The laws of thermodynamics were first developed to describe large systems, such as steam engines. The Second Law of thermodynamics is perhaps the most famous, it states that the entropy of an isolated system increases with time. These laws work well because macroscopic systems contain an enormous number of particles. At this scale, thermodynamic behavior is an average over many particles, and small random fluctuations can be ignored. Things drastically change when we look at very small systems. If a system contains only a few particles, fluctuations become important and cannot be neglected. In addition, classical physics no longer gives an accurate description, and quantum mechanics is required instead. This raises a key question, do the laws of thermodynamics still apply at the microscopic scale, and if so, how? This question is the focus of quantum thermodynamics and the theory of open quantum systems. These fields have developed rapidly in recent few decades due to advances in experimental techniques. One important line of research revisits the original ideas of thermodynamics. Early work by scientists such as Sadi Carnot focused on heat engines. In the quantum regime, this leads to the study of quantum heat engines (QHEs) and quantum batteries (QBs). These devices aim to exploit quantum effects to perform thermodynamic tasks that are impossible for classical machines.
In my Ph.D. thesis, I have explored a collection of problems focused on optimizing the performance and highlighting the distinct quantum features of QHEs and QBs. My work involved analytical and numerical investigations of thermodynamics of quantum systems using the formalism of open quantum systems and quantum thermodynamics. The first problem I looked at was a simple four-stroke finite-time quantum Otto engine cycle (analogous to the classical Otto engine, the kind that powers our cars). Each of the four strokes occurs over a finite time. We asked whether running the engine in an asymmetric way (spending more time on one stroke than the others) could lead to better performance. Interestingly, we found that for certain asymmetric cycle durations, the engine not only produced more work output but did so more reliably [3].
The next question I considered was the role of quantum measurements (an inherently strange and important aspect of quantum theory) on the performance of QHEs. We ask "can measurement performed on quantum working substance of a quantum Otto engine actually help control or improve the engine’s performance?" By modeling different measurement schemes, we found that some types of measurements could indeed enhance the average work output and reduce fluctuations [2].
Finally, I turned to another interesting device called QBs which are tiny quantum systems that can be used to store energy. Since QBs are always in contact with the external environment, we naturally expect environmental noise to degrade their performance. But we asked whether environmental noise could actually be useful and surprisingly, the answer is yes. We showed that a carefully tuned amount of environmental noise (more specifically, dephasing, which tends to destroy quantum coherence) could help charge the QBs faster and in a more robust manner [1]. This points to a more nuanced view of noise not always as a hindrance, but sometimes as a helpful tool. Also there is a very nice article on this work on Physics World: When charging quantum batteries, decoherence is a friend, not a foe.
Rahul Shastri, Chao Jiang, Guo-Hua Xu, B. Prasanna Venkatesh, Gentaro Watanabe: Dephasing enabled fast charging of quantum batteries, npj Quantum Information 11,9 (2025)
Rahul Shastri, B. Prasanna Venkatesh: Controlling Work Output and Coherence in Finite Time Quantum Otto Engines Through Monitoring Phys. Rev. E 109, 014102 (2024)
Rahul Shastri, B. Prasanna Venkatesh: Optimization of asymmetric quantum Otto engine cycles Phys. Rev. E 106, 024123 (2022)
Controlling coherence in finite time Quantum Otto engine through monitoring - Conference talk given at Quantum Thermodynamics Down Under 2023, Brisbane, Queensland, Australia, EQUS
Quantum Thermodynamics of Quantum Thermal Machines and Quantum Batteries - Talk given at Physics Symposium, IIT Gandhinagar (India)
During my Master's degree in Physics at IIT Gandhinagar, I had the opportunity to engage in two interesting projects that gave me immense joy at that time. For my master’s thesis, I explored something known as Gauge-Higgs Unification (GHU) models, where the Higgs boson emerges as a component of 5-dimensional gauge fields through symmetry-breaking mechanisms. Alongside this, I worked on a short project on geometric phases in quantum mechanics, which offered beautiful insights into how the geometrical properties of wavefunctions can have real observable effects. I have always been fascinated by the simplicity and fundamental nature of theoretical physics, and these experiences deepened my appreciation for the mathematical structure underlying physical theories.
During this time, I also spent a summer at the Physical Research Laboratory in Ahmedabad, working on a project on Langevin equations in non-equilibrium statistical mechanics. It was the first time I encountered stochastic dynamical equations, and I was instantly drawn to how randomness and thermodynamics are linked together in describing physical systems out of equilibrium. Looking back, I realize it was a very pleasant experience and played a key role in shaping my interest in thermodynamics, which later led me to choose quantum thermodynamics as the focus of my Ph.D. research.
Immediately after completing my master’s degree and before starting my Ph.D., I worked for about a year as a Project Assistant on a project titled "Zero-Carbon Solar-powered Hydrogen Production via Plasmonic Nano-antenna Enhanced Photocatalytic Water-Splitting." The project focused on developing sustainable methods for hydrogen generation using sunlight. Working on this project gave me significant hands-on experience with tools like COMSOL Multiphysics and other simulation software to model electromagnetic fields and study the absorption and scattering properties of nanomaterials. My work involved understanding and optimizing nanomaterial geometries in the context of their application to artificial photosynthesis through photocatalytic water-splitting. This role significantly strengthened my computational and data analysis skills. It was also the first time I worked on something outside the domain of theoretical physics more on the side of engineering physics with a strong emphasis on practical applications in clean energy technologies.