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Expanding on our prior efforts to search for Lorentz invariance violation(LIV) using the linear optical polarimetry of extragalactic objects, we proposea new method that combines linear and circular polarization measurements. Whileexisting work has focused on the tendency of LIV to reduce the linearpolarization degree, this new method additionally takes into account thecoupling between photon helicities induced by some models. This coupling cangenerate circular polarization as light propagates, even if there is nocircular polarization at the source. Combining significant detections of linearpolarization of light from extragalactic objects with the absence of thedetection of circular polarization in most measurements results insignificantly tighter constraints regarding LIV. The analysis was carried outin the framework of the Standard-Model Extension (SME), an effective fieldtheory framework to describe the low-energy effects of an underlyingfundamental quantum gravity theory. We evaluate the performance of our methodby deriving constraints on the mass dimension $d=4$ CPT-even SME coefficientsfrom a small set of archival circular and linear optical polarimetryconstraints and compare them to similar constraints derived in previous workswith far larger sample sizes and based on linear polarimetry only. The newmethod yielded constraints that are an order of magnitude tighter even for ourmodest sample size of 21 objects. Based on the demonstrated gain inconstraining power from scarce circular data, we advocate for the need forfuture extragalactic circular polarization surveys.

New Constraints on Lorentz Invariance Violation from Combined Linear and Circular Optical Polarimetry of Extragalactic Sources