Implementation of superconductor/ferromagnet/ superconductor π-shifters in superconducting digital and quantum circuits

Abstract

The difference between the phases of superconducting order parameter plays in superconducting circuits the role similar to that played by the electrostatic potential difference required to drive a current in conventional circuits. This fundamental property can be altered by inserting in a superconducting circuit a particular type of weak link, the so-called Josephson π-junction having inverted current-phase relation and enabling a shift of the phase by π. We demonstrate the operation of three superconducting circuits -two of them are classical and one quantum -which all utilize such π-phase shifters realized using superconductor-ferromagnet-superconductor sandwich technology. The classical circuits are based on single-flux-quantum cells, which are shown to be scalable and compatible with conventional niobium-based superconducting electronics. The quantum circuit is a π-phase biased qubit, for which we observe coherent Rabi oscillations and compare the measured coherence time with that of conventional superconducting phase qubits. 1 arXiv:1005.1581v1 [cond-mat.supr-con]

Critical current oscillations of elliptical Josephson junctions with single-domain ferromagnetic layers

“…That result has been confirmed by numerous groups since the initial discovery, using a wide variety of weak and strong ferromagnetic materials [3][4][5][6][7][8][9] . There have been several proposals to use π-junctions as new components in either classical or quantum information processing circuits [10][11][12][13][14][15][16] .…”

Experimental and theoretical analysis of the upper critical field in ferromagnet–superconductor–ferromagnet trilayers

“…Among a variety of layered superconducting systems, superconductor-ferromagnet (S/F) metallic hybrids attract special attention because of rich physics of these objects [7,8,9] as well as promising perspectives for their applications in superconducting spintronics [10,11,12]. Based on the unique properties of S/F hybrids, several kinds of device physics were proposed such as proximity-effect superconducting spin-switching [13,14,15,16,17] and Josephson current switching [18,19,20] actuated by an external magnetic field.…”

Controllable 0–π Josephson junctions containing a ferromagnetic spin valve

“…For example, a Josephson junction containing a ferromagnetic material can exhibit an intrinsic phase shift of π in its ground state for certain thicknesses of the material 1 . Such 'π-junctions' were first realized experimentally in 2001 (refs 2,3), and have been proposed as circuit elements for both high-speed classical superconducting computing and for quantum computing [4][5][6][7][8][9][10] . Here we demonstrate experimentally that the phase state of a Josephson junction containing two ferromagnetic layers can be toggled between 0 and π by changing the relative orientation of the two magnetizations.…”

“…One can now start to envisage new types of superconducting circuits using switchable 0-π junctions such as This SQUID-based memory cell has high critical current when the controllable S/F/S junction is in the 0-state and low critical current in the π-state. If the critical current of the S/F/S junction is larger than those of the two S/I/S junctions, then only the latter switch into the voltage state when the read current is applied, whereas the S/F/S junction acts as a passive phase shifter 10 . The read time of the memory cell, τ read ≈h/eI c R N , is then determined by the faster S/I/S junctions with higher I c R N product.…”