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

外部分流高间隙约瑟夫森结：设计、制造和噪声测量

基本信息

批准号：
EP/D029872/1
负责人：
Zoe Barber
金额：
$ 25.37万
依托单位：
University of Cambridge
依托单位国家：
英国
项目类别：
Research Grant
财政年份：
2006
资助国家：
英国
起止时间：
2006 至无数据
项目状态：
已结题

来源：
https://gtr.ukri.org/projects?ref=EP%2FD029872%2F1
关键词：
Externally Shunted Gap Josephson Junctions

项目摘要

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.

20世纪90年代末，S对宇宙微波背景辐射和遥远的超新星的测量证实，宇宙中大约70%的能量是引力排斥暗能量的形式。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波动，它是由量子力学振子的有限零点能量HF/2(其中f是振子频率)引起的。目前，许多实验和理论天体物理和宇宙学研究都集中在确认暗能量的来源上。然而，贝克和麦基最近发表的一篇文章表明，暗能量可能可以在实验室中使用阻性分流约瑟夫森结(RS-JJ)来测量。通过约瑟夫森结中的非线性混合可以测量低温下阻性分流中的真空起伏。如果真空涨落是暗能量的来源，宇宙中暗能量密度的有限值(由天文观测测量)为这种阻性分流中量子涨落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。上世纪80年代初，在完全不同的背景下进行了约瑟夫森结量子噪声的测量。最值得注意的是，伯克利小组测量了RS-JJ的零点涨落频谱，但最高只有0.6太赫兹。这些测量的频率上限是由该实验中使用的铅合金超导体的能隙决定的。因此，我们建议对RS-JJ中的量子噪声进行测量，这些超导体具有足够大的禁带能，可以测量到1.69 THz以上的全部噪声谱。不幸的是，现在可以重复和可重复制造的Nb结的截止频率至多为1.5太赫兹。有两个候选的超导体家族是可行的Nb的替代品：氮化物和铜酸盐。氮化物结的截止频率约为2.5太赫兹，在可达的测量温度下，这应该会产生约1.69太赫兹的足够低的准粒子电流噪声。铜酸盐超导体的能隙比氮化物高一个数量级，但由于d波对对称，在低于间隙电压的电压下存在有限的准粒子隧穿。通过在氮化物和铜酸盐上进行实验，我们将对两个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。这将对真空波动假说给予无可辩驳的证实(或反驳)。