Session N45: Heat in the Quantum World

Long-range dissipation and nontopological edge currents in charge-neutral graphene

Van der Waals heterostructures display numerous unique electronic properties. Nonlocal measurements, wherein a voltage is measured at contacts placed far away from the expected classical flow of charge carriers, have been widely employed in search of novel transport mechanisms, including dissipationless spin and valley transport, topological charge-neutral currents, hydrodynamic flows, and helical edge modes. Monolayer, bilayer, and few-layer graphene, transition-metal dichalcogenides, and moiré superlattices were found to display pronounced nonlocal effects. However, the origin of these effects is hotly debated. Graphene, in particular, exhibits giant nonlocality at charge neutrality, a striking behavior that attracted competing explanations. Utilizing a superconducting quantum interference device on a tip (SQUID-on-tip) for nanoscale thermal and scanning gate imaging, we demonstrate that the commonly-occurring charge accumulation at graphene edges leads to giant nonlocality, producing narrow conductive channels that support long-range currents. Unexpectedly, while the edge conductance has little impact on the current flow in zero magnetic field, it leads to field-induced decoupling between edge and bulk transport at moderate fields. The resulting giant nonlocality at charge neutrality and away from it produces exotic flow patterns, sensitive to edge disorder, in which charges can flow against the global electric field. The observed one-dimensional edge transport is generic and nontopological and is expected to support nonlocal transport and dissipation in many electronic systems, offering insight into the numerous controversies and linking them to long-range guided electronic states at system edges.   

"Heat transport in a single quantum dot junction"

The proper management of dissipation and heat flows in quantum nanoelectronic devices will become of growing importance, in particular in view of preserving coherence in quantum information processing applications. Single-quantum-dot junctions, which involve electron transport through a single quantum level, are a key element for quantum electronics. We study the thermoelectric and thermal transport properties of such devices. We demonstrate that the gate potential can be used to modulate the heat carried by electrons across a single quantum level, which remains however significantly below the naive expectation from the Wiedemann-Franz prediction. Although a general theory for heat transport through a quantum dot junction, accounting for on-site interactions, is not available, the heat transport data are well captured at the conductance resonances by non-interacting scattering theory. Eventually, going beyond quasi-equilibrium conditions, the potential of time-resolved thermometry for the calorimetric detection of out-of-equilibrium processes and fluctuations in quantum devices will be discussed.   

Phonons as transporters of quantum information

Phonons, and in particular surface acoustic wave phonons, have been proposed as a means to coherently couple distant solid-state quantum systems. Recent experiments have shown that superconducting qubits can control and detect individual phonons in a resonant structure, enabling the coherent generation and measurement of complex stationary phonon states. Here, I will describe our recent experiments demonstrating the deterministic emission and capture of itinerant surface acoustic wave phonons, enabling the phonon-mediated transfer of quantum states between two qubits, including the high-fidelity quantum entanglement of two superconducting qubits. If time permits, I will also describe a follow-on experiment that shows the phonon version of the optical ``quantum eraser'' experiment.   

Fluctuation theorems for nonequilibrium quantum systems

Since their discovery, the fluctuation theorems have become the hallmark results of classical and quantum stochastic thermodynamics. These theorems quantify that negative fluctuations of the entropy production are exponentially unlikely. Obviously, to derive such theorems adequate definitions of the entropy production need to be obtained first. In this talk, I will outline a few recent results that highlight the subtleties and potentially surprising insight that arises from considering nonequilibrium quantum dynamics.   

Phonon heat transfer across a vacuum through quantum fluctuations

In quantum mechanics, quantum fields are never at rest but constantly fluctuate even at zero temperature. These fluctuations are attributed as the cause of various extraordinary physical phenomena, such as spontaneous emission, Hawking radiation, and the Casimir force. Recent theories predict that quantum fluctuations of electromagnetic fields can assist phonon coupling across a vacuum gap and result in a new form of heat transfer. In this talk, we present the first experimental observation of such a bizarre phenomenon. We use nanomechanical systems to realize strong Casimir phonon coupling, and observe thermal energy exchange between individual phonon modes by monitoring their thermal Brownian motions. Control experiments were performed to eliminate the consequences of other effects such as thermal radiation and electrostatic interaction. Our experiment reveals a new mechanism of heat transfer in addition to the conventional conduction, convection and thermal radiation. It opens up new opportunities to study nanoscale energy transport and quantum thermodynamics.