Thermodynamics of continuously measured superconducting qubits: heat flow and control

The transformation of energy in the forms of heat and work pertains to everyday life and is a crucial aspect in the efficiency of machines. In fact, the laws of thermodynamics, which govern these energy transformations, are so fundamental that have their say in almost all branches of physics. The first law acknowledges that heat is energy to be accounted for in energy conservation. The second law of thermodynamics qualitatively distinguishes heat from other forms of energy by associating it to entropy, a measurement of the "lack of information" about a system, and by stating that entropy grows in macroscopic systems.
The generality of these statements stems from general statistical properties of macroscopic objects with a large number of degrees of freedoms. However, the technological advances in engineering and operating nanoscale objects like molecular machines, forces us to rethink the implications of thermodynamics for microscopic few-particle systems, where thermal fluctuations are significant. Here the laws of thermodynamics can be reformulated in terms of probabilistic equations, known as fluctuation theorems, which account for rare microscopic events, like those where entropy decreases, which are instead washed away by statistics in the macroscopic word.
The formulation and experimental verification of these theorems have been a success of stochastic thermodynamics in the past decade. The nanoscale world, however, challenges us further with quantum mechanical processes emerging at this scale, and devices built upon them. How do we include quantum fluctuations into the laws of thermodynamics? Current research is advancing on this front with some success by analyzing quantum machines operating between classical thermal sources, to identify genuine quantum effects and generalize the definitions of heat and work for quantum processes. The main problem is that in quantum mechanics even measuring the energy of an isolated system is a deterministic process, and that measuring a specified variable, e.g. work along quantum evolution, comes with unavoidable back-action that needs to be taken into account.
In this project, we set aside the usual thermodynamic setup where a system is coupled to a thermal bath and focus instead on the measurement process, where a detector monitoring the system is the reservoir with which the system exchanges energy. This kind of configuration allows us to focus on the role of quantum measurement, and it brings new aspects into play, like the fact of dealing with an out-of-equilibrium environment, and the thermodynamic role of the information gained during the measurement. It also comes with the possibility of short-term experimental realizations, since quantum detector's readout is experimentally available, as opposed to thermal baths' readout.
The project will set-up the tools to deal with the thermodynamics of quantum measurement and use them to engineer heat flow detectors and possibly heat flow engineering at the nanoscale.

The goal of the project is the development of fundamental science, and its impact will be primarily academic. Nonetheless, the results and the development of the project will have a societal as well as a long-term economic and technological impact.

Academic impact

The objectives of the project are timed to have an immediate impact on the community working on quantum thermodynamics, where studies on the role of quantum measurement are growing fast. The objectives also have the potential to influence other branches of physics, like quantum computation and quantum optics, and other disciplines like chemistry. In this way, the project can contribute to maintaining the leading position of UK research in the area of quantum thermodynamics, and more broadly in the field of quantum technologies. Moreover, the theme of energy and its manipulation the project is about is of strategic growing importance in plans for UK research developments.

Economic and Technological impact

The possibility of controlling heat flow by elementary quantum processes has a potential technological impact: It might help in optimizing the design of nanoscale machines and circuitry, or even in optimizing operations and running protocols for nanodevices. However, the specific output of the project is at the level of proof-of-principle of elementary protocols, and, though early contact with industrial enterprises might be possible, a foreseeable technological impact is beyond the project time-scales.

People and skills

The project will provide a comprehensive training of a PDRA in an active topic of current research in condensed matter theory. The PDRA will acquire high-level scientific competences in analytical modeling, numerical simulations, Besides the specific scientific competences, the PDRA will gain expertise in scientific communication via written publications and talk delivery as well as in team working skills. These benefits will extend to MPhys and Ph.D. students who will be working on projects spin-offed and related to the proposed research.

Societal impact

The projects bring about new ideas which, besides their scientific value, pertain to concepts like heat, work, and energy of interest for everybody in everyday life. How these common-sense concepts have to be modified at microscopic quantum level is of general interest. The development of the project is accompanied by a related outreach activity. This consist in the production of high-quality, informative material, dedicated websites, public lectures, and contributes to public-facing websites. Particular care is given to the outreach impact on the local community by participation in outreach programs in schools including schools visits and popular talks and taking part in LU activities like STEM Taster Days.