Innovative metamaterials and quantum amplifiers for very high mass dark matter axion searches

The identification of the ubiquitous dark matter that comprises 85% of all matter in the Universe is one of the premier questions in all of science today. The first suggestion for the dark matter in the 1930's came from astronomical observations in the guise of anomalously high velocities of galaxies in bound clusters, which pointed to a much larger source of gravity than could be accounted by the visible galaxies themselves. Evidence sharpened dramatically in the 1960's and 1970's with precise measurements of the rotation speed of spiral galaxies themselves; at this point the conclusion of a dominant unseen component of matter in the Universe became nearly inescapable. But today, nearly a century after the first evidence for dark matter was uncovered, we have no idea as to its identity.

Our understanding of cosmology however, is intrinsically bound up with our understanding of particle physics, which determined the evolution of our Universe in its earliest history after the Big Bang. It was not surprising then that plausible candidates for the dark matter emerged in the form of long-lived, weakly interacting particle fields resulting from extensions of the Standard Model of particle physics – which while very successful, is well-known to be incomplete. One leading candidate is a very heavy particle, perhaps a hundred times the proton mass, termed the WIMP (weakly interacting massive particle) which can arise in many beyond-Standard Model scenarios, but so far, no evidence has been found for them. Another is an ultralight particle called the axion, which may weigh only a trillionth of the mass of an electron, but could account for the local dark matter in our galaxy by its enormous local density, ten trillion per cubic centimeter or more. The search for the axion is the motivation for our proposal.

Forty years ago, a theorist, Pierre Sikivie, proposed that the axion could be detected by its conversion to a single microwave photon in a strong magnetic field, and that the conversion power could be resonantly boosted inside a microwave cavity. The experiment he proposed was thus a kind of sophisticated radio, by which an extraordinarily weak signal could be detected by slowly tuning the microwave cavity, and listening for a signal that might only be a billionth of a billionth of a Watt. Such experiments have been carried out now for more than three decades, in a narrow window of mass (the resonance condition is that the frequency of the microwave cavity must equal the mass), and with the largest and strongest superconducting magnets available, and state-of-the art microwave cavities and amplifiers, such a signal could be rendered observable. The US and now global effort to find the axion as the dark matter has both driven and leveraged advances in quantum sensing – inventing amplifiers that work at the very limits of quantum mechanics, and by inspired quantum enhancement strategies, building receivers that actually circumvent the limits on irreducible noise set by quantum mechanics.

New insights in theory however, suggest that the axion may lie at much higher frequencies than we are currently searching at. Microwave cavities for frequencies at 10-100 gigahertz become too small to produce useful conversion power, and we do not have satisfactory quantum amplifier designs at those frequencies either. We propose here a coordinated attack on the problem. The Berkeley team, with a long record in providing innovations in microwave cavities for axion experiments, have begun to carry out R&D on metamaterials designed with arrays of fine wires, which would replace the microwave cavities of the current experiments. As pointed out in a recent paper by a theory group at Stockholm University, wire-array metamaterials could be made both arbitrarily large, and arbitrarily high in frequency. Initial results are promising, but many questions and challenges remain to be surmounted. The Colorado group, world leaders in quantum devices, propose to push up the maximum frequency of their amplifiers by an order of magnitude or more. The Colorado group was responsible for the world's first dark matter experiment that exploited squeezed-states of the vacuum to circumvent the Standard Quantum Limit on noise, an achievement that, in the world of fundamental physics, only LIGO, which discovered gravitational radiation, had done previously. Oak Ridge National Laboratory will join with both Berkeley and Colorado to provide support for test and characterization of both metamaterials as well as amplifiers. This activity will also be a tremendous magnet for new, young and diverse talent to the field.