Scientists have devised several ways to trap light and save it. But suppose you want to keep your pet light beam for ten seconds or a minute—an eternity in the light-storage game. In a vacuum, Harvard University physicist Lene Hau created a tiny cloud of sodium ions so cold that their movements synchronized.
She then shot a light beam into the cloud at the same time she fired a laser of a different frequency into the cloud from the side. This laser confused the electrons of the sodium atoms and kept them from absorbing the primary light beam. Hau showed that if you block the laser at the correct moment, the light beam stops, though its particulars are still there, encoded in the imprint of the electrons. If you turn the laser back on, the beam will resume its travels and shoot out the other side. Thus, light can, in essence, be frozen.
Hau was even able to transfer the imprint of the light beam into a different cloud and reconstitute it up to a minute later. Crazy stuff.
The remaining question, we suppose, is Why? All three scenarios have lower output power when the trapping charge separation rates are low, since this creates a bottleneck in the cycle and limits the size of the photocurrent.
However, the ratchet states hold a great advantage over the other scenarios in this bottlenecked region, since this is precisely the situation in which excitations need to be held for some time by the ring before extraction is possible.
The NP scenario performs poorly in this case, since excitations in all likelihood decay before being extracted. Indeed, the relative power output of ratcheting over NP, shown in the left panel of Figure 5 , therefore rises to as high as a factor of As previously discussed, this ratcheting enhancement over the dark-state protection relies on the absorber being in radiative equilibrium with the sun.
By contrast, for a bare molecular ring antenna under direct solar illumination, the effective absorption rate is estimated to be several orders of magnitude lower see SI. This renders the probability of ratcheting so low so as to no longer yield an appreciable advantage.
Nevertheless, the dark-state protection mechanism remains operational and advantageous cf. Referring to Figure 4 , we see that the ratchet power output in this region is already close to its maximum.
Even so, if the extraction rate was arbitrarily tunable, then even a moderate bottleneck could be avoided completely and the main advantage of ratcheting and dark-state protection could be removed. The severity of the bottleneck is likely to be inversely proportional to the number of reaction centers.
A photocell design exploiting ratcheting in the moderate bottleneck regime would be likely then to generate optimal power per unit volume of material, and similar design principles will apply to artificially designed molecular light harvesting systems see SI for a quantitative discussion. The choice of interaction strength S is also important.
This dependence arises because the size of S determines the gap between bright and dark or ratchet states. Consequently, larger values of S entail more directed dissipation into the lower states of each band, boosting the occupation of ratchet states.
However, at the same time, increasing S leads to a lower ratchet state energy and so a lower trap energy and, hence, to a voltage drop. The trade-off between these two competing influences leads to a maximum in ratchet performance. Ratcheting scores consistently slightly below NP, however, the difference between the two remains small in the regime of interest, and ratcheting remains advantageous with an overall power output that is substantially higher than NP.
As we discuss in detail in the SI , ratcheting continues to convey an advantage in the presence of moderate levels of various real-world imperfections, such as site-energy disorder, nonradiative recombination, and exciton—exciton annihilation.
In particular, we find that for modest disorder, that is, when the variation in site energies is less than the coupling strength, the performance of the ratchet model is broadly unaffected.
As a consequence, there seems to be neither much of an advantage nor a fundamental drawback associated with moving to rings comprising more than four sites. We have investigated the light-harvesting properties of coupled ring structures, inspired by the molecular rings that serve as antennae in photosynthesis.
Considering a vibrational as well as an electromagnetic environment allows the system to explore the full Hilbert space rather than just the restricted subset of Dicke ladder states. We have shown that the off-ladder states possess interesting and desirable properties, which can be harnessed for enhancing both the current and the power of a ring-based photocell device.
Dark-state protection is available under ambient conditions i. Several possible systems could be used to observe the effect also see SI. These range from tailor-made demonstrator structures comprising superconducting qubits, 23 where the radiation field is in the microwave range. Closer in spirit to what we have proposed are macrocyclic molecules 24 with optical transitions, or NV 25 and SiV 26 centers in diamond, which have optical transitions that enable the study of the electronic dynamics at the single center level.
Our approach generalizes existing concepts for dark-state protection 4, 5 to arbitrary numbers of sites and importantly includes multiexciton states, which introduce the ratcheting effect as a distinct additional mechanism for enhancing the overall light-harvesting performance. The optical ratchet enhancement is particularly well suited to situations where exciton extraction and conversion represent the bottleneck of a photocell cycle.
In future work, it would be interesting to explore combining optical ratcheting with enhancements of the primary absorption process, for example, by exploiting the phenomena of stimulated absorption 27 or superabsorption.
Supporting Information. Author Information. The authors declare no competing financial interest. Organic photovoltaics Energy Environ. Royal Society of Chemistry. A review. This technol. Here, we introduce the energy and environmental science community to the basic concepts of org. In order to find an upper theoretical limit for the efficiency of p-n junction solar energy converters, a limiting efficiency, called the detailed balance limit of efficiency, has been calcd.
The efficiency is also calcd. Efficiencies at the matched loads were calcd. The max. Actual junctions do not obey the predicted current-voltage relation, and reasons for the difference and its relevance to efficiency are discussed. American Physical Society. The fundamental limit to photovoltaic efficiency is widely thought to be radiative recombination which balances radiative absorption.
The authors here show that it is possible to break detailed balance via quantum coherence, as in the case of lasing without inversion and the photo-Carnot quantum heat engine. This yields, in principle, a quantum limit to photovoltaic operation which can exceed the classical one. The present work is in complete accord with the laws of thermodn. Artificially implementing the biol. Here, we develop such a paradigm and present a model photocell based on the nanoscale architecture and mol.
Quantum interference of photon absorption and emission induced by the dipole-dipole interaction between mol. This result opens a promising new route for designing artificial light-harvesting devices inspired by biol. Delocalized quantum states enhance photocell efficiency Phys.
The high quantum efficiency of photosynthetic complexes has inspired researchers to explore new routes to utilize this process for photovoltaic devices. Quantum coherence has been demonstrated to play a crucial role in this process. Herein, we propose a three-dipole system as a model of a new photocell type which exploits the coherence among its three dipoles.
We have proved that the efficiency of such a photocell is greatly enhanced by quantum coherence. We have also predicted that the photocurrents can be enhanced by about These results suggest a promising novel design aspect of photosynthesis-mimicking photovoltaic devices.
Energy dissipation and decoherence are at first glance harmful to acquiring the long exciton lifetime desired for efficient photovoltaics. In the presence of both optically forbidden namely, dark and allowed bright excitons, however, they can be instrumental, as suggested in photosynthesis.
By simulating the quantum dynamics of exciton relaxations, we show that the optimized decoherence that imposes a quantum-to-classical crossover with the dissipation realizes a dramatically longer lifetime. In an example of a carbon nanotube, the exciton lifetime increases by nearly 2 orders of magnitude when the crossover triggers a stable high population in the dark excitons.
Conventional photocells suffer a fundamental efficiency threshold imposed by the principle of detailed balance, reflecting the fact that good absorbers must necessarily also be fast emitters. This limitation can be overcome by "parking" the energy of an absorbed photon in a dark state which neither absorbs nor emits light.
Here we argue that suitable dark states occur naturally as a consequence of the dipole-dipole interaction between two proximal optical dipoles for a wide range of realistic mol. We develop an intuitive model of a photocell comprising two light-absorbing mols. We conclude by describing a road map for identifying suitable mol.
B , , — DOI: American Chemical Society. Excitation-energy transfer EET has been obsd. BChls , in the light-harvesting antenna complex LH2 of photosynthetic purple bacteria. This rapid EET cannot be understood within the framework of F. The present work shows that it can be rationalized on the basis of a recently proposed formula for EET between mol. The formula differs from F. Excited states of B are regarded as excitonic, while those of B as monomeric.
The exciton-phonon coupling was taken into account over all orders for B and within a self-consistent second-order perturbation for B The authors demonstrate that this rapid EET occurs to optically forbidden exciton states of B, without total transition dipole, due to strong interaction of a transition dipole on a BChl in B with those on nearby BChls in B Dynamic localization of electronic excitation in photosynthetic complexes revealed with chiral two-dimensional spectroscopy.
There is no corresponding record for this reference. Superradiance of Frenkel excitons in linear systems Phys. B: Condens. Matter Mater. Fermion excited states in one-dimensional molecular aggregates with site disorder: Nonlinear optical response Phys. Elsevier B. Compared to the preceding version, we have introduced numerous new features, enhanced performance, and made changes in the Application Programming Interface API for improved functionality and consistency within the package, as well as increased compatibility with existing conventions used in other scientific software packages for Python.
The most significant new features include efficient solvers for arbitrary time-dependent Hamiltonians and collapse operators, support for the Floquet formalism, and new solvers for Bloch-Redfield and Floquet-Markov master equations. Here we introduce these new features, demonstrate their use, and give a summary of the important backward-incompatible API changes introduced in this version. Bath-induced coherence and the secular approximation Phys.
A: At. Superabsorption of light via quantum engineering Nat. Higgins, K. Nature Publishing Group. Almost 60 years ago Dicke introduced the term superradiance to describe a signature quantum effect: N atoms can collectively emit light at a rate proportional to N2. Structures that superradiate must also have enhanced absorption, but the former always dominates in natural systems.
Fang and his colleagues found that by properly engineering the time delay and wave parameters, they could send in two photons and trap one with more than percent probability. With improved parameters, they expect that, in principle, a perfect trapping is possible. The result provides an alternative example for investigating quantum dynamics in a nonlinear system. In turn, this can inform broad areas of research involving quantum many-body physics, where systems are composed of numerous quantum mechanically interacting particles.
For example, quantum computers need to store a photon and retrieve it when needed. Because photons move at the speed of light and cannot stop, we need to slow them down so that they can be stored. Now, we have a new, verifiable mechanism to store a photon. Fang acknowledged that the team's photon scattering work also differs because of its influence by non-Markovian dynamics, which can be difficult to address because of how previous states influence the subsequent states in a system.
In non-Markovian dynamics with delay effects, our study presents a model system with similar physics that can be solved numerically to let physicists infer and examine what happens in these systems. Ultimately, Fang noted, there is high potential to take advantage of the BIC, such as for creating a two-qubit entangling gate for quantum computers or even envisioning long-distance communication over quantum networks.
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May 15, Exciting the BIC localized between two distant qubits coupled to a one-dimensional waveguide: schematic for one system that permits the existence of BIC when the qubits are separated by multiple half-resonant wavelengths.
Credit: Brookhaven National Laboratory. Explore further.
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