@article{21009, abstract = {We demonstrate that periodically driven quantum rotors provide a promising and broadly applicable platform to implement multigap topological phases, where groups of bands can acquire topological invariants due to non-Abelian braiding of band degeneracies. By adiabatically varying the periodic kicks to the rotor we find nodal-line braiding, which causes sign flips of topological charges of band nodes and can prevent them from annihilating, indicated by nonzero values of the patch Euler class. In particular, we report on the emergence of an anomalous Dirac string phase arising in the strongly driven regime, a truly out-of-equilibrium phase of the quantum rotor. This phase emanates from braiding processes involving all (quasienergy) gaps and manifests itself with edge states at zero angular momentum. Our results reveal direct applications in state-of-the-art experiments of quantum rotors, such as linear molecules driven by periodic far-off-resonant laser pulses or artificial quantum rotors in optical lattices, whose extensive versatility offers precise modification and observation of novel non-Abelian topological properties.}, author = {Karle, Volker and Lemeshko, Mikhail and Bouhon, Adrien and Slager, Robert-Jan and Ünal, F. Nur}, issn = {2469-9934}, journal = {Physical Review A}, number = {1}, publisher = {American Physical Society}, title = {{Anomalous multigap topological phases in periodically driven quantum rotors}}, doi = {10.1103/db9d-9bns}, volume = {113}, year = {2026}, } @article{18821, abstract = {Even though the one-dimensional contact interaction requires no regularization, renormalization methods have been shown to improve the convergence of numerical calculations considerably. In this work, we compare and contrast these methods: “the running coupling constant” where the two-body ground-state energy is used as a renormalization condition, and two effective interaction approaches that include information about the ground as well as excited states. In particular, we calculate the energies and densities of few-fermion systems in a harmonic oscillator with the configuration-interaction method and compare the results based upon renormalized and bare interactions. We find that the use of the running coupling constant instead of the bare interaction improves convergence significantly. A comparison with an effective interaction, which is designed to reproduce the relative part of the energy spectrum of two particles, showed a similar improvement. The effective interaction provides an additional improvement if the center-of-mass excitations are included in the construction. Finally, we discuss the transformation of observables alongside the renormalization of the potential, and demonstrate that this might be an essential ingredient for accurate numerical calculations.}, author = {Brauneis, Fabian and Hammer, Hans Werner and Reimann, Stephanie M. and Volosniev, Artem}, issn = {2469-9934}, journal = {Physical Review A}, number = {1}, publisher = {American Physical Society}, title = {{Comparison of renormalized interactions using one-dimensional few-body systems as a testbed}}, doi = {10.1103/PhysRevA.111.013303}, volume = {111}, year = {2025}, } @article{19502, abstract = {Alkali dimers, Ak2, located on the surface of a helium nanodroplet, are set into rotation through the polarizability interaction with a nonresonant 1-ps-long laser pulse. The time-dependent degree of alignment is recorded using femtosecond-probe-pulse-induced Coulomb explosion into a pair of Ak+ fragment ions. The results, obtained for Na2, K2, and Rb2 in both the ground state 11Σ+g and the lowest-lying triplet state 13Σ+u, exhibit distinct, periodic revivals with a gradually decreasing amplitude. The dynamics differ from that expected for dimers had they behaved as free rotors. Numerically, we solve the time-dependent rotational Schrödinger equation, including an effective mean-field potential to describe the interaction between the dimer and the droplet. The experimental and simulated alignment dynamics agree well and their comparison enables us to determine the effective rotational constants of the alkali dimers with the exception of Rb2(13Σ+u) that only exhibits a prompt alignment peak but no subsequent revivals. For Na2(13Σ+u), K2(11Σ+g), K2(13Σ+u) and Rb2(11Σ+g), the alignment dynamics are well-described by a 2D rotor model. We ascribe this to a significant confinement of the internuclear axis of these dimers, induced by the orientation-dependent droplet-dimer interaction, to the tangential plane of their residence point on the droplet.}, author = {Kristensen, Henrik H. and Kranabetter, Lorenz and Ghazaryan, Areg and Schouder, Constant A. and Hansen, Emil and Jensen, Frank and Zillich, Robert E. and Lemeshko, Mikhail and Stapelfeldt, Henrik}, issn = {2469-9934}, journal = {Physical Review A}, number = {3}, publisher = {American Physical Society}, title = {{Nonadiabatic laser-induced alignment dynamics of alkali-metal dimers on the surface of a helium droplet}}, doi = {10.1103/PhysRevA.111.033114}, volume = {111}, year = {2025}, } @article{19733, abstract = {One of the most striking quantum phenomena is superposition, where one particle simultaneously inhabits different states. Most methods to verify coherent superposition are indirect, in that they require the distinct states to be recombined. Here, we adapt an xor game, in which a “test” photon is placed in a superposition of two orthogonal spatial modes, and each mode is sent to separated parties who perform local measurements on their modes without reinterfering the original modes. We show that by using a second identical “measurement” photon the parties are nonetheless able to verify if the test photon was placed in coherent superposition of the two spatial modes. We then turn this game into a resource-efficient verification scheme, obtaining a confidence that the particle is superposed which approaches unity exponentially fast. We demonstrate our scheme using a single photon, obtaining a 99% confidence that the particle is superposed with only 37 copies. Our work shows the utility of xor games to verify quantum resources, allowing us to efficiently detect quantum superposition without reinterfering the superposed modes.}, author = {Kun, Daniel and Strömberg, Karl T and Spagnolo, Michele and Dakić, Borivoje and Rozema, Lee A. and Walther, Philip}, issn = {2469-9934}, journal = {Physical Review A}, number = {5}, publisher = {American Physical Society}, title = {{Direct and efficient detection of quantum superposition}}, doi = {10.1103/PhysRevA.111.L050402}, volume = {111}, year = {2025}, } @article{18629, abstract = {We study a three-dimensional Gross-Pitaevskii equation that describes a static impurity in a dipolar Bose-Einstein condensate. Our focus is on the interplay between the shape of the impurity and the anisotropy of the medium manifested in the energy and the density of the system. Without external confinement, properties of the system are derived with basic analytical approaches. For a system in a harmonic trap, the model is investigated numerically, using the split-step Crank-Nicolson method. Our results demonstrate that the impurity self-energy is minimized when its shape more closely aligns with the anisotropic character of the bath; in particular a prolate deformed impurity aligned with the direction of the dipoles has the smallest self-energy for a repulsive impurity. Our work complements studies of impurities in Bose gases with zero-range interactions and paves the way for studies of dipolar polarons with a Gross-Pitaevskii equation.}, author = {Shukla, Neelam and Volosniev, Artem and Armstrong, Jeremy R.}, issn = {2469-9934}, journal = {Physical Review A}, number = {5}, publisher = {American Physical Society}, title = {{Anisotropic potential immersed in a dipolar Bose-Einstein condensate}}, doi = {10.1103/PhysRevA.110.053317}, volume = {110}, year = {2024}, } @article{15004, abstract = {The impulsive limit (the “sudden approximation”) has been widely employed to describe the interaction between molecules and short, far-off-resonant laser pulses. This approximation assumes that the timescale of the laser-molecule interaction is significantly shorter than the internal rotational period of the molecule, resulting in the rotational motion being instantaneously “frozen” during the interaction. This simplified description of the laser-molecule interaction is incorporated in various theoretical models predicting rotational dynamics of molecules driven by short laser pulses. In this theoretical work, we develop an effective theory for ultrashort laser pulses by examining the full time-evolution operator and solving the time-dependent Schrödinger equation at the operator level. Our findings reveal a critical angular momentum, lcrit, at which the impulsive limit breaks down. In other words, the validity of the sudden approximation depends not only on the pulse duration but also on its intensity, since the latter determines how many angular momentum states are populated. We explore both ultrashort multicycle (Gaussian) pulses and the somewhat less studied half-cycle pulses, which produce distinct effective potentials. We discuss the limitations of the impulsive limit and propose a method that rescales the effective matrix elements, enabling an improved and more accurate description of laser-molecule interactions.}, author = {Karle, Volker and Lemeshko, Mikhail}, issn = {2469-9934}, journal = {Physical Review A}, number = {2}, publisher = {American Physical Society}, title = {{Modeling laser pulses as δ kicks: Reevaluating the impulsive limit in molecular rotational dynamics}}, doi = {10.1103/PhysRevA.109.023101}, volume = {109}, year = {2024}, } @article{15167, abstract = {We perform a diagrammatic analysis of the energy of a mobile impurity immersed in a strongly interacting two-component Fermi gas to second order in the impurity-bath interaction. These corrections demonstrate divergent behavior in the limit of large impurity momentum. We show the fundamental processes responsible for these logarithmically divergent terms. We study the problem in the general case without any assumptions regarding the fermion-fermion interactions in the bath. We show that the divergent term can be summed up to all orders in the Fermi-Fermi interaction and that the resulting expression is equivalent to the one obtained in the few-body calculation. Finally, we provide a perturbative calculation to the second order in the Fermi-Fermi interaction, and we show the diagrams responsible for these terms.}, author = {Al Hyder, Ragheed and Chevy, F. and Leyronas, X.}, issn = {2469-9934}, journal = {Physical Review A}, number = {3}, publisher = {American Physical Society}, title = {{Exploring beyond-mean-field logarithmic divergences in Fermi-polaron energy}}, doi = {10.1103/PhysRevA.109.033315}, volume = {109}, year = {2024}, } @article{14553, abstract = {Quantum state tomography is an essential component of modern quantum technology. In application to continuous-variable harmonic-oscillator systems, such as the electromagnetic field, existing tomography methods typically reconstruct the state in discrete bases, and are hence limited to states with relatively low amplitudes and energies. Here, we overcome this limitation by utilizing a feed-forward neural network to obtain the density matrix directly in the continuous position basis. An important benefit of our approach is the ability to choose specific regions in the phase space for detailed reconstruction. This results in a relatively slow scaling of the amount of resources required for the reconstruction with the state amplitude, and hence allows us to dramatically increase the range of amplitudes accessible with our method.}, author = {Fedotova, Ekaterina and Kuznetsov, Nikolai and Tiunov, Egor and Ulanov, A. E. and Lvovsky, A. I.}, issn = {2469-9934}, journal = {Physical Review A}, number = {4}, publisher = {American Physical Society}, title = {{Continuous-variable quantum tomography of high-amplitude states}}, doi = {10.1103/PhysRevA.108.042430}, volume = {108}, year = {2023}, } @article{12819, abstract = {Reaching a high cavity population with a coherent pump in the strong-coupling regime of a single-atom laser is impossible due to the photon blockade effect. In this Letter, we experimentally demonstrate that in a single-atom maser based on a transmon strongly coupled to two resonators, it is possible to pump over a dozen photons into the system. The first high-quality resonator plays the role of a usual lasing cavity, and the second one presents a controlled dissipation channel, bolstering population inversion, and modifies the energy-level structure to lift the blockade. As confirmation of the lasing action, we observe conventional laser features such as a narrowing of the emission linewidth and external signal amplification. Additionally, we report unique single-atom features: self-quenching and several lasing thresholds.}, author = {Sokolova, Alesya and Kalacheva, D. A. and Fedorov, G. P. and Astafiev, O. V.}, issn = {2469-9934}, journal = {Physical Review A}, number = {3}, publisher = {American Physical Society}, title = {{Overcoming photon blockade in a circuit-QED single-atom maser with engineered metastability and strong coupling}}, doi = {10.1103/PhysRevA.107.L031701}, volume = {107}, year = {2023}, } @article{12914, abstract = {We numerically study two methods of measuring tunneling times using a quantum clock. In the conventional method using the Larmor clock, we show that the Larmor tunneling time can be shorter for higher tunneling barriers. In the second method, we study the probability of a spin-flip of a particle when it is transmitted through a potential barrier including a spatially rotating field interacting with its spin. According to the adiabatic theorem, the probability depends on the velocity of the particle inside the barrier. It is numerically observed that the probability increases for higher barriers, which is consistent with the result obtained by the Larmor clock. By comparing outcomes for different initial spin states, we suggest that one of the main causes of the apparent decrease in the tunneling time can be the filtering effect occurring at the end of the barrier.}, author = {Suzuki, Fumika and Unruh, William G.}, issn = {2469-9934}, journal = {Physical Review A}, number = {4}, publisher = {American Physical Society}, title = {{Numerical quantum clock simulations for measuring tunneling times}}, doi = {10.1103/PhysRevA.107.042216}, volume = {107}, year = {2023}, } @article{13233, abstract = {We study the impact of finite-range physics on the zero-range-model analysis of three-body recombination in ultracold atoms. We find that temperature dependence of the zero-range parameters can vary from one set of measurements to another as it may be driven by the distribution of error bars in the experiment, and not by the underlying three-body physics. To study finite-temperature effects in three-body recombination beyond the zero-range physics, we introduce and examine a finite-range model based upon a hyperspherical formalism. The systematic error discussed in this Letter may provide a significant contribution to the error bars of measured three-body parameters.}, author = {Agafonova, Sofya and Lemeshko, Mikhail and Volosniev, Artem}, issn = {2469-9934}, journal = {Physical Review A}, number = {6}, publisher = {American Physical Society}, title = {{Finite-range bias in fitting three-body loss to the zero-range model}}, doi = {10.1103/PhysRevA.107.L061304}, volume = {107}, year = {2023}, } @article{13125, abstract = {The quantum approximate optimization algorithm (QAOA) is a variational quantum algorithm, where a quantum computer implements a variational ansatz consisting of p layers of alternating unitary operators and a classical computer is used to optimize the variational parameters. For a random initialization, the optimization typically leads to local minima with poor performance, motivating the search for initialization strategies of QAOA variational parameters. Although numerous heuristic initializations exist, an analytical understanding and performance guarantees for large p remain evasive.We introduce a greedy initialization of QAOA which guarantees improving performance with an increasing number of layers. Our main result is an analytic construction of 2p + 1 transition states—saddle points with a unique negative curvature direction—for QAOA with p + 1 layers that use the local minimum of QAOA with p layers. Transition states connect to new local minima, which are guaranteed to lower the energy compared to the minimum found for p layers. We use the GREEDY procedure to navigate the exponentially increasing with p number of local minima resulting from the recursive application of our analytic construction. The performance of the GREEDY procedure matches available initialization strategies while providing a guarantee for the minimal energy to decrease with an increasing number of layers p. }, author = {Sack, Stefan and Medina Ramos, Raimel A and Kueng, Richard and Serbyn, Maksym}, issn = {2469-9934}, journal = {Physical Review A}, number = {6}, publisher = {American Physical Society}, title = {{Recursive greedy initialization of the quantum approximate optimization algorithm with guaranteed improvement}}, doi = {10.1103/physreva.107.062404}, volume = {107}, year = {2023}, } @article{11591, abstract = {We investigate the deterministic generation and distribution of entanglement in large quantum networks by driving distant qubits with the output fields of a nondegenerate parametric amplifier. In this setting, the amplifier produces a continuous Gaussian two-mode squeezed state, which acts as a quantum-correlated reservoir for the qubits and relaxes them into a highly entangled steady state. Here we are interested in the maximal amount of entanglement and the optimal entanglement generation rates that can be achieved with this scheme under realistic conditions taking, in particular, the finite amplifier bandwidth, waveguide losses, and propagation delays into account. By combining exact numerical simulations of the full network with approximate analytic results, we predict the optimal working point for the amplifier and the corresponding qubit-qubit entanglement under various conditions. Our findings show that this passive conversion of Gaussian into discrete-variable entanglement offers a robust and experimentally very attractive approach for operating large optical, microwave, or hybrid quantum networks, for which efficient parametric amplifiers are currently developed.}, author = {Agustí, J. and Minoguchi, Y. and Fink, Johannes M and Rabl, P.}, issn = {2469-9934}, journal = {Physical Review A}, number = {6}, publisher = {American Physical Society}, title = {{Long-distance distribution of qubit-qubit entanglement using Gaussian-correlated photonic beams}}, doi = {10.1103/PhysRevA.105.062454}, volume = {105}, year = {2022}, } @article{11592, abstract = {We compare recent experimental results [Science 375, 528 (2022)] of the superfluid unitary Fermi gas near the critical temperature with a thermodynamic model based on the elementary excitations of the system. We find good agreement between experimental data and our theory for several quantities such as first sound, second sound, and superfluid fraction. We also show that mode mixing between first and second sound occurs. Finally, we characterize the response amplitude to a density perturbation: Close to the critical temperature both first and second sound can be excited through a density perturbation, whereas at lower temperatures only the first sound mode exhibits a significant response.}, author = {Bighin, Giacomo and Cappellaro, Alberto and Salasnich, L.}, issn = {2469-9934}, journal = {Physical Review A}, number = {6}, publisher = {American Physical Society}, title = {{Unitary Fermi superfluid near the critical temperature: Thermodynamics and sound modes from elementary excitations}}, doi = {10.1103/PhysRevA.105.063329}, volume = {105}, year = {2022}, } @article{11997, abstract = {We study the fate of an impurity in an ultracold heteronuclear Bose mixture, focusing on the experimentally relevant case of a ⁴¹K - ⁸⁷Rb mixture, with the impurity in a ⁴¹K hyperfine state. Our paper provides a comprehensive description of an impurity in a BEC mixture with contact interactions across its phase diagram. We present results for the miscible and immiscible regimes, as well as for the impurity in a self-bound quantum droplet. Here, varying the interactions, we find exotic states where the impurity localizes either at the center or at the surface of the droplet. }, author = {Bighin, Giacomo and Burchianti, A. and Minardi, F. and Macrì, T.}, issn = {2469-9934}, journal = {Physical Review A}, number = {2}, publisher = {American Physical Society}, title = {{Impurity in a heteronuclear two-component Bose mixture}}, doi = {10.1103/PhysRevA.106.023301}, volume = {106}, year = {2022}, } @article{18191, abstract = {Large-scale quantum devices provide insights beyond the reach of classical simulations. However, for a reliable and verifiable quantum simulation, the building blocks of the quantum device require exquisite benchmarking. This benchmarking of large-scale dynamical quantum systems represents a major challenge due to lack of efficient tools for their simulation. Here, we present a scalable algorithm based on neural networks for Hamiltonian tomography in out-of-equilibrium quantum systems. We illustrate our approach using a model for a forefront quantum simulation platform: ultracold atoms in optical lattices. Specifically, we show that our algorithm is able to reconstruct the Hamiltonian of an arbitrary sized bosonic ladder system using an accessible amount of experimental measurements. We are able to significantly increase the previously known parameter precision.}, author = {Valenti, Agnes and Jin, Guliuxin and Leonard, Julian and Huber, Sebastian D. and Greplova, Eliska}, issn = {2469-9934}, journal = {Physical Review A}, number = {2}, publisher = {American Physical Society}, title = {{Scalable Hamiltonian learning for large-scale out-of-equilibrium quantum dynamics}}, doi = {10.1103/physreva.105.023302}, volume = {105}, year = {2022}, } @article{13997, abstract = {We investigate theoretically the strong-field regime of light-matter interactions in the topological-insulator class of quantum materials. In particular, we focus on the process of nonperturbative high-order harmonic generation from the paradigmatic three-dimensional topological insulator bismuth selenide (Bi2Se3) subjected to intense midinfrared laser fields. We analyze the contributions from the spin-orbit-coupled bulk states and the topological surface bands separately and reveal a major difference in how their harmonic yields depend on the ellipticity of the laser field. Bulk harmonics show a monotonic decrease in their yield as the ellipticity increases, in a manner reminiscent of high harmonic generation in gaseous media. However, the surface contribution exhibits a highly nontrivial dependence, culminating with a maximum for circularly polarized fields. We attribute the observed anomalous behavior to (i) the enhanced amplitude and the circular pattern of the interband dipole and the Berry connections in the vicinity of the Dirac point and (ii) the influence of the higher-order, hexagonal warping terms in the Hamiltonian, which are responsible for the hexagonal deformation of the energy surface at higher momenta. The latter are associated directly with spin-orbit-coupling parameters. Our results thus establish the sensitivity of strong-field-driven high harmonic emission to the topology of the band structure as well as to the manifestations of spin-orbit interaction.}, author = {Baykusheva, Denitsa Rangelova and Chacón, Alexis and Kim, Dasol and Kim, Dong Eon and Reis, David A. and Ghimire, Shambhu}, issn = {2469-9934}, journal = {Physical Review A}, number = {2}, publisher = {American Physical Society}, title = {{Strong-field physics in three-dimensional topological insulators}}, doi = {10.1103/physreva.103.023101}, volume = {103}, year = {2021}, } @article{10631, abstract = {We combine experimental and theoretical approaches to explore excited rotational states of molecules embedded in helium nanodroplets using CS2 and I2 as examples. Laser-induced nonadiabatic molecular alignment is employed to measure spectral lines for rotational states extending beyond those initially populated at the 0.37 K droplet temperature. We construct a simple quantum-mechanical model, based on a linear rotor coupled to a single-mode bosonic bath, to determine the rotational energy structure in its entirety. The calculated and measured spectral lines are in good agreement. We show that the effect of the surrounding superfluid on molecular rotation can be rationalized by a single quantity, the angular momentum, transferred from the molecule to the droplet.}, author = {Cherepanov, Igor and Bighin, Giacomo and Schouder, Constant A. and Chatterley, Adam S. and Albrechtsen, Simon H. and Muñoz, Alberto Viñas and Christiansen, Lars and Stapelfeldt, Henrik and Lemeshko, Mikhail}, issn = {2469-9934}, journal = {Physical Review A}, number = {6}, publisher = {American Physical Society}, title = {{Excited rotational states of molecules in a superfluid}}, doi = {10.1103/PhysRevA.104.L061303}, volume = {104}, year = {2021}, } @article{10545, abstract = {Classical models with complex energy landscapes represent a perspective avenue for the near-term application of quantum simulators. Until now, many theoretical works studied the performance of quantum algorithms for models with a unique ground state. However, when the classical problem is in a so-called clustering phase, the ground state manifold is highly degenerate. As an example, we consider a 3-XORSAT model defined on simple hypergraphs. The degeneracy of classical ground state manifold translates into the emergence of an extensive number of Z2 symmetries, which remain intact even in the presence of a quantum transverse magnetic field. We establish a general duality approach that restricts the quantum problem to a given sector of conserved Z2 charges and use it to study how the outcome of the quantum adiabatic algorithm depends on the hypergraph geometry. We show that the tree hypergraph which corresponds to a classically solvable instance of the 3-XORSAT problem features a constant gap, whereas the closed hypergraph encounters a second-order phase transition with a gap vanishing as a power-law in the problem size. The duality developed in this work provides a practical tool for studies of quantum models with classically degenerate energy manifold and reveals potential connections between glasses and gauge theories.}, author = {Medina Ramos, Raimel A and Serbyn, Maksym}, issn = {2469-9934}, journal = {Physical Review A}, number = {6}, publisher = {American Physical Society}, title = {{Duality approach to quantum annealing of the 3-variable exclusive-or satisfiability problem (3-XORSAT)}}, doi = {10.1103/physreva.104.062423}, volume = {104}, year = {2021}, } @article{9242, abstract = {In the recent years important experimental advances in resonant electro-optic modulators as high-efficiency sources for coherent frequency combs and as devices for quantum information transfer have been realized, where strong optical and microwave mode coupling were achieved. These features suggest electro-optic-based devices as candidates for entangled optical frequency comb sources. In the present work, I study the generation of entangled optical frequency combs in millimeter-sized resonant electro-optic modulators. These devices profit from the experimentally proven advantages such as nearly constant optical free spectral ranges over several gigahertz, and high optical and microwave quality factors. The generation of frequency multiplexed quantum channels with spectral bandwidth in the MHz range for conservative parameter values paves the way towards novel uses in long-distance hybrid quantum networks, quantum key distribution, enhanced optical metrology, and quantum computing.}, author = {Rueda Sanchez, Alfredo R}, issn = {2469-9934}, journal = {Physical Review A}, number = {2}, publisher = {American Physical Society}, title = {{Frequency-multiplexed hybrid optical entangled source based on the Pockels effect}}, doi = {10.1103/PhysRevA.103.023708}, volume = {103}, year = {2021}, }