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

在20世纪90年代后期，对宇宙微波背景辐射和遥远超新星的测量证实，宇宙中大约70%的能量是以引力排斥的暗能量的形式存在的。这种暗能量不仅是宇宙加速膨胀的原因，也是大爆炸的驱动力。这种暗能量的一个可能来源是真空涨落，它产生于量子力学振荡器的有限零点能量hf/2（其中f是振荡器频率）。目前，许多天体物理学和宇宙学的实验和理论研究都集中在确定暗能量的来源上，然而，Beck和Mackey最近发表的一篇文章提出了在实验室中使用连续分流的约瑟夫森结（RS-JJ's）测量暗能量的可能性。在低温下电阻分流器中的真空波动可以通过约瑟夫森结内的非线性混合来测量。如果真空涨落是暗能量的原因，那么宇宙中暗能量密度的有限值（通过天文观测测量）就为这个电阻分流器中的量子涨落频谱设定了频率上限。Beck和Mackey计算出截止频率的上限为1.69 THz。20世纪80年代初，约瑟夫森结中量子噪声的测量是在一个完全不同的背景下进行的。最值得注意的是，这项工作中RS-JJ的零点波动谱是由Berkeley小组测量的，但最高仅为0.6 THz。这些测量的频率上限由实验中使用的铅合金超导体的差距能量决定。在更高的频率下，准粒子的隧穿主导了所有其他的电子processes. Then，我们建议进行测量的量子噪声在RS-JJ的制造使用超导体具有足够大的间隙能量，可以测量的全部噪声频谱高达和超过1.69太赫兹。不幸的是，现在可以重复和可再现地制造的铌结的截止频率最多为1.5 THz。有两类候选超导体是铌的可行替代品：氮化物和铜酸盐。氮化物结的截止频率约为2.5 THz，在可接近的测量温度下，这应该在1.69 THz左右产生足够低的准粒子电流噪声。铜氧化物超导体具有比氮化物高一个数量级的能隙，但由于d波配对对称性，在小于能隙差距电压的电压下，存在有限的准粒子隧穿。通过对氮化物和铜酸盐进行实验，我们将在两种非常不同的材料系统中对可能的截止频率进行两次独立的测量。这将无可辩驳地证实（或实际上反驳）真空涨落假说。

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.