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A. Albrecht

Publications and source records attributed to A. Albrecht.

6 recordsLinked to original sources
arXiv2017

Exponential improvement in photon storage fidelities using subradiance and "selective radiance" in atomic arrays

A central goal within quantum optics is to realize efficient interactions between photons and atoms. A fundamental limit in nearly all applications based on such systems arises from spontaneous emission, in which photons are absorbed by atoms and then re-scattered into undesired channels. In typical treatments of atomic ensembles, it is assumed that this re-scattering occurs independently, and at a rate given by a single isolated atom, which in turn gives rise to standard limits of fidelity in applications such as quantum memories or quantum gates. However, this assumption can be violated. In particular, spontaneous emission of a collective atomic excitation can be significantly suppressed through strong interference in emission. Thus far the physics underlying the phenomenon of subradiance and techniques to exploit it have not been well-understood. In this work, we provide a comprehensive treatment of this problem. First, we show that in ordered atomic arrays in free space, subradiant states acquire an interpretation in terms of optical modes that are guided by the array, which only emit due to scattering from the ends of the finite chain. We also elucidate the properties of subradiant states in the many-excitation limit. Finally, we introduce the new concept of selective radiance. Whereas subradiant states experience a reduced coupling to all optical modes, selectively radiant states are tailored to simultaneously radiate efficiently into a desired channel while scattering into undesired channels is suppressed, thus enabling an enhanced atom-light interface. We show that these states naturally appear in chains of atoms coupled to nanophotonic structures, and we analyze the performance of photon storage exploiting such states. We find that selectively radiant states allow for a photon storage error that scales exponentially better with number of atoms than previously known bounds.

quant-ph↗
arXiv2004

Weak Lensing and Supernovae: Complementary Probes of Dark Energy

Weak lensing observations and supernova observations, combined with CMB observations, can both provide powerful constraints on dark energy properties. Considering statistical errors only, we find luminosity distances inferred from 2000 supernovae and large-scale (l < 1000) angular power spectra inferred from redshift-binned cosmic shear maps place complementary constraints on w_0 and w_a where w(z) = w_0 + w_a(a-1). Further, each set of observations can constrain higher-dimensional parameterizations of w(z); we consider eigenmodes of the w(z) error covariance matrix and find such datasets can each constrain the amplitude of about 5 w(z) eigenmodes. We also consider another parameterization of the dark energy.

astro-ph↗
arXiv2002

Exponentially Large Extra Dimensions

We show how the presence of a very light scalar with a cubic self-interaction in six dimensions can stabilize the extra dimensions at radii which are naturally exponentially large, $r \sim \ell \exp [(4π)^3/g^2]$, where $\ell$ is a microscopic physics scale and $g$ is the (dimensionless) cubic coupling constant. The resulting radion mode of the metric becomes a very light degree of freedom whose mass, $m \sim 1/(M_p r^2)$ is stable under radiative corrections. For $1/r \sim 10^{-3}$ eV the radion is extremely light, $m \sim 10^{-33}$ eV. Its couplings cause important deviations from General Relativity in the very early universe, but naturally evolve to phenomenologically acceptable values at present.

hep-th↗
arXiv2001

Natural Quintessence and Large Extra Dimensions

We examine the late-time (nucleosynthesis and later) cosmological implications of brane-world scenarios having large (millimeter sized) extra dimensions. In particular, recent proposals for understanding why the extra dimensions are so large in these models indicate that moduli like the radion appear (to four-dimensional observers) to be extremely light, with a mass of order 10^{-33} eV, allowing them to play the role of the light scalar of quintessence models. The radion-as-quintessence solves a long-standing problem since its small mass is technically natural, in that it is stable against radiative corrections. Its challenges are to explain why such a light particle has not been seen in precision tests of gravity, and why Newton's constant has not appreciably evolved since nucleosynthesis. We find the couplings suggested by stabilization models can provide explanations for both of these questions. We identify the features which must be required of any earlier epochs of cosmology in order for these explanations to hold.

astro-ph↗
arXiv1997

A detail study of defect models for cosmic structure formation

We calculate predictions from wide class of `active' models of cosmic structure formation which allows us to scan the space of possible defect models. We calculate the linear cold dark matter power spectrum and Cosmic Microwave Background (CMB) anisotropies over all observable scales using a full linear Einstein-Boltzmann code. Our main result, which has already been reported, points to a serious problem reconciling the observed amplitude of the large-scale galaxy distribution with the COBE normalization. Here, we describe our methods and results in detail. The problem is present for a wide range of defect parameters, which can be used to represent potential differences among defect models, as well as possible systematic numerical errors. We explicitly examine the impact of varying the defect model parameters and we show how the results substantiate these conclusions. The standard scaling defect models are in serious conflict with the current data, and we show how attempts to resolve the problem by considering non-scaling defects or modified stress-energy components would require radical departures from what has become the standard picture.

astro-ph↗