In this work, we investigate multivortex beams with an infinite topological charge. Such optical vortices contain an array of an infinite, but countable, number of phase singularities (isolated intensity nulls), which typically have the unitary topological charge and are located either uniformly (or not uniformly) on a straight line (or on several crossing straight lines) in the beam transverse cross section. Such optical vortices are form invariant and their transverse intensity distribution is conserved on propagation, changing only in scale and rotation. The orbital angular momentum of such optical vortices is finite, since only a finite number of screw dislocations is located within the area of the notable intensity of the Gaussian beam, while the other phase singularities are in the periphery (and in the infinity), where the intensity is very small. Increasing the Gaussian beam waist radius leads to a parabolic growth of the orbital angular momentum of such beams.

We theoretically study an optomechanical system which consists of a two-sided cavity and a mechanical membrane that is placed outside of it. The membrane is positioned close to one of its mirrors, and the cavity is coupled to the external light field through the other mirror. Our study is focused on the regime where the dispersive optomechanical coupling in the system vanishes. Such a regime is found to be possible if the membrane is less reflecting than the adjacent mirror, yielding a potentially very strong dissipative optomechanical coupling. Specifically, if the absolute values of amplitude transmission coefficients of the membrane and the mirror, $t$ and $t_m$, respectively, obey the condition $t_m^2<t\ll t_m\ll 1$, the dissipative coupling constant of the setup exceeds the dispersive coupling constant for an optomechanical cavity of the same length. The dissipative coupling constant and the corresponding optomechanical cooperativity of the proposed system are also compared with those of the Michelson-Sagnac interferometer and the so-called “membrane-at-the-edge” system, which are known for a strong optomechanical dissipative interaction. It is shown that under the above condition, the system proposed here is advantageous in both aspects. It also enables an efficient realization of the two-port configuration, which was recently proposed as a promising optomechanical system, providing, among other benefits, a possibility of quantum limited optomechanical measurements in a system which does not suffer from any optomechanical instability.

Quantum Rabi model (QRM) is fascinating not only because of its broad relevance but also due to its few-body quantum phase transition. In practice, both the bias and the nonlinear coupling in QRM are important controlling parameters in experimental setups. We study the interplay of the bias and the nonlinear interaction with the linear coupling in the ground state which exhibits various patterns of symmetry breaking and different orders of transitions. Several situations of tricriticalities are unveiled in the low-frequency limit and at finite frequencies. We find that the full quantum-mechanical effect leads to transitions, tricriticalities, quadruple points, and a fine structure of spontaneous symmetry breaking, which are much beyond the semiclassical picture. We clarify the underlying mechanisms by analyzing the energy competitions and the essential changeovers of the quantum states, which enables us to extract most analytic phase boundaries.

We study the motion of a charged particle with magnetic moment in external electromagnetic fields utilizing covariant unification of Gilbertian and Amperian descriptions of a particle magnetic dipole moment. Considering the case of a current loop, our approach is verified by comparing classical dynamics with the classical limit of relativistic quantum dynamics. We obtain the motion of a charged particle in the presence of an external linearly polarized electromagnetic (laser) plane-wave field incorporating the effect of spin dynamics. For specific laser-particle initial configurations, we determine that the Stern-Gerlach force can have a cumulative effect on the trajectory of charged particles.

The so-called eigenvalue-eigenstate link states that no property can be associated to a quantum system unless it is in an eigenstate of the corresponding operator. This precludes the assignation of properties to unmeasured quantum systems in general. This arbitrary limitation of orthodox quantum mechanics generates many puzzling situations such as for example the impossibility to uniquely define a work distribution, an essential building block of quantum thermodynamics. Alternatively, modal theories (e.g., Bohmian mechanics) provide an ontology that always allows one to define intrinsic properties, i.e., properties of quantum systems that are detached from any possible measuring context. We prove here that Aharonov, Albert, and Vaidman's notion of a weak value can always be identified with an intrinsic dynamical property of a quantum system defined in a certain modal theory. Furthermore, the fact that weak values are experimentally accessible (as an ensemble average of weak measurements which are postselected by a strong measurement) strengthens the idea that understanding the intrinsic (unperturbed) dynamics of quantum systems is possible and useful in a given modal theory. As examples of the physical soundness of these intrinsic properties, we discuss three intrinsic Bohmian properties, viz., the dwell time, the work distribution, and the quantum noise at high frequencies.

We study the energy-transfer process in quantum battery systems consisting of multiple central spins and bath spins. Here with “quantum battery” we refer to the central spins, whereas the bath serves as the “charger.” For the single-central-spin battery, we analytically derive the time evolutions of the energy transfer and the charging power with arbitrary number of bath spins. For the case of multiple central spins in the battery, we find the scaling-law relation between the maximum power $P_{\max }$ and the number of central spins $N_B$. It approximately satisfies a scaling law relation $P_{\max }\propto N_B^{\alpha }$, where scaling exponent $\alpha$ varies with the bath spin number $N$ from the lower bound $\alpha =1/2$ to the upper bound $\alpha =3/2$. The lower and upper bounds correspond to the limits $N\to 1$ and $N\gg N_B$, respectively. In thermodynamic limit, by applying the Holstein-Primakoff transformation, we rigorously prove that the upper bound is $P_{\max }=0.72BA\sqrt{N}{N}_B^{3/2}$, which shows the same advantage in scaling of a recent charging protocol based on the Tavis-Cummings model. Here $B$ and $A$ are the external magnetic field and coupling constant between the battery and the charger.

The Pauli exclusion operator (PEO), which ensures proper symmetry of the eigenstates of multi-electron systems with respect to the exchange of each pair of electrons, is introduced. Once PEO is added to the Hamiltonian, no additional constraints on the multi-electron wave function due to the Pauli exclusion principle are needed. For two-electron states in two dimensions (2D) the PEO can be expressed in a closed form in terms of momentum operators, while in the position representation PEO is a nonlocal operator. Generalizations of PEO for multi-electron systems are introduced. Several approximations to PEO are discussed. Examples of analytical and numerical calculations of PEO are given for the isotropic and anisotropic Hooke's atom in 2D. The application of approximate and kernel forms of PEO for calculations of energies and states in a 2D Hooke's atom are analyzed. The relation of PEO to standard variational calculations with the use of Slater determinant is discussed.

Device-independent quantum key distribution (DI-QKD) offers the strongest form of security against eavesdroppers bounded by the laws of quantum mechanics. However, a practical implementation is still pending due to the requirement of combinations of visibility and detection efficiency that are beyond those possible with current technology. This mismatch motivates the search for DI-QKD protocols that can close the gap between theoretical and practical security. In this work, we present two DI-QKD protocols whose security relies on Bell inequalities with more than two inputs and two outputs. We show that, for maximally entangled states and perfect visibility, a protocol based on a Bell inequality with three inputs and four outputs requires a slightly lower detection efficiency than the protocols based on Bell inequalities with two inputs and two outputs.

Conditional multiqubit gates are a key component for elaborate quantum algorithms. In a recent work, Rasmussen et al. [Phys. Rev. A 101, 022308 (2020)] proposed an efficient single-step method for a prototypical multiqubit gate, a Toffoli gate, based on a combination of Ising interactions between control qubits and an appropriate driving field on a target qubit. Trapped ions are a natural platform to implement this method, since Ising interactions mediated by phonons have been demonstrated in increasingly large ion crystals. However, the simultaneous application of these interactions and the driving field required for the gate results in undesired entanglement between the qubits and the motion of the ions, reducing the gate fidelity. In this work, we propose a solution based on adiabatic switching of these phonon-mediated Ising interactions. We study the effects of imperfect ground-state cooling and use spin-echo techniques to undo unwanted phase accumulation in the achievable fidelities. For gates coupling to all axial modes of a linear crystal, we calculate high-fidelity $(>99\%)$ $N$-qubit rotations with $N=3–7$ ions cooled to their ground state of motion and a gate time below 1 ms. Finally, we study the effect of laser intensity fluctuations and find that the proposed gate requires intensity stabilization with subpercentage noise levels. The high fidelities obtained also for large crystals could make the gate competitive with gate-decomposed, multistep variants of the $N$-qubit Toffoli gate, at the expense of requiring ground-state cooling of the ion crystal.

The temporal evolution of a quantum system can be characterized by quantum process tomography, a complex task that consumes a number of physical resources scaling exponentially with the number of subsystems. An alternative approach to the full reconstruction of a quantum channel is the selective and efficient quantum process tomography, a method that allows estimating, individually and up to the required accuracy, each element of the matrix that describes the process, using only a polynomial amount of resources. The implementation of this protocol is closely related to the possibility of building a complete set of mutually unbiased bases (MUBs), whose existence is known only when the dimension of the Hilbert space is the power of a prime number. However, an extension of the method that uses tensor products of maximal sets of MUBs has been recently introduced. Here we explicitly describe how to implement the algorithm for a selective and efficient estimation of a quantum process in a nonprime power dimension and conduct, an experimental verification of the method in a Hilbert space of dimension $d=6$. This is the smallest space for which a complete set of MUBs is not known to exist, but it can be decomposed as a tensor product of two Hilbert spaces of dimensions $D_1=2$ and $D_2=3$, in which a complete set of MUBs is already known. The six-dimensional states were codified in the discretized transverse linear momentum of photons. The state preparation and detection stages are dynamically programed with the use of only-phase spatial light modulators, in a versatile experimental setup that allows one to implement the algorithm in any finite dimension.

It is known from ensemble measurements that rubidium atoms trapped in solid parahydrogen have favorable properties for quantum sensing of magnetic fields. To use a single rubidium atom as a quantum sensor requires a technique capable of efficiently measuring the internal state of a single atom, such as laser-induced fluorescence. In this work we search for laser-induced fluorescence from ensembles of rubidium atoms trapped in solid parahydrogen and, separately, in solid neon. In parahydrogen we find no evidence of fluorescence over the range explored, and we place upper limits on the radiative branching ratio. In neon, we observe laser-induced fluorescence, measure the spectrum of the emitted light, and measure the excited-state lifetime in the matrix. Bleaching of atoms from the excitation light is also reported.

Stochastic modeling is an essential component of the quantitative sciences, with hidden Markov models (HMMs) often playing a central role. Concurrently, the rise of quantum technologies promises a host of advantages in computational problems, typically in terms of the scaling of requisite resources such as time and memory. HMMs are no exception to this, with recent results highlighting quantum implementations of deterministic HMMs exhibiting superior memory and thermal efficiency relative to their classical counterparts. In many contexts, however, nondeterministic HMMs are viable alternatives; compared to them, the advantages of current quantum implementations do not always hold. Here, we provide a systematic prescription for constructing quantum implementations of nondeterministic HMMs that reestablish the quantum advantages against this broader class. Crucially, we show that whenever the classical implementation suffers from thermal dissipation due to its need to process information in a time-local manner, our quantum implementations will both mitigate some of this dissipation and achieve an advantage in memory compression.

We explore coherent control of Penning and associative ionization in cold collisions of metastable ${\mathrm{He}}^{*}\left(2{}^{3}S\right)$ atoms via the quantum interference between different states of the ${\mathrm{He}}_2^{*}$ collision complex. By tuning the preparation coefficients of the initial atomic spin states, we can benefit from the quantum interference between molecular channels to maximize or minimize the cross sections for Penning and associative ionization. In particular, we find that we can enhance the ionization ratio by 30% in the cold regime. This work is significant for the coherent control of chemical reactions in the cold and ultracold regimes.

We introduce an alternative definition of the main frequency of an ultrashort laser pulse—the principal frequency ${\omega }_P$. This parameter is complementary to the most accepted and widely used carrier frequency ${\omega }_0$. Given the fact that these ultrashort pulses, also known as transients, have a temporal width comprising only a few cycles of the carrier wave, corresponding to a spectral bandwidth $\mathrm{\Delta }\omega$ covering several octaves, ${\omega }_P$ describes, in a more precise way, the dynamics driven by these sources. We present examples where, for instance, ${\omega }_P$ is able to correctly predict the high-order harmonic cutoff independent of the carrier envelope phase. This is confirmed by solving the time-dependent Schrödinger equation in reduced dimensions, supplemented with the time-analysis of the quantum spectra, where it is possible to observe how the subcycle electron dynamics is better described using ${\omega }_P$. The concept of ${\omega }_P$, however, can be applied to a large variety of scenarios, not only within the strong-field physics domain.

We investigate few-photon ionization of lithium atoms prepared in the polarized $2p(m_{\ell }=+1)$ state when subjected to femtosecond light pulses with left- or right-handed circular polarization at wavelengths between 665 and 920 nm. We consider whether ionization proceeds more favorably for the electric field co- or counter-rotating with the initial electronic current density. Strong asymmetries are found and quantitatively analyzed in terms of “circular dichroism” (CD). While the intensity dependence of the measured CD values is rather weak throughout the investigated regime, a very strong sensitivity on the center wavelength of the incoming radiation is observed. While the co-rotating situation overall prevails, the counter-rotating geometry is strongly favored around 800 nm due to the $2p\text{−}3s$ resonant transition, which can only be driven by counter-rotating fields. The observed features provide insights into the helicity dependence of light-atom interactions, and on the possible control of electron emission in atomic few-photon ionization by polarization-selective resonance enhancement.

We present in this paper a Herriott-cavity-assisted closed-loop Xe isotope gas comagnetometer. In this system, ${}^{129}\mathrm{Xe}$ and ${}^{131}\mathrm{Xe}$ atoms are pumped and probed by polarized Rb atoms and continuously driven by oscillating magnetic fields, whose frequencies are kept on resonance by phase-locked loops. Different from other schemes, we use a Herriott cavity to improve the Rb magnetometer sensitivity instead of the parametric modulation method, and this passive method is designed to improve the system stability while maintaining the sensitivity. This system has demonstrated an angle random walk of $0.06{}^{\circ }/{\mathrm{h}}^{1/2}$ and a bias instability of $0.2{}^{\circ }$/h (0.15 $\mu \mathrm{Hz}$) with a bandwidth of 1.5 Hz. By adding a closed-loop Rb isotope comagnetometer, we can extend this system to dual simultaneously working comagnetometers sharing the same cell. This extended system has wide applications in precision measurements, where we can simultaneously and independently measure the coupling of anomalous fields with proton spin and neutron spin.

Optical feedback in a semiconductor laser was recently used for photonic microwave square-wave generation by invoking quasiperiodicity for repeated switching between a stable stage of constant intensity and an unstable stage of intensity oscillations. Based on the Lang-Kobayashi model for a laser with external-cavity feedback, we investigate such stable-unstable switching dynamics. First, the model is verified to produce the stable-unstable switching dynamics as the feedback strength and delay are adjusted. The model agrees with former experiments in spite of previous doubts. Then, the analytical Hopf boundary for the minimum linewidth mode is derived to be matching the boundaries of the switching regions. Moreover, the trajectory in the plane of instantaneous frequency and gain is analyzed during the stable stage, where the laser is found to follow an ellipse in visiting different external cavity modes. Most importantly, an analytical derivation yields the switching period $\tau +{\tau }_{\varepsilon }$ that is slightly expanded from the feedback delay time $\tau$, where the expansion time ${\tau }_{\varepsilon }$ is found to be inversely proportional to the feedback strength. Such a switching period expansion explains the generation of a shifted frequency component below $1/\tau$ in quasiperiodic laser dynamics. The results generally contribute to understanding the switching dynamics in photonic microwave generation.

An analytic approach to investigate the zero-temperature time evolution of the Jaynes-Cummings system with cavity losses is developed. With realistic coupling between the cavity and the environment assumed, a simple master equation is derived, leading to the explicit analytic solution for the resonant case. This solution is suitable for analyses not only on single-excitation states but also on many-excitation states, which enables us to investigate the photon coherent state and to observe sharp collapses and revivals under dissipation. For the off-resonant case, on the other hand, the present study presents an analytic, systematic method instead. We examine the small and large detuning limits and discuss the condition where the widely used phenomenological treatment is justified. Explicit evaluations of the time evolutions for various initial states with finite detuning are also presented.

In this work, we study the measurement transition for a coherent squeezed pointer state through a transition factor $\mathrm{\Gamma }$ that involves a system-pointer coupling by using an arbitrary measured observable $A$. In addition, we show that the shift in the pointer's position and momentum establishes a relationship with a new value defined as the transition value, which generalizes the weak value as well as the conditional expectation value. Furthermore, a strategy is introduced to achieve different measurement regimes by just adjusting the $r$ and ${\varphi }_{\xi }$ parameters of the coherent squeezed pointer state, opening an interesting way to test quantum mechanics foundations. Our scheme has been theoretically applied in a trapped ion illuminated by a bichromatic laser beam, with a high potential to be implemented in future experimental setups.