  We propose a new device to create a tunable all-electric spin polarizer: a
quantum point contact (QPC) with four gates -- two in-plane side gates in
series. The first pair of gates, near the source, is asymmetrically biased to
create spin polarization in the QPC channel. The second set of gates, near the
drain, is symmetrically biased and that bias is varied to maximize the amount
of spin polarization in the channel. The range of common mode bias on the first
set of gates over which maximum spin polarization can be achieved is much
broader for the four gate structure compared with the case of a single pair of
in-plane side gates.
