Noise_thermometry_at_ultra_low_temperatures

Research
Cite this article: Rothfuss D, Reiser A, Fleischmann A, Enss C. 2016 Noise thermometry at ultra-low temperatures. Phil. Trans. R. Soc. A 374: 20150051. /10.1098/rsta.2015.0051 Accepted: 23 October 2015 One contribution of 16 to a Theo Murphy meeting issue ‘Towards implementing the new kelvin’. Subject Areas: low temperature physics, thermodynamics Keywords: noise thermometry, superconducting quantum interference devices, cross correlation Author for correspondence: C. Enss e-mail: christian.enss@kip.uni-heidelberg.de
1. Introduction
The precise determination of the equilibrium thermodynamic temperature at ultra-low temperatures remains 2016 The Author(s) Published by the Royal Society. All rights reserved.
See discussions, stats, and author profiles for this publication at: https:///publication/257955673
Noise thermometry at ultra low temperatures
SEE PROFILE
53
Some of the authors of this publication are also working on these related projects:
Electron Capture in Ho-163 - ECHo View project
Order parameter symmetry of the strongly hole doped Fe pnictides View project
Downloaded from / on November 4, 2016
a non-trivial experimental challenge and only a few types of thermometers have been successfully used in this range [1,2]. The lower end of the provisional low-temperature scale PLTS 2000 [3] is set by the Néel temperature of solid 3 He, TN = 0.902 mK, which means that thermodynamic temperatures below this point have to be established individually at each laboratory by the use of suitable primary thermometers. Although many properties of solids and quantum liquids depend on temperature below 1 mK there is a clear shortage of methods that allow the determination of temperature in primary mode. Most widely used is pulsed 195 Pt nuclear magnetic resonance (NMR), in which the spin temperature of the platinum nuclei just before the first pulse is determined and taken as the thermodynamic temperature assuming that the Pt nuclear spins were in thermal equilibrium with the conduction electrons. Over more than three decades, this has proved to be a reliable method to determine ultra-low temperatures. However, one disadvantage of this technique is the fact that considerable heat is deposited during the pulse sequence. In recent years, efforts have been increased to establish a second technique based on thermal noise for measuring ultra-low temperatures in primary mode or at least in relative primary mode. One of these techniques is based on the measurement of thermal current noise originating from a temperature-independent resistor. Thermally driven fluctuations of the voltage across a resistor were discovered experimentally by Johnson in 1927 [4,5] and were explained by Nyquist shortly afterwards [6]. Independent of the resistor material in the classical limit (hf kB T) the mean square voltage noise is given by
Noise thermometry at ultra-low temperatures

D. Rothfuss, A. Reiser, A. Fleischmann and C. Enss
Kirchhoff-Institut für Physik, Universität Heidelberg, Im Neuenheimer Feld 227, 69120 Heidelberg, Germany
Article in Applied Physics Letters · July 2013
DOI: 10.1063/1.4816760
CITATIONS
READS
9
4 authors, including: A. Fleischmann Universität Heidelberg
11
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 lowtemperature 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.