One significant disadvantage for superconducting circuitry, despite its great promise, is that it operates well only at very low temperatures, well into the milliKelvin regime. The most effective current refrigeration technique requires an expensive and difficult to operate cryogenic apparatus known as the dilution refrigerator. The dilution refrigerator has been the workhorse of the low temperature researcher for half a century or so and has become a highly developed instrument, it is widely used at Royal Holloway and our own group has three. However, it is not the sort of equipment that could easily be installed in hospitals or airports where sensitive detection devices might need to operate with minimal maintenance, low power needs, small footprint and simple protocols.
Another way to proceed when concerned with quantum devices is to cool only the sensitive detector or quantum computing circuit and not several kilograms of metal. This reduces the problem to cooling only a circuit that contains a few million atoms and thus requires much less power to be cooled. So we require a cooling mechanism that can be nano-fabricated and attached to our devices, it also has to be compatible with all requirements of cooling power, minimal temperature and noise levels.
The normal metal/insulator/superconductor (NIS) junction provides the necessary cooling mechanism. When biased with the appropriate voltage NIS junctions display cooling. This is because the most energetic electrons of the metal can be selectively extracted using the superconducting gap as an energy selective filter, leaving only the lower energy electrons in the metal. Collisions subsequently establish a temperature in the remaining electron system that is necessarily lower. This mechanism of cooling by evaporation is analogous to the way a cup of coffee can be cooled, by blowing across the top and removing the most energetic molecules, those in the vapour.
Nano-refrigerators based on NIS technology have been shown to work in principle in pioneering laboratory based experiments, The goal of this project is to develop nano-refrigerators to the point where they can be implemented in practice for cooling practical circuits such as qubits and quantum sensitive radiation detectors. We need also to ensure that the operation of the refrigerator does not interfere with the quantum devices it is cooling.