Talks Wednesday June 29

Postdoc @ Institute for Cross-Disciplinary Physics and Complex Systems, IFISC (UIB-CSIC) 

Wednesday June 29 – 12.30 BST

Non-Abelian Quantum Transport and Thermosqueezing Effects 

Nonequilibrium classical systems support the transport of particles (e.g., electrons) and heat. Combining these two currents can lead to interesting physical applications, known as thermoelectricity. These include the Seebeck and Peltier effects, which are used in several electronic devices. Some examples are thermocouples and thermopiles useful for measuring temperature, or thermoelectric generators, used for recycling waste heat in power plants and automobiles. Quantum systems, in addition to heat and particles, support the transport of other types of excitations, with more exotic properties. These include magnetization currents, or radiation squeezing, a widely used property in quantum technologies with roots in the uncertainty principle. 
By combining heat and squeezing currents, we derive a set of thermosqueezing effects, completely analogous to the thermoelectric case [1].  Our key insight is to use generalized Gibbs ensembles with noncommuting charges as the basic building blocks and strict charge-preserving unitaries in a collisional setup, building from previous considerations on squeezed thermal reservoirs [2]. These effects may have applications for new sensing technologies and for heat-to-work conversion in the quantum regime. Furthermore, an entire theoretical formalism for studying the joint transport of currents associated with observables that do not commute at the quantum level (the so-called non-Abelian charges) is formulated. This framework develops on the theory of linear transport processes, put forth by Onsager almost nine decades ago, and extends it to more general kinds of quantum processes. Remarkably, it is demonstrated that, within this framework, quantum coherence in the form of noncommutativity can lead to a reduction in dissipation. Therefore, the results presented here open new avenues for the thermodynamic description and exploitation of quantum effects. 

[1] Gonzalo Manzano, Juan M.R. Parrondo, and Gabriel T. Landi, PRX Quantum 3, 010304 (2022). 
[2] Gonzalo Manzano, Phys. Rev. E 98, 042123 (2018). 

Postdoc @ LENS – University of Florence 

Wednesday June 29 – 12.00 BST

Measuring negative quasiprobabilities with a diamond spin 

One of the most fascinating issues of quantum thermodynamics is how to capture the effect of quantum coherence and quantum correlations on thermodynamic processes occurring at the nanoscale. 
This is a challenging question, because conventional methods to study energy exchange such as the two-point-measurement (TPM) scheme unavoidably destroy quantum coherence terms in the initial state [1]. A possible solution to this problem is the introduction of Kirkwood-Dirac quasiprobabilities (KDQ). Negative and/or complex values of the KDQ have been shown to imply proofs of non-classicality [2], but such values have not been observed yet in experiments. 
Here we resort to the solid-state spin platform based on optically-active NV center in diamond,  recently employed in studies on energy fluctuation relations [3-5], to demonstrate a weak-TPM protocol [6] to measure the KDQ, both in a two- and a three-level system. We also explore the possibility of implementing an interferometric scheme for the measurement of KQP and moments of their statistics, using a nuclear spin as an ancillary system. Our results represent the first experimental measurement of KDQ using a weak-TPM scheme. 

[1] M. Perarnau-Llobet, E. Bäumer, K. V. Hovhannisyan, M. Huber, A. Acin, PRL 118, 070601 (2017) 
[2] A. Levy, and M. Lostaglio, PRX Quantum 1, 010309 (2020) 
[3] S. Hernández-Gómez, S. Gherardini, F. Poggiali, F. S.Cataliotti, A. Trombettoni, P. Cappellaro, and N. Fabbri, Phys. Rev. Research 2, 023327 (2020) 
[4] S. Hernández-Gómez, N. Staudenmaier, M. Campisi, N. Fabbri, New J. Phys. 23 065004 (2021) 
[5] S. Hernández-Gómez, S. Gherardini, N. Staudenmaier, F. Poggiali, M. Campisi, A. Trombettoni, F. S. Cataliotti, P. Cappellaro 
[6] A. Belenchia, S. Gherardini, S. Hernández-Gómez, N. Fabbri,  A. Levy, M. Lostaglio, in preparation. 

Faculty @ CNRS 

Wednesday June 29 – 11.30 BST

Quantum technologies need a quantum energy initiative

Quantum technologies are currently the object of high expectations from governments and private companies, as they hold the promise to shape safer and faster ways to exchange and treat information. However, despite its major potential impact for industry and society, the question of their energetic footprint has remained in a blind spot of current deployment strategies. In this talk, i will present why quantum technologies must urgently plan for the creation and structuration of a transverse quantum energy initiative, connecting quantum thermodynamicists, computer scientists, experimenters and engineers. Such initiative is the only path towards sustainable quantum technologies, help reducing the cost of classical information processing, and possibly bring out an energetic quantum advantage. 
 
arXiv:2111.09241

Faculty @ Aalto University, Finland

Wednesday June 29 – 10.45 BST

Counting electrons and photons in nanocircuits – experiments on non-equilibrium thermodynamics 

I describe recent experiments in our laboratory on how non-equilibrium excitations in low temperature superconductors can be observed by counting emitted electrons one-by-one [1]. Interesting statistics and long-term relaxation of rare events reveal new features of quasiparticle excitations in the system, precisely characterized in the experiment, but not fully in line with earlier understanding. In the second part of the talk I describe our current efforts to move from steady-state bolometric measurements of thermal microwave photons [2] to observation of single events by a nano-calorimeter. I describe the sensitive thermometer on the electronic absorber of this calorimeter achieving the fundamental lower bound of energy fluctuations arising from fluctuation-dissipation theorem [3]. The detector is in principle capable of detecting absorption events of single photons in a circuit in a continuous manner. We describe a recent proposal of splitting the energy of the photon to two absorbers to significantly boost the signal-to-noise ratio in a temperature cross-correlation measurement [4]. 

[1] E. T. Mannila, P. Samuelsson et al., Nat. Phys. 18, 145 (2022). 
[2] JP and Bayan Karimi, Rev. Mod. Phys. 93, 041001 (2021). 
[3] Bayan Karimi et al., Nat. Commun. 11, 367 (2020). 
[4] JP and Bayan Karimi, Phys. Rev. X 12, 011026 (2022). 

Postgraduate @ University of Rochester 

Wednesday June 29 – 9.45 BST

Stochastic path-integral analysis of the continuously monitored quantum harmonic oscillator

We look at the evolution of a quantum harmonic oscillator in general Gaussian states undergoing simultaneous weak continuous position and momentum measurements. The conditional state dynamics can be described in terms of stochastic diffusive evolution of the position and momentum expectation values. We extend the Chantasri-Dressel-Jordan stochastic path integral formalism (Chantasri et al., 2013, 2015) to this continuous variable system and construct a stochastic action and Hamiltonian, thereby characterizing the statistics of the measurement process. This stochastic path integral formalism helps us find the most-likely state dynamics and the final state probability densities of the system undergoing measurements. Numerical simulations confirm our analytical results. Our findings provide insights into the energetics of the measurement process, motivating their importance for quantum measurement engines/refrigerators construction. 

[1] T. Karmakar, P. Lewalle, and A. N. Jordan, PRX Quantum, 3, 010327 (2022). 
[2] A. Chantasri, J. Dressel, and A. N. Jordan, Phys. Rev. A 88, 042110 (2013). 
[3] A. Chantasri and A. N. Jordan, Phys. Rev. A 92, 032125 (2015). 

Faculty @ Indian Institute of Science Education and Research (IISER) Pune

Wednesday June 29 – 9.15 BST

Universal bounds on fluctuations in thermal machines and its connection to thermodynamic uncertainty relation

Trade-off relations involving the relative uncertainty of currents and the associated entropy production have been of enormous interest for the past few years in the field of non-equilibrium stochastic thermodynamics. It is now realized, for example, that the optimization of heat engines should balance power, efficiency as well as power fluctuations.  In this talk, I will talk about multi-affinity-driven continuous thermal machines and show that the relative fluctuations of individual currents for thermal machines are not independent but follow strict bounds, giving rise to a new universal bound for the mean efficiency of thermal machines and is tighter than the well known Carnot bound as predicted by macroscopic thermodynamics. I will also talk about the extension of this bound for time-reversal symmetry-breaking systems. 
 
1. Universal bounds on fluctuations in continuous thermal machines, S. Saryal et al Phys. Rev. Lett, 127, 190603 (2021). 
2. Bounds on fluctuations for finite-time quantum Otto cycle, S. Saryal and Bijay Agarwalla, Phys. Rev. E. Lett 103 L060103 (2021). 
3. Universal bounds on fluctuations for machines with broken time-reversal symmetry, Accepted in Phys. Rev. E, arxiv: 2110.05297 
S. Saryal, S. Mohanta, and Bijay Kumar Agarwalla 
4. Universal bounds on fluctuations for machines with broken time-reversal symmetry, S. Saryal, S. Mohanta, and Bijay Agarwalla, arXiv: 2110.05297. 

Faculty @ UCL

Wednesday June 29 – 8.45 BST

Zeno effects and entropy production for Brownian trajectories of a physical density matrix

If we regard the (reduced) density matrix of an open quantum system as a representation of real-world physical properties and not merely as a provider of probabilities for strong projective measurements, then it makes sense to imagine its time evolution to be stochastic, as it responds to physical interactions with an underspecified environment with many similar degrees of freedom. The uncertain evolution of the state of any physical system may be characterised by the production of stochastic entropy, and an ensemble of evolutions of a density matrix can be similarly treated. We use the quantum state diffusion framework to describe such continuous Brownian trajectories. We show that various stochastic couplings between the density matrix and its environment can lead to behaviour that corresponds to thermalisation or to eigenstate selection by measurement of an observable, with none of the interpretational issues that often distinguish these two effects. By regarding the density matrix as a stochastic physical property, we study simple two- and three-level systems undergoing deterministic Hamiltonian evolution and stochastic environmental disturbance, and demonstrate Zeno effects and entropy production that would not be apparent in a more traditional ensemble-averaged description of density matrix evolution.

Faculty @ RIKEN, Japan

Wednesday June 29 – 8.00 BST

Effective cancellation of the current-dissipation trade-off by quantum coherence

In recent years, various kinds of trade-off relations between the current and the entropy production have been derived in the field of stochastic thermodynamics. One of the important insights that can be gained from these trade-off relations is that a simultaneous maximization of the thermodynamic efficiency and the output power of heat engines is not possible. While the trade-off relations have been successful in many applications ranging from biomolecular processes to thermoelectric junctions, the effect of quantum coherence was not clear. Here, we focus on the current-dissipation trade-off relation which sets a bound on both enhancing the heat current and suppressing the entropy production, simultaneously. By analyzing the effect of coherence on the current-dissipation ratio, we find that coherence between different energy eigenstates always increases the entropy production, thereby worsening the current-dissipation ratio. On the other hand, we find that coherence between degenerate states may improve the current-dissipation ratio. When the amount of coherence between degenerate states is large enough, the trade-off relation is effectively cancelled, and the heat current scales at the same order as the particle number while suppressing the entropy production at a constant order. By utilizing the above scaling analysis, we show a quantum heat engine model that asymptotically reaches the Carnot efficiency with finite output power, and explain how this finding is consistent with the power-efficiency trade-off relation. 

 
[H. Tajima and K. Funo, Phys. Rev. Lett. 127, 190604 (2021)].