Periodic Reporting for period 1 - QFluctTrans (Thermodynamics of Quantum Transport)
Okres sprawozdawczy: 2016-04-01 do 2018-03-31
The continuous technological improvement of electronics depends on the miniaturization of the underlying components. They have reached the points where the building blocks are molecules. These molecular devices operate in the mesoscopic regime where quantum mechanics and thermodynamics dominate together. The electrons on the scale of molecules behave following the laws of quantum mechanics, while the heat flow through the devices has thermodynamic fluctuations that limit how much heat can flow. This a big challenge to industrial applications, as our current engineering principles are not suitable for this. In this project, we addressed this issue directly by developing new theoretical tools to describe this regime successfully. Such advancements will be crucial in all sorts of thermal and electrical devices.
The main objectives of this project where to overcome the limitations of state-of-the-art by using the tools from quantum non-equilibrium thermodynamics to understand the fundamental effects of fluctuations in transport in realistic molecular devices. We focused on two concrete challenges that combined complete the objective. First, we used tools from information theory to understand quantum coherences as a novel thermodynamic forces in mesoscopic devices. We showed how this was able to unlock new control techniques. The second challenge was to use standard techniques for molecular devices such to model transport in these devices. We made significant progress in this challenge. Together, the outcome of the two challenges gives us some ideas of how to understand the role of quantum decoherence by ab-initio of specific realistic molecular devices.
The main objectives of this project where to overcome the limitations of state-of-the-art by using the tools from quantum non-equilibrium thermodynamics to understand the fundamental effects of fluctuations in transport in realistic molecular devices. We focused on two concrete challenges that combined complete the objective. First, we used tools from information theory to understand quantum coherences as a novel thermodynamic forces in mesoscopic devices. We showed how this was able to unlock new control techniques. The second challenge was to use standard techniques for molecular devices such to model transport in these devices. We made significant progress in this challenge. Together, the outcome of the two challenges gives us some ideas of how to understand the role of quantum decoherence by ab-initio of specific realistic molecular devices.
First, we focused on understanding the role of quantum coherence in simple transport devices. We used techniques from quantum walks and quantum transport to understand toy models of particle transport, and the role of coherences in them. We then added quantum decoherence, also know as dephasing. This dephasing can come, for example, from electron-phonon couplings. We studied its role in different geometries, to show how dephasing and quantum interference can be used to control transport in some devices in general. This provides a new quantum way to contro heat and electronic flow and was published in one paper. We then used more powerful ab-initio techniques for more realistic and interesting thermo-electric molecular devices. We demonstrated that the current and heat flows are not only dictated by the temperature and potential gradient, but also by the external action of a local quantum observer that controls the coherence of the device. Depending on how and where the observation took place, the direction of heat and particle currents were independently controlled. In fact, we showed that the current and heat flow in a quantum material can go against the natural temperature and voltage gradients. Dynamical quantum observation offers new possibilities for the control of quantum transport far beyond classical thermal reservoirs. Through the concept of local projections, we illustrate how we can create and directionality control the injection of currents (electronic and heat) in nanodevices. This scheme provides novel strategies to construct quantum devices with application in thermoelectrics, spintronic injection, phononics, and sensing among others. This was published as another major publications
Second, we developed new techniques to fundamentally characterize and understand unknown quantum processes, such as those coming from molecular devices. There was no systematic way to describe a quantum process with memory solely in terms of experimentally accessible quantities. We develope a universal framework to characterize arbitrary non-Markovian quantum processes. We show how a multitime non-Markovian process can be reconstructed experimentally, and that it has a natural representation as a many-body quantum state, where temporal correlations are mapped to spatial ones. Moreover, this state is expected to have an efficient matrix-product-operator form in many cases. Our framework constitutes a systematic tool for the effective description of memory-bearing open-system evolutions in quantum transport devices. We also derived a necessary and sufficient condition for a quantum process to be Markovian which coincides with the classical one in the relevant limit. Our condition unifies all previously known definitions for quantum Markov processes by accounting for all potentially detectable memory effects. We then derive a family of measures of non-Markovianity with clear operational interpretations which is crucial for understanding the underlying dynamics and fluctuations.
Lastly, we then used these things known to start to develop some new TD-DFT functionals. We have tested some final temperature open system functionals, are currently iterating them to improve them for publication.
Second, we developed new techniques to fundamentally characterize and understand unknown quantum processes, such as those coming from molecular devices. There was no systematic way to describe a quantum process with memory solely in terms of experimentally accessible quantities. We develope a universal framework to characterize arbitrary non-Markovian quantum processes. We show how a multitime non-Markovian process can be reconstructed experimentally, and that it has a natural representation as a many-body quantum state, where temporal correlations are mapped to spatial ones. Moreover, this state is expected to have an efficient matrix-product-operator form in many cases. Our framework constitutes a systematic tool for the effective description of memory-bearing open-system evolutions in quantum transport devices. We also derived a necessary and sufficient condition for a quantum process to be Markovian which coincides with the classical one in the relevant limit. Our condition unifies all previously known definitions for quantum Markov processes by accounting for all potentially detectable memory effects. We then derive a family of measures of non-Markovianity with clear operational interpretations which is crucial for understanding the underlying dynamics and fluctuations.
Lastly, we then used these things known to start to develop some new TD-DFT functionals. We have tested some final temperature open system functionals, are currently iterating them to improve them for publication.
We went beyond the state-of-the-art in many significant ways. First, the role of quantum decoherence was recognized, but not fully understood as a control parameters in quantum transport. We showed in two publications new ways, geometries and principles of how it actually works. A crucial part of this understanding was the generalization of non-equilibrium quantum thermodynamic concepts to include coherences. In this we showed new and powerful ways where decoherence can control the magnitude and direction of electrical currents and heat currents.
Also we made ground breaking work on characterizing quantum processes in the non-Markovian regime. The stated of the art lacked an operational description has hindered advances in understanding physical, chemical, and biological processes. There often unjustified theoretical assumptions are beingmade to render a dynamical description tractable. This has led to theories plagued with unphysical results and no consensus on what a quantum Markov (memoryless) process is. We went beyond this by developing a universal framework to characterize arbitrary non-Markovian quantum processes. We showed how a multitime non-Markovian process can be reconstructed experimentally, and that it has a natural representation as a many-body quantum state, where temporal correlations are mapped to spatial ones. Moreover, this framework allowed for a systematic tool for the effective description of memory-bearing open-system evolutions, including a quantum Markovianity condition that includes and goes beyond all done before. This was published in two separate publications.
Finally, we tested the existing open quantum system TD-DFT existing functional for a simple system, and from this, started to develop new functionals that can be used for transport.
All these together provide new theoretical tools to understand, characterize and control quantum transport in mesoscopic system. This will lead to new electronic devices.
Also we made ground breaking work on characterizing quantum processes in the non-Markovian regime. The stated of the art lacked an operational description has hindered advances in understanding physical, chemical, and biological processes. There often unjustified theoretical assumptions are beingmade to render a dynamical description tractable. This has led to theories plagued with unphysical results and no consensus on what a quantum Markov (memoryless) process is. We went beyond this by developing a universal framework to characterize arbitrary non-Markovian quantum processes. We showed how a multitime non-Markovian process can be reconstructed experimentally, and that it has a natural representation as a many-body quantum state, where temporal correlations are mapped to spatial ones. Moreover, this framework allowed for a systematic tool for the effective description of memory-bearing open-system evolutions, including a quantum Markovianity condition that includes and goes beyond all done before. This was published in two separate publications.
Finally, we tested the existing open quantum system TD-DFT existing functional for a simple system, and from this, started to develop new functionals that can be used for transport.
All these together provide new theoretical tools to understand, characterize and control quantum transport in mesoscopic system. This will lead to new electronic devices.