Heat management for efficient quantum computers
Born in the 19th century when scientists were first discovering how to build and operate steam engines, thermodynamics is a branch of physics that deals with the energy and work of a system. The field has several successes to notch up that have literally transformed our lives, with applications ranging from refrigerators and air conditioners to jet planes. Until recently, thermodynamics has been mostly applied to large systems described by the laws of classical physics. However, with modern computers miniaturising down to the nanoscale and the quantum regime, scientists have radically changed their view that thermodynamics concerns itself with only big systems and started realising that it can hold at all scales. Quantum thermodynamics is an emerging field that studies the relation between thermodynamics and quantum mechanics. In this context, seemingly established interpretations of heat and work have to be revised. Quantum heat and work are subject to quantum fluctuations, which are described by mathematical relations called fluctuation relations. Against this backdrop, the EU-funded project NEQUFLUX (Nonequilibrium quantum fluctuations in superconducting devices) conducted feasible experiments to test and apply quantum fluctuation relations for developing efficient quantum nanodevices. Just like the steam engine, nanodevices can be viewed as quantum heat engines, taking heat from a hot source (a resistor), expending part of it to work (in the form of photons) and dumping the rest in a cold source (again a resistor). The engine can also work as a quantum refrigerator, thereby making it possible, for example, to cool parts of a microchip. Special operations called quantum gates manage heat transport in the chip. The experiment conducted allowed measurements of quantum heat and work and their fluctuations. The device operated at very low temperature and was based on superconducting circuits. Another part of the work was geared towards developing a device featuring similar superconducting circuits that use external energy to transport charge from one place of a superconducting wire patterned on the chip to another. In the superconductor, electrons bind in pairs, called Cooper pairs, and the device allows manipulating and transporting such pairs, even in the presence of thermal noise in the wires. Overall, NEQUFLUX investigated methods of leveraging the technology already developed for quantum computation with superconducting circuits to steer and manage heat and charge in these circuits rather than information.
Keywords
Quantum computers, quantum thermodynamics, fluctuation relations, NEQUFLUX, Cooper pairs