Radio Frequency SQUID Research Articles

Novel Stable and Reliable Readout Electronics for HTS rf SQUID

Conventional readout electronics for radio-frequency SQUIDs exhibit long-term instabilities due to a temperature-dependent frequency drift of their voltage controlled oscillator. A novel electronics with a quartz-controlled synthesizer was developed to solve this problem. The frequency can be adjusted from 450 to 900 MHz with 2 ppm frequency stability, the amplitude range covers more than 60 dB. An adjustable phase shifter for demodulation is implemented to compensate for variable cable length. The total amplification is about 100 dB. The electronics with touchpanel and remote USB control is commercially available with up to three preamplifier units. Because thermal drift is eliminated, it is well suited for outdoor applications, e.g. in geophysics.

Superconducting phase qubit based on the Josephson oscillator with strong anharmonicity

We propose a superconducting phase qubit on the basis of the radio-frequency SQUID with the screening parameter value $\beta_L = (2\pi/\Phi_0)LI_c \approx 1$, biased by a half flux quantum $\Phi_e=\Phi_0/2$. Significant anharmonicity ($> 30%$) can be achieved in this system due to the interplay of the cosine Josephson potential and the parabolic magnetic-energy potential that ultimately leads to the quartic polynomial shape of the well. The two lowest eigenstates in this global minimum perfectly suit for the qubit which is insensitive to the charge variable, biased in the optimal point and allows an efficient dispersive readout. Moreover, the transition frequency in this qubit can be tuned within an appreciable range allowing variable qubit-qubit coupling.

Analysis of Some Nondestructive Evaluation Experiments Using Eddy Currents

Nondestructive evaluation (NDE) is often conducted by inducing eddy currents in the materials to be examined, and characterizing those currents by the magnetic fields that they produce. Most papers describe experimental results produced by such methods; very few theoretical approaches have been proposed. In the present article, we develop some particularly very simple but ad-hoc analytical studies, notably in the case of various calibrated flaws such as cracks. The validity of the models has been checked with a classical High Tc radio frequency SQUID, which is described. Theory and measurement have been found to be in good agreement.

Low frequency noise of high- Tc radio-frequency SQUIDs based on grain boundary Josephson junctions

Noise properties of single-layer washer-type high- T c YBa 2Cu 3O 7− x rf SQUIDs with 30° grain boundary Josephson junctions have been investigated. We show experimentally that the additional low-frequency noise is produced by the magnetic flux dynamics in the “unwanted” junction in high- T c rf SQUIDs which is formed by the grain boundary running through the washer of the SQUIDs on bicrystal substrates. In order to decrease the low-frequency noise we have designed rf SQUIDs with two strongly asymmetrical Josephson junctions.

Signal properties of radio frequency SQUID with a finite amplitude of the second harmonic in the current-phase relationship

The signal properties of radio frequency (rf) SQUIDs with a finite amplitude of the second harmonic in the current-phase relationship have been theoretically and experimentally investigated. Both amplitude as well as phase detection has been considered in order to readout the SQUID's output signal. We observed peculiarities of the rf SQUID response of a SQUID with YBCO 45 degree grain boundary Josephson junction. We found that the current-phase relationship contains a finite amplitude of the second harmonic. Comparison of the theoretical and experimental results is given. It is shown that the sign of the amplitude of the second harmonic can be determined from the shape of experimental curves, and the value of this amplitude is defined by the positions of extrema of these curves.

A likelihood ratio test applied to a radio-frequency SQUID Hamiltonian

In this Letter, we propose the use of a likelihood ratio test to distinguish between different models for the capacitance of the radio-frequency SQUID ring, treated as a macroscopic quantum object in thermal equilibrium with its environment.

High frequency coupling coefficient between the coplanar resonator and radio frequency SQUID

The coupling coefficient (k/sup 2/) between the superconducting coplanar resonator and the flip chip coupled washer radio frequency SQUID was calculated using computer simulation. The coplanar resonator is formed by two coplanar lines surrounding a flux concentrator with each line having a slit. For a 3 mm/spl times/3 mm washer SQUID with a 100 /spl mu/m/spl times/100 /spl mu/m loop coupled to a coplanar resonator with the slits on opposite sides, we estimated k/sup 2/=7.1/spl times/10/sup -3/. However, if the same SQUID is coupled to a coplanar resonator with parallel slits and with a short circuit at the 90/spl deg/ position with respect to the opening slit, k/sup 2/ of only 0.5/spl times/10/sup -3/ is obtained, which indicates that the k/sup 2/ is strongly dependent on the resonator layout. The simulation also shows that k/sup 2/ is dependent on the SQUID washer size and the separation distance between the washer SQUID and the coplanar resonator.

Zero-dispersion stochastic resonance in a model for a superconducting quantum interference device

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 SQUID) can be substantially enhanced, over a wide range of frequencies, by the addition of noise. This manifestation of zero-dispersion stochastic resonance (ZDSR) 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: (a) an asymptotic expression describing ZDSR for the general case in the limit of weak dissipation; (b) a method for the numerical calculation of fluctuation spectra in bistable or multistable underdamped systems.

High-temperature superconducting SQUID as calibrator of ac voltage dividers and LF voltmeters

The performance of a radio-frequency SQUID made of high-temperature superconducting ceramics as a calibrator of ac voltage dividers and LF voltmeters at frequencies up to 1 MHz is investigated. The measurement technique and errors are analyzed. The obtained results suggest that in principle the high-temperature superconducting device can reproduce the voltage divider scale with an error less than 10−3 dB within an input signal range from several microvolts to several volts.

Staircase pattern in a superfluid He-II torsional-oscillator analog of the radio-frequency SQUID.

We are developing a superfluid [sup 4]He analog of the rf SQUID. A thin partition with a micrometric orifice is placed inside a toroidal container filled with liquid helium; ac and dc superflows through the orifice are induced by rotating the whole torus, which is the inertial member of a torsional oscillator. The torus oscillation amplitude vs drive curve shows a small but reproducible staircase pattern, which is detectable between 1.4 and 2.05 K, but is not modulated by a superimposed steady rotation. We discuss this behavior in terms of the occurrence of multiple phase slips.

Non-linear behaviour in the rf-SQUID magnetometer

Using a particular example, the radio frequency SQUID magnetometer we introduce a novel type of non-linear system, where a non-linearity arises in a linear classical oscillator due to the presence of a macroscopic quantum object. The non-linear behaviour found in this system is discussed and related to possible experimental investigation of the joint system, and of the quantum object in particular.

Planar microwave biased radio frequency SQUIDs with a cryogenic preamplifier

We report on the design and performance of a planar microwave rf-SQUID readout by a cryogenic preamplifier. The SQUID-sensor consists of a planar half-wavelength microstrip resonator, into which the SQUID-loop is integrated. The pump frequency is 1.7 GHz. The cryogenic preamplifier employs a microwave high electron mobility transistor (HEMT), located close to the SQUID sensor. Measurements of the flux noise and the fractional step rise parameter were carried out in the temperature range between 4.2 and 1.5 K. In an open loop white flux noise values of 2×10−6 Φ0/√Hz were obtained at 4.2 K.

Performance of Bi-Sr-Ca-Cu-O thick film rf SQUIDs at liquid nitrogen temperatures

Periodic oscillations in the V-Φ characteristics have been observed up to 97 K in radio frequency SQUIDs fabricated from screen printed Bi-Sr-Ca-Cu-O thick films. The SQUID has been operated successfully in the flux-locked-loop mode at 77 K. Stability of the SQUID output and its noise performance have been found to improve considerably when the SQUID is operated in the flux-locked-loop mode as compared to that in the open-loop mode. The spectral noise density, in the white noise region, is 5×10−4 Φ0/√Hz at 77 K for the flux-locked-loop mode.

Macroscopic quantum interference and superconducting glass state in YBa2(Cu1−xFex)3O7−δ

Macroscopic quantum phenomena and superconducting glass state have been observed simultaneously in a random system of linked Josephson junctions in lumps of suitable high temperature superconducting ceramics Y-Ba-Cu-O. This multiply connected Josephson medium was used for radio-frequency SQUID operation at temperatures up to 90 K. The effect of nonergodic glass state on SQUID behaviour readily was observed. The presence of a wide range of relaxation times in glass state leads to an appreciable noise of 1/f type in the SQUID output signal at f ≲ 103 Hz.

Thermal magnetization noise as a function of frequency in amorphous ferromagnets

We report the observation of 1/frequency magnetization noise in toroidal strip-wound cores of amorphous ferromagnetic alloys at liquid-helium temperatures and in the frequency range 0.1 Hz–1 kHz. The materials tested are Co-based alloys, and exhibit very low magnetostriction and good soft magnetic properties. The experimental apparatus consists of a radio frequency SQUID magnetometer coupled to the sample by a superconducting flux transformer which is devised to allow measurements of its complex inductance. We show that the magnetization noise obeys in detail the fluctuation-dissipation theorem. A temperature-dependent partial roll-off of the complex permeability is found at ∼100 Hz, possibly linked to pinning wells for domain-wall motion. The link between 1/frequency noise and magnetic viscosity is briefly discussed.

Noise measurements in a two-hole radio frequency SQUID at liquid-nitrogen temperature

The authors report measurements of the noise in a two-hole RF SQUID fabricated from YBCO by an erosion technique. The noise was measured from 10 mHz to 1 kHz. Above 25 Hz the noise is essentially white with an RMS value of 1.3*10-3 phi 0 Hz-1/2 and is dominated by the noise of the amplifier. Below 1 Hz the noise power rises as 1/f1.3.

Two hole SQUID behaviour in a bulk YBCO sample at 77K

We describe the development of a 2-hole radio frequency SQUID made from a single disc of YBa 2Cu 3O 7−y. The device displays single flux quantum sensitivity and an estimated noise level of 1 × 10 −3φ o/Hz above 50Hz.

Radio-frequency SQUID operation using a ceramic high-temperature superconductor

The recent discovery1 of ceramic superconductors with high transition temperatures opens the way for the development of superconducting devices operating at liquid-nitrogen temperature (77 K). Of particular interest is the possibility of constructing a radio-frequency superconducting quantum interference device (r.f. SQUID), for use as an extremely sensitive magnetometer2. The operation of the r.f. SQUID is also of interest as a means of investigating superconductivity in the new ceramic materials. Here we report the construction of an r.f. SQUID based on the multi-phase high-temperature superconducting ceramic Y1.2Ba0.8CuO4 (ref. 1). We have used this as a magnetometer operating at 4.2 K and have observed unusual SQUID behaviour which may be due to the presence of many weakly coupled superconducting paths within the material1,3. We have also observed SQUID behaviour at temperatures as high as 45 K.