arXiv subjects

Xuan Ye

Publications and source records attributed to Xuan Ye.

16 recordsLinked to original sources

arXiv2026

Time Crystals in Coupled Exciton-Polariton Condensates

In this paper, we show that time crystals can emerge in coupled exciton-polariton condensates without periodic external driving, enabled instead by incoherent gain and dissipation channels inherent to semiconductor microcavities. We present a full quantum description of these processes that recovers the established effective theory at the mean-field level. We analytically determine the mean-field phase diagram for the time-crystalline phase and find that its emergence requires the ratio of Kerr nonlinearity to nonlinear dissipation to exceed $\sqrt{5/4}$. Within this regime, the periodic oscillation of the particle numbers forms an attractor that is insensitive to the initial conditions. Numerical bifurcation diagrams reveal transitions between the time-crystalline phase and various steady phases, in excellent agreement with the analytical results. Using Bogoliubov perturbation theory, we evaluate the leading-order quantum corrections and find that, over a wide parameter range, these corrections remain periodic and much smaller than the mean-field background, thereby establishing the robustness of the time crystal.

cond-mat.quant-gas↗

arXiv2026

Regularized vacuum stress tensor of a scalar field as the inflaton or dark energy

We study the regularized vacuum stress tensor of scalar fields in maximally symmetric spacetime and assess the feasibility of driving primordial inflation or current cosmic acceleration by analyzing the existence of solutions to the Friedmann equation. We find that a conformally coupled scalar field with mass of order $10$ $M_{\text{pl}}$ can be a candidate for both the inflaton and dark energy, suggesting that these two components may have the same quantum origin. In contrast, a minimally coupled scalar field cannot serve as either the inflaton or dark energy regardless of its mass.

gr-qc↗

arXiv2026

Primordial Black Hole Formation in Dust-Radiation Bouncing Cosmologies

Primordial black holes (PBHs) provide a unique probe of the early Universe and may have an enhanced abundance in bouncing cosmologies, where a long contracting phase can amplify perturbations. We develop a unified framework to study PBH formation in dust-radiation bouncing cosmologies, focusing on the classical contracting phase so that the results are insensitive to bounce details. We compute the curvature power spectrum for an extremely small dust equation of state using a stable semi-analytical (adiabatic) method, derive the Jeans length of the two-fluid system using dynamical-system analysis and the WKB approximation, and extend the three-zone model from the single- to the two-fluid case to model local collapse. We implement two collapse criteria to obtain the curvature perturbation threshold for PBH formation and estimate PBH mass fractions for benchmark masses spanning low-mass ($10^{-17} M_{\odot}$) to supermassive ($10^{13} M_{\odot}$) scales. The critical curvature threshold is extremely small and nearly mass-independent over a broad range $(\zeta_c \sim 10^{-21}$ for $10^{-14}$ to $10^{13} M_{\odot})$, with deviations only near dust-radiation equality. Nevertheless, the square root of the curvature power spectrum at the relevant formation times is many orders of magnitude smaller, yielding vanishingly small PBH mass fractions across the benchmark masses. Compared with the pure-dust case, radiation pressure and the two-fluid collapse conditions significantly suppress PBH production, implying that substantial PBH formation in dust-radiation bouncing cosmologies would require additional mechanisms to amplify curvature perturbations.

gr-qc↗

arXiv2025

Adiabatic and point-splitting regularization of spin-1/2 field in de Sitter space

We study the regularization of a spin-1/2 fieldin the vacuum state in de Sitter space. We find that the 2nd order adiabatic regularization is sufficient to remove all UV divergences for the spectral stress tensor, as well as for the power spectrum. The regularized vacuum stress tensors of the massive field is maximally symmetric with the energy density remaining negative, and behaves as a ``negative" cosmological constant. In the massless limit it reduces smoothly to the zero stress tensor of the massless field, and there is no trace anomaly. We also perform the point-splitting regularization in coordinate space, and obtain the analytical, regularized correlation function and stress tensor, which agree with those from the adiabatic regularization. In contrast, the 4th order regularization is an oversubtraction, and changes the sign of the vacuum energy density. In the massless limit the 4th order regularized auto-correlation becomes singular and the regularized stress tensor does not reduce to the zero stress tensor of the massless field. These difficulties tell that the 4th order regularization is inadequate for the spin-1/2 massive field.

gr-qc↗

arXiv2025

Gravitational effects on Hong-Ou-Mandel interference in terrestrial laboratory

In this study, we investigate how Earth's gravitational field affects Hong-Ou-Mandel (HOM) interference experiments conducted in a terrestrial laboratory. To second order, we calculate the relativistic time delay from the null geodesic equation (particle perspective), while the phase shift and the associated effective time delay are derived from the Klein-Gordon equation (wave perspective). Since gravity influences both the temporal and spatial parts of the phase shift, these two time delays differ and lead to different coincidence probabilities. The previous HOM experiment conducted on a rotating platform suggests that the wave perspective can explain the experimental results. We further explore the frame dragging and redshift effects in an arbitrarily oriented rectangular interferometer under two distinct scenarios with different photon paths, measuring one effect in each scenario. We find that both effects can be amplified by increasing the number of light loops. Additionally, we emphasize that the next-to-leading order Sagnac effect, arising from gravitational acceleration, is comparable to the Thomas precession, the geodetic effect, and the Lense-Thirring effect. To detect the leading order Sagnac effect and the redshift effect caused by gravitational acceleration, we estimate the number of loops that photons should travel in the interferometer. Furthermore, we propose that the difference between two HOM patterns can be used as a probe to detect gravitational effects on quantum systems.

gr-qc↗

arXiv2025

The Conserved Effective Stress Tensor of Gravitational Wave

We present a detailed study of the effective stress tensor of gravitational wave (GW) as the source for the background Einstein equation and examine three candidates in literature. The second order perturbed Einstein tensor $G^{(2)}_{\mu\nu}$, up to a coefficient, proposed by Brill, Hartle, and Isaacson, has long been known to be covariantly nonconserved with respect to the background spacetime. We observe that $G^{(2)}_{\mu\nu}$ is not a true tensor on the background spacetime. More importantly, we find that, by expressing $G^{(2)}_{\mu\nu}$ in terms of the perturbed Hilbert-Einstein actions, the nonconserved part of $G^{(2)}_{\mu\nu}$ is actually canceled out by the perturbed fluid stress tensors in the back-reaction equation, or is vanishing in absence of fluid. The remaining part of $G^{(2)}_{\mu\nu}$ is just the conserved effective stress tensor $\tau_{\mu\nu}$ proposed by Ford and Parker. As the main result, we derive $\tau_{\mu\nu}$ for a general curved spacetime by varying the GW action and show its conservation using the equation of GW. The stress tensor $T_{\text{MT}}^{\mu\nu}$ proposed by MacCallum and Taub was based on an action $J_2$. We derive $T_{\text{MT}}^{\mu\nu}$ and find that it is nonconserved, and that $J_2$ does not give the correct GW equation in presence of matter. The difficulty with $J_2$ is due to a background Ricci tensor term, which should be also canceled out by the fluid term or vanishing in absence of fluid. We also demonstrate these three candidates in a flat Robertson-Walker spacetime. The conserved $\tau_{\mu\nu}$ has a positive energy density spectrum, and is adequate for the back-reaction in a perturbation scheme, while the two nonconserved stress tensors have a negative spectrum at long wavelengths and are unphysical.

gr-qc↗

arXiv2025

Quantum ensemble learning with a programmable superconducting processor

Quantum machine learning is among the most exciting potential applications of quantum computing. However, the vulnerability of quantum information to environmental noises and the consequent high cost for realizing fault tolerance has impeded the quantum models from learning complex datasets. Here, we introduce AdaBoost.Q, a quantum adaptation of the classical adaptive boosting (AdaBoost) algorithm designed to enhance learning capabilities of quantum classifiers. Based on the probabilistic nature of quantum measurement, the algorithm improves the prediction accuracy by refining the attention mechanism during the adaptive training and combination of quantum classifiers. We experimentally demonstrate the versatility of our approach on a programmable superconducting processor, where we observe notable performance enhancements across various quantum machine learning models, including quantum neural networks and quantum convolutional neural networks. With AdaBoost.Q, we achieve an accuracy above 86% for a ten-class classification task over 10,000 test samples, and an accuracy of 100% for a quantum feature recognition task over 1,564 test samples. Our results demonstrate a foundational tool for advancing quantum machine learning towards practical applications, which has broad applicability to both the current noisy and the future fault-tolerant quantum devices.

quant-ph↗

arXiv2024

Regularized stress tensor of vector fields in de Sitter space

We study the Stueckelberg field in de Sitter space, which is a massive vector field with the gauge fixing (GF) term $\frac{1}{2\zeta} (A^\mu\,_{;\, \mu})^2$. We obtain the vacuum stress tensor, which consists of the transverse, longitudinal, temporal, and GF parts, and each contains various UV divergences. By the minimal subtraction rule, we regularize each part of the stress tensor to its pertinent adiabatic order. The transverse stress tensor is regularized to the 0th adiabatic order, the longitudinal, temporal, and GF stress tensors are regularized to the 2nd adiabatic order. The resulting total regularized vacuum stress tensor is convergent and maximally-symmetric, has a positive energy density, and respects the covariant conservation, and thus can be identified as the cosmological constant that drives the de Sitter inflation. Under the Lorenz condition $A^\mu\,_{;\, \mu}=0$, the regularized Stueckelberg stress tensor reduces to the regularized Proca stress tensor that contains only the transverse and longitudinal modes. In the massless limit, the regularized Stueckelberg stress tensor becomes zero, and is the same as that of the Maxwell field with the GF term, and no trace anomaly exists. If the order of adiabatic regularization were lower than our prescription, some divergences would remain. If the order were higher, say, under the conventional 4th-order regularization, more terms than necessary would be subtracted off, leading to an unphysical negative energy density and the trace anomaly simultaneously.

gr-qc↗

arXiv2024

Primordial Black Hole Formation in a Dust Bouncing Model

Linear scalar cosmological perturbations have increasing spectra in the contracting phase of bouncing models. We study the conditions for which these perturbations may collapse into primordial black holes and the hypothesis that these objects constitute a fraction of dark matter. We compute the critical density contrast that describes the collapse of matter perturbations in the flat-dust bounce model with a parametric solution, obtained from the Lemaitre-Tolman-Bondi metric that represents the spherical collapse. We discuss the inability of the Newtonian gauge to describe perturbations in contracting models as the perturbative hypothesis does not hold in such cases. We carry the calculations for a different Gauge choice and compute the perturbations power spectra numerically. Finally, assuming a Gaussian distribution, we compute the primordial black hole abundance with the Press-Schechter formalism and compare it with observational constraints. From our analysis, we conclude that the primordial black hole formation in a dust-dominated contracting phase does not lead to a significant mass fraction of primordial black holes in dark matter today.

astro-ph.CO↗

arXiv2023

Maxwell field with gauge fixing term in the radiation- and matter-dominant stages: exact solution and stress tensor

We study the Maxwell field with a general gauge fixing (GF) term in the radiation-dominant (RD) and matter-dominant (MD) stages of expanding Universe, as a continuation to the previous work in de Sitter space. We derive the exact solutions, perform the covariant canonical quantization and obtain the stress tensor in the Gupta-Bleuler (GB) physical states, which is independent of the GF constant and is also invariant under the quantum residual gauge transformation. The transverse stress tensor is similar in all flat Robertson-Walker spacetimes, and its vacuum part is $\propto k^4$ and becomes zero after the 0th-order adiabatic regularization. The longitudinal-temporal stress tensor, in both RD and MD stages, is zero due to a cancelation between the longitudinal and temporal parts in the GB states, and so is the particle part of the GF stress tensor. The vacuum GF stress tensor, in the RD stage, contains $k^4,k^2$ divergences and becomes zero by the 2nd-order regularization, however, in the MD stage, contains $k^4, k^2, k^0$ divergences and becomes zero by the 4th-order regularization. So, the order of adequate regularization depends not only upon the type of fields, but also upon the background spacetimes. In summary, in both the RD and MD stages, like in de Sitter space, the total regularized vacuum stress tensor is zero, only the transverse photon part remains, and this holds independent of the GF term.

gr-qc↗

arXiv2022

Maxwell field with gauge fixing term in de Sitter space: exact solution and stress tensor

The Maxwell field with a general gauge fixing (GF) term is nontrivial, not only the longitudinal and temporal modes are mixed up in the field equations, but also unwanted consequences might arise from the GF term. We derive the complete set of solutions in de Sitter space, and implement the covariant canonical quantization which restricts the residual gauge transformation down to a quantum residual gauge transformation. Then, in the Gupta-Bleuler (GB) physical state, we calculate the stress tensor which is amazingly independent of the gauge fixing constant and is also invariant under the quantum residual gauge transformation. The transverse components are simply the same as those in the Minkowski spacetime, and the transverse vacuum stress tensor has only one UV divergent term ($\propto k^4$), which becomes zero by the 0th-order adiabatic regularization. The longitudinal-temporal stress tensor in the GB state is zero due to a cancelation between the longitudinal and temporal parts. More interesting is the stress tensor of the GF term. Its particle contribution is zero due to the cancelation in the GB state, and its vacuum contribution is twice that of a minimally-coupling massless scalar field, containing $k^4$ and $k^2$ divergences. After the 2nd-order adiabatic regularization, the GF vacuum stress tensor becomes zero too, so that there is no need to introduce a ghost field, and the zero GF vacuum stress tensor can not be a possible candidate for the cosmological constant. Thus, all the physics predicted by the Maxwell field with the GF term will be the same as that without the GF term. We also carry out analogous calculation in the Minkowski spacetime, and the stress tensor is similar to, but simpler than that in de Sitter space.

gr-qc↗

arXiv2022

Point-splitting regularization of the stress tensor of a coupling scalar field in de Sitter space

We perform the point-splitting regularization on the vacuum stress tensor of a coupling scalar field in de Sitter space under the guidance from the adiabatically regularized Green's function. For the massive scalar field with the minimal coupling $\xi=0$, the 2nd order point-splitting regularization yields a finite vacuum stress tensor with a positive, constant energy density, which can be identified as the cosmological constant that drives de Sitter inflation. For the coupling $\xi\ne 0$, we find that, even if the regularized Green's function is continuous, UV and IR convergent, the point-splitting regularization does not automatically lead to an appropriate stress tensor. The coupling $\xi R$ causes log divergent terms, as well as higher-order finite terms which depend upon the path of the coincidence limit. After removing these unwanted terms by extra treatments, the 2nd-order regularization for small couplings $\xi \in(0,\frac{1}{7.04})$, and respectively the 0th-order regularization for the conformal coupling $\xi=\frac16$, yield a finite, constant vacuum stress tensor, in analogy to the case $\xi=0$. For the massless field with $\xi=0$ or $\xi=\frac16$, the point-splitting regularization yields a vanishing vacuum stress tensor, and there is no conformal trace anomaly for $\xi=\frac16$. If the 4th-order regularization were taken, the regularized energy density for general $\xi$ would be negative, which is inconsistent with the de Sitter inflation, and the regularized Green's function would be singular at the zero mass, which is unphysical. In all these cases, the stress tensor from the point-splitting regularization is equal to that from the adiabatic one.

gr-qc↗

arXiv2020

Transmissive Metagrating for Arbitrary Wavefront Shaping Over the Full Visible Spectrum

Metagratings have been shown to form an agile and efficient platform for extreme wavefront manipulation, going beyond the limitations of gradient metasurfaces. Previous approaches for transmissive metagratings have resorted on compound asymmetric inclusions to achieve single-channel near-perfect diffraction. However, such complex inclusions are sensitive to geometric parameters and lack the flexibility for arbitrary phase modulation, restricting applications to beam deflection. Here, we show perfect unitary diffraction in all-dielectric transmissive metagratings using rectangular inclusions by tailoring their multipole interferences. Using this principle, we experimentally demonstrate analog phase profile encoding of a hologram through displacement modulation of CMOS-compatible silicon nitride nanobars, manifesting broadband and wide-angle high diffraction efficiencies for both polarizations and across the entire visible range. Featured with extreme angle/wavelength/polarization tolerance and alleviated structural complexity for both design and fabrication, our demonstration unlocks the full potential of metagrating-based wavefront manipulation for a variety of practical applications.

physics.optics↗

arXiv2020

Adiabatic regularization and Green's function of a scalar field in de Sitter space: Positive energy spectrum and no trace anomaly

In the conventional adiabatic regularization the vacuum ultraviolet divergences of a quantum field in curved spacetime are removed by subtracting the $k$-mode of the stress tensor to the 4th-order. For a scalar field in de Sitter space, we find that the 4th-order regularized spectral energy density is negative. Moreover, the 2nd-order regularization for minimal coupling ($ξ=0$) and the 0th-order regularization for conformal coupling ($ξ=\frac16$) yield a positive and UV-convergent spectral energy density and power spectrum. The regularized stress tensor in the vacuum is maximally symmetric and can drive inflation, while its $k$-modes representing the primordial fluctuations are nonuniformly distributed. Conventional regularization of a Green's function in position space is generally plagued by a log IR divergence. Only in the massless case with $ξ=0$ or $\frac16$, we can directly regularize the Green's functions and obtain vanishing results that agree with the adiabatic regularization results. In this case, the regularized power spectrum and stress tensor are both zero, and no trace anomaly exists. To overcome the log IR divergence problem in the massive cases with $ξ=0$ and $\frac16$, we perform Fourier transformation of the regularized power spectra and obtain the regularized analytical Green's functions which are IR- and UV-convergent.

gr-qc↗

arXiv2019

Full-color complex-amplitude vectorial holograms based on multi-freedom metasurfaces

Phase, polarization, amplitude and frequency represent the basic dimensions of light, playing crucial roles for both fundamental light-mater interactions and all major optical applications. Metasurface emerges as a compact platform to manipulate these knobs, but previous metasurfaces have limited flexibility to simultaneous control them. Here, we introduce a multi-freedom metasurface that can simultaneously and independently modulate phase, polarization and amplitude in an analytical form, and further realize frequency multiplexing by a k-space engineering technique. The multi-freedom metasurface seamlessly combine geometric Pancharatnam-Berry phase and detour phase, both of which are frequency-independent. As a result, it allows complex-amplitude vectorial hologram at various frequencies based on the same design strategy, without sophisticated nanostructure searching of massive size parameters. Based on this principle, we experimentally demonstrate full-color complex-amplitude vectorial meta-holograms in the visible with a metal-insulator metal architecture, unlocking the long-sought full potential of advanced light field manipulation through ultrathin metasurfaces.

physics.optics↗

arXiv2019

A massless scalar field in Robertson-Walker spacetimes: Adiabatic regularization and Green's function

We study adiabatic regularization of a coupling massless scalar field in general spatially flat Robertson-Walker (RW) spacetimes. For the conformally-coupling, the 0th-order regularized power spectrum and 0th-order regularized stress tensor are zero, and no trace anomaly exists in general RW spacetimes. This is a new result which extents those found in de Sitter space. For the minimally-coupling, the regularized spectra are also zero in the radiation-dominant stage, the matter-dominant stage, and de Sitter space as well. The vanishing of these adiabatically regularized spectra are also confirmed by direct regularization of the Green's functions. For a general coupling and general RW spacetimes, the regularized spectra can be negative under the conventional prescription. By going to higher order of regularization, the spectra will generally become positive, but will also acquire IR divergence which is inevitable for a massless field. To avoid the IR divergence, the inside-horizon regularization is applied. By these procedures, one will eventually achieve nonnegative, UV- and IR-convergent power spectrum and spectral energy density.

gr-qc↗