Showing 1–50 of 127 results for author: Segal, D

Design Principles for Enhanced Quantum Transport with Site-Dependent Noise

Authors: Maggie Lawrence, Elise Wang, Dvira Segal

Abstract: Environmental noise can enhance transport, an effect known as environmental noise-assisted quantum transport. Most theoretical studies focus on optimizing system parameters under spatially uniform system-environment coupling. Here, instead, we optimize the environmental noise itself by allowing for site-dependent dephasing. We investigate steady-state transport in one-dimensional lattices with eit… ▽ More Environmental noise can enhance transport, an effect known as environmental noise-assisted quantum transport. Most theoretical studies focus on optimizing system parameters under spatially uniform system-environment coupling. Here, instead, we optimize the environmental noise itself by allowing for site-dependent dephasing. We investigate steady-state transport in one-dimensional lattices with either ramped or disordered energy landscapes, considering both short- and long-range coherent tunneling. In the absence of environmental effects, in the thermodynamic limit these systems can exhibit localization, and thus suppressed transport, arising from destructive interference. Using a Lindblad master equation framework, we implement local dephasing optimized to maximize steady-state population flux. We find that for ramp potentials, short-range tunneling favors selective dephasing on alternating sites, whereas long-range tunneling benefits from a dephasing profile that increases with distance from the injection site. In energetically disordered systems, strongly detuned sites require enhanced local dephasing under short-range tunneling to facilitate transport. In all cases, we find that site-optimized dephasing allows higher transport efficiency than uniform dephasing, and it is accompanied by increased spatial delocalization of the steady state. Our results provide microscopic insight into the interplay between coherent dynamics and environmental noise. Dephasing broadens energy levels locally, helping to overcome detuning and destructive interference. More generally, we establish spatially-structured environmental noise as a strategy for controlling both quantum transport and state coherence in open systems. △ Less

Submitted 24 April, 2026; originally announced April 2026.

Comments: 23 pages, 18 figures

Impossibility of Refrigeration and Engine Operation in Minimal Qubit Repeated-Interaction Models

Authors: Gabrielle Barsky-Giles, Alessandro Prositto, Matthew Gerry, Dvira Segal

Abstract: We investigate the operation of a qubit as a quantum thermal device within the repeated interaction framework, allowing for strong system-bath coupling and finite interaction times. We analyze two minimal models: an alternating-coupling setup, in which the qubit sequentially interacts with hot and cold baths, and a simultaneous-coupling setup, where both baths interact with the qubit during each c… ▽ More We investigate the operation of a qubit as a quantum thermal device within the repeated interaction framework, allowing for strong system-bath coupling and finite interaction times. We analyze two minimal models: an alternating-coupling setup, in which the qubit sequentially interacts with hot and cold baths, and a simultaneous-coupling setup, where both baths interact with the qubit during each collision. For the alternating model, we obtain an exact analytical solution for the limit-cycle state, valid for arbitrary coupling strengths and collision durations. Using this solution, we rigorously prove a no-go theorem for quantum refrigeration. We further demonstrate that, although work can be generated locally at individual system-bath contacts, the total work over a cycle is always nonpositive, precluding engine operation. In the absence of work, the model describes pure heat conduction, for which we derive a closed-form expression for the heat current and show that it exhibits a nonmonotonic turnover behavior. The simultaneous-coupling model is analyzed perturbatively. In the short-collision-time limit, it reproduces the same steady-state behavior as the alternating model, reinforcing the generality of the constraints identified. Our results establish fundamental limitations on qubit-based quantum thermal machines operating under Markovian repeated interactions and highlight the need for enriched models to realize functional quantum thermal devices. △ Less

Submitted 20 February, 2026; originally announced February 2026.

Tuning molecular thermal conductance through endgroup modification and halogen substitution

Authors: Jonathan J. Wang, Dvira Segal

Abstract: We demonstrate tuning of the phononic thermal conductance in single molecules with carbon-chain backbones through modifications of terminal groups and halogen substitution of hydrogen atoms. Our simulations focus on intrinsic molecular properties, and we employ a workflow based on {\it ab initio} molecular dynamics, enabling the training and development of machine-learned interatomic potentials. T… ▽ More We demonstrate tuning of the phononic thermal conductance in single molecules with carbon-chain backbones through modifications of terminal groups and halogen substitution of hydrogen atoms. Our simulations focus on intrinsic molecular properties, and we employ a workflow based on {\it ab initio} molecular dynamics, enabling the training and development of machine-learned interatomic potentials. These potentials are subsequently used in classical nonequilibrium molecular dynamics simulations to extract thermal conductance coefficients. Replacing terminal methyl groups with amine, sulfur, or halogen substituents leads to pronounced changes in thermal conductance: bromine-terminated chains exhibit the lowest conductance, whereas amine and methyl-terminated chains show the highest. Additionally, single-atom substitution of hydrogen by fluorine or other halogens along the alkane backbone significantly reduces thermal transport. Finally, our simulations of the length dependence of thermal conductance in alkane chains containing 3-12 carbon atoms reveal its saturation beyond eight carbon atoms. Together, our findings show that simple chemical modifications offer a versatile route to controlling phononic heat flow in single molecules. △ Less

Submitted 13 December, 2025; originally announced December 2025.

Compact cavity-dressed Hamiltonian framework at arbitrarily strong light-matter coupling

Authors: Jakub Garwoła, Dvira Segal

Abstract: We present a non-perturbative Hamiltonian mapping method for quantum systems strongly coupled to a quantized field mode (cavity), yielding compact closed-form representations of hybrid light-matter systems. The mapping method builds on an entangling transformation of photonic and atomic degrees of freedom. By truncating the resulting cavity-dressed Hamiltonian (CDH) to successively larger excitati… ▽ More We present a non-perturbative Hamiltonian mapping method for quantum systems strongly coupled to a quantized field mode (cavity), yielding compact closed-form representations of hybrid light-matter systems. The mapping method builds on an entangling transformation of photonic and atomic degrees of freedom. By truncating the resulting cavity-dressed Hamiltonian (CDH) to successively larger excitation sectors, we construct a series of compact models that converge to the exact limit, outpacing conventional approaches even in the challenging resonant and ultrastrong light-matter regime. The mapping principle also applies to multimode cavities coupled to matter through noncommuting operators and to leaky cavities. We benchmark the CDH framework on the quantum Rabi model, demonstrating accurate spectral predictions in both weak and strong coupling regimes, together with converging ground-state and thermal observables. We study the Dicke-Heisenberg lattice model and determine its phase diagram under resonant and strong light-matter coupling, achieving significant computational savings over brute-force simulations and identifying cavity-mediated spin correlations both analytically and numerically. The closed-form and compactness of the CDH provide both physical insight and enhanced computational efficiency, facilitating studies of strongly coupled hybrid light-matter systems. △ Less

Submitted 3 June, 2026; v1 submitted 14 November, 2025; originally announced November 2025.

Comments: 23 pages, 13 figures

Journal ref: Phys. Rev. Research 8, L022039 - Published 2 June, 2026

Separation of relaxation timescales via strong system-bath coupling: Dissipative three-level system as a case study

Authors: Brett Min, Matthew Gerry, Dvira Segal

Abstract: We analytically demonstrate that strong system-bath coupling separates the relaxation dynamics of a dissipative quantum system into two distinct regimes: a short-time dynamics that, as expected, accelerates with increasing coupling to the environment, and a slow dynamics that, counterintuitively, becomes increasingly prolonged at sufficiently strong coupling. Using the reaction-coordinate polaron-… ▽ More We analytically demonstrate that strong system-bath coupling separates the relaxation dynamics of a dissipative quantum system into two distinct regimes: a short-time dynamics that, as expected, accelerates with increasing coupling to the environment, and a slow dynamics that, counterintuitively, becomes increasingly prolonged at sufficiently strong coupling. Using the reaction-coordinate polaron-transform mapping, we uncover the general mechanism behind this effect and derive accurate expressions for both relaxation timescales. Numerical simulations confirm our analytical predictions. From a practical perspective, our results suggest that strong coupling to a dissipative bath can autonomously generate and sustain long-lived quantum coherences, offering a promising strategy for bath-engineered quantum state preparation. △ Less

Submitted 15 July, 2025; originally announced July 2025.

Comments: 15 pages, 8 figures

Journal ref: Phys. Rev. A 112, 062226 (2025)

Thermal state preparation by repeated interactions at and beyond the Lindblad limit

Authors: Carlos Ramon-Escandell, Alessandro Prositto, Dvira Segal

Abstract: We study the nature of thermalization dynamics and the associated preparation (simulation) time under the repeated interaction protocol uncovering a generic anomalous, Mpemba-like trend. As a case study, we focus on a three-level system and analyze its dynamics in two complementary regimes, where the system-ancilla interaction strength is either large or small. Focusing on the estimation of the si… ▽ More We study the nature of thermalization dynamics and the associated preparation (simulation) time under the repeated interaction protocol uncovering a generic anomalous, Mpemba-like trend. As a case study, we focus on a three-level system and analyze its dynamics in two complementary regimes, where the system-ancilla interaction strength is either large or small. Focusing on the estimation of the simulation time, we derive closed-form expressions for the minimum number of collisions, or minimal simulation time, required to achieve a thermal state, which is within $ε$ distance to the target thermal state. At zero temperature, we analytically identify a set of points (interaction strength $\times$ their duration) that minimize the simulation time. At nonzero temperature, we observe a Mpemba-like effect: Starting from a maximally mixed state, thermalization to an intermediate-temperature state takes longer than to a lower-temperature one. We provide an accurate analytical approximation for this phenomenon and demonstrate its occurrence in larger systems and under randomized interaction strengths. The prevalence of the Mpemba effect in thermal state preparation presents a significant challenge for preparing states in large systems, an open problem calling for new strategies. △ Less

Submitted 13 June, 2025; originally announced June 2025.

Journal ref: Phys. Rev. Research 7, 043012 (2025)

Optimal qubit-mediated quantum heat transfer via noncommuting operators and strong coupling effects

Authors: Marlon Brenes, Jakub Garwoła, Dvira Segal

Abstract: Heat transfer in quantum systems is a current topic of interest due to emerging quantum technologies that attempt to miniaturize engines and examine fundamental aspects of thermodynamics. In this work, we consider heat transfer between two thermal reservoirs in which a central spin degree of freedom mediates the process. Our objective is to identify the system-bath coupling operators that maximize… ▽ More Heat transfer in quantum systems is a current topic of interest due to emerging quantum technologies that attempt to miniaturize engines and examine fundamental aspects of thermodynamics. In this work, we consider heat transfer between two thermal reservoirs in which a central spin degree of freedom mediates the process. Our objective is to identify the system-bath coupling operators that maximize heat transfer at arbitrary system-bath coupling strengths. By employing a Markovian embedding method in the form of the reaction-coordinate mapping, we study numerically heat transfer at arbitrary system-bath coupling energy and for general system-bath coupling operators between the baths and the central qubit system. We find a stark contrast in the conditions required for optimal heat transfer depending on whether the system is weakly or strongly coupled to the heat baths. In the weak-coupling regime, optimal heat transfer requires identical coupling operators that facilitate maximum sequential transport, resonant with the central qubit. In contrast, in the strong-coupling regime, noncommuting system-bath coupling operators between the hot and cold reservoirs are necessary to achieve optimal heat transfer. We further employ the Effective Hamiltonian theory and gain partial analytical insights into the observed phenomena. We discuss the limitations of this approximate method in capturing the behavior of the heat current for noncommuting coupling operators, calling for its future extensions to capture transport properties in systems with general interaction Hamiltonians. △ Less

Submitted 24 June, 2025; v1 submitted 7 January, 2025; originally announced January 2025.

Comments: Journal version

Journal ref: Phys. Rev. B 111, 235440, 2025

Mixed-state phase transitions in spin-Holstein models

Authors: Brett Min, Yuxuan Zhang, Yuxuan Guo, Dvira Segal, Yuto Ashida

Abstract: Understanding coupled electron-phonon systems is one of the fundamental issues in strongly correlated systems. In this work, we aim to extend the notion of mixed-state phases to the realm of coupled electron/spinphonon systems. Specifically, we consider a two-dimensional cluster Hamiltonian locally coupled to a set of single bosonic modes with arbitrary coupling strength. First, we adopt a pure-st… ▽ More Understanding coupled electron-phonon systems is one of the fundamental issues in strongly correlated systems. In this work, we aim to extend the notion of mixed-state phases to the realm of coupled electron/spinphonon systems. Specifically, we consider a two-dimensional cluster Hamiltonian locally coupled to a set of single bosonic modes with arbitrary coupling strength. First, we adopt a pure-state framework and examine whether a ground state phase transition out of the symmetry-protected topological phase can be captured using the standard polaron unitary transformation. This approach involves restricting the analysis to the low-energy manifold of the phonon degrees of freedom. We find that the pure-state approach fails to detect the anticipated transition to a topologically trivial phase at strong spin-phonon coupling. Next, we turn to a mixed-state picture. Here, we analyze mixed states of the model obtained by tracing out the phonons degrees of freedom. We employ two distinct diagnostics for mixed-state phase transitions: (i) the von Neumann conditional mutual information (CMI) and (ii) the Rényi-2 CMI. We argue that both measures detect signatures of mixed-state phase transitions, albeit at different critical spin-phonon coupling strengths, corresponding to subtly distinct notions of the mixed-state phases. △ Less

Submitted 3 December, 2024; originally announced December 2024.

Comments: 18 pages, 4 figures

Journal ref: Phys. Rev. B 111, 115123 (2025)

Machine learning delta-T noise for temperature bias estimation

Authors: Matthew Gerry, Jonathan J. Wang, Joanna Li, Ofir Shein-Lumbroso, Oren Tal, Dvira Segal

Abstract: Delta-T shot noise is activated in temperature-biased electronic junctions, down to the atomic scale. It is characterized by a quadratic dependence on the temperature difference and a nonlinear relationship with the transmission coefficients of partially opened conduction channels. In this work, we demonstrate that delta-T noise, measured across an ensemble of atomic-scale junctions, can be utiliz… ▽ More Delta-T shot noise is activated in temperature-biased electronic junctions, down to the atomic scale. It is characterized by a quadratic dependence on the temperature difference and a nonlinear relationship with the transmission coefficients of partially opened conduction channels. In this work, we demonstrate that delta-T noise, measured across an ensemble of atomic-scale junctions, can be utilized to estimate the temperature bias in these systems. Our approach employs a supervised machine learning algorithm to train a neural network with input features being the scaled electrical conductance, the delta-T noise, and the mean temperature. Due to limited experimental data, we generate synthetic datasets, designed to mimic experiments. The neural network, trained on these synthetic data, was subsequently applied to predict temperature biases from experimental datasets. Using performance metrics, we demonstrate that the mean bias -- the deviation of predicted temperature differences from their true value -- is less than 1 K for junctions with conductance up to 4$G_0$. Our study highlights that, while a single delta-T noise measurement is insufficient for accurately estimating the applied temperature bias due to noise contributions from other sources, averaging over an ensemble of junctions enables predictions within experimental uncertainties. This demonstrates that machine learning approaches can be utilized for estimation of temperature biases, and similarly other stimuli in electronic junctions. △ Less

Submitted 29 November, 2024; originally announced December 2024.

Journal ref: J. Chem. Phys. 162, 084108 (2025)

Measurement-induced entanglement transition in chaotic quantum Ising chain

Authors: Manali Malakar, Marlon Brenes, Dvira Segal, Alessandro Silva

Abstract: We numerically investigate the robustness against various perturbations of measurement-induced phase transition in monitored quantum Ising models in the no-click limit, where the dynamics is described by a non-Hermitian Hamiltonian. We study perturbations that break the integrability and/or the symmetry of the model, as well as modifications in the measurement protocol, characterizing the resultin… ▽ More We numerically investigate the robustness against various perturbations of measurement-induced phase transition in monitored quantum Ising models in the no-click limit, where the dynamics is described by a non-Hermitian Hamiltonian. We study perturbations that break the integrability and/or the symmetry of the model, as well as modifications in the measurement protocol, characterizing the resulting chaos and lack of integrability through the Dissipative Spectral Form Factor (DSFF). We show that while the measurement-induced phase transition and its properties appear to be broadly insensitive to lack of integrability and breaking of the $\mathbb{Z_2}$ symmetry, a modification of the measurement basis from the transverse to the longitudinal direction makes the phase transition disappear altogether. △ Less

Submitted 31 October, 2024; v1 submitted 11 July, 2024; originally announced July 2024.

Journal ref: Phys. Rev. B 110, 134316 (2024)

Random walk with horizontal and cyclic currents

Authors: Joanna Li, Matthew Gerry, Israel Klich, Dvira Segal

Abstract: We construct a minimal two-chain random walk model and study the information that fluctuations of the flux and higher cumulants can reveal about the model: its structure, parameters, and whether it operates under nonequilibrium conditions. The two coupled chains allow for both horizontal and cyclic transport. We capture these processes by deriving the cumulant generating function of the system, wh… ▽ More We construct a minimal two-chain random walk model and study the information that fluctuations of the flux and higher cumulants can reveal about the model: its structure, parameters, and whether it operates under nonequilibrium conditions. The two coupled chains allow for both horizontal and cyclic transport. We capture these processes by deriving the cumulant generating function of the system, which characterizes both horizontal and cyclic transport in the long time limit. First, we show that either the horizontal or the cyclic currents, along with their higher-order cumulants, can be used to unravel the intrinsic structure and parameters of the model. Second, we investigate the "zero current" situation, in which the {\it horizontal} current vanishes. We find that fluctuations of the horizontal current reveal the nonequilibrium condition at intermediate bias, while the cyclic current remains nonzero throughout. We also show that in nonequilibrium scenarios close to the zero {\it horizontal} current limit, the entropy production rate is more tightly lower-bounded by the relative noise of the {\it cyclic} current, and vice versa. Finally, simulations of transport before the steady state sets in allow for the extraction of the interchain hopping rate. Our study, illustrating the information concealed in fluctuations, could see applications in chemical networks, cellular processes, and charge and energy transport materials. △ Less

Submitted 4 July, 2024; originally announced July 2024.

Role of Bath-Induced Many-Body Interactions in the Dissipative Phases of the Su-Schrieffer-Heeger Model

Authors: Brett Min, Kartiek Agarwal, Dvira Segal

Abstract: The Su-Schrieffer-Heeger chain is a prototype example of a symmetry-protected topological insulator. Coupling it non-perturbatively to local thermal environments, either through the intercell or the intracell fermion tunneling elements, modifies the topological window. To understand this effect, we employ the recently developed reaction-coordinate polaron transform (RCPT) method, which allows trea… ▽ More The Su-Schrieffer-Heeger chain is a prototype example of a symmetry-protected topological insulator. Coupling it non-perturbatively to local thermal environments, either through the intercell or the intracell fermion tunneling elements, modifies the topological window. To understand this effect, we employ the recently developed reaction-coordinate polaron transform (RCPT) method, which allows treating system-bath interactions at arbitrary strengths. The effective system Hamiltonian, which is obtained via the RCPT, exposes the impact of the baths on the SSH chain through renormalization of tunneling elements and the generation of many-body interaction terms. By performing exact diagonalization and computing the ensemble geometric phase, a topological invariant applicable even to systems at finite temperature, we distinguish the trivial band insulator (BI) from the topological insulator (TI) phases. Furthermore, through the RCPT mapping, we are able to pinpoint the main mechanism behind the extension of the parameter space for the TI or the BI phases (depending on the coupling scheme, intracell or intercell), which is the bath-induced, dimerized, many-body interaction. We also study the effect of on-site staggered potentials on the SSH phase diagram, and discuss extensions of our method to higher dimensions. △ Less

Submitted 19 June, 2024; originally announced June 2024.

Comments: 17 pages, 7 figures

Journal ref: Phys. Rev. B 110, 125415 (2024)

Bath-induced interactions and transient dynamics in open quantum systems at strong coupling: Effective Hamiltonian approach

Authors: Marlon Brenes, Brett Min, Nicholas Anto-Sztrikacs, Nir Bar-Gill, Dvira Segal

Abstract: Understanding the dynamics of dissipative quantum systems, particularly beyond the weak coupling approximation, is central to various quantum applications. While numerically exact methods provide accurate solutions, they often lack the analytical insight provided by theoretical approaches. In this study, we employ the recently-developed method dubbed the effective Hamiltonian theory to understand… ▽ More Understanding the dynamics of dissipative quantum systems, particularly beyond the weak coupling approximation, is central to various quantum applications. While numerically exact methods provide accurate solutions, they often lack the analytical insight provided by theoretical approaches. In this study, we employ the recently-developed method dubbed the effective Hamiltonian theory to understand the dynamics of system-bath configurations without resorting to a perturbative description of the system-bath coupling energy. Through a combination of mapping steps and truncation, the effective Hamiltonian theory offers both analytical insights into signatures of strong couplings in open quantum systems and a straightforward path for numerical simulations. To validate the accuracy of the method, we apply it to two canonical models: a single spin immersed in a bosonic bath and two noninteracting spins in a common bath. In both cases, we study the transient regime and the steady state limit at nonzero temperature, and spanning system-bath interactions from the weak to the strong regime. By comparing the results of the effective Hamiltonian theory with numerically exact simulations, we show that although the former overlooks non-Markovian features in the transient equilibration dynamics, it correctly captures non-perturbative bath-generated couplings between otherwise non-interacting spins as observed in their synchronization dynamics and correlations. Altogether, the effective Hamiltonian theory offers a powerful approach to understanding strong coupling dynamics and thermodynamics, capturing the signatures of such interactions in both relaxation dynamics and in the steady state limit. △ Less

Submitted 25 June, 2024; v1 submitted 5 March, 2024; originally announced March 2024.

Comments: Journal version

Journal ref: J. Chem. Phys. 160, 244106 (2024)

Thermal transport in fullerene-based molecular junctions: Molecular dynamics simulations

Authors: Joanna Li, Jonathan J. Wang, Dvira Segal

Abstract: We investigate phonon thermal transport of fullerene-based single-molecule junctions by employing classical molecular dynamics simulations. The thermal conductances of fullerene monomers, dimers, and trimers are computed through three distinct molecular dynamics methods, by following the equilibration dynamics in one method, and using two other nonequilibrium simulation methods. We discuss technic… ▽ More We investigate phonon thermal transport of fullerene-based single-molecule junctions by employing classical molecular dynamics simulations. The thermal conductances of fullerene monomers, dimers, and trimers are computed through three distinct molecular dynamics methods, by following the equilibration dynamics in one method, and using two other nonequilibrium simulation methods. We discuss technical aspects of the simulation techniques, and show that their predictions for the thermal conductance agree. Our simulations reveal that the thermal conductance of fullerene monomer and dimer junctions are similar, yet the thermal conductance of trimer junctions is significantly reduced. This study could assist in the design of high-performing thermoelectric junctions, where low thermal conductance is desired. △ Less

Submitted 8 February, 2024; originally announced February 2024.

Bath-engineering magnetic order in quantum spin chains: An analytic mapping approach

Authors: Brett Min, Nicholas Anto-Sztrikacs, Marlon Brenes, Dvira Segal

Abstract: Dissipative processes can drive different magnetic orders in quantum spin chains. Using a non-perturbative analytic mapping framework, we systematically show how to structure different magnetic orders in spin systems by controlling the locality of the attached baths. Our mapping approach reveals analytically the impact of spin-bath couplings, leading to the suppression of spin splittings, bath-dre… ▽ More Dissipative processes can drive different magnetic orders in quantum spin chains. Using a non-perturbative analytic mapping framework, we systematically show how to structure different magnetic orders in spin systems by controlling the locality of the attached baths. Our mapping approach reveals analytically the impact of spin-bath couplings, leading to the suppression of spin splittings, bath-dressing and mixing of spin-spin interactions, and emergence of non-local ferromagnetic interactions between spins coupled to the same bath, which become long-ranged for a global bath. Our general mapping method can be readily applied to a variety of spin models: We demonstrate (i) a bath-induced transition from antiferromangnetic (AFM) to ferromagnetic ordering in a Heisenberg spin chain, (ii) AFM to extended Neel phase ordering within a transverse-field Ising chain with pairwise couplings to baths, and (iii) a quantum phase transition in the fully-connected Ising model. Our method is non-perturbative in the system-bath coupling. It holds for a variety of non-Markovian baths and it can be readily applied towards studying bath-engineered phases in frustrated or topological materials. △ Less

Submitted 11 January, 2024; originally announced January 2024.

Comments: (4.5 pages + 4 figures) main text (13 pages + 3 figures) supplemental material

Journal ref: Phys. Rev. Lett. 132, 266701 (2024)

Challenges in molecular dynamics simulations of heat exchange statistics

Authors: Jonathan J. Wang, Matthew Gerry, Dvira Segal

Abstract: We study heat exchange in temperature-biased metal-molecule-metal molecular junctions by employing the LAMMPS atomic molecular dynamics simulator. Generating the nonequilibrium steady state with Langevin thermostats at the boundaries of the junction, we show that the {\it average} heat current across a gold-alkanedithiol-gold nanojunction behaves correctly-physically, with the thermal conductance… ▽ More We study heat exchange in temperature-biased metal-molecule-metal molecular junctions by employing the LAMMPS atomic molecular dynamics simulator. Generating the nonequilibrium steady state with Langevin thermostats at the boundaries of the junction, we show that the {\it average} heat current across a gold-alkanedithiol-gold nanojunction behaves correctly-physically, with the thermal conductance value matching the literature. In contrast, the {\it full probability distribution function} for heat exchange, as generated by the simulator, violates the fundamental fluctuation symmetry for entropy production. We trace this failure back to the implementation of the thermostats and the expression used to calculate the heat exchange. To rectify this issue and produce the correct statistics, we introduce single-atom thermostats as an alternative to conventional many-atom thermostats. Once averaging heat exchange over the hot and cold thermostats, this approach successfully generates the correct probability distribution function, which we use to study the behavior of both the average heat current and its noise. We further examine the thermodynamic uncertainty relation in the molecular junction and show that it holds, albeit demonstrating nontrivial trends. Our study points to the need to carefully implement nonequilibrium molecular dynamics solvers in atomistic simulation software tools for future investigations of noise phenomena in thermal transport. △ Less

Submitted 13 November, 2023; originally announced November 2023.

Effective-Hamiltonian theory: An approximation to the equilibrium state of open quantum systems

Authors: Nicholas Anto-Sztrikacs, Brett Min, Marlon Brenes, Dvira Segal

Abstract: We extend and benchmark the recently-developed Effective-Hamiltonian (EFFH) method [PRX Quantum $\bf{4}$, 020307 (2023)] as an approximation to the equilibrium state ("mean-force Gibbs state") of a quantum system at strong coupling to a thermal bath. The EFFH method is an approximate framework. Through a combination of the reaction-coordinate mapping, a polaron transformation and a controlled trun… ▽ More We extend and benchmark the recently-developed Effective-Hamiltonian (EFFH) method [PRX Quantum $\bf{4}$, 020307 (2023)] as an approximation to the equilibrium state ("mean-force Gibbs state") of a quantum system at strong coupling to a thermal bath. The EFFH method is an approximate framework. Through a combination of the reaction-coordinate mapping, a polaron transformation and a controlled truncation, it imprints the system-bath coupling parameters into the system's Hamiltonian. First, we develop a $\textit{variational}$ EFFH technique. In this method, system's parameters are renormalized by both the system-bath coupling parameters (as in the original EFFH approach) and the bath's temperature. Second, adopting the generalized spin-boson model, we benchmark the equilibrium state from the EFFH treatment against numerically-exact simulations and demonstrate a good agreement for both polarization and coherences using the Brownian spectral function. Third, we contrast the (normal and variational) EFFH approach with the familiar (normal and variational) polaron treatment. We show that the two methods predict a similar structure for the equilibrium state, albeit the EFFH approach offers the advantage of simpler calculations and closed-form analytical results. Altogether, we argue that for temperatures comparable to the system's frequencies, the EFFH methodology provides a good approximation for the mean-force Gibbs state in the full range of system-bath coupling, from ultraweak to ultrastrong. △ Less

Submitted 26 July, 2023; originally announced July 2023.

Comments: 21 pages, 5 figures

Journal ref: Phys. Rev. B 108, 115437 (2023)

Multi-spin probes for thermometry in the strong-coupling regime

Authors: Marlon Brenes, Dvira Segal

Abstract: We study the sensitivity of thermometric probes that are composed of $N$ spins coupled to a sample prepared at temperature $T$. Our analysis extends beyond the weak-coupling limit into the strong sample-probe coupling regime. In particular, sample-induced interactions between each of the spins are generated via strong coupling effects and are not fine-tuned amongst each body composing the probe. B… ▽ More We study the sensitivity of thermometric probes that are composed of $N$ spins coupled to a sample prepared at temperature $T$. Our analysis extends beyond the weak-coupling limit into the strong sample-probe coupling regime. In particular, sample-induced interactions between each of the spins are generated via strong coupling effects and are not fine-tuned amongst each body composing the probe. By employing the reaction-coordinate mapping to evaluate the non-canonical equilibrium state of the probe at finite coupling, we compute the thermometric sensitivity via the quantum Fisher information through the equilibrium state itself. We find that for single-spin probes $(N = 1)$, temperature sensitivity decreases in the regime of weak-to-intermediate coupling strength, however, as the coupling increases we observe much higher sensitivity of the probe in the low-temperature regime. Furthermore, as long as $N > 1$, there exist optimal values of the sample-probe interaction energy that allow one to attain enhanced thermometric sensitivity when compared to the maximum achieved precision obtained from thermal Gibbs states at weak coupling, particularly in the regime of low temperature. Finally, we show that this enhanced sensitivity may be observed from suboptimal measurements. △ Less

Submitted 3 October, 2023; v1 submitted 9 July, 2023; originally announced July 2023.

Comments: Journal version

Journal ref: Phys. Rev. A 108, 032220 (2023)

Random walks on modular chains: Detecting structure through statistics

Authors: Matthew Gerry, Dvira Segal

Abstract: We study kinetic transport through modular networks consisting of alternating domains using both analytical and numerical methods. We demonstrate that the mean velocity is insensitive to the local structure of the network, and it indicates only on global, structural-averaged properties. However, by examining high-order cumulants characterizing the kinetics, we reveal information on the degree of i… ▽ More We study kinetic transport through modular networks consisting of alternating domains using both analytical and numerical methods. We demonstrate that the mean velocity is insensitive to the local structure of the network, and it indicates only on global, structural-averaged properties. However, by examining high-order cumulants characterizing the kinetics, we reveal information on the degree of inhomogeneity of blocks and the size of repeating units in the network. Specifically, in unbiased diffusion, the kurtosis is the first transport coefficient that exposes structural information, whereas in biased chains, the diffusion coefficient already reveals structural motifs. Nevertheless, this latter dependence is weak and it disappears at both low and high biasing. Our study demonstrates that high order moments of the population distribution over sites provide information about the network structure that is not captured by the first moment (mean velocity) alone. These results are useful towards deciphering mechanisms and determining architectures underlying long-range charge transport in biomolecules and biological and chemical reaction networks. △ Less

Submitted 16 May, 2023; originally announced May 2023.

Comments: 13 pages, 10 figures

Variational Quantum Eigensolvers in the Era of Distributed Quantum Computers

Authors: Ilia Khait, Edwin Tham, Dvira Segal, Aharon Brodutch

Abstract: The computational power of a quantum computer is limited by the number of qubits available for information processing. Increasing this number within a single device is difficult; it is widely accepted that distributed modular architectures are the solution to large scale quantum computing. The major challenge in implementing such architectures is the need to exchange quantum information between mo… ▽ More The computational power of a quantum computer is limited by the number of qubits available for information processing. Increasing this number within a single device is difficult; it is widely accepted that distributed modular architectures are the solution to large scale quantum computing. The major challenge in implementing such architectures is the need to exchange quantum information between modules. In this work, we show that a distributed quantum computing architecture with {\it limited} capacity to exchange information between modules can accurately solve quantum computational problems. Using the example of a variational quantum eignesolver with an ansatz designed for a two-module (dual-core) architecture, we show that three inter-module operations provide a significant advantage over no inter-module (or serially executed) operations. These results provide a strong indication that near-term {\it modular} quantum processors can be an effective alternative to their monolithic counterparts. △ Less

Submitted 27 February, 2023; originally announced February 2023.

Comments: 4.5 pages + SI. Comment are welcomed

Particle current statistics in driven mesoscale conductors

Authors: Marlon Brenes, Giacomo Guarnieri, Archak Purkayastha, Jens Eisert, Dvira Segal, Gabriel Landi

Abstract: We propose a highly-scalable method to compute the statistics of charge transfer in driven conductors. The framework can be applied in situations of non-zero temperature, strong coupling to terminals and in the presence of non-periodic light-matter interactions, away from equilibrium. The approach combines the so-called mesoscopic leads formalism with full counting statistics. It results in a gene… ▽ More We propose a highly-scalable method to compute the statistics of charge transfer in driven conductors. The framework can be applied in situations of non-zero temperature, strong coupling to terminals and in the presence of non-periodic light-matter interactions, away from equilibrium. The approach combines the so-called mesoscopic leads formalism with full counting statistics. It results in a generalised quantum master equation that dictates the dynamics of current fluctuations and higher order moments of the probability distribution function of charge exchange. For generic time-dependent quadratic Hamiltonians, we provide closed-form expressions for computing noise in the non-perturbative regime of the parameters of the system, reservoir or system-reservoir interactions. Having access to the full dynamics of the current and its noise, the method allows us to compute the variance of charge transfer over time in non-equilibrium configurations. The dynamics reveal that in driven systems, the average noise should be defined operationally with care over which period of time is covered. △ Less

Submitted 22 August, 2023; v1 submitted 24 November, 2022; originally announced November 2022.

Comments: Journal version

Journal ref: Phys. Rev. B 108, L081119 (2023)

Effective Hamiltonian theory of open quantum systems at strong coupling

Authors: Nicholas Anto-Sztrikacs, Ahsan Nazir, Dvira Segal

Abstract: We present the reaction-coordinate polaron-transform (RCPT) framework for generating effective Hamiltonian models to treat nonequilibrium open quantum systems at strong coupling with their surroundings. Our approach, which is based on two exact transformations of the Hamiltonian followed by its controlled truncation, ends with a new Hamiltonian with a weakened coupling to the environment. This new… ▽ More We present the reaction-coordinate polaron-transform (RCPT) framework for generating effective Hamiltonian models to treat nonequilibrium open quantum systems at strong coupling with their surroundings. Our approach, which is based on two exact transformations of the Hamiltonian followed by its controlled truncation, ends with a new Hamiltonian with a weakened coupling to the environment. This new effective Hamiltonian mirrors the initial one, except that its parameters are dressed by the system-bath couplings. The power and elegance of the RCPT approach lie in its generality and in its mathematical simplicity, allowing for analytic work and thus profound understanding of the impact of strong system-bath coupling effects on open quantum system phenomena. Examples interrogated in this work include canonical models for quantum thermalization, charge and energy transport at the nanoscale, performance bounds of quantum thermodynamical machines such as absorption refrigerators and thermoelectric generators, as well as the equilibrium and nonequilibrium behavior of many-body dissipative spin chains. △ Less

Submitted 10 November, 2022; originally announced November 2022.

Comments: 29 pages, 12 figures

Simulations of Heat Transport in Single-Molecule Junctions: Investigations of the Thermal Diode Effect

Authors: Jonathan J. Wang, Jie Gong, Alan J. H. McGaughey, Dvira Segal

Abstract: With the objective to understand microscopic principles governing thermal energy flow in nanojunctions, we study phononic heat transport through metal-molecule-metal junctions using classical molecular dynamics (MD) simulations. Considering a single-molecule gold-alkanedithiol-gold junction, we first focus on aspects of method development and compare two techniques for calculating thermal conducta… ▽ More With the objective to understand microscopic principles governing thermal energy flow in nanojunctions, we study phononic heat transport through metal-molecule-metal junctions using classical molecular dynamics (MD) simulations. Considering a single-molecule gold-alkanedithiol-gold junction, we first focus on aspects of method development and compare two techniques for calculating thermal conductance: (i) The Reverse Nonequilibrium MD (RNEMD) method, where heat is inputted and extracted at a constant rate from opposite metals. In this case, the thermal conductance is calculated from the nonequilibrium temperature profile that is created on the junction. (ii) The Approach-to-Equilibrium MD (AEMD) method, with the thermal conductance of the junction obtained from the equilibration dynamics of the metals. In both methods, simulations of alkane chains of growing size display an approximate length-independence of the thermal conductance, with calculated values matching computational and experimental studies. The RNEMD and AEMD methods offer different insights on thermal transport, and we discuss their relative benefits and shortcomings. Assessing the potential application of molecular junctions as thermal diodes, the alkane junctions are made spatially asymmetric by modifying their contact regions with the bulk, either by using distinct endgroups or by replacing one of the Au contacts by Ag. Anharmonicity is built into the system within the molecular force-field. Using the RNEMD method, we show that, while the temperature profile strongly varies (compared to the gold-alkanedithiol-gold junctions) due to these structural modifications, the thermal diode effect is inconsequential in these systems -- unless one goes to very large thermal biases. This finding suggests that one should seek molecules with considerable internal anharmonic effects for developing nonlinear thermal devices. △ Less

Submitted 13 September, 2022; originally announced September 2022.

Simulating time evolution on distributed quantum computers

Authors: Finn Lasse Buessen, Dvira Segal, Ilia Khait

Abstract: We study a variation of the Trotter-Suzuki decomposition, in which a Hamiltonian exponential is approximated by an ordered product of two-qubit operator exponentials such that the Trotter step size is enhanced for a small number of terms. Such decomposition directly reflects hardware constraints of distributed quantum computers, where operations on monolithic quantum devices are fast compared to e… ▽ More We study a variation of the Trotter-Suzuki decomposition, in which a Hamiltonian exponential is approximated by an ordered product of two-qubit operator exponentials such that the Trotter step size is enhanced for a small number of terms. Such decomposition directly reflects hardware constraints of distributed quantum computers, where operations on monolithic quantum devices are fast compared to entanglement distribution across separate nodes using interconnects. We simulate non-equilibrium dynamics of transverse-field Ising and XY spin chain models and investigate the impact of locally increased Trotter step sizes that are associated with an increasingly sparse use of the quantum interconnect. We find that the overall quality of the approximation depends smoothly on the local sparsity and that the proliferation of local errors is slow. As a consequence, we show that fast local operations on monolithic devices can be leveraged to obtain an overall improved result fidelity even on distributed quantum computers where the use of interconnects is costly. △ Less

Submitted 15 August, 2022; originally announced August 2022.

Comments: 6+5 pages, 5+6 figures

Journal ref: Phys. Rev. Research 5, L022003 (2023)

Long-Range Charge Transport in Homogeneous and Alternating-Rigidity Chains

Authors: Francisco Lai Liang, Dvira Segal

Abstract: We study the interplay of intrinsic-electronic and environmental factors on long-range charge transport across molecular chains with up to $N\sim 80$ monomers. We describe the molecular electronic structure of the chain with a tight-binding Hamiltonian. Thermal effects in the form of electron decoherence and inelastic scatterings are incorporated with the Landauer-Büttiker probe method. In short c… ▽ More We study the interplay of intrinsic-electronic and environmental factors on long-range charge transport across molecular chains with up to $N\sim 80$ monomers. We describe the molecular electronic structure of the chain with a tight-binding Hamiltonian. Thermal effects in the form of electron decoherence and inelastic scatterings are incorporated with the Landauer-Büttiker probe method. In short chains of up to 10 units we observe the crossover between coherent (tunneling, ballistic) motion and thermally-assisted conduction, with thermal effects enhancing the current beyond the quantum coherent limit. We further show that unconventional (non monotonic with size) transport behavior emerges when monomer-to-monomer electronic coupling is made large. In long chains, we identify a different behavior, with thermal effects suppressing the conductance below the coherent-ballistic limit. With the goal to identify a minimal model for molecular chains displaying unconventional and effective long-range transport, we simulate a modular polymer with alternating regions of high and low rigidity. Simulations show that, surprisingly, while charge correlations are significantly affected by structuring environmental conditions, reflecting charge delocalization, the electrical resistance displays an averaging effect, and it is not sensitive to this patterning. We conclude by arguing that efficient long-range charge transport requires engineering both internal electronic parameters and environmental conditions. △ Less

Submitted 30 May, 2022; originally announced May 2022.

Quantum Thermal Transport Beyond Second Order with the Reaction Coordinate Mapping

Authors: Nicholas Anto-Sztrikacs, Felix Ivander, Dvira Segal

Abstract: Standard quantum master equation techniques such as the Redfield or Lindblad equations are perturbative to second order in the microscopic system-reservoir coupling parameter $λ$. As a result, characteristics of dissipative systems, which are beyond second order in $λ$, are not captured by such tools. Moreover, if the leading order in the studied effect is higher-than-quadratic in $λ$, a second-or… ▽ More Standard quantum master equation techniques such as the Redfield or Lindblad equations are perturbative to second order in the microscopic system-reservoir coupling parameter $λ$. As a result, characteristics of dissipative systems, which are beyond second order in $λ$, are not captured by such tools. Moreover, if the leading order in the studied effect is higher-than-quadratic in $λ$, a second-order description fundamentally fails, even at weak coupling. Here, using the reaction coordinate (RC) quantum master equation framework, we are able to investigate and classify higher-than-second order transport mechanisms. This technique, which relies on the redefinition of the system-environment boundary, allows for the effects of system-bath coupling to be included to high orders. We study steady-state heat current beyond second-order in two models: The generalized spin-boson model with non-commuting system-bath operators and a three-level ladder system. In the latter model heat enters in one transition and it is extracted from a different one. Crucially, we identify two transport pathways: (i) System's current, where heat conduction is mediated by transitions in the system, with the heat current scaling as $j_q \propto λ^2$ to lowest order in $λ$. (ii) Inter-bath current, with the thermal baths directly exchanging energy between them, facilitated by the bridging quantum system. To the lowest order in $λ$, this current scales as $j_q \propto λ^4$. These mechanisms are uncovered and examined using numerical and analytical tools. We contend that the RC mapping brings, already at the level of the mapped Hamiltonian, much insights on transport characteristics. △ Less

Submitted 16 March, 2022; v1 submitted 11 March, 2022; originally announced March 2022.

Comments: 13 pages, 7 figures. We added a missing term to equations 11 and 12

Absence and recovery of cost-precision tradeoff relations in quantum transport

Authors: Matthew Gerry, Dvira Segal

Abstract: The operation of many classical and quantum systems in nonequilibrium steady state is constrained by cost-precision (dissipation-fluctuation) tradeoff relations, delineated by the thermodynamic uncertainty relation (TUR). However, coherent quantum electronic nanojunctions can escape such a constraint, showing finite charge current and nonzero entropic cost with vanishing current fluctuations. Here… ▽ More The operation of many classical and quantum systems in nonequilibrium steady state is constrained by cost-precision (dissipation-fluctuation) tradeoff relations, delineated by the thermodynamic uncertainty relation (TUR). However, coherent quantum electronic nanojunctions can escape such a constraint, showing finite charge current and nonzero entropic cost with vanishing current fluctuations. Here, we analyze the absence, and restoration, of cost-precision tradeoff relations in fermionic nanojunctions under different affinities: voltage and temperature biases. With analytic work and simulations, we show that both charge and energy currents can display the absence of cost-precision tradeoff if we engineer the transmission probability as a boxcar function -- with a perfect transmission and hard energy cutoffs. Specifically for charge current under voltage bias, the standard TUR may be immediately violated as we depart from equilibrium, and it is exponentially suppressed with increased voltage. However, beyond idealized, hard-cutoff energy-filtered transmission functions, we show that realistic models with soft cutoffs or imperfect transmission functions follow cost-precision tradeoffs, and eventually recover the standard TUR sufficiently far from equilibrium. The existence of cost-precision tradeoff relations is thus suggested as a generic feature of realistic nonequilibrium quantum transport junctions. △ Less

Submitted 16 February, 2022; originally announced February 2022.

Strong system-bath coupling reshapes characteristics of quantum thermal machines

Authors: Felix Ivander, Nicholas Anto-Sztrikacs, Dvira Segal

Abstract: We study the performance of quantum absorption refrigerators, paradigmatic autonomous quantum thermal machines, and reveal central impacts of strong couplings between the working system and the thermal baths. Using the reaction coordinate quantum master equation method, which treats system-bath interactions beyond weak coupling, we discover that reshaping of the window of performance is a central… ▽ More We study the performance of quantum absorption refrigerators, paradigmatic autonomous quantum thermal machines, and reveal central impacts of strong couplings between the working system and the thermal baths. Using the reaction coordinate quantum master equation method, which treats system-bath interactions beyond weak coupling, we discover that reshaping of the window of performance is a central outcome of strong system-bath couplings. This alteration of the cooling window stems from the dominant role of parameter renormalization at strong couplings. We further show that strong coupling admits direct transport pathways between the thermal reservoirs. Such beyond-second-order transport mechanisms are typically detrimental to the performance of quantum thermal machines. Our study reveals that it is inadequate to claim for either a suppression or an enhancement of the cooling performance at strong coupling when analyzed against a single parameter and in a limited domain. Rather, a comprehensive approach should be adopted so as to uncover the reshaping of the operation window. △ Less

Submitted 9 November, 2021; originally announced November 2021.

Bounds on fluctuations for ensembles of quantum thermal machines

Authors: Matthew Gerry, Na'im Kalantar, Dvira Segal

Abstract: We study universal aspects of fluctuations in an ensemble of noninteracting continuous quantum thermal machines in the steady state limit. Considering an individual machine, such as a refrigerator, in which relative fluctuations (and high order cumulants) of the cooling heat current to the absorbed heat current, $η^{(n)}$, are upper-bounded, $η^{(n)}\leq η_C^n$ with $n\geq 2$ and $η_C$ the Carnot… ▽ More We study universal aspects of fluctuations in an ensemble of noninteracting continuous quantum thermal machines in the steady state limit. Considering an individual machine, such as a refrigerator, in which relative fluctuations (and high order cumulants) of the cooling heat current to the absorbed heat current, $η^{(n)}$, are upper-bounded, $η^{(n)}\leq η_C^n$ with $n\geq 2$ and $η_C$ the Carnot efficiency, we prove that an {\it ensemble} of $N$ distinct machines similarly satisfies this upper bound on the relative fluctuations of the ensemble, $η_N^{(n)}\leq η_C^n$. For an ensemble of distinct quantum {\it refrigerators} with components operating in the tight coupling limit we further prove the existence of a {\it lower bound} on $η_N^{(n)}$ in specific cases, exemplified on three-level quantum absorption refrigerators and resonant-energy thermoelectric junctions. Beyond special cases, the existence of a lower bound on $η_N^{(2)}$ for an ensemble of quantum refrigerators is demonstrated by numerical simulations. △ Less

Submitted 8 September, 2021; originally announced September 2021.

Comments: Na'im Kalantar and Matthew Gerry contributed equally to this work and are joint "first authors"

Optimal control of quantum thermal machines using machine learning

Authors: Ilia Khait, Juan Carrasquilla, Dvira Segal

Abstract: Identifying optimal thermodynamical processes has been the essence of thermodynamics since its inception. Here, we show that differentiable programming (DP), a machine learning (ML) tool, can be employed to optimize finite-time thermodynamical processes in a quantum thermal machine. We consider the paradigmatic quantum Otto engine with a time-dependent harmonic oscillator as its working fluid, and… ▽ More Identifying optimal thermodynamical processes has been the essence of thermodynamics since its inception. Here, we show that differentiable programming (DP), a machine learning (ML) tool, can be employed to optimize finite-time thermodynamical processes in a quantum thermal machine. We consider the paradigmatic quantum Otto engine with a time-dependent harmonic oscillator as its working fluid, and build upon shortcut-to-adiabaticity (STA) protocols. We formulate the STA driving protocol as a constrained optimization task and apply DP to find optimal driving profiles for an appropriate figure of merit. Our ML scheme discovers profiles for the compression and expansion strokes that are superior to previously-suggested protocols. Moreover, using our ML algorithm we show that a previously-employed, intuitive energetic cost of the STA driving suffers from a fundamental flaw, which we resolve with an alternative construction for the cost function. Our method and results demonstrate that ML is beneficial both for solving hard-constrained quantum control problems and for devising and assessing their theoretical groundwork. △ Less

Submitted 27 August, 2021; originally announced August 2021.

Comments: 5 pages + 5 figures, SM: 3 page + 1 figure. Comments are welcomed!

Journal ref: Phys. Rev. Research 4, L012029 2022

Quantum flicker noise in atomic and molecular junctions

Authors: Ofir Shein-Lumbroso, Junjie Liu, Abhay Shastry, Dvira Segal, Oren Tal

Abstract: We report on a quantum form of electronic flicker noise in nanoscale conductors that contains valuable information on quantum transport. This noise is experimentally identified in atomic and molecular junctions, and theoretically analyzed by considering quantum interference due to fluctuating scatterers. Using conductance, shot noise, and flicker noise measurements, we show that the revealed quant… ▽ More We report on a quantum form of electronic flicker noise in nanoscale conductors that contains valuable information on quantum transport. This noise is experimentally identified in atomic and molecular junctions, and theoretically analyzed by considering quantum interference due to fluctuating scatterers. Using conductance, shot noise, and flicker noise measurements, we show that the revealed quantum flicker noise uniquely depends on the distribution of transmission channels, a key characteristic of quantum conductors. This dependence opens the door for the application of flicker noise as a diagnostic probe for fundamental properties of quantum conductors and many-body quantum effects, a role that up to now has been performed by the experimentally less-accessible shot noise. △ Less

Submitted 23 August, 2021; originally announced August 2021.

Periodically-driven quantum thermal machines from warming up to limit cycle

Authors: Junjie Liu, Kenneth A. Jung, Dvira Segal

Abstract: Theoretical treatments of periodically-driven quantum thermal machines (PD-QTMs) are largely focused on the limit-cycle stage of operation characterized by a periodic state of the system. Yet, this regime is not immediately accessible for experimental verification. Here, we present a general thermodynamic framework that can handle the performance of PD-QTMs both before and during the limit-cycle s… ▽ More Theoretical treatments of periodically-driven quantum thermal machines (PD-QTMs) are largely focused on the limit-cycle stage of operation characterized by a periodic state of the system. Yet, this regime is not immediately accessible for experimental verification. Here, we present a general thermodynamic framework that can handle the performance of PD-QTMs both before and during the limit-cycle stage of operation. It is achieved by observing that periodicity may break down at the ensemble average level, even in the limit-cycle phase. With this observation, and using conventional thermodynamic expressions for work and heat, we find that a complete description of the first law of thermodynamics for PD-QTMs requires a new contribution, which vanishes only in the limit-cycle phase under rather weak system-bath couplings. Significantly, this contribution is substantial at strong couplings even at limit cycle, thus largely affecting the behavior of the thermodynamic efficiency. We demonstrate our framework by simulating a quantum Otto engine building upon a driven resonant level model. Our results provide new insights towards a complete description of PD-QTMs, from turn-on to the limit-cycle stage and, particularly, shed light on the development of quantum thermodynamics at strong coupling. △ Less

Submitted 21 June, 2021; v1 submitted 19 June, 2021; originally announced June 2021.

Comments: submitted. Comments and suggestions are highly welcome

Journal ref: Phys. Rev. Lett. 127, 200602 (2021)

Boosting quantum battery performance by structure engineering

Authors: Junjie Liu, Dvira Segal

Abstract: Quantum coherences, correlations and collective effects can be harnessed to the advantage of quantum batteries. Here, we introduce a feasible structure engineering scheme that is applicable to spin-based open quantum batteries. Our scheme, which builds solely upon a modulation of spin energy gaps, allows engineered quantum batteries to exploit spin-spin correlations for mitigating environment-indu… ▽ More Quantum coherences, correlations and collective effects can be harnessed to the advantage of quantum batteries. Here, we introduce a feasible structure engineering scheme that is applicable to spin-based open quantum batteries. Our scheme, which builds solely upon a modulation of spin energy gaps, allows engineered quantum batteries to exploit spin-spin correlations for mitigating environment-induced aging. As a result of this advantage, an engineered quantum battery can preserve relatively more energy as compared with its non-engineered counterpart over the course of the storage phase. Particularly, the excess in stored energy is independent of system size. This implies a scale-invariant passive protection strategy, which we demonstrate on an engineered quantum battery with staggered spin energy gaps. Our findings establish structure engineering as a useful route for advancing quantum batteries, and bring new perspectives on efficient quantum battery designs. △ Less

Submitted 13 April, 2021; originally announced April 2021.

Comments: 6+6 pages, 3+5 figures, submitted. Comments and suggestions are welcome

Universal Bounds on Fluctuations in Continuous Thermal Machines

Authors: Sushant Saryal, Matthew Gerry, Ilia Khait, Dvira Segal, Bijay Kumar Agarwalla

Abstract: We study bounds on ratios of fluctuations in steady-state time-reversal heat engines controlled by multi affinities. In the linear response regime, we prove that the relative fluctuations (precision) of the output current (power) is always lower-bounded by the relative fluctuations of the input current (heat current absorbed from the hot bath). As a consequence, the ratio between the fluctuations… ▽ More We study bounds on ratios of fluctuations in steady-state time-reversal heat engines controlled by multi affinities. In the linear response regime, we prove that the relative fluctuations (precision) of the output current (power) is always lower-bounded by the relative fluctuations of the input current (heat current absorbed from the hot bath). As a consequence, the ratio between the fluctuations of the output and input currents are bounded both from above and below, where the lower (upper) bound is determined by the square of the averaged efficiency (square of the Carnot efficiency) of the engine. The saturation of the lower bound is achieved in the tight-coupling limit when the determinant of the Onsager response matrix vanishes. Our analysis can be applied to different operational regimes, including engines, refrigerators, and heat pumps. We illustrate our findings in two types of continuous engines: two-terminal coherent thermoelectric junctions and three-terminal quantum absorption refrigerators. Numerical simulations in the far-from-equilibrium regime suggest that these bounds apply more broadly, beyond linear response. △ Less

Submitted 24 March, 2021; originally announced March 2021.

Strong coupling effects in quantum thermal transport with the reaction coordinate method

Authors: Nicholas Anto-Sztrikacs, Dvira Segal

Abstract: We present a semi-analytical approach for studying quantum thermal energy transport beyond the weak system-bath coupling regime. Our treatment, which results in a renormalized, effective Hamiltonian model is based on the reaction coordinate method. In our technique, applied to the nonequilibrium spin-boson model, a collective coordinate is extracted from each environment and added into the system… ▽ More We present a semi-analytical approach for studying quantum thermal energy transport beyond the weak system-bath coupling regime. Our treatment, which results in a renormalized, effective Hamiltonian model is based on the reaction coordinate method. In our technique, applied to the nonequilibrium spin-boson model, a collective coordinate is extracted from each environment and added into the system to construct an enlarged system. After performing additional Hamiltonian's truncation and transformation, we attain an effective two-level system with renormalized parameters, which is weakly coupled to its environments, thus can be simulated using a perturbative Markovian quantum master equation approach. We compare the heat current characteristics in our method to other techniques, and demonstrate that we properly capture strong system-bath signatures such as the turnover behavior of the heat current as a function of system-bath coupling strength. We further investigate the thermal diode effect and demonstrate that strong couplings moderately improve the rectification ratio relative to the weak coupling limit. The effective Hamiltonian method that we developed here offers fundamental insight into the strong coupling behavior, and is computationally economic. Applications of the method towards studying quantum thermal machines are anticipated. △ Less

Submitted 9 March, 2021; originally announced March 2021.

Comments: 19 pages, 10 figures

Harmonic chains and the thermal diode effect

Authors: Na'im Kalantar, Bijay Kumar Agarwalla, Dvira Segal

Abstract: Harmonic oscillator chains connecting two harmonic reservoirs at different constant temperatures cannot act as thermal diodes, irrespective of structural asymmetry. However, here we prove that perfectly harmonic junctions can rectify heat once the reservoirs (described by white Langevin noise) are placed under temperature gradients, which are asymmetric at the two sides, an effect that we term "te… ▽ More Harmonic oscillator chains connecting two harmonic reservoirs at different constant temperatures cannot act as thermal diodes, irrespective of structural asymmetry. However, here we prove that perfectly harmonic junctions can rectify heat once the reservoirs (described by white Langevin noise) are placed under temperature gradients, which are asymmetric at the two sides, an effect that we term "temperature-gradient harmonic oscillator diodes". This nonlinear diode effect results from the additional constraint -- the imposed thermal gradient at the boundaries. We demonstrate the rectification behavior based on the exact analytical formulation of steady state heat transport in harmonic systems coupled to Langevin baths, which can describe quantum and classical transport, both regimes realizing the diode effect under the involved boundary conditions. Our study shows that asymmetric harmonic systems, such as room-temperature hydrocarbon molecules with varying side groups and end groups, or a linear lattice of trapped ions may rectify heat by going beyond simple boundary conditions. △ Less

Submitted 26 February, 2021; originally announced March 2021.

Journal ref: Phys. Rev. E 103, 052130 (2021)

Coherences and the thermodynamic uncertainty relation: Insights from quantum absorption refrigerators

Authors: Junjie Liu, Dvira Segal

Abstract: The thermodynamic uncertainty relation, originally derived for classical Markov-jump processes, provides a trade-off relation between precision and dissipation, deepening our understanding of the performance of quantum thermal machines. Here, we examine the interplay of quantum system coherences and heat current fluctuations on the validity of the thermodynamics uncertainty relation in the quantum… ▽ More The thermodynamic uncertainty relation, originally derived for classical Markov-jump processes, provides a trade-off relation between precision and dissipation, deepening our understanding of the performance of quantum thermal machines. Here, we examine the interplay of quantum system coherences and heat current fluctuations on the validity of the thermodynamics uncertainty relation in the quantum regime. To achieve the current statistics, we perform a full counting statistics simulation of the Redfield quantum master equation. We focus on steady-state quantum absorption refrigerators where nonzero coherence between eigenstates can either suppress or enhance the cooling power, compared with the incoherent limit. In either scenario, we find enhanced relative noise of the cooling power (standard deviation of the power over the mean) in the presence of system coherence, thereby corroborating the thermodynamic uncertainty relation. Our results indicate that fluctuations necessitate consideration when assessing the performance of quantum coherent thermal machines. △ Less

Submitted 1 December, 2020; v1 submitted 29 November, 2020; originally announced November 2020.

Comments: 14 pages, 11 figures, comments and suggestions are welcome

Journal ref: Phys. Rev. E 103, 032138 (2021)

On the definitions and simulations of vibrational heat transport in nanojunctions

Authors: Na'im Kalantar, Bijay Kumar Agarwalla, Dvira Segal

Abstract: Thermal transport through nanosystems is central to numerous processes in chemistry, material sciences, electrical and mechanical engineering, with classical molecular dynamics as the key simulation tool. Here we focus on thermal junctions with a molecule bridging two solids that are maintained at different temperatures. The classical steady state heat current in this system can be simulated in di… ▽ More Thermal transport through nanosystems is central to numerous processes in chemistry, material sciences, electrical and mechanical engineering, with classical molecular dynamics as the key simulation tool. Here we focus on thermal junctions with a molecule bridging two solids that are maintained at different temperatures. The classical steady state heat current in this system can be simulated in different ways, either at the interfaces with the solids, which are represented by thermostats, or between atoms within the conducting molecule. We show that while the latter, intramolecular definition feasibly converges to the correct limit, the molecule-thermostat interface definition is more challenging to converge to the correct result. The problem with the interface definition is demonstrated by simulating heat transport in harmonic and anharmonic one-dimensional chains illustrating unphysical effects such as thermal rectification in harmonic junctions. △ Less

Submitted 28 August, 2020; originally announced August 2020.

Quantum Nondemolition Photon Counting With a Hybrid Electromechanical Probe

Authors: Junjie Liu, Hsing-Ta Chen, Dvira Segal

Abstract: Quantum nondemolition (QND) measurements of photons is a much pursued endeavor in the field of quantum optics and quantum information processing. Here we propose a novel hybrid optoelectromechanical platform that integrates a cavity system with a hybrid electromechanical probe for QND photon counting. Building upon a mechanical-mode-mediated nonperturbative electro-optical dispersive coupling, our… ▽ More Quantum nondemolition (QND) measurements of photons is a much pursued endeavor in the field of quantum optics and quantum information processing. Here we propose a novel hybrid optoelectromechanical platform that integrates a cavity system with a hybrid electromechanical probe for QND photon counting. Building upon a mechanical-mode-mediated nonperturbative electro-optical dispersive coupling, our protocol performs the QND photon counting measurement by means of the current-voltage characteristics of the probe. In particular, we show that the peak voltage shift of the differential conductance is linearly dependent on the photon occupation number, thus providing a sensitive measure of the photon number, especially in the strong optomechanical coupling regime. Given that our proposed hybrid system is compatible with state-of-the-art experimental techniques, we discuss its implementations and anticipate applications in quantum optics and polariton physics. △ Less

Submitted 14 November, 2020; v1 submitted 25 August, 2020; originally announced August 2020.

Comments: 7+4 pages, 2 figures, comments and suggestions are welcome

Journal ref: Phys. Rev. A 102, 061501 (2020)

Sharp negative differential resistance from vibrational mode softening in molecular junctions

Authors: Junjie Liu, Dvira Segal

Abstract: We unravel the critical role of vibrational mode softening in single-molecule electronic devices at high bias. Our theoretical analysis is carried out with a minimal model for molecular junctions, with mode softening arising due to quadratic electron-vibration couplings, and by developing a mean-field approach. We discover that the negative sign of the quadratic electron-vibration coupling coeffic… ▽ More We unravel the critical role of vibrational mode softening in single-molecule electronic devices at high bias. Our theoretical analysis is carried out with a minimal model for molecular junctions, with mode softening arising due to quadratic electron-vibration couplings, and by developing a mean-field approach. We discover that the negative sign of the quadratic electron-vibration coupling coefficient can realize at high voltage a sharp negative differential resistance (NDR) effect with a large peak-to-valley ratio. Calculated current-voltage characteristics, obtained based on ab initio parameters for a nitro-substituted oligo(phenylene ethynylene) junction, agree very well with measurements. Our results establish that vibrational mode softening is a crucial effect at high voltage, underlying NDR, a substantial diode effect, and the breakdown of current-carrying molecular junctions. △ Less

Submitted 22 May, 2020; originally announced May 2020.

Comments: Main text: 6 pages, 3 figures, 1 table; Supplemental material: 9 pages, 8 figures

Journal ref: Nano Lett. 20. 6128 (2020)

Dissipation-engineering of nonreciprocal quantum dot circuits: An input-output approach

Authors: Junjie Liu, Dvira Segal

Abstract: Nonreciprocal effects in nanoelectronic devices offer unique possibilities for manipulating electron transport and engineering quantum electronic circuits for information processing purposes. However, a lack of rigorous theoretical tools is hindering this development. Here, we provide a general input-output description of nonreciprocal transport in solid-state quantum dot architectures, based on q… ▽ More Nonreciprocal effects in nanoelectronic devices offer unique possibilities for manipulating electron transport and engineering quantum electronic circuits for information processing purposes. However, a lack of rigorous theoretical tools is hindering this development. Here, we provide a general input-output description of nonreciprocal transport in solid-state quantum dot architectures, based on quantum optomechanical analogs. In particular, we break reciprocity between coherently-coupled quantum dots by dissipation-engineering in which these (so-called) primary dots are mutually coupled to auxiliary, damped quantum dots. We illustrate the general framework in two representative multiterminal noninteracting models, which can be used as building blocks for larger circuits. Importantly, the identified optimal conditions for nonreciprocal behavior hold even in the presence of additional dissipative effects that result from local electron-phonon couplings. Besides the analysis of the scattering matrix, we show that a nonreciprocal coupling induces unidirectional electron flow in the resonant transport regime. Altogether, our analysis provides the formalism and working principles towards the realization of nonreciprocal nanoelectronic devices. △ Less

Submitted 11 April, 2020; originally announced April 2020.

Comments: 18 pages, 10 figures, comments and suggestions are welcome

Journal ref: Phys. Rev. B 102, 125416 (2020)

Thermodynamic uncertainty relation in atomic-scale quantum conductors

Authors: Hava Meira Friedman, Bijay Kumar Agarwalla, Ofir Shein-Lumbroso, Oren Tal, Dvira Segal

Abstract: The thermodynamic uncertainty relation (TUR) is expected to hold in nanoscale electronic conductors, when the electron transport process is quantum coherent and the transmission probability is constant (energy and voltage independent). We present measurements of the electron current and its noise in gold atomic-scale junctions and confirm the validity of the TUR for electron transport in realistic… ▽ More The thermodynamic uncertainty relation (TUR) is expected to hold in nanoscale electronic conductors, when the electron transport process is quantum coherent and the transmission probability is constant (energy and voltage independent). We present measurements of the electron current and its noise in gold atomic-scale junctions and confirm the validity of the TUR for electron transport in realistic quantum coherent conductors. Furthermore, we show that it is beneficial to present the current and its noise as a TUR ratio in order to identify deviations from noninteracting-electron coherent dynamics. △ Less

Submitted 1 February, 2020; originally announced February 2020.

Comments: 9 pages, 5 figures

Journal ref: Phys. Rev. B 101, 195423 (2020)

Experimental study of the thermodynamic uncertainty relation

Authors: Soham Pal, Sushant Saryal, Dvira Segal, T. S. Mahesh, Bijay Kumar Agarwalla

Abstract: A cost-precision trade-off relationship, the so-called thermodynamic uncertainty relation (TUR), has been recently discovered in stochastic thermodynamics. It bounds certain thermodynamic observables in terms of the associated entropy production. In this work, we experimentally study the TUR in a two-qubit system using an NMR setup. Each qubit is prepared in an equilibrium state, but at different… ▽ More A cost-precision trade-off relationship, the so-called thermodynamic uncertainty relation (TUR), has been recently discovered in stochastic thermodynamics. It bounds certain thermodynamic observables in terms of the associated entropy production. In this work, we experimentally study the TUR in a two-qubit system using an NMR setup. Each qubit is prepared in an equilibrium state, but at different temperatures. The qubits are then coupled, allowing energy exchange (in the form of heat). Using the quantum state tomography technique we obtain the moments of heat exchange within a certain time interval and analyze the relative uncertainty of the energy exchange process. We find that generalized versions of the TUR, which are based on the fluctuation relation, are obeyed. However, the specialized TUR, a tighter bound that is valid under specific dynamics, is violated in certain regimes of operation, in excellent agreement with analytic results. Altogether, this experiment-theory study provides a deep understanding of heat exchange in quantum systems, revealing favorable noise-dissipation regimes of operation. △ Less

Submitted 18 December, 2019; originally announced December 2019.

Comments: 9 pages, 6 figures

Journal ref: Phys. Rev. Research 2, 022044 (2020)

Generalized input-output method: A new route to quantum transport junctions

Authors: Junjie Liu, Dvira Segal

Abstract: In this work, we put forward a generalized input-output method (GIOM) for studying charge transport in molecular junctions accounting for strong electron-vibration interactions and including electronic and phononic environments. The method radically expands the scope of the input-output theory, which was originally proposed to treat quantum optic problems. Based on the GIOM we derive a Langevin-ty… ▽ More In this work, we put forward a generalized input-output method (GIOM) for studying charge transport in molecular junctions accounting for strong electron-vibration interactions and including electronic and phononic environments. The method radically expands the scope of the input-output theory, which was originally proposed to treat quantum optic problems. Based on the GIOM we derive a Langevin-type equation of motion for molecular operators, which posses a great generality and accuracy, and permits the derivation of a stationary charge current expression involving only two types of transfer rates. Furthermore, we devise the so-called "Polaron Transport in Electronic Resonance" (PoTER) approximation, which allows to feasibly simulate electron dynamics in generic tight-binding models with strong electron-vibration interactions. For short chains, the charge current reduces to known limits and reasonably agrees with exact numerical simulations. For extended junctions the current displays a turnover from phonon-assisted to phonon-suppressed transport. Nevertheless, the onset of ohmic behavior requires extensions beyond the PoTER approximation. As an additional application, we consider a cavity-coupled molecule junction. Here we identify a cavity-induced suppression of charge current in the single-site case, and observe signatures of polariton formation in the current-voltage characteristics in the strong light-matter coupling regime. A critical understanding gained from the GIOM-PoTER scheme is that the single-site vibrationally-coupled model is deceptively simpler, and amenable to approximations than multi-site models. Therefore, benchmarking of methods should not be concluded with the single-site case. The work manifests that the input-output framework, which is normally employed in quantum optics, can serve as a powerful and feasible tool in the realm of electron transport junctions. △ Less

Submitted 26 November, 2019; originally announced November 2019.

Comments: 25 pages, 7 figures, comments are welcome

Journal ref: Phys. Rev. B 101, 155406 (2020)

Cooling condition for multilevel quantum absorption refrigerators

Authors: Hava Meira Friedman, Dvira Segal

Abstract: Models for quantum absorption refrigerators serve as test beds for exploring concepts and developing methods in quantum thermodynamics. Here, we depart from the minimal, ideal design and consider a generic multilevel model for a quantum absorption refrigerator, which potentially suffers from lossy processes. Based on a full-counting statistics approach, we derive a formal cooling condition for the… ▽ More Models for quantum absorption refrigerators serve as test beds for exploring concepts and developing methods in quantum thermodynamics. Here, we depart from the minimal, ideal design and consider a generic multilevel model for a quantum absorption refrigerator, which potentially suffers from lossy processes. Based on a full-counting statistics approach, we derive a formal cooling condition for the refrigerator, which can be feasibly evaluated analytically and numerically. We exemplify our approach on a three-level model for a quantum absorption refrigerator that suffers from different forms of non-ideality (heat leakage, competition between different cooling pathways), and examine the cooling current with different designs. This study assists in identifying the cooling window of imperfect thermal machines. △ Less

Submitted 20 September, 2019; originally announced September 2019.

Comments: 12 pages, 4 figures, 2 appendices

Journal ref: Phys. Rev. E 100, 062112 (2019)

Thermodynamic uncertainty relation in thermal transport

Authors: Sushant Saryal, Hava Friedman, Dvira Segal, Bijay Kumar Agarwalla

Abstract: We use the fundamental nonequilibrium steady-state fluctuation symmetry and derive a condition on the validity of the thermodynamic uncertainty relation (TUR) in thermal transport problems, classical or quantum alike. We test this condition and study the breakdown of the TUR in different thermal transport junctions of bosonic and electronic degrees of freedom. First, we prove that the TUR is valid… ▽ More We use the fundamental nonequilibrium steady-state fluctuation symmetry and derive a condition on the validity of the thermodynamic uncertainty relation (TUR) in thermal transport problems, classical or quantum alike. We test this condition and study the breakdown of the TUR in different thermal transport junctions of bosonic and electronic degrees of freedom. First, we prove that the TUR is valid in harmonic oscillator junctions. In contrast, in the nonequilibrium spin-boson model, which realizes many-body effects, it is satisfied in the Markovian limit, but violations arise as we tune (reduce) the cutoff frequency of the thermal baths, thus observing non-Markovian dynamics. Finally, we consider heat transport by noninteracting electrons in a tight-binding chain model. Here we show that the TUR is feasibly violated by tuning e.g. the hybridization energy of the chain to the metal leads. These results manifest that the validity of the TUR relies on the statistics of the participating carriers, their interaction, and the nature of their couplings to the macroscopic contacts (metal electrodes, phonon baths). △ Less

Submitted 24 July, 2019; originally announced July 2019.

Comments: 10 pages, 3 figures

Journal ref: Phys. Rev. E 100, 042101 (2019)

Origin of the Anomalous Electronic Shot Noise in Atomic-Scale Junctions

Authors: Anqi Mu, Ofir Shein Lumbroso, Oren Tal, Dvira Segal

Abstract: Fluctuations pose fundamental limitations in making sensitive measurements, yet at the same time, noise unravels properties that are inaccessible at the level of the averaged signal. In electronic devices, shot noise arises from the discrete nature of charge carriers and it increases linearly with the applied voltage according to the celebrated Schottky formula. Nonetheless, measurements of shot n… ▽ More Fluctuations pose fundamental limitations in making sensitive measurements, yet at the same time, noise unravels properties that are inaccessible at the level of the averaged signal. In electronic devices, shot noise arises from the discrete nature of charge carriers and it increases linearly with the applied voltage according to the celebrated Schottky formula. Nonetheless, measurements of shot noise in atomic-scale junctions at high voltage reveal significant nonlinear (anomalous) behavior, which varies from sample to sample, and has no specific trend. Here, we provide a viable, unifying explanation for these diverse observations based on the theory of quantum coherent transport. Our formula for the anomalous shot noise relies on---and allows us to resolve---two key characteristics of a conducting junction: The structure of the transmission function at the vicinity of the Fermi energy and the asymmetry of the bias voltage drop at the contacts. We tested our theory on high voltage shot noise measurements on Au atomic scale junctions and demonstrated a quantitative agreement, recovering both the enhancement and suppression of shot noise as observed in different junctions. The good theory-experiment correspondence supports our modelling and emphasizes that the asymmetry of the bias drop on the contacts is a key factor in nanoscale electronic transport, which may substantially impact electronic signals even in incomplex structures. △ Less

Submitted 18 July, 2019; originally announced July 2019.

Comments: arXiv admin note: text overlap with arXiv:1902.06312

Thermodynamic uncertainty relation in quantum thermoelectric junctions

Authors: Junjie Liu, Dvira Segal

Abstract: Recently, a thermodynamic uncertainty relation (TUR) has been formulated for classical Markovian systems demonstrating trade-off between precision (current fluctuation) and cost (dissipation). Systems that violate the TUR are interesting as they overcome another trade-off relation concerning the efficiency of a heat engine, its power, and its stability (power fluctuations). Here, we analyze the ro… ▽ More Recently, a thermodynamic uncertainty relation (TUR) has been formulated for classical Markovian systems demonstrating trade-off between precision (current fluctuation) and cost (dissipation). Systems that violate the TUR are interesting as they overcome another trade-off relation concerning the efficiency of a heat engine, its power, and its stability (power fluctuations). Here, we analyze the root, extent, and impact on performance of TUR violations in quantum thermoelectric junctions at steady state. Considering noninteracting electrons, first we show that only the "classical" component of the current noise, arising from single-electron transfer events follows the TUR. The remaining, "quantum" part of current noise is therefore responsible for the potential violation of TUR in such quantum systems. Next, focusing on the resonant transport regime we determine the parameter range in which the violation of the TUR can be observed---for both voltage-biased junctions and thermoelectric engines. We illustrate our findings with exact numerical simulations of a serial double quantum dot system. Most significantly, we demonstrate that the TUR always holds in noninteracting thermoelectric generators when approaching the thermodynamic efficiency limit. △ Less

Submitted 26 April, 2019; originally announced April 2019.

Journal ref: Phys. Rev. E 99, 062141 (2019)

Photon statistics of a double quantum dot micromaser: Quantum treatment

Authors: Bijay Kumar Agarwalla, Manas Kulkarni, Dvira Segal

Abstract: A semiconductor single-atom micromaser consists of a microwave cavity coupled to a gain medium, a double quantum dot driven out of equilibrium by a bias voltage. The masing threshold of this system was recently probed by measuring photon statistics in the cavity [Y-Y. Liu \textit{et al}, Phys. Rev. Lett. 119, 097702 (2017)]. In this paper, we develop an in-depth, rigorous understanding of this exp… ▽ More A semiconductor single-atom micromaser consists of a microwave cavity coupled to a gain medium, a double quantum dot driven out of equilibrium by a bias voltage. The masing threshold of this system was recently probed by measuring photon statistics in the cavity [Y-Y. Liu \textit{et al}, Phys. Rev. Lett. 119, 097702 (2017)]. In this paper, we develop an in-depth, rigorous understanding of this experiment and related works. First, we use a semiclassical theory and study transmission spectroscopy. This approach allows us to derive the masing threshold condition for arbitrary temperature and voltage bias, and expose microscopic principles required for realizing photon gain and thereby a photon amplifier. Next, by employing the quantum master equation approach we extend the Scully-Lamb quantum theory of a laser to the present setup, and investigate the statistics of emitted photons below and above the masing threshold as a function of experimentally tunable parameters. Although our focus is primarily on hybrid quantum dot circuit - quantum electrodynamics systems, our approach is adaptable to other light-matter systems where the gain medium consists of a mesoscopic structure. △ Less

Submitted 19 March, 2019; originally announced March 2019.

Comments: 14 pages, 7 figures

Journal ref: Phys. Rev. B 100, 035412 (2019)

Anomalous electronic shot noise in resonant tunneling junctions

Authors: Anqi Mu, Dvira Segal

Abstract: We study the behavior of shot noise in resonant tunneling junctions far from equilibrium. Quantum-coherent elastic charge transport can be characterized by a transmission function, that is the probability for an incoming electron at a given energy to tunnel through a potential barrier. In systems such as quantum point contacts, electronic shot noise is oftentimes calculated based on a constant (en… ▽ More We study the behavior of shot noise in resonant tunneling junctions far from equilibrium. Quantum-coherent elastic charge transport can be characterized by a transmission function, that is the probability for an incoming electron at a given energy to tunnel through a potential barrier. In systems such as quantum point contacts, electronic shot noise is oftentimes calculated based on a constant (energy independent) transmission probability, a good approximation at low temperatures and under a small bias voltage. Here, we generalize these investigations to far from equilibrium settings by evaluating the contributions of electronic resonances to the electronic current noise. Our study extends canonical expressions for the voltage-activated shot noise and the recently discovered delta-T noise to the far from equilibrium regime, when a high bias voltage or a temperature difference is applied. In particular, when the Fermi energy is located on the shoulder of a broad resonance, we arrive at a formula for the shot noise revealing anomalous-nonlinear behavior at high bias voltage. △ Less

Submitted 17 February, 2019; originally announced February 2019.