  Significant achievements in the reduction of classical-noise floor will allow
macroscopic systems to prepare nearly Heisenberg-Limited quantum states through
a continuous measurement, i.e. conditioning. In order to probe the conditional
quantum state and confirm quantum dynamics, we propose use of an optimal
time-domain variational measurement, in which the homodyne detection phase
varies in time. This protocol allows us to characterize the macroscopic quantum
state below the Heisenberg Uncertainty -- i.e. Quantum Tomography -- and the
only limitation comes from readout loss which enters in a similar manner as the
frequency-domain variational scheme proposed by Kimble et al.. In the case of
no readout loss, it is identical to the back-action-evading scheme invented by
Vyatchanin et al. for detecting gravitational-wave (GW) signal with known
arrival time. As a special example and to motivate Macroscopic Quantum
Mechanics (MQM) experiments with future GW detectors, we mostly focus on the
free-mass limit -- the characteristic measurement frequency is much higher than
the oscillator frequency -- and further assume the classical noises are
Markovian, which captures the main feature of a broadband GW detector. Besides,
we consider verifications of Einstein-Podolsky-Rosen (EPR) type entanglements
between macroscopic test masses in GW detectors, which enables to test one
particular version of Gravity Decoherence conjectured by Diosi and Penrose.
