Quantum Optomechanics on Multiple Mass Scales

Massachusetts Institute Of Technology, Cambridge MA

Investigators

Abstract

The motion of everyday objects like humans, tennis balls and cars is very well described by the the laws of classical mechanics, first laid out by Isaac Newton. But the microscopic world of atoms and their constituent particles is more accurately described by quantum mechanics. These two regimes of physics - classical and quantum - are well understood, but what happens at the boundary between the two regimes? Is there even a clear, well-defined boundary? When does an object transition from classical to quantum behavior? And why should we care? Scientists have come to understand that quantum behavior is present in objects of all sizes, shapes and compositions, but is usually masked by thermal noise - the constant jittering of atoms that make up the objects due to heat energy stored and released. Removing thermal noise to reveal quantum mechanical behavior is very difficult even for a small collection of atoms, and certainly very hard for much larger objects. But it can be done, initially just for a few special atoms, and recently for increasingly larger objects. Macroscopic objects that exhibit quantum behavior are useful not only for studying the classical-quantum boundary, but also have some very practical applications. Among these applications is generating exotic quantum states of light, called "squeezed" states, that can be used to make more precise measurements than ordinary light. Interferometric gravitational wave detectors, such as the Advance LIGO ones that discovered gravitational waves from colliding black holes, rely on measuring the positions of mirrors with sub-attometer (less than one billionth of a billionth of a meter) precision. To further improve their sensitivity, one can inject squeezed states of light into these instruments. This work pertains to making these special states of light by strongly coupling laser light to movable mirrors whose motion is dominated by quantum mechanical effects rather than thermal noise. To make a squeezed state, it is necessary to find a way to correlate two usually uncorrelated properties of the light - a combination of the amplitude and phase. This is usually done by passing light through a nonlinear optical material whose refractive index depends on the strength of the electric field (amplitude of the light), such that amplitude fluctuations get imprinted on the phase of the light as it passes through that material. In this work the PI uses an alternative, and relatively unexplored, method of using optomechanical coupling. An intense light beam is incident on a movable mirror. The amplitude fluctuations of the light drive the mirror position due to radiation pressure. The mirror position fluctuations are imprinted on the phase of the light reflected from the mirror, thus correlating amplitude and phase fluctuations. The setup comprises an optical cavity where one mirror is a nano-fabricated cantilevered oscillator made of a GaAlAs heterostructure Bragg reflector. The other mirror is ablated and coated on to an optical fiber tip. The aim is to generate squeezed light in a broad audio frequency band using this apparatus. To do so, it is necessary to reduce the thermal noise of the mirror oscillator enough for its motion to be dominated by quantum fluctuations, which requires a highly optimized oscillator design that may also be cryogenic cooled. As with any quantum-limited experiment, a variety of other classical noises, such as seismic and acoustic vibrations, classical intensity and phase noise of the optical system must be reduced and controlled.