In relativity, there is no absolute notion of simultaneity because two clocks that are in different places can always be desynchronized by a Lorentz boost. Here, we explore the implications of this effect for the quantum theory of unstable particles. We show that when a wave function is boosted, its tails travel one to the past and the other to the future. As a consequence, in the new frame of reference, the particle is in a quantum superposition decayed $+$ nondecayed, where the property decayedness is entangled with the position. Since a particle cannot be localized in a region smaller than the Compton wavelength, there is a nonzero lower bound on this effect, which is fundamental in nature. The surprising implication is that, in a quantum world, decay probabilities can never be Lorentz invariant. We show that this insight was the missing ingredient to reconcile the seemingly conflicting views about time dilation in relativistic quantum mechanics and quantum field theory.

Nonrelativistic particles that are effectively confined to two dimensions can, in general, move on curved surfaces, allowing dynamical phenomena beyond what can be described with scalar potentials or even vector gauge fields. Here we consider a simple case of piecewise uniform curvature: a particle moves on a plane with a spherical extrusion. Depending on the latitude at which the sphere joins the plane, the extrusion can range from an infinitesimal bump to a nearly full sphere that just touches the plane. Free classical motion on this surface of piecewise uniform curvature follows geodesics that are independent of velocity, while quantum mechanical scattering depends on energy. We compare classical, semiclassical, and fully quantum problems, which are all exactly solvable, and show how semiclassical analysis explains the complex quantum differential cross section in terms of interference between two classical trajectories: the sphere on a plane acts as a kind of double slit.

In this paper, we present an experimentally feasible protocol to generate the cat states in the microwave resonator coupled to a superconducting qubit. The setup employs a detuned, time-dependent parametric drive to squeeze the resonator mode so that an adjustable qubit-resonator coupling strength can be obtained. Therefore, based on the transitionless tracking algorithm, we can design control pulses to generate the qubit-resonator entangled states with high fidelity in the laboratory frame. Then, the even and odd cat states can be further obtained by performing measurement on the superconducting qubit. Compared to the scheme [Chen et al., Phys. Rev. Lett. 126, 023602 (2021)], the present protocol is realized in the regime of weak parametric drive. In the case, squeezing-induced noise can be reduced so that the fidelity of the generated state can be improved. Numerical simulations indicate that the present protocol is well executed under experimentally available parameters. Thus, the protocol is feasible with the present state of the art in microwave superconducting circuits.

Kernel methods are a cornerstone of classical machine learning. The idea of using quantum computers to compute kernels has recently attracted attention. Quantum embedding kernels (QEKs), constructed by embedding data into the Hilbert space of a quantum computer, are a particular quantum kernel technique that is particularly suitable for noisy intermediate-scale quantum devices. Unfortunately, kernel methods face three major problems: Constructing the kernel matrix has quadratic computational complexity in the number of training samples, choosing the right kernel function is nontrivial, and the effects of noise are unknown. In this work, we addressed the latter two. In particular, we introduced the notion of trainable QEKs, based on the idea of classical model optimization methods. To train the parameters of the QEK, we proposed the use of kernel-target alignment. We verified the feasibility of this method, and showed that for our experimental setup we could reduce the training error significantly. Furthermore, we investigated the effects of device and finite sampling noise, and we evaluated various mitigation techniques numerically on classical hardware. We took the best performing strategy and evaluated it on data from a real quantum processing unit. We found that using this mitigation strategy demonstrated an increased kernel matrix quality.

We design a Bell inequality that is violated by correlations obtained from the ground states of an $XXZ$ spin-1 chain with on-site anisotropies at the region of phase transition. In order to detect such correlations in spin-1 systems we exploit the formalism of the generalized Bell inequality via the use of multipartite and high-order correlations. We observe a sharp violation in the vicinity of the quantum phase transition between the so-called large-$D$ and AFM phase. Interestingly, the violation of our Bell inequality is manifested by the change in the $XXZ$ spin-1 chain ground state to a Greenberger-Horne-Zeilinger-like state in the critical region. Our results provide a characterization of quantum phase transition via the violation of Bell-type constraint by correlations in the $XXZ$ spin-1 chain with multibody correlations and high-order measurements.

Variational quantum algorithms (VQAs) are among the most promising algorithms in the era of noisy intermediate scale quantum devices. Such algorithms are constructed using a parametrization $U(\theta$) with a classical optimizer that updates the parameters $\theta$ in order to minimize a cost function $C$. For this task, in general the gradient descent method, or one of its variants, is used. This is a method where the circuit parameters are updated iteratively using the cost function gradient. However, several works in the literature have shown that this method suffers from a phenomenon known as the barren plateaus (BPs). In this paper, we propose a method to mitigate BPs. In general, the parameters $\theta$ used in the parametrization $U$ are randomly generated. In our method they are obtained from a classical neural network (CNN). We show that this method, besides to being able to mitigate BPs during startup, is also able to mitigate the effect of BPs during the VQA training. In addition, we also show how this method behaves for different CNN architectures.

In the present work, we determine the $C_5$ coefficients along with their uncertainties due to quadrupole polarization effects of all the alkali atoms interacting with a metal (Au), a semiconductor (Si), and four dielectric materials (${\mathrm{SiO}}_2, {\mathrm{SiN}}_x$, yttrium aluminum garnet, and sapphire). The required dynamic electric quadrupole ($E2$) polarizabilities are evaluated by calculating $E2$ matrix elements of a large number of transitions in the alkali atoms by employing a relativistic coupled-cluster method. A significant contribution towards the long-range van der Waals potential is made by the quadrupole polarization effects. Our finding shows that contributions from the $C_5$ coefficients to the atom-wall interaction potentials are pronounced at short distances (1–10 nm). The $C_3$ coefficients of a Fr atom interacting with the above material media are also reported. These results could be useful in understanding the interactions of alkali atoms trapped in different material bodies during high-precision measurements.

Electric-field-controlled Förster-resonance energy transfer (FRET) between Rydberg helium (He) atoms and ground-state ammonia (${\mathrm{NH}}_3$) molecules has been studied at translational temperatures below 100 mK. The experiments were performed in an intrabeam collision apparatus with pulsed supersonic beams of ${\mathrm{NH}}_3$ seeded in He. A range of Förster resonances, between triplet Rydberg states in He with principal quantum numbers of 38, 39, and 40, and the inversion intervals in ${\mathrm{NH}}_3$ were investigated. Resonance widths as low as $100\pm 20$ MHz were observed for Rydberg-Rydberg transitions with electric dipole transition moments of 3270 D. These widths result from binary collisions at a mean center-of-mass speed of $19.3\pm 2.6$ m/s. For transitions in which the initially prepared Rydberg states were strongly polarized, with large induced static electric dipole moments, van der Waals interactions between the collision partners increased the resonance widths to $\sim 750$ MHz. From measurements of the rate of FRET for the narrowest resonances observed, a density of ${\mathrm{NH}}_3$ of $(9.4\pm 0.3)\times {10}^9{\mathrm{cm}}^{-3}$ in the upper ground-state inversion sublevel in the interaction region of the apparatus was determined nondestructively.

The spin-orbit interaction of light is a crucial concept for understanding the electromagnetic properties of a material and realizing the spin-controlled manipulation of optical fields. Achieving these goals requires a complete description of spin-dependent optical phenomena in the context of vector-wave mechanics. We develop an extended Dirac theory for optical fields in generic media, which was found to be akin to a non-Hermitian chiral extension of massive fermions with anomalous magnetic momenta moving in an external pseudomagnetic field. This similarity allows us to investigate the optical behaviors of a material by effective field-theory methods and can find wide applications in metamaterials, photonic topological insulators, etc. We demonstrate this method by studying the spin-orbit interaction of structured light in a spin-degenerate medium and inhomogeneous isotropic medium, which leads to both spin-orbital Hall effects and spin-to-orbital angular momentum conversion. Of importance, our approach provides simple and clear physical insight into the spin-orbit interaction of light in generic media, and could potentially bridge our understanding of topological insulators between electronic and photonic systems.

In this work, a simplified propagation model is proposed to specifically study the propagation effects of the driving laser pulse on the terahertz radiation in a gaseous medium for a short propagation distance. The simulations indicate that, due to the atomic ionization process and the energy redistribution between the laser field and the ionized electrons, the bichromatic laser pulses experience significant energy decaying, blue-shift, and a trailing edge broadening. As a result, the behaviors of the ionized electrons in the deformed laser field lead to the blue-shift and the multipeak structures in the terahertz radiation spectrum. Further investigations show that the differences of the terahertz spectrum for different relative phases of the fundamental and the second harmonic fields are gradually fading away as the driving laser pulse propagates. All these results strongly suggest that the propagation effects should be taken into consideration while it is generally disregarded.

Squeezed states are a versatile class of quantum states with applications ranging from quantum computing to high-precision detection. We propose a method for generating tunable squeezed vacuum states of light with multiple modes encoded in frequency bins. Our method uses custom-engineered spontaneous parametric down-conversion pumped by a pulse-shaped pump field. The multimode squeezed states are generated in a single pass and can be tuned in real time by adjusting the properties of the pump field. Exploring new quantum states of light, encoded in new degrees of freedom, can be a fruitful path toward discovering new quantum applications.

Complex absorbing potentials are frequently employed in quantum calculations. One of the advantages that such absorbers provide is the ability to attenuate outgoing waves in simulations of unbound systems, thus allowing for truncating the numerical domain. Here we argue that the absorber may also be used to probe outgoing waves so that physical information about absorbed particles may be retained. Moreover, under certain conditions, the physical information extracted via the absorber is subject to loss in coherence, as is also the case when collapsing the wave function upon measurement. Both these aspects demonstrate clearly how, and when, the effect of introducing a complex absorbing potential corresponds to that of a detector.

An indefinite causal order, where the causes of events are not necessarily in past events, is predicted by the process matrix framework. A fundamental question is how these nonseparable causal structures can be related to the thermodynamic phenomena. Here, we approach this problem by considering the existence of two cooperating local Maxwell's demons which try to exploit the presence of global correlations and indefinite causal order to optimize the extraction of work. Thus we prove that it is possible to have a larger probability to lower the local energy to zero if causal inequalities are violated and that more average work can be extracted with respect to a definite causal order. However, for noninteracting parties, for the system considered the work extractable cannot be larger than the definite causal order bound.

We study the properties of multipartite quantum correlation (MQC) in a one-dimensional spin-1/2 $XY$ chain, where the three-spin reduced states are focused on and the four introduced MQC measures are based on entanglement negativity and entanglement of formation. It is found that, even in the Ising case, the three-spin subsystems have long-range MQCs and the tripartite quantum correlations beyond the nearest-neighbor three spins can detect the quantum phase transition and obey the finite-size scaling around the critical point. Furthermore, in the $XY$ model, we show that the two selected MQCs can indicate exactly the factorization point of the ground state for the anisotropic model in the thermodynamic and finite-size cases. Moreover, the spatial distribution of MQC based on entanglement negativity can attain a much larger range by tuning the anisotropic parameter, and the newly defined MQC based on entanglement of formation can detect the bound entanglement in the three-spin subsystems when the entanglement negativity loses its efficacy.

Coherence is a defining property of quantum theory that accounts for quantum advantage in many quantum information tasks. Although many coherence quantifiers were introduced in various contexts, the lack of efficient methods to estimate them restricts their applications. In this paper, we tackle this problem by proposing one universal method to provide measurable bounds for most current coherence quantifiers. Our method is motivated by the observation that the distance between the state of interest and its diagonal parts in the reference basis, which lies at the heart of the coherence quantifications, can be readily estimated by the disturbance effect and uncertainty of the reference measurement. Thus, our method of bounding coherence provides a feasible and broadly applicable avenue for detecting coherence, facilitating its further practical applications.