Collective Quantum Effects Research Articles

Molecular beam epitaxy of GaN/AlGaN quantum wells on bulk GaN substrate in the step-flow or step meandering regime: Influence on indirect exciton diffusion

GaN/AlxGa1−xN quantum wells were grown by molecular beam epitaxy on high quality bulk (0001) GaN substrates. The quantum well thickness was set in the 6–8 nm range to favor the photoluminescence emission of indirect excitons. Indeed, such excitons are known to be spatially indirect due to the presence of the internal electric field which spatially separates the electron and hole wave functions. The growth conditions were optimized in view of minimizing the photoluminescence peak broadening. In particular, the impact of growth temperature (up to 900 °C) on the surface morphology, structural, and photoluminescence properties was studied. The diffusion of indirect excitons on the scale of tens of micrometers was measured with a micro-photoluminescence setup equipped with a spatially resolved detection. A dedicated model and its analysis allow us to extract from these measurements the exciton diffusion constant and to conclude on the optimum growth conditions for the GaN/AlxGa1−xN quantum well structures suited for studies of quantum collective effects in indirect exciton liquids.

Collective modes and quantum effects in two-dimensional nanofluidic channels.

Nanoscale fluid transport is typically pictured in terms of atomic-scale dynamics, as is natural in the real-space framework of molecular simulations. An alternative Fourier-space picture, that involves the collective charge fluctuation modes of both the liquid and the confining wall, has recently been successful at predicting new nanofluidic phenomena such as quantum friction and near-field heat transfer, that rely on the coupling of those fluctuations. Here, we study the charge fluctuation modes of a two-dimensional (planar) nanofluidic channel. Introducing confined response functions that generalize the notion of surface response function, we show that the channel walls exhibit coupled plasmon modes as soon as the confinement is comparable to the plasmon wavelength. Conversely, the water fluctuations remain remarkably bulk-like, with significant confinement effects arising only when the wall spacing is reduced to 7 Å. We apply the confined response formalism to predict the dependence of the solid-water quantum friction and thermal boundary conductance on channel width for model channel wall materials. Our results provide a general framework for Coulomb interactions of fluctuating matter under nanoscale confinement.

Multiband Quantum Materials

Quantum materials are defined by the emergence of new properties resulting from collective quantum effects and by holding promise for their quantum applications. Novel superconductors, from high-Tc cuprates and iron-based superconductors to twisted monolayers, exhibit a higher level of emergent complexity, with a multiband electronic structure playing a pivotal role in their comprehension and potential applications. Here, we provide a brief overview of key multiband effects in these superconductors and topological semimetals, offering guidelines for the theory-assisted development of new quantum materials and devices.

Collective Microwave Response for Multiple Gate-Defined Double Quantum Dots.

We fabricate and characterize a hybrid quantum device that consists of five gate-defined double quantum dots (DQDs) and a high-impedance NbTiN transmission resonator. The controllable interactions between DQDs and the resonator are spectroscopically explored by measuring the microwave transmission through the resonator in the detuning parameter space. Utilizing the high tunability of the system parameters and the high cooperativity (Ctotal > 17.6) interaction between the qubit ensemble and the resonator, we tune the charge-photon coupling and observe the collective microwave response changing from linear to nonlinear. Our results present the maximum number of DQDs coupled to a resonator and manifest a potential platform for scaling up qubits and studying collective quantum effects in semiconductor-superconductor hybrid cavity quantum electrodynamics systems.

Influence of Collective Nuclear Vibrations on Initial State Eccentricities in Pb + Pb Collisions

We study within the Monte Carlo Glauber model the influence of collective quantum effects in the Pb nucleus on the azimuthal anisotropy coefficients $\epsilon_{2,3}$ in Pb+Pb collisions at the LHC energies. To account for the quantum effects, we modify the sampling of the nucleon positions by applying suitable filters that guarantee that the colliding nuclei have the mean squared quadrupole and octupole moments consistent with the ones extracted from the experimental quadrupole and octupole strength functions for the Pb nucleus with the help of the energy weighted sum rule. Our Monte Carlo Glauber model with the modified sampling of the nucleon positions leads to $\epsilon_2\{2\}/\epsilon_3\{2\}\approx 0.8$ at centrality $\lesssim 1$\%, which allows to resolve the $v_2$-to-$v_3$ puzzle.

Decay and renormalization of a longitudinal mode in a quasi-two-dimensional antiferromagnet

An ongoing challenge in the study of quantum materials, is to reveal and explain collective quantum effects in spin systems where interactions between different modes types are important. Here we approach this problem through a combined experimental and theoretical study of interacting transverse and longitudinal modes in an easy-plane quantum magnet near a continuous quantum phase transition. Our inelastic neutron scattering measurements of Ba2FeSi2O7 reveal the emergence, decay, and renormalization of a longitudinal mode throughout the Brillouin zone. The decay of the longitudinal mode is particularly pronounced at the zone center. To account for the many-body effects of the interacting low-energy modes in anisotropic magnets, we generalize the standard spin-wave theory. The measured mode decay and renormalization is reproduced by including all one-loop corrections. The theoretical framework developed here is broadly applicable to quantum magnets with more than one type of low energy mode.

Algebraic harmony and probabilities in genomes. Long-range coherence in quantum code biology

According to the founders of quantum mechanics and quantum biology P. Jordan and E. Schrödinger, the main difference between living and inanimate objects is the dictatorial influence of genetic molecules on the whole living organism. Code biology can make a valuable contribution to understanding this dictatorial influence of genetic molecules whose ensemble is endowed with many interconnected alphabets and codes. The paper is devoted to probability rules of nucleotide sequences of single-stranded DNA in eukaryotic and prokaryotic genomes. These rules are connected with n-plets alphabets of DNA whose nucleotide sequences are considered as bunches of many parallel texts written in interconnected n-plets alphabets. The rules draw attention to genomic phenomena of special tetragroupings of n-plets and new genomic symmetries. A generalization of the second Chargaff's rule is described. They show the existence of long-range coherence in genomic DNA sequences and reveal new connections of structural features of genomic sequences with formalisms of quantum mechanics and quantum informatics. The author supposes that the received results are related to the known vibration-resonance theory of G. Frohlich about long-range coherence in biological systems, that is, about collective quantum effects there. The possible influence of the described genetiс probability phenomena on the genetically inherited physiological structures is noted and discussed.

Quantum phase transition of a two-dimensional quadrupolar system

Ensembles with long-range interactions between particles are promising for revealing strong quantum collective effects and many-body phenomena. Here we study the ground-state phase diagram of a two-dimensional Bose system with quadrupolar interactions using a diffusion Monte Carlo technique. We predict a quantum phase transition from a gas to a solid phase. The Lindemann ratio and the condensate fraction at the transition point are $\gamma=0.269(4)$ and $n_0/n=0.031(4)$, correspondingly. We observe the strong rotonization of the collective excitation branch in the vicinity of the phase transition point. Our results can be probed using state-of-the-art experimental systems of various nature, such as quasi-two-dimensional systems of quadrupolar excitons in transition metal dichalcogenide (TMD) trilayers, quadrupolar molecules, and excitons or Rydberg atoms with quadrupole moments induced by strong magnetic fields.

Outstanding nobility observed in Cu5 clusters reveals the key role of collective quantum effects

Outstanding nobility observed in Cu5 clusters reveals the key role of collective quantum effects

Harmonic Progression in Bioinformatics and Recurrent Series in Inherited Biostructures

The article is devoted to the study of new approaches to the development of mathematical models in genetic biomechanics, which studies the structural relationships of the genetic coding system with genetically inherited biological forms. More specifically, we are talking about models based on the recurrent harmonic progression whose connection with the information sequences of DNA molecules in the genomes of higher and lower organisms was recently revealed. In particular, the article describes previously unknown connections of the function of natural logarithms with the structures of the molecular genetic system, which allow modelling the main psychophysical logarithmic law by Weber-Fechner and also many other logarithmic structures in genetically inherited biological systems. In physics, the harmonic progression is traditionally considered, first of all, as related to standing waves in resonators. Our results are correlated with Frohlich’s vibration-resonant theory about collective quantum effects and long-range communication in biological systems.

Collective Nuclear Vibrations and Initial State Shape Fluctuations in Central Pb + Pb Collisions: Resolving the v2 to v3 Puzzle

We have studied, for the first time, the influence of the collective quantum effects in the nuclear wave functions on the azimuthal anisotropy coefficients $\epsilon_{2,3}$ in the central Pb+Pb collisions at the LHC energies. With the help of the energy weighted sum rule we demonstrate that the classical treatment with the Woods-Saxon nuclear density overestimates the mean square quadrupole moment of the $^{208}$Pb nucleus by a factor of $\sim 2.2$. The Monte-Carlo Glauber simulation of the central Pb+Pb collisions accounting for the restriction on the quadrupole moment leads to $\epsilon_2/\epsilon_3\approx 0.8$ which allows to resolve the $v_2$-to-$v_3$ puzzle.

Achieving the Scaling Limit for Nonequilibrium Green Functions Simulations.

The dynamics of strongly correlated fermions following an external excitation reveals extremely rich collective quantum effects. Examples are fermionic atoms in optical lattices, electrons in correlated materials, and dense quantum plasmas. Presently, the only quantum-dynamics approach that rigorously describes these processes in two and three dimensions is the nonequilibrium Green functions (NEGF) method. However, NEGF simulations are computationally expensive due to their T^{3} scaling with the simulation duration T. Recently, T^{2} scaling was achieved with the generalized Kadanoff-Baym ansatz (GKBA), for second-order Born (SOA) selfenergies, which has substantially extended the scope of NEGF simulations. Here we demonstrate that GKBA-NEGF simulations can be performed with order T^{1} scaling, both for SOA and GW selfenergies, and point out the remarkable capabilities of this approach.

COLLECTIVE NUCLEAR VIBRATIONS AND INITIAL STATE SHAPE flUCTUATIONS IN CENTRAL PB + PB COLLISIONS: RESOLVING THE V2 TO V3 PUZZLE

We have studied, for the first time, the influence of the collective quantum effects in the nuclear wavefunctions on the azimuthal anisotropy coefficients ∈2,3 in the central Pb + Pb collisions at the LHC energies. With the help of the energy weighted sum rule, we demonstrate that the classical treatment with the Woods—Saxon nuclear density overestimates the mean square quadrupole moment of the 208Pb nucleus by a factor of ~2.2. The Monte Carlo Glauber simulation of the central Pb + Pb collisions accounting for the restriction on the quadrupole moment leads to ∈2/∈3 ≈ 0.8 which allows to resolve the v2-to-v3 puzzle.

Investigating New Reactivities Enabled by Polariton Photochemistry.

We perform quantum dynamics simulations to investigate new chemical reactivities enabled by cavity quantum electrodynamics. The quantum light-matter interactions between the molecule and the quantized radiation mode inside an optical cavity create a set of hybridized electronic-photonic states, so-called polaritons. The polaritonic states adapt the curvatures from both the ground and the excited electronic states, opening up new possibilities to control photochemical reactions by exploiting intrinsic quantum behaviors of light-matter interactions. With quantum dynamics simulations, we demonstrate that the selectivity of a model photoisomerization reaction can be controlled by tuning the photon frequency of the cavity mode or the light-matter coupling strength, providing new ways to manipulate chemical reactions via the light-matter interaction. We further investigate collective quantum effects enabled by coupling the quantized radiation mode to multiple molecules. Our results suggest that in the resonance case, a photon is recycled among molecules to enable multiple excited state reactions, thus effectively functioning as a catalyst. In the nonresonance case, molecules emit and absorb virtual photons to initiate excited state reactions through fundamental quantum electrodynamics processes. These results from quantum dynamics simulations reveal basic principles of polariton photochemistry as well as promising reactivities that take advantage of intrinsic quantum behaviors of photons.

II-VI Based Inorganic-Organic Hybrid Quantum Structures with High Degree of Structural Ordering, Long-Term Stability, and Novel Properties

Translational symmetry ensures phase coherency of a physical process among different units of a crystal, and thus produces collective quantum effects beyond the sum of the units. Any significant physical and/or chemical fluctuation, which typically exists in a semiconductor alloy or self-assembled or artificially grown nanostructure array, would hinder our ability to study and use the collective behavior relying on this coherency. Man-made structures remain as one of the central interests since semiconductor superlattices were proposed by Esaki and Tsu in 1970. Unfortunately, man-made structures with both genuine long- and short-range order are rare. Here a new family of inorganic-organic hybrid quantum structures provides us a unique opportunity to explore coherency quantum phenomena in man-made structures. These self-assembled hybrid structures are ultra-thin slabs or atomic chains of II-VI semiconductors interconnected or coordinated by small organic molecules, exhibiting very high degree of structural perfectness comparable to a typical binary semiconductor, as manifested in a very small XRD and Raman linewidth. They exhibit several extraordinary properties, for instance, exceedingly strong excitonic absorption, large exciton binding energies, and zero thermal expansion. By varying either the inorganic or organic component, one can tune the coupling between the inorganic units to achieve quasi-1-D, quasi-2-D, and 3-D super-structures with tailored material properties for different basic research interests and potential applications (e.g., room temperature exciton-polariton condensation, high efficiency UV emission and detection, p-type transparent conducting material). Furthermore, some hybrid structures, such as β-ZnTe(en)0.5, have been found to retain the same structural perfectness after being stored under ambient condition over a decade, despite others show varying degrees of structural degradation. This finding indicates that the long-term stability of the material depends on both the intrinsic effect, the strength of chemical bonding, and the extrinsic effect, such as the presence of defects in the initial structure.

Open quantum systems with local and collective incoherent processes: Efficient numerical simulations using permutational invariance

The permutational invariance of identical two-level systems allows for an exponential reduction in the computational resources required to study the Lindblad dynamics of coupled spin-boson ensembles evolving under the effect of both local and collective noise. Here we take advantage of this speedup to study several important physical phenomena in the presence of local incoherent processes, in which each degree of freedom couples to its own reservoir. Assessing the robustness of collective effects against local dissipation is paramount to predict their presence in different physical implementations. We have developed an open-source library in Python, the Permutational-Invariant Quantum Solver (PIQS), which we use to study a variety of phenomena in driven-dissipative open quantum systems. We consider both local and collective incoherent processes in the weak, strong, and ultrastrong-coupling regimes. Using PIQS, we reproduced a series of known physical results concerning collective quantum effects and extended their study to the local driven-dissipative scenario. Our work addresses the robustness of various collective phenomena, e.g., spin squeezing, superradiance, quantum phase transitions, against local dissipation processes.

Enhancing the Charging Power of Quantum Batteries.

Can collective quantum effects make a difference in a meaningful thermodynamic operation? Focusing on energy storage and batteries, we demonstrate that quantum mechanics can lead to an enhancement in the amount of work deposited per unit time, i.e., the charging power, when N batteries are charged collectively. We first derive analytic upper bounds for the collective quantum advantage in charging power for two choices of constraints on the charging Hamiltonian. We then demonstrate that even in the absence of quantum entanglement this advantage can be extensive. For our main result, we provide an upper bound to the achievable quantum advantage when the interaction order is restricted; i.e., at most k batteries are interacting. This constitutes a fundamental limit on the advantage offered by quantum technologies over their classical counterparts.

Collective proton transfer in ordinary ice: local environments, temperature dependence and deuteration effects.

The transfer of multiple protons in hydrogen-bonded networks usually occurs one proton at a time. At sufficiently high temperatures, each proton transfers via thermally activated hopping along its hydrogen bond, thereby moving a charge defect through the network. At low temperatures, quantum-mechanical tunnelling might set in instead, thus avoiding hopping over the energy barriers. In the case of several transferring protons, independent thermal hopping or quantum tunnelling of the individual protons becomes less favourable because of a significant creation of charge defects. In individual molecules or hydrogen-bonded molecular complexes, for instance, double proton transfer is often found to be concerted. Multiple proton transfer that avoids charge defects can occur in cyclic topologies built from several hydrogen bonds that allow for directional chains of proton transfer. This requires perfect proton order within these rings, which imposes handedness and thus chirality, and changes parity upon transfer of all protons. Ordinary ice, which is hexagonal ice Ih, is the most stable form of crystalline ice obtained upon freezing liquid water at ambient pressure and consists of interconnected six rings of oxygen atoms that host six protons each. These hexagonal rings remain proton disordered even down to low temperatures, as heralded by the residual entropy of ice Ih. However, owing to combinatorics, a certain number of these six rings is proton ordered in macroscopic crystals. These chiral hexameric rings might support coherent tunnelling of the hosted protons. Indeed, there is some evidence in the recent literature, both experimental and simulational, that correlated tunnelling of all six protons might be possible in proton-ordered six rings in ice Ih if temperatures are low enough. In this Perspective, the key ideas and previous findings will be reviewed in the light of relevant experiments with a focus on available ab initio path integral simulation work supplemented with additional data provided herein.

Observation of the exciton Mott transition in the photoluminescence of coupled quantum wells

Indirect excitons in coupled quantum wells have long radiative lifetimes and form a cold quasi-two-dimensional population suitable for studying collective quantum effects. Here we report the observation of the exciton Mott transition from an insulating (excitons) to a conducting (ionized electron-hole pairs) phase, which occurs gradually as a function of carrier density and temperature. The transition is inferred from spectral and time-resolved photoluminescence measurements around a carrier density of $2\times10^{10} \mathrm{cm}^{-2}$ and temperatures of $12-16 \mathrm{K}$. An externally applied electric field is employed to tune the dynamics of the transition via the quantum-confined Stark effect. Our results provide evidence of a gradual nature of the exciton Mott transition.

Coherence-Induced Reversibility and Collective Operation of Quantum Heat Machines via Coherence Recycling

Collective behavior where a set of elements interact and generate effects that are beyond the reach of the individual non interacting elements, are always of great interest in physics. Quantum collective effects that have no classical analogue are even more intriguing. In this work we show how to construct collective quantum heat machines and explore their performance boosts with respect to regular machines. Without interactions between the machines the individual units operate in a stochastic, non-quantum manner. The construction of the collective machine becomes possible by introducing two simple quantum operations: coherence extraction and coherence injection. Together these operations can harvest coherence from one engine and use it to boost the performance of a slightly different engine. For weakly driven engines we show that the collective work output scales quadratically with the number of engines rather than linearly. Eventually, the boost saturates and the scaling becomes linear. Nevertheless, even in saturation, work is still significantly boosted compared to individual operation. To study the reversibility of the collective machine we introduce the 'entropy pollution' measure. It is shown that there is a regime where the collective machine is N times more reversible while producing N times more work, compared to the individual operation of N units. Moreover, the collective machine can even be more reversible than the most reversible unit in the collective. This high level of reversibility becomes possible due to a special symbiotic mechanism between engine pairs.