We propose a tangible experimental scheme for demonstrating quantum entanglement between swift electrons and light, relying on coherent cathodoluminescence for photon generation in a transmission electron microscope, and a quantum eraser setup for formation and verification of entanglement. The entanglement of free electrons with light is key to developing free-electron quantum optics and its potential applications such as quantum sensing, novel photonic and electron state generation, and entanglement between free electrons.

Dynamic circuit operations (measurements with feedforward) are important components for future quantum computing efforts, but lag behind gates in the availability of characterization methods. Here we introduce a series of dynamic circuit benchmarking routines based on interleaving dynamic circuit operation blocks $\mathcal{F}$ in one-qubit randomized benchmarking sequences of data qubits. $\mathcal{F}$ spans between the set of data qubits and a measurement qubit and may include feedforward operations based on the measurement. We identify six candidate operation blocks, such as preparing the measured qubit in $|0\rangle$ and performing a $Z$-Pauli on the data qubit conditioned on a measurement of “1”. Importantly, these blocks provide a methodology to accumulate readout assignment errors in a long circuit sequence. We also show the importance of dynamical decoupling in reducing $ZZ$ crosstalk and measurement-induced phase errors during dynamic circuit blocks. When measured on an IBM Eagle device with appropriate dynamical decoupling, the results are consistent with measurement assignment error and the decoherence of the data qubit as the leading error sources.

We study the fragmentation dynamics of CO molecules in slow- (0.16 a.u. ${\mathrm{Xe}}^{4+}$) and intermediate- (1.41 a.u. ${\mathrm{He}}^{2+}$) velocity electron capture collisions. The kinetic energy release distributions of the ${\mathrm{C}}^{+}+{\mathrm{O}}^{+}$ fragmentation channel show the variation of energy depositions during the collisions for the associated capture paths. We have also observed molecular-axis orientation-dependent variations in the angle-differential cross sections for the double-electron capture and transfer ionization processes. A simple mathematical model based on Young's optical double-slit experiment has been invoked. The variations are attributed to the molecular double-slit-type interference phenomena arising from the interference between the two outgoing de Broglie waves formed by electron capture. The asymmetry in the interference pattern due to the scattering phase gives insight into the role of the quantum phenomena in characterizing such collision dynamics. Our study thus provides general evidence of the wave-particle duality in low to highly perturbative ion-molecule collisions.

We develop an approach for efficient and scalable simulations of measurement and control of quantum systems built upon existing phase-space methods, namely, the truncated Wigner approximation.. We benchmark against existing particle-filter methods by simulating measurement-based feedback cooling in a two-mode system, whose low-dimensional nature permits a computation of an exact solution. The advantage of our method is multimode scalability, which we demonstrate through the successful simulation of measurement-based feedback cooling of an incoherent quasi-one-dimensional thermal ensemble to quantum degeneracy. As the underlying principle of our approach exploits a general correspondence between measurement and coherent feedback, we anticipate it is also applicable across a broad range of other quantum control scenarios.

Quantum correlations play a prominent role in numerous applications, from quantum communications and networking to enhanced sensing. I propose using optical soliton splitting and inelastic soliton collision to generate pulse pairs with highly correlated photon number fluctuations. The fluctuations are studied using the linearized quantum fluctuation theory of vector solitons. I show that the interpulse correlations arise in the process of inelastic soliton interactions initiated by the polarization mode dispersion and modulation of the fiber dispersion. The inelastic soliton interactions produce pulses with highly correlated quantum fluctuations in the orthogonal polarization components of the field. In the time domain, the pulses have a four-lobe correlation pattern. In the frequency domain, the correlation functions acquire a “checkerboard” structure with alternating positive and negative regions. Correlations between the separated pulses arise only with appropriate pulse width, group velocity dispersion, and polarization mode dispersion. The generation of pulses with high intrapulse correlations offers the possibility of producing entangled sources for continuous-variable quantum communication and computation.

We investigate the reduction of measurement-added noise in force sensing by analyzing its power spectral density (PSD) within a hybrid optomechanical system. The setup is comprised of an optomechanical cavity equipped with a movable mirror which acts as the mechanical oscillator, a stationary semi-transparent mirror, a superconducting qubit, and an optical parametric amplifier (OPA). By utilizing the concept of coherent quantum noise cancellation (CQNC), we derive the conditions necessary for complete cancellation of back-action force, thereby enhancing force sensitivity. Furthermore, with the gradual increase in the OPA pump gains, we suppress the sensitivity beyond the standard quantum limit (SQL) at a lower value of laser power. The removal of back-action noise, along with the reduction of shot noise, improves force detection capabilities, thereby surpassing the standard quantum limit associated with weak force detection.

We investigate dark solitons and their interactions in one-dimensional self-defocusing media with fourth-order diffraction (FOD). Anomalous FOD supports bifurcated dark solitons when the coefficient of FOD exceeds a threshold. Dark solitons are stable (unstable) against perturbations when FOD is normal (anomalous). FOD also controls flexibly interactions of well-separated parallel dark solitons, leading to the formation of bound states of parallel solitons pairs which are always repulsive in conventional local nonlinear media with standard (quadratic) diffraction.

The Purcell effect describes the enhancement of the spontaneous emission rate of an emitter near a resonant structure. However, evaluating the Purcell factor quantitatively and empirically is difficult due to the difficulties in measuring the electromagnetic near field of an optical resonance for calculation of the exact effective modal volume, especially with non-Hermitian resonators. Therefore, we propose an analytical approach to circumvent the need to measure the near field and predict the Purcell enhancement with spectral parameters, which can be directly measured in the far field or fitted from such spectral measurements. Our proposed model predicts the averaged Purcell enhancement by metasurfaces on a photoluminescent medium, and is verified with experimental measurements and numerical simulations of nanoparticle arrays coupled to a fluorescent thin film. The model directly analyzes the photoluminescence enhancement and extraction efficiency of the metasurface, and can be generalized to work with an arbitrarily shaped photoluminescent medium that is coupled to a resonator. This discovery provides a practical and accessible way to understand the underlying mechanisms of photoluminescence enhancement and will facilitate optimization of metasurfaces for efficient extraction of the enhanced luminescence.

Tripartite entanglement as a remarkable resource in quantum information science has been extensively investigated in hybrid quantum systems, whereas it is generally weak and prone to be suppressed by noise, restricting its practical application in quantum technologies. Here, we propose how to enhance the tripartite entanglement among magnons, photons, and phonons in a hybrid cavity-magnon optomechanical system by exploiting a synergistic effect of the optical parametric amplification (OPA) and mechanical parametric amplification (MPA). We find that in the case of individually applying the OPA or MPA, the tripartite entanglement can be enhanced by several folds. Remarkably, upon suitably tuning the phase matching of the two parametric fields presented simultaneously, the strengths of the entanglement can be further enhanced due to the constructive interference between the OPA and MPA. We also show that our method can improve the robustness of the entanglement against thermal noise. Our work provides a promising method for manipulating the entanglement with easy tunability and may serve as a useful tool for the enhancement and protection of fragile quantum resources.

The activation of Bell nonlocality is a protocol that enables the violation of a Bell inequality from a system that initially did not allow for any such violation because the state of the system was Bell-local. This activation of hidden Bell nonlocality has been demonstrated in experiments; however, while the certification of Bell nonlocality is conceptional straightforward, a statistically rigorous verification that the initial state is Bell-local has not yet been achieved. This is due to two key obstacles: The lack of a method to establish a suitable confidence region from the measured data and the need for an efficient technique to verify Bell locality of all states within the confidence region. In this work, we address both challenges. We introduce a confidence polytope in the form of a hyperoctahedron and provide a computationally efficient method to verify whether a quantum state admits a local hidden state model, thus being unsteerable and, consequently, Bell-local. Using these methods, we find that a statistically rigorous certification of hidden Bell nonlocality needs of the order of ${10}^9$ samples.

Recently, distributed sensing has been extensively studied using squeezed states. While this is an excellent development, it is desirable to investigate the use of other quantum probes, such as entangled states of light. In this paper, we focus on distributed sensing, i.e., estimating multiple unknown phases at different spatial nodes using multiphoton polarization-entangled Greenberger-Horne-Zeilinger states distributed across different nodes. We utilize tools of quantum metrology and calculate the quantum Fisher information matrix (QFIM). However, the QFIM turns out to be singular, hindering the determination of quantum Cramér-Rao bounds for the parameters of interest. Recent experiments have contended with a weaker form of the Cramér-Rao bound, which does not require the inversion of the QFIM. It is desirable to understand how relevant these weaker bounds are and how closely they approach the exact Cramér-Rao bounds. We thus analyze the reason for this singularity and, by removing a redundant phase, obtain a nonsingular QFIM, allowing us to derive exact quantum Cramér-Rao bounds. Using the nonsingular QFIM, we show that the arithmetic average of the distributed phases is Heisenberg limited. We demonstrate that quantum metrological bounds can be saturated by projective measurements, enabling us to determine the Fisher information matrix, which is also singular. We then show how this singularity can be resolved.

The detection and estimation of quantum entanglement are the essential issues in the theory of quantum entanglement. We construct matrices based on the realignment of density matrices and the vectorization of the reduced density matrices, from which a family of separability criteria are presented for both bipartite and multipartite systems. Moreover, tighter lower bounds of concurrence and convex-roof extended negativity are derived. Criteria are also given to detect the genuine tripartite entanglement. Lower bounds of the concurrence of genuine tripartite entanglement are presented. By detailed examples we show that our results are better than the corresponding ones in identifying and estimating quantum entanglement as well as genuine multipartite entanglement.

The optimal rate at which information can be sent through a quantum channel when the transmitted signal must simultaneously carry some minimum amount of energy is characterized. To do so, we introduce the quantum-classical analog of the capacity-power function, and we generalize results in classical information theory for transmitting classical information through noisy channels. We show that the capacity-power function for a classical-quantum channel, for both unassisted and private protocol, is concave, and we also prove additivity for unentangled and uncorrelated ensembles of input signals for such channels. This implies that we do not need regularized formulas for calculation. We show that these properties also hold for all noiseless channels when we restrict the set of input states to be pure quantum states. For general channels, we geometrically prove that the capacity-power function is piecewise concave, with further support from numerical simulations. Further, we connect channel capacity to properties of random quantum states. In particular, we obtain analytical expressions for the capacity-power function for the case of noiseless channels using properties of random quantum states under an energy constraint and concentration phenomena in large Hilbert spaces.

Present-day state-of-the-art ab initio many-body calculations on $f$-block-containing cold molecules rely heavily on perturbative approaches for spin-orbit coupling and exclude a substantial part of the atomic transitions in the $f$ and $d$ shells. Here, we demonstrate the importance of proper relativistic treatment of the $f$ and $d$ shells in Yb-containing diatomics and the inclusion of the $f\to d$ transition to obtain accurate spectra of ${\mathrm{YbLi}}^{+}$. We focus on state-of-the-art relativistic many-body calculations for the Yb atom's ground and excited states and the ${\mathrm{YbLi}}^{+}$ potential energy surface. For that purpose, we exploit various quantum many-body methods, namely, a spin-free and four-component implementation of the coupled-cluster singles and doubles (CCSD) model and its equation-of-motion (EOM) extensions, spin-free complete-active-space self-consistent field, and internally contracted multireference (MR) configuration interaction approaches. We oppose scalar relativistic calculations to four-component variants to support the reliability of our EOM-CCSD study and shed new light on the interplay between these systems' spin-orbit coupling and the proper treatment of relativistic effects. Finally, we provide new reference potential-energy curves for ground and excited states. Additionally, we compute spectroscopic constants and analyze the Einstein coefficients.

We explore effective approaches for describing the dynamics of induced interactions among two noninteracting distinguishable impurities (Bose polarons) when their couplings to the host species are switched on. First, we evaluate the time-dependent characteristics of each polaron through reduced single-particle models. Their validity is ultimately judged by comparing them to observables obtained within a many-body variational approach. We argue that utilizing a time-dependent optimization process with the effective mass and frequency being fitting parameters leads to an accurate matching on the level of one-body observables with the prediction of the many-body method. Next, we construct two-body effective models, which, in addition to the effective parameters of the single polaron, include a contact interaction potential modeling the induced interactions. It is shown that the instantaneous coupling strength obtained with a time-dependent optimization process allows for an accurate description of the impurities two-body correlation dynamics when compared to the many-body calculations. In this sense, it is possible to describe the dynamical crossover from correlated to anticorrelated impurity patterns related to the transition from attractive to repulsive induced interactions. Our results should facilitate the description of strongly particle-imbalanced mixtures via reduced models and hence their experimental understanding.

Bosonic two-mode squeezed states are paradigmatic entangled states in continuous variable systems, which have broad applications in quantum information processing. In this work, we propose a photon-phonon squeezing protocol assisted by a Kerr magnon within a hybrid cavity magnomechanical system. We construct an effective Hamiltonian that accounts for photon-phonon squeezing through strong photon-magnon interaction and modulation over the driving on the photon mode. The effective Hamiltonian can be confirmed by the diagonalization of the system's Liouvillian superoperator. With the effective Hamiltonian and quantum Langevin equation, we provide a rigorous theoretical solution for the dynamical process of squeezing generation. Our finding indicates that the asymptotic stationary squeezing can be obtained by optimizing the squeezing quadrature operator, even when the covariance matrix of the system still varies with time. This squeezing level can exceed the maximum value under stable conditions. Moreover, our analysis reveals that a proper Kerr nonlinearity of the magnon can further promote the squeezing generation. Our work provides an extendable framework for generating squeezed states of two Gaussian modes with indirect coupling.

Quantum work capacitances and maximal asymptotic work/energy ratios are figures of merit characterizing the robustness against noise of work extraction processes in quantum batteries formed by collections of quantum systems. In this paper we establish a direct connection between these functionals and, exploiting this result, we analyze different types of noise models mimicking self-discharging (amplitude damping channels), thermalization (generalized amplitude damping channels), and dephasing effects (simultaneous phase-amplitude damping channels). In this context we show that input quantum coherence can significantly improve the storage performance of noisy quantum batteries and that the maximum output ergotropy is not always achieved by the maximum available input energy.

We investigate the interaction of a transmon qubit with a classical gravitational field. Exploiting the generic phenomena of the gravitational redshift and Aharonov-Bohm phase, we show that entangled quantum states dephase with a universal rate. The gravitational phase shift is expressed in terms of a quantum computing noise channel. We give a measurement protocol based on a modified phase estimation algorithm which is linear in the phase drift, which is optimal for measuring the small phase that is acquired from the gravitation channel. Additionally, we propose qubit-based platforms as quantum sensors for precision gravitometers and mechanical strain gauges as an example of this phenomenon's utility. We estimate a sensitivity for measuring the local gravitational acceleration to be $\delta g/g\sim {10}^{-7}$. This paper demonstrates that classical gravitation has a nontrivial influence on quantum computing hardware and provides an illustration of how quantum computing hardware may be utilized for purposes other than computation. While we focus on superconducting qubits, we point to the universal nature of gravitational phase effects for all quantum platforms.