Precision MeasurementsResearch at LIGO MIT is focused on increasing the sensitivity of advanced gravitational-wave detectors. These cutting-edge facilities operate at quantum limits of sensitivity, which requires special techniques to overcome the Standard Quantum Limit.
Quantum squeezed light is a valuable resource for any precision measurement experiment reaching standard limits of sensitivity. There is a strong interest in the community to develop a compact squeezed light source that could be used as a tool to push the sensitivity further into the quantum realm. There are also various proposals to use squeezed light for quantum repeaters for communication protocols.
LIGO MIT is proud of squeezed light technology routinely used in our lab, therefore compact squeezer project aims to make it more accessible. As a first step, we are planning to develop a compact shoe-box size continuous-wave fiber-coupled squeezer as a reliable quantum resource easy-to-use and ready-to-go. Technical limitations make procuring compact squeezer challenging, but incredibly rewarding.Read more
Frequency Dependent Squeezing
Since 2019, the injection of squeezed states of light, is being used to improve the shot-noise limit to the sensitivity of the Advanced LIGO detectors, at frequencies above ∼50 Hz. Below this frequency, quantum backaction, in the form of radiation pressure induced motion of the mirrors, degrades the sensitivity.
To simultaneously reduce shot noise at high frequencies and quantum radiation pressure noise at low frequencies requires a quantum noise filter cavity with low optical losses to rotate the squeezed quadrature as a function of frequency. At MIT, we demonstrated frequency dependent squeezing using a 16m long filter cavity prototype in 2020 . In 2023, 300m filter cavities were installed at the LIGO sites and are now used for broadband quantum enhancement in LIGO.
Coating Thermal Noise
LIGO's astronomical reach is directly limited in its most sensitive frequency band by coating thermal noise (CTN). CTN is noise caused by the thermal motion of the surface of the mirrors that form the 4 km long optical cavities. At LIGO-MIT we design new dielectric coatings for these mirrors, utilizing novel blends of materials and nanotechnology, and characterize their properties using a unique multi-mode high-finesse optical cavity.
GRAVITES is an experiment designed to explore the intricate relationship between quantum mechanics and general relativity. Its primary objective is to examine the behavior of entangled quantum states in accordance with the predictions of general relativity. The experiment utilizes a large-scale optical fiber interferometer to measure the gravitational redshift-induced phase on single photons and pairs of entangled photons. This is achieved by vertically displacing one of the two arms of the Mach-Zehnder interferometer. Once established as a high-precision measurement tool, this versatile interferometer can be employed as a testbed for other experiments that explore the intersection of quantum field theory and general relativity. Additionally, it holds potential applications in the field of quantum key distribution.
Typically, squeezed light is produced using a specially made non-linear crystal. Alternatively, we are exploring using the radiation pressure mediated interaction between light and micromechanical mirrors to achieve squeezed light through a different method. This optomechanical squeezer has the potential to be less complex, minaturizable, scalable, and less susceptible to environmental noises than its non-linear crystal conunterpart.
An older generation of the micromechanical mirrors used in the experiment can be found in the image to the right. This chip contains approximately 50 individual mirrors each nearly the diameter of a strand of hair. The radiation pressure of light is a small force so small mirrors are needed for it to be noticeable.
Optical polarimetry for dark matter detection
Ultralight fields, such as axions, are an increasingly popular dark matter candidate. If such fields exist, they may induce faint oscillations in the behavior of ordinary matter or of light. We are building an Axion-like Dark-matter Birefringent Cavity (ADBC) to search for pseudoscalar fields that could alter the polarization of highly stable laser light. This experiment complements other astrophysical and laboratory efforts such as ABRACADABRA, and sets the stage for larger, more powerful, and more sensitive optical polarimetry searches for dark matter.
Point Absorber Limits
Higher optical power leads to less quantum shot noise. However, any defects on the optics coating such as metallic dust of a few-μm size would absorb laser power and cause thermoelastic deformation under strong irradiation of laser light, also known as the point absorber effect. The undesired deformation of the optics increases losses and limits the amount of achievable power fundamentally. At MIT, we provide analysis that sets the requirements of point absorber to achieve high optical power/low quantum noise, and explore technical possibilities to meet these requirements.