To integrate nanoscale experiments into sub-millikelvin cryostats will require new technology. The difficulties are largely those of making thermal contact to the electron gases in the nanostructures. This is especially true with semiconductor nanostructures. At ultralow temperatures such substrates become effective thermal vacua and thermal contact is restricted to the pathways via the metallic leads to the circuits

The only quantity which matters in cooling such circuits is the ratio of the heat leak into the circuit material to the thermal contact to the refrigerant. Both quantities have to be attacked in parallel. First we can make great efforts to reduce the external heat leak. With the best current refrigerators we can create enclosures which are so well insulated that the heat leak into an isolated non-conductor is already at the level set by the background radioactive heating (largely from nearby constructional concrete). Metallic samples experience additional heating from eddy currents generated by motion in stray magnetic fields. However, these can also be reduced to a level below 4 pW per mole which translates to ~10⁻²⁴ watts into a micron cube sample.

The real difficulties come when we attach leads, as this immediately connects the outside world. We have to take this problem very seriously and start with the best electrical filtering possible, which fortunately is being pursued with vigour in JRA2. Secondly we must enhance the thermal contact to the refrigerator. In a semiconductor 2DEG, for example, the substrate makes no contribution to thermal contact at the lowest temperatures which runs entirely via the leads. Using ideas from BASEL and ULANC we can thermally anchor each lead directly in the mixing chamber liquid with sintered silver pads and then furnish each lead with its own mini nuclear stage to absorb any final incoming energy in the nuclear bath. Finally we can envisage completely new tailor-made nanoscale structures independent of conventional semiconductors. Ideal candidates for microkelvin cooling are carbon-nanotubes and graphene structures which can be directly immersed in superfluid 3He where there is a dense 3He quasiparticle gas making orders of magnitude better contact directly to the structures. Finally, we should exploit the increasing scattering lengths with falling temperatures to make "macroscopic" metallic circuits which are still able to maintain electron coherence over the scale of the sample. This will make thermal contact easier, but will require high-purity materials and new features, such as electro-polishing to minimise coherence-limiting processes at the surfaces, but this is looking far ahead.

In nanostructures, cooling the phonon bath and measuring its temperature is a challenge. We propose the realization of nanoscale single crystal silicon membrane on which a S/I/N junction and a thermometer can be deposited. The purpose is to cool down the phonon thermal bath using the SIN junction as a nanocooler. Very low temperatures can be reached on such a device where the heat capacity of the membrane is very small (less than 10⁻¹⁵ J/K).

This task aims to make low millikelvin and microkelvin experiments accessible to any laboratory whether it has the infrastructure for dealing with liquid nitrogen and helium refrigerants or not. The principal aim is to generate the knowhow leading to the production of prototype systems requiring no external support services other than power and which can be operated automatically without the operator needing any specialist millikelvin knowledge. Thus, as well as the design aim of reaching ultralow temperatures, the model needs to be compact, reliable, and simple to use.

To this end we envisage an ultimate design where the operational cool-down sequence is fully automatic with single-button initiation. Since the dilution refrigerator will operate in a cryogen-free environment we can dispense with low-temperature vacuum seals and simply open the experimental space by releasing a single room-temperature o-ring flange.

A compact dilution refrigerator, with an integrated pulsed-tube cooler, will act as the precooling stage to 10 mK temperatures. As a novel initiative, we will integrate the nuclear cooling stage, driven by a "dry" superconducting solenoid, with the compact dilution refrigerator. The whole system will be designed to be inherently vibration free which is an important consideration bearing in mind that the initial cooling is provided by a pulsed-tube cooler. We have already demonstrated that vibration amplitudes below 0.1 μm at the pulse-tube frequency are possible.

Once the concept has been successfully demonstrated we envisage that such machines will provide ready access to the milli- and microkelvin temperature regime for any laboratory without the need for any specialist knowledge or support. We also envisage that these machines will be in demand for other purposes beyond the ambit of this application.

Using the combined knowledge and expertise of the applicants we are also planning an entirely new advanced microkelvin refrigerator facility intended exclusively for condensed-matter and nanoscale experiments at milli- and microkelvin temperatures. This will be sited at ULANC in a purpose-built 90+ m² laboratory hall with 7 m clearance and a 3 m dewar pit dedicated to this project, which is supported by 400k€ from the UK Science Research Investment Fund. The access-giving laboratories in this consortium have a very large fraction of the world expertise and capability in carrying out experiments at sub-millikelvin temperatures. We propose to build on this unique European resource by pooling our existing knowledge along with the technology developed in task 2 above to make this the most advanced sub-microkelvin facility for nanokelvin studies that current knowledge will allow.

The project will involve the development of new designs of nuclear cooling stages and new in-mixing-chamber experimental platforms with special attention to improving thermal contact, good isolation and filtering but based on the lowest possible temperature cooling stages which can be achieved.