An electron trapped in a magnetic field will listen for the whispers of dark matter. Eighty-four percent of the universe’s matter is invisible—its identity is unknown. This project addresses that gap by using a single electron, held in a cryogenic Penning trap, as a quantum sensor. The electron’s motion will be monitored for tiny disturbances caused by two leading dark matter candidates: axions and millicharged particles. Axions would solve a long-standing puzzle in particle physics, while millicharged particles could explain why hydrogen in the early universe cooled faster than expected. If successful, APTE will perform the first search for axions at a mass of 0.128 meV and for millicharged particles with charges as small as 10⁻⁷ e—regions where no data currently exist. This is fundamental science. It will not immediately change a smartphone or a power grid. But similar precision measurements of single particles have, in the past, led to technologies like atomic clocks and magnetic resonance imaging. A confirmed dark matter signal would rewrite physics; a null result would still refine the hunt.
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No one knows what 84% of the matter in the universe is, and the goal of APTE is to find out. APTE will investigate two leading types of dark matter: axions and millicharged particles. Axions not only resolve a longstanding puzzle in particle physics, but if their mass is 0.001-1 meV they are an excellent dark matter candidate. Millicharged particles arise in some theories beyond the Standard Model and have recently been suggested as a com- ponent of dark matter that could explain the anomalous cooling of hydrogen in the early universe. APTE will use an electron in a cryogenic Penning trap in an entirely new application: a quantum sensor for astroparticle physics. The radial modes of the trapped electron will be monitored for transitions caused by collisions with millicharged dark matter or the absorption of single microwave photons from axion decays. Changes in radial quantum state will be imprinted onto the trapped electron's axial frequency using a strong magnetic inhomogeneity, the same principle as used to measure the electron magnetic moment. Drawing on my experience in BASE, one of the world's most sensitive cryogenic Penning traps, I will pioneer the use of advanced phase sensitive measurement protocols of my own invention. These will be used to monitor the electron's axial frequency for phase jumps which occur when the radial mode changes. With this electron single photon counter we will be able to overcome the Stan- dard Quantum Limit which severely degrades our ability to investigate axion masses > 0.04 meV. APTE will perform a first search for dark matter axions with masses around 0.128 meV and millicharged dark matter particles with masses up to at least 1 GeV and charges 10^-1 to 10^-7 e. Both measurements probe important theories in a regime where there is currently no data and few proposed experiments. This ground-breaking project will lay the foundations for a future program of axion searches, as well as other applications in fundamental physics.
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