arXiv subjects

Chih-Wei Wang

Publications and source records attributed to Chih-Wei Wang.

16 recordsLinked to original sources

arXiv2026

Non-Hermitian Generalization of Bloch Sphere in Spacetime Algebra

We establish a geometric generalization of the Bloch sphere for two-level quantum systems with non-Hermitian Hamiltonians using the Spacetime Algebra (STA) formulation. By lifting the state density operator from the even subalgebra to the full STA, we show that the state space expands from the unit 2-sphere to a future light cone. The non-unitary time evolution generated by a general non-Hermitian Hamiltonian corresponds to proper orthochronous Lorentz transformations on the null vectors. We classify the Hamiltonian dynamics into four distinct geometric classes: spatial rotations (corresponding to $\mathcal{PT}$-symmetric systems), pure boosts (anti-$\mathcal{PT}$-symmetric systems), null rotations (exceptional points), and general mixtures. We also use this formulation to study several results from PT-symmetric quantum mechanics, including the topological features of the exceptional points.

quant-ph↗

arXiv2025

Quantum Control of Thermal Emission from Photonic Crystals with Two-Level Atoms

Thermal light engineering is a field of considerable interest and potential. We study quantum light-matter interactions in a one-dimensional photonic crystal with two-level atoms as the active medium, replacing classical oscillators in traditional blackbody models. In a thermal bath with pumping, these atoms modulate thermal emission via interactions with photonic modes. The model with quantum two-level systems enables the processes of spontaneous emission, stimulated absorption, and stimulated emission. Equilibrium and nonequilibrium regimes depend on competition between pumping and thermal relaxation rates. Strong light-matter interaction and photon decay govern dynamics and steady states. In equilibrium, with a high thermal relaxation rate, photon numbers are initially determined by spontaneous emission and later stabilize due to stimulated absorption, influenced by light-matter interaction strength. In-band-gap photons reach steady states at a time scale of one or two orders of magnitude longer than outside-band-gap photons. Interestingly, for a strong light-matter interaction, all photons in the equilibrium regimes show Planckian radiation, regardless of their frequencies in or out of the band gaps. Band-gap suppression of thermal emission is more pronounced with weaker light-matter interaction or larger photon decay. In the nonequilibrium regime, the dynamics of photon numbers exhibit a multi-time-scale process transitioning to steady states due to strong pumping and stimulated processes. Steady-state electron populations of two-level atoms deviate from the Fermi-Dirac distribution, and the steady-state photon numbers exhibit super-Planckian emission. These findings enable quantum control of thermal emission spectra, which is relevant for reducing thermal noise in quantum computing or enhancing radiative cooling.

quant-ph↗

arXiv2024

Polarized Superradiance from CsPbBr3 Quantum Dot Superlattice with Controlled Inter-dot Electronic Coupling

Cooperative emission of photons from an ensemble of quantum dots (QDs) as superradiance can arise from the electronically coupled QDs with a coherent emitting excited state. This contrasts with superfluorescence (Dicke superradiance), where the cooperative photon emission occurs via a spontaneous buildup of coherence in an ensemble of incoherently excited QDs via their coupling to a common radiation mode. While superfluorescence has been observed in perovskite QD systems, reports of superradiance from the electronically coupled ensemble of perovskite QDs are rare. Here, we demonstrate the generation of polarized superradiance with a very narrow linewidth (<5 meV) and a large redshift (~200 meV) from the electronically coupled CsPbBr3 QD superlattice achieved through a combination of strong quantum confinement and ligand engineering. In addition to photon bunching at low excitation densities, the superradiance is polarized in contrast to the uncoupled exciton emission from the same superlattice. This finding suggests the potential for obtaining polarized cooperative photon emission via anisotropic electronic coupling in QD superlattices even when the intrinsic anisotropy of exciton transition in individual QDs is weak.

cond-mat.mes-hall↗

arXiv2022

Photoemission of the Upconverted Hot Electrons in Mn-doped CsPbBr$_3$ Nanocrystals

Hot electrons play a crucial role in enhancing the efficiency of photon-to-current conversion or photocatalytic reactions. In semiconductor nanocrystals, energetic hot electrons capable of photoemission can be generated via the upconversion process involving the dopant-originated intermediate state, currently known only in Mn-doped cadmium chalcogenide quantum dots. Here, we report that Mn-doped CsPbBr3 nanocrystals are an excellent platform for generating hot electrons via upconversion that can benefit from various desirable exciton properties and the structural diversity of metal halide perovskites (MHP). 2-dimensional Mn-doped CsPbBr$_3$ nanoplatelets are particularly advantageous for hot electron upconversion due to the strong exciton-dopant interaction mediating the upconversion process. Furthermore, nanoplatelets reveal evidence for the hot electron upconversion via long-lived dark exciton in addition to bright exciton that may enhance the upconversion efficiency. This study not only establishes the feasibility of hot electron upconversion in MHP host but also demonstrates the potential merits of 2-dimensional MHP nanocrystals in hot electron upconversion.

physics.chem-ph↗

arXiv2022

Industry Characteristics and Financial Risk Spillovers

This paper proposes a new measure of tail risk spillover. The empirical application provides evidence of significant volatility and tail risk spillovers from the financial sector to many real economy sectors in the U.S. economy in the period from 2001 to 2011. These spillovers increase in crisis periods. The conditional coexceedance in a given sector is positively related to its amount of debt financing, and negatively related to its relative valuation and investment. Real economy sectors which require substantial external financing, and whose value and investment activity are relatively lower, are prime candidates for depreciation in the wake of crisis in the financial sector.

q-fin.RM↗

arXiv2022

Measuring Systemic Risk: Common Factor Exposures and Tail Dependence Effects

We model systemic risk using a common factor that accounts for market-wide shocks and a tail dependence factor that accounts for linkages among extreme stock returns. Specifically, our theoretical model allows for firm-specific impacts of infrequent and extreme events. Using data on the four sectors of the U.S. financial industry from 1996 to 2011, we uncover two key empirical findings. First, disregarding the effect of the tail dependence factor leads to a downward bias in the measurement of systemic risk, especially during weak economic times. Second, when these measures serve as leading indicators of the St. Louis Fed Financial Stress Index, measures that include a tail dependence factor offer better forecasting ability than measures based on a common factor only.

q-fin.RM↗

arXiv2018

The Density Operators of Qubit Systems in the Multiparticle Spacetime Algebra

We provide a method to write down the density operator for any pure state of multi-qubit systems in the multiparticle spacetime algebra (MSTA) introduced by Doran, Gull, and Lasenby. Using the MSTA formulation, we analyze several aspects of quantum mechanics in a geometrical way including the Bell inequality and the dynamics of two coupled qubits. Lastly, we provide a natural way to construct the local unitary invariants in the MSTA. Using these invariants, we analyze the space of the two-qubit and three-qubit pure states with local and non-local degrees of freedom separated.

quant-ph↗

arXiv2016

Short- and long-range corrected hybrid density functionals with the D3 dispersion corrections

We propose a short- and long-range corrected (SLC) hybrid scheme employing 100% Hartree-Fock (HF) exchange at both zero and infinite interelectronic distances, wherein three SLC hybrid density functionals with the D3 dispersion corrections (SLC-LDA-D3, SLC-PBE-D3, and SLC-B97-D3) are developed. SLC-PBE-D3 and SLC-B97-D3 are shown to be accurate for a very diverse range of applications, such as core ionization and excitation energies, thermochemistry, kinetics, noncovalent interactions, dissociation of symmetric radical cations, vertical ionization potentials, vertical electron affinities, fundamental gaps, and valence, Rydberg, and long-range charge-transfer excitation energies. Relative to omegaB97X-D, SLC-B97-D3 provides significant improvement for core ionization and excitation energies and noticeable improvement for the self-interaction, asymptote, energy-gap, and charge-transfer problems, while performing similarly for thermochemistry, kinetics, and noncovalent interactions.

physics.chem-ph↗

arXiv2013

Modular Properties of 3D Higher Spin Theory

In the three-dimensional sl(N) Chern-Simons higher-spin theory, we prove that the conical surplus and the black hole solution are related by the S-transformation of the modulus of the boundary torus. Then applying the modular group on a given conical surplus solution, we generate a 'SL(2,Z)' family of smooth constant solutions. We then show how these solutions are mapped into one another by coordinate transformations that act non-trivially on the homology of the boundary torus. After deriving a thermodynamics that applies to all the solutions in the 'SL(2,Z)' family, we compute their entropies and free energies, and determine how the latter transform under the modular transformations. Summing over all the modular images of the conical surplus, we write down a (tree-level) modular invariant partition function.

hep-th↗

arXiv2010

Split Flows in Bubbled Geometries

We propose a procedure to clarify part of the physical sector in the five dimensional bubble geometries based on ideas similar to the split attractor flow conjecture proposed by Denef. This procedure involves building some simple tree-like graphs that we call skeletons without referring to the moduli space. The skeleton (tree) exists if and only if it passes the existence conditions which are purely based on some local CTC's (closed timelike curves) checking. Then, we propose the conjecture similar to Denef's version which states that every existing skeleton (tree) should correspond to some solution in which the global absence of CTC's is ensured. Furthermore, we propose two pictures to identify this correspondence explicitly and use some numerical examples to show how this procedure works. We also analyze the physical sector of the simplest bubbled supertube and see how the existence conditions constrain the charge parameter space.

hep-th↗

arXiv2008

Plumbing the Abyss: Black Ring Microstates

We construct the first smooth, horizonless ``microstate geometries'' that have the same charges, dipole charges and angular momenta as a BPS black ring whose horizon is macroscopic. These solutions have exactly the same geometry as black rings, except that the usual infinite throat is smoothly capped off at a very large depth. If the solutions preserve a U(1)x U(1) isometry, then this depth is limited by flux quantization but if this symmetry is broken then the throat can be made arbitrarily deep by tuning classical, geometric moduli. Interpreting these ``abysses'' (smooth microstate geometries of arbitrary depth) from the point of view of the AdS-CFT correspondence suggests two remarkable alternatives: either stringy effects can eliminate very large regions of a smooth low-curvature supergravity solution, or the D1-D5-P CFT has quantum critical points. The existence of solutions whose depth depends on moduli also enables us to define ``entropy elevators,'' and these provide a new tool for studying the entropy of BPS and near-BPS black holes.

hep-th↗

arXiv2006

Mergers and Typical Black Hole Microstates

We use mergers of microstates to obtain the first smooth horizonless microstate solutions corresponding to a BPS three-charge black hole with a classically large horizon area. These microstates have very long throats, that become infinite in the classical limit; nevertheless, their curvature is everywhere small. Having a classically-infinite throat makes these microstates very similar to the typical microstates of this black hole. A rough CFT analysis confirms this intuition, and indicates a possible class of dual CFT microstates. We also analyze the properties and the merging of microstates corresponding to zero-entropy BPS black holes and black rings. We find that these solutions have the same size as the horizon size of their classical counterparts, and we examine the changes of internal structure of these microstates during mergers.

hep-th↗

arXiv2006

The Foaming Three-Charge Black Hole

We find a very large set of smooth horizonless geometries that have the same charges and angular momenta as the five-dimensional, maximally-spinning, three-charge, BPS black hole (J^2 = Q^3). Our solutions are constructed using a four-dimensional Gibbons-Hawking base space that has a very large number of two-cycles. The entropy of our solutions is proportional to Q^(1/2). In the same class of solutions we also find microstates corresponding to zero-entropy black rings, and these are related to the microstates of the black hole by continuous deformations.

hep-th↗

arXiv2005

Sliding Rings and Spinning Holes

We construct smooth supergravity solutions describing a BPS black ring with a BPS black hole centered at an arbitrary distance above the ring. We find that as one moves the black hole the entropy of the ring remains constant, but the angular momentum coming from the supergravity fluxes changes. Our solutions also show that in order to merge a BPS black ring with a BPS black hole one has to increase one of the angular momenta of the ring, and that the result of the merger is always a BMPV black hole. We also find a class of mergers that are thermodynamically reversible, and comment on their physics.

hep-th↗

arXiv2005

Black Rings with Varying Charge Density

We find the general five-dimensional, supersymmetric black ring solutions in M-theory based upon a circular ring, but with arbitrary, fluctuating charge distributions around the ring. The solutions have three arbitrary charge distribution functions, but their asymptotic charges and angular momenta only depend upon the total charges on the ring. The arbitrary density fluctuations thus represent "hair." By varying the charge distributions one can continuously change the entropy of these black rings; to our knowledge this is the first solution in which the entropy depends on classical moduli. We also show that there is a family of solutions, with two arbitrary functions, for which the horizon remains rotationally invariant, and yet the complete solution breaks rotational symmetry. If the horizon area is set to zero then one obtains families of supertube solutions. We find that our general solutions are governed by three harmonic functions that may be thought of as classical excitations of a string. The horizon area provides a natural Lorentz metric on these excitations, and the constancy of the rotational invariance of the horizon imposes a set of Virasoro constraints.

hep-th↗

arXiv1998

Chiral structure of the solutions of the Ginsparg-Wilson relation

We analyse the structure of solutions of the Ginsparg-Wilson relation for lattice Dirac operator in topologically trivial gauge sector. We show that the properties of such solutions relating to the perturbative stability of the pole of the fermion propagator as well as to the structure of the Yukawa models based on these solutions are solely determined by the non-local chirally invariant part of these Dirac operators. Depending on the structure of this part, the pole in the fermion propagator may or may not be stable under radiative corrections. We illustrate this by explicit examples.

hep-lat↗