Talks Monday June 27

Postdoc @ Jagiellonian University, Kraków 

Monday June 27 – 12.30 BST

Optimizing thermalizations 

The standard dynamical approach to quantum thermodynamics is based on Markovian master equations describing the thermalization of a system weakly coupled to a large environment. Here, based on the newly introduced notion of continuous thermomajorization, we obtain necessary and sufficient conditions for the existence of such a thermalization process transforming between given initial and final energy populations of the system. These include standard entropy production relations as a special case, but also yield a complete, continuous family of entropic functions that need to monotonically increase during the dynamics. Importantly, we significantly simplify these conditions by identifying a finitely verifiable set of constraints governing non-equilibrium transformations under master equations. What is more, the framework is also constructive, i.e., it returns an explicit set of elementary controls realizing any allowed transformation. We also present an algorithm that in a finite number of steps allows one to construct all energy distributions achievable from a given initial state via Markovian thermalization processes. The algorithm can be deployed to solve complex optimization problems in out-of-equilibrium setups and it returns explicit elementary control sequences realizing optimal transformations. We illustrate this by finding optimal protocols in the context of cooling, work extraction and catalysis. The same tools also allow one to quantitatively assess the role played by memory effects in the performance of thermodynamic protocols. We obtained exhaustive solutions on a laptop machine for systems with dimension d ≤ 7, but with heuristic methods one could access much higher d. 
 
arXiv:2111.12130 

Faculty @ Institute for Basic Science

Monday June 27 – 12.00 BST

Effects of measurements on quantum devices

In this talk, I’ll introduce a method, known as repeated contacts, that allows us to infer the sum of the values of an observable taken during contacts with a pointer state [1]. I’ll first show the superiority of this approach with the results of the same number of generalized Gaussian measurements of the considered observable. The proposed repeated contact approach could be beneficial to measure the work output of a quantum Otto heat engine and I’ll demonstrate this using a two-level system as the working substance [2]. I’ll discuss the imperfect thermalizing engine under Landau-Zener work strokes and show that the repeated contact scheme is minimally invasive and gives enhanced performance metrics (efficiency, power output, and reliability) as compared to the standard repeated generalized Gaussian measurements. Lastly, I’ll consider the charging of a quantum battery using an Otto engine and demonstrate that periodic measurement on the battery speeds up its charging of internal energy, but sacrifices the ergotropy [3]. Overall, I’ll show the importance of considering the effects of quantum measurements using different measurement protocols on the performance of a machine. 

[1] J. Thingna and P. Talkner, Phys. Rev. A 102, 012213 (2020). 
[2] J. Son, P. Talkner, and J. Thingna Phys. Rev. X Quantum 2, 040328 (2021). 
[3] J. Son, P. Talkner, and J. Thingna (in preparation). 

Faculty @ Hebrew University 

Monday June 27 – 11.30 BST

Controlling the uncontrollable: Quantum control of open system dynamics 

Control of an open quantum system is essential in contemporary science and technology. We demonstrate such control employing a thermodynamically consistent framework. We specifically address the fact that the control drive modifies the interaction with environment. The effect is incorporated within the dynamical equation, leading to control dependent dissipation. Thermodynamics is reflected by a unidirectional flow of energy to the environment. The dynamical equations of motion serves as the key element for open system control. The control paradigm is displayed by analyzing 
entropy changing state to state transformations, heating and cooling N-levels systems. A crucial task is generating quantum gates under dissipative conditions. We demonstrate both non-unitary reset maps with complete memory loss and a universal set of single and double qubit unitary gates. 

Faculty @ Ecole Normale Supérieure, Lyon, France

Monday June 27 – 10.45 BST

Energetics of the driving field during a single qubit gate and a measurement powered engine

Qubits are physical, a quantum gate thus not only acts on the information carried by the qubit but also on its energy. What is then the corresponding flow of energy between the qubit and the controller that implements the gate? In this talk, we exploit a superconducting platform to answer this question in the case of a quantum gate realized by a resonant drive field. During the gate, the superconducting qubit becomes entangled with the microwave drive pulse so that there is a quantum superposition between energy flows. We measure the energy change in the drive field conditioned on the outcome of a projective qubit measurement. We demonstrate that the drive’s energy change associated with the measurement backaction can exceed by far the energy that can be extracted by the qubit. This can be understood by considering the qubit as a weak measurement apparatus of the driving field. 

In the second part of the talk, we will discuss an experiment that realizes an engine able to extract work from the measurement backaction of a qubit. The extracted work is directly measured in the reflection of a resonant field that drives the qubit.

Faculty @ School of Mathematics, Statistics and Physics, Newcastle

Monday June 27 – 9.45 BST

Quantum critical battery: Harnessing non-adiabatic excitations 

Crossing a quantum critical point in finite time challenges the adiabatic condition due to the closing of the energy gap, which ultimately results in the formation of excitations. Such non-adiabatic excitations are typically deemed detrimental in many scenarios, and consequently several strategies have been put forward to circumvent their formation.  Here, however, we show how these non-adiabatic excitations — originated from the failure to meet the adiabatic condition due to the presence of a quantum critical point — can be controlled and thus harnessed to perform certain tasks advantageously. We focus on closed cycles reaching the quantum critical point of fully-connected model to analyse a quantum battery that is loaded by approaching a quantum critical point. We show that the stored and extractable work increases exponentially via repeating cycles. Our results can readily be implemented in different experimental platforms as well as highlight the rich interplay between quantum thermodynamics and critical nonequilibrium dynamics. 
 
O. Abah,  G. De Chiara, M. Paternostro and R. Puebla, Harnnessing non-adiabatic excitations promoted by a quantum critical point, arXiv:2105.00362 

Postgraduate @ Trinity College Dublin 

Monday June 27 – 9.15 BST

Dephasing-enhanced performance in quasiperiodic thermal machines 

Understanding and controlling quantum transport in low-dimensional systems is pivotal for heat management at the nanoscale. One promising strategy to obtain the desired transport properties is to engineer particular spectral structures. In this work we are interested in quasiperiodic disorder — incommensurate with the un- derlying periodicity of the lattice — which induces fractality in the energy spectrum. A well known example is the Fibonacci model which, despite being non-interacting, yields anomalous diffusion with a continuously varying dynamical exponent smoothly crossing over from superdiffusive to subdiffusive regime as a function of potential strength. We study the finite-temperature electric and heat transport of this model in linear response in the absence and in the presence of dephasing noise due to inelastic scattering. The dephasing causes both thermal and electric transport to become diffusive, thereby making thermal and electrical conductivities finite in the thermodynamic limit. Thus, in the subdiffusive regime it leads to enhancement of transport. We find that the thermal and electric conductivities have multiple peaks as a function of dephasing strength. Remarkably, we observe that the thermal and electrical conductivities are not proportional to each other, a clear violation of Wiedemann-Franz law, and the position of their maxima can differ. We argue that this feature can be utilized to enhance performance of quantum thermal machines. In particular, we show that by tuning the strength of the dephasing noise we can enhance the performance of the device in regimes where it acts as an autonomous refrigerator. 

Postgraduate @ Aalto University 

Monday June 27 – 8.45 BST

Effect of high impedance environment on photonic heat transport

Due to the fast growth of quantum technologies, heat transport, regardless of its carriers in such systems, has become an intensive subject of study. This work focuses on studying microwave photon-mediated heat transport and its deviation from the quantum limit in systems with a small Josephson junction immersed into a high impedance environment. The variations of the quantum limit are due to Coulomb blockade effects caused by the high impedance environment. For our purpose, the environment’s resistance is greater than the resistance quantum 6.45 KΩ. Therefore, these platforms are suitable for studying exchange heat between the two environments, thanks to the correspondence between Plank’s black body radiation and Johnson-Nysquist noise yielding from the resistive elements. Photonic heat transport in this type of superconducting circuit has been studied theoretically [1-4] and experimentally [5,6] in the low impedance regime. The experimental work succeeded by Meschke et al. [5] was demonstrated heat conductance due to photons. However, the maximum limit imposed by the universal quantum limit of thermal conductance did not reach due to the impedance mismatch provided by the element used to tune the heat flow. Afterward, Timofeev et al. [6] experimentally demonstrated the vital role of impedance matching to get the heat transport quantum limited. In the present work, we study phenomena that limit quantum heat conductance when a better impedance matching for a tunable circuit such as Ref [5] is implemented. We have obtained the photonic heat conduction closer to the quantum limit by introducing higher impedance resistors. 

1.      D. R. Schmidt, R. J. Schoelkopf, and A. N. Cleland, Photon-mediated thermal relaxation of electrons in nanostructures, Phys. Rev. Lett. 93, 045901 (2004). 
2.      T. Ojanen and A. P. Jauho, Mesoscopic photon heat transistor, Phys. Rev. Lett. 100, 155902  (2008). 
3.      L. M. Pascal, H. Courtois, and F. W. Hekking, Circuit approach to photonic heat transport, Phys. Rev. B 83, 125113 (2011). 
4.      G. Thomas, J. P. Pekola, and D. S. Golubev, Photonic heat transport across a Josephson junction, Phys. Rev. B 100, 94508 (2019). 
5.      M. Meschke, W. Guichard, and J. P. Pekola, Single-mode heat conduction by photons, Nature 444, 187 (2006). 
6.      A. V. Timofeev, M. Helle, M. Meschke, M. Möttönen, and J. P. Pekola, Phys. Rev. Lett. 102, 200801 (2009). 

Faculty @ Chalmers University, Sweden

Monday June 27 – 8.00 BST

Heat and charge currents across a QPC: opportunities for thermoelectrics and readout of screening effects

Quantum point contacts have a prominent role in electronic devices and act as beam splitters and energy filters. They are therefore simple examples for thermoelectric mesoscopic devices, where they were shown to maximise the possible power output. In this talk, I will present some of their properties for thermoelectric steady state heat engines and hot-carrier photovoltaics. In particular, I will show, how QPC-based thermoelectric heat engines perform in terms of precision, demonstrating a novel constraint of the “thermodynamic uncertainty relation” in the linear response regime of an operating engine [1]. I will furthermore show how the efficiency of energy conversion is influenced when the energy distribution of the heat source is described by a non thermal state as it is the case for example in hot-carrier solar cells [2]. 

When the QPC based thermoelectric device is operated at large voltage and/or temperature biases, the transmission of the QPC is modified due to screening effects. These modifications, which can have electrostatic as well as quantum origin, strongly depend on different conductor properties and are typically hard to access from experiments. In the second part of my talk, I will present a proposal how to readout these screening effects by exploiting tunable corrections to the thermoelectric linear-response coefficients, which occur when an additional ac signal is applied via a third terminal [3]. 

[1] S. Kheradsoud, N. Dashti, M. Misiorny, P. P. Potts, J. Splettstoesser, P. Samuelsson: Entropy 21, 777 (2019). 

[2] L. Tesser. R. S. Whitney, J. Splettstoesser, in preparation. 

[3] N. Dashti, M. Acciai, S. Kheradsoud, M. Misiorny, P. Samuelsson, J. Splettstoesser: Phys. Rev. Lett. 127, 246802 (2021).