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Ultracold Collisions and Fundamental Physics with Strontium
The success of strontium-based optical lattice clocks in the last five years has led to a recommendation by BIPM of strontium as a future standard of frequency and time. Due to the excellent agreement between three international labs, the strontium optical clock transition is the best agreed-upon optical frequency to date. We use the international optical clock data to limit present-day drift of fundamental constants and their coupling to the ambient gravitational potential. Strontium lattice clocks are still making rapid progress and promise a large signal-to-noise improvement over single-ion-based frequency standards by employing O(104) atoms. Reaching quantum-projection-noise limited measurement requires a careful study and control of the many-body interactions in the system. We measure interactions between ultracold fermions at the 10-17 level and relate them to s-wave collisions due to a loss of indistinguishability during the spectroscopic process. This new understanding of the many-body effects will increase the precision of current optical lattice clock systems and can lead to the accuracy level that has so far been pioneered only in single particle (trapped ion) systems. A second generation strontium system is used to control ultracold interactions in an otherwise ideal gas of bosonic 88Sr via the optical Feshbach resonance effect. These new measurement and control capabilities pave the way to reach atomic shot-noise limited optical clock performance without detrimental effects from large atom numbers
Characterizing quantum instruments: from non-demolition measurements to quantum error correction
In quantum information processing quantum operations are often processed
alongside measurements which result in classical data. Due to the information
gain of classical measurement outputs non-unitary dynamical processes can take
place on the system, for which common quantum channel descriptions fail to
describe the time evolution. Quantum measurements are correctly treated by
means of so-called quantum instruments capturing both classical outputs and
post-measurement quantum states. Here we present a general recipe to
characterize quantum instruments alongside its experimental implementation and
analysis. Thereby, the full dynamics of a quantum instrument can be captured,
exhibiting details of the quantum dynamics that would be overlooked with common
tomography techniques. For illustration, we apply our characterization
technique to a quantum instrument used for the detection of qubit loss and
leakage, which was recently implemented as a building block in a quantum error
correction (QEC) experiment (Nature 585, 207-210 (2020)). Our analysis reveals
unexpected and in-depth information about the failure modes of the
implementation of the quantum instrument. We then numerically study the
implications of these experimental failure modes on QEC performance, when the
instrument is employed as a building block in QEC protocols on a logical qubit.
Our results highlight the importance of careful characterization and modelling
of failure modes in quantum instruments, as compared to simplistic
hardware-agnostic phenomenological noise models, which fail to predict the
undesired behavior of faulty quantum instruments. The presented methods and
results are directly applicable to generic quantum instruments.Comment: 28 pages, 21 figure
Heat and Charge Transport Properties of MgB2
A polycrystalline sample of the MgB_2 superconductor was investigated by
measurements of the electrical resistivity, the thermopower and the thermal
conductivity in the temperature range between 1.8K and 300K in zero magnetic
field. The electrical resistivity shows a superconducting transition at
T_c=38.7K and, similarly to borocarbides, a T^2.4 behaviour up to 200K. The
electron diffusion thermopower and its bandstructure-derived value indicate the
dominant hole character of the charge carriers. The total thermopower can be
explained by the diffusion term renormalized by a significant electron-phonon
interaction and a phonon drag term. In the thermal conductivity, for decreasing
temperature, a significant decrease below T_c is observed resulting in a T^3
behaviour below 7K. The reduced Lorenz number exhibits values smaller than 1
and a characteristic minimum which resembles the behaviour of non-magnetic
borocarbides.Comment: 7 pages, 5 figures; added references and minor changes; accepted for
publication in Physica
Characterizing large-scale quantum computers via cycle benchmarking
Quantum computers promise to solve certain problems more efficiently than
their digital counterparts. A major challenge towards practically useful
quantum computing is characterizing and reducing the various errors that
accumulate during an algorithm running on large-scale processors. Current
characterization techniques are unable to adequately account for the
exponentially large set of potential errors, including cross-talk and other
correlated noise sources. Here we develop cycle benchmarking, a rigorous and
practically scalable protocol for characterizing local and global errors across
multi-qubit quantum processors. We experimentally demonstrate its practicality
by quantifying such errors in non-entangling and entangling operations on an
ion-trap quantum computer with up to 10 qubits, with total process fidelities
for multi-qubit entangling gates ranging from 99.6(1)% for 2 qubits to 86(2)%
for 10 qubits. Furthermore, cycle benchmarking data validates that the error
rate per single-qubit gate and per two-qubit coupling does not increase with
increasing system size.Comment: The main text consists of 6 pages, 3 figures and 1 table. The
supplementary information consists of 6 pages, 2 figures and 3 table
A model of decay
We suggest a parameterization of the matrix element for decay using kinematic variables convenient for experimental
analysis. The contributions of intermediate - and -states up
to spin 3 have been taken into account. The angular distributions for each
discussed hypothesis have been obtained and analysed using Monte-Carlo
simulation.Comment: 24 pages, 9 figures, 1 table; V2: text in some places improved and
acknowledgments adde
Bostonia: The Boston University Alumni Magazine. Volume 10
Founded in 1900, Bostonia magazine is Boston University's main alumni publication, which covers alumni and student life, as well as university activities, events, and programs
Quantitative high-throughput phenotypic screening of pediatric cancer cell lines identifies multiple opportunities for drug repurposing
Drug repurposing approaches have the potential advantage of facilitating rapid and cost-effective development of new therapies. Particularly, the repurposing of drugs with known safety profiles in children could bypass or streamline toxicity studies. We employed a phenotypic screening paradigm on a panel of well-characterized cell lines derived from pediatric solid tumors against a collection of ∼3,800 compounds spanning approved drugs and investigational agents. Specifically, we employed titration-based screening where compounds were tested at multiple concentrations for their effect on cell viability. Molecular and cellular target enrichment analysis indicated that numerous agents across different therapeutic categories and modes of action had an antiproliferative effect, notably antiparasitic/protozoal drugs with non-classic antineoplastic activity. Focusing on active compounds with dosing and safety information in children according to the Children's Pharmacy Collaborative database, we identified compounds with therapeutic potential through further validation using 3D tumor spheroid models. Moreover, we show that antiparasitic agents induce cell death via apoptosis induction. This study demonstrates that our screening platform enables the identification of chemical agents with cytotoxic activity in pediatric cancer cell lines of which many have known safety/toxicity profiles in children. These agents constitute attractive candidates for efficacy studies in pre-clinical models of pediatric solid tumors
Experimental quantification of spatial correlations in quantum dynamics
Correlations between different partitions of quantum systems play a central role in a variety of many-body quantum systems, and they have been studied exhaustively in experimental and theoretical research. Here, we investigate dynamical correlations in the time evolution of multiple parts of a composite quantum system. A rigorous measure to quantify correlations in quantum dynamics based on a full tomographic reconstruction of the quantum process has been introduced recently [Á. Rivas et al., New Journal of Physics, 17(6) 062001 (2015).]. In this work, we derive a lower bound for this correlation measure, which does not require full knowledge of the quantum dynamics. Furthermore we also extend the correlation measure to multipartite systems. We directly apply the developed methods to a trapped ion quantum information processor to experimentally characterize the correlations in quantum dynamics for two- and four-qubit systems. The method proposed and demonstrated in this work is scalable, platform-independent and applicable to other composite quantum systems and quantum information processing architectures. We apply the method to estimate spatial correlations in environmental noise processes, which are crucial for the performance of quantum error correction procedures
The Hilbert-Schmidt Theorem Formulation of the R-Matrix Theory
Using the Hilbert-Schmidt theorem, we reformulate the R-matrix theory in
terms of a uniformly and absolutely convergent expansion. Term by term
differentiation is possible with this expansion in the neighborhood of the
surface. Methods for improving the convergence are discussed when the
R-function series is truncated for practical applications.Comment: 16 pages, Late
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