This work is aimed at bringing a micromechanical system into the fully quantum regime, which would result in a breakthrough in the field of opto/nanomechanical systems. It would greatly broaden our notion of which physical systems can exhibit quantum effects and allow us to explore the physics associated with quantum optics and mesoscopic condensed matter in an entirely new type of system (i.e., mechanical), whose coupling to readout devices and the environment is qualitatively different from present-day quantum systems. The project uses ultrasensitive millimeter-scale membranes dispersively coupled to high-finesse optical cavities. This coupling is strong enough to laser-cool the membrane to its vibrational ground state, to observe the quantum back-action of displacement measurements, and to produce squeezed light. At the same time this coupling can be tuned to coax especially subtle quantum effects from the optomechanical system; by realizing a strong "position squared" readout we will observe the quantization of energy in the membrane's vibration and quantum jumps between the membrane's energy eigenstates. These goals cover a wide conceptual range, but they represent different facets of the same optomechanical coupling which can be realized in a single device.
These experiments are relevant to ultrasensitive instruments in a variety of fields. Quantum limited displacement measurements are relevant to astrophysical gravitational wave searches, as is the production of squeezed light. Micromechanical devices cooled to their ground state could serve as exceptionally sensitive detectors, particularly when coupled to a readout capable of registering the devices' individual quantum excitations. The conceptual simplicity of these systems, combined with the possibility of using them to explore exotic quantum phenomena on a macroscopic scale makes them appealing. The past few years have seen a rapid increase in the number of students, postdocs, and PIs working in this field, and an increased level of interest from scientific and general audiences. This work will serve as an excellent basis for training undergraduate, graduate, and postdoctoral students in important scientific techniques, and will prepare them for a wide range of careers in applied or fundamental research.
The primary scientific goal of this project was to observe quantum mechanical effects in a millimeter-sized object. Although we did not achieve this goal, we made substantial progress towards it, and believe that the technical innovations we have developed as a result of this work will allow us to achieve our scientific goals in the near future. Specifically, we have developed an apparatus that takes our original experiment (built under a previous award, 2005 - 2008) and improves it in several ways. Our original experiment consisted of an optical cavity (a pair of mirrors facing each other, such that photons become trapped between them) with a flexible membrane inside the cavity. The membrane is typically 1 mm square and 50 nm thick. As photons bounce back and forth between the cavity mirrors, they sometimes also reflect off the membrane. Each such reflection delivers a kick to the membrane. Since these kicks are delivered by individual quantum objects (i.e., photons), they are capable of inducing quantum behavior in the membrane’s motion. Observing quantum behavior in the motion of such a large object is a major goal in the field of optomechanics. In practice, achieving this goal requires a very sophisticated device: the mirrors forming the cavity must be capable of trapping a photon for as many bounces as possible (this number is known as the "finesse"); the membrane must be cooled to the lowest possible temperature, and it would be preferable to have an instrument that is sensitive not to the membrane’s displacement, but to the square of its displacement. Under our previous award (2005 – 2008), we built a device in which the cavity finesse was 15,000, the temperature of the device was 300 K, and the "displacement squared" measurement had a strength of 30 kHz/nm^2. In the present award (2008 – 2011), we increased the finesse to by more than a factor of three to 50,000, we decreased the temperature by almost a factor of one thousand to 0.4 Kelvin, and we increased the "displacement squared" measurement strength by a factor of one thousand to 30 MHz/nm^2. In principle these improvements should have allowed us to observe quantum effects in the motion of the membrane, but we found that noise in our laser beam and vibrations inside our cryostat prevented us from reaching our goal. We are now modifying our apparatus to decrease the laser noise and vibrations. The primary "broader impact" goal of this work was to train young scientists. Several graduate and postdoctoral students have received intense training during this project. They have learned cutting edge techniques in laser optics, fiber optics, cryogenics, signal processing and data analysis. Three graduate students supported by this work have received their Ph.D.’s and gone on to prestigious postdoctoral positions as well as private sector positions. The postdoctoral student who was supported by this award has gone on to start his own lab at McGill University as a tenure-track faculty member.
