  Measurement is integral to quantum information processing and communication;
it is how information encoded in the state of a system is transformed into
classical signals for further use. In quantum optics, measurements are
typically destructive, so that the state is not available afterwards for
further steps - crucial for sequential measurement schemes. The development of
practical methods for non-destructive measurements on optical fields is
therefore an important topic for future practical quantum information
processing systems. Here we show how to measure the presence or absence of the
vacuum in a quantum optical field without destroying the state, implementing
the ideal projections onto the respective subspaces. This not only enables
sequential measurements, useful for quantum communication, but it can also be
adapted to create novel states of light via bare raising and lowering
operators.
