This award supports an experimental program to generate and characterize quantum states arising from the interaction of light with macroscopic mechanical oscillators, with the goal of furthering the understanding of the fundamental limits of quantum measurement, as well as improving the performance of interferometric gravitational-wave detectors. The two main experiments are: (i) a meter-scale interferometer with 1 gram mirrors suspended as pendulums, high circulating power, and a quantum-limited optical readout, and (ii) a centimeter-long Fabry-Perot cavity with a cryogenically cooled 250 nanogram cantilevered micromirror. The objectives of these experiments are: (1) Direct observation quantum radiation pressure (backaction) noise; (2) Observation and manipulation of optomechanically induced transparency that arises when optomechanical coupling introduces sufficient optical rigidity to renormalize the dressed mechanical states into optically-bright and optically-dark modes of motion; (3) Observation of ponderomotive squeezing; (4) Observation of ground state cooling of a macroscopic object; (5) Reaching and surpassing the free-particle Standard Quantum Limit; (6) Observation of quantum entanglement, arising from radiation pressure induced coupling of the motion of the mirror and the quantum radiation field. None of these phenomena have been observed experimentally to date. The main purpose of this program is to further the understanding of radiation-pressure-dominated interferometers, an important feature of future gravitational-wave detectors. Equally attractive is the prospect of exploring the fundamental physics of quantum correlations due to optical-mechanical couplings in a macroscopic mechanical oscillator system.
Interferometric gravitational wave detectors, such as those of the Laser Interferometer Gravitational-wave Observatory (LIGO), are seeking to detect gravitational waves emitted by violent cosmic events such as supernova explosions and collisions of neutron stars and black holes. Since gravitational waves are completely distinct from electromagnetic radiation, direct detection of gravitational waves is expected to open a new window into the Universe and provide opportunities to study cosmic phenomena that are "invisible" using light alone. Gravitational waves from astrophysical sources cause microscopic distortions of spacetime that can be measured by an interferometer whose mirrors are suspended as pendulums to isolate them from all other effects beside the gravitational wave. The changes in arm length, typically of order 1/1000 the size of a proton, are detected by very precise measurement of the interference pattern of the laser light reflected from each 4 kilometer long arm of the interferometer. Quantum fluctuations of the light arising from the discrete nature of photons limit the sensitivity of gravitational wave detectors. This so-called shot noise, due to the random quantum fluctuations of the light, limits the precision with which the interference pattern, and hence the gravitational wave signal, can be measured. Similarly, radiation pressure noise limits the sensitivity due to the interferometer mirrors being "kicked" by the fluctuating momentum of the photons that is transferred to the mirrors when the laser light reflects from them. Studying ways to characterize and circumvent this noise limit is essential to making further improvements in sensitivity of gravitational wave detectors and also allows for the study of fundamental quantum effects, such as squeezing and entanglement, in macroscopic mechanical systems. The broader impact of this research program lies in its scientific and its personnel diversity. The scientific diversity arises from the necessarily cross-disciplinary nature of the proposed research: it combines the techniques and formalism of quantum optics and quantum measurement theory with gravitational wave detection. The personnel diversity is the outcome of aggressive recruitment of women and minority students by the PI (herself a member of minority groups), through her own efforts as well as those of the outreach programs of the LIGO Laboratory and MIT. In addition, the sub-quantum-noise-limit measurements are popular with students and generate considerable enthusiasm with the public as well. The proposed experiments share common technologies with quantum information, quantum control, and mesoscopic condensed matter physics (nano- and micro-mechanical oscillators).
