We consider how to tell the time-ordering associated with measurement data from quantum experiments at two times and any number of qubits. We define an arrow of time inference problem. We consider conditions on the initial and final states that are symmetric or asymmetric under time reversal. We represent the spatiotemporal measurement data via the pseudodensity matrix space-time state. There is a forward process which is completely positive and trace-preserving (CPTP) and a reverse process which is obtained via an alternative recovery map based on inverting unitary dilations. For asymmetric conditions, the protocol determines whether the data are consistent with the unitary dilation recovery map or the CPTP map. For symmetric conditions, the recovery map yields a valid CPTP map and the experiment may take place in either direction. We also discuss adapting the approach to the Leifer-Spekkens or Process matrix space-time states.

Quantum coherence, nonlocality, and contextuality are key resources for quantum advantage in metrology, communication, and computation. We introduce a graph-based approach to derive classicality inequalities that bound local, noncontextual, and coherence-free models, offering a unified description of these seemingly disparate quantum resources. Our approach generalizes recently proposed basis-independent coherence witnesses, and it recovers all noncontextuality inequalities of the exclusivity graph approach. Moreover, violations of certain classicality inequalities witness preparation contextuality. We describe an algorithm to find all such classicality inequalities, and we use it to analyze some of the simplest scenarios.

Quantum coherence is the key resource in quantum technologies, including faster computing, secure communication, and advanced sensing. Its quantification and detection are, therefore, paramount within the context of quantum information processing. Having certain prior knowledge of the observables may enhance the efficiency of coherence detection. In this work, we posit that the trace of the observables is a known quantity. Our investigation confirms that this assumption indeed extends the scope of coherence-detection capabilities. Utilizing this prior knowledge of the trace of the observables, we establish a series of coherence-detection criteria. We investigate the detection capabilities of these coherence criteria from diverse perspectives and ultimately ascertain the existence of four distinct and inequivalent criteria. These findings contribute to the deepening of our understanding of coherence-detection methodologies, thereby potentially opening new avenues for advancements in quantum technologies.

The shareability of quantum correlations among the constituent parties of a multiparty quantum system is restricted by the quantum information theoretic concept called monogamy. Depending on the multiparty quantum systems, different measures of quantum correlations show disparate signatures for monogamy. We characterize the shareability of quantum correlations, from both entanglement-separability and information-theoretic kinds, in a multiparty quantum spin system containing two- and three-body interactions with respect to its system parameters and external applied magnetic field. Monogamy score in this system exhibits both monogamous and nonmonogamous traits depending on the quantum correlation measure, strengths of system parameters, and external magnetic field. The percentage of nonmonogamous states when the information-theoretic quantum correlations are considered is higher than that of the entanglement-separability kind in allowed ranges of these variables. The integral powers of the quantum correlation measures for which the nonmonogamous states become monogamous are identified.

We show a general method to estimate with optimum precision, i.e., the best precision determined by the light-matter interaction process, a set of parameters that characterize a phase object. The method is derived from ideas presented by Pezze et al. [Phys. Rev. Lett. 119, 130504 (2017)]. Our goal is to illuminate the main characteristics of this method as well as its applications to the physics community probably not familiar with the formal quantum language usually employed in works related to quantum estimation theory. First, we derive precision bounds for the estimation of the set of parameters characterizing the phase object. We compute the Crámer-Rao lower bound for two experimentally relevant types of illumination: a multimode coherent state with mean photon number $N$ and $N$ copies of a multimode single-photon quantum state. We show under which conditions these two models are equivalent. Second, we show that the optimum precision can be achieved by projecting the light reflected or transmitted from the object onto a set of modes with engineered spatial shape. We describe how to construct these modes and demonstrate explicitly that the precision of the estimation using these measurements is optimum. As an example, we apply these results to the estimation of the height and sidewall angle of a cliff-like nanostructure, an object relevant in the semiconductor industry for the evaluation of nanofabrication techniques.

We study the effects of the Breit interaction on angular emission of Auger electrons emitted from the nonradiative Auger decay $1s2s^{2}2p_{1/2}J=1\to 1s^{2}2s{J}_f=1/2$ of berylliumlike high-$Z$ ions excited by electron impact. The Breit interaction is found to contribute to lowering the anisotropy of the Auger electrons, and this effect behaves less prominently with increasing nuclear charge and impact energy for heavier ions than ${\mathrm{Re}}^{71+}$. This finding is surprisingly different from the conclusion drawn for the case of x-ray photon emission, and, indeed, rather contradicts our common understanding of the role of the Breit interaction in excitation and decay of highly charged heavy ions, for which the contribution of the Breit interaction is ever expected to be more prominent as the nuclear charge and impact energy increase. Our detailed analysis reveals that such an unexpected behavior results from a constructive or more destructive competition of the effects of the Breit interaction on both the excitation and Auger decay of the autoionizing level $1s2s^{2}2p_{1/2}J=1$ of high-$Z$ ions.

High-order harmonic generation in ${\mathrm{H}}_3^{+}$ in a strong midinfrared laser pulse is studied by numerically simulating a four-dimensional time-dependent Schrödinger equation including two active electrons. Once one electron is tunneling out, the other bound electron hops in ${\mathrm{H}}_3^{2+}$. At rescattering, if the bound electron may hop to the opposite site compared to that at the ionization moment, the high-order harmonic generation is suppressed and a valley is formed in high-order harmonic spectra. Such harmonic suppression can be used to extract the ultrafast electron localization of the bound electron in ${\mathrm{H}}_3^{2+}$, as well as timing the rescattering moment. Our findings thus shed light on the intricate interplay between electrons, and provide valuable insights into fundamental ultrafast mechanisms.

The interferometer is a vital technology in achieving coherent control and measurement in quantum systems. In this study, we present a few-phonon interferometer based on an optomechanical crystal cavity, which is cooled down with several phonons even less. Through the phonon counting, the competition between the thermal and coherent phonons is demonstrated by the interference. The interference visibility is monotonically increased with increasing the coherent phonons. However, the interference on second-order correlation $g^{\left(2\right)}\left(0\right)$ occurs in reverse because of the different statistical properties of thermal and coherent phonons, which also leads to an unnatural interference visibility with great coherent phonons. The investigation of this correlation interference technology will advance the development of cavity optomechanics systems in the quantum precision sensing and quantum information processing fields.

A high-quality narrowband polarization-entangled source in the telecom band is preferred to avoid frequency dispersion for long-distance transmission in optical fibers and to efficiently couple with telecom-band quantum memories. Here, we report narrowband, telecom-band, polarization-entangled photon pair generation based on the superposition of single-longitudinal-mode photon pairs from two monolithic nonlinear crystal cavities in a passively stable interferometer based on beam displacers. The photon pairs generated from the cavities exhibit a high coincidence to accidental coincidence ratio of 20 000 and a bandwidth below 500 MHz. Two-photon polarization interference, Bell inequality, and quantum state tomography are performed to indicate the high quality of the entangled source. The current configuration demonstrates greater stability than traditional free space cavity-enhanced polarization-entangled state generation, which is promising for quantum communication applications.

The quantum approximate optimization algorithm (QAOA) is an appealing proposal to solve NP problems on noisy intermediate-scale quantum (NISQ) hardware. Making NISQ implementations of the QAOA resilient to noise requires short ansatz circuits with as few controlled-not (cnot) gates as possible. Here we present the dynamic adaptive quantum approximate optimization algorithm (Dynamic-ADAPT-QAOA). Our algorithm significantly reduces the circuit depth and the cnot count of standard ADAPT-QAOA, a leading proposal for near-term implementations of the QAOA. Throughout our algorithm, the decision to apply cnot-intensive operations is made dynamically, based on algorithmic benefits. Using density-matrix simulations, we benchmark the noise resilience of ADAPT-QAOA and Dynamic-ADAPT-QAOA. We compute the gate-error probability $p_{\text{gate}}^{★}$ below which these algorithms provide, on average, more accurate solutions than the classical, polynomial-time approximation algorithm by Goemans and Williamson. For small systems with six to ten qubits, we show that $p_{\text{gate}}^{★}>{10}^{-3}$ for Dynamic-ADAPT-QAOA. Compared to standard ADAPT-QAOA, this constitutes an order-of-magnitude improvement in noise resilience. This improvement should make Dynamic-ADAPT-QAOA viable for implementations on superconducting NISQ hardware, even in the absence of error mitigation.

The structure of quantum mechanics forbids a bipartite scenario for masking quantum information, however, it allows multipartite maskers. The Latin squares are found to be closely related to a series of tripartite maskers. This adds another item, significantly different from the original no-cloning theorem, to the no-go theorems. On the other hand, anyonic excitations in two dimensions exhibit exotic collective behaviors of quantum physics, and open the avenue of fault-tolerant topological quantum computing. Here, we give the Latin-square construction of Abelian and Ising anyons in the Kitaev model and study the maskable space configuration in anyonic space. The circling and braiding of Kitaev anyons are masking operations on extended hyperdisks in anyonic space. We also realize quantum information masking in a teleportation way in the Kitaev Ising anyon model.

We consider how the theory of optimal quantum measurements determines the maximum information available to the receiving party of a quantum key distribution (QKD) system employing linearly independent but nonorthogonal quantum states. Such a setting is characteristic of several practical QKD protocols. Due to nonorthogonality, the receiver is not able to discriminate unambiguously between the signals. To understand the fundamental limits that this imposes, the quantity of interest is the maximum mutual information between the transmitter (Alice) and the receiver, whether legitimate (Bob) or an eavesdropper (Eve). To find the optimal measurement—taken individually or collectively—we use a framework based on operator algebra and general results derived from singular-value decomposition, achieving optimal solutions for von Neumann measurements and positive operator-valued measures (POVMs). The formal proof and quantitative analysis elaborated for two signals allow us to conclude that optimal von Neumann measurements are uniquely defined and provide a higher information gain compared to POVMs. Interestingly, collective measurements not only do not provide additional information gain with respect to individual ones but also suffer from a gain reduction in the case of POVMs.

We investigate the distributed dense coding (DC) protocol, involving multiple senders and a single or two receivers under the influence of non-Markovian noise, acting on the encoded qubits transmitted from the senders to the receiver(s). We compare the effects of non-Markovianity on DC for both the dephasing and depolarizing channels. In the case of dephasing channels, we illustrate that for some classes of states, high non-Markovian strength can eradicate the negative influence of noisy channels which is not observed for depolarizing noise. Furthermore, we incorporate randomness into the noise models by replacing the Pauli matrices with random unitaries and demonstrate the constructive impact of stochastic noise models on the quenched averaged dense coding capacity. Interestingly, we report that the detrimental effect of non-Markovian depolarizing channels in the DC protocol can be eliminated when randomness is added to the channel.

This article reports on the application of a long-pulse near-Fourier-transform-limited ultraviolet (UV) laser to a high-precision spectroscopic investigation of the $n\mathrm{p}$ and $n\mathrm{f}$ Rydberg series converging to the ${\left(5p\right)}^5{}^{2}P_{3/2}$ ground state of ${\mathrm{Xe}}^{+}$. These series were measured by pulsed-field ionization following single-photon absorption from the ${\left(5p\right)}^5{\left(6s\right)}^1{}^{3}P_2$ metastable state of Xe in a doubly skimmed supersonic expansion. They were analyzed by multichannel quantum-defect theory to determine the ionization energies of all natural isotopes of Xe, and to quantify the isotopic shifts of all natural isotopes and the hyperfine structure in the spectra of ${}^{129}\mathrm{Xe}$ ($I=1/2$) and ${}^{131}\mathrm{Xe}$ ($I=3/2$). Special attention was given to the determination of precise and accurate transition frequencies by careful consideration of systematic uncertainties originating from the DC- and AC-Stark effects, the first- and second-order Doppler effects, the photon-recoil shift, and the calibration of the laser frequency including the analysis of frequency chirps during the laser pulses.

On the basis of optimal control theory, we numerically study how to optimally manipulate molecular vibrational dynamics by using cycle-averaged polarizability interactions induced by mildly intense nonresonant laser (NR) pulses. As the essential elements to be controlled are the probability amplitudes, namely, the populations and the relative phases of the vibrational eigenstates, we consider three fundamental control objectives: selective population transfer, wave-packet shaping that requires both population control and relative-phase control, and wave-packet deformation suppression that solely requires relative-phase control while avoiding population redistribution. The nontrivial control of wave-packet deformation suppression is an extension of our previous study on wave-packet spreading suppression. Focusing on the vibrational dynamics in the $B$ state of ${\mathrm{I}}_2$ as a case study, we adopt optimal control simulations and model analyses under the impulsive excitation approximation to systematically examine how to achieve the control objectives with shaped NR pulses. Optimal solutions are always given by NR pulse trains, in which each pulse interval and each pulse intensity are adjusted to cooperate with the vibrational dynamics to effectively utilize the quantum interferences to realize the control objectives with high probability.

We use octave-spanning, phase-shaped supercontinuum generation as an analog computing element in a neural network. We can perform standard machine learning tasks such as autoencoding by embedding the physical device within a virtual neural network which converts input data to optical parameters, and measured spectra to output data. To understand the computing potential of the supercontinuum, we fully measure a small subset of input parameter space. We test universal function approximation by forming a basis set of delta functions from linear combinations of the physically measured functions. This approach provides a more general way to estimate the computational power of a physical operation than performing a specific task.

We present the realization of a vectorial perturbation method based on the Born series applied to strong electromagnetic scattering problems. We present the general theoretical formalism and show a semianalytical implementation for scattering by diffraction gratings. We are particularly interested in the strong scattering regime, where the Born series is known to wildly (namely, exponentially) diverge. By applying Padé approximation to the vectorial Born series, we are able to obtain accurate results from divergent Born series. The method we present has the inherent benefit of being close to the actual physical mechanism behind the formation of a scattered signal, as the solution is built step by step from a sequence of multiple-scattering events. This helps in the understanding of signal formation, which is a key element in inverse scattering problems that are relevant to optical metrology, among others.

A Stokes parameter representation for two-photon systems is developed to calculate cross sections for both entangled and classically correlated mixed states scattering off unpolarized and polarized Compton electrons. The cross section of Compton scattering for pairs of maximally entangled annihilation photons, generated by the disintegration of para-positronium, is compared with classical analogs. An analysis of the symmetrical properties and basis independence of these systems is conducted to elucidate the observed differences. We propose a method to establish an upper bound for identifying correlations influenced by entanglement. Furthermore, we calculate the cross section for Compton scattering of annihilation photons from spin-polarized electrons. A qualitative analysis reveals a contradiction to the contemporary assumption that Compton scattering acts as an entanglement kill switch. These findings contribute to a broader understanding of photon systems and their behavior in various scattering scenarios. Although the primary focus of this article is on Compton scattering, the formalism can be adapted to other interactions, provided that the Mueller matrix representation of the interaction is available.

Analyzing the nonclassicality of correlations generated in $n$-local quantum networks is an important branch of study in quantum information theory. Speaking of nonclassicality, violation of an $n$-local inequality ensures non $n$-locality of corresponding network correlations. Being correlation-based, violation of any such inequality is experimentally feasible. The present work explores whether applying suitable local unitary operations can aid violation of any $n$-local inequality. Observations have ensured the utility of local unitary operations in enhancing the detection efficiency of bilocal inequality and $n$-local inequality (for $n=3,4$) in the star configuration. In an $n$-local quantum network, such operations correspond to local basis change applied by at least one party over its joint state of qubits received from the sources. In an $n$-local network, with all the parties performing any possible local unitary operation, correlations not violating an $n$-local inequality (compatible with the network considered), are first characterized. Such characterization is then exploited to provide examples of local unitary operations aiding violation of existing $n$-local inequalities for $n=2$, 3, and 4, specifically. Interestingly, there exist two qubit unitary operations, for which nontrilocal correlations can be simulated even when one of the sources distribute pure product state in star network. Quantum gates associated with the useful unitary operations are provided.

We consider few-body systems in which only a certain subset of the particle-particle interactions is resonant. We characterize each subset by a unitary graph in which the vertices represent distinguishable particles and the edges resonant two-body interactions. Few-body systems whose unitary graph is connected will collapse unless a repulsive three-body interaction is included. We find two categories of graphs, distinguished by the kind of three-body repulsion necessary to stabilize the associated system. Each category is characterized by whether the graph contains a loop or not: for tree-like graphs (graphs containing a loop) the three-body force renormalizing them is the same as in the three-body system with two (three) resonant interactions. We show numerically that this conjecture is correct for the four-body case as well as for a few five-body configurations. We explain this result in the four-body sector qualitatively by imposing Bethe-Peierls boundary conditions on the pertinent Faddeev-Yakubovsky decomposition of the wave function.