Rydberg Single Photon Detection for Probing 0.1-10 meV Dark Matter with BREAD

TL;DR

The paper proposes a Rydberg-based single-photon detector (SPD) integrated with the Broadband Reflector Experiment for Axion Detection (BREAD) to probe light bosonic dark matter in the 0.1–10 meV mass range. It leverages BREAD's focusing to convert DM-induced photons into detectable electrons and explores two sensing modalities: resonant Rydberg–Rydberg transitions (0.1–1 meV) read out by state-selective field ionization, and direct Rydberg ionization (1–10 meV) for broadband coverage, with dielectric-layer stacks enhancing photon production. The authors derive photon-absorption and ionization rates using Fermi’s golden rule, relate the rates to experimental geometry via $E_{DM}^2 = 2 R_{DM} m_{DM} / A_{focus}$, and discuss detection efficiencies and timescales, showing that high absorption efficiency is achievable in the high-Q BREAD mode. Projected sensitivities for axions and dark photons are presented for ~1000 days per decade in mass, with and without dielectric layers, illustrating competitive reach in the THz DM parameter space and highlighting the method's complementarity to prior BREAD approaches.

Abstract

We introduce a Rydberg-based single photon detector (SPD) for probing dark matter in the 0.1-10 meV mass range (20 GHz-2 THz). The Rydberg SPD absorbs photons produced and focused by the BREAD dish antenna and trades them for free, detectable electrons. At the lower end of the mass range, photons drive Rydberg-Rydberg transitions, which are read out via state-selective ionization. At higher masses, they directly ionize the Rydberg atoms.

Figures (3)

• Figure 1: Schematic of the Rydberg SPD setup. (a) BREAD uses a parabolic reflector to focus photons emitted perpendicular to the barrel surface onto a focal point, increasing the photon intensity at the focus. (b) A Rydberg beam is prepared by exciting electrons to a high principal quantum number. When the photon energy exceeds the ionization threshold $\omega > I_n$ the photons directly ionize the Rydberg atoms, producing detectable free electrons. (c) The target photon energy matches the transition energy between Rydberg states, $\omega = \omega_n$. During the sensing stage, the Rydberg beam is exposed to photons from the BREAD setup. Downstream, the Rydberg state is read out using selective field ionization (SFI). In SFI, an external electric field is scanned across the ionization threshold of the relevant states, and the resulting ionization is detected.
• Figure 2: Dark matter mass range accessible using Rydberg states with principal quantum number $n$. Top: Rydberg transition coverage (without scanning) for $n=30, 31, \cdots, 65$. We include transitions of $n\rightarrow n+i$ within the 0.1-1 meV range. The inset shows the distribution of mass gaps across all available transitions. The largest uncovered gap in mass coverage is 0.02 meV, which can be bridged using a perturbative scanning scheme in about $0.02\,{\rm meV}\times Q_{\rm DM}/m_{\rm DM}\sim4\times10^4$ steps for $Q_{\rm DM}\sim10^6$. Bottom: Ionization is efficient for photons with frequencies above the ionization threshold, with a power-law fall-off at higher frequencies. This enables broadband sensitivity using only a small number of atoms. Using four different Rydberg states allows coverage of the 0.1--1 meV DM mass range.
• Figure 3: Projected Rydberg SPD sensitivities in the BREAD experiment for axion (left) and dark photon (right) dark matter. We assume SNR=5, DCR=0.01 Hz, 1000 days of measurement to cover the 0.1--1 meV range, and an additional 1000 days to probe the 1--10 meV range. Orange lines correspond to the BREAD dish antenna setup, and green lines include additional dielectric layers that further enhance the photon production rate and also broaden the photon linewidth. Details are discussed in the main text. Existing constraints are shown in light gray (astrophysical and cosmological) and light purple (laboratory-based). Existing limits are adapted from Ref. AxionLimits and references therein. Dark purple constraints come from a pilot BREAD experiment, dubbed GigaBREAD, which used a custom coaxial horn antenna at the focal point with a low-noise radio-frequency receiver to probe dark photons BREAD:2023xhc and axions GigaBREAD:2025lzq in the 44--52 ${\rm \mu eV}$ range (10.7--12.5 GHz).