Critical current noise investigations in underdamped Josephson devices
An experimental investigation of the critical current noise in underdamped niobium based Josephson devices by a technique based on the switching current measurements has been performed. By sweeping the junction with a current ramp the measurement of the critical current switching using the standard time of flight technique has been carried out to analyze the data in order to extract the current noise.The measurements refer to Josephson junctions having an area ranging from (4 × 4) to (40 × 40) μm2 in the temperature range from 4.2 to 1.2 K. The experimental results show a linear behavior of the current white noise from both the junction area and the temperature. These measurements provide very useful information about the intrinsic noise of Josephson devices involving SQUIDs and qubits.
Noise theory of dc-nanoSQUIDs based on Dayem nanobridges
The nano-SQUIDs based on nano-Dayem bridges are widely employed in the study of small spin systems but being the current-phase relationship (CPR) of them non-sinusoidal, the theoretical predictions based on the standard SQUID theory usually is inapplicable. In this field, we have been developed a noise theory of nonobridges based nano-SQUIDs. We have computed the main characteristics including current-voltage and voltage-magnetic flux characteristics, magnetic flux-to-voltage transfer factor, and spectral densities of voltage and magnetic flux noise in the range of sinusoidal limit to hysteretic one. The CPRs have been computed by using the theory of Josephson weak links based on the Ginzburg–Landau equation. The results show a dependence of the magnetic flux noise spectral density on a power of bridge length and coherence length ratio, involving a degradation of about 5 times in the two extreme cases and are consistent with experimentally measured magnetic flux noises reported in the literature. These results provide useful information for both device physics and their applications.
Performance of nano-superconducting quantum interference devices for small spin cluster detection
Performance of nano-SQUIDs has been investigated in view of their employment in the detection of small spin populations. The analysis has been focused on nano-SQUID sensors having a square loop with a side length of 200 nm. We calculate the spin sensitivity and the magnetic response relative to the single Bohr magneton (single spin) as a function of its position within the SQUID hole. The results show that the SQUID response depends strongly on the spin position; the ratio between the spin sensitivity evaluated in the center of the loop and the minimum one is as high as a factor of 3 for a spin at a reasonable distance of 10 nm from the SQUID plane. Furthermore, the magnetic flux due to several hundred of spins has been evaluated by considering different random spin distributions within the SQUID hole. Due to the both non-uniform SQUID response and the random distribution process, the results show a statistical uncertainty which has been evaluated as a function of the spin number. The estimated information are very useful to optimize the sensor performance in view of the most nanomagnetism applications.
Performances of compact integrated superconducting magnetometers for biomagnetic imaging
Performances of compact fully integrated SQUID magnetometers have been investigated in view of their employment in large multichannel systems for biomagnetic imaging. The analysis has been focused on SQUID sensors having a pickup loop side length of 3 and 4 mm based on a design aimed to maximize the magnetic flux transferred from the detection coil to the SQUID in comparison with a magnetometer with 9 mm side length having a suitable sensitivity for biomagnetic applications.The performance study has been consisted in the computation of the magnetic responses to a current dipole which is the most fundamental approach used in biomagnetism. The results have shown that the dipole current sensitivity of 4 mm long side magnetometers is suitable for application in multichannel systems for magnetoencephalography and magnetocardiography.
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