SUPERCONDUCTING ELECTRONICS INTRODUCTION Superconductivity was discovered in 1911 in the laboratory of Heike Kamerlingh Onnes after he succeeded to liquefy helium (for which he was awarded the Nobel Prize in Phyics in 1913), but its application to electronics only really appeared after the prediction and observation of the Josephson effect in the 1960s (1, 2). Two advances in the mid-1980s led to superconducting electronics gaining popularity: the discovery of oxide superconductors with transition temperatures above the boiling point of liquid nitrogen (77 K) (3) and the development of reliable Nb/AlO x /Nb whole wafer tunnel junctions (4). Superconducting electronics (5) are divided into two regimes: low-temperature superconductor (LTS) devices fabricated with superconductors such as niobium, niobium nitride or magnesium diboride, operating at temperatures below roughly 30 K, and high-temperature superconductor (HTS) devices fabricated with oxide superconductors such as YBa 2 Cu 3 O 7-δ (YBCO) and operating at temperatures above 30 K. The need to cool superconducting electronics to cryogenic temperatures continues to limit the widespread uptake of such electronics, but superior detection capability, noise performance, switching speed and energy efficiency make superconducting electronics valuable in applications where the cryocooling penalty is of little or no consequence. Superconductors exhibit two fundamental characteris- tics, namely zero dc electrical resistance and the exclusion of magnetic fields, called the Meissner-Ochsenfeld effect (6), or Meissner effect for short. Zero dc electrical resistance means that superconducting wires and cables can conduct electricity or transmit small signals without loss. This makes high-current applications possible, such as powerful electromagnets for magnetic resonance imaging (MRI) or the super powerful 11.8 Tesla magnetic fields planned for the ITER Tokamak fusion reactor (7). Superconducting electronics can also be powered without losses in the bias network. The Meissner effect makes magnetic levitation possible in applications that range from suspending bearingless motors to high speed trains. This exclusion of magnetic fields from a superconductor requires that all current flow must occur on the surface of the superconductor, but in practice the surface current penetrates the superconductor to a finite depth, called the penetration depth. A phenomenological theory of superconductivity was derived from Maxwell’s equations and Newton’s laws by the brothers Fritz and Heinz London in 1935 (8). The London equations (for a detailed review, see (9)) are sufficient to describe the Meissner effect, and lead to the equation for penetration depth λ = ( ) 0 2 * * * µ q n m , (1) where the mass m * of a Cooper pair is twice the electron mass, the Cooper pair density n * is half that of the electron density, and the Cooper pair q * charge is twice that of the electron charge. The permeability of free space, µ 0 , is 4π×10 -7 H/m. The penetration depth is frequency independent, unlike the skin depth of normal metals. The London equations are sufficient to model the electromagnetic characteristics of most superconducting electronics. The mechanism for low-temperature superconductivity was not completely understood until 1957, when John Bardeen, Leon Cooper and Robert Schrieffer proposed a microscopic theory (10) in which electrons in a superconductor pair up to form so-called Cooper pairs. Through electron interaction with the crystal lattice of the superconductor, an attractive potential is created that overcomes Coulomb electrical repulsion between the paired electrons. These electron pairs do not behave as point particles; rather their influence extends over about a micrometre. There are a large number of Cooper pairs inside the sphere of influence of any pair, so that pairs interact, are correlated with one another and move together. For this work, Bardeen, Cooper and Schrieffer received the Nobel Prize in Physics in 1972. A host of new possibilities for superconducting electronics opened up when Brian Josephson predicted Cooper pair tunnelling through thin barriers in 1962 (1). It was confirmed experimentally the next year (2), and Josephson was awarded the Nobel Prize in Physics in 1973, jointly with Leo Esaki and Ivar Giaever for their work on the discovery of tunnelling phenomena. For decades, the maximum critical temperature of any known superconductor was about 22 K to 23 K, well below the 77 K barrier that would allow relatively inexpensive liquid nitrogen cooling and more widespread applications. This changed abruptly in 1986 when Georg Bednorz and