arXiv subjects

Paul Brumer

Publications and source records attributed to Paul Brumer.

At least 19 recordsLinked to original sources

arXiv2026

Exact dynamics of first-order system-bath coherence in bilinear bosonic models

We investigate the exact dynamics of first-order system--bath coherence induced by excitation exchange in bilinear bosonic models. By solving the linear Heisenberg equations, we obtain the exact evolution of system and bath operators and evaluate the first-order coherence between the system mode and the collective bath mode directly coupled to it. We analyze how this coherence depends on initial occupations, coupling strength, spectral width, and detuning, showing that its buildup and oscillatory behavior are closely related to excitation exchange and reservoir memory. We further examine controlled coherence dynamics under leakage-elimination-operator inspired modulation of the system frequency. The results show that random modulation can suppress excitation leakage and maintain finite and relatively stable coherence fluctuations at long times. These results clarify the evolution and control of first-order system--bath coherence in settings where a localized bosonic mode is coupled to a structured bosonic reservoir.

quant-ph↗

arXiv2026

Coherent Control of Energy Transport at Room Temperature in a Noisy Bath

Coherent control of energy transport in a non-equilibrium steady-state (NESS) in a reaction-center-connected donor-acceptor pair is proposed. The pigments are considered to be continuously interacting with incoherent radiation and a phonon bath while being driven by phase-controlled coherent fields. Coherent excitation of the donor-acceptor pair is shown to induce interference between excitation pathways, resulting in phase dependent modulation of the flux. As a consequence one can enhance or suppress energy transfer via interference, e.g. an optical energy switch. The persistence of such interference enables coherent control at a NESS in dissipative regime suggests an extension of the operational scope of quantum control from traditional transient domain with low dissiaption to noisy environment NESS at room temperature.

quant-ph↗

arXiv2026

Identical-Particle Symmetry-Enabled Complete Coherent Control of Ultracold Atomic and Molecular Collisions

We show that exchange symmetry in collisions of identical particles enables symmetry-protected coherent control of the total scattering cross section. For identical fermions, antisymmetrization enforces complete phase synchronization of the contributing scattering channels, yielding maximal control visibility. For identical bosons, synchronization persists but with reduced visibility due to additional exchange (satellite) contributions. Collisions of distinguishable particles lack this symmetry-imposed phase locking, leading to lower controllability and visibility. We elucidate these principles through coupled-channel quantum-scattering calculations for lithium-lithium collisions, comparing the $^{6}\mathrm{Li}$-$^{6}\mathrm{Li}$ (identical fermions), $^{7}\mathrm{Li}$-$^{7}\mathrm{Li}$ (identical bosons), and $^{6}\mathrm{Li}$-$^{7}\mathrm{Li}$ (distinguishable) systems. Furthermore, in the identical particle cases, symmetry-enforced synchronization enables full control over the parity of the final state at any collisional energy. This mechanism is broadly applicable to identical-particle collisions, including homonuclear molecules for which established approaches -- DC electric fields, or microwave shielding -- are ineffective or unavailable.

physics.atom-ph↗

arXiv2026

General framework for quantifying entanglement production in ultracold molecular collisions and chemical reactions

Entanglement, a defining feature of quantum mechanics, arises naturally from interactions between molecular systems. Yet the precise nature and quantification of entanglement in the products of molecular collisions and reactions remain largely unexplored. Here, we show that coupling between the external (motional) and internal degrees of freedom of the colliding molecules generates diverse forms of product-state entanglement: discrete-discrete, continuum-continuum, and hybrid discrete-continuum. We develop a general theoretical framework to quantify these entanglement forms directly from scattering S-matrix elements and identify a novel class of entangled states-multimode hybrid cat states, that exhibit multimode discrete-continuum entanglement. Although applicable at arbitrary collision energies, the formalism is illustrated in the ultracold and cold regimes for inelastic Rb+SrF and Rb+Sr$^+$ collisions, as well as the chemical reaction F+HD $\rightarrow$ HF+D, DF+H. We demonstrate that entanglement can be efficiently controlled near magnetic Feshbach resonances, paving the way for precise magnetic control of product-state entanglement generation in ultracold molecular collisions.

physics.atom-ph↗

arXiv2025

Mixed State Entanglement Via the Cauchy-Schwarz Inequality

The entanglement properties of mixed states are of great importance in the study of open quantum systems and quantum information science, but commonly used entanglement measures, such as negativity, can be difficult to apply or connect to physical properties of the system. We introduce the Cauchy-Schwarz Violation (CSV) Condition, which has a simple dependence on the populations and coherences of the density operator. A sufficient condition for entanglement, it provides a more direct connection to the physical characteristics of the system such as its symmetries. We illustrate the often surprising insights gained from the CSV condition by applying it to the Jaynes-Cummings Model, the Quantum Rabi Model, and an open-system Quantum Rabi Model.

quant-ph↗

arXiv2025

Leveraging Reactant Entanglement in the Coherent Control of Ultracold Bimolecular Chemical Reactions

Entanglement is a crucial resource for achieving quantum advantages in quantum computation, quantum sensing, and quantum communication. As shown in this Letter, entanglement is also a valuable resource for the coherent control of the large class of bimolecular chemical reactions. We introduce an entanglement-enhanced coherent control scheme, in which the initial preparation of the superposition state is divided into two steps: the first entangles the reactants, and the second is responsible for coherent control. This approach can overcome the limitations of traditional coherent control of scattering caused by non-interfering pathways, known as satellite terms. By tuning the amount of entanglement between reactants, the visibility of coherent control in chemical reactions can be modulated and optimized. Significantly, there exists an optimal amount of entanglement, which ensures complete indistinguishability of the reaction pathways, maximizing the extent of control. This entanglement-enhanced coherent control scheme is computationally illustrated using the ultracold KRb + KRb reaction, where a perfect control over the parity of the product rotational states is achieved.

physics.atom-ph↗

arXiv2025

Parametric Hypersensitivity and Transport in the Steady-State Open-System Holstein Model

We demonstrate that the nonequilibrium steady state (NESS) of an open-system Holstein model with linear bias displays extreme sensitivity to the closed system parameters. This sensitivity is shown to correspond to avoided crossings in the closed system spectrum, as previously demonstrated in the Rabi model. We then develop a kinetic model to analyze the effects of environmental parameters on NESS hypersensitivity. This reveals that hypersensitivity only exists in intermediate environmental parameter regimes, a prediction that is verified numerically. The inherent spatial character of the Holstein model offers a natural connection to transport, revealing that transport properties in the steady-state regime can be optimized by simultaneously coordinating the closed- and open-system parameters.

quant-ph↗

arXiv2024

Time-Reversal Symmetry-Protected Coherent Control of Ultracold Molecular Collisions

Coherent control of atomic and molecular scattering relies on the preparation of colliding particles in superpositions of internal states, establishing interfering pathways that can be used to tune the outcome of a scattering process. However, incoherent addition of different partial wave contributions to the integral cross sections (partial wave scrambling), commonly encountered in systems with complex collisional dynamics, poses a significant challenge, often limiting control. This work demonstrates that time-reversal symmetry can overcome these limitations by constraining the relative phases of S-matrix elements, thereby protecting coherent control against partial wave scrambling, even for collisions mediated by highly anisotropic interactions. Using the example of ultracold O$_2$-O$_2$ scattering, we show that coherent control is robust against short-range dynamical complexity. Furthermore, the time-reversal symmetry also protects the control against a distribution of collisional energy. These findings show that ultracold scattering into the final states that are time-reversal-invariant, such as the J = 0, M = 0 rotational state, can always be optimally controlled by using time-reversal-invariant initial superpositions. Beyond the ultracold regime, we observe significant differences in the controllability of crossed-molecular beam vs. trap experiments with the former being easier to control, emphasizing the cooperative role of time-reversal and permutation symmetries in maintaining control at any temperature. These results open new avenues for the coherent control of complex inelastic collisions and chemical reactions both in and outside of the ultracold regime.

physics.atom-ph↗

arXiv2024

Machine Learning Optimization of non-Kasha Behavior and of Transient Dynamics in Model Retinal Isomerization

Designing a model of retinal isomerization in Rhodopsin, the first step in vision, that accounts for both experimental transient and stationary state observables is challenging. Here, multi-objective Bayesian optimization is employed to refine the parameters of a minimal two-state-two-mode (TM) model describing the photoisomerization of retinal in Rhodopsin. With an appropriate selection of objectives, the optimized retinal model predicts excitation wavelength-dependent fluorescence spectra that closely align with experimentally observed non-Kasha behavior in the non-equilibrium steady state. Further, adjustments to the potential energy surface within the TM model reduce the discrepancies across the time domain. Overall, agreement with experimental data is excellent.

physics.bio-ph↗

arXiv2024

Non-Markovian environment induced anomaly in steady state quantum coherence

Environment induced steady state quantum coherence (SSQC) is a captivating phenomenon that challenges conventional understandings of decoherence. In this letter, we delve into the foundational aspects of environment-induced SSQC, shedding light on its emergence within the framework of system-bath interactions. Starting from a microscopic system-bath coupled model, we investigate the dependence of SSQC on environmental memory effects, bath temperature, system-bath coupling strength, and squeezing parameters. Our findings reveal that the environment not only acts as a generator but also as a disruptor of SSQC. A peak will exist for a non-Markovian bath, which is a result of competition between these two mechanisms. Interestingly, the peak disappears in Markovian case. Additionally, we observe that the generated SSQC can be further amplified through environment squeezing.

quant-ph↗

arXiv2024

Magnetic Feshbach resonances in ultracold atom-molecule collisions

We report numerically exact quantum scattering calculations on magnetic Feshbach resonances in ultracold, strongly anisotropic atom-molecule [Rb($^2$S) + SrF($^2\Sigma^+$)] collisions based on state-of-the-art ab initio potential energy surfaces. We find broad resonances mediated by the intermolecular spin-exchange interaction, as well as narrow resonances due to the intramolecular spin-rotation interaction, which are unique to atom-molecule collisions. Remarkably, the density of resonances in atom-molecule collisions is not much higher than that in atomic collisions despite the presence of a dense manifold of molecular rotational states, which can be rationalized by analyzing the adiabatic states of the collision complex.

physics.atom-ph↗

arXiv2024

Frequency-Dependent Vibronic Effects in Steady State Energy Transport

The interplay between electronic and intramolecular high-frequency vibrational degrees of freedom is ubiquitous in natural light-harvesting systems. Recent studies have indicated that an intramolecular vibrational donor-acceptor frequency difference can enhance energy transport. Here, we analyze the extent to which different intramolecular donor-acceptor vibrational frequencies affect excitation energy transport in equilibrium (coherent light excitation) and the more natural nonequilibrium steady state (incoherent light excitation) configurations. It is found that if the Huang-Rhys factors remain constant, the acceptor population increases when the intramolecular vibrational frequency of the acceptor exceeds that of the donor. The increase in the acceptor population due to the vibrational frequency difference is higher for higher values of the Huang-Rhys factors or the vibronic coupling strengths. However, the nonequilibrium steady state results show that the vibrational donor-acceptor frequency difference does not significantly enhance energy transport in the natural scenario of incoherent light excitation and under biologically relevant parameters. Insight about a potential mechanism to optimize energy transfer in the NESS based on increasing the harvesting time at the reaction center is analyzed.

physics.chem-ph↗

arXiv2024

Interference is in the eye of the beholder: application to the coherent control of collisional processes

Interference is widely regarded as a foundational attribute of quantum mechanics. However, for a given experimental arrangement, interference can either contribute or not contribute to the outcome depending upon the basis in which it is measured. This observation is both foundational and particularly relevant to coherent control of molecular processes, an approach based upon quantum interference. Here we address this issue and its relevance to controlling molecular processes via the "coherent control scattering (CCS) matrix", a formalism that allows an analysis of modifications in interference structure resulting from a change of basis. This analysis reveals that the change in interference structure can be attributed to the non-commutativity of the transformation matrix with the CCS matrix, and the non-orthogonality of the transformation. Additionally, minimal interference is shown to be associated with the CCS eigenbasis, and that the Fourier transform of the eigenvectors of the CCS matrix provides the maximal interference and hence the best coherent control. The change of controllability through a change of basis is illustrated with an example of $^{85}$Rb+ $^{85}$Rb scattering. In addition, the developed formalism is applied to explain recent experimental results on He + D$_2$ inelastic scattering demonstrating the presence or absence of interference depending on the basis.

physics.atom-ph↗

arXiv2023

Population Oscillations and Ubiquitous Coherences in multilevel quantum systems driven by incoherent radiation

We consider incoherent excitation of multilevel quantum systems, e.g. molecules with multiple vibronic states. We show that (1) the geometric constraints of the matter-field coupling operator guarantee that noise-induced coherences will be generated in all systems with four or more energy eigenstates and (2) noise-induced coherences can lead to population oscillations due to quantum interference via coherence transfer between pairs of states in the ground and excited manifolds. Our findings facilitate the experimental detection of noise-induced coherent dynamics in complex quantum systems.

quant-ph↗

arXiv2023

Multichannel quantum defect theory with a frame transformation for ultracold atom-molecule collisions in magnetic fields

We extend the powerful formalism of multichannel quantum defect theory combined with a frame transformation to ultracold atom-molecule collisions in magnetic fields. By solving the coupled-channel equations with hyperfine and Zeeman interactions omitted at short range, the extended theory enables a drastically simplified description of the intricate quantum dynamics of ultracold molecular collisions in terms of a small number of short-range parameters. We apply the formalism to ultracold Mg + NH collisions in a magnetic field, achieving a 10$^4$-fold reduction in computational effort.

physics.chem-ph↗

arXiv2023

Robust Coherent Control of Bimolecular Collisions beyond the Ultracold Regime

Quantum coherent control of bimolecular collisions beyond the ultracold regime can face a major challenge due to the incoherent addition of different partial wave contributions to the total scattering cross section. These contributions become increasingly numerous as the collision energy increases, leading to a loss of overall control. Here, we overcome this limitation by leveraging the recently discovered Partial Wave Phase Locking (PWPL) effect, which synchronizes the oscillations of all partial wave contributions. By using rigorous quantum scattering calculations, we demonstrate that PWPL enables coherent control of spin exchange in ion-atom collisions, far outside the ultracold regime, even with as many as 5000 partial wave contributions. The predicted extent of control is sufficient to be measurable in cold atom-ion hybrid experiments.

physics.atom-ph↗

arXiv2023

Signatures of Non-universal Quantum Dynamics of Ultracold Chemical Reactions of Polar Alkali-dimer Molecules with Alkali-metal Atoms: Li($^2$S) +NaLi($a^3\Sigma^+$) $\to$ Na($^2$S) + Li$_2$($a^3\Sigma_u^+$)

Ultracold chemical reactions of weakly bound triplet-state alkali-metal dimer molecules have recently attracted much experimental interest. We perform rigorous quantum scattering calculations with a new $ab\, initio$ potential energy surface to explore the chemical reaction of spin-polarized NaLi($a^3\Sigma^+$) and Li($^2$S) to form Li$_2$($a^3\Sigma_u^+$) and Na($^2$S). The reaction is exothermic, and proceeds readily at ultralow temperatures. Significantly, we observe strong sensitivity of the total reaction rate to small variations of the three-body part of the Li$_2$Na interaction at short range, which we attribute to a relatively small number of open Li$_2$($a^3\Sigma_u^+$) product channels populated in the reaction. This provides the first signature of highly non-universal dynamics seen in rigorous quantum reactive scattering calculations of an ultracold exothermic insertion reaction involving a polar alkali-dimer molecule, opening up the possibility of probing microscopic interactions in atom+molecule collision complexes via ultracold reactive scattering experiments.

physics.atom-ph↗

arXiv2023

Coherent Control of ultracold molecular collisions: The role of resonances

We consider the coherent control of ultracold molecule-molecule scattering, impacted by a dense set of rovibrational resonances. To characterize the resonance spectrum, a rudimentary model based on multichannel quantum defect theory has been used to study the control of the scattering cross section and the reaction rate. Complete control around resonance energies is shown to be possible, but thermal averaging over a large number of resonances significantly reduces the extent of control of reaction rates due to the random distribution of optimal control parameters between resonances. We show that measuring the extent of coherent control could be used to extract meaningful information about the relative contribution of direct scattering versus collision complex formation, as well as about the statistical regime.

physics.atom-ph↗