Draft:Quantum capacitance detector

Quantum Capacitance Detector (QCD)

The Quantum Capacitance Detector (QCD) is one of the most sensitive far-infrared (far-IR) detector ever developed, leveraging advanced superconducting technology to achieve unprecedented sensitivity. The device operates on the principle of detecting tiny changes in quantum capacitance induced by interactions between photons and superconducting materials.

The architecture of the QCD involves a superconducting absorber that receives infrared or submillimeter radiation through an antenna. This radiation provides the energy needed to break Cooper pairs in the absorber, generating phonons and nonequilibrium quasiparticles. These quasiparticles diffuse to the junctions of the single Cooper-pair box (SCB). The SCB is capacitively coupled to an LC oscillator or a microwave resonator, which measures the state of the SCB. When a single quasiparticle tunnels onto the SCB island, it switches the parity of the island, effectively changing the gate charge by one electron. This transition shifts the SCB far from its degeneracy point, altering its capacitance. This abrupt capacitance change results in a significant frequency shift in the oscillator.

The QCD's extraordinary sensitivity is reflected in its noise equivalent power (NEP)Noise-equivalent power, which is below 10^−20 W/(Hz)^1/2. This makes the QCD capable of individual far-IR photon counting, crucial for precise scientific measurements.

In addition to its sensitivity, the QCD benefits from being readable in the frequency domain, similar to the Microwave Kinetic Inductance Detector (MKID)inductance detector. This readout method supports large-scale multiplexing using established fabrication and measurement techniques, essential for creating extensive detector arrays.

Applications of the QCD are particularly significant in far-infrared and submillimeter astronomy, where large arrays of sensitive, low-noise detectors are necessary to achieve scientific objectives. The QCD meets these needs by enabling the precise measurement and analysis of far-IR and submillimeter radiation from astronomical sources.