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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.

Abstract:
Hot electron processes at metallic heterojunctions are central to optical-to-chemical or electrical energy transduction. Ultrafast nonlinear photoexcitation of graphite (Gr) has been shown to create hot thermalized electrons at temperatures corresponding to the solar photosphere in less than 25 fs. Plasmonic resonances in metallic nanoparticles are also known to efficiently generate hot electrons. Here we deposit Ag nanoclusters (NC) on Gr to study the ultrafast hot electron generation and dynamics in their plasmonic heterojunctions by means of time-resolved two-photon photoemission (2PP) spectroscopy. By tuning the wavelength of p-polarized femtosecond excitation pulses, we find an enhancement of 2PP yields by 2 orders of magnitude, which we attribute to excitation of a surface-normal Mie plasmon mode of Ag/Gr heterojunctions at 3.6 eV. The 2PP spectra include contributions from (i) coherent two-photon absorption of an occupied interface state (IFS) 0.2 eV below the Fermi level, which electronic structure calculations assign to chemisorption-induced charge transfer, and (ii) hot electrons in the π*-band of Gr, which are excited through the coherent screening response of the substrate. Ultrafast pump–probe measurements show that the IFS photoemission occurs via virtual intermediate states, whereas the characteristic lifetimes attribute the hot electrons to population of the π*-band of Gr via the plasmon dephasing. Our study directly probes the mechanisms for enhanced hot electron generation and decay in a model plasmonic heterojunction.

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