We show that at least $2kTln2$ of heat dissipation per qubit (in the register of a simulated circuit) occurs in measurement-based quantum computation according to Landauer's principle. This result is derived by using only the fundamental fact that quantum physics respects the no-signaling principle.

A long-standing problem on the classical capacity of bosonic Gaussian channels has recently been resolved by proving the minimum output entropy conjecture. It is also known that the ultimate capacity quantified by the Holevo bound can be achieved asymptotically by using an infinite number of channels. However, it is less understood to what extent the communication capacity can be reached if one uses a finite number of channels, which is a topic of practical importance. In this paper, we study the capacity of Gaussian communication, i.e., employing Gaussian states and Gaussian measurements to encode and decode information under a single-channel use. We prove that the optimal capacity of single-channel Gaussian communication is achieved by one of two well-known protocols, i.e., coherent-state communication or squeezed-state communication, depending on the energy (number of photons) as well as the characteristics of the channel. Our result suggests that the coherent-state scheme known to achieve the ultimate information-theoretic capacity is not a practically optimal scheme for the case of using a finite number of channels. We find that overall the squeezed-state communication is optimal in a small-photon-number regime whereas the coherent-state communication performs better in a large-photon-number regime.

Position-based quantum cryptography (PBQC) allows a party to use its geographical location as its only credential to implement various cryptographic protocols. Such a protocol may lead to important applications in practice. Although it has been shown that any PBQC protocol is breakable if the adversaries pre-share an arbitrarily large entangled state, the security of PBQC in the bounded-quantum-storage model is still an open question. In this paper, we study the performance of various PBQC protocols over a lossy channel under the assumption that no entanglement is pre-shared between adversaries. By introducing the decoy state idea, we show that an extended Bennett-Brassard-1984–type PBQC protocol implemented with a weak coherent source and realistic single-photon detectors can tolerate an overall loss (including both the channel loss and the detection efficiency) of 13 dB if the intrinsic quantum bit error rate is 1%. We also study a few continuous variable PBQC protocols and show that they suffer from a 3-dB loss limitation.

In this paper we study an archetypical scenario in which repulsive Casimir-Polder forces between an atom or molecule and two macroscopic bodies can be achieved. This is an extension of previous studies of the interaction between a polarizable atom and a wedge, in which repulsion occurs if the atom is sufficiently anisotropic and close enough to the symmetry plane of the wedge. A similar repulsion occurs if such an atom passes a thin cylinder or a wire. An obvious extension is to compute the interaction between such an atom and two facing wedges, which includes as a special case the interaction of an atom with a conducting screen possessing a slit, or between two parallel wires. To this end we further extend the electromagnetic multiple-scattering formalism for three-body interactions. To test this machinery we reinvestigate the interaction of a polarizable atom between two parallel conducting plates. In that case, three-body effects are shown to be small and are dominated by three- and four-scattering terms. The atom-wedge calculation is illustrated by an analogous scalar situation, described in the Appendix. The wedge-wedge-atom geometry is difficult to analyze because this is a scale-free problem. However, it is not so hard to investigate the three-body corrections to the interaction between an anisotropic atom or nanoparticle and a pair of parallel conducting cylinders and show that the three-body effects are very small and do not affect the Casimir-Polder repulsion at large distances between the cylinders. Finally, we consider whether such highly anisotropic atoms needed for repulsion are practically realizable. Since this appears rather difficult to accomplish, it may be more feasible to observe such effects with highly anisotropic nanoparticles.

A theoretical investigation of the feasibility of laser-cooling the ${}^{24}\mathrm{Mg}{}^{19}\mathrm{F}$ molecule is performed using ab initio calculations. The low-lying electronic states are determined by the multireference configuration-interaction (MRCI) method, where the Davidson modification (+Q) with the Douglas-Kroll-Hess scalar relativistic correction is also taken into account. The calculated spectroscopic constants are in excellent agreement with the available experimental data. The Franck-Condon factors (FCFs), radiative lifetimes, and radiative widths are verified by calculating the potential energy curves and the transition dipole moment of the $A{}^2\Pi \left({\nu }^{\prime }\right)\to X{}^2{\Sigma }^{+}\left(\nu \right)$ transition. Our calculation indicates that the $A{}^2\Pi ({\nu }^{\prime }=0)\to X{}^2{\Sigma }^{+}(\nu =0)$ transition provides highly diagonally distributed FCFs $(f_{00}=0.917)$ and a short radiative lifetime $(\tau =7.96\mathrm{ns})$ for the $A{}^2\Pi ({\nu }^{\prime }=0)$ state, which is short enough for rapid laser cooling. The required cooling wavelengths are in the ultraviolet region. A comprehensive scheme demonstrates the possibility of laser-cooling MgF. Moreover, the $C{}^2{\Sigma }^{+}$ state is confirmed to be a Rydberg state at the MRCI level, which is in line with experimental conjecture. The $B{}^2\Pi$ and $D{}^2{\Sigma }^{+}$ states are also reported, but they have not been observed to date.

In this work, we introduce a classical trajectory Monte Carlo (CTMC) methodology, specially conceived to provide a more accurate representation of charge-exchange processes between highly charged ions and $\mathrm{H}\left(1s\right)$ and ${\text{H}}^{*}(n=2)$. These processes are of particular relevance in power fusion reactor programs, for which charge-exchange spectroscopy has become a useful plasma diagnostics tool. To test the methodology, electron-capture reactions from these targets by ${\mathrm{C}}^{6+},{\mathrm{N}}^{7+}$, and ${\mathrm{O}}^{8+}$ are studied at impact energies in the $10\text{–}150\text{keV}/\mathrm{amu}$ range. State-selective cross sections are contrasted with those predicted by the standard microcanonical formulation of the CTMC method, the CTMC method with an energy variation of initial binding energies that produces an improved radial electron density, and the atomic orbital close-coupling method. The present results are found in to be much better agreement with the quantum-mechanical results than the results of former formulations of the CTMC method.

We obtained a simple theoretical unified parameter for the characterization of rotational population propagation of diatomic molecules in a periodic train of terahertz optical pulses around the condition of so-called quantum resonance. The parameter comprises the peak intensity and interval between the pulses, and the level energies of the initial and final rotational states of the molecule. Using the unified parameter, we can predict the upper and lower boundaries of probability localization on the rotational level network, including the effect of centrifugal distortion. The unified parameter was tentatively derived from an analytical expression obtained by performing rotating-wave approximation and spectral decomposition of the time-dependent Schrödinger equation under an assumption of time-order invariance. The validity of the parameter was confirmed by comparison with numerical simulations for isotope-selective rotational excitation of KCl molecules.

Experimental realizations of disorder in optical lattices generate a distribution of the Bose-Hubbard (BH) parameters, like on-site potentials, hopping strengths, and interaction energies. We analyze this distribution for bosons in a two-dimensional bichromatic quasiperiodic potential by determining the generalized Wannier functions and calculating the corresponding BH parameters. Using a local mean-field cluster analysis, we study the effect of the corresponding disorder on the phase diagrams. We find a substantial amount of disorder in the hopping strengths, which produces strong deviations from the phase diagram of the disordered BH model with purely random on-site potentials.

We study the generation of high-harmonic radiation by Bloch electrons in a model transparent solid driven by a strong midinfrared laser field. We solve the single-electron time-dependent Schrödinger equation (TDSE) using a velocity-gauge method [M. Korbman et al., New J. Phys. 15, 013006 (2013)] that is numerically stable as the laser intensity and number of energy bands are increased. The resulting harmonic spectrum exhibits a primary plateau due to the coupling of the valence band to the first conduction band, with a cutoff energy that scales linearly with field strength and laser wavelength. We also find a weaker second plateau due to coupling to higher-lying conduction bands, with a cutoff that is also approximately linear in the field strength. To facilitate the analysis of the time-frequency characteristics of the emitted harmonics, we also solve the TDSE in a time-dependent basis set, the Houston states [J. B. Krieger and G. J. Iafrate, Phys. Rev. B 33, 5494 (1986)], which allows us to separate interband and intraband contributions to the time-dependent current. We find that the interband and intraband contributions display very different time-frequency characteristics. We show that solutions in these two bases are equivalent under a unitary transformation but that, unlike the velocity-gauge method, the Houston state treatment is numerically unstable when more than a few low-lying energy bands are used.

We analyze the occurrence of normal-mode coupling (NMC) in bimodal lasers attributed to the collective interaction of the cavity field with a mesoscopic number of quantum dots (QDs). In contrast to the conventional NMC, here we observe locking of the frequencies and splitting of the linewidths of the system's eigenmodes in the coherent coupling regime. The theoretical analysis of the incoherent regime is supported by experimental observations where the emission spectrum of one of the orthogonally polarized modes of a bimodal QD micropillar laser demonstrates a distinct two-peak structure.

We study entanglement creation between two independent $\mathit{XX}$ chains, which are repeatedly coupled locally to spin-1/2 Bell pairs. We show analytically that in the steady state the entanglement of the Bell pairs is perfectly transferred to the chains, generating large-scale interchain pair correlations. However, before the steady state is reached, within a growing causal region around the interacting locus the chains are found in a current driven nonequilibrium steady state (NESS). In the NESS, the chains cross entanglement decays exponentially with respect to the distance to the boundary sites with a typical length scale which is inversely proportional to the driving current.

Quantum metrology is a general term for methods to precisely estimate the value of an unknown parameter by actively using quantum resources. In particular, some classes of entangled states can be used to significantly suppress the estimation error. Here we derive a formula for rigorously evaluating an upper bound for the estimation error in a general setting of quantum metrology with arbitrary finite data sets. Unlike in the standard approach, where lower bounds for the error are evaluated in an ideal setting with almost infinite data, our method rigorously guarantees the estimation precision in realistic settings with finite data. We also prove that our upper bound shows the Heisenberg limit scaling whenever the linearized uncertainty, which is a popular benchmark in the standard approach, shows it. As an example, we apply our result to a Ramsey interferometer, and numerically show that the upper bound can exhibit the quantum enhancement of precision for finite data.

There are different results in the literature for the interaction energy of two separated nonidentical atoms in the case when one of the atoms is prepared in an excited state. Moreover, there are different ways to define this interaction energy. If the interaction energy is defined as a shift in the energy of the initial state of the combined atomic system, it is possible to carry out a time-dependent calculation that provides an unambiguous method for obtaining this shift. The time-dependent calculations lead to an interaction energy that is an oscillatory function of the interatomic separation, in contrast to the nonoscillatory behavior that is predicted using an alternative theory based on time-independent perturbation theory.

Rotated quadratures carry the phase-dependent information of the electromagnetic field, so they are somehow conjugate to the photon number. We analyze this noncanonical pair, finding an exact uncertainty relation, as well as a couple of weaker inequalities obtained by relaxing some restrictions of the problem. We also find the intelligent states saturating that relation and complete their characterization by considering extra constraints on the second-order moments of the variables involved. Using these moments, we construct performance measures tailored to diagnose photon-added and Schrödinger-cat-like states, among others.

Quantum metrology and quantum information necessitate a profound study of suitable states. Attenuations induced by free-space communication links or fluctuations in the generation of such states limit the quantum enhancement in possible applications. For this reason we investigate quantum features of mixtures of so-called NOON states propagating in atmospheric channels. First, we show that noisy NOON states can still yield a phase resolution beyond classical limitations. Second, we identify entanglement of noisy NOON states after propagation in fluctuating loss channels. To do so, we apply the partial transposition criterion. Our theoretical analysis formulates explicit bounds which are indispensable for experimental verification of quantum entanglement and applications in quantum metrology.

We propose a generalization of the color codes based on finite groups $G$. For non-Abelian groups, the resulting model supports non-Abelian anyonic quasiparticles and topological order. We examine the properties of these models such as their relationship to Kitaev quantum double models, quasiparticle spectrum, and boundary structure.

Distinguishing assigned quantum states with assigned probabilities via quantum measurements is a crucial problem for the transmission of classical information through quantum channels. Measurement operators maximizing the probability of correct discrimination have been characterized by Helstrom, Holevo, and Yuen since the 1970s. On the other hand, closed-form solutions are available only for particular situations enjoying high degrees of symmetry. As a suboptimal solution to the problem, measurement operators, directly determined from states and probabilities and known as square-root measurements (SRMs), were introduced by Hausladen and Wootters. These operators were also recognized to be optimal for pure states equipped with geometrical uniform symmetry (GUS). In this paper we discuss the optimality of the SRM and find necessary and sufficient conditions in order that SRM maximize the correct decision probabilities for the set of states formed by several constellations of GUS states. The results are applied to some specific examples concerning double constellations of quantum phase-shift keying and pulse-position modulation states, with possible applications to practical systems of quantum communications.

The $I\left(\left|\Omega \right|=1\right)–X{}^1{\Sigma }^{+}\left(0,0\right)$ band near 512 nm of thorium oxide has been recorded and analyzed field free and in the presence of both static electric and magnetic fields. The determined $T_{00}$, $B$, and $q$ field-free parameters for the $I\left(\right|\Omega |=1)(v=0)$ state are $(\text{in}\mathrm{c}{\mathrm{m}}^{-1})19539.3823\pm 0.0003,\mathrm{0.328\; 69}\pm \mathrm{0.000\; 03}$, and 0.001 54 ± 0.000 05. The Zeeman-induced shifts and splittings of the low-$J$ lines were analyzed to determine $g$ factors and suggest that the $I\left(\right|\Omega |=1)(v=0)$ state has a dominant ${}^1\Pi$ character. The Stark tuning of the low-$J$ lines was analyzed to determine the permanent electric dipole moment ${\mu }_{\mathrm{el}}$ for the $I\left(\right|\Omega |=1)(v=0)$ state of 4.25 ± 0.02 D.

Attosecond-magnetic-field-pulse generation is investigated using numerical solutions of the time-dependent Schrödinger equation for oriented ${{\mathrm{H}}_2}^{+}$ excited and ionized by intense $2\times {10}^{16} \mathrm{W}/{\mathrm{cm}}^2$ circularly polarized attosecond UV pulses. The results show that localized attosecond-magnetic-field pulses $B$ at the molecular center $\left(\mathbf{r}=\mathbf{0}\right)$ decrease in intensity with increasing attosecond-pulse wavelength, following a classical model. Magnetic-field minima are obtained at a specific laser-pulse wavelength $\lambda =55$ nm, which is attributed to ionization suppression. It is found that spatially localized coherent circular electron currents and wave packets are created and induce magnetic-field minima. At $\lambda =55$ nm, coherent excitation between the ground state and Rydberg states is created, giving rise to partial Rabi oscillations in population and doublets in molecular above-threshold-ionization photoelectron energy spectra. Pulse intensities are shown to influence these effects on the attosecond time scale through population variations.