In this era of precision cosmology, measurements suggest that ordinary matter represents only a fraction of the total matter density in the Universe. The rest, whose presence we only infer gravitationally, is unknown in its nature, and is termed Cold Dark Matter, for which a particle called a light axion is a well-motivated candidate. Should axions constitute the dark matter of our Milky Way halo, they may be detected through their conversion into a narrow Radio Frequency (RF) signal in a microwave-cavity resonator permeated by a magnetic field. The experiment being performed at Yale, ADMX-HF (Axion Dark Matter eXperiment - High Frequency), was designed to serve both as an innovation test-bed to develop and transition new cavity designs and quantum-limited photon detection schemes, and as a pathfinder to take first data in the 10-100 micro-electron Volt mass range.
R&D on superconducting thin-films, and squeezed-state and single-photon detection in quantum electronics will produce unanticipated spin-offs. ADMX-HF has proven to be a great attractor for young talent. This award can be expected to extend this record. In addition, it is planned to establish a permanent exhibit on dark-sector science at the Chabot Space and Science Center, or the Oakland Museum of California, utilizing prototypes and artifacts from many of the historic dark matter, dark energy and cosmic microwave background experiments.
UC Berkeley and U. Colorado/JILA are assisting Yale by taking the lead on cavity and amplifier development, respectively. The three universities have successfully delivered on ADMX-HF, which recently performed its first data run. This represented the inaugural use of Josephson Parametric Amplifiers (JPAs) in the microwave cavity experiment, and operation with a dilution refrigerator. With this award, Berkeley and Colorado plan two lines of R&D: (i) Berkeley will pursue boosting the cavity Q by incorporating multilayer structures of thin-film superconductors into the cavity design, as well as exploring Photonic Band Gap (PBG) structures as practical resonators for higher frequency operation; and (ii) Colorado/JILA will focus on reducing amplifier noise below the Standard Quantum Limit (SQL), initially by incorporating a JPA-based squeezed-vacuum state receiver. This would be only the second application of a squeezed-state receiver, beside the GEO/LIGO gravity wave interferometers. Later, photon detector systems based on superconducting qubits inside of a cavity will be developed and deployed. Technical deliverables anticipated are to (a) extend the mass range of the search to 12 GHz (~50 micro-eV), and (b) deliver an improvement in power sensitivity by an order of magnitude, and thus approach sensitivity to Dine-Fischler-Srednicki-Zhitnitsky (DFSZ) axions. This will dramatically improve the discovery potential of the axion search in the most promising mass region.
