arXiv subjects

Hyeyoun Chung

Publications and source records attributed to Hyeyoun Chung.

12 recordsLinked to original sources

arXiv2017

Quasinormal ringing on the brane

While the linear behavior of gravity in braneworld models is well understood, much less is known about full non-linear gravitational effects. Even when they agree at the linear level, these could be expected to distinguish braneworlds from a lower-dimensional theory with no brane. Black holes are a good testing ground for such studies, as they are nonlinear solutions that would be expected to reflect the background geometry. In particular, we assess the role of black hole quasinormal modes in gravitational experiments devised to be sensitive to the existence of the brane, in a lower-dimensional setting where we have analytical control. We compute quasinormal modes of brane-localized black holes and find that they follow the entropy of the corresponding black hole. This observation allows us to conclude that, surprisingly, the scattering problem we consider, at least in some regimes, does not distinguish between non-linear gravitational effects of black holes in AdS space with a brane and black holes in a spacetime of one lower dimension.

hep-th↗

arXiv2014

Measures for a Multidimensional Multiverse

We explore the phenomenological implications of generalizing measures to a multidimensional multiverse. We consider a simple model in which the vacua are nucleated from a $D$-dimensional parent spacetime through dynamical compactification of the extra dimensions, and compute the geometric contribution to the probability distribution of observations within the multiverse for each measure. We then study how the shape of this probability distribution depends on the timescales for the existence of observers, for vacuum domination, and for curvature domination ($t_{obs}, t_Λ,$ and $t_c$, respectively.) In this work we restrict ourselves to bubbles with positive cosmological constant, $Λ$. In the case of the causal patch cutoff, when the bubble universes have $p+1$ large spatial dimensions with $p \geq 2$, the shape of the probability distribution is such that we obtain the coincidence of timescales $t_{obs} \sim t_Λ \sim t_c$. Moreover, the size of the cosmological constant is related to the size of the landscape. However, the exact shape of the probability distribution is different in the case $p = 2$, compared to $p \geq 3$. In the case of the fat geodesic measure, the result is even more robust: the shape of the probability distribution is the same for all $p \geq 2$, and we once again obtain the coincidence $t_{obs} \sim t_Λ \sim t_c$. These results require only very mild conditions on the prior probability of the distribution of vacua in the landscape. Our work shows that the observed double coincidence of timescales is a robust prediction even when the multiverse is generalized to be multidimensional; that this coincidence is not a consequence of our particular universe being (3+1)-dimensional; and that this observable cannot be used to preferentially select one measure over another in a multidimensional multiverse.

hep-th↗

arXiv2012

Instability of Near-Extremal Black Holes in N=2, d=4 Supergravity

As a precursor to studying the bound states of multiple non-extremal black holes in $\mathcal{N}=2$, $d=4$ supergravity, we investigate the stability of a near-extremal D0-D4 black hole in the probe limit, when the parameters of the black hole solution lie within a certain regime. We determine whether it is possible to form bound states of this "core" non-extremal black hole with BPS probe particles, and whether it is possible for the "core" black hole to decay by the emission of such BPS probes either to a local minimum of the probe potential, or spatial infinity. We first carry out a qualitative analysis of the probe potential to determine when quantum tunneling of probes from the black hole is possible. We then find the wavefunction of the scattered probe by using the WKB approximation to solve the Dirac equation in the black hole background, and use this solution to compute the tunneling amplitude.

hep-th↗

arXiv2012

Tunneling between single and multi-centered black hole configurations

We find a gravitational instanton that connects an initial state corresponding to a single-centered extremal Reissner-Nordstrom (ERN) black hole configuration, to a final state corresponding to a multi-centered configuration. This instanton is interpreted as describing quantum tunneling between the two different black hole solutions. We evaluate the Euclidean action for this instanton and find that the amplitude for the tunneling process is equal to half the difference in entropy between the initial and final configurations.

gr-qc↗

arXiv2012

Hawking Radiation and Entropy from Horizon Degrees of Freedom

We study the thermodynamic properties of horizons using the dynamical description of the gravitational degrees of freedom at a horizon found in a previous work. We use the action of the horizon degrees of freedom to posit an ansatz Liouville action in the near-horizon region, and calculate the horizon entropy using the Cardy formula. We also couple the gravitational degrees of freedom at the horizon to a classical scalar background, and show that Hawking radiation is produced.

gr-qc↗

arXiv2011

Fragmentation of black hole scaling throats in N=2 supergravity

We find an instanton analogous to the Brill instanton that describes the fragmentation of a single-centered black hole scaling throat of charge $Γ_1 + Γ_2 + Γ_3$ in N=2 supergravity to three disconnected throats of charges $Γ_1, Γ_2, Γ_3$, in the limit where the intersection products between the charges of the three throats satisfies $<Γ_1, Γ_2> \lll <Γ_2, Γ_3>, <Γ_3, Γ_1,>$. We evaluate the Euclidean action for this instanton and find that the amplitude for the tunneling process is proportional to the difference in entropy between the initial and final configurations.

gr-qc↗

arXiv2011

Dynamics of Diffeomorphism Degrees of Freedom at a Horizon

We define a set of boundary conditions that ensure the presence of a null hypersurface with the essential characteristics of a horizon, using the formalism of weakly isolated horizons as a guide. We then determine the diffeomorphisms that preserve these boundary conditions, and derive a dynamical action for these diffeomorphisms in a neighbourhood of the horizon. The action is similar to that of Liouville theory, and the equation of motion of the gravitational degrees of freedom approaches that of a free two-dimensional conformal field theory in the near-horizon region.

gr-qc↗

arXiv2010

The Effective Fluid Approach to Cosmological Nonlinearities: Applications to Preheating

In [arXiv:1004.2488], Baumann et al. present a new formalism for studying cosmological systems where the characteristic scale of non-linearities is much smaller than the Hubble scale. By integrating out the short-wavelength modes, it is possible to obtain an effective theory of long-wavelength perturbations that is described by an imperfect fluid evolving in an FRW background. As the long-wavelength perturbations remain small even when the short-scale dynamics are non-linear, the tools of linear perturbation theory may be applied. The work in [arXiv:1004.2488] deals only with matter in the form of a pressureless perfect fluid with zero anisotropic stress, and also assumes that the short-scale gravitational dynamics are Newtonian. In this work we extend this formalism to the case of a perfect fluid with pressure, and in particular to the case of preheating after inflation, where the matter content of the universe can be modeled by two coupled scalar fields. We discard the assumption that the short-scale gravitational dynamics are Newtonian. We find that our results differ from Baumann et al.'s even when the pressure is set to zero, which suggests that relaxing their assumptions creates appreciable changes in the long-wavelength effective theory. We derive equations of motion for the total density perturbation and matter velocities during preheating, as well as linearized Einstein equations for the long-wavelength metric perturbations. We also present the equations governing the effective long-wavelength scalar field dynamics.

astro-ph.CO↗

arXiv2010

Asymptotic Symmetries of Rindler Space at the Horizon and Null Infinity

We investigate the asymptotic symmetries of Rindler space at null infinity and at the event horizon using both systematic and ad hoc methods. We find that the approaches that yield infinite-dimensional asymptotic symmetry algebras in the case of anti-de Sitter and flat spaces only give a finite-dimensional algebra for Rindler space at null infinity. We calculate the charges corresponding to these symmetries and confirm that they are finite, conserved, and integrable, and that the algebra of charges gives a representation of the asymptotic symmetry algebra. We also use relaxed boundary conditions to find infinite-dimensional asymptotic symmetry algebras for Rindler space at null infinity and at the event horizon. We compute the charges corresponding to these symmetries and confirm that they are finite and integrable. We also determine sufficient conditions for the charges to be conserved on-shell, and for the charge algebra to give a representation of the asymptotic symmetry algebra. In all cases, we find that the central extension of the charge algebra is trivial.

gr-qc↗

arXiv2008

The Study of Entangled States in Quantum Computation and Quantum Information Science

This thesis explores the use of entangled states in quantum computation and quantum information science. Entanglement, a quantum phenomenon with no classical counterpart, has been identified as an important and quantifiable resource in many areas of theoretical quantum information science, including quantum error correction, quantum cryptography, and quantum algorithms. We first investigate the equivalence classes of a particular class of entangled states (known as graph states due to their association with mathematical graphs) under local operations. We prove that for graph states corresponding to graphs with neither cycles of length 3 nor 4, the equivalence classes can be characterized in a very simple way. We also present software for analyzing and manipulating graph states. We then study quantum error-correcting codes whose codewords are highly entangled states. An important area of investigation concerning QECCs is to determine which resources are necessary in order to carry out any computation on the code to an arbitrary degree of accuracy, while simultaneously maintaining a high degree of resistance to noise. We prove that transversal gates, which are designed to prevent the propagation of errors through a system, are insufficient to achieve universal computation on almost all QECCs. Finally, we study the problem of creating efficient quantum circuits for creating entangling measurements. Entangling measurements can be used to harness the apparent extra computing power of quantum systems by allowing us to extract information about the global, collective properties of a quantum state using local measurements. We construct explicit quantum circuits that create entangling measurements, and show that these circuits scale polynomially in the input parameters.

quant-ph↗

arXiv2008

Subsystem stabilizer codes cannot have a universal set of transversal gates for even one encoded qudit

A long-standing open problem in fault-tolerant quantum computation has been to find a universal set of transversal gates. As three of us proved in arXiv: 0706.1382, such a set does not exist for binary stabilizer codes. Here we generalize our work to show that for subsystem stabilizer codes in $d$ dimensional Hilbert space, such a universal set of transversal gates cannot exist for even one encoded qudit, for any dimension $d$, prime or nonprime. This result strongly supports the idea that other primitives, such as quantum teleportation, are necessary for universal fault-tolerant quantum computation, and may be an important factor for fault tolerance noise thresholds.

quant-ph↗

arXiv2007

Local unitary versus local Clifford equivalence of stabilizer and graph states

The equivalence of stabilizer states under local transformations is of fundamental interest in understanding properties and uses of entanglement. Two stabilizer states are equivalent under the usual stochastic local operations and classical communication criterion if and only if they are equivalent under local unitary (LU) operations. More surprisingly, under certain conditions, two LU equivalent stabilizer states are also equivalent under local Clifford (LC) operations, as was shown by Van den Nest et al. [Phys. Rev. \textbf{A71}, 062323]. Here, we broaden the class of stabilizer states for which LU equivalence implies LC equivalence ($LU\Leftrightarrow LC$) to include all stabilizer states represented by graphs with neither cycles of length 3 nor 4. To compare our result with Van den Nest et al.'s, we show that any stabilizer state of distance $δ=2$ is beyond their criterion. We then further prove that $LU\Leftrightarrow LC$ holds for a more general class of stabilizer states of $δ=2$. We also explicitly construct graphs representing $δ>2$ stabilizer states which are beyond their criterion: we identify all 58 graphs with up to 11 vertices and construct graphs with $2^m-1$ ($m\geq 4$) vertices using quantum error correcting codes which have non-Clifford transversal gates.

quant-ph↗