Improved Absolute Polarization Calibrator for BICEP CMB Polarimeters

¹ School of Physics and Astronomy, Cardiff University, Cardiff, CF24 3AA, United Kingdom
² Kavli Institute for Particle Astrophysics & Cosmology, Stanford University, Stanford, CA 94305, USA
³ SLAC National Accelerator Laboratory, Menlo Park, CA 94025, USA
⁴ Department of Physics and Astronomy, University of British Columbia, Vancouver, British Columbia, V6T 1Z1, Canada
⁵ Center for Astrophysics | Harvard & Smithsonian, Cambridge, MA 01238, USA
⁶ Department of Physics, California Institute of Technology, Pasadena, CA 91125, USA
⁷ Department of Physics, University of Cincinnati, Cincinnati, OH 45221, USA
⁸ Department of Physics, Stanford University, Stanford, California 94305, USA
⁹ Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, USA
¹⁰ Kavli Institute for Cosmological Physics, University of Chicago, Chicago, IL 60637, USA
¹¹ National Institute of Standards and Technology, Boulder, CO 80305, USA
¹² High-Energy Physics Division, Argonne National Laboratory, Lemont, IL, 60439, USA
¹³ School of Physics and Astronomy, University of Minnesota, Minneapolis, MN 55455, USA
¹⁴ Service des Basses Températures, Commissariat à l’Énergie Atomique, 38054 Grenoble, France
¹⁵ Department of Physics, Harvard University, Cambridge, MA 02138, USA
¹⁶ Department of Physics, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801, USA
¹⁷ Department of Astronomy, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801, USA
¹⁸ Centre for Theoretical Cosmology, DAMTP, University of Cambridge, Cambridge CB3 0WA, UK
¹⁹ Kavli Institute for Cosmology Cambridge, Cambridge CB3 0HA, UK
²⁰ Minnesota Institute for Astrophysics, University of Minnesota, Minneapolis, MN 55455, USA
²¹ Department of Physics, Boston University, Boston, MA 02215, USA

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Published online: 20 February 2026

Abstract

Cosmic birefringence is a hypothesized parity violation in electromagnetism that predicts a frequency-independent polarization rotation as light propagates. This would rotate the light from the Cosmic Microwave Background, producing an unexpected EB correlation. However, cosmic birefringence angle is degenerate with instrument polarization angle, and breaking this degeneracy requires an absolute polarization calibration. We calibrate the BICEP3 telescope (a 95GHz CMB polarimeter) by observing a rotating polarized source (RPS) with both the telescope and a small test receiver called the In-Situ Absolute Angle Calibrator (ISAAC).