Quantized vortices carry the angular momentum in rotating superfluids, and are key to the phenomenon of quantum turbulence. Advances in ultracold-atom technology enable quantum turbulence to be studied in regimes with both experimental and theoretical control, unlike the original contexts of superfluid helium experiments. While much work has been performed with bosonic systems, detailed studies of fermionic quantum turbulence are nascent, despite wide applicability to other contexts such as rotating neutron stars. In this paper, we present a large-scale study of quantum turbulence in rotating fermionic superfluids using an accurate time-dependent density functional theory called the superfluid local density approximation. We identify two different modes of turbulent decay in the dynamical equilibration of a rotating fermionic superfluid, and contrast these results with a computationally simpler description provided by the Gross-Pitaevskii equation, which we find can qualitatively reproduce these decay mechanisms if dissipation is explicitly included. These results demonstrate that dissipation mechanisms intrinsic to fermionic superfluids play a key role in differentiating fermionic from bosonic turbulence, which manifests by enhanced damping of Kelvin waves.

We describe a class of minimal diffraction-free and dispersion-free superluminal wave packets ($X$ waves and focused $X$ waves) that can carry a given amount of units $m$ of orbital angular momentum (OAM) per photon. Even if their frequency spectrum is wide enough to synthesize a unipolar pulse, OAM imposes a temporal pulse shape with as many zeros as units of OAM, and therefore $\left|m\right|/2$ temporal oscillations. All the frequencies in the broadband spectrum are displayed, like in a rainbow, from the bluer ones in the vicinity of the vortex to the redder ones in the wave periphery. At the same time, the whole wave packet experiences a blueshift proportional to $\left|m\right|$. As a result of the radial redshift and the OAM blueshift, the color of the brilliant ring around the central vortex is independent of OAM, and solely determined by the broadband source spectrum. Given the very peculiar properties of $X$ waves with OAM, their generation and use, instead of standard Laguerre-Gauss modes, could improve the performance of OAM-based communication and quantum cryptographic systems, as well as the efficiency of the generation of high harmonics and attosecond pulses with OAM.

We investigate interesting symmetry properties verified by the down-converted beams produced in optical parametric amplification with structured light. We show that the Poincaré sphere symmetry, previously demonstrated for first-order spatial modes, translates to a multiple Poincaré sphere structure for higher orders. Each one of these multiple spheres is associated with a two-dimensional subspace defined by a different value of the orbital angular momentum. Therefore, the symmetry verified by first-order modes is reproduced independently in each subspace. This effect can be useful for parallel control of independently correlated beams.

The theory of non-Hermitian systems and the theory of quantum deformations have attracted a great deal of attention in the past decades. In general, non-Hermitian Hamiltonians are constructed by an ad hoc manner. Here, we study the ($2+1$) Dirac oscillator and show that in the context of the $\kappa$-deformed Poincaré-Hopf algebra its Hamiltonian is non-Hermitian but has real eigenvalues. The non-Hermiticity stems from the $\kappa$-deformed algebra. From the mapping in Bermudez et al., Phys. Rev. A 76, 041801(R) (2007), we propose the $\kappa$-Jaynes-Cummings and $\kappa$-anti-Jaynes-Cummings models, which describe an interaction between a two-level system with a quantized mode of an optical cavity in the $\kappa$-deformed context. We find that the $\kappa$ deformation modifies the Zitterbewegung frequencies and the collapses and revivals of quantum oscillations. In particular, the total angular momentum in the $z$ direction is not conserved anymore, as a direct consequence of the deformation.

Attenuating a quantum state using a beam splitter will introduce noise and decoherence. Here we show that heralding techniques can be used to attenuate Schrödinger cat states and squeezed vacuum states without any noise or decoherence [Mičuda et al., Phys. Rev. Lett. 109, 180503 (2012)]. Noiseless attenuation also preserves quantum interference effects in nonclassical states such as squeezed vacuum states.

We derive entropic inseparability criteria for the phase-space representation of quantum states. In contrast to criteria involving differential entropies of marginal phase-space distributions, our criteria are based on a joint distribution known as the Husimi $Q$ distribution. This distribution is experimentally accessible in cold atoms, circuit QED architectures, and photonic systems, and bears practical advantages compared to the detection of marginals. We exemplify the strengths of our entropic approach by considering several classes of non-Gaussian states where second-order criteria fail. We show that our criteria certify entanglement in previously undetectable regions, highlighting the strength of using the Husimi $Q$ distribution for entanglement detection.

We study intraenvironmental classical and quantum correlations in a thermal machine, which is modeled as a driven quantum system coupled with thermal reservoirs. We compute the mutual information, the quantum discord, and the entanglement between two parts of the environment formed by oscillators centered around two different frequencies. We show that there are only two processes that generate time-extensive correlations in the long-time limit. First, there is a resonant process which is responsible for the transport of excitations between different environmental modes due to the absorption (or emission) of energy from (or into) the driving field. Second, there is a nonresonant process that transforms the energy from the external driving into pairs of excitations in two environmental modes. We show that there is a regime when the mutual information and the quantum discord between the parts of the environment correlated by these two processes grow quadratically in time, while entanglement production is time extensive.

We calculate hard spin-independent three-loop radiative corrections to energy levels in muonium and positronium which are due to radiative corrections with polarization insertions in two-photon exchange diagrams. These corrections could be relevant for the new generation of precise $1S\text{−}2S$ and $2S\text{−}2P$ measurements in muonium and positronium.

The influence of the blackbody radiation field on the $g$ factor of light hydrogenlike ions is considered within the framework of quantum electrodynamics at finite temperature for bound states. One-loop thermal corrections are examined for a wide range of temperatures. The numerical results for $1s, 2s, 2p_{1/2}$, and $2p_{3/2}$ states are presented. It is shown that for excited states, finite temperature corrections to the bound-electron $g$ factor are close to the level of current experimental uncertainty even at room temperatures and can be discerned within the measurements anticipated in the near future.

It is known that the Lifshitz theory of the Casimir force comes into conflict with measurement data if the response of conduction electrons in metals to electromagnetic fluctuations is described by the well-tested dissipative Drude model. The same theory is in very good agreement with measurements of the Casimir force from graphene, whose spatially nonlocal electromagnetic response is derived from the first principles of quantum electrodynamics. Here we propose spatially nonlocal phenomenological dielectric functions of metals which lead to nearly the same response as the standard Drude model to the propagating waves, but to a different response in the case of evanescent waves. Unlike some previous suggestions of this type, the response functions considered here depend on all components of the wave vector, as is most natural in the formalism of specular reflection used. It is shown that these response functions satisfy the Kramers-Kronig relations. We derive respective expressions for the surface impedances and reflection coefficients. The obtained results are used to compute the effective Casimir pressure between two parallel plates, the Casimir force between a sphere and a plate, and the Casimir force gradient in configurations of the most precise experiments performed with both nonmagnetic (Au) and magnetic (Ni) test bodies. It is shown that in all cases (Au-Au, Au-Ni, and Ni-Ni test bodies) the predictions of the Lifshitz theory found by using the dissipative nonlocal response functions are in as good agreement with the measurement data as was reached previously with the dissipationless plasma model. Possible developments and applications of these results are discussed.

Encoding a dimension in the internal degree of freedom of an atom provides an interesting tool for quantum simulation, facilitating the realization of artificial gauge fields. We propose an extension of the synthetic dimension toolbox, making it possible to encode two dimensions within a large atomic spin. The protocol combines first- and second-order spin couplings such that the spin projection $m$ and the remainder $r=m$ (mod 3) of its Euclidian division by 3 act as orthogonal coordinates on a synthetic cylinder. It is suited for an implementation with lanthanide atoms, which feature a large electronic spin and narrow optical transitions for applying the required spin couplings. This method is useful for simulating geometries with periodic boundary conditions and engineering various types of topological systems evolving in high dimensions.

Particles made of two fermions can in many cases be treated as elementary bosons, but the conditions for this treatment to be valid are nontrivial. The so-called “coboson formalism” is a powerful tool to tackle compositeness effects relevant for instance for exciton physics and ultracold atomic dimers. A key element of this theory is an ansatz for the ground state of $N$ pairs, built from the single-pair ground state combined with the exclusion principle. We show that this ansatz can fail in one-dimensional systems which fulfill the conditions expected to make the ansatz valid. Nevertheless, we also explain how coboson theory can recover the correct ground state. Thus, our work highlights limitations and strengths of the formalism and leads to a better treatment of composite bosons.

We investigate the energy-velocity dispersion relation of spin solitons with arbitrary initial velocities. Based on the dispersion relation, we further characterize the inertial mass of a spin soliton and find two critical velocities to distinguish between the positive mass and negative mass. One critical velocity separates the initial negative and positive masses, and the other denotes the velocity for negative-positive mass transition during an acceleration process. With the aid of the two critical velocities, we further propose two possible ways to observe the pure negative mass effect of a spin soliton. Moreover, the relations between the soliton's width and its velocity are discussed according to the variational results. We report that the soliton width can become narrower with larger moving speed (when a spin soliton possesses positive mass), in sharp contrast to the dark solitons reported before. These results would motivate experiments to observe negative mass effects of solitons in Bose-Einstein condensates.

Frequency correlations are a versatile and powerful tool which can be exploited to perform spectral analysis of objects whose direct measurement might be unfeasible. This is achieved through a so-called ghost spectrometer that can be implemented with quantum and classical resources alike. While there are some known advantages associated to either choice, an analysis of their metrological capabilities has not yet been performed. Here we report on the metrological comparison between a quantum and a classical ghost spectrometer. We perform the estimation of the transmittivity of a bandpass filter using frequency-entangled photon pairs. Our results show that a quantum advantage is achievable, depending on the values of the transmittivity and on the number of frequency modes analyzed.

The generation of non-Abelian geometric phases from a system of evanescently coupled waveguides is considered within the framework of nonorthogonal coupled-mode theory. Here, we study an experimentally feasible tripod arrangement of waveguides that contain dark states from which a nontrivial $U\left(2\right)$ mixing can be obtained by means of an adiabatic parameter variation. We investigate the influence of higher-order contributions beyond nearest-neighbor coupling as well as self-coupling on the stability of a non-Abelian $U\left(3\right)$ phase generated from an optical tetrapod setup. Our results indicate that, despite the mode nonorthogonality, the symmetry of dark states protects the geometric evolution of light from distortion.

The factorized form of the unitary coupled-cluster (UCC) approximation is one of the most promising methodologies to prepare trial states for strongly correlated systems within the variational quantum eigensolver framework. The factorized form of the UCC Ansatz can be systematically applied to a reference state to generate the desired entanglement. The difficulty associated with such an approach is the requirement of simultaneously entangling a growing number of qubits, which quickly exceeds the hardware limitations of today's quantum machines. In particular, while circuits for singles and double excitations can be performed on current hardware, higher-rank excitations require too many gate operations. In this work, we propose a set of schemes that trade off using extra qubits for a reduced gate depth to decompose high-rank UCC excitation operators into significantly lower depth circuits. These results will remain useful even when fault-tolerant machines are available to reduce the overall state-preparation circuit depth.

We study bidirectional teleportation while explicitly taking into account a mixed environment. This environment initially causes pure dephasing decoherence of the Bell state which assists teleportation. We find that when teleportation is performed in one direction it is accompanied by a transfer of correlations into the teleported qubit state. In the other direction, if no new decoherence process occurs then not only the state of the qubit, but also its correlations with the environment are teleported with unit fidelity. These processes do not depend on the measurement outcome during teleportation and do not differentiate between classical and quantum decoherence. If, on the other hand, the second teleportation step is preceded by decoherence of the Bell state then the situation is much more complicated. Teleportation and transfer of correlations occur simultaneously, yielding different teleported qubit-environment states for different measurement outcomes. These states can differ in the degree of coherence of the teleported qubit, but only for an entangling Bell-state-environment interaction in the first step can they have different amounts of qubit-environment entanglement. In the extreme case, one of the teleported qubit states can be entangled with the environment while the other is separable.

Digital signatures are widely used for providing security of communications. At the same time, the security of currently deployed digital signature protocols is based on unproven computational assumptions. An efficient way to ensure an unconditional (information-theoretic) security of communication is to use quantum key distribution (QKD), whose security is based on laws of quantum mechanics. In this work we develop an unconditionally secure signature scheme that guarantees authenticity and transferability of arbitrary length messages in a QKD network. In the proposed setup, the QKD network consists of two subnetworks: (i) an internal network that includes the signer and with limitation on the number of malicious nodes and (ii) an external network that has no assumptions on the number of malicious nodes. A consequence of the absence of the trust assumption in the external subnetwork is the necessity of assistance from internal subnetwork recipients for the verification of message-signature pairs by external subnetwork recipients. We provide a comprehensive security analysis of the developed scheme, perform an optimization of the scheme parameters with respect to the secret key consumption, and demonstrate that the developed scheme is compatible with the capabilities of currently available QKD devices.