We explore the relative robustness of single-mode squeezing and entanglement (which are quantum resources interconvertible via passive optics) for two-mode Gaussian states under different dissipative environments. When the individual modes interact with identical local baths, entanglement and squeezing decay at the same rate. However, when only one of the modes interacts with a local bath, the comparative robustness of entanglement and squeezing depends on the initial squeezing of the state. Similarly, when the system interacts with a global bath, the robustness of entanglement and squeezing depends on the initial squeezing. Thus depending on the nature of dissipative environments and the initial squeezing of the state, one can select the more robust form of resource out of squeezing and entanglement to store quantumness. This can be used to effectively enhance the performance of various quantum information processing protocols based on continuous variable Gaussian states.

Zero-noise extrapolation (ZNE) is a widely used technique for gate-error mitigation on near-term quantum computers because it can be implemented in software and does not require knowledge of the quantum computer noise parameters. Traditional ZNE requires a significant resource overhead in terms of quantum operations. A recent proposal using a targeted (or random) instead of fixed identity insertion method (RIIM versus FIIM) requires significantly fewer quantum gates for the same formal precision. We start by showing that RIIM can allow for ZNE to be deployed on deeper circuits than FIIM but requires many more measurements to achieve the same level of statistical uncertainty. We develop two extensions to FIIM and RIIM. The list identity insertion method allows us to mitigate the error from certain controlled-not (cnot) gates, typically those with the largest error. The set identity insertion method naturally interpolates between the measurement-efficient FIIM and the gate-efficient RIIM, allowing us to trade off fewer cnot gates for more measurements. Finally, we investigate a way to boost the number of measurements, namely, to run ZNE in parallel, utilizing as many quantum devices as are available. We explore the performance of RIIM in a parallel setting where there is a nontrivial spread in noise across sets of qubits within or across quantum computers.

We perform a calculation of the nuclear magnetic shielding in HD and HT molecules, with complete and perturbative accounts for nuclear masses. From the difference in shielding, we obtain the deuteron and triton magnetic moments in agreement with the CODATA value, with the accuracy limited only by nuclear magnetic resonance measurements. Most importantly, our calculations indicate a potential for improved determination of nuclear magnetic moments.

We formulate a three-dimensional semiclassical model to address triple and double ionization in three-electron atoms driven by intense infrared laser pulses. During time propagation, our model fully accounts for the Coulomb singularities, the magnetic field of the laser pulse, and the motion of the nucleus at the same time as for the motion of the three electrons. The framework we develop is general and can account for multielectron ionization in strongly driven atoms with more than three electrons. To avoid unphysical autoionization arising in classical models of three or more electrons, we replace the Coulomb potential between pairs of bound electrons with effective Coulomb potentials. The Coulomb forces between electrons that are not both bound are fully accounted for. We develop a set of criteria to determine when electrons become bound during time propagation. We compare ionization spectra obtained with the model developed here and with the Heisenberg model that includes a potential term restricting an electron from closely approaching the core. Such spectra include the sum of the electron momenta along the direction of the laser field as well as the correlated electron momenta. We also compare these results with experimental ones.

We investigate theoretically the nonresonant excitation of vibrational levels in polar molecules by unipolar radiation pulses of duration much shorter than the characteristic period of the molecule's vibration. We consider several profiles of the potential of the interaction of atoms in a diatomic molecule and derive analytically the probabilities of the molecule's transition to excited vibrational states when driven by subcycle unipolar pulses. It is shown that the excitation efficiency is governed by the electric pulse area so that unipolar half-cycle pulses turn out to be the most efficient ones. We introduce the characteristic scale of the electric pulse area, which serves as a measure of the pulse action on the vibrational states of the molecule. The results are generalized to the interaction of excited vibrational and rotational states and it is shown that the behavior of the vibrational levels' populations versus the electric pulse area as well as the introduced characteristic scale stays valid.

The emerging hybrid cavity optomagnonic system is a very promising quantum information processing platform for its strong or ultrastrong photon-magnon interaction on the scale of micrometers in the experiment. In this paper, the superfluid–Mott-insulator quantum phase transition in a two-dimensional cavity optomagnonic array system has been studied based on this characteristic. The analytical solution of the critical hopping rate is obtained by the mean-field approach, second-order perturbation theory, and Landau second-order phase transition theory. The numerical results show that the increasing coupling strength and the positive detunings of the photon and the magnon favor the coherence and then the stable areas of Mott lobes are compressed correspondingly. Moreover, the analytical results agree with the numerical ones when the total excitation number is lower. Finally, an effective repulsive potential is constructed to exhibit the corresponding mechanism. The results obtained here provide an experimentally feasible scheme for characterizing the quantum phase transitions in a cavity optomagnonic array system, which will offer valuable insight for quantum simulations.

The physical meaning of the quantum weak value still remains a matter of debate. Originally introduced by Aharonov et al. [Phys. Rev. Lett. 60, 1351 (1988).] as another counterintuitive feature of quantum mechanics, it eventually evolved into a practical tool that is widely used. The theoretical framework in which weak values were introduced was given by von Neumann's model of quantum measurements. In this model, a system is submitted to measurement by coupling it to a “pointer,” or meter. In the weak-coupling regime, one may perform a low-order Taylor expansion for the evolution operator of the system and meter. This standard approach ties weak values with weak couplings and weak measurements. We report closed-form expressions that can be used to untie weak values from weak measurements. The regime of strong measurements thereby becomes accessible to weak values, without leaving the framework in which the latter were originally introduced. The reported results should also help us to better understand the physical meaning of weak values.

Distance to uncontrollability is a crucial concept in classical control theory. Here, we introduce quantum distance to uncontrollability as a measure of how close a universal quantum system is to a nonuniversal one. This allows us to provide a quantitative version of the quantum speed limit, decomposing the bound into geometric and dynamical components. We consider several physical examples including globally controlled solid state qubits, scrambling of quantum information, and a cross-Kerr system, showing that the quantum distance to uncontrollability provides a precise meaning to spectral crowding, weak interactions, and other bottlenecks to universality. We suggest that this measure should be taken into consideration in the design of quantum technology.

The core objective of machine-assisted scientific discovery is to learn physical laws from experimental data without prior knowledge of the systems in question. In the area of quantum physics, making progress towards these goals is significantly more challenging due to the curse of dimensionality as well as the counterintuitive nature of quantum mechanics. Here we present the QNODE, a latent neural ordinary differential equation (ODE) trained on expectation values of closed and open-quantum-systems dynamics. It can learn to generate such measurement data and extrapolate outside of its training region that satisfies the von Neumann and time-local Lindblad master equations for closed and open quantum systems, respectively, in an unsupervised means. Furthermore, the QNODE rediscovers quantum-mechanical laws such as the Heisenberg's uncertainty principle in a data-driven way, without any constraint or guidance. Additionally, we show that trajectories that are generated from the QNODE that are close in its latent space have similar quantum dynamics while preserving the physics of the training system.

Practical quantum computation requires highly efficient implementation of quantum logic gates. The fan-out gate represents such a type of multiqubit controlled gate with one qubit controlling a number of target qubits. Recent efforts for fan-out gates using Rydberg atoms include adiabatic operations along with the dark-state dynamics of Rydberg states [M. Khazali and K. Mølmer, Phys. Rev. X 10, 021054 (2020)] and implementation based on the asymmetric Rydberg-Rydberg interaction [J. T. Young et al., Phys. Rev. Lett. 127, 120501 (2021)]. Inspired by these advances, we propose a fast-operation scheme for geometric multiqubit fan-out gates, in which the target qubits can be executed with arbitrary operations based on the asymmetric Rydberg-Rydberg interaction and time-optimal control technology. One of the main favorable features of our scheme is its performance in a nonadiabatic way and with the time-optimal control technology, by which the shortest smooth geometric path is found. Another feature, that the target qubits could be executed with arbitrary geometric quantum operations, makes our scheme general and useful. Therefore, our scheme provides a promising alternative route toward scalable fault-tolerant quantum information processing based on Rydberg atoms.

We report a measurement of the radium ion's $7p{}^2{P}_{3/2}$ state lifetime $\tau =4.78\left(3\right)$ ns. The measured lifetime is in good agreement with theoretical calculations and will enable a determination of the differential scalar polarizability of the narrow linewidth $7s{}^2{S}_{1/2}\to 6d{}^2{D}_{5/2}$ optical clock transition.

We study laser-induced dissociation of a metastable ${\mathrm{NO}}^{2+}$ ion-beam target into ${\mathrm{N}}^{+}+{\mathrm{O}}^{+}$, focusing on the prominent contribution by molecules breaking parallel to the polarization at high peak laser intensity $(\sim {10}^{15}$ $\mathrm{W}/{\mathrm{cm}}^2)$. Our experimental results and time-dependent Schrödinger equation calculations show that, contrary to commonly held intuition that electronic transitions always prevail, the dominant process underlying this highly aligned dissociation is a multiphoton permanent-dipole transition involving only the electronic ground state and leading to its vibrational continuum. Strong-field permanent-dipole transitions should thus be considered generally, as they may play a significant role in other heteronuclear molecules. Moreover, their role should only grow in importance for longer wavelengths, a trending direction in ultrafast laser studies.

In this paper, we prove that the anisotropic Kepler problem in two-body motion can be analogous to optical non–rotationally-symmetric Eaton lens. According to the numerical calculation and simulation results, when the mass ratio of anisotropic Kepler problem $\mu >1$, the corresponding non–rotationally-symmetric Eaton lens can simulate and reproduce the periodic orbits and chaotic characteristics of anisotropic Kepler problem in geometric optics and wave optics, respectively. In the extended study, when a beam is incident to the non–rotationally-symmetric Eaton lens from air, different from the light properties in traditional Eaton lens, the deflected beam is no longer along 180°, and may have scatterings in other directions, which are influenced by the incident direction and mass ratio μ. The results provide a successful reference for the analogy between optical and mechanical phenomena.

Hyperentanglement, the simultaneous and independent entanglement of quantum particles in multiple degrees of freedom, is a powerful resource that can be harnessed for efficient quantum information processing. In photonic systems, the two degrees of freedom (DOFs) often used to carry quantum and classical information are polarization and frequency, thanks to their robustness in transmission, both in free space and in optical fibers. Telecom-band hyperentangled photons generated in optical fibers are of particular interest because they are compatible with existing fiber-optic infrastructure and can be distributed over fiber networks with minimal loss. Here, we experimentally demonstrate the generation of telecom-band biphotons hyperentangled in both the polarization and frequency DOFs using a periodically poled silica fiber and observe entanglement concurrences above 0.95 for both polarization and frequency DOFs. Furthermore, by concatenating a Hong-Ou-Mandel interference test for frequency entanglement and full state tomography for polarization entanglement in a single experiment, we can demonstrate simultaneous entanglement in both the polarization and frequency DOFs. The states produced by our hyperentanglement source can enable protocols such as dense coding and high-dimensional quantum key distribution.

Motivated by a recent prediction to engineer the dispersion relation of a waveguide constructed from atomic components [Phys. Rev. Res. 4, 013080 (2022)], we explore the possibility of creating directional transport in an open, collective quantum system. We characterize the optical response of this atomic waveguide through a scattering-matrix formalism built upon theories of photoelectric detection that allows us to find the conditions for directional mode-to-mode transmission to occur. We find that directional waveguides allow for an efficient outcoupling of light by reducing backscattering channels at the edges. This reduced backscattering is seen to play a major role in the dynamics when disorder is included numerically. A directional waveguide is shown to be more robust to localization, but at the cost of increased radiative losses.

We develop a framework that provides a straightforward approach to fully exploit the permutational symmetry of identical multilevel systems. By taking into account the permutational symmetry, we outline a simple scheme that allows us to map the dynamics of $N$ identical $d$-level systems to the dynamics of $d$ bosonic modes with $N$ particles, achieving an exponential reduction on the dimensionality of the problem in a simple and straightforward way. In particular, we consider the Lindblad dynamics of several identical multilevel systems interacting with a common subsystem under the action of collective dissipation terms.

Prediction of the equilibrium populations in quantum dynamics simulations of molecules exposed to black-body radiation has proved challenging for semiclassical treatments, with the usual Ehrenfest and Maxwell-Bloch methods exhibiting serious failures. In this context, we explore the behavior of a recently introduced semiclassical model of light-matter interaction derived from a dissipative Lagrangian [C. M. Bustamante, E. D. Gadea, A. Horsfield, T. N. Todorov, M. C. González Lebrero, and D. A. Scherlis, Phys. Rev. Lett. 126, 087401 (2021)]. It is shown that this model reproduces the Boltzmann populations for two-level systems, predicting the black-body spectra in approximate agreement with Planck's distribution. In multilevel systems, small deviations from the expected occupations are seen beyond the first excited level. By averaging over fast oscillations, a rate equation is derived from the dissipative equation of motion that makes it possible to rationalize these deviations. Importantly, it enables us to conclude that this model will produce the correct equilibrium populations provided the occupations of the lowest levels remain close to unity, a condition satisfied at low temperature or small excitations.

Entanglement between atoms and microwave photons in a superconducting coplanar waveguide (SCW) can enable hybrid quantum devices and interface static and flying qubits. We study a one-step controlled-$Z$(cz) gate between a neutral atom trapped near a SCW and a microwave mode in the SCW, which is an extension of the gate proposed in J. D. Pritchard et al., Phys. Rev. A 89, 010301(R) (2014). The gate protocol is simple and requires one laser pulse for exciting a transition between the ground and Rydberg states of the neutral atom.

In this work we propose a theory of temperature estimation of quantum systems, which is applicable in the regime of non-negligible prior temperature uncertainty and limited measurement data. In this regime the problem of establishing a well-defined measure of estimation precision becomes nontrivial. Furthermore, the construction of a suitable criterion for optimal measurement design must be reexamined to account for the prior uncertainty. We propose a fully Bayesian approach to temperature estimation based on the concept of thermodynamic length, which solves both these problems. As an illustration of this framework, we consider thermal spin-$1/2$ particles and investigate the fundamental difference between two cases: on the one hand, when the spins are probing the temperature of a heat reservoir and, on the other, when the spins themselves constitute the sample.

The effects of the spatiotemporal degrees of freedom on the practical implementation of an SU(1,1) interferometer are investigated. A recently developed Wigner functional approach is used to obtain the phase sensitivity of such an SU(1,1) interferometer in terms of all the spatiotemporal degrees of freedom. It reveals how experimental scale parameters affect the performance of the interferometer. The analysis provides information that would be useful for quantum metrology applications.