Externally-Shunted High-Gap Josephson Junctions: Design, Fabrication and Noise Measurements

In the late 1990's measurements of the cosmic microwave background radiation and distant supernovae confirmed that around 70% of the energy in the universe is in the form of gravitationally-repulsive dark energy. This dark energy is not only responsible for the accelerating expansion of the universe but also was the driving force for the big bang. A possible source of this dark energy is vacuum fluctuations which arise from the finite zero-point energy of a quantum mechanical oscillator, hf/2 (where f is the oscillator frequency). Much experimental and theoretical astrophysics and cosmology research is currently focussed on confirming the source of dark energy.A recent publication by Beck and Mackey, however, suggests the possibility that dark energy may be measured in the laboratory using resistively-shunted Josephson junctions (RS-JJ's). Vacuum fluctuations in the resistive shunt at low temperatures can be measured by non-linear mixing within the Josephson junction. If vacuum fluctuations are responsible for dark energy, the finite value of the dark energy density in the universe (as measured by astronomical observations) sets an upper frequency limit on the spectrum of the quantum fluctuations in this resistive shunt. Beck and Mackey calculated an upper bound on this cut-off frequency of 1.69 THz.Measurements of quantum noise in Josephson junctions were performed in a quite different context in the early 1980's. Most notably for this work, the spectrum of zero-point fluctuations in RS-JJ's was measured by the Berkeley group, but only up to 0.6 THz. The upper frequency limit of these measurements was dictated by the gap energy of the lead-alloy superconductors used in that experiment. At higher frequencies tunnelling of quasiparticles dominates over all other electronic processes.We therefore propose to perform measurements of the quantum noise in RS-JJ's fabricated using superconductors with sufficiently large gap energies that the full noise spectrum up to and beyond 1.69 THz can be measured. Unfortunately niobium junctions, which may now be repeatably and reproducibly fabricated, have a cut-off frequency of, at best, 1.5 THz. There are two candidate families of superconductor which present themselves as viable alternatives to niobium: the nitrides and the cuprates. Nitride junctions have cut-off frequencies of around 2.5 THz, which should give sufficiently low quasiparticle current noise around 1.69 THz at accessible measurement temperatures. Cuprate superconductors have an energy gap an order of magnitude higher than the nitrides, but here there is finite quasiparticle tunnelling at voltages less than the gap voltage, due to the d-wave pairing symmetry. By performing experiments on both the nitrides and the cuprates we will have two independent measurements of the possible cut-off frequency in two very different materials systems. This would give irrefutable confirmation (or indeed refutation) of the vacuum fluctuations hypothesis.