Talks Thursday June 30

Faculty @ University of Maryland, US

Thursday June 30 – 18.30 BST

Towards reconciliation of completely positive open system dynamics with equilibration postulate

Why do chaotic quantum many-body systems thermalize internally? The eigenstate thermalization hypothesis (ETH) explains why if the Hamiltonian lacks degeneracies. If the Hamiltonian conserves one quantity (“charge”), the ETH implies thermalization within an eigenspace of the charge—in a microcanonical subspace. But, as was recently pointed out in quantum thermodynamics, quantum systems can have charges that fail to commute with each other and so share no eigenbasis; microcanonical subspaces may not exist. Worse, the Hamiltonian will have degeneracies, so the ETH need not imply thermalization. We adapt the ETH to noncommuting charges by positing a non-Abelian ETH and invoking the approximate microcanonical subspace introduced in quantum thermodynamics. We apply the non-Abelian ETH in calculating local observables’ time-averaged. In many cases, we prove, the time average thermalizes. However, we find anomalous corrections to thermal predictions under a physically reasonable assumption. This work bridges noncommuting charges, recently on the rise in quantum thermodynamics, to the ETH, a cornerstone of many-body physics.

Murthy, Babakhani, Iniguez, Srednicki, and NYH, arXiv:2206.05310
(2022).
NYH and Majidy, npj Quantum Information 8, 10 (2022).
Kranzl, Lasek, Joshi, Kalev, Blatt, Roos, and NYH, arXiv:2202.04652
(2022).

Faculty @ MIT, US

Thursday June 30 – 17.45 BST

Machine-learning-accelerated Bose-Einstein condensation

Machine learning is emerging as a technology that can enhance physics experiment and data analysis. Here, we apply machine learning to accelerate the production of a Bose-Einstein condensate across the phase transition from a classical to a quantum gas. Starting from a room-temperature gas and optimizing laser cooling (Raman sideband cooling) using a Bayesian approach with up to 55 control parameters, we prepare a condensate of rubidium atoms in less than 0.6 seconds; the fastest condensation to date. We find that the choice of cost function for the algorithm strongly influences the trade-off between large and pure condensates. We anticipate that many other physics experiments with complex nonlinear system dynamics or involving phase transitions can be significantly enhanced by a similar machine-learning approach.

Postdoc @ Queen’s University Belfast 

Thursday June 30 – 17.15 BST

The Thermodynamics of Quantum Coherence and Quantum Thermal Machines 

The impact of quantum resources, mainly quantum coherence, on the operation of thermodynamic tasks is considered one of the leading subjects of research in quantum thermodynamics. In our work, we study the effects of quantum coherent baths on the functioning of a quantum thermal machine by using a collision model framework in the continuous time limit. We find that consuming coherences from the baths allows the machine to function with efficiencies beyond the Carnot bound and allows it to perform useful tasks simultaneously including refrigeration and  generation of work, providing a new genre of device dubbed hybrid refrigerator. 
 
K. Hammam, H. Leitch, Y. Hassouni, G. De Chiara, Exploiting coherence for quantum thermodynamic advantage, (2022), arXiv:2202.07515 

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) 

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. 

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. 

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 

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.