We propose a scheme to enhance quantum entanglement in an optomechanical system by exploiting the so-called Duffing nonlinearity. Our model system consists of two mechanically coupled mechanical resonators, both driven by an optical field. One resonator supports Duffing nonlinearity, while the other does not. The resonators are coupled to each other via the so-called phonon-hopping mechanism. The hopping rate is $\theta$-phase dependent that induces exceptional point singularities in the system. Interestingly, while the resonator with Duffing nonlinearity exhibits vanishing entanglement with light, we observe an increase in entanglement between light and the other mechanical resonator. This enhanced entanglement persists longer against thermal fluctuations compared to the one without the nonlinearity. Additionally, this entanglement features a sudden death and revival phenomenon, where the peaks happen at multiples of $\theta =\frac{\pi }{2}$. This work opens another avenue for exploiting nonlinear resources to generate strong quantum entanglement, paving the way for advancements in quantum information processing, quantum sensing, and quantum computing within complex systems.

In this paper, we address the use of a calcite crystal-based local detector for the discrimination of the orbital angular momentum of quantum radiation produced by parametric down-conversion, using only a portion of the beam. Specifically, we propose and experimentally demonstrate that discrimination can be achieved by exploiting the introduction of a precise and controlled spatial shift between two replicas of the state within the crystals. This approach utilizes a robust and intrinsically stable monolithic configuration, obviating the need for feedback mechanisms or thermal drift compensation. Our method offers a promising avenue for enhancing the reliability and efficiency of quantum communication systems, and we believe that this technology could significantly advance the development of quantum communication techniques, where information encoding is based on orbital angular momentum, or spatially distributed orbital angular momentum detection.

An accessible tabletop source for the production of entangled x rays is crucial for the field of high-energy quantum optics. Here, we present a detailed analysis of the entanglement and polarization of the two photons emitted by an electron in an intense laser wave (nonlinear double-Compton scattering), by working within the framework of strong-field QED. By identifying contributions to the emission probability stemming from the electron being either on-shell or off-shell between the two-photon emissions, we show that the entangled photons are generated via the off-shell contribution for moderate beam energies, which are realizable on a tabletop setup. We propose an experiment to produce and isolate pairs of entangled x rays, through spectral filtering.

We show how to implement a tunable beam splitter and Hong-Ou-Mandel interference in the Su-Schrieffer-Heeger chain by manipulating the topological edge states adiabatically. The boson initially injected in the single end of the chain can be transferred to the double end with a tunable proportion that depends on the dynamical phases accumulated during the adiabatic evolution. We also observe Hong-Ou-Mandel interference via the tunable (50:50) beam splitter and achieve a spatially entangled two-particle NOON state. We demonstrate the robustness of our proposal under chiral- and time-reversal-symmetry-preserving disorder. However, chiral symmetry is scarce for realist systems. Therefore, we demonstrate that Hong-Ou-Mandel interference is robust to inversion-symmetric disorder breaking the chiral symmetry, highlighting the protection of inversion symmetry. More importantly, the inversion symmetry violated by static disorder can be restored for more common situations where disorder becomes time dependent, giving rise to the Hong-Ou-Mandel interference protected by temporal-average-inversion symmetry. Our approach provides an alternative way to study quantum effects in topological matter with potential applications..

Compared to general quantum states, the sparse states arise more frequently in the field of quantum computation. In this work we consider the preparation for $n$-qubit sparse quantum states with $s$ nonzero amplitudes and propose two algorithms. The first algorithm uses $O(ns/{log}_{2}n+n)$ gates, improving upon previous methods by $O\left({log}_{2}n\right)$. Moreover, the classical runtime of this algorithm is optimal. We further establish a matching lower bound for any algorithm that is not amplitude aware and employs at most $poly\left(n\right)$ ancillary qubits. The second algorithm is tailored for binary strings that exhibit a short Hamiltonian path. An application is encoding the input data into a state with specified Hamming weight $k$ in quantum machine learning, for which our algorithm constructs a circuit of size $O(\left(\genfrac{}{}{0pt}{}{n}{k}\right){log}_{2}n)$. This surpasses previous results by $O(k/{log}_{2}n)$ and is close to the lower bound $O(\left(\genfrac{}{}{0pt}{}{n}{k}\right))$. The classical runtime is also nearly optimal. Both algorithms shrink the existing gap theoretically and provide increasing advantages numerically.

We propose a device consisting of $m$ Mach-Zehnder interferometers and realizing sorting of the first $2^m$ temporal Hermite-Gauss modes of light passing through it by adjusting the accumulated temporal Gouy phase acquired by every mode. This mode-order-dependent phase shift is achieved by a fractional Fourier transform realized by a time lens in one of the interferometer's arms. We consider application of such a sorter with just two interferometers to sorting the Schmidt modes of a photon pair generated in spontaneous parametric downconversion and find the theoretical lower bound on the cross-talk probability of 5.5%.

We present an alternative route to realize coherent perfect absorption (CPA) and subnatural narrowing by vacuum-induced coupling (VIC) between excited levels of an atomic ensemble in an optical nanofiber (ONF) cavity quantum electrodynamics (CQED) system. By employing the VIC arising from quantum interference between the spontaneous emission pathways from the upper excited doublet to one common ground state in the three-level $V$-type atoms, a very narrow absorption resonance can be observed in the generated dark-state spectral window owing to quantum destructive interference between the spontaneous emission pathways. The spectroscopic output feature exhibits a reduced linewidth, which is much narrower than both the empty-cavity linewidth and the atomic transition linewidth. As a check, the approximate analytical results for the ONF cavity-output power and linewidth of interest agree well with the numerical simulations. Furthermore, we find that the CPA can be used to probe the existence of the VIC effect. By tuning the relative phase between the two input light fields appropriately, the conversion between the CPA and coherent perfect transmission (CPT) can be realized in both the linear and nonlinear excitation regimes, i.e., the relative phase can switch the ONF CQED system from a perfect photon absorber to a nearly perfect photon transmitter. Also, we analyze the optical bistability of the ONF CQED system and its phase dependence in the nonlinear excitation regime. Such interference control of the CPA-to-CPT (or vice versa) scheme proposed here may have potential applications in optical logical devices for optical computing and quantum information processing.

The study of quantum nonlocal correlations in networks (QNCNs), beyond the standard Bell theory with a single source, involves simulating large-scale quantum communications. The detectability of QNCNs depends on the input sources and selected measurements, and more essentially on the network structures. In this paper, we prove that same-size acyclic networks with different independence degrees can generate distinct sets of QNCNs in the device-independent scenario. We then distinguish two acyclic networks with different independence degrees by comparing the QNCNs generated from them. Furthermore, we show quantum demonstrations of the aforementioned notions in two-qubit-source networks. We investigate the quantitative influence of different network topologies on the detection of QNCNs in acyclic networks with Werner states as sources. Finally, we study how to maintain high independence degrees in networks formed by connecting two networks. This makes the correlations on the newly formed quantum network more likely to be nonlocal. Our research provides an alternative perspective for the well-known graph classification problem, and establishes an intrinsic connection between the graph theory and the foundation of quantum mechanics.

Photodissociation is one of the most important photoinduced chemical reactions. It occurs when the potential energy curve along a chemical bond is repulsive in an excited state. Typically, “ballistic” ultrafast dissociation leads to the broadening of absorption resonances and the smearing out of vibrational fine-structure. We report on the photodissociation of ${\mathrm{H}}_2\mathrm{O}$ in the $\tilde{B} 2{}^{1}A_1$ electronic state, characterized by a $|3a_1^{-1}4a_1^1\rangle$ configuration, which can be reached via resonant inelastic x-ray scattering or direct ultraviolet absorption. In both cases the spectra show narrow vibrational resonances, in spite of the dissociative character of the state. We find that “delayed” dissociation pathways, caused by reflection of the nuclear wave packet, are responsible for this effect. In spite of the analogous topology of the potential energy surfaces of the core- and valence-excited states, the reflection of the wave packet takes place only in the latter. The two-dimensional wave packet of the O-H stretching coordinates becomes trapped in a “cavity” near the Franck-Condon region, resulting from a mismatch between the OH vibrational frequency in the cavity and the one at the dissociation limit.

We show how the directional collective response of atomic arrays to light can be exploited for the dissipative generation of entangled atomic states, relevant for, e.g., quantum metrology. We consider an atomic array illuminated by a paraxial beam of a squeezed-vacuum field and demonstrate that quantum-squeezing correlations are dissipatively transferred to the array atoms, resulting in an atomic spin-squeezed steady state. We find that the entanglement transfer efficiency and hence the degree of spin squeezing are determined by the resonant optical reflectivity of the array. Considering realistic cases of finite-size array and illuminating beam, we find how the spin-squeezing strength scales with system parameters, such as the number of layers in the array and its spatial overlap with the beam. We discuss applications in atomic clocks in both optical and microwave domains.

The ordered atomic array emerges as a versatile atom-photon interface for exquisite control of light-matter interaction at the nanoscale. In particular, the one-dimensional atom-waveguide system can achieve strong coupling between a single atom and a photon, where two atomic mirrors coupled to waveguide act as a high-finesse cavity. In this work we show that under the strong-coupling regime, the mirror atoms arranged in the form of a Su-Schrieffer-Heeger chain are advantageous in decoherence suppression compared to a nontopological Bragg configuration, manifesting a significant increase in the lifetime of dressed states, substantial narrowing of the spectral linewidth, and order-of-magnitude enhancement of the cooperativity of light-matter interaction. Furthermore, subradiant dressed states with a decay rate smaller than the emission rate of a free-space atom are found in the dissipationless atomic cavity with a few mirror atoms, which cannot be achieved with a Bragg atomic mirror. A theory of the local density of states (LDOS) of the atomic cavity is presented and reveals that the decoherence suppression is directly related to the negative LDOS in a wide frequency range contributed by the bulk modes of the topological atomic cavity. This peculiar feature of the LDOS indicates the destructive quantum interference of photon scattering by the topological edge- and bulk-state channels. Our work demonstrates the great potential of topology in engineering quantum states and enhancing quantum coherence for future applications in quantum computing and quantum information processing.

We investigate two key aspects of quantum systems by using the Tavis-Cummings dimer system as a platform. The first aspect involves unraveling the relationship between the phenomenon of self-trapping (or lack thereof) and integrability (or quantum chaos). Second, we uncover the possibility of mixed behavior in this quantum system using diagnostics based on random matrix theory and make an in-depth study of classical-quantum correspondence. The setup chosen for the study is precisely suited as it (i) enables a transition from delocalized to self-trapped states and (ii) has a well-defined classical limit, thereby amenable to studies involving classical-quantum conjectures. The obtained classical model in itself has rich chaotic and ergodic properties which were probed via maximal Lyapunov exponents. Furthermore, we present aspects of chaos in the corresponding open quantum system and make connections with non-Hermitian random matrix theory.

Noise affects the performance of quantum technologies, hence the importance of elaborating operative figures of merit that can capture its impact in exact terms. In quantum metrology, the introduction of the Fisher-information measurement noise susceptibility now allows one to quantify the robustness of measurement for single-parameter estimation. Here we extend this notion to the multiparameter quantum estimation scenario. We provide its mathematical definition in the form of a semidefinite program. Although a closed formula could not be found, we further derive an upper and a lower bound to the susceptibility. We then apply these techniques to two paradigmatic examples of multiparameter estimation: the joint estimation of phase and phase diffusion and the estimation of the different parameters describing the incoherent mixture of optical point sources. Our figure of merit provides clear indications on conditions allowing or hampering robustness of multiparameter measurements.

We use the spatial degree of freedom of light modes to construct optical analogs of generalized quantum coherent states for Hermite- and Laguerre–Gauss modes. Our optical analogs preserve the statistical properties of their quantum counterparts, encoded in their amplitude and phase distributions. We explore three basic symmetries that provide generalized displaced, rotated, and squeezed coherent states. Given the substantial interest in squeezed states for probing matter, we believe that the optical analogs introduced here have significant implications for optical sensing. Specifically, the single-particle nature of our spatial modes makes them robust candidates for sensing photosensitive materials. Overall, our approach opens the door to optical metrology and sensing protocols that mimic those already existing in the quantum realm, and facilitates further exploration of the quantum state zoo through classical optical analogs.

Photon pairs generated from spontaneous parametric down-conversion are a well-established method to realize entangled bipartite photonic systems. Laguerre-Gaussian modes, which carry orbital angular momentum (OAM), are commonly exploited to engineer high-dimensional entangled quantum states. For Hilbert spaces with dimension $d>2$, maximally entangled states (MESs) help to improve the capacity and security of quantum communication protocols, among several other promising features. However, the direct generation of MESs in well-defined high-dimensional subspaces of the infinite OAM basis has remained a challenge. Here, we formalize how the spatial distribution of the pump beam and the nonlinear profile of the crystal can be simultaneously utilized to generate MESs without additional spatial filtering of OAM modes within a subspace. We illustrate our approach with maximally entangled qutrits $(d=3)$ and ququints $(d=5)$.

We introduce a hybrid quantum-classical algorithm to compute the Green function for strongly correlated electrons on noisy intermediate-scale quantum (NISQ) devices. The technique consists in the construction of a nonorthogonal excitation basis composed of a set of single-particle excitations generated by occupation-number operators. The excited sectors of the Hamiltonian in this basis can then be measured on the quantum device and a classical postprocessing procedure yields the Green function in the Lehmann representation. The technique allows for noise filtering, a useful feature for NISQ devices. To validate the approach, we carry out a set of proof-of-principle calculations on the single-band Hubbard model on IBM quantum hardware. For a two-site system we find good agreement between the results of quantum simulations and the exact result for the local spectral function. This proof of principle also shows that the noise filtering provides a reliable way to get rid of satellite peaks present in the spectral weight obtained from a NISQ device. A simulation of a four-site system carried out on classical hardware suggests that the approach can achieve similar accuracy for larger systems.