Work measurement as a generalized quantum measurement

We present a new method to measure the work $w$ performed on a driven quantum system and to sample its probability distribution $P(w)$. The method is based on a simple fact that remained unnoticed until now: Work on a quantum system can be measured by performing a generalized quantum measurement at a single time. Such measurement, which technically speaking is denoted as a POVM (positive operator valued measure) reduces to an ordinary projective measurement on an enlarged system. This observation not only demystifies work measurement but also suggests a new quantum algorithm to efficiently sample the distribution $P(w)$. This can be used, in combination with fluctuation theorems, to estimate free energies of quantum states on a quantum computer.Comment: 4 page

A Wigner quasiprobability distribution of work

In this article we introduce a quasiprobability distribution of work that is based on the Wigner function. This construction rests on the idea that the work done on an isolated system can be coherently measured by coupling the system to a quantum measurement apparatus. In this way, a quasiprobability distribution of work can be defined in terms of the Wigner function of the apparatus. This quasidistribution contains the information of the work statistics and also holds a clear operational definition. Moreover, it is shown that the presence of quantum coherence in the energy eigenbasis is related with the appearance of characteristics related to non-classicality in the Wigner function such as negativity and interference fringes. On the other hand, from this quasiprobability distribution it is straightforward to obtain the standard two-point measurement probability distribution of work and also the difference in average energy for initial states with coherences.Comment: 11 pages, 3 figure

Quantum–classical correspondence in spin–boson equilibrium states at arbitrary coupling

The equilibrium properties of nanoscale systems can deviate significantly from standard thermodynamics due to their coupling to an environment. We investigate this here for the θ-angled spin–boson model, where we first derive a compact and general form of the classical equilibrium state including environmental corrections to all orders. Secondly, for the quantum spin–boson model we prove, by carefully taking a large spin limit, that Bohr’s quantum–classical correspondence persists at all coupling strengths. This shows, for the first time, the validity of the quantum–classical correspondence for an open system and gives insight into the regimes where the quantum system is well-approximated by a classical one. Finally, we provide the first classification of the coupling parameter regimes for the spin–boson model, from weak to ultrastrong, both for the quantum case and the classical setting. Our results shed light on the interplay of quantum and mean force corrections in equilibrium states of the spin–boson model, and will help draw the quantum to classical boundary in a range of fields, such as magnetism and exciton dynamics

A new group of Archaic Greek Pottery (“Group H”) manufactured in Tartessos (Huelva, SW of Spain)

A la producción por griegos residentes en Huelva de vasos cerámicos caracterizados por una pasta verdosa amarillenta puede sumarse un segundo grupo, convencionalmente denominado “Grupo H”, caracterizado por pastas amarillentas, anaranjadas o rojizas y frecuentes engobes rojos. El estudio preliminar de dos especímenes había ya apuntado una producción local de este nuevo grupo. Una ampliación de los análisis mediante activación neutrónica de otros 10 ejemplares establece de forma concluyente una estrecha semejanza con la composición química de los depósitos de arcillas próximos al hábitat. Asimismo, como en el grupo de pasta verdosa amarillenta, fue detectada una llamativa contaminación de algunos fragmentos cerámicos por metales preciosos, oro y especialmente plata. La representación de ambos grupos sugiere que un significativo porcentaje de los vasos cerámicos griegos arcaicosTo the production by Greek residents in Huelva of pottery characterised by yellowish-green clay, a second ceramic group distinguished by yellowish, orange or reddish clay, and frequently red slipped, conventionally nominated “Group H” can now be added. The preliminary study of two pieces had already aimed at a local production of the new group. This fact is supported by the results of an additional study based on the Neutron Activation Analyses of 10 ceramic specimens conclusively indicating a close resemblance to the chemical composition of clays from local deposits near the habitat. Likewise, as in the group of the yellowish-green clay, a striking contamination with precious metals, gold and especially silver, was detected in some of the fragments. The representation of both groups suggests that a significant percentage of the Archaic Greek vessels found in Huelva was manufactured in sit

Tutorial on the stochastic simulation of dissipative quantum oscillators

This is the author accepted manuscript.Data availability statement: SpiDy.jl is publicly available at github.com/quantum-exeter/SpiDy.jl. The code to produce Figs. 1 - 4 and associated data is available from CRH upon reasonable requestGeneric open quantum systems are notoriously difficult to simulate unless one looks at specific regimes. In contrast, classical dissipative systems can often be effectively described by stochastic processes, which are generally less computationally expensive. Here, we use the paradigmatic case of a dissipative quantum oscillator to give a pedagogic introduction into the modelling of open quantum systems using quasiclassical methods, i.e. classical stochastic methods that use a ‘quantum’ noise spectrum to capture the influence of the environment on the system. Such quasiclassical methods have the potential to offer insights into the impact of the quantum nature of the environment on the dynamics of the system of interest whilst still being computationally tractable.Engineering and Physical Sciences Research Council (EPSRC)University of ExeterFoundational Questions Institute FundRoyal Societ

Thermodynamically optimal protocols for dual-purpose qubit operations

This is the final version. Available on open access from the American Physical Society via the DOI in this record. Code availability: The code used to produce the data in Figs. 2 and 3 is available upon reasonable request to JD, [email protected] processing, quantum or classical, relies on channels transforming multiple input states to different corresponding outputs. Previous research has established bounds on the thermodynamic resources required for such operations, but no protocols have been specified for their optimal implementation. For the insightful case of qubits, we here develop explicit protocols to transform two states in an energetically optimal manner. We first prove conditions on the feasibility of carrying out such transformations at all, and then quantify the achievable work extraction. Our results uncover a fundamental incompatibility between the thermodynamic ideal of slow, quasistatic processes and the information-theoretic requirement to preserve distinguishability between different possible output states.Engineering and Physical Sciences Research Council (EPSRC)Foundational Questions Institute FundDeutsche ForschungsgemeinschaftRoyal Societ

Optimal finite-time heat engines under constrained control

This is the final version. Available on open access from the American Physical Society via the DOI in this recordWe optimize finite-time stochastic heat engines with a periodically scaled Hamiltonian under experimentally motivated constraints on the bath temperature T and the scaling parameter λ. We present a general geometric proof that maximum-efficiency protocols for T and λ are piecewise constant, alternating between the maximum and minimum allowed values. When λ is restricted to a small range and the system is close to equilibrium at the ends of the isotherms, a similar argument shows that this protocol also maximizes output power. These results are valid for arbitrary dynamics. We illustrate them for an overdamped Brownian heat engine, which can experimentally be realized using optical tweezers with stiffness λ.China Scholarship CouncilFundational Questions InstituteCaixa FoundationGovernment of SpainFundacio CellexFundacio Mir-PuigGeneralitat de Catalunya (CERCA)Engineering and Physical Sciences Research Council (EPSRC)Royal SocietySwiss National Science FoundationHumboldt foundationCzech Science Foundatio

Thermodynamics and optimal protocols of multidimensional quadratic Brownian systems

This is the final version. Available on open access from IOP Publishing via the DOI in this recordData availability statement; All data that support the findings of this study are included within the article (and any supplementary files).We characterize finite-time thermodynamic processes of multidimensional quadratic overdamped systems. Analytic expressions are provided for heat, work, and dissipation for any evolution of the system covariance matrix. The Bures-Wasserstein metric between covariance matrices naturally emerges as the local quantifier of dissipation. General principles of how to apply these geometric tools to identify optimal protocols are discussed. Focusing on the relevant slow-driving limit, we show how these results can be used to analyze cases in which the experimental control over the system is partial.Humboldt FoundationCzech Science Foundation'la Caixa' FoundationGovernment of SpainSevero OchoaFundacio CellexFundacio Mir-PuigGeneralitat de CatalunyaFundational Questions Institute FundEngineering and Physical Sciences Research Council (EPSRC)China Scholarship CouncilSwiss National Science Foundatio