Faculty Publications: August, 2017

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Title:
Weil-Petersson geometry on the space of Bridgeland stability conditions
Authors:
Fan, Yu-Wei; Kanazawa, Atsushi; Yau, Shing-Tung
Publication:
eprint arXiv:1708.02161
Publication Date:
08/2017
Origin:
ARXIV
Keywords:
Mathematics - Algebraic Geometry, High Energy Physics - Theory, Mathematics - Differential Geometry, 53D37, 14J33, 14J32, 18E30
Comment:
21 pages. Comments are welcome!
Bibliographic Code:
2017arXiv170802161F

Abstract

Inspired by mirror symmetry, we investigate some differential geometric aspects of the space of Bridgeland stability conditions on a Calabi-Yau triangulated category. The aim is to develop theory of Weil-Petersson geometry on the stringy K\"ahler moduli space. A few basic examples are studied. In particular, we identify our Weil-Petersson metric with the Bergman metric on a Siegel modular variety in the case of the self-product of an elliptic curve.

 

Title:
The Pandora multi-algorithm approach to automated pattern recognition of cosmic-ray muon and neutrino events in the MicroBooNE detector
Publication:
eprint arXiv:1708.03135
Publication Date:
08/2017
Origin:
ARXIV
Keywords:
High Energy Physics - Experiment, Physics - Data Analysis, Statistics and Probability
Comment:
Preprint to be submitted to The European Physical Journal C
Bibliographic Code:
2017arXiv170803135M

Abstract

The development and operation of Liquid-Argon Time-Projection Chambers for neutrino physics has created a need for new approaches to pattern recognition in order to fully exploit the imaging capabilities offered by this technology. Whereas the human brain can excel at identifying features in the recorded events, it is a significant challenge to develop an automated, algorithmic solution. The Pandora Software Development Kit provides functionality to aid the design and implementation of pattern-recognition algorithms. It promotes the use of a multi-algorithm approach to pattern recognition, in which individual algorithms each address a specific task in a particular topology. Many tens of algorithms then carefully build up a picture of the event and, together, provide a robust automated pattern-recognition solution. This paper describes details of the chain of over one hundred Pandora algorithms and tools used to reconstruct cosmic-ray muon and neutrino events in the MicroBooNE detector. Metrics that assess the current pattern-recognition performance are presented for simulated MicroBooNE events, using a selection of final-state event topologies.

 

Title:
Motional Ground State Cooling Outside the Lamb-Dicke Regime
Authors:
Yu, Yichao; Hutzler, Nicholas R.; Zhang, Jessie T.; Liu, Lee R.; Rosenband, Till; Ni, Kang-Kuen
Publication:
eprint arXiv:1708.03296
Publication Date:
08/2017
Origin:
ARXIV
Keywords:
Physics - Atomic Physics, Condensed Matter - Quantum Gases, Physics - Chemical Physics
Comment:
5 pages, 4 figures
Bibliographic Code:
2017arXiv170803296Y

Abstract

We report Raman sideband cooling of a single sodium atom to its three-dimensional motional ground state in an optical tweezer. Despite a large Lamb-Dicke parameter, high initial temperature, and large differential light shifts between the excited state and the ground state, we achieve a ground state population of 81(4)% after 100 ms of cooling, for the 85% of atoms that survive cooling and re-imaging. Our technique includes addressing high-order sidebands, where several motional quanta are removed by a single laser pulse, and fast modulation of the optical tweezer intensity. We demonstrate that Raman sideband cooling to the 3D motional ground state is possible, even without tight confinement and low initial temperature.

 

Title:
Photonic Band Structure of Two-dimensional Atomic Lattices
Authors:
Perczel, Janos; Borregaard, Johannes; Chang, Darrick E.; Pichler, Hannes; Yelin, Susanne F.; Zoller, Peter; Lukin, Mikhail D.
Publication:
eprint arXiv:1708.03413
Publication Date:
08/2017
Origin:
ARXIV
Keywords:
Quantum Physics, Physics - Atomic Physics, Physics - Optics
Comment:
15 pages, 7 figures
Bibliographic Code:
2017arXiv170803413P

Abstract

Two-dimensional atomic arrays exhibit a number of intriguing quantum optical phenomena, including subradiance, nearly perfect reflection of radiation and long-lived topological edge states. Studies of emission and scattering of photons in such lattices require complete treatment of the radiation pattern from individual atoms, including long-range interactions. We describe a systematic approach to perform the calculations of collective energy shifts and decay rates in the presence of such long-range interactions for arbitrary two-dimensional atomic lattices. As applications of our method, we investigate the topological properties of atomic lattices both in free-space and near plasmonic surfaces.

 

Title:
Fermionic spinon theory of square lattice spin liquids near the N\'eel state
Authors:
Thomson, Alex; Sachdev, Subir
Publication:
eprint arXiv:1708.04626
Publication Date:
08/2017
Origin:
ARXIV
Keywords:
Condensed Matter - Strongly Correlated Electrons, High Energy Physics - Theory
Comment:
32 pages, 5 figures, 12 tables
Bibliographic Code:
2017arXiv170804626T

Abstract

Quantum fluctuations of the N\'eel state of the square lattice antiferromagnet are usually described by a $\mathbb{CP}^1$ theory of bosonic spinons coupled to a U(1) gauge field, and with a global SU(2) spin rotation symmetry. Such a theory also has a confining phase with valence bond solid (VBS) order, and upon including spin-singlet charge 2 Higgs fields, deconfined phases with $\mathbb{Z}_2$ topological order possibly intertwined with discrete broken global symmetries. We present dual theories of the same phases starting from a mean-field theory of fermionic spinons moving in $\pi$-flux in each square lattice plaquette. Fluctuations about this $\pi$-flux state are described by 2+1 dimensional quantum chromodynamics (QCD$_3$) with a SU(2) gauge group and $N_f=2$ flavors of massless Dirac fermions. It has recently been argued by Wang et al. (arXiv:1703.02426) that this QCD$_3$ theory describes the N\'eel-VBS quantum phase transition. We introduce adjoint Higgs fields in QCD$_3$, and obtain fermionic dual descriptions of the phases with $\mathbb{Z}_2$ topological order obtained earlier using the bosonic $\mathbb{CP}^1$ theory. We also present a fermionic spinon derivation of the monopole Berry phases in the U(1) gauge theory of the VBS state. The global phase diagram of these phases contains multi-critical points, and our results imply new boson-fermion dualities between critical gauge theories of these points.

 

Title:
All-optical nanoscale thermometry with silicon-vacancy centers in diamond
Authors:
Nguyen, Christian T.; Evans, Ruffin E.; Sipahigil, Alp; Bhaskar, Mihir K.; Sukachev, Denis D.; Agafonov, Viatcheslav N.; Davydov, Valery A.; Kulikova, Liudmila F.; Jelezko, Fedor; Lukin, Mikhail D.
Publication:
eprint arXiv:1708.05419
Publication Date:
08/2017
Origin:
ARXIV
Keywords:
Quantum Physics, Condensed Matter - Mesoscale and Nanoscale Physics
Comment:
5 pages, 3 figures
Bibliographic Code:
2017arXiv170805419N

Abstract

We demonstrate an all-optical thermometer based on an ensemble of silicon-vacancy centers (SiVs) in diamond by utilizing a temperature dependent shift of the SiV optical zero-phonon line transition frequency, $\Delta\lambda/\Delta T= 6.8\,\mathrm{GHz/K}$. Using SiVs in bulk diamond, we achieve $70\,\mathrm{mK}$ precision at room temperature with a sensitivity of $360\,\mathrm{mK/\sqrt{Hz}}$. Finally, we use SiVs in $200\,\mathrm{nm}$ nanodiamonds as local temperature probes with $521\,\mathrm{ mK/\sqrt{Hz}}$ sensitivity. These results open up new possibilities for nanoscale thermometry in biology, chemistry, and physics, paving the way for control of complex nanoscale systems.

 

Title:
A bright nanowire single photon source based on SiV centers in diamond
Authors:
Marseglia, L.; Saha, K.; Ajoy, A.; Schröder, T.; Englund, D.; Jelezko, F.; Walsworth, R.; Pacheco, J. L.; Perry, D. L.; Bielejec, E. S.; Cappellaro, P.
Publication:
eprint arXiv:1708.05782
Publication Date:
08/2017
Origin:
ARXIV
Keywords:
Physics - Applied Physics, Quantum Physics
Comment:
15 pages, 5 figures
Bibliographic Code:
2017arXiv170805782M

Abstract

The practical implementation of many quantum technologies relies on the development of robust and bright single photon sources that operate at room temperature. The negatively charged silicon-vacancy (SiV-) color center in diamond is a possible candidate for such a single photon source. However, due to the high refraction index mismatch to air, color centers in diamond typically exhibit low photon out-coupling. An additional shortcoming is due to the random localization of native defects in the diamond sample. Here we demonstrate deterministic implantation of Si ions with high conversion efficiency to single SiV- centers, targeted to fabricated nanowires. The co-localization of single SiV- centers with the nanostructures yields a ten times higher light coupling efficiency than for single SiV- centers in bulk diamond. This enhanced photon out-coupling, together with the intrinsic scalability of the SiV- creation method, enables a new class of devices for integrated photonics and quantum science.

 

Title:
Scale-Dependent Galaxy Bias from Massive Particles with Spin during Inflation
Authors:
Moradinezhad Dizgah, Azadeh; Dvorkin, Cora
Publication:
eprint arXiv:1708.06473
Publication Date:
08/2017
Origin:
ARXIV
Keywords:
Astrophysics - Cosmology and Nongalactic Astrophysics, High Energy Physics - Phenomenology, High Energy Physics - Theory
Bibliographic Code:
2017arXiv170806473M

Abstract

The presence of additional particles during inflation leads to non-Gaussianity in late-time correlators of primordial curvature perturbations. The shape and amplitude of this signal depend on the mass and spin of the extra particles. Constraints on this distinct form of primordial non-Gaussianity, therefore, provide a wealth of information on the particle content during inflation. We investigate the potential of upcoming galaxy surveys in constraining such a signature through its impact on the observed galaxy power spectrum. Primordial non-Gaussianity of various shapes induces a scale-dependent bias on tracers of large-scale structure, such as galaxies. Using this signature we obtain constraints on massive particles during inflation, which can have non-zero spins. In particular, we show that the prospects for constraining particles with spins 0 and 1 are promising, while constraining particles with spin 2 from power spectrum alone seems challenging. We show that the multi-tracer technique can significantly improve the constraints from the power spectrum by at least an order of magnitude. Furthermore, we analyze the effect of non-linearities due to gravitational evolution on the forecasted constraints on the masses of the extra particles and the amplitudes of the imprinted non-Gaussian signal. We find that gravitational evolution affects the constraints by less than a factor of 2.

 

Title:
A Novel Stretch Energy Minimization Algorithm for Equiareal Parameterizations
Authors:
Yueh, Mei-Heng; Lin, Wen-Wei; Wu, Chin-Tien; Yau, Shing-Tung
Publication:
eprint arXiv:1708.07391
Publication Date:
08/2017
Origin:
ARXIV
Keywords:
Computer Science - Graphics, Computer Science - Computational Geometry
Comment:
29 pages, 15 figures
Bibliographic Code:
2017arXiv170807391Y

Abstract

Surface parameterizations have been widely applied to computer graphics and digital geometry processing. In this paper, we propose a novel stretch energy minimization (SEM) algorithm for the computation of equiareal parameterizations of simply connected open surfaces with a very small area distortion and a highly improved computational efficiency. In addition, the existence of nontrivial limit points of the SEM algorithm is guaranteed under some mild assumptions of the mesh quality. Numerical experiments indicate that the efficiency, accuracy, and robustness of the proposed SEM algorithm outperform other state-of-the-art algorithms. Applications of the SEM on surface remeshing and surface registration for simply connected open surfaces are demonstrated thereafter. Thanks to the SEM algorithm, the computations for these applications can be carried out efficiently and robustly.

 

Title:
Wang and Yau's Quasi-Local Energy for a Maximally Rotating Black hole
Authors:
Miller, Warner A.; Ray, Shannon; Wang, Mu-Tao; Yau, Shing-Tung
Publication:
eprint arXiv:1708.07532
Publication Date:
08/2017
Origin:
ARXIV
Keywords:
General Relativity and Quantum Cosmology
Comment:
34 pages, 12 figures
Bibliographic Code:
2017arXiv170807532M

Abstract

We explore the Wang and Yau (W-Y) quasi-local energy (QLE) for a family of spacelike surfaces in a Kerr spacetime. Their approach is an extension of the Hamilton-Jacobi approach developed by Brown and York (B-Y). W-Y broadened their definition for a surface $\mathcal{S}$ from its isometric embedding into Euclidean 3-space to its isometric embedding into Minkowski spacetime. This extension solves earlier problems and provides a QLE for a larger class of surfaces in spacetimes. In order to explore this new definition of QLE we examine a family of spacelike 2-surfaces $\mathcal{S}_r$ of constant radii in a $t=constant$ hypersurface of Boyer-Lindquist coordinates. There exists a critical radius $r^*$ below which these 2-surfaces are not globally embeddable in Euclidean 3-space. Within this subcritical region, the original application of B-Y is not defined. However, since these surfaces are time-flat, $\tau=0$ is a solution to the W-Y Euler-Lagrange equation and reproduces the same complex-valued QLE of B-Y. These surfaces are excluded from a W-Y positivity condition. Nevertheless, we see no reason why such surfaces should not have a well defined QLE. Therefore, we explore their formalism in this domain to gain clues for its possible extension. We find that W-Y QLE functional yields a global minimum for each of these subcritical surfaces and it is not a solution of the W-Y Euler-Lagrange equation. Our numerical results suggest that this global QLE minimum lies on a boundary separating admissible energies from inadmissible complex energies.

 

Title:
Statistical mechanics of specular reflections from fluctuating membranes and interfaces
Authors:
Azadi, Amir; Nelson, David R.
Publication:
eprint arXiv:1708.08154
Publication Date:
08/2017
Origin:
ARXIV
Keywords:
Condensed Matter - Soft Condensed Matter
Bibliographic Code:
2017arXiv170808154A

Abstract

We study the density of specular reflection points in the geometrical optics limit when light scatters off fluctuating interfaces and membranes in thermodynamic equilibrium. We focus on the statistical mechanics of both capillary-gravity interfaces (characterized by a surface tension) and fluid membranes (controlled by a bending rigidity) in thermodynamic equilibrium in two dimensions. Building on work by Berry, Nye, Longuet-Higgins and others, we show that the statistics of specular points is fully characterized by three fundamental length scales, namely, a correlation length $\xi$, a microscopic length scale $\ell$ and the overall size $L$ of the interface or membrane. By combining a scaling analysis with numerical simulations, we confirm the existence of a scaling law for the density of specular reflection points, $n_{spec}$, in two dimensions, given by $n_{spec}\propto\ell^{-1}$ in the limit of thin fluctuating interfaces with the interfacial thickness $\ell\ll\xi_I$. The density of specular reflections thus diverges for fluctuating interfaces in the limit of vanishing thickness and shows no dependance on the interfacial capillary-gravity correlation length $\xi_I$. Although fluid membranes under tension also exhibit a divergence in $n_{spec}\propto\left(\xi_M\ell\right)^{-1/2}$, the number of specular reflections in this case can grow by decreasing the membrane correlation length $\xi_{M}$.

 

Title:
Induced Ellipticity for Inspiraling Binary Systems
Authors:
Randall, Lisa; Xianyu, Zhong-Zhi
Publication:
eprint arXiv:1708.08569
Publication Date:
08/2017
Origin:
ARXIV
Keywords:
General Relativity and Quantum Cosmology, Astrophysics - High Energy Astrophysical Phenomena, High Energy Physics - Phenomenology, High Energy Physics - Theory
Comment:
36 pages
Bibliographic Code:
2017arXiv170808569R

Abstract

Although gravitational waves tend to erase eccentricity of an inspiraling binary system, ellipticity can be generated in the presence of surrounding matter. We present a semi-analytical method for understanding the eccentricity distribution of binary black holes in the presence of a supermassive black hole in a galactic center. Given a matter distribution, we show how to determine the resultant eccentricity analytically in the presence of both tidal forces and evaporation up to one cutoff and one matter-distribution-independent function, paving the way for understanding the environment of detected inspiraling black holes. We furthermore generalize Kozai-Lidov dynamics to situations where perturbation theory breaks down for short time intervals, allowing more general angular momentum exchange, such that eccentricity is generated even when all bodies orbit in the same plane.

 

Title:
The silicon-vacancy spin qubit in diamond: quantum memory exceeding ten milliseconds and single-shot state readout
Authors:
Sukachev, Denis D.; Sipahigil, Alp; Nguyen, Christian T.; Bhaskar, Mihir K.; Evans, Ruffin E.; Jelezko, Fedor; Lukin, Mikhail D.
Publication:
eprint arXiv:1708.08852
Publication Date:
08/2017
Origin:
ARXIV
Keywords:
Quantum Physics
Bibliographic Code:
2017arXiv170808852S

Abstract

The negatively-charged silicon-vacancy (SiV$^-$) color center in diamond has recently emerged as a promising system for quantum photonics. Its symmetry-protected optical transitions enable creation of indistinguishable emitter arrays and deterministic coupling to nanophotonic devices. Despite this, the longest coherence time associated with its electronic spin achieved to date ($\sim 250$ ns) has been limited by coupling to acoustic phonons. We demonstrate coherent control and suppression of phonon-induced dephasing of the SiV$^-$ electronic spin coherence by five orders of magnitude by operating at temperatures below 500 mK. By aligning the magnetic field along the SiV$^-$ symmetry axis, we demonstrate spin-conserving optical transitions and single-shot readout of the SiV$^-$ spin with 89% fidelity. Coherent control of the SiV$^-$ spin with microwave fields is used to demonstrate a spin coherence time $T_2$ of 13 ms and a spin relaxation time $T_1$ exceeding 1 s at 100 mK. These results establish the SiV$^-$ as a promising solid-state candidate for the realization of scalable quantum networks.

 

Title:
Physical and geometric constraints explain the labyrinth-like shape of the nasal cavity
Authors:
Zwicker, David; Ostilla-Mónico, Rodolfo; Lieberman, Daniel E.; Brenner, Michael P.
Publication:
eprint arXiv:1708.08966
Publication Date:
08/2017
Origin:
ARXIV
Keywords:
Physics - Biological Physics, Physics - Fluid Dynamics, Quantitative Biology - Tissues and Organs
Comment:
7 pages, 4 figures
Bibliographic Code:
2017arXiv170808966Z

Abstract

The nasal cavity is a vital component of the respiratory system that heats and humidifies inhaled air in all vertebrates. Despite this common function, the shapes of nasal cavities vary widely across animals. To understand this variability, we here connect nasal geometry to its function by theoretically studying the airflow and the associated scalar exchange that describes heating and humidification. We find that optimal geometries, which have minimal resistance for a given exchange efficiency, have a constant gap width between their side walls, but their overall shape is restricted only by the geometry of the head. Our theory explains the geometric variations of natural nasal cavities quantitatively and we hypothesize that the trade-off between high exchange efficiency and low resistance to airflow is the main driving force shaping the nasal cavity. Our model further explains why humans, whose nasal cavities evolved to be smaller than expected for their size, become obligate oral breathers in aerobically challenging situations.

 

Title:
Area, entanglement entropy and supertranslations at null infinity
Authors:
Kapec, Daniel; Raclariu, Ana-Maria; Strominger, Andrew
Publication:
Classical and Quantum Gravity, Volume 34, Issue 16, article id. 165007 (2017). (CQGra Homepage)
Publication Date:
08/2017
Origin:
IOP
DOI:
10.1088/1361-6382/aa7f12
Bibliographic Code:
2017CQGra..34p5007K

Abstract

The area of a cross-sectional cut Σ of future null infinity ( \newcommand{\ip}{{{ I}}^+} \ip ) is infinite. We define a finite, renormalized area by subtracting the area of the same cut in any one of the infinite number of BMS-degenerate classical vacua. The renormalized area acquires an anomalous dependence on the choice of vacuum. We relate it to the modular energy, including a soft graviton contribution, of the region of \newcommand{\ip}{{{ I}}^+} \ip to the future of Σ . Under supertranslations, the renormalized area shifts by the supertranslation charge of Σ . In quantum gravity, we conjecture a bound relating the renormalized area to the entanglement entropy across Σ of the outgoing quantum state on \newcommand{\ip}{{{ I}}^+} \ip .

 

Title:
Scrutinizing the unresolved x-ray background in the CDFS field via transdimensional sampling
Authors:
Daylan, Tansu; Feder-Staehle, Richard; Finkbeiner, Douglas P.
Affiliation:
AA(Harvard University), AB(Harvard University), AC(Harvard University)
Publication:
American Astronomical Society, HEAD meeting #16, id.113.04
Publication Date:
08/2017
Origin:
AAS
Abstract Copyright:
(c) 2017: American Astronomical Society
Bibliographic Code:
2017HEAD...1611304D

Abstract

We analyse the Chandra Deep Field South (CDFS) data using a novel inference framework called probabilistic cataloging, where fair samples are taken from the posterior distribution of the catalog space of point sources in the field consistent with the merged CDFS exposure. This requires constructing a Markov chain of samples via transdimensional proposals involving births and deaths as well as within-model proposals. Our point source model is also hierarchical, allowing the shape of the flux distribution of the point sources to be inferred simultaneously. By marginalizing over point sources below the detection threshold, robust uncertainty estimates can be placed on the flux and color distribution of Active Galactic Nuclei (AGNs) as well as the normalization and spectrum of the truly isotropic background. We validate our method using simulated deep Chandra exposures and show that the isotropic background emission can be constrained at the 10% level, which is a result that takes into account its covariance with unresolved AGNs and the particle background of Chandra. We compare our method to results derived from fluctuation analyses, i.e., study of the 1-point function of photon counts across the image, and discuss how our results relate to the AGN population synthesis models.

 

Title:
Complementarity and stability conditions
Authors:
Georgi, Howard
Affiliation:
AA(Center for the Fundamental Laws of Nature, ThePhysics Laboratories, Harvard University, Cambridge, MA 02138, United States)
Publication:
Physics Letters B, Volume 771, p. 558-562.
Publication Date:
08/2017
Origin:
ELSEVIER
Abstract Copyright:
(c) 2017 Elsevier Science B.V. All rights reserved.
DOI:
10.1016/j.physletb.2017.06.008
Bibliographic Code:
2017PhLB..771..558G

Abstract

We discuss the issue of complementarity between the confining phase and the Higgs phase for gauge theories in which there are no light particles below the scale of confinement or spontaneous symmetry breaking. We show with a number of examples that even though the low energy effective theories are the same (and trivial), discontinuous changes in the structure of heavy stable particles can signal a phase transition and thus we can sometimes argue that two phases which have different structures of heavy particles that cannot be continuously connected and thus the phases cannot be complementary. We discuss what this means and suggest that such ;stability conditions; can be a useful physical check for complementarity


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