Superconducting Materials & Devices
The possible detection of an imprint by inflation on the polarization of the cosmic microwave background (CMB) excited both astrophysicists and the general public around the world in March 2014 when BICEP2 published its results.
Inflation may produce a background of gravitational waves that produce a characteristic “swirly” CMB polarization signal called a B-mode pattern. The BICEP2 telescope, led by researchers at Caltech, Harvard, U. Minnesota, and Stanford, reported a B-mode pattern at 150 GHz. The extreme sensitivity needed to make this measurement was provided by JPL/MDL transition-edge sensor (TES) bolometer arrays, and an extensive three-year observation campaign from the South Pole.
This led to a collaborative effort to determine the nature of the signal by many in the astrophysics community, including a collaboration between BICEP2 and the ESA/NASA Planck satellite, which observed the sky using earlier generations of MDL devices. Deep multi-frequency data are needed to separate the signal into CMB and Galactic components, the most significant Galactic signal arising from emission from interstellar dust, which is not well measured in polarization.
In January 2015, the search for inflationary polarization took to the sky. The SPIDER balloon experiment launched carrying six focal plane arrays (each the size of BICEP2), with three observing at 150 GHz and three focal planes at 95 GHz. All six focal planes employ current TES bolometer arrays. The SPIDER balloon mission will increase the coverage area on the sky at these two frequencies. Balloon-borne observations represent the closest environment to a satellite, with similar scanning measurements and a hostile radiation environment, representing an important milestone for advancing the readiness of CMB detector technology.
Concurrently, two different South Pole telescopes, Keck Array and BICEP3, are utilizing TES bolometer arrays. The Keck telescope array has been observing the same coverage of sky that BICEP2 observed, at both 150 GHz and 95 GHz, for the last three years. The Keck Array combines five BICEP2-style telescopes in a common barrel, making Keck a more sensitive multi-frequency system. The Keck Array will report on the signal observed by BICEP2 in the same region of sky in early 2015, and new results at 95 GHz later in the year. Two new focal plane units were installed during the short Antarctic summer to add measurements at a third frequency band at 220 GHz.
The BICEP3 telescope is also being fielded for its first season of observations in 2015. BICEP3 uses modular bolometer units developed at JPL to build a truly enormous focal plane collecting twice as much light as all five Keck Array telescopes combined. BICEP3 will use this technology to make sensitive observations at 95 GHz, expected to be the world’s most sensitive instrument at this frequency when the system is fully operational. The same modular technology should be ideal for assembling and testing a large multi-frequency focal plane that will be needed in a future satellite mission to measure CMB polarization to fundamental limits.