Abstract In this work, we experimentally investigate the lower bound of the thermodynamic uncertainty relation (TUR) using a quantum Maxwell’s demon, specifically modeled as a Szilard engine. A quantum circuit representing the Szilard engine was designed and implemented on an IBM quantum computer that showcases the integration of quantum information processing. The circuit was executed multiple times to extract currents associated with various thermodynamic quantities, allowing for an estimation of the thermodynamic uncertainty product. TUR establishes a universal trade-off between the precision of thermodynamic currents (such as heat, work, or entropy) and entropy production rate in non-equilibrium systems, placing a universal bound on their fluctuations. Our findings indicate that the TUR’s lower bound is upheld in the quantum regime, with the uncertainty product consistently exceeding the theoretical limit of two. This result reaffirms the robustness of the thermodynamic uncertainty relation in quantum thermodynamic systems.
We analyze an engine whose working fluid consists of a single quantum particle, paralleling Szilard's construction of a classical single-particle engine. Following his resolution of Maxwell's Second Law paradox using the latter, which turned on physically instantiating the demon (control subsystem), the quantum engine's design mirrors the classically-chaotic Szilard Map that operates a thermodynamic cycle of measurement, thermal-energy extraction, and memory reset. Focusing on the thermodynamic costs to observe and control the particle and comparing these in the quantum and classical limits, we detail the thermodynamic tradeoffs behind Landauer's Principle for information-processing-induced thermodynamic dissipation in the quantum and classical regimes. In particular, and as found with the classical engine, we show that the sum of the thermodynamic costs over a cycle obeys a generalized Landauer Principle, exactly balancing energy extraction from the heat bath. Thus, the quantum engine obeys the Second Law. However, the quantum engine does so via substantially different mechanisms: classically measurement and erasure determine the thermodynamics, while in the quantum implementation the cost of partition insertion is key.
The observation of macroscopic quantum tunneling (MQT) manifests the quantumness of macroscopic variables, such as the phase difference across a Josephson junction (JJ). Since then, MQT has become a hallmark experiment to show the quantum nature of JJ-based devices. It has also paved a way to test other macroscopic quantum effects and has led to engineering a variety of artificial atoms, i.e., qubits. However, in the progress of quantum circuits for different applications, it is crucial to understand the influence of thermal and quantum fluctuations, dissipation, and noise to various quantum devices. Thus, the role of the factors mentioned above and the junction parameters
EJ (Josephson coupling energy) and
EC (charging energy) can be scrutinized by understanding the dynamics of the fictitious phase particle. Similarly, in the application perspective, Szilard engine based on SET device are too slow to convert heat into work and are not efficient in-terms of power performance. To overcome the drawbacks, we proposed a high-efficiency fast Szilard engine based on a flux logic device (FLD) and investigated its property numerically and optimize its performance. \\\\ In the first part of the dissertation, we studied the phase dynamics of moderately damped Josephson junctions. The time of flight technique is used to measure the switching current distributions (SCDs). Different dynamical states, thermal activation (TA), MQT, and phase diffusion (PD) are identified from SCDs through well-defined criteria. We observed unambiguous evidence of MQT when temperature is below
∼0.5-0.8 K. The importance of the tilt in the phase dynamics of the junctions, which has been overlooked in previous studies, is clarified. We found that the junction with larger
EJ but the same
ωp0 requires greater tilt and lower temperature to enter MQT regime. \\ \\ In the second part of the dissertation, to overcome the limitation of the SET based Szilard engine, we used superconducting quantum interference device (SQUID) as a one-bit flux logic cell to realize a fast Szilard engine for optimal power. In the past decade, due to technological advancement, the study of energy fluctuations in nano-scale systems such as optical or electrical traps and single-electron tunneling (SET) devices has transformed Szilard's engine to experimental realizations. However, the heat engine realized are slow to convert heat into work limiting their performance in speed and power. The SQUID-based flux logic device (FLD) has a high characteristic frequency,
ωp/2$\pi \sim
10^{10}$ Hz, making it an excellent candidate for realizing a high-efficiency Szilard engine with unprecedented performance. We showed that, on average, the proposed heat engine can extract k
BTln2 of heat per cycle from the thermal reservoir in the quasi-static (QS) limit. In addition, we showed that the FLD-based Szilard engine delivers maximum power when operated about 200 times faster than the threshold speed of quasi-static operation. Our result demonstrates that Szilard's engine's performance based on FLD exceeds other nano-device implementations by orders of magnitude. \\
The physical nature of computation imposes fundamental limits on the energy efficiency of information processing, as formalized by Landauer's principle, which establishes a minimum thermodynamic cost for information erasure. This dissertation investigates near-Landauer-limit computation using superconducting Quantum Flux Parametrons (QFPs). We first establish the theoretical foundations of physical computation, including logical and thermodynamic reversibility, stochastic thermodynamics, and the relationship between information, entropy, and energy dissipation. Building upon these principles, we develop and analyze computational protocols based on QFPs and Coupled QFPs (CQFPs). Numerical simulations demonstrate thermodynamically efficient bit erasure, bit-flip operations, and a superconducting realization of the Szilard Engine. We further demonstrate universal combinational logic through a NAND gate, as well as signal fan-out through a copier gate and memory through a D-latch, providing the essential primitives required for scalable digital computation. A central result of this work is the demonstration that momentum, conventionally regarded as a source of error in superconducting logic, can instead be exploited as a computational resource. By operating QFP circuits in the underdamped regime, momentum-assisted protocols achieve simultaneous improvements in speed, fidelity, and energy efficiency. These results establish QFP-based superconducting circuits as a promising platform for near-Landauer-limit computation and provide a foundation for future momentum-assisted superconducting dynamical computing architectures.
Maxwell's demon explores the role of information in physical processes.Employing information about microscopic degrees of freedom, this "intelligent observer" is capable of compensating entropy production (or extracting work), apparently challenging the second law of thermodynamics.In a modern standpoint, it is regarded as a feedback control mechanism and the limits of thermodynamics are recast incorporating information-to-energy conversion.We derive a trade-off relation between information-theoretic quantities empowering the design of an efficient Maxwell's demon in a quantum system.The demon is experimentally implemented as a spin-1=2 quantum memory that acquires information, and employs it to control the dynamics of another spin-1=2 system, through a natural interaction.Noise and imperfections in this protocol are investigated by the assessment of its effectiveness.This realization provides experimental evidence that the irreversibility in a nonequilibrium dynamics can be mitigated by assessing microscopic information and applying a feed-forward strategy at the quantum scale.
With a class of quantum heat engines which consists of two-energy-eigenstate systems undergoing, respectively, quantum adiabatic processes and energy exchanges with heat baths at different stages of a cycle, we are able to clarify some important aspects of the second law of thermodynamics. The quantum heat engines also offer a practical way, as an alternative to Szilard's engine, to physically realize Maxwell's demon. While respecting the second law on the average, they are also capable of extracting more work from the heat baths than is otherwise possible in thermal equilibrium.