Presently, our knowledge of the Universe comes from observing the light that is radiated by stars; from the spectacular light shows that accompany the explosive births and deaths of stars; and ancient light from the Big Bang itself. But is not be the only messenger to carry the secrets of the distant Universe to us. Many of these spectacular events also emit gravitational waves. Gravitational waves are ripples in the fabric of space and time that were predicted to exist by Einstein in 1916. They are very faint, however, and have never been directly observed. My work involves making instruments, based on very sensitive interferometry using lasers to sense the position of test masses that move in response to gravitational waves (see LIGO).
The sensitivity of a detector determines how far into the distant skies it can "see." Over the years, as the LIGO detectors have become increasingly more sensitive in the quest to see ever more distant events in our Universe, the very basic laws of quantum mechanics have become an impediment to the LIGO mission. This leads us to the focus of the Quantum Measurement group at MIT.
The effect of a gravitational-wave on interferometers like those of LIGO is to cause the distances between the mirrors of the interferometer to change at the frequency of the GW. This relative displacement of mirrors can be measured as a phase shift of the light traveling in the arms of the interferometer using the interference of the laser light at the anti-symmetric port of the interferometer. The principle is straightforward, but the relative displacements being measured are one-thousandth the size of an atomic nucleus, i.e. 10-18 m for present-day detectors, and another factor of 10 smaller in next-generation detectors.
Since laser light is used to make precise measurements of the positions of the mirrors of the interferometer, the noise on the light sets important limits on how well the displacements can be measured.