In 1935, Schrodinger attempted to demonstrate the limitations of quantum mechanics using a thought experiment in which a cat is put in a quantum superposition of alive and dead states. The idea remained an academic curiosity until the 1980s when it was proposed that, under suitable conditions, a macroscopic object with many microscopic degrees of freedom could behave quantum mechanically, provided that it was sufficiently decoupled from its environment. Although much progress has been made in demonstrating the macroscopic quantum behaviour of various systems such as superconductors, nanoscale magnets, laser-cooled trapped ions, photons in a microwave cavity and C60 molecules, there has been no experimental demonstration of a quantum superposition of truly macroscopically distinct states. Here we present experimental evidence that a superconducting quantum interference device (SQUID) can be put into a superposition of two magnetic-flux states: one corresponding to a few microamperes of current flowing clockwise, the other corresponding to the same amount of current flowing anticlockwise.

Abstract-Superconductor digital electronics using Josephson junctions as ultrafast switches and magnetic-flux encoding of information was proposed over 30 years ago as a sub-terahertz clock frequency alternative to semiconductor electronics based on complementary metal-oxide-semiconductor (CMOS) transistors. Recently, interest in developing superconductor electronics has been renewed due to a search for energy saving solutions in applications related to high-performance computing. The current state of superconductor electronics and fabrication processes are reviewed in order to evaluate whether this electronics is scalable to a very large scale integration (VLSI) required to achieve computation complexities comparable to CMOS processors. A fully planarized process at MIT Lincoln Laboratory, perhaps the most advanced process developed so far for superconductor electronics, is used as an example. The process has nine superconducting layers: eight Nb wiring layers with the minimum feature size of 350 nm, and a thin superconducting layer for making compact high-kineticinductance bias inductors. All circuit layers are fully planarized using chemical mechanical planarization (CMP) of SiO 2 interlayer dielectric. The physical limitations imposed on the circuit density by Josephson junctions, circuit inductors, shunt and bias resistors, etc., are discussed. Energy dissipation in superconducting circuits is also reviewed in order to estimate whether this technology, which requires cryogenic refrigeration, can be energy efficient. Fabrication process development required for increasing the density of superconductor digital circuits by a factor of ten and achieving densities above 10 7 Josephson junctions per cm 2 is described.

We develop a fast Magnetic Josephson Junction (MJJ)a superconducting ferromagnetic device for a scalable high-density cryogenic memory compatible in speed and fabrication with energy-efficient Single Flux Quantum (SFQ) circuits. We present experimental results for Superconductor-Insulator-Ferromagnet-Superconductor (SIFS) MJJs with high characteristic voltage I c R n of >700 V proving their applicability for superconducting circuits. By applying magnetic field pulses, the device can be switched between MJJ logic states. The MJJ I c R n product is only ~30% lower than that of conventional junction coproduced in the same process, allowing for integration of MJJ-based and SIS-based ultra-fast digital SFQ circuits operating at tens of gigahertz.High speed, low power superconducting Rapid Single Flux Quantum (RSFQ) digital circuits have already found their applications in Digital-RF systems impacting communications and signal intelligence applications [1,2]. Recently, a new energy-efficient generation of RSFQ circuits, eSFQ and ERSFQ logics, offered a way to overcome the low energy efficiency of conventional technologies for the next generation of supercomputers [3]. However, the practical applications of these superconducting digital technologies will inevitably be very limited without compatible in speed and signal levels, high-capacity, energy-efficient Random Access Memory (RAM). The largest superconducting RAM demonstrated to date, a 4 kbit RAM [4], is insufficient for practical applications and hardly compatible with SFQ-type circuits.The low density of superconducting memory is directly related to a relatively large size of memory cells based on SFQ storage loops coupled to address lines via transformers which are difficult to scale [5][6][7][8]. The required ac power posed an additional implementation problem for achieving larger capacity RAM integrated circuits [8]. In order to get around the low capacity of superconductor RAMs, hybrid superconductor-semiconductor schemes were pursued [9,10]. However, these approaches can address only limited applications and cannot satisfy the need for a fast, energy efficient memory in a close proximity to the digital circuits, preferably on the same chip. Alternatively, combining superconducting elements with ferromagnetic layers and dots was suggested to achieve higher density of superconducting memory [11,12]. However, these ideas did not go beyond initial concepts nor address compatibility with SFQ circuits.Recently, we have introduced a memory cell based on Magnetic Josephson Junction (MJJ), a Josephson switching device with ferromagnetic (F) layer(s). The MJJ critical current can change and retain its value by ferromagnet magnetization, so that a memory element size is defined by the scalable small MJJ device [13]. With achieving MJJ switching speed comparable to that of conventional JJs, both types of junctions can be integrated into a single circuit operating in an SFQ non-hysteretic switching regime, enabling a low power and high speed memory operation. Since such ...

No abstract