QuTHERA
Single-photon detection of dark-photon dark matter with superconducting qubits
T ~ 20 mK
infrared-
tight
connectors
dark
photon, γ′
photon, γ
~THz
~ 8 cm
Cu hemisphere, Au-plated
lid
Si lens
cavity
qubit chip
probe tone, f ~ GHz
1
2
photon, γ
~THz
qubit
island
ground
plane
Zoom: qubit chip
H = 4EC(n − ng)² − EJ cos φ
offset charge ng, in units of 2e
photon, γ
~THz
ng → ng + ½
Quasiparticle
island
ground
plane
substrate
Josephson
junction
Cooper pair
delocalized over ξ
Zoom: Josephson junction
Transmission, |S21|
even ng
odd ng
charge
dispersion, ε
~MHz
linewidth, κ
~MHz
ng →
ng + ½
Probe frequency, f (GHz)
Conversion. A dark photon γ′ passes the shielding and converts at the Cu hemisphere into a terahertz photon γ, emitted normal to the surface.
Focusing. Normal emission from the whole surface converges on the dish centre, where the Si lens couples it onto the qubit chip.
Absorption. The pier of the qubit island acts as a double-slot antenna and delivers the photon to the Josephson junction.
Pair breaking. The photon (hν > 2Δ) breaks a Cooper pair delocalized across the junction, leaving one quasiparticle on each side.
Parity flip. The island charge changes by one electron (ng → ng + ½), so the resonance jumps by ε and the probe transmission steps up.
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