Semifluxon Molecule under Control

Josephson junctions with a phase drop of π in the ground state allow us to create vortices of supercurrent carrying only half of the magnetic flux quantum Φ₀≈2.07×10^-15Wb. Such semifluxons have twofold degenerate ground states denoted ↑ (with flux +Φ₀/2 and supercurrent circulating clockwise) and ↓ (with flux -Φ₀/2 and supercurrent circulating counterclockwise). We investigate a molecule consisting of two coupled semifluxons in a 0-π-0 long Josephson junction. The fluxes (polarities) of semifluxons are measured by two on-chip SQUIDs. By varying the dc bias current applied to the 0-π-0 junction, we demonstrate controllable manipulation and switching between two states, ↑↓ and ↓↑, of a semifluxon molecule. These results provide a major step towards employing semifluxons as bits or qubits for classical and quantum digital electronics.