  A parity meter projects the state of two qubits onto two subspaces with
different parities, the states in each parity class being indistinguishable. It
has application in quantum information for its entanglement properties. In our
work we consider the electronic Mach-Zehnder interferometer (MZI) coupled
capacitively to two double quantum dots (DQDs), one on each arm of the MZI.
These charge qubits couple linearly to the charge in the arms of the MZI. A key
advantage of an MZI is that the qubits are well separated in distance so that
mutual interaction between them is avoided. Assuming equal coupling between
both DQDs and the arms and the same bias for each DQD, this setup usually
detects three different currents, one for the odd states and two for each even
state. Controlling the magnetic flux of the MZI, we can operate the MZI as a
parity meter: only two currents are measured at the output, one for each parity
class. In this configuration, the MZI acts as an ideal detector, its Heisenberg
efficiency being maximal. For a class of initial states, the initially
unentangled DQDs become entangled through the parity measurement process with
probability one.
