Project Activities > Joint Research Activities > JRA4

Novel methods and devices for ultra low temperature measurements

Objectives and expected impact

Description of work

Many standard methods in low temperature physics are not directly suited for use at ultralow temperatures. The main reason is the strong thermal decoupling and the requirement of extremely low parasitic heating. This is especially true in studies of small nanosamples near their quantum mechanical ground state. Fundamentally new approaches are needed to overcome these obstacles to open up new frontiers in this field of research. The ultimate goal is to develop measurement techniques limited only by the laws of quantum mechanics. They are useful at sub-mK temperatures where the thermal noise of environment becomes smaller than the quantum noise at relatively low frequencies of f > 20 MHz.

One line of development will be based on the idea of transforming conventional measuring methods into contactless setups by utilizing inductive, capacitive and optical coupling methods. Avoiding direct contact of wires and measuring cables at the samples can reduce parasitic heat flow by many orders of magnitude. Therefore we will design and demonstrate ultra sensitive techniques to measure specific heat, thermal conductivity and sound velocity by consequent implementation of contactless methods. In addition, we will utilize new types of filtered leads developed in JRA2 to suppress high frequency noise.

Another general requirement for many experiments at ultralow temperatures is the use of ultra sensitive low temperature amplifiers. For many applications the optimal choice are SQUID amplifiers. Therefore we intend to develop SQUID amplifiers for various low and high frequency applications, which can be operated at mK temperatures with an energy sensitivity close to the quantum limit.

Finally, thermometry is an essential part of any microkelvin experiment. Studies on nanosize samples at ultra low temperatures are, however, hampered by the lack of convenient thermometers. We intend to make a serious effort to solve this problem by developing suitable nanothermometers for ultra-low temperatures. We will transfer our knowledge on SQUID amplifiers to develop noise thermometry of nanosamples. Coulomb blockade thermometry, invented already in 1994 by some of the consortium partners (TKK), will be further developed to work also at sub-mK. Microkelvin experiments in nanosamples in semiconductor materials (pursued in JRA1) require on-chip thermometry measuring directly, in-situ the temperature of the electrons in the sample. We will develop a suitable quantum dot thermometer to allow temperature measurements at sub-mK temperatures in semiconductor nanosamples. In addition, a compact ultralow temperature 195Pt NMR thermometer will be realized.

To meet these objectives, the following activities will be implemented:

> Task 1: Contactless measurement of thermal, dielectric, magnetic and acoustic properties
> Task 2a: SQUID amplifiers for microkelvin measurements
> Task 2b: High frequency SQUID amplifiers at the quantum limit
> Task 3a: Noise thermometer
> Task 3b: Ultra low temperature 195Pt NMR - Thermometer
> Task 3c: Coulomb blockade thermometer for nanosamples