in Asymmetric Superconducting Quantum Interference Devices

Mizugaki Yoshinao, Kazuyuki Katoh

Gen Uehara, Masanori Higuchi

G. Testa, E. Sarnelli, S. Pagano +2 more

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.

Gen Uehara, Naoki Matsuda, Kunio Kazami +1 more

An asymmetric bias current injection method is developed to reduce bias-caused noise in Drung-type SQUID (superconducting quantum interference devices) without adding extra coils. A measuring scheme for ∂Φ/∂I b is tested, and it is demonstrated that the reduction is from 100 to 150 times that attained by the conventional SQUID biasing method. It is revealed that the direction of APF (additional positive feedback) coil winding is important in attaining this reduction.

Susan A. E. Berggren

M. Rudolph, J. Nagel, J. M. Meckbach +5 more

We have investigated asymmetrically shunted Nb/Al-AlO$_x$/Nb direct current (dc) superconducting quantum interference devices (SQUIDs). While keeping the total resistance $R$ identical to a comparable symmetric SQUID with $R^{-1} = R_1^{-1} + R_2^{-1}$, we shunted only one of the two Josephson junctions with $R = R_{1,2}/2$. Simulations predict that the optimum energy resolution $ε$ and thus also the noise performance of such an asymmetric SQUID can be 3--4 times better than that of its symmetric counterpart. Experiments at a temperature of 4.2\,K yielded $ε\approx 32\,\hbar$ for an asymmetric SQUID with an inductance of $22\,\rm{pH}$. For a comparable symmetric device $ε= 110\,\hbar$ was achieved, confirming our simulation results.