Quantum batteries, quantum systems for energy storage, have gained interest due to their potential scalable charging power density. A quantum battery proposal based on the Dicke model has been explored using organic microcavities, which enable a cavity-enhanced energy-transfer process called superabsorption. However, energy-storage lifetime in these devices is limited by fast radiative emission losses, worsened by superradiance. Here, we demonstrate a promising approach to extend the energy-storage lifetime of Dicke quantum batteries using molecular triplet states. We examine a type of multilayer microcavity where an active absorption layer transfers energy to the molecular triplets of a storage layer, identifying two regimes based on exciton-polariton resonances. We tested one of these mechanisms by fabricating and characterizing five devices across a triplet-polariton resonance, showing that triplet population is maximized when the lower polariton and triplet state are isoenergetic. We found that one of these devices can store energy for 40.3 ± 0.4 μ s , a 10 3 -fold increase in storage time compared to previous demonstrations. We conclude by discussing potential optimization outlooks for this class of devices.
Quantum batteries, quantum systems for energy storage, have gained interest due to their potential scalable charging power density. A quantum battery proposal based on the Dicke model has been explored using organic microcavities, which enable a cavity-enhanced energy-transfer process called superabsorption. However, energy-storage lifetime in these devices is limited by fast radiative emission losses, worsened by superradiance. Here, we demonstrate a promising approach to extend the energy-storage lifetime of Dicke quantum batteries using molecular triplet states. We examine a type of multilayer microcavity where an active absorption layer transfers energy to the molecular triplets of a storage layer, identifying two regimes based on exciton-polariton resonances. We tested one of these mechanisms by fabricating and characterizing five devices across a triplet-polariton resonance, showing that triplet population is maximized when the lower polariton and triplet state are isoenergetic. We found that one of these devices can store energy for 40.3 ± 0.4 μ s , a 10 3 -fold increase in storage time compared to previous demonstrations. We conclude by discussing potential optimization outlooks for this class of devices.
Excitonic quantum batteries, based on organic fluorescent molecules embedded in optical microcavities, offer a room-temperature platform for studying collective effects in energy storage and developing applications. Recent experiments have offered evidence of superabsorption, a collective enhancement to the light absorption rate of organic molecules which leads to a scalable power density. However, they have also highlighted the challenge posed by rapid radiative decay of fluorescent molecules, which limits the energy storage lifetime. Current strategies to overcome this trade-off focus on controlling the coupling between the absorbing manifold and that used for energy storage. In this chapter, we review three implementations of this design principle: transferring energy from optically excited states to long-lived dark triplet states, generating triplet pairs and higher-spin states through singlet exciton fission, and forming charge-separated states. We discuss each mechanism from both theoretical and experimental perspectives, with particular emphasis on recent device implementations that have extended storage times by several orders of magnitude. We conclude with a cross-platform outlook on the role of metastable states across coherent and room-temperature implementations, from neutral atom arrays to masers and colour centres.
A collaboration spanning Australia and Europe used a microcavity structure, an organic dye molecule confined between mirrors, to realise superabsorption experimentally, the principle at the heart of the quantum battery. Ultrafast transient absorption measurements showed superextensive behaviour: as the concentration of molecules (their number) rose, both the charging power and the stored energy per molecule went up while the charging rise time shortened. The work finds that decoherence (dephasing) is not an obstacle but a necessary condition for holding energy stably in place, which offers a new insight for the design of practical quantum batteries. [Quantum Biology Society] In 1954 the physicist Robert Dicke predicted superradiance: when N atoms sit close together, the waves of light they emit interfere constructively, so the time for emission scales as 1/N and the peak emission power scales as N². The effect has since been confirmed broadly across quantum systems ranging from quantum dots to nitrogen-vacancy (NV) centres in diamond. The reverse process, superabsorption, has been a far harder problem. The difficulty lies in engineering the precise environment in which such behaviour can occur and in monitoring charging timescales short enough to be hard to probe at all, and until recently the effect had been seen only for small numbers of atoms. There is a further problem on the storage side: in a closed system, the same coherent effects that produce fast charging also produce fast discharging. Even so, superabsorption is the conceptual foundation of the quantum battery, a device whose charging time is inversely related to its capacity, so that a larger battery charges faster. It has long been an open goal in the field. A paper published in Science Advances in January 2022, from a team led by James Q. Quach at the University of Adelaide in Australia and Tersilla Virgili at IFN-CNR and the Politecnico di Milano in Italy, along with collaborators in the UK, demonstrated superabsorption in a microcavity built around an organic dye. ■ Dye Meets Mirrors: Demonstrating Superextensive Charging The researchers built a microcavity by inserting a 185 nm thick film containing the fluorescent dye Lumogen-F Orange (LFO), dispersed in a polystyrene matrix, between two distributed Bragg reflectors (DBRs), the dielectric mirrors that confine the light. They then ran ultrafast transient absorption spectroscopy on samples with the dye concentration (and therefore the number of molecules, N) set at 0.5%, 1%, 5% and 10%. Injecting a pump pulse of under 20 fs and following the charging of energy on a femtosecond timescale, they found superextensive scaling, with the highest-concentration 10% cavity at N = 16.0 × 10¹⁰. Charging power per molecule rises: going from 1% to 10% concentration, the peak charging power per molecule (P_max) climbed steeply from 0.060 to 0.791 eV/ps. Stored energy per molecule increases: the peak stored energy per molecule (E_max) likewise grew nearly tenfold, from 0.011 to 0.108 eV. Charging rise time shortens: the rise time to reach half the maximum energy (τ) fell from 0.118 to 0.094 ps. Because E_max and P_max are per-molecule quantities, their increase with N is precisely what makes the behaviour superextensive: the whole grows faster than in proportion to its size. As the paper puts it, a single microcavity containing N molecules takes less time to charge than N single-molecule microcavities would, even if the latter were all charged at once, and it would store more energy than those N separate cavities combined. This is the core principle of the quantum battery, that gathering everything into one large battery charges faster and more efficiently than charging many small ones separately. Two qualifications are worth keeping in view. The experiment directly provides the time dependence of the signal, but the absolute scale of the energy density is difficult to estimate from first principles, so the authors extracted it by fitting to their theoretical model; the values above come from that fit. And the modelling identifies three operating regimes, with the scaling not the same across all the experiments: charging power density is superextensive in every regime, while superextensive energy density scaling and subextensive charging time show up in different regimes. ■ A Paradoxical Conclusion: Decoherence Locks the Energy In The most provocative and physically deepest conclusion in the paper concerns the way energy is stored. In a closed quantum system perfectly sealed off from its surroundings, under unitary dynamics, energy drawn in rapidly by superabsorption is superradiated straight back out at the same rate. Charging is fast, but storage is impossible. The authors identify the mechanism that resolves this as decoherence driven by environmental noise, dephasing to be precise. When dephasing occurs, energy in the optically active bright mode is transferred into inactive dark modes that cannot emit into the cavity. Absorption stays fast while decay becomes slow, so the energy is held until it can be used. In the theoretical model, the presence of these decay channels gives rise to ratchet states capable of absorbing but not emitting light, which is what allows the energy to be stored stably. Decoherence, always the thing to be overcome in quantum information science, turns out to be a working requirement for a quantum battery: the paper states that purely closed unitary dynamics is insufficient for a practical device, and that retaining energy calls for finely tuned decoherence processes. ■ Limits and Outlook The study demonstrates the charging of a prototype quantum battery convincingly, but it carries clear limits. No energy extraction: the paper demonstrates charging and trapping the energy, and no further. A practical mechanism for drawing the stored energy out into an external device, that is, discharging, was not demonstrated. The authors do point to existing approaches, such as inserting charge transport layers between the active layer and the cavity layers so that the top cavity becomes a cathode and the bottom an anode. A concentration ceiling: above roughly 10% dye concentration, quenching sets in, driven by aggregation of the dye molecules and intermolecular states such as excimers, and performance degrades. (The authors gesture at biological molecules as a possible way around this, noting that in proteins such as green fluorescent protein (GFP) the active chromophore sits inside a structural cage that suppresses exciton-exciton quenching, and suggesting such materials might open a route to higher concentrations.) In short, this work has opened a path for building superextensive absorption into the design of organic optoelectronic devices. A commercial quantum battery remains a long way off. But the fact alone that collective quantum behaviour across more than 10¹⁰ molecules was demonstrated in air at room temperature, rather than under cryogenic temperatures and ultra-high vacuum, makes this a striking milestone. As the authors put it, the idea of superextensive charging may have wide-reaching consequences for sensing and for energy capture and storage technologies. #Superabsorption #Superradiance #QuantumBattery #DickeModel #Microcavity #Polariton #Decoherence #Dephasing #RatchetStates #UltrafastSpectroscopy #OrganicDye #QuantumBiology #ScienceAdvances Source (Science Advances): https://doi.org/10.1126/sciadv.abk3160 Theoretical precursor (Nature Communications 2014, open access): https://doi.org/10.1038/ncomms5705
Quantum batteries, which are quantum systems to be used for the storage and transformation of energy, have been recently attracting research interest. A promising candidate for their investigation is the Dicke model, which describes an ensemble of two-level systems interacting with a single-mode electromagnetic wave in a resonator cavity. In order to charge the battery, a coupling between the ensemble of two-level systems and resonator cavity should be turned off at a certain moment of time. This moment of time is chosen in such a way that the energy gets fully stored in an ensemble of two-level systems. In our previous works we have investigated a bound-luminosity superradiant state of the extended Dicke model and found analytical expressions for the dynamics of coherent energy transfer between the superradiant condensate and the ensemble of two-level systems. Here, using our previous results, we have derived analytically the superlinear law for the quantum battery charging power $P\ensuremath{\sim}{N}^{3/2}$ as a function of the number N of two-level systems in the battery, and also the N dependence for the charging time ${t}_{c}\ensuremath{\sim}{N}^{\ensuremath{-}1/2}$. The N exponent 3/2 of the charging power is in quantitative correspondence with the recent result 1.541 obtained numerically by other authors. The physics of Dicke quantum battery charging is considered in detail.
Abstract Quantum batteries harness the unique properties of quantum mechanics to enhance energy storage compared to conventional batteries. In particular, they are predicted to undergo superextensive charging, where batteries with larger capacity actually take less time to charge. Up until now however, they have not been experimentally demonstrated, due to the challenges in quantum coherent control. Here we implement an array of two-level systems coupled to a photonic mode to realise a Dicke quantum battery. Our quantum battery is constructed with a microcavity formed by two dielectric mirrors enclosing a thin film of a fluorescent molecular dye in a polymer matrix. We use ultrafast optical spectroscopy to time resolve the charging dynamics of the quantum battery at femtosecond resolution. We experimentally demonstrate superextensive increases in both charging power and storage capacity, in agreement with our theoretical modelling. We find that decoherence plays an important role in stabilising energy storage, analogous to the role that dissipation plays in photosynthesis. This experimental proof-of-concept is a major milestone towards the practical application of quantum batteries in quantum and conventional devices. Our work opens new opportunities for harnessing collective effects in light-matter coupling for nanoscale energy capture, storage, and transport technologies, including the enhancement of solar cell efficiencies.
The rate at which matter emits or absorbs light can be modified by its environment, as markedly exemplified by the widely studied phenomenon of superradiance. The reverse process, superabsorption, is harder to demonstrate because of the challenges of probing ultrafast processes and has only been seen for small numbers of atoms. Its central idea—superextensive scaling of absorption, meaning larger systems absorb faster—is also the key idea underpinning quantum batteries. Here, we implement experimentally a paradigmatic model of a quantum battery, constructed of a microcavity enclosing a molecular dye. Ultrafast optical spectroscopy allows us to observe charging dynamics at femtosecond resolution to demonstrate superextensive charging rates and storage capacity, in agreement with our theoretical modeling. We find that decoherence plays an important role in stabilizing energy storage. Our work opens future opportunities for harnessing collective effects in light-matter coupling for nanoscale energy capture, storage, and transport technologies.
We analyse a two-cavity architecture, in which a lossy cavity hosting N qubits is coherently coupled to an auxiliary cavity, as a resource for the storage phase of an open quantum battery at non-zero temperature. Within a local Lindblad treatment in the resonant configuration, we find that the inter-cavity coupling enhances the suppression of self-discharging across every initial preparation, battery size, and temperature we examine, with the protection degrading smoothly as the mean thermal occupation increases. For a single qubit, the energy-basis coherence of a pure superposition leads to better long-time retention than fully excited state, highlighting the beneficial role of quantum coherence in protecting stored energy against thermal degradation. For two-qubit batteries, Bell-state preparations exhibit enhanced long-time ergotropy retention compared with the fully excited state, while the inclusion of qubit-qubit interactions produces only a weak dependence on the interaction type and strength within the parameter regime considered. Extending the analysis to multi-qubit GHZ-charged batteries with all-to-all Heisenberg interactions, we find that the normalized retained ergotropy increases monotonically with the number of qubits. This behavior is consistent with the collective enhancement of the qubit-cavity coupling in the symmetric Dicke manifold, indicating that larger quantum batteries can benefit from improved protection against self-discharge. These findings establish cavity-assisted protection as a promising strategy for mitigating self-discharging and realizing of long-lived quantum batteries in experimentally accessible platforms.