Ultralow temperature thermometry with nanoscale devices

The goal of cooling materials and structures ever closer to absolute zero temperature has led to significant discoveries in physics and has prompted the development of many new technologies. For example, the phenomena of superfluidity and superconductivity showed that quantum-mechanical effects dominate the behaviour of certain materials at low temperatures. The discovery of the quantum Hall effect has given a new metrological standard for defining voltage, and the discovery of Coulomb blockade may soon allow the ampere to be redefined using devices that generate electrical current one electron at a time. Cooling to very low temperatures can better allow us to observe and control certain materials and structures at a quantum-mechanical level. This continues to drive research in low temperature physics and underpins many efforts to realise new quantum technologies such as quantum computation and advanced sensors.

Present refrigeration technologies allow certain materials to be cooled extremely close to absolute zero. The limit for continuous cooling is around 1 millikelvin, using dilution refrigeration. Additional cooling based on nuclear demagnetisation refrigeration allows some materials can be cooled to less than a hundredth of this temperature. The biggest challenge in using either of these methods to cool an arbitrary sample is making a good thermal connection between the sample and the refrigerator. At low temperatures thermal connections between materials become very small. This can mean, for instance, that the electrons in the metal wires contacting an on-chip device are at a different temperature to that of the chip, and neither are as cold as the refrigerator. This a particular problem in the field of nanoelectronics where the sample has a tiny active volume with a very weak thermal connection to its surroundings. At present, it is extremely challenging to cool nanoelectronic samples significantly below 10 millikelvin.

This project will combine techniques from ultralow temperature physics and nanotechnology to develop new devices that can measure the temperature of electrons in nanoelectronic structures below 1 millikelvin. These thermometers will then be used to build a platform for reaching temperatures of 1 millikelvin or below in arbitrary nanoelectronic samples. Three different thermometers will be studied, before the most promising one is selected for the final stage of the project. All of the thermometers will be essential diagnostic tools throughout the project, informing the development of electrical filters, thermal shielding and refrigeration methods.

The new thermometry techniques will give us a better understanding of nanoscale structures in a currently inaccessible temperature range. This is likely to be a significant benefit to many active areas of research in low temperature physics, quantum computing, nanoscience and metrology.

The main impact of this project will be new instrumentation and methodology to enable the study of nanoelectronic structures at currently inaccessible temperatures below 1 millikelvin. In addition to advancing the fundamental understanding of heat flow in nanoscale structures, this project aims to improve key enabling technologies - thermometry and refrigeration - that have broad utility in low temperature science and the growing area of Quantum Technologies. Quantum Technologies are anticipated to improve the performance of a variety of sensors with benefits ranging from improved medical imaging to resource exploration. Many proposed quantum technologies, such as enhanced magnetic field sensors and bolometers based on superconducting materials, will need to be operated at sub-kelvin temperatures or even colder. This project is therefore closely aligned with the EPSRC Grand Challenge "Quantum Physics for New Quantum Technologies" and recent UK government investment in Quantum Technologies. The current push to develop practical Quantum Technologies in a short timescale (five years) makes the work proposed in this project especially timely. As well helping to realise the benefits of Quantum Technologies, the importance of improved sensing, metrology, and quantum technologies will be communicated to the general public through publication and outreach activities.

During this project graduate students will be trained in highly desirable skills such as nanofabrication and cryogenic techniques, including skills that are valuable in other sectors such as electronics, low noise measurements, data analysis and scientific computing.

The UK already has a strong commercial presence in low temperature technologies, particularly in the construction of dilution refrigerators. New techniques and knowledge arising from this project are expected have benefits for this market and these will be explored through existing industrial collaborations within and outside the UK. As well as helping to maintain the UK's position in supplying millikelvin refrigerators, these collaborations can explore a potential new market in sub-millikelvin refrigeration for nanoscale samples and devices.