The view exists that the Bell inequality is a mere inconsistent application of classical concepts to a well-established quantum world. In the article, “Nonlocality claims are inconsistent with Hilbert-space quantum mechanics” [Phys. Rev. A 101, 022117 (2020)], R. B. Griffiths advocates for quantum theory's locality. Although Griffiths presents valuable insights in favor of quantum mechanics' local character, he argues the Bell inequality is an inconsistent application of classical physics to quantum mechanics. We explain why a correct assessment of the Bell inequality does not, in fact, conflict with Griffiths' views of quantum locality and, on the contrary, it already contemplates them. Hence, Bell inequality inconsistency is not necessary for Griffiths's quantum locality to hold.

In Phys. Rev. A 101, 022117 (2020), it was argued that Bell inequalities are based on classical not quantum physics, and, hence, their violation in experiments provides no support for the claimed existence of peculiar nonlocal and superluminal influences in the real (quantum) world. This Reply to Lambare's Comment, Phys. Rev. A 104, 066201 (2021), on that paper seeks to clarify some issues related to the correct use of Hilbert space quantum mechanics for identifying the microscopic causes of later macroscopic measurement outcomes, a matter not properly addressed by Bell, who used classical hidden variables in place of the Hilbert subspaces (equivalently, their projectors) employed by von Neumann in Mathematical Foundations of Quantum Mechanics.

We study theoretically the deformation of the Fermi surface (FS) of a three-dimensional gas of Rydberg-dressed ${}^6\mathrm{Li}$ atoms. The laser dressing to high-lying Rydberg $D$ states results in angle-dependent soft-core-shaped interactions whose anisotropy is described by multiple spherical harmonics. We show that this can drastically modify the shape of the FS and that its deformation depends on the interplay between the Fermi momentum $k_F$ and the reciprocal momentum $\overline{k}$ corresponding to the characteristic soft-core radius of the dressing-induced potential. When $k_F<\overline{k}$, the dressed interaction stretches a spherical FS into an ellipsoid. When $k_F\gtrsim \overline{k}$, complex deformations are encountered which exhibit multipolar characteristics. We analyze the formation of Cooper pairs around the deformed FS and show that they occupy large orbital angular momentum states ($p, f$, and $h$ wave) coherently. Our study demonstrates that Rydberg dressing to high angular momentum states may pave a route toward the investigation of unconventional Fermi gases and multiwave superconductivity.

We used a direct laser writing technique to build annular waveguides inside a glassy blade. These waveguides have shown the capability of generating orbital angular momentum beams with different topological charges in the same structure. Here we demonstrate that, after choosing suitable waveguide parameters, one may tune the topological charge of a beam just by tilting the input wavefront. This work opens opportunities for optical communications using spatial degrees of freedom multiplexed beams and for generation of tunable angular momentum at the microscale for integrability of lab on chips.

Quantum correlations encoded in photonic Laguerre-Gaussian modes were shown to be related to the Gouy-phase shifts [Kawase et al., Phys. Rev. Lett. 101, 050501 (2008)], allowing for a nondestructive manipulation of photonic quantum states. In this work, we exploit the relation between phase-space correlations of biphotons produced by spontaneously parametric down-conversion (SPDC) as encoded in the logarithmic negativity (LN) and the Gouy phase as they are diffracted through an asymmetrical double-slit setup. Using an analytical approach based on a double-Gaussian approximation for type-I SPDC biphotons, we show that measurements of Gouy-phase differences provide information on their phase-space entanglement variation, governed by the physical parameters of the experiment and expressed by the LN via covariance matrix elements.

The number of outcomes is a defining property of a quantum measurement, in particular, if the measurement cannot be decomposed into simpler measurements with fewer outcomes. Importantly, the number of outcomes of a quantum measurement can be irreducibly higher than the dimension of the system. The certification of this property is possible in a semi-device-independent way either based on a Bell-like scenario or by utilizing the simpler prepare-and-measure scenario. Here we show that in the latter scenario the minimal scheme for certifying an irreducible three-outcome qubit measurement requires three state preparations and only two measurements and we provide experimentally feasible examples for this minimal certification scheme. We also discuss the dimension assumption characteristic of the semi-device-independent approach and to what extent it can be mitigated.

Noise dominates every aspect of near-term quantum computers, rendering it exceedingly difficult to carry out even small computations. In this paper we are concerned with the modeling of noise in noisy intermediate-scale quantum computers. We focus on three error groups that represent the main sources of noise during a computation and present quantum channels that model each source. We engineer a noise model that combines all three noise channels and simulates the evolution of the quantum computer using its calibrated error rates. We run various experiments of our model, showcasing its behavior compared to other noise models and an IBM quantum computer. We find that our model provides a better approximation of the quantum computer's behavior than the other models. Following this, we use a genetic algorithm to optimize the parameters used by our noise model, bringing the behavior of the model even closer to the quantum computer. Finally, a comparison between the pre- and postoptimization parameters reveals that, according to our model, certain operations can be more or less erroneous than the hardware-calibrated parameters show.

We estimate the energy distribution of positrons produced in the interaction of ultrarelativistic electrons with a high-intensity laser beam. The underlying trident process is factorized on the probabilistic level. That is, we deploy a two-step mechanism for the formation of electron-positron pairs. In the first step, a high-energy photon is produced as a result of nonlinear Compton scattering. In the second step, an electron-positron pair is created by the nonlinear (multiphoton) Breit-Wheeler process.

We work out the effective scaling approach to frictionless quantum quenches in a one-dimensional Bose gas trapped in a harmonic trap. The effective scaling approach produces an auxiliary equation for the scaling parameter interpolating between the noninteracting and the Thomas-Fermi limits. This allows us to implement a frictionless quench by inversely engineering the smooth trap frequency, as compared to the two-jump bang control. Our result is beneficial to design a shortcut-to-adiabaticity expansion of trapped Bose gases for arbitrary values of interaction, and can be directly extended to the three-dimensional case.

The scattering of light by localized three-dimensional dielectrics having gain and loss defies the usual Born approximation, since the material can increase the amplitude of the incident field within the scatterer, making the weak-scattering assumption invalid. The convergence of the Born series is rarely discussed in analytical treatments, as the state of exhaustion is reached after calculating the very first few terms of the series. Even if all the terms are obtained, the series will certainly be of a divergent type in general, thus invalidating the equality of the scattered field to its Taylor-series representation. We present here a simple localized material model of a dielectric having parity-time ($\mathcal{PT}$) symmetry, consisting of a $\mathcal{PT}$-symmetric dipole, such that all the terms in the Born series are analytically evaluated and a closed-form expression is obtained in the far-zone approximation. The scattered field is then analyzed by using Padé approximants in order to obtain convergent representations of the scattered radiation and to compare with the exact solution. This allows us to study the role of the gain and loss parameter in strong-scattering regimes and to demonstrate the remarkable properties of Padé approximants when applied to scattering.

Enforcing stable phase relations among all laser comb modes is a powerful way to control broad-band emitted radiation with only two actuators. This is an established technique for pulsed mode-locked lasers covering even more than an octave with several thousand modes. Recently, generation of combs has been demonstrated in miniaturized sources, like microresonators for the near-infrared and quantum cascade lasers for the mid- and far-infrared region. Despite the lack of a thorough theoretical background, these sources have been recently characterized by a Fourier-transform analysis of comb emission (FACE). This technique is suitable for comb characterization in any spectral region and regardless of the temporal emission profile. Here we provide a comprehensive and complete FACE theoretical framework, clarifying the details of the data analysis, giving access to more detailed results, and improving the overall reach of the technique itself. This will foster a widespread use of this approach for a full characterization of any comb emission.

In this paper, we study the exact dynamics of open quantum systems in the presence of dissipationless localized bound states and periodic driving fields. We show that different from the static adjustment of the system parameters to control the existence of localized bound states in open quantum systems, the periodic driving can either modulate the dynamics of the existing localized bound states or make some of them disappear due to the sideband generation from the driving that makes electrons transit between the localized bound states and continuous states. This analysis is also different from the widely used Floquet theory in the study of the driving effect on open system dynamics. We also find the conditions for the protection of the dissipationless localized bound states from the driving and for the manipulation of quantum coherence between localized bound states using the driving. The results for the dynamics of localized bound states have a potential application in controlling the quantum state against decoherence for the sake of its sensitivity to the fundamental frequency of the driving field and its strength.

Continuous-variable cluster states allow for fault-tolerant measurement-based quantum computing when used in tandem with the Gottesman-Kitaev-Preskill (GKP) encoding of a qubit into a bosonic mode. For quad-rail-lattice macronode cluster states, whose construction is defined by a fixed, low-depth beam splitter network, we show that a Clifford gate and GKP error correction can be simultaneously implemented in a single teleportation step. We give explicit recipes to realize the Clifford generating set, and we calculate the logical gate-error rates given finite squeezing in the cluster-state and GKP resources. We find that logical error rates of ${10}^{-2}$–${10}^{-3}$, compatible with the thresholds of topological codes, can be achieved with squeezing of 11.9–13.7 dB. The protocol presented eliminates noise present in prior schemes and puts the required squeezing for fault tolerance in the range of current state-of-the-art optical experiments. Finally, we show how to produce distillable GKP magic states directly within the cluster state.

Variational quantum algorithms such as the quantum approximate optimization algorithm (QAOA) are particularly attractive candidates for implementation on near-term quantum processors. As hardware realities such as error and qubit connectivity will constrain achievable circuit depth in the near future, new ways to achieve high performance at low depth are of great interest. In this work, we present a modification to QAOA that adds additional variational parameters in the form of freedom of the rotation axis in the $XY$ plane of the mixer Hamiltonian. Via numerical simulation, we show that this leads to a drastic performance improvement over standard QAOA at finding solutions to the MaxCut problem on graphs of up to seven qubits. Furthermore, we explore the $Z$-phase error mitigation properties of our modified Ansatz, its performance under a realistic error model for a neutral atom quantum processor, and the class of problems it can solve in a single round.

In realistic metrology, entangled probes are more sensitive to noise, especially for a correlated environment. The precision of parameter estimation with entangled probes is even lower than that of the unentangled ones in a correlated environment. In this paper we propose a measurement scheme with only one auxiliary qubit, which can selectively offset the impact of environmental noise under this situation. We analyze the estimation precision of our scheme and find out that it approaches the Heisenberg limit when prepared in a proper auxiliary state. We further discuss employing auxiliary states to improve the precision of measurement in other environment models such as a partially correlated environment. In order to verify our scheme, we apply a recently developed quantum algorithm to simulate the quantum dynamics of our proposal and show that it outperforms the other proposals with less resources.

Circular dichroism is a widely used technique for investigating optically chiral molecules, especially for biomolecules. It is thus of great importance that these parameters be estimated precisely so that the molecules with desired functionalities can be designed. In order to surpass the limits of classical measurements, we need to probe the system with quantum light. We develop the quantum Fisher information matrix (QFIM) for precision estimates of the circular dichroism and the optical rotary dispersion for a variety of input quantum states of light that are easily accessible in the laboratory. The Cramer-Rao bounds for all four chirality parameters are obtained from the QFIM for (a) single photon input states with a specific linear polarization and for (b) NOON states having two photons with both either left polarized or right polarized. The QFIM bounds, using quantum light, are compared with bounds obtained for classical light beams, i.e., beams in coherent states. Quite generally, both the single-photon state and the NOON state exhibit superior precision in the estimation of absorption and phase shift in relation to a coherent source of comparable intensity, especially in the weak-absorption regime. In particular, the NOON state naturally offers the best precision among the three. We compare QFIM bounds with the error sensitivity bounds, as the latter are relatively easier to measure whereas the QFIM bounds require full state tomography.

We present an experimental and theoretical study of the absorption and emission spectra of Yb atoms in a solid Ne matrix at a resolution of 0.025 nm. Five absorption bands were identified as due to transitions from the $4f^{14}5d^{0}6s^2{}^1{S}_0$ ground-state configuration to $4f^{14}5d^{0}6s6p$ and $4f^{13}5d^{1}6s^2$ configurations. The two lowest-energy bands were assigned to outer-shell transitions to $6s6p{}^3{P}_1$ and ${}^1{P}_1$ atomic states and displayed the structure of a broad doublet and an asymmetric triplet, respectively. The remaining three higher-frequency bands were assigned to inner-shell transitions to distinct $J=1$ states arising from the $4f^{13}5d^{1}6s^2$ configuration and were highly structured with narrow linewidths. A classical simulation was performed to identify the stability and symmetry of possible trapping sites in the Ne crystal. It showed that the overarching $1+2$ structure of the high-frequency bands could be predominantly ascribed to crystal-field splitting in the axial field of a 10-atom vacancy of $C_{4v}$ symmetry. Their prominent substructures were shown to be manifestations of phonon sidebands associated with the zero-phonon lines on each crystal-field state. Unprecedented for a metal–rare-gas system, resolution of individual phonon states on an allowed electronic transition was possible under excitation spectroscopy which reflects the semiquantum nature of solid Ne. In contrast to the absorption spectra, emission spectra produced by steady-state excitation into the ${}^1{P}_1$ absorption band consisted of simple, unstructured fluorescence bands.

We report numerical simulations of positronium experiments designed to measure the $n=2$ fine-structure intervals. The simulations include all possible interference effects between all 20 states in the $n=1$ and laser-excited $n=2$ manifolds as well as representations of the electric and magnetic fields present in the waveguides used in the experiments. We find that rf wave reflection from the vacuum chamber walls is a possible explanation of previously observed line-shape distortions and shifts. We also characterized several systematic effects, including those arising from quantum interference, that are likely to be significant for future measurements.