The existence of contextuality in quantum mechanics is a fundamental departure from the classical description of the world. Currently, the quest to identify scenarios which cannot be more contextual than quantum theory is at the forefront of research in quantum contextuality. In this work we experimentally test two inequalities, which are capable of revealing fully contextual quantum correlations, on a Hilbert space of dimension 8 and 4, respectively, on an NMR quantum information processor. The projectors associated with the contextuality inequalities are reformulated in terms of Pauli operators, which can be determined in an NMR experiment. We analyze the behavior of each inequality under rotation of the underlying quantum state, which unitarily transforms it to another pure state.

We show that two unknown quantum states, either pure or mixed, can be bidirectionally “teleported” counterfactually. In the process of information exchange, there is no physical particle traveling between two communicators. Unlike conventional quantum teleportation, our protocol requires neither prearranged entangled pairs nor Bell measurements. We entangle and disentangle a photon and an atom through multiple measurements in a special sequence. These multiple measurements can smoothly but reversibly control system evolution. The total effect of multiple measurements can be considered as a unitary time evolution operator. Therefore, our protocol can be regarded as a counterfactual quantum swap gate. Based on our approach, all known conventional quantum gates can be counterfactualized.

We demonstrate the role of measurement back action of a coherent spin environment in the dynamics of a spin (qubit) coupled to it, by inducing nonclassical (quantum random walk–like) statistics on its measurement trajectory. We show how the long lifetime of the spin bath correlates with measurements of the qubit over many repetitions. We use nitrogen vacancy centers in diamond as a model system and projective single-shot readout of the electron spin at low temperatures to simulate these effects. The employed measurement control could be used for purification of spin ensembles to desired quantum states.

Nonlocal advantage of quantum coherence (NAQC) based on coherence complementarity relations is generally viewed as a stronger nonclassical correlation than Bell nonlocality. An arbitrary two-qubit state with NAQC must be an entangled state, which demonstrates that the criterion of NAQC can also be regarded as an entanglement witness. In this paper, we experimentally investigate the NAQC for Bell-diagonal states with high fidelity in an optics-based platform. We perform local measurements on a subsystem in three mutually unbiased bases and reconstruct the density matrices of the measured states by a quantum state tomography process. By analyzing characteristics of the $l_1$ norm, relative entropy, and skew information of coherence with parameters of quantum states, NAQC for the quantum states is accurately captured, and it shows that our experimental results are well compatible with the theoretical predictions. Our results have shown that NAQC could be a new kind of entanglement witness and a useful physical resource for quantum information processing.

The positivity of the partial transpose is in general only a necessary condition for separability. There exist quantum states that are not separable, but nevertheless are positive under the partial transpose. States of this type are known as bound entangled states, meaning that these states are entangled but they do not allow distillation of pure entanglement by means of local operations and classical communication. We present a parametrization of a class of $2\times 2$ bound entangled Gaussian states for bipartite continuous-variable quantum systems with two modes on each side. We propose an experimental protocol for preparing a particular bound entangled state in quantum optics. We then discuss the robustness properties of this protocol with respect to the occupation number of thermal inputs and the degrees of squeezing.

We have considered the Raman scattering in molecular media. Applying two laser fields in a two-photon resonance with vibrational transition, we have studied the role of rotational levels for excitation of vibrational coherence. It is shown that the molecular vibrational coherence strongly depends on the effect of coherent population trapping for rotational levels. The obtained results are important for applications of Raman spectroscopy to molecular detection in engineering, chemical, and biological applications.

The flat-top optical filter is an important component in practical optical applications, but it is very challenging for experimental realization in the integrated photonic platform. Here, we propose a robust and high-efficiency optical filter with wide-band flat top, based on the adiabatic conversion of light in coupled waveguides with a chirped modulation of the gap. The bandwidth of the filter can be $110\mathrm{nm}$ for telecom wavelengths, but owing to the adiabatic criteria, the length of the device is about $20\mathrm{mm}$. To narrow the bandwidth and reduce the footprint of the device simultaneously, an additional apodization modulation of the geometry is introduced, and a $6\text{−}\mathrm{nm}$ bandwidth can be realized with a device length of $4\mathrm{mm}$. The influences of structure parameters on the device performance are also analyzed, and the results show that the device is robust to the variation of the structure parameters. The proposed integrated bandpass filter is feasible for experiments, and thus it would be useful in both classical and quantum information processing on photonic chips.

A chirped parametrically driven discrete nonlinear Schrödinger equation is discussed. It is shown that the system allows two resonant excitation mechanisms, i.e., successive two-level transitions (ladder climbing) or a continuous classical-like nonlinear phase locking (autoresonance). Two-level arguments are used to study the ladder-climbing process, and semiclassical theory describes the autoresonance effect. The regimes of efficient excitation in the problem are identified and characterized in terms of three dimensionless parameters describing the driving strength, the dispersion nonlinearity, and the Kerr-type nonlinearity, respectively. The nonlinearity alters the borderlines between the regimes and their characteristics.

In this work, we study the dynamics of a two-level system (qubit) embedded in a finite-temperature structured bath under periodical nondemolition measurements. The system under measurements will reach a quasisteady state, whose effective temperature can be maintained lower than that of the surrounding environment. We study the influence of the environmental modes from different regimes of frequency on the qubit. The spectrum of the bath consisting of a large number of bosonic harmonic oscillators can be approximately divided into three parts according to their effects of cooling or heating. Due to the spectral analysis over the structured bath based on a time-convolutionless master equation beyond the rotating-wave approximation, we propose a necessary cooling condition for the bath in the context of quantum nonselective and nondemolition measurements. It consists of two parts: (i) the logarithmic derivative of the spectrum around the system transition frequency should be large enough, at least larger than one-half of the inverse temperature of the bath; (ii) the spectrum should have a sharp high-frequency cutoff that cannot be far detuning from the system transition frequency. From this condition, we find that environments with two popular types of spectra, i.e., the modified Lorentzian models and the super-Ohmic models, are available for cooling the open quantum system.

The study of ontology (hidden variables) provides for a vital ground on which significant nonclassical features of quantum theory are revealed. One such nonclassical ontic feature is preparation contextuality (PC) and advantage in oblivious communication tasks is one of its operational signatures. This article primarily pursues the ontic feature underlying quantum advantage in communication complexity (CC). We construct oblivious communication tasks tailored to given CC problems. We upper bound the classical success probability of these oblivious communication tasks, obtaining preparation noncontextual inequalities. We use the very states and measurements responsible for advantage in CC problems along with the orthogonal mixtures of these states to orchestrate an advantageous protocol for the associated oblivious communication tasks and the violation of the associated inequalities, thereby unveiling PC. To showcase the vitality of our results, we find a criterion for unbounded violation of these inequalities and demonstrate the same for two widely studied CC problems.

The Clifford hierarchy of unitary operators is a foundational concept for universal quantum computation. It was introduced to show that universal quantum computation can be realized via quantum teleportation, given access to certain standard resources. While the full structure of the hierarchy is still not understood, Cui et al. [S. X. Cui et al., Phys. Rev. A 95, 012329 (2017)] recently described the structure of diagonal unitaries in the hierarchy. They considered diagonal unitaries whose action on a computational basis qudit state is described by a $2^k\mathrm{th}$ root of unity raised to some polynomial function of the state, and they established the level of such unitaries in the hierarchy as a function of $k$ and the degree of the polynomial. For qubit systems, we consider $k\mathrm{th}$-level diagonal unitaries that can be described just by quadratic forms of the state over the ring ${\mathbb{Z}}_{2^k}$ of integers modulo $2^k$. The quadratic forms involve symmetric matrices over ${\mathbb{Z}}_{2^k}$ that can be used to efficiently describe all two-local and certain higher locality diagonal gates in the hierarchy. We also provide explicit algebraic descriptions of their action on Pauli matrices, which establishes a natural recursion to diagonal unitaries from lower levels. The result involves symplectic matrices over ${\mathbb{Z}}_{2^k}$ and hence our perspective unifies a subgroup of diagonal gates in the Clifford hierarchy with the binary symplectic framework for gates in the Clifford group. We augment our description with simple examples for certain standard gates. In addition to demonstrating structure, these formulas might prove useful in applications such as (i) classical simulation of quantum circuits, especially via the stabilizer rank approach, (ii) synthesis of logical non-Clifford unitaries, specifically alternatives to expensive magic state distillation, and (iii) decomposition of arbitrary unitaries beyond the $\mathrm{Clifford}+T$ set of gates, perhaps leading to shorter depth circuits. Our results suggest that some nondiagonal gates in the hierarchy might also be understood by generalizing other binary symplectic matrices to integer rings.

We assess the quality of a source of allegedly pure two-qubit states using both standard tomography and methods inspired by device-independent self-testing. Even when the detection and locality loopholes are open, the latter methods can dispense with modeling of the system and the measurements. However, due to finite sample fluctuations, the estimated probability distribution usually does not satisfy the no-signaling conditions exactly. We implement data analysis that is robust against these fluctuations. We demonstrate a high ratio $f_s/f_t\approx 0.988$ between the fidelity estimated from self-testing and that estimated from full tomography, proving the high performance of self-testing methods.

Among the various versions of the twin-field quantum key distribution (TF-QKD) protocol [M. Lucamarini, Z. Yuan, J. Dynes, and A. Shields, Nature (London) 557, 400 (2018)] that can overcome the rate-distance limit, the TF-QKD without phase postselection proposed by Cui et al. [Phys. Rev. Appl. 11, 034053 (2019)] is an elegant TF-QKD that can provide high key rates since the postselection of global phases has been removed. However, the achievable distance of this variant is shorter than that of the original phase-matching QKD [X. Ma, P. Zeng, and H. Zhou, Phys. Rev. X 8, 031043 (2018)]. In this paper, we propose a method for improving its performance by introducing an additional decoy mode. The upper bound of the information leakage can be more tightly estimated; hence, both the key rate and the achievable distance are significantly improved. Interestingly, the operation of the proposed additional decoy mode is the same as that of the code mode; hence, it does not introduce difficulties into the experimental system. In addition, the improvement is substantial with finite decoy states, which is meaningful in practice.

We employ a technique that combines the configuration interaction method with the singles-doubles coupled-cluster method to perform calculation of the energy levels, transition amplitudes, lifetimes, $g$ factors, and magnetic dipole and electric quadrupole hyperfine structure constants for many low-lying states of neutral actinium. We find very good agreement with existing experimental energy levels and make accurate predictions for missing levels. It has been noted that some of the levels were previously misidentified; our analysis supports this claim. If spectroscopy is performed with ${}^{225}\mathrm{Ac}$, our calculations will lead to values for nuclear structure constants. The accuracy of this can be constrained by comparing with ${}^{227}\mathrm{Ac}$.

We report on a measurement of the $CP$-violating permanent electric dipole moment (EDM) of the neutral ${}^{129}\mathrm{Xe}$ atom. Our experimental approach is based on the detection of the free precession of co-located nuclear spin-polarized ${}^3\mathrm{He}$ and ${}^{129}\mathrm{Xe}$ samples. The EDM measurement sensitivity benefits strongly from long spin coherence times of several hours achieved in diluted gases and homogeneous weak magnetic fields of about 400 nT. A finite EDM is indicated by a change in the precession frequency, as an electric field is periodically reversed with respect to the magnetic guiding field. Our result $(-4.7\pm 6.4)\times {10}^{-28}e\mathrm{cm}$, is consistent with zero and is used to place a new upper limit on the ${}^{129}\mathrm{Xe}$ EDM: $|d_{\text{Xe}}|<1.5\times {10}^{-27}e\mathrm{cm}$ (95% C.L.). We also discuss the implications of this result for various $CP$-violating observables as they relate to theories of physics beyond the standard model.

In this work we present a combined experimental and theoretical study of $K$-shell ionization by electrons with energies close to the threshold. The ionization cross sections of the $K$ shells of Te, Ta, and Bi atoms have been measured up to 100 keV with uncertainties ranging from 4% to 8%. In turn, calculations have been done using the subconfiguration average distorted-wave (SCADW) method, which includes the full two-body retarded electromagnetic interaction between the projectile and target electrons. The predictions of the SCADW method are in good agreement with the experimental data. In contrast, theoretical cross sections based on first-order perturbation theory where the transverse interaction is computed with plane waves instead of distorted waves underestimate the SCADW values as well as the experimental data. The difference between the two investigated ab initio formalisms grows with atomic number, being 3% for Te, 15% for Ta, and 25% for Bi. An additional comparison of both theoretical approaches with recent measurements for Au $K$ supports the conclusion that the SCADW method reproduces well the experimental $K$-shell ionization cross section of atoms with intermediate to large $Z$ near the threshold.

We revisit the excitation-tunneling process in nonsequential double ionization (NSDI) of helium in an 800-nm laser field. The correlated two-electron momentum distributions are calculated by using the improved quantitative rescattering (QRS) model, in which the lowering of the threshold energy due to the presence of an electric field at the instant of recollision is taken into account. In the framework of the QRS model, the correlated two-electron momentum distributions for excitation-tunneling in NSDI can be factorized as a product of the returning-electron wave packet (RWP) and the field-free differential cross section (DCS) for electron impact excitation of the parent ion multiplied by the tunneling ionization rate of electrons in the excited states. The RWPs, which describe the momentum distribution of the returning electrons, are obtained within the strong-field approximation for high-order above-threshold ionization. The DCSs for electron impact excitation of ${\mathrm{He}}^{+}$ are calculated using the state-of-the-art many-electron $R$-matrix theory, and the tunneling ionization rates for electrons in the excited states are evaluated by solving the time-dependent Schrödinger equation. The calculated correlated two-electron momentum distribution shows that the fourfold symmetry with regard to the parallel momentum components is broken. This is in contradiction to the prevalent view that the correlation pattern for excitation-tunneling in NSDI is symmetric with respect to the coordinate axes. By including the recollisional ($e,2e$) process, the predicted correlated two-electron momentum distributions are found to be in good qualitative agreement with experiment.

We consider a weakly interacting Bose-Einstein condensate that rotates in either a harmonic, or a weakly anharmonic trapping potential. Performing numerical calculations, we investigate the behavior of the gas in these two cases as the angular momentum, or equivalently as the rotational frequency of the trap increases. While in the case of a purely harmonic potential the gas makes a transition from the mean-field regime to the correlated Laughlin regime, in the case of anharmonic confinement the mean-field approximation remains always valid. We compare our derived results in these two cases, using both the mean-field approximation, as well as the diagonalization of the many-body Hamiltonian considering a small atom number.

A ring of dipole-coupled quantum emitters positioned at subwavelength distances possesses only a few radiant but many subradiant collective excitations with lifetimes growing exponentially with the atom number. These exhibit a three-dimensionally confined spatial radiation field pattern and form nanoscale high-$Q$ optical resonators. Tailoring size, orientation, and distance between two rings allows for increasing the ratio of coherent ring-to-ring energy transfer versus free-space emission by orders of magnitude. In particular, the more dark and delocalized over several sites are excitons, the higher is their fidelity for transport to a second ring.

We propose a scheme to simultaneously cool two mechanical oscillators in a dissipatively coupled optomechanical system. Calculating the mean phonon numbers of the two mechanical oscillators in steady state, we find that there are two cooling regions. Both mechanical oscillators can be cooled efficiently in the two cooling regions. We also find that when the coupling strength between the two mechanical oscillators is large, the second mechanical oscillator can be cooled to a temperature lower than the first mechanical oscillator. This feature can be understood as the entanglement between two phononic supermodes of the two mechanical oscillators. When the coupling strength is small, the second mechanical oscillator can also be cooled, but cannot reach a temperature lower than the first mechanical oscillator, as the thermal fluctuations of reservoir cannot be ignored. The quantum cooling limits of the two mechanical oscillators are discussed for both the strong and weak couplings between the two mechanical oscillators.