Abstract
Abstract : Superconducting quantum detector structures were formulated, designed, fabricated and tested. As designed, the expected photoresponse depends on the superconducting current's kinetic inductance. The kinetic inductance, inversely proportional to the Cooper pair density, changes with photoabsorption. Photons with sufficient energy to breaks Cooper pairs, modulate the superconducting current's kinetic inductance. The modulation depends on the photoabsorbed photon flux density and the quasiparticle excitation lifetime. Such a photoresponse is consistent with a quantum detector. For maximum response, we have fabricated photodetector structures with significantly reduced geometrical inductance and maximum kinetic inductance. These quantum detectors were monolithic with Josephson-junction readout circuits. A multi-level YBa2Cu3O(7-delta) processes was used for fabrication. These devices were tested at cryogenic temperatures in a specially designed and fabricated facility. Electrically, the SQUID read out performed well. Modulation of the SQUID critical current was successful, revealing that we successfully fabricated these detector structures. This modulation was achieved by injecting a dc current into one half of the quantum photodetector, thereby changing the average phase across Josephson junctions from pi/2. This phase change reduced the SQUID's critical current. The quasiparticle lifetime was too short to measure a quantum response at low frequencies. The experimentally observed short quasiparticle lifetime may be due to intrinsic or extrinsic effects. Given the relative immaturity of high temperature superconducting fabrication technology, it is not unreasonable to expect material defects to severely reduce the quasiparticle lifetime and hence mask any low frequency quantum responses. A second possibility is that unlike the BCS superconductors, the quasiparticle lifetimes in high temperature superconductors do not exponentially increase with lower temperatures. c
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