Starting from a Lagrangian, the electromagnetic field is quantized in the presence of a body rotating along its axis of symmetry. Response functions and fluctuation-dissipation relations are obtained. A general formula for rotational friction and power radiated by a rotating dielectric body is obtained in terms of the dyadic Green's tensor. The Hamiltonian is determined and possible generalizations are discussed. As an example, the rotational friction and power radiated by a spherical dielectric in the vicinity of a semi-infinite dielectric plane is obtained and discussed in some limiting cases.

Passive decoy-state quantum key distribution (QKD) systems, proven to be more desirable than the active ones in some scenarios, also have the problem of device imperfections like intensity fluctuations. In this paper, the formula of key generation rate of the passive decoy-state protocol using the weak coherent pulse (WCP) source with intensity fluctuation is given, and then the influence of intensity fluctuations on the performance of the passive decoy-state protocol is rigorously characterized. From numerical simulations, it can be seen that intensity fluctuations have non-negligible influence on the performance of the passive decoy-state QKD protocol with WCP source. Most importantly, our simulations show that, under the same deviation of intensity fluctuations, the passive decoy-state method performs better than the active one decoy-state method and is close to the active vacuum + weak decoy-state method.

We present the development of a real-space and projected congruent transformation method for treating electron correlation in chemical systems. This method uses an explicitly correlated function for performing congruent transformation on the electronic Hamiltonian. As a result of this transformation, the electronic Hamiltonian is transformed into a sum of two-, three-, four-, five-, and six-particle operators. Efficient computational implementation of these many-particle operators continues to be challenging for application of the congruent transformation approach for many-electron systems. In this work, we present a projected congruent transformed Hamiltonian (PCTH) approach to avoid computation of integrals involving operators that couple more than two particles. The projected congruent transformation becomes identical to the real-space congruent transformation in the limit of infinite basis size. However, for practical calculations, the projection is always performed on a finite-dimensional space. We show that after representing the contributing expressions of the PCTH in terms of diagrams, it is possible to identify a subset of diagrams that can be summed up to infinite order. This technique, denoted as partial infinite-order summation (PIOS), partly alleviates the limitation from the finite-basis representation of the PCTH method. The PCTH and PCTH-PIOS methods were applied to an isoelectronic series of 10-electron systems $(\mathrm{Ne},\mathrm{HF},{\mathrm{H}}_2\mathrm{O},{\mathrm{NH}}_3,{\mathrm{CH}}_4)$ and results were compared with configuration interaction (CISD) calculations. The results indicate that the PCTH-PIOS method can treat electron-electron correlations while avoiding explicit construction and diagonalization of the Hamiltonian matrix.

An analytic operator solution of a generalized quantum mechanical Hamiltonian of two-mode Bose-Einstein condensates (BECs) is obtained for short-time dynamics and the same is used to investigate the nonclassical properties of the modes present in the system. Nonclassical characters are observed by means of single-mode and intermodal quadrature squeezing, single-mode and intermodal sub-Poissonian boson statistics, and intermodal entanglement. In addition to the traditionally studied lower-order nonclassical properties, signatures of higher-order nonclassical characters of two-mode BEC systems are also obtained by investigating the possibility of higher-order antibunching and higher-order entanglement. The mutual relation among the observed nonclassicalities and their evolution (variation) with time and the ratio of the single boson tunneling amplitude ($\varepsilon$) and the coupling constant for the intramodal interaction ($\kappa$) are also reported.

A detailed scheme is established for the direct generation of optical vortices, signifying light endowed with orbital angular momentum. In contrast to common techniques based on the tailored conversion of the wave front in a conventional beam, this method provides for the direct spontaneous emission of photons with the requisite field structure. This form of optical emission results directly from the electronic relaxation of a delocalized exciton state that is supported by a ringlike array of three or more nanoscale chromophores. An analysis of the conditions leads to a general formulation revealing a requirement for the array structure to adhere to one of a restricted set of permissible symmetry groups. It is shown that the coupling between chromophores within each array leads to an energy level splitting of the exciton structure, thus providing for a specific linking of exciton phase and emission wavelength. For emission, arrays conforming to one of the given point-group families’ doubly degenerate excitons exhibit the specific phase characteristics necessary to support vortex emission. The highest order of exciton symmetry, corresponding to the maximum magnitude of electronic orbital angular momentum supported by the ring, provides for the most favored emission. The phase properties of the emission produced by the relaxation of such excitons are exhibited on plots which reveal the azimuthal phase progression around the ring, consistent with vortex emission. It is proven that emission of this kind produces electromagnetic fields that map with complete fidelity onto the phase structure of a Laguerre-Gaussian optical mode with the corresponding topological charge. The prospect of direct generation paves the way for practicable devices that need no longer rely on the modification of a conventional laser beam by a secondary optical element. Moreover, these principles hold promise for the development of a vortex laser, also based on nanoscale exciton decay, enabling the production of coherent radiation with a tailor-made helical wave front.

We propose a comprehensive physical model explaining the conical character of broadband terahertz generation from femtosecond two-color laser-induced air plasma filaments. We show, that, in contrast to other models, emission is always conical, resulting from phase matching of the radiation produced inside the filament combined with a partial back reflection of the generated terahertz field from the filament itself due to the frequency-dependent critical plasma density. The obtained conical angle varies from 2${}^{\circ }$ to 10${}^{\circ }$, depending on the plasma density distribution and filament length. Unlike previously proposed models, our model shows good agreement with our experiments as well as a wide range of experimental findings from the literature.

This paper shows that we cannot clone a given entangled state of a bipartite system with stochastic local operation and classical communication (SLOCC). It is well known that we cannot create entanglement from scratch under SLOCC; an interesting interpretation of our work implies that even with another entangled state being a catalyst, we cannot realize such a process either.

It is shown that noncentrosymmetric materials with bulk second-order nonlinear susceptibility can be used to generate strongly antibunched radiation at an arbitrary wavelength, solely determined by the resonant behavior of suitably engineered coupled microcavities. The proposed scheme exploits the unconventional photon blockade of a coherent driving field at the input of a coupled cavity system, where one of the two cavities is engineered to resonate at both fundamental and second harmonic frequencies, respectively. Remarkably, the unconventional blockade mechanism occurs with reasonably low quality factors at both harmonics, and does not require a sharp doubly resonant condition for the second cavity, thus proving its feasibility with current semiconductor technology.

Feynman's formulation of quantum theory is remarkable in its combination of formal simplicity and computational power. However, as a formulation of the abstract structure of quantum theory, it is incomplete as it does not account for most of the fundamental mathematical structure of the standard von Neumann–Dirac formalism such as the unitary evolution of quantum states. In this paper, we show how to reconstruct the entirety of the finite-dimensional quantum formalism starting from Feynman's rules with the aid of a single physical postulate, the no-disturbance postulate. This postulate states that a particular class of measurements have no effect on the outcome probabilities of subsequent measurements performed. We also show how it is possible to derive both the amplitude rule for composite systems of distinguishable subsystems and Dirac's amplitude-action rule, each from a single elementary and natural assumption, by making use of the fact that these assumptions must be consistent with Feynman's rules.

Joint measurability of sharp quantum observables is determined pairwise, and so can be captured in a graph. We prove the converse: any graph whose vertices represent sharp observables and whose edges represent joint measurability is realized by quantum theory. This leads us to show that it is not always possible to use Neumark dilation to turn unsharp observables into sharp ones with the same joint measurability relations.

Quantum error-correcting codes have a distance parameter, conveying the minimum number of single spin errors that could cause error correction to fail. However, the success thresholds of the finite per-qubit error rate that have been proven for the likes of the toric code require them to work well beyond this limit. We argue that, without the assumption of being below the distance limit, the success of error correction is not only contingent on the noise model, but what the noise model is believed to be. Any discrepancy must adversely affect the threshold rate, and risks invalidating existing threshold theorems. We prove that for the two-dimensional (2D) toric code, suitable thresholds still exist by utilizing a mapping to the 2D random bond Ising model.

We report a refined determination of muonium 1$S$-2$S$ transition frequency and its isotope shift with deuterium by recalibrating the iodine reference lines using an optical frequency comb. The reference lines for the muonium and deuterium 1$S$-2$S$ transitions are determined with a precision of $2.4\times {10}^{-10}$ and $1.7\times {10}^{-10}$, respectively. An updated muonium-deuterium 1$S$-2$S$ isotope-shift frequency is derived from these references to be 11 203 464.9(9.2)(4.0) MHz, in agreement with an updated bound-state quantum-electrodynamics prediction based on 2010 adjustments of Committee on Data for Science and Technology and 2.3 times better in the systematic uncertainty than V. Meyer et al. [Phys. Rev. Lett. 84, 1136 (2000)].

Data on the $(5s_{1/2}+5p_{1/2})1_g$ ${}^{87}{\text{Rb}}_2$ state under the $D_1$ limit, provided by a photoassociation experiment on cold atoms, have been analyzed by an improved LeRoy-Bernstein (LRB) approach including linear and nonlinear terms and provide a $c_6$ value of the potential. To do that, using a model for hyperfine structure shifts, we have first subtracted the hyperfine effects in the split lines and deduced the vibrational energies. Then, we have compared three LRB-type models to fit the data. We conclude that the second-order improved LRB is well suited and allows us to deduce an experimental value of $c_6$ [$c_6=(15.14\pm 0.05)\times {10}^4$ a.u.].

In this article, the electronic interaction of medium-energy ions with hafnium dioxide is studied. Stopping cross sections for He ions in the energy range from 30 to 160 keV have been measured in backscattering experiments from thin films of HfO${}_2$ on Si using time-of-flight medium-energy ion scattering. The observed energy loss for helium ions is found to be high compared to expectations from earlier results for hydrogen in HfO${}_2$, a result which indicates a contribution from energy-loss processes that is different from direct excitation of electron-hole pairs in close collisions. Furthermore, data exhibit a significant deviation from velocity proportionality. A discussion of the results, together with data from studies for H and He in SiO${}_2$, show characteristic differences which are traced back to different electronic structures of the target materials and their influence on the charge-exchange channels that are active in the interaction with helium. Charge exchange, in turn, via shifted mean-charge states, will influence the observed ionization along the ion track. The results can furthermore serve as reference values for ion-beam-based depth profiling at medium and low ion energies.

We compute ground-state fidelity of the one-dimensional Bose-Hubbard model at unit filling factor. To this aim, we apply the density matrix renormalization group algorithm to systems with open and periodic boundary conditions. We find that fidelity differs significantly in the two cases and study its scaling properties in the quantum critical regime.

We derive a theoretical model which describes Bose-Einstein condensation in an open driven-dissipative system. It includes external pumping of a thermal reservoir, finite lifetime of the condensed particles, and energy relaxation. The coupling between the reservoir and the condensate is described with semiclassical Boltzmann rates. This results in a dissipative term in the Gross-Pitaevskii equation for the condensate, which is proportional to the energy of the elementary excitations of the system. We analyze the main properties of a condensate described by this hybrid Boltzmann–Gross-Pitaevskii model, namely, dispersion of the elementary excitations, bogolon distribution function, first-order coherence, dynamic and energetic stability, and drag force created by a disorder potential. We find that the dispersion of the elementary excitations of a condensed state fulfills the Landau criterion of superfluidity. The condensate is dynamically and energetically stable as longs as it moves at a velocity smaller than the speed of excitations. First-order spatial coherence of the condensate is found to decay exponentially in one dimension and with a power law in two dimensions, similarly with the case of conservative systems. The coherence lengths are found to be longer due to the finite lifetime of the condensate excitations. We compare these properties with those of a condensate described by the popular “diffusive” models in which the dissipative term is proportional to the local condensate density. In the latter, the dispersion of excitations is diffusive which as soon as the condensate is put into motion implies finite mechanical friction and can lead to an energetic instability.

We apply the number-conserving Bogoliubov theory to spinor Bose-Einstein condensates and show that the Goldstone magnon causes instability leading to fragmentation. Unlike the dynamical instability, where modes with complex eigenfrequencies grow exponentially, here the zero-energy mode exhibits algebraic growth. We also point out that a small fraction of thermally excited atoms enhances the fragmentation dynamics.

Casimir-type forces, such as those between two neutral conducting plates, or between a sphere, atom, or molecule and a plate, have been widely studied and are becoming of increasing significance, for example, in nanotechnology. A key challenge is to better understand, from a fundamental microscopic approach, why the Casimir force is in some circumstances attractive and in others repulsive. Here, we study the Casimir-Polder forces experienced by small quantum systems such as atoms or molecules in an optical cavity. In order to make the problem more tractable, we work in a (1+1)-dimensional setting, we take into account only the ground state and the first excited state of the atom, and we model the electromagnetic field as a scalar field with Dirichlet or Neumann boundary conditions. This allows us to determine the conditions for the Casimir force to be attractive or repulsive for individual atoms, namely, through the interplay of paramagnetic and diamagnetic vacuum effects. We also study the microscopic-macroscopic transition, finding that as the number of atoms in the cavity is increased, the atoms start to affect the Casimir force exerted on the cavity walls similarly to a dielectric medium.

Optical limiters are designed to transmit low-intensity light, while blocking the light with excessively high intensity. A typical passive limiter absorbs excessive electromagnetic energy, which can cause its overheating and destruction. We propose the concept of a photonic reflective limiter based on resonance transmission via a localized mode. Such a limiter does not absorb the high-level radiation, but rather reflects it back to space. Importantly, the nearly total reflection occurs within a broad frequency range and direction of incidence. The same concept can be applied to infrared and microwave frequencies.

We introduce quantum information engines that extract work from quantum states and a single thermal reservoir. They may operate under three general conditions—(1) unitarily steered evolution (US), driven by a restricted set of available Hamiltonians; (2) irreversible thermalization (IT), and (3) isothermal relaxation (IR)—and hence are called USITIR machines. They include novel engines without traditional feedback control mechanisms, as well as versions which also include them. Explicit constructions of USITIR engines are presented for one- and two-qubit states and their maximum extractable work is computed, which is optimal. Optimality is achieved when the notions of controllable thermalizability and density matrix controllability are fulfilled. Then many-body extensions of USITIR engines are also analyzed and conditions for optimal work extraction are identified. When they are not met, we measure their lack of optimality by means of newly defined uncontrollable entropies, which are explicitly computed for some selected examples. This includes cases of distinguishable and indistinguishable particles.