Rothfuss D, Reiser A, Fleischmann A, Enss C
Kirchhoff-Institut für Physik, Universität Heidelberg, Im Neuenheimer Feld 227, 69120 Heidelberg, Germany.
Kirchhoff-Institut für Physik, Universität Heidelberg, Im Neuenheimer Feld 227, 69120 Heidelberg, Germany
Philos Trans A Math Phys Eng Sci. 2016 Mar 28;374(2064):20150051. doi: 10.1098/rsta.2015.0051.
The options for primary thermometry at ultra-low temperatures are rather limited. In practice, most laboratories are using (195)Pt NMR thermometers in the microkelvin range. In recent years, current sensing direct current superconducting quantum interference devices (DC-SQUIDs) have enabled the use of noise thermometry in this temperature range. Such devices have also demonstrated the potential for primary thermometry. One major advantage of noise thermometry is the fact that no driving current is needed to operate the device and thus the heat dissipation within the thermometer can be reduced to a minimum. Ultimately, the intrinsic power dissipation is given by the negligible back action of the readout SQUID. For thermometry in low-temperature experiments, current noise thermometers and magnetic flux fluctuation thermometers have proved to be most suitable. To make use of such thermometers at ultra-low temperatures, we have developed a cross-correlation technique that reduces the amplifier noise contribution to a negligible value. For this, the magnetic flux fluctuations caused by the Brownian motion of the electrons in our noise source are measured inductively by two DC-SQUID magnetometers simultaneously and the signals from these two channels are cross-correlated. Experimentally, we have characterized a thermometer made of a cold-worked high-purity copper cylinder with a diameter of 5 mm and a length of 20 mm for temperatures between 42 μK and 0.8 K. For a given temperature, a measuring time below 1 min is sufficient to reach a precision of better than 1%. The extremely low power dissipation in the thermometer allows continuous operation without heating effects.
在极低温下进行原级测温的选择相当有限。实际上，大多数实验室在微开尔文温度范围内使用（195）铂核磁共振温度计。近年来，电流传感直流超导量子干涉器件（DC - SQUIDs）使得在这个温度范围内能够使用噪声测温法。这类器件也展现出了原级测温的潜力。噪声测温法的一个主要优点是，操作该器件无需驱动电流，因此温度计内部的热耗散可降至最低。最终，本征功耗由读出超导量子干涉器件可忽略不计的反作用给出。对于低温实验中的测温，电流噪声温度计和磁通量涨落温度计已被证明是最合适的。为了在极低温下使用这类温度计，我们开发了一种互相关技术，可将放大器噪声贡献降低到可忽略不计的值。为此，我们用两个直流超导量子干涉器件磁力计同时感应测量由噪声源中电子的布朗运动引起的磁通量涨落，并对这两个通道的信号进行互相关处理。在实验中，我们对一个由直径5毫米、长度20毫米的冷加工高纯铜圆柱体制成的温度计进行了42微开尔文至0.8开尔文温度范围的特性表征。对于给定温度，测量时间低于1分钟就足以达到优于1%的精度。温度计极低的功耗允许其持续运行而无加热效应。
