  The advanced nanoscale integration available in silicon complementary
metal-oxide-semiconductor (CMOS) technology provides a key motivation for its
use in spin-based quantum computing applications. Initial demonstrations of
quantum dot formation and spin blockade in CMOS foundry-compatible devices are
encouraging, but results are yet to match the control of individual electrons
demonstrated in university-fabricated multi-gate designs. We show here that the
charge state of quantum dots formed in a CMOS nanowire device can be sensed by
using floating gates to electrostatically couple it to a remote single electron
transistor (SET) formed in an adjacent nanowire. By biasing the nanowire and
gates of the remote SET with respect to the nanowire hosting the quantum dots,
we controllably form ancillary quantum dots under the floating gates, thus
enabling the demonstration of independent control over charge transitions in a
quadruple (2x2) quantum dot array. This device overcomes the limitations
associated with measurements based on tunnelling transport through the dots and
permits the sensing of all charge transitions, down to the last electron in
each dot. We use effective mass theory to investigate the necessary
optimization of the device parameters in order to achieve the tunnel rates
required for spin-based quantum computation.
