Photon pair production at flavour factories with per mille accuracy

We present a high-precision QED calculation, with 0.1% theoretical accuracy, of two photon production in e⁺e⁻$e^{+}{e}^{-}$ annihilation, as required by more and more accurate luminosity monitoring at flavour factories. The accuracy of the approach, which is based on the matching of exact next-to-leading order corrections with a QED Parton Shower algorithm, is demonstrated through a detailed analysis of the impact of the various sources of radiative corrections to the experimentally relevant observables. The calculation is implemented in the latest version of the event generator BabaYaga, available for precision simulations of photon pair production at e⁺e⁻$e^{+}{e}^{-}$ colliders of moderately high energies.     

Grassmann phase space theory and the Jaynes–Cummings model

The Jaynes-Cummings model of a two-level atom in a single mode cavity is of fundamental importance both in quantum optics and in quantum physics generally, involving the interaction of two simple quantum systems—one fermionic system (the TLA), the other bosonic (the cavity mode). Depending on the initial conditions a variety of interesting effects occur, ranging from ongoing oscillations of the atomic population difference at the Rabi frequency when the atom is excited and the cavity is in an n$n$-photon Fock state, to collapses and revivals of these oscillations starting with the atom unexcited and the cavity mode in a coherent state. The observation of revivals for Rydberg atoms in a high-Q microwave cavity is key experimental evidence for quantisation of the EM field. Theoretical treatments of the Jaynes-Cummings model based on expanding the state vector in terms of products of atomic and n$n$-photon states and deriving coupled equations for the amplitudes are a well-known and simple method for determining the effects.     

Advances in relativistic molecular quantum mechanics

A quantum mechanical equation HΨ=EΨ$H\Psi =E\Psi$ is composed of three components, viz., Hamiltonian H$H$, wave function Ψ$\Psi$, and property E(λ)$E\left(\lambda \right)$, each of which is confronted with fundamental issues in the relativistic regime, e.g., (1) What is the most appropriate relativistic many-body Hamiltonian? How to solve the resulting equation? (2) How does the relativistic wave function behave at the coalescence of two electrons? How to do relativistic explicit correlation? (3) How to formulate relativistic properties properly?, to name just a few. It is shown here that the charge-conjugated contraction of Fermion operators, dictated by the charge conjugation symmetry, allows for a bottom-up construction of a relativistic Hamiltonian that is in line with the principles of quantum electrodynamics (QED). Various approximate but accurate forms of the Hamiltonian can be obtained based entirely on physical arguments. In particular, the exact two-component Hamiltonians can be formulated in a general way to cast electric and magnetic fields, as well as electron self-energy and vacuum polarization, into a unified framework. While such algebraic two-component Hamiltonians are incompatible with explicit correlation, four-component relativistic explicitly correlated approaches can indeed be made fully parallel to the nonrelativistic counterparts by virtue of the ‘extended no-pair projection’ and the coalescence conditions. These findings open up new avenues for future developments of relativistic molecular quantum mechanics. In particular, ‘molecular QED’ will soon become an active and exciting field.     

Superluminality and UV completion

The idea that the existence of a consistent UV completion satisfying the fundamental axioms of local quantum field theory or string theory may impose positivity constraints on the couplings of the leading irrelevant operators in a low-energy effective field theory is critically discussed. Violation of these constraints implies superluminal propagation, in the sense that the low-frequency limit of the phase velocity vph(0)$v_{\mathrm{ph}}(0)$ exceeds c  . It is explained why causality is related not to vph(0)$v_{\mathrm{ph}}(0)$ but to the high-frequency limit vph(∞)$v_{\mathrm{ph}}(\infty )$ and how these are related by the Kramers–Kronig dispersion relation, depending on the sign of the imaginary part of the refractive index Imn(ω)$\mathrm{Im}n(\omega )$ which is normally assumed positive. Superluminal propagation and its relation to UV completion is investigated in detail in three theories: QED in a background electromagnetic field, where the full dispersion relation for n(ω)$n(\omega )$ is evaluated numerically and the role of the null energy condition T_μνkμkν?0$T_{\mu \nu }{k}^{\mu }{k}^{\nu }?0$ is highlighted; QED in a background gravitational field, where examples of superluminal low-frequency phase velocities arise in violation of the positivity constraints; and light propagation in coupled laser–atom Λ  -systems exhibiting Raman gain lines with Imn(ω)<0$\mathrm{Im}n(\omega )<0$. The possibility that a negative Imn(ω)$\mathrm{Im}n(\omega )$ must occur in quantum field theories involving gravity to avoid causality violation, and the implications for the relation of IR effective field theories to their UV completion, are carefully analysed.     

Effective cavity pumping from weakly coupled quantum dots

We derive the effective cavity pumping and decay rates for the master equation of a quantum dot–microcavity system in the presence of N$N$ weakly coupled dots. We show that the in-flow of photons is not linked to the out-flow by thermal equilibrium relationships.     

Probing Planckian physics in de Sitter space with quantum correlations

We study the quantum correlation and quantum communication channel of both free scalar and fermionic fields in de Sitter space, while the Planckian modification presented by the choice of a particular α$\alpha$-vacuum has been considered. We show the occurrence of degradation of quantum entanglement between field modes for an inertial observer in curved space, due to the radiation associated with its cosmological horizon. Comparing with standard Bunch–Davies choice, the possible Planckian physics causes some extra decrement on the quantum correlation, which may provide the means to detect quantum gravitational effects via quantum information methodology in future. Beyond single-mode approximation, we construct proper Unruh modes admitting general α$\alpha$-vacua, and find a convergent feature of both bosonic and fermionic entanglements. In particular, we show that the convergent points of fermionic entanglement negativity are dependent on the choice of α$\alpha$. Moreover, an one-to-one correspondence between convergent points H_c$H_c$ of negativity and zeros of quantum capacity of quantum channels in de Sitter space has been proved.     

Electron interactions in an antidot in the integer quantum Hall regime

A quantum antidot, a submicron depletion region in a two-dimensional electron system, has been actively studied in the past two decades, providing a powerful tool for understanding quantum Hall systems. In a perpendicular magnetic field, electrons form bound states around the antidot. Aharonov–Bohm resonances through such bound states have been experimentally studied, showing interesting phenomena such as Coulomb charging, h/2e$h/2e$ oscillations, spectator modes, signatures of electron interactions in the line shape, Kondo effect, etc. None of them can be explained by a simple noninteracting electron approach. Theoretical models for the above observations have been developed recently, such as a capacitive-interaction model for explaining the h/2e$h/2e$ oscillations and the Kondo effect, numerical prediction of a hole maximum-density-droplet antidot ground state, and spin-density-functional theory for investigating the compressibility of antidot edges. In this review, we summarize such experimental and theoretical works on electron interactions in antidots.     

The dynamics of quantum correlation with two controlled qubits under classical dephasing environment

We discuss the quantum correlation dynamics of two qubits controlled through the application of π$\pi$-pulses under classical dephasing non-Markovian environment. It is shown that the quantum discord (QD) and one-norm geometric quantum discord (one-norm GQD) between the two qubits, which are prepared initially in the three-parameter-X$X$-type quantum states, depend strongly on non-Markovian properties and the time difference between adjacent pulses. The freezing time of discord and one-norm GQD can be lengthened for appropriate pulse separate time and pulse numbers. And the freezing time of one-norm GQD is longer slightly than QD for both Markovian and non-Markovian cases. What is more, we find that double sudden changes of one-norm GQD can appear only for some initial parameters when π$\pi$-pulses are applied.     

Sharing of classical and quantum correlations via XY interaction

The sharing of classical and quantum correlations via XY interaction is investigated. The model includes two identical networks consisting of n$n$ nodes, the i$i$th node of one network sharing a correlated state with the j$j$th node of the other network, while all other nodes are initially unconnected. It is shown that classical correlation, quantum discord as well as entanglement can be shared between any two nodes of the network via XY interaction and that quantum information can be transferred effectively between them. It is found that there is no simple dominating relation between the quantum correlation and entanglement in inertial system.     

Improved quantum state transfer via quantum partially collapsing measurements

In this work, we present a general scheme to improve quantum state transfer (QST) by taking advantage of quantum partially collapsing measurements. The scheme consists of a weak measurement performed at the initial time on the qubit encoding the state of concern and a subsequent quantum reversal measurement at a desired time on the destined qubit. We determine the strength q_r$q_r$ of the post quantum reversal measurement as a function of the strength p$p$ of the prior weak measurement and the evolution time t$t$ so that near-perfect QST can be achieved by choosing p$p$ close enough to 1, with a finite success probability, regardless of the evolution time and the distance over which the QST takes place. The merit of our scheme is twofold: it not only improves QST, but also suppresses the energy dissipation, if any.     

The Delayed-Choice Quantum Eraser Leaves No Choice

International Journal of Theoretical Physics - A realizable delayed-choice quantum eraser, using a modified Mach-Zehnder (MZ) interferometer and polarization entangled photons, is theoretically...     

A phenomenology analysis of the tachyon warm inflation in loop quantum cosmology

We investigate the warm inflation condition in loop quantum cosmology. In our consideration, the system is described by a tachyon field interacted with radiation. The exponential potential function, V(?)=V₀e−α?$V(?)=V_0{e}^{-\alpha ?}$, with the same order parameters V₀$V_0$ and α, is taken as an example of this tachyon warm inflation model. We find that, for the strong dissipative regime, the total number of e-folds is less than the one in the classical scenario, and for the weak dissipative regime, the beginning time of the warm inflation will be later than the tachyon (cool) inflation.     

Possible corrections to quantum mechanical predictions in a hidden variable model

We derive possible corrections to the statistical predictions of quantum mechanics in measurement over an ensemble of identically prepared systems based on a hidden variable model of quantization developed in the previous work. The corrections are characterized by a dimensionless parameter σ$\sigma$ and the prediction of quantum mechanics is reproduced in the formal limit σ→0$\sigma \to 0$. Quantum mechanics is argued to be reliable for a sufficiently low quantum number.     

Cosmological dynamics with modified induced gravity on the normal DGP branch

In this Letter we investigate cosmological dynamics on the normal branch of a DGP-inspired scenario within a phase space approach where induced gravity is modified in the spirit of f(R)$f(R)$-theories. We apply the dynamical system analysis to achieve the stable solutions of the scenario in the normal DGP branch. Firstly, we consider a general form of the modified induced gravity and we show that there is a standard de Sitter point in phase space of the model. Then we prove that this point is stable attractor only for those f(R)$f(R)$ functions that account for late-time cosmic speed-up.