|
Conditional measurement opens the possibility to generate
a wide range of nonclassical states of light. Quantum correlation
between two modes -a signal and a trigger- is the prerequisite for
this general procedure. The information provided by the trigger
conditions the measurement of the signal and causes a nonunitary
state reduction. This state is generated nonlocally only when a
trigger condition is fulfiled.
If state reduction by conditional measurement was intensively
studied in the photon counting regime, no scheme has been suggested
so far to generate nonclassical bright beams. We propose a new
and simple setup to conditionally prepare a bright amplitude squeezed
beam and report on its experimental implementation. Our scheme
is based on a non-degenerate optical parametric oscillator (ND-OPO)
above threshold which is well known
to generate bright twin beams with very large quantum noise intensity
correlations inside the bandwidth of the cavity.
Thanks to this high amount of correlations, the direct measurement
of the idler can condition efficiently the readout of the signal.
When the idler takes a given value, the signal is
taken into account. Other measurements are discarded. Our calculations
show that -whatever is its initial excess noise- the signal is
reduced to a quadrature-amplitude squeezed state and the
Fano factor of the resulting gaussian noise distribution is twice
the remaining noise on the
intensity difference. That means the squeezing of the reduced
state is equal to the twins correlation minus 3 dB. Essentially,
conditional state reduction is successful with a probability
inferior to one. In the case of conditioning on one value, the
generation probability goes to zero if the correlations are not
ideal. Instead of a single triggering value, one can consider
a given band around the mean of the idler. In that case, the conditional
squeezing is dependent on the width of the selection band and
the success rate can be improved. For a narrow band relative to
the relevant parameter which is the standard deviation of a same
power coherent state, the squeezing is only slightly reduced in
comparison with the single value conditioning.
|
