Steven Anton

Very Large Scale Integration of Nano-Patterned YBa2Cu3O7-delta Josephson Junctions in a Two-Dimensional Array

Very large scale integration of Josephson junctions in a two-dimensional series-parallel array has been achieved by ion irradiating a YBa(2)Cu(3)O(7-delta) film through slits in a nanofabricated mask created with electron beam lithography and reactive ion etching. The mask consisted of 15820 high aspect ratio (20:1), 35 nm wide slits that restricted the irradiation in the film below to form Josephson junctions. Characterizing each parallel segment k, containing 28 junctions, with a single critical current I(ck) we found a standard deviation in I(ck) of about 16%.

Low-frequency critical current noise in Josephson junctions induced by temperature fluctuations

We demonstrate a spurious contribution to low-frequency critical current noise in Josephson junctions—normally attributed to charge trapping in the barrier—arising from temperature instabilities inherent in cryogenic systems. These temperature fluctuations modify the critical current via its temperature dependence. Cross-correlations between measured temperature and critical current noise in Al-AlOx-Al junctions show that, despite excellent temperature stability, temperature fluctuations induce observable critical current fluctuations. Particularly, because 1/f critical current noise has decreased with improved fabrication techniques in recent years, it is important to understand and eliminate this additional noise source.

Mean square flux noise in SQUIDs and qubits: numerical calculations

The performance of SQUIDs and superconducting qubits based on magnetic flux is degraded by the presence of magnetic flux noise with a spectral density scaling approximately inversely with frequency. It is generally accepted that the noise arises from the random reversal of spins on the surface of the superconductors. We introduce a numerical method of calculating the mean square flux noise from independently fluctuating spins on the surface of thin-film loops of arbitrary geometry. By reciprocity, is proportional to ?B(r)2?, where B(r) is the magnetic field generated by a circulating current around the loop and r varies over the loop surface. By discretizing the loop nonuniformly, we efficiently and accurately compute the current distribution and resulting magnetic field, which may vary rapidly across the loop. We use this method to compute in a number of scenarios in which we systematically vary physical parameters of the loop. We compare our simulations to an earlier analytic result predicting that in the limit where the loop radius R is much greater than the linewidth W. We further show that the previously neglected contribution of edge spins to is significant?even dominant?in narrow-linewidth loops.

Magnetic field calculations in the vicinity of superconductive circuit structures

Modeling the magnetic field generated by currents in thin-film superconducting structures is useful to a broad range of applications. Present methods calculate the magnetic field around such structures from two-dimensional current sheets. We present an efficient and accurate algorithm to calculate the magnetic field at arbitrary locations in three dimensions, both internal and external to the structures. We use a combination of FastHenry and InductEx to calculate the current density, from which we compute the magnetic field semi-analytically using Biot-Savarts law. As practical examples, we employ the algorithm to compute (i) the mean square flux noise in square SQUID and qubit loops and (ii) the dc bias current-induced magnetic field in real digital circuit layouts.