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Stefan Evans

Publications and source records attributed to Stefan Evans.

17 recordsLinked to original sources
arXiv2026

Non-Hermitian photon number filtering using N00N state Bloch oscillations

We explore Bloch oscillations of $N=1$ and $N=2$ photonic N00N states, using an array of linearly growing effective index waveguides with a simulated asymmetric loss profile. Starting with equal probability input $N=1$ and $N=2$ N00N states, and siphoning off a portion of the waveguides near the half Bloch period, we selectively output specific photon number states. Tuning the input phase, we achieve dynamic switching between 2-photon dominated (80\% of the output) and 1-photon dominated (90\% of the output) cases. This offers a path to improved state preparation in photonic circuits used in computing and networking.

quant-ph↗
arXiv2026

Quantum Radar Cross Section with two-photon entangled states

We study two-photon entangled states for quantum radar cross section (QRCS), which is an extension of a single-photon QRCS formula. Since signal-idler entanglement does not provide any enhancement of the QRCS [Brandsema's PhD Thesis (2017)], we focus on signal-signal entanglement and derive the corresponding biphoton QRCS. We show that it can provide an enhancement over the single-photon QRCS and two-photon separable QRCS, where the performance is evaluated for various two-dimensional target geometries in monostatic/bistatic configurations. Furthermore, using the double-Gaussian approximation, we derive QRCS formula for biphoton states with arbitrary degree of entanglement and compute the resulting scattering patterns.

quant-ph↗
arXiv2026

Noise mitigation in quantum enhanced fiber optic gyroscopes

We analyze noise in a quantum-enhanced fiber optic gyroscope (FOG), focusing on one of the leading sources of phase uncertainty - uncorrelated photon saturation. Taking a squeezed state input as a source for N00N states, we compute the uncorrelated false coincidence counts at the optimal phase bias, and determine an upper limit to the squeezed amplitude $ξ$ which allows for sub-shot noise precision. As examples, we apply parameters of present-day quantum FOG experiments, and determine the maximum possible precision enhancement based on their respective $ξ$ and optimal phase bias points. Aiming to future FOG setups with higher N00N state fluxes, our result highlights the need to transition to multimode states to bypass the $ξ$ limitation, such as photon pairs generated by the dynamical Casimir effect.

quant-ph↗
arXiv2025

Analysis of polymerized superconducting circuits

We apply polymer quantization, a quantization technique sometimes used in high energy physics, to several superconducting circuits including: transmons, transmission line resonators, and LC circuits. In the case of transmon qubits and transmission line resonators, experimental predictions are very close to what is found with canonical quantization, though in this approach constant charge offsets can be interpreted as quantization ambiguities. In the case of LC circuits, polymer quantization predicts nonlinearities which are not present in the canonical approach. Based on this analysis we design and analyze a qubit which uses a meander inductor instead of a Josephson junction. Implications for qubit performance and fabrication are discussed. Given a choice for an effective phase operator, relevant parameters such as anharmonicity, frequency, and dispersive shifts are calculated for this meander inductor based qubit.

quant-ph↗
arXiv2024

Axion-induced Casimir force between nuclei and dynamical axion pair creation

We study the interaction between axions and nuclei by combining the Peccei-Quinn mechanism with results from quantum chromo-dynamics (QCD) which imply that the QCD condensates are reduced within nuclear matter. Thus, the effective axion mass is also reduced, yielding a finite axion-nucleon scattering cross section. Even in the absence of real axions, this interaction would manifest itself in a Casimir type attraction between two nuclei. Finally, accelerated nuclei can create entangled pairs of axions via the dynamical Casimir effect (or as signatures of the Unruh effect).

hep-ph↗
arXiv2024

Everlasting interaction: polarization summation without a Landau pole

We propose an external field approach to evaluating effective action allowing the interaction to act everywhere at all times (everlasting). Requiring that the asymptotic gauge fields are always-interacting, we implement displacement fields encoding polarization corrections into the derivation of effective action. The result is a novel polarization summation for one-cut reducible loop diagrams, which can be applied to two cases: transient quasi-constant electromagnetic fields, and everlasting interactions. In the first case, a perturbative expansion of our result recovers the Schwinger-Dyson reducible diagram series with a Landau pole. The everlasting summation evaluated in nonperturbative fashion removes the Landau pole, providing a new avenue for modeling strongly interacting theories.

hep-ph↗
arXiv2023

Search for axion resonances in vacuum birefringence with three-beam collisions

We consider birefringent (i.e., polarization changing) scattering of x-ray photons at the superposition of two optical laser beams of ultra-high intensity and study the resonant contributions of axions or axion-like particles, which could also be short-lived. Applying the specifications of the Helmholtz International Beamline for Extreme Fields (HIBEF), we find that this set-up can be more sensitive than previous light-by-light scattering (birefringence) or light-shining-through-wall experiments in a certain domain of parameter space. By changing the pump and probe laser orientations and frequencies, one can even scan different axion masses.

hep-ph↗
arXiv2023

Improving Euler-Heisenberg-Schwinger effective action with dressed photons

We implement a longstanding proposal by Weisskopf to apply virtual polarization corrections to the in/out external fields in study of the Euler-Heisenberg-Schwinger effective action. Our approach requires distinguishing the electromagnetic and polarization fields based on mathematical tools developed by Białynicki-Birula, originally for the Born-Infeld action. Our solution is expressed as a differential equation where the one-loop effective action serves as input. As a first result of our approach, we recover the higher-order one-cut reducible loop diagrams discovered by Gies and Karbstein.

hep-ph↗
arXiv2022

Singular properties of QED vacuum response to applied quasi-constant electromagnetic fields

Employing the Bogoliubov coefficient summation method and introducing the gyromagnetic ratio $g\neq 2$ we derive an explicit functional form of $\mathfrak{Im}V^\mathrm{EHS}_g$, the imaginary part of Euler-Heisenberg-Schwinger (EHS) type effective action. We show that $\mathfrak{Im}V^\mathrm{EHS}_g$ is periodic in $g$ for any (quasi-)constant electromagnetic field configuration, and equal to the imaginary part obtained using a periodic in $g$ Ramanujan integrand in the proper time representation of $V^\mathrm{EHS}_g$. This validates the Ramanujan representation of $V^\mathrm{EHS}_g$ for both real and imaginary parts and allows writing the effective action in a suitably modified Schwinger proper time format. As a function of the ratio $b/a$ between ${\mathcal{B}} \to b$ and ${\mathcal{E}}\to a$ covariant generalizations of EM fields, we explore the singular properties of $\mathfrak{Im}V^\mathrm{EHS}_g$ at $g=2\pm 4k, k=0,\pm1,\pm2\ldots$ involving the pseudoscalar $ab\equiv \vec{\mathcal{E}}\cdot\vec{\mathcal{B}}$ in perturbative and nonperturbative behavior. We study the $e^-e^+$-decay vacuum instability, incorporating the physical value of $g-2$ vertex diagrams when summing infinite irreducible loops. We obtain an effective expansion parameter $χ_b=αb/2a$ ($α=e^2/4π$), characterizing the onset of nonperturbative in $g-2$ suppression of vacuum instability. We demonstrate the $χ_b$ domains for which perturbative expansion in $α$ breaks down: The EM vacuum subject to critical electric field strength is stabilized in magnetic-dominated \lq magnetar\rq\ environments. Considering separately the case of ${\mathcal{E}}$ and ${\mathcal{B}}$ fields, we generalize to all $g$ the temperature representation of the $V^\mathrm{EHS}_g$ effective action.

hep-ph↗
arXiv2022

A Cusp in QED at g=2

We explore nonperturbative properties of QED allowing a gyromagnetic ratio $g\ne g_{\rm D}\equiv 2$. We study the effective action $V_{\mathrm{eff}}$ for an arbitrarily strong constant and homogeneous field. Using the external field method, we find a cusp as a function of the gyromagnetic factor $g$ in: a) The QED $b_0$-renormalization group coefficient; b) A subclass of light-light scattering coefficients obtained in the long wavelength limit expansion. We recognize possibility of asymptotic freedom in an Abelian theory for certain domains of $g$.

hep-ph↗
arXiv2022

Emergence of periodic in magnetic moment effective QED action

We evaluate for the inhomogeneous static electric Sauter step potential the imaginary part of the emerging homogeneous in electric field effective Euler-Heisenberg-Schwinger action sourced by vacuum fluctuations of a charged particle with magnetic moment of arbitrary strength. The result is convergent for all values of gyromagnetic ratio $g$, periodic in $g$, with a cusp at $g=2$. We consider the relation to the QED beta-function which is also found to be periodic in $g$. We confirm presence of asymptotic freedom conditions using this novel method and document a wider range of $g$-values for which asymptotic freedom is present.

hep-ph↗
arXiv2021

Particle production at a finite potential step: Transition from Euler-Heisenberg to Klein paradox

Spontaneous pair production for spin-$1/2$ and spin-$0$ particles is explored in a quantitative manner for a static $\tanh$-Sauter potential step (SS), evaluating the imaginary part of the effective action. We provide finite-valued per unit-surface results, including the exact sharp-edge Klein paradox (KP) limit, which is the upper bound to pair production. At the vacuum instability threshold the spin-$0$ particle production can surpass that for the spin-$1/2$ rate. Presenting the effect of two opposite sign Sauter potential steps creating a well we show that spin-$0$ pair production, contrary to the case of spin-$1/2$, requires a smoothly sloped wall.

hep-ph↗
arXiv2020

Electron EM-mass melting in strong fields

We study the response of the electron mass to an externally applied electrical field. As a consequence of nonlinear electromagnetic (EM) effective action, the mass of a particle diminishes in the presence of an externally applied electric field. We consider modification of the muon anomalous magnetic moment $g-2$ due to electron loop insert in higher order. Since the virtual electron pair is in close proximity to the muon, it experiences strong field phenomena. We show that the current theory-experiment muon $g-2$ discrepancy could originate in the (virtual) electron mass \textit{non-perturbative} modification by the strong muon EM field. The magnitude of the electron mass modification can be also assessed via enhancement of $e^+e^-$-pair production in strong fields.

hep-ph↗
arXiv2018

Virtual axion-like particle complement to Euler-Heisenberg-Schwinger action

We modify action in an external electromagnetic field to include effects of virtual axion-like particle (ALP) excitations. A measurable addition to QED-Euler-Heisenberg-Schwinger (EHS) action is obtained and incorporated into experimental constraints placed on ALP mass and coupling to two photons. The regime of these constraints in which the ALP vacuum effect surpasses the EHS effect is characterized. We show that probing of the virtual vacuum effect offers an alternative method in search for physics related to ALPs.

hep-ph↗
arXiv2018

Vacuum stabilized by anomalous magnetic moment

An analytical result for Euler-Heisenberg effective action, valid for electron spin $g-$factor $|g|<2$, was extended to the domain $|g|>2$ via discovered periodicity of the effective action. This allows for a simplified computation of vacuum instability modified by the electrons measured $g=2.002319$. We find a strong suppression of vacuum decay into electron positron pairs when magnetic fields are dominant. The result is reminiscent of mass catalysis by magnetic fields.

hep-ph↗
arXiv2018

Strong fields and neutral particle magnetic moment dynamics

Interaction of magnetic moment of point particles with external electromagnetic fields experiences unresolved theoretical and experimental discrepancies. In this work we point out several issues within the relativistic quantum mechanics and the QED and we describe effects related to a new covariant classical model of magnetic moment dynamics. Using this framework we explore the invariant acceleration experienced by neutral particles coupled to an external plane wave field through the magnetic moment: we study the case of ultra relativistic Dirac neutrinos with magnetic moment in the range of $10^{-11}$ to $10^{-20}$ $μ_\mathrm{B}$; and we address the case of slowly moving neutrons. We explore how critical accelerations for neutrinos can be experimentally achieved in laser-pulse interactions. The radiation of accelerated neutrinos can serve as an important test distinguishing between Majorana and Dirac nature of neutrinos.

hep-ph↗