TY - JOUR AB - 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. AU - Karle, Volker AU - Lemeshko, Mikhail ID - 15004 IS - 2 JF - Physical Review A SN - 2469-9926 TI - Modeling laser pulses as δ kicks: Reevaluating the impulsive limit in molecular rotational dynamics VL - 109 ER - TY - JOUR AB - 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. AU - Al Hyder, Ragheed AU - Chevy, F. AU - Leyronas, X. ID - 15167 IS - 3 JF - Physical Review A SN - 2469-9926 TI - Exploring beyond-mean-field logarithmic divergences in Fermi-polaron energy VL - 109 ER - TY - JOUR AB - 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. AU - Sokolova, Alesya AU - Kalacheva, D. A. AU - Fedorov, G. P. AU - Astafiev, O. V. ID - 12819 IS - 3 JF - Physical Review A SN - 2469-9926 TI - Overcoming photon blockade in a circuit-QED single-atom maser with engineered metastability and strong coupling VL - 107 ER - TY - JOUR AB - 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. AU - Suzuki, Fumika AU - Unruh, William G. ID - 12914 IS - 4 JF - Physical Review A SN - 2469-9926 TI - Numerical quantum clock simulations for measuring tunneling times VL - 107 ER - TY - JOUR AB - 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. AU - Agafonova, Sofya AU - Lemeshko, Mikhail AU - Volosniev, Artem ID - 13233 IS - 6 JF - Physical Review A SN - 2469-9926 TI - Finite-range bias in fitting three-body loss to the zero-range model VL - 107 ER - TY - JOUR AB - 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. AU - Fedotova, Ekaterina AU - Kuznetsov, Nikolai AU - Tiunov, Egor AU - Ulanov, A. E. AU - Lvovsky, A. I. ID - 14553 IS - 4 JF - Physical Review A SN - 2469-9926 TI - Continuous-variable quantum tomography of high-amplitude states VL - 108 ER - TY - JOUR AB - 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. AU - Sack, Stefan AU - Medina Ramos, Raimel A AU - Kueng, Richard AU - Serbyn, Maksym ID - 13125 IS - 6 JF - Physical Review A SN - 2469-9926 TI - Recursive greedy initialization of the quantum approximate optimization algorithm with guaranteed improvement VL - 107 ER - TY - JOUR AB - 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. AU - Agustí, J. AU - Minoguchi, Y. AU - Fink, Johannes M AU - Rabl, P. ID - 11591 IS - 6 JF - Physical Review A SN - 2469-9926 TI - Long-distance distribution of qubit-qubit entanglement using Gaussian-correlated photonic beams VL - 105 ER - TY - JOUR AB - 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. AU - Bighin, Giacomo AU - Cappellaro, Alberto AU - Salasnich, L. ID - 11592 IS - 6 JF - Physical Review A SN - 2469-9926 TI - Unitary Fermi superfluid near the critical temperature: Thermodynamics and sound modes from elementary excitations VL - 105 ER - TY - JOUR AB - 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. AU - Bighin, Giacomo AU - Burchianti, A. AU - Minardi, F. AU - Macrì, T. ID - 11997 IS - 2 JF - Physical Review A SN - 2469-9926 TI - Impurity in a heteronuclear two-component Bose mixture VL - 106 ER - TY - JOUR AB - 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. AU - Rueda Sanchez, Alfredo R ID - 9242 IS - 2 JF - Physical Review A SN - 2469-9926 TI - Frequency-multiplexed hybrid optical entangled source based on the Pockels effect VL - 103 ER - TY - JOUR AB - 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. AU - Medina Ramos, Raimel A AU - Serbyn, Maksym ID - 10545 IS - 6 JF - Physical Review A SN - 2469-9926 TI - Duality approach to quantum annealing of the 3-variable exclusive-or satisfiability problem (3-XORSAT) VL - 104 ER - TY - JOUR AB - 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. AU - Cherepanov, Igor AU - Bighin, Giacomo AU - Schouder, Constant A. AU - Chatterley, Adam S. AU - Albrechtsen, Simon H. AU - Muñoz, Alberto Viñas AU - Christiansen, Lars AU - Stapelfeldt, Henrik AU - Lemeshko, Mikhail ID - 10631 IS - 6 JF - Physical Review A SN - 2469-9926 TI - Excited rotational states of molecules in a superfluid VL - 104 ER - TY - JOUR AB - 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. AU - Baykusheva, Denitsa Rangelova AU - Chacón, Alexis AU - Kim, Dasol AU - Kim, Dong Eon AU - Reis, David A. AU - Ghimire, Shambhu ID - 13997 IS - 2 JF - Physical Review A SN - 2469-9926 TI - Strong-field physics in three-dimensional topological insulators VL - 103 ER - TY - JOUR AB - Attosecond delays between photoelectron wave packets emitted from different electronic shells are now well established. Is there any delay between electrons originating from the same electronic shell but leaving the cation in different fine-structure states? This question is relevant for all attosecond photoemission studies involving heavy elements, be it atoms, molecules or solids. We answer this fundamental question by measuring energy-dependent delays between photoelectron wave packets associated with the 2P3/2 and 2P1/2 components of the electronic groundstates of Xe+ and Kr+. We observe delays reaching up to 33±6 as in the case of Xe. Our results are compared with two state-of-the-art theories. Whereas both theories quantitatively agree with the results obtained for Kr, neither of them fully reproduces the experimental results in Xe. Performing delay measurements very close to the ionization thresholds, we compare the agreement of several analytical formulas for the continuum-continuum delays with experimental data. Our results show an important influence of spin-orbit coupling on attosecond photoionization delays, highlight the requirement for additional theory development, and offer a precision benchmark for such work. AU - Jordan, I. AU - Huppert, M. AU - Pabst, S. AU - Kheifets, A. S. AU - Baykusheva, Denitsa Rangelova AU - Wörner, H. J. ID - 14009 IS - 1 JF - Physical Review A SN - 2469-9926 TI - Spin-orbit delays in photoemission VL - 95 ER -