Opportunities for testing quantum mechanics are unique at
a 
--factory. Some of the tests that can be performed at Da
ne \
are described here. In the past,
few checks of quantum mechanics have been made
in neutral kaon physics and none with a two-kaon system.
At a 
--factory, several such tests are possible.
The quantity we proposed to check is the amount of interference
predicted between two different physical processes.
When there are two different
mechanisms, 1 and 2, by which a system can transit from
one state to another,
the resulting probability of transition P in classical physics
is the sum of the
transition probabilities, 
and 
, due to the individual processes,
In quantum mechanics, when the initial and the final states are pure
single states and when the system does not interact with the environment,
what one is supposed to add are not probabilities, but probability
amplitudes, 
and 
The interference between the two processes results from that property.
For the tests of concern here, we express the transition probability P as
where 
and 
are the individual transition
probabilities of each process, the last term
represents the amplitude of the interference effect, and
is a parameter which we call the ``decoherence parameter''.
can be measured by measuring P.
If there is no decoherence mechanism,
quantum mechanics predicts 
,
thus 
measures the amount of violation of
the theoretical prediction.
The tests proposed here are designed
to yield a low upper limit for 
if the predictions of quantum mechanics
are 100% correct.
1--
is analogous to what is called ``visibility'' in optics.
If the initial or the final state included in the sample of events
is not pure, the interference effect is reduced.
Background can fake a violation. The advantage of a 
--factory
is the possibility to have initial kaon systems in quite pure states,
final decay modes that are theoretically equivalent
to pure states, and practically no known decoherence mechanism.
