  The spin-vortex-induced loop current (SVILC) is a nano-sized loop current
predicted to exist in the CuO$_2$ plane in the bulk of hole-doped cuprate
superconductors. It is a persistent loop current protected by the topological
winding number associated with the wave function. It exists around a
spin-vortex created by the itinerant electrons with a doped hole at its center.
The direction of each SVILC can be either clockwise (winding number is -1) or
counterclockwise (winding number is +1) and the winding number with no current
(winding number is zero) is forbidden by the singlevalued requirement of the
wave function with respect to the electron coordinates. Recently, it has been
demonstrated, theoretically, that this degree-of-freedom can be used for
qubits. Coupling of neighboring qubits by external current feeding is confirmed
to be realizable. This means that nano-sized couplers of SVILC qubits using
feeding external currents are realizable. The size of couplers of SVILC qubits
can be conparable or smaller than that of trapped ion qubits. Couper size of
SVILC qubits is decided by the range of spin vortices in CuO$_2$ plane and
current distribution, thus, this is tunable by feeding external current and
substituting Cu atoms in barrier atoms. That of trapped ion qubits is limited
by the distance that combined vibration occurs or laser range with respect to
the coordinates. In the present work, We demonstrated splitting energy levels
by external feeding current of three qubit system of SVILC qubits. This means
that nano-sized qubit differentiator can be realized, and noise by static
magnetic field can be cut off, and this may enable the realizing fully-fault
tolerant quantum computers by SVILC qubits. Moreover, the possibility of
downscaling of them is shown.
