Quantum Feedback Thermal Machines: Role of Measurement and Feedback in quantum thermal machines

In the first part, we will discuss the thermodynamic aspects of a single qubit-based device, powered by weak quantum measurements, and feedback controlled by a quantum Maxwell's demon. We will discuss both discrete and time-continuous operations of the measurement-based device at finite temperatures of the reservoir. In the discrete example where a demon acquires information via discrete weak measurements, we find that the thermodynamic variables including the heat exchanged, extractable work, and the entropy produced are completely determined by an information-theoretic measure of the demon's perceived arrow of time. We will also discuss a realistic time-continuous operation of the device where the feedback is applied after a sequence of weak measurements. In the time-continuous limit, we derive the exact finite-time statistics of work, heat, and entropy changes along individual quantum trajectories of the quantum measurement process, and relate them to the demon's arrow of time.

In the second part, we will investigate coupled-qubit-based thermal machines powered by quantum measurements and feedback. We will consider two different versions of the machine:(1) a quantum Maxwell’s demon, where the coupled-qubit system is connected to a detachable single shared bath, and (2) a measurement-assisted refrigerator, where the coupled-qubit system is in contact with a hot and cold bath. In the quantum Maxwell’s demon case, we will discuss both discrete and continuous measurements. We find that the power output from a single qubit-based device can be improved by coupling it to the second qubit. We will further find that the simultaneous measurement of both qubits can produce higher net heat extraction compared to two setups operated in parallel where only single-qubit measurements are performed. In the refrigerator case, we will use continuous measurement and unitary operations to power the coupled-qubit-based refrigerator. We find that the cooling power of a refrigerator operated with swap operations can be enhanced by performing suitable measurements.

In the final part, I will discuss our ongoing research works on optimizing measurement and feedback-based quantum thermal machines using reinforcement learning.