Understanding the structure of nonlocal correlations is important in many fields ranging from fundamental questions of physics to device-independent cryptography. We present a protocol that can convert extremal two-party–two-input nonlocal no-signaling boxes of any type into any other extremal two-party–two-input nonlocal no-signaling box perfectly. Our results are exact, and even though the number of required boxes cannot be determined in advance, their expected number is finite. Our protocol is adaptive and demonstrates for the first time the usefulness of using no-signaling boxes in different causal orders by the parties.

We probe the use of optical frequency combs to place lower bounds on anomalous amplification of a weak-value-measured pulse delay, potentially reaching a theoretical temporal resolution of better than ${10}^{-34}$ s. Owing to the interferometric behavior of weak values, we show that anomalous weak value amplification of a time delay is not equivalent to a temporal linear phase ramp. We show that the anomalous weak value is a rearrangement of amplitudes that generates an apparent shift that can be measured in direct detection, but does not change the actual frequency offset of a spectral distribution measurable in coherent detection. This implies that high-precision heterodyne beatnote frequency measurements of an optical frequency comb cannot be used in combination with the weak value amplification, at least in its currently conceived form. This unfortunate feature sheds light on the nature of anomalous weak values in shifting the pointer in a conjugate basis.

Unextendible product bases (UPBs) provide a versatile tool with various applications across different areas of quantum information theory. Their comprehensive characterization is thus of great importance and has been a subject of vital interest for over two decades now. An open question asks about the existence of UPBs, which are genuinely unextendible, i.e., they are not extendible even with biproduct vectors. In other words, the problem is to verify whether there exist genuinely entangled subspaces (GESs), subspaces composed solely of genuinely multiparty entangled states, complementary to UPBs. We solve this problem in the negative for many sizes of UPBs in different multipartite scenarios. In particular, in the all-important case of equal local dimensions, we show that there are always forbidden cardinalities for such UPBs, including the minimal ones corresponding to GESs of the maximal dimensions.

We present a simple theoretical model to calculate multiple-ionization cross sections (MICS) of Ne and Ne-like molecules (molecules with ten electrons as ${\mathrm{H}}_2\mathrm{O}, \mathrm{C}{\mathrm{H}}_4, \mathrm{N}{\mathrm{H}}_3$, and HF) for proton and light-ion impact, taking into account both direct and postcollisional emissions. In this paper, we tackle the case of water molecules, relevant to investigate the radiobiological effects of ion impact on living matter. The theory is developed in the framework of the independent electron model (IEM). To keep the model as simple as possible, we describe the impact parameter dependence of the single-particle ionization probabilities required by the IEM through decreasing exponential functions for each target orbital. We obtain the parameters of the exponential functions from the total net-ionization cross sections for each orbital by applying either the continuum distorted wave-eikonal initial-state (CDW-EIS) approximation or Rudd's model. We then calculate the contribution of Auger postcollisional electron emission to MICS by using the Ne postcollisional emission probabilities. This postcollisional treatment offers a very simple alternative to the much more complex molecular evaluation of postcollisional relaxation and provides results in close agreement with experimental data for proton and other light ions. We also demonstrate the relevance of considering postcollisional emission for water molecules after the $2a_1$ ($2s$ Ne-like) orbital direct ionization.

Both tunneling and Feshbach resonances occur near the second dissociation threshold of ${\mathrm{H}}_2$. There are three channels, $\mathrm{H}\left(1s\right)+\mathrm{H}\left(2p_{1/2}\right)$, $\mathrm{H}\left(1s\right)+\mathrm{H}\left(2s_{1/2}\right)$, and $\mathrm{H}\left(1s\right)+\mathrm{H}\left(2p_{3/2}\right)$, which correlate to the $2p\sigma B{}^1{\mathrm{\Sigma }}_u^{+}$, $3p\sigma B^{\prime }{}^1{\mathrm{\Sigma }}_u^{+}$, and $2p\pi C{}^1{\mathrm{\Pi }}_u^{}$ states, respectively. For the 2pπC state, there is a potential barrier near the dissociation limit, which accommodates bound states ${\upsilon }^{\prime }=13,J^{\prime }=2,3$ with energies above the threshold. The tunneling dissociation results in $\mathrm{H}\left(1s\right)+\mathrm{H}\left(2p_{3/2}\right)$, while Feshbach resonances produce $\mathrm{H}\left(1s\right)+\mathrm{H}\left(2s_{1/2}\right)$ or $\mathrm{H}\left(1s\right)+\mathrm{H}\left(2p_{1/2}\right)$. We measured the rotationally resolved spectra and determined the corresponding spectroscopic parameters. To study the dissociation dynamics, the spin-orbit branching ratios, and the fragment anisotropy parameters along the line profiles have been measured by a combination of Rydberg state ionization, velocity map ion imaging, and delay-time-curve methods. For the predissociation of the $2p\pi C{}^1{\mathrm{\Pi }}_u^{+}\left({\upsilon }^{\prime }=13,J^{\prime }=2\right)$ state with fragment kinetic energy of $0.7\mathrm{c}{\mathrm{m}}^{\text{–}1}$, only the fragment $\mathrm{H}\left(2p_{1/2}\right)$ was observed. For the $2p\pi C{}^1{\mathrm{\Pi }}_u^{-}\left({\upsilon }^{\prime }=13,J^{\prime }=3\right)$ state with fragment kinetic energy of $36\mathrm{c}{\mathrm{m}}^{\text{–}1}$, the branching ratio between $\mathrm{H}\left(2p_{1/2}\right)$ and $\mathrm{H}\left(2p_{3/2}\right)$ was found to be close to the diabatic limit, and the predissociation should occur via tunneling and coupling with a triplet state near the dissociation limit. For the $2p\pi C{}^1{\mathrm{\Pi }}_u^{+},{\upsilon }^{\prime }=13,J^{\prime }=3$ state, the predissociation should occur via tunneling, and coupling with the $3p\sigma B^{\prime }{}^1{\mathrm{\Sigma }}_u^{+}$ and $2p\sigma B{}^1{\mathrm{\Sigma }}_u^{+}$ states.

We revisit strong-field ionization of aligned ${\mathrm{O}}_2, {\mathrm{CO}}_2$, and ${\mathrm{CS}}_2$ molecules in light of recent advances in the field of strong-field physics, in particular the inclusion of multielectron polarization in the numerical solution of the time-dependent Schrödinger equation (TDSE) within the singe-active-electron approximation. Multielectron polarization is modeled by the introduction of a long-range induced dipole term based on the polarizability of the cation, and a field at short distances that counteracts the applied external field and leads to a vanishing time-dependent interaction within a certain cutoff radius. For the probed molecules, the main effect of including multielectron polarization is the reduction of the total ionization yields (TIYs), and for molecules with large polarizability of their cation (${\mathrm{CO}}_2$ and ${\mathrm{CS}}_2$), the alignment angle of maximum TIY will shift. The photoelectron momentum distributions and above-threshold ionization spectra show little imprint of the multielectron polarization associated with the long-range part of the laser-induced dipole potential. For ${\mathrm{CO}}_2$ and ${\mathrm{CS}}_2$, the inclusion of multielectron polarization and the associated induced dipole potential in the TDSE model gives alignment-resolved distributions of total ionization yields which are in better agreement with the available experimental results.

The total ionization rate of biharmonic ($\omega +3\omega$) ionization is studied within the independent particle approximation and the third-order perturbation theory. Particular attention is paid to how the polarization of the biharmonic light field affects the total rate. The ratios of the biharmonic ionization rates for linearly and circularly polarized beams as well as for corotating and counter-rotating elliptically polarized beams are analyzed, and how they depend on the beam parameters, such as photon frequency or phase between $\omega$ and $3\omega$ light beams. We show that the interference of the biharmonic ionization amplitudes determines the dominance of a particular beam polarization over another and that it can be controlled by an appropriate choice of beam parameters. Furthermore, we demonstrate our findings for the ionization of neon $L$ shell electrons.

The dark-mode effect is a stubborn obstacle for ground-state cooling of multiple degenerate mechanical modes optomechanically coupled to a common cavity-field mode. Here we propose an auxiliary-cavity-mode method for simultaneous ground-state cooling of two degenerate or near-degenerate mechanical modes by breaking the dark mode. We find that the introduction of the auxiliary cavity mode not only breaks the dark-mode effect, but also provides a new cooling channel to extract the thermal excitations stored in the dark mode. Moreover, we study the general physical-coupling configurations for breaking the dark mode in a generalized network-coupled four-mode optomechanical system consisting of two cavity modes and two mechanical modes. We find the analytical dark-mode-breaking condition in this system. This method is general and it can be generalized to break the dark-mode effect and to realize the simultaneous ground-state cooling in a multiple-mechanical-mode optomechanical system. We also demonstrate the physical mechanism behind the dark-mode breaking by studying the breaking of dark-state effect in the $N\text{−}\mathrm{type}$ four-level atomic system. Our results not only provide a general method to control various dark-mode and dark-state effects in physics, but also present an opportunity to the study of macroscopic quantum phenomena and applications in multiple-mechanical-resonator systems.

This work presents a quantum theory of the nonlinear optical process of second-harmonic generation (SHG) in one-dimensional microresonators. More specifically, we show how the manipulation of vacuum field fluctuations in high-confinement systems, leading to a spectrally (and spatially) modulated commutation relation for the photon's generalized formulations of their creation and annihilation operators, deeply affects SHG behavior and gives rise to a threshold level. The two main effects the modulated commutator has on this optical process are an inhibition of the SHG process at low pumping level and a significant (cubic) amplification of the second-harmonic signal production rate once the threshold is overcome (finally reaching the usual quadratic dependence at sufficiently high pumping level). Our predictions, which represent a concrete picture of a fractional quantum system, could be used to probe vacuum field fluctuations present in high-confinement microresonators and emphasize the fundamental importance of vacuum field fluctuations.

The collective dynamics of two noninteracting two-level emitters, which are coupled to a structured wave guide that supports two-photon bound states, is investigated. Tuning the energy of the two emitters such that they are in resonance with the two-photon bound-state energy band, we identify parameter regimes where the system displays fractional populations and essentially undamped Rabi oscillations. The Rabi oscillations, which have no analog in the single-emitter dynamics, are attributed to the existence of a collective polaron-like photonic state that is induced by the emitter-photon coupling. The full dynamics is reproduced by a two-state model in which the photonic polaron interacts with the state $|e,e,\text{vac}\rangle$ (two emitters in their excited state and empty wave guide) through a Rabi coupling frequency that depends on the emitter separation. Our work demonstrates that emitter-photon coupling can lead to an all-to-all momentum space interaction between two-photon bound states and tunable non-Markovian dynamics, opening up exciting directions for emitter arrays coupled to a waveguide.

Does the remote measurement disturbance of the quantum state of a system $B$ by a measurement on system $A$ entangled with $B$ constitute a real disturbance, i.e., an objective alteration, of $B$ in an operational sense? Employing information-theoretic criteria motivated by operational considerations alone, we argue that the disturbance in question is real for a subset of steerable correlations. This result highlights the distinction between quantum no-signaling and the relativistic signal locality. It furthermore suggests a natural reason why a convex operational theory should be nonsignaling, namely, to ensure consistency between the properties of reduced systems and those of single systems.

We investigate the possibility of realizing a loophole-free violation of Bell's inequality using deterministic single-photon sources. We provide a detailed analysis of a scheme to achieve such violations over long distances with immediate extensions to device-independent quantum key distribution. We investigate the effect of key experimental imperfections that are unavoidable in real-world single-photon sources including the finite degree of photon indistinguishability, single-photon purity, and the overall source efficiency. We benchmark the performance requirements to state-of-the-art deterministic single-photon sources based on quantum dots in photonic nanostructures and find that experimental realizations appear to be within reach. We also evaluate the requirements for a postselected version of the protocol, which relaxes the demanding requirements with respect to the source efficiency.

Triorthogonal codes are a class of quantum error-correcting codes used in magic state distillation protocols. We classify all triorthogonal codes with $n+k\le 38$, where $n$ is the number of physical qubits and $k$ is the number of logical qubits of the code. We find 38 distinguished triorthogonal subspaces, and we show that every triorthogonal code with $n+k\le 38$ descends from one of these subspaces through elementary operations such as puncturing and deleting qubits. Specifically, we associate each triorthogonal code with a Reed-Muller polynomial of weight $n+k$, and we classify the Reed-Muller polynomials of low weight using the results of Kasami, Tokura, and Azumi [IEEE Trans. Inf. Theory 16, 752 (1970); Inf. Contr. 30, 380 (1976)] and an extensive computerized search. In an Appendix independent of the main text, we improve a magic state distillation protocol by reducing the time variance due to stochastic Clifford corrections.

We study the convex combinations of the $(d+1)$-generalized Pauli dynamical maps in a Hilbert space of dimension $d$. For certain choices of the decoherence function, the maps are noninvertible, and they remain under convex combinations as well. For the case of dynamical maps characterized by the decoherence function $(1-e^{-ct})/n$ with the decoherence parameter $n$ and decay factor $c$, we evaluate the fraction of invertible maps obtained upon mixing, which is found to increase superexponentially with dimension $d$.

State preparation and measurement errors are commonly regarded as indistinguishable. The problem of distinguishing state preparation errors from measurement errors is important to the field of characterizing quantum processors. In this work, we propose a method to separately characterize state preparation and measurement errors using a different type of algorithmic cooling protocol called measurement-based algorithmic cooling (MBAC). MBAC assumes the ability to perform (potentially imperfect) projective measurements on individual qubits, which is available on many modern quantum processors. We demonstrate that MBAC can significantly reduce state preparation error under realistic assumptions, with a small overhead that can be upper bounded by measurable quantities. Thus, MBAC can be a valuable tool not only for benchmarking near-term quantum processors but also for improving the performance of quantum processors in an algorithmic manner.

Quantum coherence is a fundamental resource in modern quantum physics with important applications in quantum technologies. In composite quantum systems, various forms of coherence emerge and play an important role, such as the global coherence, coherence of local subsystems, and the recently introduced mutual coherence. We investigate states that maximize the mutual coherence in various subspaces of the overall two-qubit Hilbert space and discover a nontrivial asymmetric optimal state in the three-dimensional subspace. We experimentally generate this optimal state from two factorized photonic qubits by a strictly incoherent probabilistic quantum operation that projects the input state onto the desired three-dimensional subspace. For comparison, we also experimentally test the preparation of states with maximal mutual coherence by unitary transformations of input product states. These proof-of-principle tests demonstrate the initial steps of control of mutual quantum coherence in qubit systems.

We develop techniques to probe the dynamics of quantum information and implement them experimentally on an IBM superconducting quantum processor. Our protocols adapt shadow tomography for the study of time-evolution channels rather than of quantum states and rely only on single-qubit operations and measurements. We identify two unambiguous signatures of quantum information scrambling, neither of which can be mimicked by dissipative processes, and relate these to many-body teleportation. By realizing quantum chaotic dynamics in experiment, we measure both signatures and support our results with numerical simulations of the quantum system. We additionally investigate operator growth under this dynamics and observe behavior characteristic of quantum chaos. As our methods require only a single quantum state at a time, they can be readily applied on a wide variety of quantum simulators.

Correlated electron materials, such as superconductors and magnetic materials, are regarded as fascinating targets in quantum computing. However, the quantitative resources, specifically the number of quantum gates and qubits, required to perform a quantum algorithm to simulate correlated electron materials remain unclear. In this study, we estimate the resources required for the Hamiltonian simulation algorithm for correlated electron materials, specifically for organic superconductors, iron-based superconductors, binary transition-metal oxides, and perovskite oxides, using the fermionic swap network. The effective Hamiltonian derived using the ab initio downfolding method is adopted for the Hamiltonian simulation, and a procedure for the resource estimation by using the fermionic swap network for the effective Hamiltonians including the exchange interactions is proposed. For example, in the system for the ${10}^2$ unit cells, the estimated numbers of gates per Trotter step and qubits are approximately ${10}^7$ and ${10}^3$, respectively, on average for the correlated electron materials. Furthermore, our results show that the number of interaction terms in the effective Hamiltonian, especially for the Coulomb interaction terms, is dominant in the gate resources when the number of unit cells constituting the whole system is up to ${10}^2$, whereas the number of fermionic swap operations is dominant when the number of unit cells is more than ${10}^3$.

Integrated total and state-selective electron-capture cross sections are calculated for bare beryllium ion scattering on atomic hydrogen initially configured in the $2\ell m$ states (where $n=2, \ell$, and $m$ are the principal, angular momentum, and magnetic quantum numbers, respectively). This is done using the wave-packet convergent close-coupling approach which solves the three-body Schrödinger equation by employing a two-center expansion for the total scattering wave function. These calculations are performed within a projectile-energy range of 1 to 500 keV/u. The results suggest that at low energies, collisions with hydrogen in each of the $2\ell m$ states produce a total electron-capture cross section approximately an order of magnitude larger than for scattering on the ground state. However, as projectile energy increases, the cross section for capture from the excited states falls well below the $\mathrm{H}(1s$) electron capture cross section. A possible reason for this observation could be related with the way the target electron radial densities are distributed in different initial states. The results obtained in this work are compared to previous calculations where available. In terms of the $n$-resolved charge-exchange cross sections, significant disagreement is found between our results and some preceding calculations available in the literature to warrant further investigations.