arXiv subjects

S. F. Huelga

Publications and source records attributed to S. F. Huelga.

At least 19 recordsLinked to original sources

arXiv2025

Unlocking Heisenberg Sensitivity with Sequential Weak Measurement Preparation

We propose a state preparation protocol based on sequential measurements of a central spin coupled with a spin ensemble, and investigate the usefulness of the generated multi-spin states for quantum enhanced metrology. Our protocol is shown to generate highly entangled spin states, devoid of the necessity for non-linear spin interactions. The metrological sensitivity of the resulting state surpasses the standard quantum limit, reaching the Heisenberg limit under symmetric coupling strength conditions. We also explore asymmetric coupling strengths, identifying specific preparation windows in time for optimal sensitivity. Our findings introduce a novel method for generating large-scale, non-classical, entangled states, enabling quantum-enhanced metrology within current experimental capabilities.

quant-ph↗

arXiv2023

Non-perturbative treatment of open-system multi-time expectation values in Gaussian bosonic environments

We determine the conditions for the equivalence between the multi-time expectation values of a general finite-dimensional open quantum system when interacting with, respectively, an environment undergoing a free unitary evolution or a discrete environment under a free evolution fixed by a proper Gorini-Kossakowski-Lindblad-Sudarshan generator. We prove that the equivalence holds if both environments are bosonic and Gaussian and if the one- and two-time correlation functions of the corresponding interaction operators are the same at all times. This result leads to a non-perturbative evaluation of the multi-time expectation values of operators and maps of open quantum systems interacting with a continuous set of bosonic modes by means of a limited number of damped modes, thus setting the ground for the investigation of open-system multi-time quantities in fully general regimes.

quant-ph↗

arXiv2022

Exact Simulation of Pigment-Protein Complexes Unveils Vibronic Renormalization of Electronic Parameters in Ultrafast Spectroscopy

The primary steps of photosynthesis rely on the generation, transport, and trapping of excitons in pigment-protein complexes (PPCs). Generically, PPCs possess highly structured vibrational spectra, combining many discrete intra-pigment modes and a quasi-continuous of protein modes, with vibrational and electronic couplings of comparable strength. The intricacy of the resulting vibronic dynamics poses significant challenges in establishing a quantitative connection between spectroscopic data and underlying microscopic models. Here we show how to address this challenge using numerically exact simulation methods by considering two model systems, namely the water-soluble chlorophyll-binding protein of cauliflower and the special pair of bacterial reaction centers. We demonstrate that the inclusion of the full multi-mode vibronic dynamics in numerical calculations of linear spectra leads to systematic and quantitatively significant corrections to electronic parameter estimation. These multi-mode vibronic effects are shown to be relevant in the longstanding discussion regarding the origin of long-lived oscillations in multidimensional nonlinear spectra.

physics.chem-ph↗

arXiv2019

Multi-color quantum control for suppressing ground state coherences in two-dimensional electronic spectroscopy

The measured multi-dimensional spectral response of different light harvesting complexes exhibits oscillatory features which suggest an underlying coherent energy transfer. However, making this inference rigorous is challenging due to the difficulty of isolating excited state coherences in highly congested spectra. In this work, we provide a coherent control scheme that suppresses ground state coherences, thus making rephasing spectra dominated by excited state coherences. We provide a benchmark for the scheme using a model dimeric system and numerically exact methods to analyze the spectral response. We argue that combining temporal and spectral control methods can facilitate a second generation of experiments that are tailored to extract desired information and thus significantly advance our understanding of complex open many-body structure and dynamics.

quant-ph↗

arXiv2015

Sensing in the Presence of Observed Environments

Sensing in the presence of environmental noise is a problem of increasing practical interest. In a master equation description, where the state of the environment is unobserved, the effect of signal and noise is described by system operators only. In this context it is well-known that noise that is orthogonal on an external signal can be corrected for without perturbing the signal, while similarly efficient strategies for non-orthogonal signal and noise operators are not known. Here we make use of the fact that system-environment interaction typically arises via local two-body interactions describing the exchange of quanta between system and environment, which are observable in principle. That two-body-interactions are usually orthogonal on system operators, allows us to develop error corrected sensing supported by the observation of the quanta that are emitted into the environment. We describe such schemes and outline a realistic proof-of-principle experiment in an ion trap set-up.

quant-ph↗

arXiv2015

Phase-dependent exciton transport and energy harvesting from thermal environments

Non-Markovian effects in the evolution of open quantum systems have recently attracted widespread interest, particularly in the context of assessing the efficiency of energy and charge transfer in nanoscale biomolecular networks and quantum technologies. With the aid of many-body simulation methods, we uncover and analyse an ultrafast environmental process that causes energy relaxation in the reduced system to depend explicitly on the phase relation of the initial state preparation. Remarkably, for particular phases and system parameters, the net energy flow is uphill, transiently violating the principle of detailed balance, and implying that energy is spontaneously taken up from the environment. A theoretical analysis reveals that non-secular contributions, significant only within the environmental correlation time, underlie this effect. This suggests that environmental energy harvesting will be observable across a wide range of coupled quantum systems.

physics.chem-ph↗

arXiv2014

Dephasing-assisted transport in linear triple quantum dots

Environmental noise usually hinders the efficiency of charge transport through coherent quantum systems; an exception is dephasing-assisted transport (DAT). We show that linear triple quantum dots in a transport configuration and subjected to pure dephasing exhibit DAT if the coupling to the drain reservoir exceeds a threshold. DAT occurs for arbitrarily weak dephasing and the enhancement can be directly controlled by the coupling to the drain. Moreover, for specific settings, the enhanced current is accompanied by a reduction in relative shot noise. We identify the quantum Zeno effect and long-distance tunnelling as underlying dynamical processes involved in dephasing-assisted and -suppressed transport. Our analytical results are obtained by using the density matrix formalism and the characteristic polynomial approach to full counting statistics.

cond-mat.mes-hall↗

arXiv2013

Origin of long-lived oscillations in 2D-spectra of a Quantum Vibronic Model: Electronic vs Vibrational coherence

We demonstrate that the coupling of excitonic and vibrational motion in biological complexes can provide mechanisms to explain the long-lived oscillations that have been obtained in non linear spectroscopic signals of different photosynthetic pigment protein complexes and we discuss the contributions of excitonic versus purely vibrational components to these oscillatory features. Considering a dimer model coupled to a structured spectral density we exemplify the fundamental aspects of the electron-phonon dynamics, and by analyzing separately the different contributions to the non linear signal, we show that for realistic parameter regimes purely electronic coherence is of the same order as purely vibrational coherence in the electronic ground state. Moreover, we demonstrate how the latter relies upon the excitonic interaction to manifest. These results link recently proposed microscopic, non-equilibrium mechanisms to support long lived coherence at ambient temperatures with actual experimental observations of oscillatory behaviour using 2D photon echo techniques to corroborate the fundamental importance of the interplay of electronic and vibrational degrees of freedom in the dynamics of light harvesting aggregates.

physics.chem-ph↗

arXiv2013

Experimental Demonstration of non-Markovian Dynamics via a Temporal Bell-like Inequality

We assess non-Markovianity of a quantum open-system dynamics through the violation of temporal bell-like inequalities in a controllable Nuclear Magnetic Resonance system. We investigate experimentally the connections between the violation of the addressed temporal Bell-like inequality and the non-divisibility of the effective evolution of our system, which we fully characterize in a broad range of experimentally controllable regimes. Finally, we investigate the link between our approach and Leggett-Garg-like inequalities based on time-translational two-time correlation functions. These results open up interesting perspectives for the effective, non-tomographic characterization of dynamical evolution by combining tools of different nature.

quant-ph↗

arXiv2013

Vibrations, Quanta and Biology

Quantum biology is an emerging field of research that concerns itself with the experimental and theoretical exploration of non-trivial quantum phenomena in biological systems. In this tutorial overview we aim to bring out fundamental assumptions and questions in the field, identify basic design principles and develop a key underlying theme -- the dynamics of quantum dynamical networks in the presence of an environment and the fruitful interplay that the two may enter. At the hand of three biological phenomena whose understanding is held to require quantum mechanical processes, namely excitation and charge transfer in photosynthetic complexes, magneto-reception in birds and the olfactory sense, we demonstrate that this underlying theme encompasses them all, thus suggesting its wider relevance as an archetypical framework for quantum biology.

physics.bio-ph↗

arXiv2013

Probing quantum coherence in qubit arrays

We discuss how the observation of population localization effects in periodically driven systems can be used to quantify the presence of quantum coherence in interacting qubit arrays. Essential for our proposal is the fact that these localization effects persist beyond tight-binding Hamiltonian models. This result is of special practical relevance in those situations where direct system probing using tomographic schemes becomes infeasible beyond a very small number of qubits. As a proof of principle, we study analytically a Hamiltonian system consisting of a chain of superconducting flux qubits under the effect of a periodic driving. We provide extensive numerical support of our results in the simple case of a two-qubits chain. For this system we also study the robustness of the scheme against different types of noise and disorder. We show that localization effects underpinned by quantum coherent interactions should be observable within realistic parameter regimes in chains with a larger number of

quant-ph↗

arXiv2013

Quantum speed limits in open system dynamics

Bounds to the speed of evolution of a quantum system are of fundamental interest in quantum metrology, quantum chemical dynamics and quantum computation. We derive a time-energy uncertainty relation for open quantum systems undergoing a general, completely positive and trace preserving (CPT) evolution which provides a bound to the quantum speed limit. When the evolution is of the Lindblad form, the bound is analogous to the Mandelstam-Tamm relation which applies in the unitary case, with the role of the Hamiltonian being played by the adjoint of the generator of the dynamical semigroup. The utility of the new bound is exemplified in different scenarios, ranging from the estimation of the passage time to the determination of precision limits for quantum metrology in the presence of dephasing noise.

quant-ph↗

arXiv2012

Exploiting structured environments for efficient energy transfer: The phonon antenna mechanism

A non-trivial interplay between quantum coherence and dissipative environment-driven dynamics is becoming increasingly recognised as key for efficient energy transport in photosynthetic pigment-protein complexes, and converting these biologically-inspired insights into a set of design principles that can be implemented in artificial light-harvesting systems has become an active research field. Here we identify a specific design principle - the phonon antenna - that demonstrates how inter-pigment coherence is able to modify and optimize the way that excitations spectrally sample their local environmental fluctuations. We place this principle into a broader context and furthermore we provide evidence that the Fenna-Matthews-Olson complex of green sulphur bacteria has an excitonic structure that is close to such an optimal operating point, and suggest that this general design principle might well be exploited in other biomolecular systems.

quant-ph↗

arXiv2012

Mappings of open quantum systems onto chain representations and Markovian embeddings

We derive a sequence of measures whose corresponding Jacobi matrices have special properties and a general mapping of an open quantum system onto 1D semi infinite chains with only nearest neighbour interactions. Then we proceed to use the sequence of measures and the properties of the Jacobi matrices to derive an expression for the spectral density describing the open quantum system when an increasing number of degrees of freedom in the environment have been embedded into the system. Finally, we derive convergence theorems for these residual spectral densities.

quant-ph↗

arXiv2012

Vibrational structures and long-lasting electronic coherence

Recent observations of beating signals in the excitation energy transfer dynamics of photosynthetic complexes have been interpreted as evidence for sustained coherences that are sufficiently long-lived for energy transport and coherence to coexist. The possibility that coherence may be exploited in biological processes has opened up new avenues of exploration at the interface of physics and biology. The microscopic origin of these long-lived coherences, however, remains to be uncovered. Here we present such a mechanism and verify it by numerically exact simulations of system-environment dynamics. Crucially, the non-trivial spectral structures of the environmental fluctuations and particularly discrete vibrational modes can lead to the generation and sustenance of both oscillatory energy transport and electronic coherence on timescales that are comparable to excitation energy transport. This suggests that the non-trivial structure of protein environments plays a more significant role for coherence in biological processes than previously believed.

quant-ph↗

arXiv2012

Coherence and Decoherence in Biological Systems: Principles of Noise Assisted Transport and the Origin of Long-lived Coherences

The quantum dynamics of transport networks in the presence of noisy environments have recently received renewed attention with the discovery of long-lived coherences in different photosynthetic complexes. This experimental evidence has raised two fundamental questions: Firstly, what are the mechanisms supporting long-lived coherences and secondly, how can we assess the possible functional role that the interplay of noise and quantum coherence might play in the seemingly optimal operation of biological systems under natural conditions? Here we review recent results, illuminate them at the hand of two paradigmatic systems, the Fenna-Matthew-Olson (FMO) complex and the light harvesting complex LHII, and present new progress on both questions. In particular we introduce the concept of the phonon antennae and discuss the possible microscopic origin or long-lived electronic coherences.

physics.bio-ph↗

arXiv2012

Quantum dynamics of bio-molecular systems in noisy environments

We discuss three different aspects of the quantum dynamics of bio-molecular systems and more generally complex networks in the presence of strongly coupled environments. Firstly, we make a case for the systematic study of fundamental structural elements underlying the quantum dynamics of these systems, identify such elements and explore the resulting interplay of quantum dynamics and environmental decoherence. Secondly, we critically examine some existing approaches to the numerical description of system-environment interaction in the non-perturbative regime and present a promising new method that can overcome some limitations of existing methods. Thirdly, we present an approach towards deciding and quantifying the non-classicality of the action of the environment and the observed system-dynamics. We stress the relevance of these tools for strengthening the interplay between theoretical and experimental research in this field.

quant-ph↗

arXiv2011

Violation of a temporal Bell inequality for single spins in solid by over 50 standard deviations

Quantum non-locality has been experimentally investigated by testing different forms of Bell's inequality, yet a loophole-free realization has not been achieved up to now. Much less explored are temporal Bell inequalities, which are not subject to the locality assumption, but impose a constrain on the system's time-correlations. In this paper, we report on the experimental violation of a temporal Bell's inequality using a nitrogen vacancy defect (NV) in diamond and provide a novel quantitative test of quantum coherence. We therefore present a new technique to initialize the electronic state of the NV with high fidelity, a necessary requirement for reliable quantum information processing and/or the implementation of protocols for quantum metrology.

quant-ph↗