The QUIET experiment is designed to search for the imprint of inflation in the Cosmic Microwave Background radiation by measuring so-called "B-mode" patterns of polarization that are expected to be produced by gravitational waves generated during inflation. This signature, which would allow the energy scale of inflation to be determined, is extremely faint, requiring instruments that are sensitive to nanoKelvin differences in radiation temperature. Measuring B-mode polarization is a significant challenge and the experiment must be carefully designed to avoid false detections from systematic effects.

The QUIET I project, led by Bruce Winstein (1943-2011), used MMIC (Millimeter Monolithic Integrated Circuit) HEMT (High Electron Mobility Transistor) amplifiers operating at 44 GHz and 90 GHz on a 1.4m telescope that was located on the Chajnantor Plateau in Chile. QUIET I served as a proof of concept for sensitive HEMT experiments, setting limits to B-modes, while detecting the brighter E-modes of polarization with high signal to noise. The figure shows the QUIET experiment in Chile and the 91 element 90 GHz focal plane.  QUIET measurements of E-modes and B-modes are also shown and compared to other state of the art experiments in the field (see QUIET Collaboration, arXiv:1207.5034, submitted to Astrophysical Journal). The parameter of interest that is measured by QUIET and other experiments is the "tensor/scalar ratio", r, which relates the perturbations of the gravitational field during inflation caused by gravitational waves to those caused by variations in the density of energy.  A key milestone reached by QUIET I was achieving the best rejection to date of systematic errors.  In QUIET I these errors were sufficiently well controlled to allow a future upgrade (QUIET II) to reach a tensor-scalar ratio of $r < 0.01$, which allows the most natural models of inflation to be investigated.

KIPAC is leading the development of QUIET phase II which will incorporate three receiver arrays: (i) a 499-element 90 GHz MMIC array, closely modeled on the 90-element array that was fielded in QUIET Phase-I campaign; (ii) a 3-element 30 GHz array; and (iii) a 10-element 37 GHz array. The lower frequencies will provide sensitive measurements of potentially confusing astrophysical foreground signals so that these can be subtracted from the 90\,GHz channel. The arrays will be mounted on a new 2.4\,m telescope similar to, but larger than, the telescope used for QUIET I. The results of QUIET I, coupled with the major improvements in QUIET detector sensitivity that have occurred since phase I was built, ensure that after 3.5 years of operation QUIET-II will have the sensitivity, the control of systematic errors, and the foreground rejection needed to reach $r < 0.01$.

QUIET Phase I was a collaboration between the University of Chicago (PI Bruce Winstein), California Institute of Technology, Columbia University, Fermi National Accelerator Lab, the Jet Propulsion Laboratory, KEK (Japan), KIPAC/Stanford University, Max-Planck Institute fur Radioastronomie (Germany), Nikhef (The Netherlands), Princeton, Oxford University (UK), University of Miami, University of Manchester (UK), University of Oslo (Norway).

QUIET Phase II is a collaboration between KIPAC/Stanford University (PI Sarah Church), California Institute of Technology, CITA (Canada), Fermi National Accelerator Lab, the Jet Propulsion Laboratory, KEK (Japan),  Nikhef (The Netherlands), University of Miami, University of Maryland,  University of Oslo (Norway).

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