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

在20世纪90年代末，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认暗能量的来源上。然而，Beck和Mackey最近发表的一篇文章表明，暗能量可以在实验室中使用电阻分流约瑟夫森结（RS-JJ）来测量。电阻分流器在低温下的真空波动可以通过约瑟夫森结内的非线性混合来测量。如果真空波动是暗能量的原因，那么宇宙中暗能量密度的有限值（通过天文观测测量）就为这种电阻分流中的量子波动的频谱设定了一个频率上限。贝克和麦基计算出这个截止频率的上限为1.69太赫兹。在20世纪80年代早期，对约瑟夫森结中的量子噪声的测量是在一个完全不同的背景下进行的。在这项工作中最值得注意的是，伯克利小组测量了RS-JJ零点波动的频谱，但只有0.6太赫兹。这些测量的频率上限是由实验中使用的铅合金超导体的间隙能决定的。在更高的频率下，准粒子的隧穿作用支配着所有其他的电子过程。因此，我们建议在使用间隙能足够大的超导体制造的RS-JJ中进行量子噪声的测量，从而可以测量到1.69 THz及以上的全噪声谱。不幸的是，铌结的截止频率最多只有1.5太赫兹，而这种结现在可以被重复和可复制地制造出来。有两个候选超导体家族，它们是铌的可行替代品：氮化物和铜酸盐。氮化物结的截止频率约为2.5太赫兹，在可达到的测量温度下，准粒子电流噪声应该足够低，约为1.69太赫兹。铜超导体具有比氮化物高一个数量级的能隙，但由于d波对对称性，在低于能隙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，我们将对两种非常不同的材料系统中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（或实际上是驳斥）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.