In quantum optics, it is common to assume that atoms are pointlike objects compared to the wavelength of the electromagnetic field they interact with. However, this dipole approximation is not always valid, e.g., if atoms couple to the field at multiple discrete points. Previous work has shown that superconducting qubits coupled to a one-dimensional waveguide can behave as such “giant atoms” and then can interact through the waveguide without decohering, a phenomenon that is not possible with small atoms. Here, we show that this decoherence-free interaction is also possible when the coupling to the waveguide is chiral, i.e., when the coupling depends on the propagation direction of the light. Furthermore, we derive conditions under which the giant atoms in such chiral architectures exhibit dark states. In particular, we show that unlike small atoms, giant atoms in a chiral waveguide can reach a dark state even without being excited by a coherent drive. We also show that in the driven-dissipative regime, dark states can be populated faster in giant atoms. The results presented here lay a foundation for applications based on giant atoms in quantum simulations and quantum networks with chiral settings.

We investigate photoelectron momentum distribution (PMD) from strong-field ionization of Ar by a polarization-gated (PG) pulse constructed by a left circularly polarized and a time-delayed right circularly polarized carrier-envelope phase stabilized few-cycle laser pulse. We experimentally demonstrate the feasibility of using a PG pulse to precisely control the interferences between electron wave packets (EWPs) released at different times within one optical cycle. In contrast to previous works where the various types of intracycle interferences are usually hardly resolved and separated, our work shows that, with this specific PG control scheme, it becomes possible to create a highly distinct interference pattern composed of well-separated structures produced by different types of intracycle interferences in the measured PMD. Quantum trajectory Monte Carlo simulations reproduce well the experimental findings and uncover the physical mechanism behind this subcycle control of EWPs with a PG pulse.

A set of multipartite orthogonal quantum states is strongly nonlocal if it is locally irreducible for every bipartition of the subsystems [S. Halder, M. Banik, S. Agrawal, and S. Bandyopadhyay, Phys. Rev. Lett. 122, 040403 (2019)]. Although this property has been shown in three-, four-, and five-partite systems, the existence of strongly nonlocal sets in $N$-partite systems remains unknown when $N\ge 6$. In this paper, we successfully show that a strongly nonlocal set of orthogonal entangled states exists in ${\left({\mathbb{C}}^d\right)}^{\otimes N}$ for all $N\ge 3$ and $d\ge 2$, which reveals the strong quantum nonlocality in general $N$-partite systems. For $N=3$ or 4 and $d\ge 3$, we present a strongly nonlocal set consisting of genuinely entangled states, which has a smaller size than any known strongly nonlocal orthogonal product set. Finally, we connect strong quantum nonlocality with local hiding of information as an application.

We investigate the feasibility of using electrons in a linear Paul trap as qubits in a future quantum computer. We discuss the necessary experimental steps to realize such a device through a concrete design proposal, including trapping, cooling, electronic detection, spin readout, and single- and multiqubit gate operations. Numeric simulations indicate that two-qubit Bell-state fidelities of order 99.99% can be achieved assuming reasonable experimental parameters.

Capturing specific kinds of quantum correlation is of paramount importance for quantum networking. Different routes can be taken to achieve this task, highlighting different aspects of such quantum correlations. Following the recent theoretical results by Yadin, Fadel, and Gessner [Nat. Commun. 12, 2410 (2021)], we demonstrate experimentally how steering manifests in the metrological abilities of a bipartite state. Our results confirm the relevance of this approach, and compare the outcome with already employed alternatives.

Nonlocality without entanglement (NLWE) is a nonlocal quantum phenomenon that arises in separable state discrimination. We show that the availability of the postmeasurement information about the prepared subensemble can affect the occurrence of NLWE in discriminating non-orthogonal nonentangled states. We provide a two-qubit state ensemble consisting of four nonorthogonal separable pure states and show that the postmeasurement information about the prepared subensemble can annihilate NLWE. We also provide another two-qubit state ensemble consisting of four nonorthogonal separable states and show that the postmeasurement information can create NLWE. Our result can provide a useful method to share or hide information using nonorthogonal separable states.

A nonrelativistic quantum approach is adopted to describe the scattering of an intense laser beam from a simple atomic system. Two forms of the corresponding transition amplitude (TA) are derived, one involving the kinetic momentum operator of the electron and the other involving the force operator. The spectral and angular distribution of the scattered radiation is expressed in terms of TA by a relation recalling the classical one. The formalism is applied to the scattering of radiation by an electron (free or bound) with the absorption of two photons from the incident laser beam. Analytic and numeric results are presented for the distributions of the emitted photons in the case of the nonlinear Compton scattering from a hydrogenic atom in the ground state.

The flux of angular momentum of the electromagnetic field of an arbitrarily moving point charge is investigated. General equations for the transfer of angular momentum at arbitrary distance from the charge are obtained, and corresponding equations in the far-field approximation are derived. An explicit expression is obtained for the flux of angular momentum in the wave zone in terms of coordinates, velocity, and acceleration of the charge. The torque that would act on an object if it absorbs the incident radiation is calculated. It is shown that this torque is proportional to the curl of the stress tensor of the electromagnetic field; in the far-field approximation the torque is proportional to the curl of the Poynting vector. Application of the obtained formulas is illustrated by the example of a rotating dipole.

We consider the propagation of light beams through disordered lattices of coupled waveguides searching for Anderson localization and investigating the evolution of nonclassical properties of injected quantum states. We assume that the beam is initially in a variety of states, such as the complementary coherent state, the reciprocal binomial state, and the polynomial state. The statistical properties of the evolved states were analyzed numerically as functions of the localization and delocalization parameters averaged over many realizations of disorder. We also numerically reconstruct the Wigner function of the output state. Interestingly, we find that high values of disorder tend to preserve the quantum properties of some input states when we look at the input waveguide despite of the coupling between it and the neighboring waveguides.

We examine the field-field and field-atom entanglements generated by Λ-type three-level atoms placed in an optical cavity driven above threshold by two coherent input optical fields. It is shown that under realistic experimental conditions entanglement between the two output cavity fields as well as between the fields and the atoms can be achieved when the two cavity fields are both one- and two-photon (Raman) resonant with the atomic transitions, as in the configuration for electromagnetically induced transparency (EIT). The entanglement has similar features to pump-signal-idler three-color entanglement in an above-threshold optical parametric oscillator [Villar et al., Phys. Rev. Lett. 97, 140504 (2006)]. This cavity-enhanced atom-field cooperative coupling could enable convenient and efficient generation of bright, nondegenerate, narrow-band entangled fields, which may find potential applications in realistic quantum communications and networking protocols.

We analyze the structure of the space of temporal correlations generated by quantum systems. We show that the temporal correlation space under dimension constraints can be nonconvex. For the general case, we provide the necessary and sufficient dimension of a quantum system needed to generate a convex correlation space for a given scenario. We further prove that this dimension coincides with the dimension necessary to generate any point in the temporal correlation polytope. As an application of our results, we derive nonlinear inequalities to witness the nonconvexity for qubits and qutrits in the simplest scenario, and present an algorithm which can help to find the minimum for a certain type of nonlinear expressions under dimension constraints.

We provide a necessary condition that a quantum measurement can be implemented by the class of protocols known as local operations and classical communication, or LOCC, including when an error is allowed but must vanish in the limit of an infinite number of rounds, a case referred to as asymptotic LOCC. Our condition unifies, extends, and provides an intuitive, geometric justification for previous results on asymptotic LOCC. We use our condition to answer a variety of long-standing, unsolved problems, including for distinguishability of certain sets of states by LOCC. These include various classes of unextendible product bases, for which we prove they cannot be distinguished by LOCC even when infinite resources are available and an asymptotically vanishing error is allowed.

Particle transport in quantum systems, which can be modeled by quantum walks on graphs, demonstrates a faster propagation advantage over the corresponding transport in classical systems. As known from several graph examples, achieving further advantages is possible by adding directional control in quantum walks. One way to introduce directional bias is via time-reversal symmetry breaking that can be achieved with chiral quantum walks, where complex phases are added to the edge weights. However, it is not known for which complex phases values and on which graphs quantum transport can be enhanced. Therefore, the classification of graph properties remains an open problem. Here we tackle this graph classification problem with a graph convolutional neural network trained on a set of simulated examples. We find that chiral quantum-walk dynamics leads to almost always faster transport on hypercubes compared to nonchiral dynamics. We connect our paper to physical implementations of quantum walks in superconducting qubits and optical waveguides. Our results open the possibility and flexibility of experimental implementations in demonstrating quantum-walk advantage.

Quantum algorithms for differential equation solving, data processing, and machine learning potentially offer an exponential speedup over all known classical algorithms. However, there also exist obstacles to obtaining this potential speedup in useful problem instances. The essential obstacle for quantum differential equation solving is that outputting useful information may require difficult postprocessing, and the essential obstacle for quantum data processing and machine learning is that inputting the data is a difficult task just by itself. In this study, we demonstrate that, when combined, these difficulties solve one another. We show how the output of quantum differential equation solving can serve as the input for quantum data processing and machine learning, allowing dynamical analysis in terms of principal components, power spectra, and wavelet decompositions. To illustrate this, we consider continuous-time Markov processes on epidemiological and social networks. These quantum algorithms provide an exponential advantage over existing classical Monte Carlo methods.

We study the fluctuations of subsystem entropies in closed quantum many-body systems after thermalization. Using a combination of analytics and numerics for both random quantum circuits and Hamiltonian dynamics, we find that the statistics of such entropy fluctuations is drastically different than in the classical setting. For instance, shortly after a system thermalizes, the probability of entropy fluctuations for a subregion is suppressed in the dimension of the Hilbert space of the complementary subregion. This suppression becomes increasingly stringent as a function of time, ultimately depending on the exponential of the Hilbert space dimension, until extremely late times when the amount of suppression saturates. We also use our results to estimate the total number of rare fluctuations at large timescales. We find that the “Boltzmann brain” paradox is largely ameliorated in quantum many-body systems, in contrast with the classical setting.

Quantum Krylov subspace diagonalization (QKSD) algorithms provide a low-cost alternative to the conventional quantum phase estimation algorithm for estimating the ground- and excited-state energies of a quantum many-body system. While QKSD algorithms typically rely on using the Hadamard test for estimating Krylov subspace matrix elements of the form $\langle {\varphi }_i\left|e^{-i\widehat{H}\tau }\right|{\varphi }_j\rangle$, the associated quantum circuits require an ancilla qubit with controlled multiqubit gates that can be quite costly for near-term quantum hardware. In this paper, we show that a wide class of Hamiltonians relevant to condensed-matter physics and quantum chemistry contain symmetries that can be exploited to avoid the use of the Hadamard test. We propose a multifidelity estimation protocol that can be used to compute such quantities, showing that our approach, when combined with efficient single-fidelity estimation protocols, provides a substantial reduction in circuit depth. In addition, we develop a unified theory of quantum Krylov subspace algorithms and present three quantum-classical algorithms for the ground- and excited-state energy estimation problems, where each algorithm provides various advantages and disadvantages in terms of total number of calls to the quantum computer, gate depth, classical complexity, and stability of the generalized eigenvalue problem within the Krylov subspace.

Heat-bath algorithmic cooling (HBAC) techniques are techniques that are used to purify a target element in a quantum system. These methods compress and transfer entropy away from the target element into auxiliary elements of the system. The performance of algorithmic cooling has been investigated under ideal noiseless conditions. However, realistic implementations are imperfect, and for practical purposes, noise should be taken into account. Here we analyze HBAC techniques under realistic noise models. Surprisingly, we find that noise can, in some cases, enhance the performance and improve the cooling limit of HBAC techniques. We numerically simulate the noisy algorithmic cooling for the two optimal strategies, partner-pairing and two-sort algorithms. We find that for both of them, in the presence of the generalized amplitude damping noise, the process converges, and the asymptotic purity can be higher than the noiseless process. This opens up new avenues for increasing the purity beyond heat-bath algorithmic cooling.

Adaptiveness is a key principle in information processing including statistics and machine learning. We investigate the usefulness adaptive methods in the framework of asymptotic binary hypothesis testing, when each hypothesis represents asymptotically many independent instances of a quantum channel, and the tests are based on using the unknown channel and observing outputs. Unlike the familiar setting of quantum states as hypotheses, there is a fundamental distinction between adaptive and nonadaptive strategies with respect to the channel uses, and we introduce a number of further variants of the discrimination tasks by imposing different restrictions on the test strategies. The following results are obtained: (1) We prove that for classical-quantum channels, adaptive and nonadaptive strategies lead to the same error exponents both in the symmetric (Chernoff) and asymmetric (Hoeffding, Stein) settings. (2) The first separation between adaptive and nonadaptive symmetric hypothesis testing exponents for quantum channels, which we derive from a general lower bound on the error probability for nonadaptive strategies; the concrete example we analyze is a pair of entanglement-breaking channels. (3) We prove, in some sense generalizing the previous statement, that for general channels adaptive strategies restricted to classical feed-forward and product state channel inputs are not superior in the asymptotic limit to nonadaptive product state strategies. (4) As an application of our findings, we address the discrimination power of an arbitrary quantum channel and show that adaptive strategies with classical feedback and no quantum memory at the input do not increase the discrimination power of the channel beyond nonadaptive tensor product input strategies.

The x-ray spectra of copper $K\alpha$ and $K\beta$ remain a topic of great interest due to the complex open-shell processes involved, with many discrepancies among theories and experiments. We present high-accuracy theory of copper $K\alpha$ and $K\beta$ diagram and satellite spectra in the multiconfiguration Dirac-Hartree-Fock method. Diagram spectra are expanded to $5s$ with simultaneous convergence of 28 000 configuration state functions (CSFs) $\left(K\alpha \right)$ and to $6g$ with simultaneous convergence of 91 000 CSFs $\left(K\beta \right)$, achieving eigenvalue convergence to $\pm 0.03$ eV or 0.000 25%, approximately a factor of 10 improvement over past work. It is necessary to invoke biorthogonalization, developments of the active space approach, analysis of markers for theoretical convergence of eigenvalues, and the question of self-consistency for both $K\alpha$ and $K\beta$ spectra. We make use of gauge convergence, eigenvalue convergence, and $A$-coefficient convergence. The satellite spectra are a major outcome—without these, it is not possible to make use of the increased accuracy of the diagram computations. The Cu $K\alpha 3d^8$ double shake satellite spectrum alone contains 1506 unique eigenvalues (transitions) and required simultaneous convergence of 593 000 CSFs. Ab initio shakeoff probabilities for $1s,2s,2p,3s,3p,3d$, and $4s$ subshells as a result of the $K$-shell photoionization process are presented. Portable spectral representations are provided in the supplemental material, discussed in the text. We investigate the meaning behind shake, shakeoff, and shakeup processes and the computational potential to investigate these at the current time. We separate the notions of total shake, single shake, and double shake, explain how these are observable in high-quality experimental spectra, and how these calculations can be used experimentally.

The high-order harmonic generation (HHG) by the interaction of an intense vortex infrared laser and a gas medium has been employed to produce the extreme ultraviolet (XUV) light beam carrying a high value orbital angular momentum (OAM). Here we extend this approach to generate the XUV light with superimposed OAM modes by mixing two different Laguerre-Gaussian (LG) modes as the driving beam. By solving the three-dimensional Maxwell's wave equations of the high-harmonic field and calculating the coherence lengths of HHG, we reveal the complex nature of phase-matching conditions in the gas medium, especially their dependence on the azimuthal angle. We demonstrate that the OAM spectra of the HHG can be tailored by varying the harmonic order, the position of gas medium, and the energy ratio of two LG beams. We also uncover that only including short-trajectory contribution in the single-atom response could broaden the OAM spectrum of the macroscopic HHG because of the featured phase-matching characteristics caused by the atomic intrinsic phase. We anticipate this work to stimulate some interests in generating an XUV vortex beam with the tunable OAM spectrum through the gaseous HHG process as well as in separating different OAM modes of an XUV vortex beam by advanced techniques.