Quasiparticle dynamics in aluminium superconducting microwave resonators
Quasiparticle dynamics in aluminium superconducting microwave resonators
复制标题
铝超导微波谐振器中的准粒子动力学
DOI：
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发表时间：
2014
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通讯作者：
P. Visser
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作者：
P. Visser
本论文描述了超导微谐振器探测器的固有局限性。在低温超导体中，大多数电子配对成所谓的库珀对，这导致了众所周知的无电阻导电。因此，超导微波谐振器具有非常高的品质因数并且对最小的信号敏感。因此，可以用这些谐振器检测库珀对数量的微小变化。超导体（例如铝）的结合能较低，这使得这种探测器适合探测亚毫米辐射，这是天文学特别感兴趣的波长范围。每个谐振器可以具有稍微不同的长度，从而具有不同的谐振频率，这使得能够使用单条读出线读出数千个谐振器（像素）。由于探测器原理依赖于对破碎的库珀对（所谓的准粒子）的数量进行计数，因此这些谐振器探测器最基本的噪声源是由于准粒子数量的波动。我们证明超导铝谐振器确实受到这些波动的限制。我们表明，对这些波动的测量还可以了解准粒子的数量及其寿命，这是超导体本身的固有属性。当温度降低时，准粒子的数量预计会呈指数下降。然而，我们观察到准粒子的数量及其寿命在低温下饱和。对这些准粒子波动作为微波读出功率的函数的测量表明，低温下过量的准粒子是由于微波功率耗散造成的。因此，本论文的第二个主要主题是微波读出信号如何影响超导体和谐振器响应。我们表明，由于读出功率吸收，谐振器响应变得强烈非线性。由于微波光子能量低于电子对破坏能量，因此微波信号应该无法破坏库珀对。我们表明，过量准粒子的产生是由于复杂的效应，即由于读出功率吸收而导致准粒子在能量上的重新分布。最后，我们报告了一项将太赫兹辐射应用于微波谐振器探测器的实验。为了证明探测器足够灵敏，可以用于冷却太空望远镜上的相机，必须在测试设置中模拟此类仪器的条件。我们观察到探测器的噪声受到光子噪声、辐射源引起的基本波动的限制，并且灵敏度满足太空望远镜的要求。
This thesis describes the intrinsic limits of superconducting microresonator detectors. In a superconductor at low temperature, most of the electrons are paired into so called Cooper pairs, which cause the well-known electrical conduction without resistance. Superconducting microwave resonators have therefore very high quality factors and are sensitive to the smallest signals. A small change in the number of Cooper pairs can therefore be detected with these resonators. The binding energy in a superconductor, such as aluminium, is low, which makes such a detector suitable for detection of submillimetre radiation, a wavelength range that is particularly interesting for astronomy. Each resonator can have a slightly different length, and thus a different resonant frequency, which enables the readout of thousands of resonators (pixels) with a single readout line. Because the detector principle relies on counting the number of broken Cooper pairs (so called quasiparticles), the most fundamental source of noise of these resonator detectors is due to fluctuations in the number of quasiparticles. We demonstrate that superconducting aluminium resonators are indeed limited by these fluctuations. We show that a measurement of these fluctuations also provides access to the number of quasiparticles and their lifetime, which are intrinsic properties of the superconductor itself. At decreasing temperatures the number of quasiparticles is expected to decrease exponentially. However, we observe that the number of quasiparticles and their lifetime saturate at low temperature. Measurements of these quasiparticle fluctuations as a function of the microwave readout power revealed that the excess quasiparticles at low temperature are due to microwave power dissipation. The second main topic of this thesis is therefore how the microwave readout signal affects the superconductor and the resonator response. We show that the resonator response becomes strongly nonlinear due to readout power absorption. Since the microwave photon energy is lower than the pair-breaking energy, the microwave signal should not be able to break Cooper pairs. We show that the creation of excess quasiparticles is due to an intricate effect, the redistribution of quasiparticles over energy due to readout power absorption. Finally, we report an experiment in which we apply terahertz radiation to a microwave resonator detector. To demonstrate that the detector is sensitive enough to be used in a camera aboard a cooled space telescope, the conditions for such an instrument have to be mimicked in the test-setup. We observe that the noise of the detector is limited by photon-noise, fundamental fluctuations due to the source of radiation, and that the sensitivity fulfils the requirements for a space telescope.