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:
Development of new techniques for measuring dielectric, magnetic, acoustic and thermal properties of samples at ultralow temperatures. Optical heating techniques, scanning SQUID probes as well as inductive and capacitive coupling schemes will be investigated for designing contactless measuring methods. As one example we mention here a new idea to measure heat capacity of amorphous solids at ultra low temperatures. The understanding of the low temperature properties of amorphous solids is of vital importance for many cutting edge technologies like solid state qubits and kinetic inductance detectors. At very low temperatures it is possible to generate polarization echoes in amorphous dielectrics. For this the sample is located in a microwave cavity and the microwave pulses are coupled in inductively. Since the echo amplitude is a steep function of temperature it can be used to determine the internal temperature of the sample without any leads. Combining this with an optical heating system allows the measurement of specific heat of such samples without electrical contacts to the sample. Many other properties can be measured in a similar way.
Development of novel SQUID systems with micro-coil input circuits as local probes of quantum matter and nanosystems at millikelvin and microkelvin temperatures. The pick-up loop may be integrated with the SQUID loop as in a miniature SQUID susceptometer or be located remotely and transformer coupled to the SQUID. The major gain from the micro-circuit is that high inductive coupling between the coil and the sample or region of interest (of comparable dimensions) can be achieved. If a mechanical actuator is implemented to move the coil the system would act as a NMR microscope, with spatial resolution limited by coil dimensions. In addition, we wish to develop SQUID amplifiers operated at mK temperatures with energy sensitivity approaching the quantum limit, using conventional pick-up coils. We also wish to approach the quantum limit at relatively low frequencies (of order 1 MHz). Our approach will be threefold. First the SQUID will be miniaturized to reduce the energy corresponding to a given flux amplitude. Secondly, the cooling of the resistive element inherent to the SQUID will be given special attention, while finally the power dissipated in the resistive element will be minimized by using an inductive detection scheme of the flux in the SQUID.
The quantum regime of SQUID amplifiers is an open problem. Is a SQUID operated at sufficiently high frequency and sufficiently low temperatures a quantum-limited amplifier? We need to understand the back action of the SQUID on the input circuit. SQUID amplifiers operating into the several hundred MHz region in flux-locked loop mode are on the immediate horizon. Clearly the first thing would be to try and observe quantum-limited noise from a resistor at low mK temperatures. The longer term objective is to achieve quantum limited SQUID amplifiers into the GHz regime for quantum computing applications.
To develop current sensing noise thermometry for the temperature range 50 microkelvin to 10 K (5 orders of magnitude in temperature, one calibration point, no cross calibration, precision independent of temperature). One way to achieve this is by the use of a high-purity noble metal as a temperature sensor, whose current fluctuations are measured inductively by a DC SQUID.
To develop a compact 195Pt NMR -Thermometer for temperatures down to 10 microkelvin. At the high temperature end (10 mK) of the scale this thermometer will be calibrated against a current sensing noise thermometer. Below 1 mK a new superconducting fixed-point device will be developed to provide calibration points. Here the rhodium transition will be utilized, for example.
Coulomb blockade thermometry (CBT) is based on detecting the non-linear conductance of a semiconductor quantum dot or of an array of tunnel junctions: the width of the conductance peak or dip around zero bias voltage is measured, and this width can be related directly to absolute temperature without calibration. We wish to develop Coulomb blockade thermometry for nano samples at the lowest possible temperatures, including both tunnel junction CBT sensors as well as GaAs quantum dot temperature sensors in semiconducting nanosamples.