Superconducting Qubits as Single THz-Photon Detectors for Bosonic Dark Matter Searches
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The detection of single photons at terahertz frequencies is notoriously difficult, since wavelengths are small, making the application of haloscopes difficult, while the photon energy is also low, making calorimetric detection impossible for current technologies. This combination defines the so-called THz gap, which extends through the laboratory search landscape for fundamental physics. In the 0.1 THz to 1 THz band, the strongest direct laboratory bound on a dark photon is χ ≲ 10^−8 over a narrow sub-band, three to four orders of magnitude weaker than the limits set by microwave haloscopes immediately below 0.1 THz.
Notably, the superconducting gap of typical qubit-platform metals lies in this same energy range. Recent measurements identify thermal radiation in this band as the dominant source of Cooper-pair breaking in state-of-the-art superconducting qubits, a harmful source of errors for superconducting quantum computers. Offset charge-sensitive superconducting qubits are currently the only sensor technology that resolves the tunneling of a single quasiparticle across a Josephson junction, which is the signal that follows the resonant absorption of a photon by the spurious antenna mode of the qubit capacitor pad. We propose to exploit this coupling — between the microwave-controllable electromagnetic qubit mode and the THz-scale antenna mode of the capacitor island — to use the qubit as a sensor of single photons sourced by fundamental physics in the THz gap. The planned demonstrator instrument is a multiplexed qubit array with narrow-band, frequency-tiled antenna modes, coupled to a free-space optical collection chain and operated as a single-photon counter. For a per-channel dark count rate of 1 Hz and an effective collection area of order centimeters squared, a one-month exposure projects a reach on the dark-photon kinetic mixing parameter four to five orders of magnitude below the strongest direct laboratory bound in this frequency range.