  We study operation of a new device, the superconducting differential double
contour interferometer (DDCI), in application for the ultra sensitive detection
of magnetic flux and for digital read out of the state of the superconducting
flux qubit. DDCI consists of two superconducting contours weakly coupled by
Josephson Junctions. In such a device a change of the critical current and the
voltage happens in a step-like manner when the angular momentum quantum number
changes in one of the two contours. The DDCI may outperform traditional
Superconducting Quantum Interference Devices when the change of the quantum
number occurs in a narrow magnetic field region near the half of the flux
quantum due to thermal fluctuations, quantum fluctuations, or the switching a
loop segment in the normal state for a while by short pulse of an external
current.
