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Sarah Shandera

Publications and source records attributed to Sarah Shandera.

At least 19 recordsLinked to original sources
arXiv2026

Excitation Flow, Positivity, and Fisher Information for Open Subsystems of an $N$-Qubit Network

We derive closed-form propagators for any $K$-qubit subsystem of a closed $N$-qubit network with a single conserved excitation. A single transition amplitude simultaneously controls excitation flow between subsystems, the positivity and complete positivity of every propagator, the entanglement entropy of every subsystem, and the quantum Fisher information for global parameters. Positivity and complete positivity coincide, determined solely by the direction of excitation flow, independently of subsystem size, coherence, or entanglement structure. A propagator is positive and completely positive if and only if it contracts the subsystem state toward its fixed point. The ensemble of propagators collectively constrains global properties inaccessible to any single subsystem. For single-qubit subsystems, we characterize the ensemble's fixed-point distribution and domain of positivity, finding a band of states that lies inside the positivity domain of every propagator yet is never visited by the physical dynamics. The quantum Fisher information decomposes into state and process contributions over any observation window $[t_1,t_2]$, with the state contribution bounded while the process contribution grows secularly. The total Fisher information is minimal when all future propagators are nonpositive and not completely positive, and near its maximum when they are positive and completely positive.

quant-ph↗
arXiv2026

From Origins to Observables: Distinguishing Dark Compact Objects with Population-Level Microlensing Signatures

While primordial black holes (PBHs) have long been a benchmark target for microlensing searches, the modern landscape of dark matter models suggests other, distinct, formation channels for compact objects made of dark matter. In the large class of self-interacting, dissipative models, dark matter has cooling channels that can enable fragmentation and gravitational collapse of some dark matter into compact objects, including black holes. The resulting populations have mass distributions, bias parameters, and abundance, spatial profile and velocity dispersion within the Milky Way that all differ from those of PBHs. We demonstrate that these population-level differences can leave imprints in the space of microlensing observables, with the differences in how the populations trace the dark matter giving the primary distinguishing lever. We discuss the possible overlap of microlensing signals from dark and baryonic lenses, and the complementarity of microlensing detection or constraints with other gravitational probes of novel populations of dark matter origin.

astro-ph.CO↗
arXiv2026

Operator dynamics in k-Markov random circuits

We demonstrate that $k$-Markov sequences of unitary gates provide low-cost handles to manipulate the rate and structure of information spreading compared to traditional random, 0-Markov, circuits. For SWAP gates and brickwork circuits, we use graph cover time to demonstrate how $k$-Markov processes can be used to control operator transport. With SWAP gates and the set of Clifford gates that can change operator weight, we show how $k$-Markov sequences can be used to manipulate scrambling time and generate novel structures of spatial-temporal correlations across a qubit network. We show that $k$-Markov circuits constructed from PSWAP gates at fixed angle are equivalent to standard brickwork circuits with PSWAP angle drawn from non-uniform distributions generated by the $k$-Markov process. In those circuits, the time evolution of the average Hamming weight and the space-time correlation structure after equilibrium again vary significantly from the 0-Markov case, depending on the transition probabilities of the process.

quant-ph↗
arXiv2025

SHH method for SIDM: An SIDM-hydro hybrid method for simulating self-interacting dark matter

We present a new scheme to couple existing numerical methods for elastic self-interacting dark matter (SIDM) to the hydrodynamic equations via a continuous function of the local Knudsen number. The method, an SIDM-hydro hybrid (SHH), allows more efficient simulation of the evolution of inhomogeneous halos deep into the regime of gravothermal collapse. With the improved efficiency gained by moving to a hydrodynamical description in high-density regions, the SHH method allows central densities of two orders of magnitude higher to be reached in considerably less simulation time than traditional methods. Our implementation should be considered as the first step toward a robust SHH method, as we interpolate the first and second moments of the Boltzmann equation in the ideal-fluid limit only. The simulation results are qualitatively similar to those found with other methods, although there are differences in the implementation of the primary physics driving the dynamics, and in the details of the resulting halo profiles. However, our results indicate that the SHH technique shows promise to investigate gravothermal collapse in diverse, dynamical environments. The method can be extended to incorporate non-ideal fluid terms and dissipation, as needed for dark-matter scenarios where interactions beyond the elastic regime may be important in the dense interiors of some halos.

astro-ph.CO↗
arXiv2025

Open-systems tools for non-thermalizing closed quantum systems

We design several examples of constrained, symmetric quantum circuit dynamics that generate non-equilibrium steady states. The qubit networks maintain local memory of the initial conditions and display inhomogeneous subsystem dynamics over long times, clearly distinguishable from approximately thermalizing networks of the same size. Each network can be described as an ensemble of open systems, a collection of qubits evolving with phase-covariant dynamics. Constraints from the conservation law and global unitary dynamics of the entire network bound the distribution of single-qubit dynamics in the ensemble, but different steady states are distinguishable by several measures. We quantify the distance of the steady states from the homogeneous steady state and further characterize them using the complexity of their mutual information networks, the volume of state space explored, a thermodynamic utility measure using extractable work, and correlated structure in the occurrence of non-completely positive qubit propagator maps.

quant-ph↗
arXiv2024

Natural disorder distributions from measurement

We consider scenarios where the dynamics of a quantum system are partially determined by prior local measurements of some interacting environmental degrees of freedom. The resulting effective system dynamics are described by a disordered Hamiltonian, with spacetime-varying parameter values drawn from distributions that are generically neither flat nor Gaussian. This class of scenarios is a natural extension of those where a fully non-dynamical environmental degree of freedom determines a universal coupling constant for the system. Using a family of quasi-exactly solvable anharmonic oscillators, we consider environmental ground states of nonlinearly coupled degrees of freedom, unrestricted by a weak coupling expansion, which include strongly quantum non-Gaussian states. We derive the properties of distributions for both quadrature and photon number measurements. Measurement-induced disorder of this kind is likely realizable in laboratory quantum systems and, given a notion of naturally occurring measurement, suggests a new class of scenarios for the dynamics of quantum systems in particle physics and cosmology.

quant-ph↗
arXiv2024

Effective dynamics of qubit networks via phase-covariant quantum ensembles

We derive a new constructive procedure to rapidly generate ensembles of phase-covariant dynamical maps that may be associated to the individual spins of a closed quantum system. We do this by first computing the single-spin dynamical maps in small XXZ networks and chains, specialized to the class of initial states that guarantees phase-covariant dynamics for each spin. Since the dynamics in any small, closed system contains oscillatory features associated to the system size, we define an averaging procedure to extract time-homogeneous dynamics. We use the the average map and the set of deviations from the average map in the exactly derived ensembles to motivate the form of distributional functions for map parameters. The distributions then straightforwardly generate arbitrary-sized ensembles of channels, constrained by a few global properties. This procedure can also generate ensembles where individual maps are not phase-covariant although the average map is, corresponding to realizations of disordered, or noisy, Hamiltonians. The construction procedure suggests new ways to realize random families of open-system dynamics, subject to constraints that require the ensemble to approximate a partition of a closed system.

quant-ph↗
arXiv2023

Spontaneously interacting qubits from Gauss-Bonnet

Building on previous constructions examining how a collection of small, locally interacting quantum systems might emerge via spontaneous symmetry breaking from a single-particle system of high dimension, we consider a larger family of geometric loss functionals and explicitly construct several classes of critical metrics which "know about qubits" (KAQ). The loss functional consists of the Ricci scalar with the addition of the Gauss-Bonnet term, which introduces an order parameter that allows for spontaneous symmetry breaking. The appeal of this method is two-fold: (i) the Ricci scalar has already been shown to have KAQ critical metrics and (ii) exact equations of motions are known for loss functionals with generic curvature terms up to two derivatives. We show that KAQ critical metrics, which are solutions to the equations of motion in the space of left-invariant metrics with fixed determinant, exist for loss functionals that include the Gauss-Bonnet term. We find that exploiting the subalgebra structure leads us to natural classes of KAQ metrics which contain the familiar distributions (GUE, GOE, GSE) for random Hamiltonians. We introduce tools for this analysis that will allow for straightfoward, although numerically intensive, extension to other loss functionals and higher-dimension systems.

quant-ph↗
arXiv2022

A Lower Bound on the Mass of Compact Objects from Dissipative Dark Matter

We study the minimum mass of dark compact objects formed in dissipative dark-matter halos and show that the simple atomic-dark-matter model consistent with all current observations can create low-mass fragments that can evolve into compact objects forbidden by stellar astrophysics. We model the collapse of the dark halo's dense core by tracing the thermo-chemical evolution of a uniform-density volume element under two extreme assumptions for density evolution: hydrostatic equilibrium and pressure-free collapse. We then compute the opacity-limited minimum fragment mass from the minimum temperature achieved in these calculations.

astro-ph.CO↗
arXiv2022

Snowmass2021 Cosmic Frontier White Paper: Probing dark matter with small-scale astrophysical observations

The current understanding of dark matter comes largely from measurements of the total matter content in the universe, from the distribution of gravitating matter on very large scales, and from rotation curves and velocity dispersions on sub-galactic scales. However, small-scale structure may well hold the key to unlocking the particle physics of the still-mysterious 85% of matter in the universe. Novel small-scale astrophysical probes of new particles and dark matter will become possible with a large data volume of high-precision measurements enabled by next-generation gravitational-wave detectors and advanced astrometry instruments. Cosmic microwave background and large-scale structure surveys will provide complementary constraints on dark matter models with unique small-scale signatures. We lay out the studies of small-scale structures and compact objects as dark matter probes, and summarize the requirements to achieve the goals.

hep-ph↗
arXiv2022

Increasing extractable work in small qubit landscapes

An interesting class of physical systems, including those associated with life, demonstrates the ability to hold thermalization at bay and perpetuate states of high free-energy compared to a local environment. In this work, we study quantum systems with no external sources or sinks for energy, heat, work, or entropy, that allow for high free-energy subsystems to form and persist. We initialize systems of qubits in mixed, uncorrelated states and evolve them subject to a conservation law. We find that four qubits make up the minimal system for which these restricted dynamics and initial conditions allow an increase in extractable work for a subsystem. On landscapes of eight co-evolving qubits, interacting in randomly selected subsystems at each step, we demonstrate that restricted connectivity and an inhomogeneous distribution of initial temperatures both lead to landscapes with longer intervals of increasing extractable work for individual qubits. We demonstrate the role of correlations that develop on the landscape in enabling a positive change in extractable work.

quant-ph↗
arXiv2022

Dark Matter In Extreme Astrophysical Environments

Exploring dark matter via observations of extreme astrophysical environments -- defined here as heavy compact objects such as white dwarfs, neutron stars, and black holes, as well as supernovae and compact object merger events -- has been a major field of growth since the last Snowmass process. Theoretical work has highlighted the utility of current and near-future observatories to constrain novel dark matter parameter space across the full mass range. This includes gravitational wave instruments and observatories spanning the electromagnetic spectrum, from radio to gamma-rays. While recent searches already provide leading sensitivity to various dark matter models, this work also highlights the need for theoretical astrophysics research to better constrain the properties of these extreme astrophysical systems. The unique potential of these search signatures to probe dark matter adds motivation to proposed next-generation astronomical and gravitational wave instruments.

hep-ph↗
arXiv2022

Classifying the non-time-local and entangling dynamics of an open qubit system

We study families of dynamical maps generated from interactions with varying degrees of symmetry. For a family of time-independent Hamiltonians, we demonstrate the relationship between symmetry, strong-coupling, perfect entanglers, non-Markovian features, and non-time-locality. We show that by perturbing the initial environment state, effective time-local descriptions can be obtained that are non-singular yet capture essential non-unitary features of the reduced dynamics. We then consider a time-dependent Hamiltonian that changes the degree of symmetry by activating a dormant degree of freedom. In this example we find that the one-qubit reduced dynamics changes dramatically. These results can inform the construction of effective theories of open systems when the larger system dynamics is unknown.

quant-ph↗
arXiv2021

Molecular Chemistry for Dark Matter II: Recombination, Molecule Formation, and Halo Mass Function in Atomic Dark Matter

Dissipative dark matter predicts rich observable phenomena that can be tested with future large-scale structure surveys. As a specific example, we study atomic dark matter, consisting of a heavy particle and a light particle charged under a dark electromagnetism. In particular, we calculate the cosmological evolution of atomic dark matter focusing on dark recombination and dark-molecule formation. We have obtained the relevant interaction-rate coefficients by re-scaling the rates for normal hydrogen, and evolved the abundances for ionized, atomic, and molecular states using a modified version of Recfast++ (which we have released publicly at https://github.com/jamesgurian/RecfastJulia). We also provide an analytical approximation for the final abundances. We then calculate the effects of the atomic dark matter on the linear power spectrum, which enter through a dark-photon diffusion and dark acoustic oscillations. At the formation time, the atomic dark matter model suppresses halo abundances on scales smaller than the diffusion scale, just like the warm dark matter models suppress the abundance below the free-streaming scale. The subsequent evolution with radiative cooling, however, will alter the halo mass function further.

astro-ph.CO↗
arXiv2021

Molecular Chemistry for Dark Matter III: DarkKROME

Dark matter that is dissipative may cool sufficiently to form compact objects, including black holes. Determining the abundance and mass spectrum of those objects requires an accurate model of the chemistry relevant for the cooling of the dark matter gas. Here we introduce a chemistry tool for dark matter, DarkKROME, an extension of the KROME software package. DarkKROME is designed to include all atomic and molecular processes relevant for dark matter with two unequal-mass fundamental fermions, interacting via a massless-photon mediated $U(1)$ force. We use DarkKROME to perform one-zone collapse simulations and study the evolution of temperature-density phase diagrams for various dark-sector parameters.

astro-ph.CO↗
arXiv2021

Molecular Chemistry for Dark Matter

Molecular cooling is essential for studying the formation of sub-structure of dissipative dark-matter halos that may host compact objects such as black holes. Here, we analyze the reaction rates relevant for the formation, dissociation, and transition of hydrogenic molecules while allowing for different values of the physical parameters: the coupling constant, the proton mass, and the electron mass. For all cases, we re-scale the reaction rates for the standard molecular hydrogen, so our results are valid as long as the dark matter is weakly coupled and one of the fermions is much heavier than the other. These results will allow a robust numerical treatment of cosmic structure, in particular for mini-halos for which molecular cooling is important, in a dissipative dark matter scenario.

astro-ph.CO↗
arXiv2020

A gravitational-wave limit on the Chandrasekhar mass of dark matter

We explore a new paradigm to study dissipative dark matter models using gravitational-wave observations. We consider a dark atomic model which predicts the formation of binary black holes such as GW190425 while obeying constraints from large-scale structure, and improving on the missing satellite problem. Using LIGO and Virgo gravitational-wave data from 12th September 2015 to 1st October 2019, we show that interpreting GW190425 as a dark matter black-hole binary limits the Chandrasekhar mass for dark matter to be below 1.4 $M_\odot$ at $> 99.9\%$ confidence implying that the dark proton is heavier than 0.95 GeV, while also suggesting that the molecular energy-level spacing of dark molecules lies near $10^{-3}$ eV and constraining the cooling rate of dark matter at low temperatures.

astro-ph.CO↗