In 2007, the initial LIGO gravitational-wave detectors achieved a noise level within a factor 10 of the "Standard Quantum Limit'' (SQL). This SQL is a sensitivity limit imposed by the Heisenberg Uncertainty
Principle, when one uses light in standard ways to monitor the motions of LIGO's massive mirrors. Advanced LIGO detectors (currently under construction) will operate very near the SQL, and third-generation detectors (e.g., LIGO-III for which R&D is now underway) must beat the SQL significantly in order to achieve another large gain in sensitivity.

In the first part of my talk, I will discuss how quantum fluctuations in LIGO's massive mirrors enforce the SQL, and I will describe designs for new, nonstandard optical configurations that could can allow us to
circumvent the SQL in upgrades to Advanced LIGO and in LIGO-III. These new configurations rely on manipulating the mirrors' dynamics via light pressure that is tailored in special ways, and rely on signal readout schemes that avoid looking at the mirrors' quantum fluctuations.

In the second part of my talk, I will discuss how to modify LIGO's experimental protocol so as to maximize the sensitivity to the mirrors' quantum fluctuations, thereby converting LIGO into a playground
for studying the quantum mechanical behavior of human-sized objects. Among the possible experiments I shall describe are the production and observation of quantum entanglement and Schrodinger-cat states in LIGO's 40 kg mirrors. The experimental strategies I will describe can also be applied to quantum mechanical experiments on smaller scales, all the way down to nanomechanical oscillators.