We experimentally demonstrate the operation of a Josephson magnetic random access memory unit cell, built with a Ni80Fe20/Cu/Ni pseudo spin-valve Josephson junction with Nb electrodes and an integrated readout SQUID in a fully planarized Nb fabrication process. We show that the parallel and anti-parallel memory states of the spin-valve can be mapped onto a junction equilibrium phase of either zero or π by appropriate choice of the ferromagnet thicknesses, and that the magnetic Josephson junction can be written to either a zero-junction or π-junction state by application of write fields of approximately 5 mT. This work represents a first step towards a scalable, dense, and power-efficient cryogenic memory for superconducting high-performance digital computing.
We demonstrate electrostatically actuated end-coupled optical waveguide devices in the indium phosphide (InP) material system. The design of a suitable layer structure and fabrication process for actuated InP-based waveguide micro-electro-mechanical systems (MEMS) is reviewed. Critical issues for optical design, such as coupling losses, are discussed and their effect on device performance is evaluated. Several end-coupled waveguide devices are demonstrated, including 1 × 2 optical switches and resonant sensors with integrated optical readout. The 1 × 2 optical switches exhibit low-voltage operation (<7 V), low crosstalk (−26 dB), reasonable loss (3.2 dB) and switching speed suitable for network restoration applications (140 µs, 2 ms settling time). Experimental characterization of the integrated cantilever waveguide resonant sensors shows high repeatability and accuracy, with a standard deviation as low as σ = 50 Hz (0.027%) for f resonant = 184.969 kHz. By performing focused-ion beam (FIB) milling on a sensor, a mass sensitivity of m/ f = 5.3 × 10 −15 g Hz −1 was measured, which is competitive with other sensors. Resonant frequencies as high as f = 1.061 MHz (Q effective = 159.7) have been measured in air with calculated sensitivity m/ f = 1.1 × 10 −16 g Hz −1. Electrostatic tuning of the resonator sensors was also examined. The prospect of developing InP MEMS devices monolithically integrated with active optical components (lasers, LEDs, photodetectors) is discussed.
Fabrication of coplanar waveguide resonators with internal quality factors near 106 remains challenging. Here, high-purity superconductors are implemented through metamorphic epitaxial aluminum that is grown via molecular beam epitaxy on silicon and sapphire substrates. X-ray diffraction and scanning transmission electron microscopy indicate an abrupt highly ordered interface that results in crystal relaxation within a few monolayers of the substrate interface and no measurable interfacial contamination. Quarter-wave coplanar waveguide resonators are fabricated using optical lithography and measured at temperatures below 100 mK. Post measurement characterization with charge contrast imaging in a scanning electron microscope identifies processing artifacts at the waveguide sidewalls, on the exposed substrate area and on the exposed aluminum surface. Of primary importance are processing induced corrosion defects on aluminum sidewalls, nanoparticle contamination, and photoresist residue that is difficult to remove without affecting the superconductor material. Likely correlations between these artifacts and the measured quality factor are discussed in context of device to device variations in resonator performance.
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