method for the numerical calculation of fluctuation spectra in bistable or multistable underdamped systems. ͓S1063-651X͑97͒08112-9͔
High-frequency stochastic resonance in SQUIDs
Physics Letters A, 1996
It is shown theoretically and by analogue electronic experiment that stochastic resonance (SR), in which a weak periodic signal can be optimally enhanced by the addition of noise of appropriate intensity, is to be anticipated in underdamped SQUIDs (superconducting quantum interference devices). It manifests under conditions quite unlike those needed for classical SR, which is restricted to low frequencies and confined to systems that are both overdamped and bistable. The zero-dispersion SR reported here can be expected over a vastly wider, tunable, range of high frequencies in highly underdamped SQUIDs that need not necessarily be bistable.
A coupled DC SQUID with low 1/f noise
IEEE Transactions on Appiled Superconductivity, 1993
A low-noise coupled DC superconducting quantum interference device (SQUID) especially optimized for low frequency is discussed. Using large Josephson junctions and a low loop inductance the contribution of the critical current fluctuation to 1/f noise can be minimized. To minimize the inductance the Josephson junctions are placed in the center of the washer close to the SQUID loop. A scaled-up
Journal of Applied Physics, 2000
We measured the amplitude-frequency characteristics of radio frequency superconducting quantum interference devices ͑rf SQUIDs͒ 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 ͑HTS͒ rf SQUIDs. These parameters were: ͑a͒ the high-frequency coupling coefficient between the rf SQUID and the tank circuit resonator, k, ͑b͒ the SQUIDs hysteretic parameter, , ͑c͒ the critical current of the Josephson junction, I c , ͑d͒ its normal resistance, R n , and ͑e͒ its noise parameter, ⌫. We found a good agreement with the values of (I c) and R n determined directly after destructively opening the SQUID loop. In accordance with the theoretical predictions, our experimental results show that at large thermal fluctuation levels (TХ77 K), rf SQUIDs with large loop inductance operate in nonhysteretic mode up to  values exceeding 3. Furthermore, we have shown that the optimal energy sensitivity is attained in the nonhysteretic mode at a value of  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.
Quantum State Engineering With the Rf-SQUID: A Brief Introduction
Arxiv preprint quant-ph/0307101, 2003
The SQUID, or superconducting quantum interference device, is a highly sensitive instrument employed for the nondestructive measurement of magnetic fields, with a host of applications in both biophysics and materials technology. It is composed of a cooled superconductive metal ring separated by a thin insulating barrier of non-superconducting metal. Electrons tunnel across the barrier to form a Josephson junction; an rf SQUID is essentially a Josephson junction with tunable current and energy. Quantum computers take advantage of the superpositional logic of quantum mechanics to allow for dramatic increases in computational efficiency. rf SQUIDs show potential for quantum computing applications by forming the qubit component of a quantum computer, through simply treating the direction of current, clockwise or counterclockwise, as the value of the bit.
Applied Physics Letters, 1999
A low-noise single-chip two-stage superconducting quantum interference device ͑SQUID͒ system with a double relaxation oscillation SQUID as the second stage has been realized. The system was operated in a direct voltage readout mode, with a closed loop bandwidth up to 1 MHz. Operated at 4.2 K, the white flux noise measured in flux locked loop was 1.3 ⌽ 0 /ͱHz, corresponding to an energy sensitivity of ⑀Ϸ27h. Owing to the large flux-to-voltage transfer of up to 3.6 mV/⌽ 0 , the room-temperature preamplifier noise did not dominate the overall flux noise.
Potential Characterization of a Double SQUID Device for Quantum Computing Experiments
IEEE Transactions on Applied Superconductivity, 2007
We report on experiments performed on a system consisting of a double SQUID (superconducting quantum interference device) built with gradiometer geometry. Two single-turn coils provide two independent control fluxes: one of these allows biasing the device and tilting the potential, while the other changes the barrier height of the potential. When the dynamics of the inner dc SQUID can be neglected, the free energy of the double SQUID, as a function of the internal magnetic flux, is just the corrugated parabola of an rf SQUID whose local minima represent metastable states for the system. Our analysis instead is substantially concerned with the interesting phenomenology generated by the static configurations of an internal two-junction interferometer and by the tunability of the internal loop inductance. Two readout systems are employed to thoroughly characterize the dynamics of our system. We investigate the dynamical response at temperatures low enough (tens of mK) to minimize the effects of thermal fluctuations concentrating the analysis on the aspects that could be relevant for macroscopic quantum coherence and computing. The results indicate that from the finite inductance of the inner loop originates a potential well generating competing processes with the tunneling between the two main wells of the rf-SQUID potential.
Magnetic field behavior of YBCO step-edge Josephson junctions in rf-washer SQUIDs
IEEE Transactions on Appiled Superconductivity, 2001
Abstruct-The suppression of the critical current in YBCO Josephson junctions by the Earth's magnetic field strongly affects the operation of SQUIDs outside magnetic shielding. Commonly, one observes a modulation of the SQUID fluxvoltage transfer function amplitude, Vsq.pp, with a period of AB,, leading to an increased white flux noise level or unstable SQUID operation. Here, we report on the investigation of AB,., of rf-SQUID sensors based on step edge junctions (SEJ) operated in a flip chip configuration with coplanar resonators with integrated flux concentrators. To investigate the origin of the suppression of Vsq-pp, we opened the SQUID loop of some samples and measured the magnetic field dependence of the critical current I,(B) directly and compared it to VSqJB). It is shown that a junction width in the submicrometer scale is required for operation of the sensors in the Earth's magnetic field.
Peculiarities of rf SQUID response in finite magnetic fields
Physica C-superconductivity and Its Applications, 2000
Single-layer washer-type high-T YBa Cu O rf SQUIDs with grain-boundary Josephson junctions, as well as low-T c 2 3 7 yx c Nb rf SQUIDs with Nb-Al O -Nb tunnel junctions, have been investigated in finite magnetic fields. It was shown 2 3
High-Tc rf SQUIDs with large inductance of quantization loop
Physica C: Superconductivity, 2004
Experimental data on signal and noise characteristics of high-T c RF SQUIDs with large inductance of quantization loop are presented. The SQUIDs were produced by a thick-film HTS-technique of painting on the Y 2 BaCuO 5 substrate. For the first time, a steady quantum interference was observed in RF SQUID with the inductance as large as L S = 6.6 nH, that is, 60 times higher than the fluctuation inductance L F ≃ 10 −10 H at the liquid nitrogen temperature T = 77 K. A new method is offered to evaluate the sensitivity of RF SQUID with optimum inductance of the quantization loop.