Atomically thin semiconducting crystals derived from new classes of layered materials have rapidly emerged to enable two-dimensional (2D) nanostructures with unusual electronic, optical, mechanical, and thermal properties. While graphene has been the forerunner and hallmark of 2D crystals, newly emerged 2D semiconductors offer intriguing, beyond-graphene, attributes. The sizable and tunable bandgaps of compound and single-element 2D semiconductors offer attractive perspectives for strong multiphysics coupling and efficient transduction across various signal domains. In this presentation, I will describe my research group’s latest efforts on investigating how mechanically active atomic layer semiconductors and their heterostructures interact with optical and electronic interrogations, and on engineering such structures into new ultrasensitive transducers and signal processing building blocks. Using single- and few-layer transition metal di-chalcogenide (TMDC) crystals, we demonstrate multimode resonant 2D nanoelectromechanical systems (NEMS) with extraordinary electrical tunability. We have also found remarkably broad dynamic range (DR~70 to 100dB) in these 2D NEMS, via deterministic measurement of device intrinsic noise floor and onset of nonlinearity. I will describe spatial mapping and visualization of mode shapes and Brownian motion in these 2D multimode resonators, along with their applications in resolving intrinsic anisotropy and structural asymmetry. I shall then discuss emerging device applications, from classical information processing technologies to 2D NEMS operating in their quantum regime.
Kater Murch (Washington University in St. Louis): Quantum Thermodynamics with Superconducting Qubits
The laws of thermodynamics are fundamental laws of nature that classify energy changes for macroscopic systems as work performed by external driving and heat exchanged with the environment. In the past decades, these principles have been successfully extended to the level of classical trajectories of microscopic systems to account for thermal fluctuations. In particular, experimentally tested generalizations of the second law, known as fluctuation theorems, quantify the occurrence of negative entropy production. The extension of thermodynamics to include quantum fluctuations faces unique challenges such as the proper identification of heat and work and clarification of the role of quantum coherence. I will present experiments that allow us to track heat and work along single quantum trajectories of a superconducting qubit evolving under continuous unitary evolution and measurement. We are able to verify the first law of thermodynamics in that the measured heat and work sum to the total energy change of the quantum system. We then verify the second law of thermodynamics in the form of the Jarzynski equality by employing a novel quantum feedback loop that cancels the heat exchanged at each point in time with additional work. Our results successfully generalize stochastic thermodynamics to the quantum regime, paving the way for future experimental and theoretical investigations of quantum information and thermodynamics.
Geometrical effects influencing the measured spin coherence and quantum phase coherence in mesoscopic structures were characterized by low-temperature spin-dependent quantum transport experiments. The findings are of possible relevance for the design of devices for quantum technologies, and have foundational aspects as well. The materials studied have strong spin-orbit interaction and are heterostructures of InSb, InAs, or InGaAs, and the semimetal Bi with its surface states. The materials were patterned into mesoscopic stadia, narrow channels or quantum interferometers, of typical size ~ 1 micron, comparable to the spin and quantum phase coherence lengths. Aharonov-Bohm experiments, antilocalization, and universal conductance fluctuations were used to quantify the spin- and quantum phase coherence lengths. Using geometrical constraints on the accumulation of quantum geometric phases, the work shows a correspondence, in a diffusive transport regime, between mesoscopic dephasing effects due to time-reversal symmetry breaking by magnetic fields, and spin decoherence due to spin-orbit interaction (Aharonov-Bohm / Aharonov-Casher correspondence). The work also reveals device-geometrical influences on quantum phase coherence from coupling to the classical environment and geometrical effects of electron-electron interactions.
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Christopher White, Caltech IQIM
I will describe a method "DMT" for approximating density operators of 1D systems as low bond dimension matrix product operators that, when combined with a standard framework for time evolution (TEBD), makes possible simulation of the dynamics of strongly thermalizing systems to arbitrary times. The method performs well for both near-equilibrium initial states (Gibbs states with spatially varying temperatures) and far-from-equilibrium initial states, including quenches across phase transitions and pure states. I will also discuss ongoing work applying the method to the diffusive-subdiffusive transition in the ergodic phase of the random-field Heisenberg model.