We study nonclassical effects in the dynamics of an open quantum system. The model involves a harmonic oscillator coupled to a reservoir of noninteracting harmonic oscillators. Different system-bath interaction schemes as well as reservoir states are considered. Particularly, the squeezed reservoirs coupled to the system through single and two quanta exchange processes are put in the spotlight. We investigate the quantumness conveyed to the system through the bath by computing a nonclassicality measure for different bath properties. The measure of nonclassicality is calculated for projective measurements both in the number state basis and in a basis formed by a set of coherent states. Our results show that in both bases the measure exhibits characteristic features for each bath state and the form of its interaction with the system. Some of those features are independent from the measurement scheme (number or coherent) and, thus, emergent from the reservoir. This allows for fingerprinting and identifying the environmental effects by tracking a given probe with appropriate measurements. Hence, it may prove useful for distinguishing different sources of decoherence.

This paper is an algorithmic study of quantum phase estimation with multiple eigenvalues. We present robust multiple-phase estimation (RMPE) algorithms with Heisenberg-limited scaling. The proposed algorithms improve significantly from the idea of single-phase estimation methods by combining carefully designed signal processing routines and an adaptive determination of runtime amplifying factors. They address both the integer-power model, where the unitary $U$ is given as a black box with only integer runtime accessible, and the real-power model, where $U$ is defined through a Hamiltonian $H$ by $U=exp(-2\pi \mathrm{i}H)$ with any real runtime allowed. These algorithms are particularly suitable for early fault-tolerant quantum computers in the following senses: (1) a minimal number of ancilla qubits are used, (2) an imperfect initial state with a significant residual is allowed, (3) the prefactor in the maximum runtime can be arbitrarily small given that the residual is sufficiently small and a gap among the dominant eigenvalues is known in advance. Even if the eigenvalue gap does not exist, the proposed RMPE algorithms can achieve the Heisenberg limit while maintaining (1) and (2).

Robust quantum state transfer (QST) is the foundation for information exchange among nodes in quantum networks. In this paper, we propose a robust QST protocol that utilizes topological edge modes in the qubit chains to encode (decode) quantum states (flying qubits). By employing qubits with tunable couplings, we construct Su-Schrieffer-Heeger (SSH) chains as the nodes of a quantum network. The end qubit of each SSH chain is dissipatively coupled to a chiral waveguide, and the dissipative strength is a constant. We refer to the SSH chain with a dissipation channel at the end qubit as the non-Hermitian SSH chain. Comparing the symmetry and energy spectra of the non-Hermitian SSH chain with those of the SSH chain, our analysis reveals that the dissipative dynamics of the topological edge state in the non-Hermitian SSH chain are governed by its imaginary spectra. The edge mode with the imaginary spectrum can be used to encode (decode) quantum states (flying qubits), thereby enabling robust QST between two remote mirrored non-Hermitian SSH chains. Our numerical simulations demonstrate that high-fidelity QST can be achieved even in the presence of coupling errors. Furthermore, we extend our analysis to consider QST in imperfect chiral waveguides, providing insights into the robustness of our protocol under realistic conditions. Our discussion is applicable to various quantum platforms and holds significant implications for constructing large-scale quantum networks.

Quantum process tomography (QPT) methods aim at identifying a given quantum process. QPT is a major quantum information processing tool, since it allows one to characterize the actual behavior of quantum gates, which are the building blocks of quantum computers. The present paper focuses on the estimation of a unitary process. This class is of particular interest because quantum mechanics postulates that the evolution of any closed quantum system is described by a unitary transformation. The standard approach of QPT is to measure copies of a particular set of predetermined (generally pure) states after they have been modified by the process to be identified. The main problem with this setup is that preparing an input state and setting it precisely to a predetermined value is challenging and thus yields errors. These errors can be decomposed into a sum of centered errors (i.e., whose average on all the copies is zero) and systematic errors that are the same for all the copies. The latter is often the main source of error in QPT. The algorithm we introduce works for any input states that make QPT theoretically possible (i.e., unless there are several solutions due to, e.g., a lack of diversity in the input states). The fact that we do not require the input states to be precisely set to predetermined values means that we can use a trick to remove the issue of systematic errors by considering that some states' copies are unknown but measured before they go through the process to be identified. We achieve this by splitting the copies of each input state into several groups and measuring the copies of the $k\mathrm{th}$ group after they have successively been transferred through $k$ instances of the process to be identified (each copy of each input state is used for only a single measurement). Using this approach, we can compute estimates of the measured states before and after they go through the process without using the knowledge we might have about the initial states. We test our algorithm with simulated data, and we assess its performance with a cnot gate on a trapped-ions qubit quantum computer.

We present a variation of a quantum algorithm for the machine learning task of classification with graph-structured data. The algorithm implements a feature extraction strategy that is based on Gaussian boson sampling (GBS), a near-term model of quantum computing. However, unlike the currently proposed algorithms for this problem, our GBS setup requires only binary (light or no light) detectors, as opposed to photon-number-resolving detectors. Binary detectors are technologically simpler and can operate near room temperature, making our algorithm much less complex and costly to implement physically. We also investigate the connection between graph theory and the Torontonian matrix function which characterizes the probabilities of binary GBS detection events.

We establish that there are properties of quantum many-body dynamics which are efficiently learnable if we are given access to out-of-time-order correlators (OTOCs), but which require exponentially many operations in the system size if we can only measure time-ordered correlators. This implies that any experimental protocol which reconstructs OTOCs solely from time-ordered correlators must be, in certain cases, exponentially inefficient. Our proofs leverage and generalize recent techniques in quantum learning theory. Along the way, we elucidate a general definition of time-ordered versus out-of-time-order experimental measurement protocols, which can be considered as classes of adaptive quantum learning algorithms. Moreover, our results provide a theoretical foundation for novel applications of OTOCs in quantum simulations.

Contemporary quantum computers encode and process quantum information in binary qubits $(d=2)$. However, many architectures include higher energy levels that are left as unused computational resources. We demonstrate a superconducting ququart $(d=4)$ processor and combine quantum optimal control with efficient gate decompositions to implement high-fidelity ququart gates. We distinguish between viewing the ququart as a generalized four-level qubit and an encoded pair of qubits, and characterize the resulting gates in each case. In randomized benchmarking experiments we observe gate fidelities $\ge 95\%$ and identify coherence as the primary limiting factor. Our results validate ququarts as a viable tool for quantum information processing.

Optically pumped magnetometers are important instruments for imaging biological magnetic signals without the need for cryogenic cooling. These magnetometers are presently available in the commercial market and utilize the principles of atomic alignment or orientation, enabling remarkable sensitivity and precision in the measurement of magnetic fields. This research focuses on utilizing a spin-aligned atomic ensemble for magnetometry at zero field. An approach is introduced which involves evaluating how the linear polarization of light rotates as it passes through the atomic vapor to null the magnetic field. Analytical expressions are derived for the resulting spin alignment and photodetection signals. Experimental results are provided, demonstrating good agreement with the theoretical predictions. The sensitivity and bandwidth of the magnetometer are characterized based on the detected polarization rotation signal. Lastly, the practical utility of the magnetometer for medical applications is demonstrated by successfully detecting a synthetic cardiac signal.

We study a variational class of generalized Ramsey protocols that include two one-axis twisting (OAT) operations, one performed before the phase imprint and the other after. In this framework, we optimize the axes of the signal imprint, the OAT interactions, and the direction of the final projective measurement. We distinguish between protocols that exhibit symmetric or antisymmetric dependencies of the spin projection signal on the measured phase. Our results show that the quantum Fisher information, which sets the limits on the sensitivity achievable with a given one-axis twisted input state, can be saturated within our class of variational protocols for almost all initial twisting strengths. By incorporating numerous protocols previously documented in the literature, our approach creates a unified framework for Ramsey echo protocols with OAT states and measurements.

Delayed electron detachment induced by resonant two-photon excitation of $\mathrm{C}{{}_7}^{-}$ is observed using an electrostatic ion storage ring. Radiative cooling during the period of more than 100 ms storage considerably narrows the initially very broad internal energy distributions resulting in the delayed detachment profiles to be well fitted by single-exponential functions. The time constants of the decay, representing the electron detachment rate constants, are found to be much smaller than the theoretically predicted ones based on the detailed balance theory employing the state density of the harmonic vibrations.

Collinear laser spectroscopy has been performed on He-like ${\mathrm{C}}^{4+}$ ions extracted from an electron-beam ion source (EBIS). In order to determine the transition frequency with the highest-possible accuracy, the line shape of the fluorescence response function was studied for pulsed and continuous ion extraction modes of the EBIS in order to optimize its symmetry and linewidth. We found that the best signal-to-noise ratio is obtained using the continuous beam mode for ion extraction. Applying frequency-comb-referenced collinear and anticollinear laser spectroscopy, we achieved a measurement accuracy of better than 2 MHz including statistical and systematic uncertainties. The origin and size of systematic uncertainties, as well as further applications for other isotopes and elements are discussed.

We report on the existence of exceptional points (EPs) in single-resonance autoionization and provide analytical expressions for their positions in parameter space, in terms of the Fano asymmetry parameter. We additionally propose a reliable method for the experimental determination of EPs, based solely on information about their ionization probability as a function of the system parameters. The links between EPs, the maxima of the asymmetric profile, and the effective decay rate of the ground state are investigated in detail. Quantitative numerical examples pertaining to the doubly excited $2s2p\left({}^{1}P\right)$ state of helium confirm the validity of our formulation and results. In addition to unveiling hidden aspects of autoionization, our treatment and results provide a benchmark for the exploration of EPs and their properties in a variety of materials exhibiting Fano profiles with a broad perspective of possible applications.

We implement a magnetic-field-sensitive multiwave atom Ramsey interferometer based on radio-frequency (RF) field-coupled Zeeman states, and we theoretically derive and experimentally measure its frequency-domain transfer function with low-pass filtering characteristics. In addition, we investigate a potential application scheme of the transfer functions for interference fringe recovery. As the measurements involve the ac magnetic field, a Raman spectrum-based solution is implemented to calibrate the actual ac magnetic field amplitude at the position of the atom inside the vacuum chamber. The methods demonstrated in this work will help to improve the performance of the atom interferometer and are also useful in Ramsey interference-based systems and spinor Bose-Einstein condensates (BEC) experiments.

We investigate the spatial beam dynamics inside a multimode graded-index fiber under Raman amplification by adopting a semianalytical variational approach. The variational analysis provides us with four coupled ordinary differential equations that govern the beam's evolution and are much faster to solve numerically compared to the full nonlinear wave equation. Their solution also provides considerable physical insight and allows us to study the impact of important nonlinear phenomena such as self-focusing and cross-phase modulation. We first show that the variational results agree well with full numerical simulations over a wide range of operating parameters. We use them to investigate the signal beam's amplification for different initial widths of the pump and signal beams. This allows us to quantify the pumping conditions under which the quality of a signal beam could be improved at the fiber's output end. Our analysis may prove beneficial in practice for two reasons. First, it can be used to scan the design parameters of an experiment in a time-efficient fashion and determine the optimum values of these parameters. Second, it can provide physical insight into nonlinear phenomena involved in the Raman-amplification process.

In this paper we propose a non-Markovian quantum channel approach to mitigating the degradation of the average fidelity in continuous-variable quantum teleportation. The non-Markovian quantum channel is modeled by an augmented system where ancillary systems are introduced to represent the internal modes of non-Markovian environments. With a proper non-Markovianity, enhanced effects of the channels on continuous-variable quantum teleportation are observed. Also, the logarithmic negativity of entangled states can be effectively maintained so that the decay of the average fidelity of quantum teleportation is mitigated. In addition, the analysis on different teleported quantum states shows that coherent states exhibit stronger robustness than those of squeezed states and cat states in quantum teleportation with a non-Markovian channel.

A set of orthogonal states is the strongest nonlocal set if it is locally stable in every bipartition, which shows the strong quantum nonlocality proposed by Halder et al. [Phys. Rev. Lett. 122, 040403 (2019)]. The existence of the strongest nonlocal sets with the minimum size is an open question. In this work, we partially solve this question by constructing the strongest nonlocal sets with the minimum size in $2\otimes 2\otimes 2\otimes 2$ and $2\otimes d_2\otimes d_3$, where $2\le d_2\le d_3$. Moreover, we also give the strongest nonlocal sets with size $d_2{d}_3+d_1-1$ in $d_1\otimes d_2\otimes d_3$ and the strongest nonlocal sets with size $d^3+d-1$ in $d\otimes d\otimes d\otimes d$. All the sizes of the strongest nonlocal sets are close to the minimum size and are smaller than all previously known constructions. As an application, our strongest nonlocal sets can be used to construct partially genuinely entangled subspaces in $d_1\otimes d_2\otimes d_3$ when $2\le d_1\le d_2\le d_3$ and $d_2\ge 3$.

Based on the optimal quantum violation of the suitable Bell's inequality, device-independent self-testing of states and observables has been reported. It is well studied that locally commuting or compatible observables cannot be used to reveal quantum nonlocality. Therefore, self-testing of commuting local observables is not possible through the Bell test. In this work, we demonstrate the self-testing of a set of mutually commuting local observables. We show that the optimal quantum violations of suitably formulated bilocality and $n$-locality inequalities in networks uniquely fix the observables of one party to be mutually commuting. In particular, we first demonstrate that in a two-input arbitrary-party star network, two commuting local observables can be self-tested. Further, by considering an arbitrary-input three-party bilocal network scenario, we demonstrate the self-testing of an unbounded number of mutually commuting local observables.

We derive finite nuclear mass and finite nuclear size corrections to the magnetic shielding in light ions. These corrections are important for the accurate determination of nuclear magnetic moments. We correct several previous formulas for the nuclear mass corrections and present improved results for the magnetic shielding in ${}^1\mathrm{H}, {}^3\mathrm{He}^{+}$, and ${}^3\mathrm{He}$. Finally, we obtain an ${}^3\mathrm{He}$ atomic magnetic moment, which serves as an accurate probe to measure magnetic fields.

We investigate the impact of nuclear motion on attosecond charge migration modes, characterized by low-frequency and long-distance hole density patterns in polyatomic molecules. We create the initial hole density using constrained density functional theory and track the subsequent electron-ion dynamics using the Ehrenfest dynamics together with the time-dependent density functional theory. Our focus is on a prototypical ${\mathrm{BrC}}_4\mathrm{H}$ system, where we examine the effects of alterations in initial nuclear geometry. We find that the nuclear motion drives and transforms the charge migration modes, underscoring the strong connection between hole patterns and nuclear dynamics. For various initial stretches or bends, these modes demonstrate considerable robustness to the nuclear motion, i.e., they do not dissipate under the classical nuclear motion throughout the computational time. We discover that the charge migration modes correspond to prominent peaks in the frequency spectra of the electronic dipole moment. Additionally, our calculations indicate that the modes primarily originate from the last four atoms near the halogen site and are predominantly attributed to the hole density closest to these nuclear trajectories. Our study offers insights into the behavior of the charge migration modes under varying nuclear geometry and may have implications for understanding related nuclear effects.