Editorial Team

Aleksander Lasek 

Postdoc @ University of Maryland 

Thursday June 30 – 16.45 BST

Experimental observation of thermalisation with noncommuting charges 

Noncommuting charges have recently emerged as an area at the intersection of quantum thermodynamics and quantum information. There is a flurry of papers being published in this rapidly developing subfield. Often, the global energy and particle number are conserved, and the system is prepared with a well-defined particle number. However, quantum evolution can also conserve quantities, or charges, that fail to commute with each other. As noncommutation underlies quantumness, such systems are of particular interest. Quantum simulators have recently enabled experimental observations of quantum many-body systems’ internal thermalisation. We initiate the experimental testing of its predictions with a trapped-ion simulator. We initialize 6–15 qubits in an approximate microcanonical subspace, a recently theorized generalisation of the microcanonical subspace for accommodating noncommuting charges. The noncommuting charges are the three spin components. We report the first experimental observation of an equilibrium state predicted within quantum-information thermodynamics in 2016: the non-Abelian thermal state. Despite the threat of decoherence breaking multiple conservation laws, thanks to our use of dynamical decoupling, our many-body system is shown to exhibit quantum-thermodynamical effects only described in theory until now. This work initiates the experimental testing of a recently emerged subfield that has so far remained theoretical. 
 
Arxiv preprint “Experimental observation of thermalisation with noncommuting charges” (2022) available at: https://arxiv.org/abs/2202.04652 
Previous theoretical work: 
N.  Yunger  Halpern,  P.  Faist,  J.  Oppenheim,  and  A.  Winter,  “Microcanonical  and  resource-theoretic  derivations  of  thethermal state of a quantum system with noncommuting charges”, Nature Commun.7, 12051 (2016) 

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Stefan Nimmrichter 

Faculty @ University of Siegen

Thursday June 30 – 15.45 BST

Coherent speed-up in the collisional charging of quantum batteries 

We study self-contained collisional models for the charging of a quantum battery by a stream of identical nonequilibrium qubit units, comparing the charging power for coherent and incoherent protocols on an initially empty battery. The battery can be an oscillator, a large spin, or any other linear energy ladder with a ground state, while the qubits are assumed to be resonant with the ladder, which obviates the need for additional work input as they exchange excitations with the battery. 
When the qubits are prepared in a population-inverted, incoherent mixture of energy eigenstates, the energy and ergotropy gain in the battery can be described by a generalized classical random walk process with level-dependent rates. We provide an upper bound on the charging power for any incoherent protocol, including adaptive charging strategies.  We show that this bound can be broken by non-adaptive protocols with qubits that contain quantum coherence, thus demonstrating a quantum speedup at the level of a single battery. 
In homogeneous ladder models with level-independent transition rates, the speedup can be attributed to quantum walk-like interference effects. In oscillator and spin batteries, the greatest speedup is reached in the limit when the charging process approximates a coherent Rabi drive. 
We show that the oscillator battery model could be realized in a state-of-the-art cavity micromaser setup, and the coherent speedup could be achieved in the presence of realistic cavity damping. 

By Editorial Team, ago

Krissia Zawadzki

Postdoc @ Royal Holloway University of London 

Thursday June 30 – 15.15 BST

Work statistics and entanglement across the superfluid-insulator transition 

Out-of-equilibrium strongly correlated systems have been shown to display interesting work fluctuations across a phase transition [1,2]. The work distribution at criticality has been investigated recently in a few models, most of which are exactly solvable, with a focus on the first and second moments after a sudden quench. Very recently, results for inhomogeneous Hubbard chains driven for a finite time indicated that the skewness of the work distribution, besides being a measure of non-Gaussianity, also captures transitions between different correlated phases [3]. Here, we explore the first three moments of the work distribution across the superfluid-insulator transition (SIT), which is well described by the attractive Fermionic Hubbard model in the presence of randomly distributed impurities [4]. The SIT can be triggered by changing either (i) the concentration of impurities or (ii) the disorder strength. We study two quench protocols implementing these two paths and discuss the impact of the entanglement and of the temperature for maximal work extraction. Our results indicate that for disorder strengths sufficiently large to overcome the Coulomb attraction, all three moments of the work distribution show a kink at the critical concentration $C_C=N/2$. This is the same point in which the entanglement is minimal or vanishes. All the effects of the transition are suppressed at high temperatures, with work being absorbed by the system and very large fluctuations. The protocol in which the SIT is triggered by route (i) is more efficient for the average work extraction and its variance is minimized at $C_C$. 

[1] “Statistics of the Work Done on a Quantum Critical System by Quenching a Control Parameter” 
A. Silva 
Phys. Rev. Lett. 101, 120603,  2008 
[2] “Work statistics and symmetry breaking in an excited-state quantum phase transition” 
Z. Mzaouali, R. Puebla, J. Goold, M. El Baz, and S. Campbell 
Phys. Rev. E 103, 032145, 2021 
[3] “Work-distribution quantumness 
and irreversibility when crossing a quantum phase transition in finite time.” 
K. Zawadzki, R. M. Serra, and I. D’Amico. 
Phys. Rev. Research, 2(3) 033167, 2020 
[4] “Superfluid-insulator transition unambiguously detected by 
entanglement in one-dimensional disordered superfluids.” 
G.A. Canella and V. V. França. Scientific reports, 9(1)1–6, 2019. 

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Ariane Soret 

Postdoc @ University of Luxembourg 

Thursday June 30 – 14.45 BST

Thermodynamic consistency in open quantum systems: from exact identities to quantum master equations 

In the context of open quantum systems, a recent effort has been put in finding alternative and general derivations of master equations which do not require the secular approximation, using partial coarse graining [1] and symmetrization  techniques [2,3,4]. These generalized master equations provide an accurate dynamic description for a larger range of parameters than the standard secular Lindblad master equations. However, the question of their thermodynamic consistency has not yet been addressed. 
Here, we study the thermodynamic consistency of quantum master equations from a general point of view. Starting with a discussion of the fluctuation theorem for work, heat and entropy production at the unitary level, we derive a quantum detailed balance condition that has to be fulfilled by any time-local master equation to ensure thermodynamic consistency. While the Redfield equation breaks such conditions, we show that the validity of the latter can be restored by using secular or beyond-secular approximation schemes, already proven to be effective in restoring the positivity of the dynamical evolution [2,3,4]. 
In addition, we study the steady state for different models of master equations, proving that the breakdown of the Redfield equation at long times is strictly connected with the violation of the quantum detailed balance condition. 
We illustrate the theoretical results by a thorough numerical analysis of the paradigmatic example of a three level system. 

[1] Gernot Schaller and Tobias Brandes. Preservation of positivity by dynamical coarse graining. Phys. Rev. A, 78:022106, Aug 2008 
[2] Krzysztof Ptaszyński and Massimiliano Esposito. Thermodynamics of quantum information flows. Physical review letters, 122(15):150603, 2019. 
[3] Gavin McCauley, Benjamin Cruikshank, Denys I Bondar, and Kurt Jacobs. Accurate lindblad-form master equation for weakly damped quantum systems across all regimes. npj Quantum Information, 6(1):1–14, 2020. 
[4] Frederik Nathan and Mark S. Rudner. Universal lindblad equation for open quantum systems. Phys. Rev. B, 102:115109, Sep 2020 

By Editorial Team, ago

Sabrina Maniscalco & Marco Cattaneo

Faculty @ University of Helsinki, Finland

Thursday June 30 – 14.00 BST

Quantum simulation of dissipative collective effects on noisy quantum computers

Dissipative collective effects are ubiquitous in quantum physics, and their relevance ranges from the study of entanglement in biological systems to noise mitigation in quantum computers. Here, we put forward the first fully quantum simulation of dissipative collective phenomena on a real quantum computer. The quantum simulation is based on the recently introduced multipartite collision model, which reproduces the action of a dissipative common environment by means of repeated interactions between the system and some ancillary qubits. First, we theoretically study the accuracy of this algorithm on near-term quantum computers with noisy gates, and we derive some rigorous error bounds which depend on the timestep of the collision model and on the gate errors. These bounds can be employed to estimate the necessary resources for the efficient quantum simulation of the collective dynamics. Then, we implement the algorithm on some IBM quantum computers to simulate superradiance and subradiance between a pair of qubits. Our experimental results successfully display the emergence of collective effects in the quantum simulation. Finally, we analyze the noise properties of the gates we employed in the algorithm by means of full process tomography. Using the state-of-the-art tools for noise analysis in quantum computers, we obtain the values of the average gate fidelity, unitarity and diamond error, and we establish a connection between them and the accuracy of the experimentally simulated state. Although the scaling of the error as a function of the number of gates is favorable, we observe that reaching the threshold for quantum fault tolerant computation is still orders of magnitude away.

By Editorial Team, ago

Gonzalo Manzano

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). 

By Editorial Team, ago

Santiago Hernández-Gómez

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. 

By Editorial Team, ago

Alexia Auffèves

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

By Editorial Team, ago

Jukka Pekola

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). 

By Editorial Team, ago

Tathagata Karmakar 

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). 

By Editorial Team, ago