The passage of time is tracked by counting oscillations of a frequency reference, such as Earth's revolutions or swings of a pendulum. By referencing atomic transitions, frequency (and thus time) can ...be measured more precisely than any other physical quantity, with the current generation of optical atomic clocks reporting fractional performance below the 10
level
. However, the theory of relativity prescribes that the passage of time is not absolute, but is affected by an observer's reference frame. Consequently, clock measurements exhibit sensitivity to relative velocity, acceleration and gravity potential. Here we demonstrate local optical clock measurements that surpass the current ability to account for the gravitational distortion of space-time across the surface of Earth. In two independent ytterbium optical lattice clocks, we demonstrate unprecedented values of three fundamental benchmarks of clock performance. In units of the clock frequency, we report systematic uncertainty of 1.4 × 10
, measurement instability of 3.2 × 10
and reproducibility characterized by ten blinded frequency comparisons, yielding a frequency difference of -7 ± (5)
± (8)
× 10
, where 'stat' and 'sys' indicate statistical and systematic uncertainty, respectively. Although sensitivity to differences in gravity potential could degrade the performance of the clocks as terrestrial standards of time, this same sensitivity can be used as a very sensitive probe of geopotential
. Near the surface of Earth, clock comparisons at the 1 × 10
level provide a resolution of one centimetre along the direction of gravity, so the performance of these clocks should enable geodesy beyond the state-of-the-art level. These optical clocks could further be used to explore geophysical phenomena
, detect gravitational waves
, test general relativity
and search for dark matter
.
Optical clocks benefit from tight atomic confinement enabling extended interrogation times as well as Doppler- and recoil-free operation. However, these benefits come at the cost of frequency shifts ...that, if not properly controlled, may degrade clock accuracy. Numerous theoretical studies have predicted optical lattice clock frequency shifts that scale nonlinearly with trap depth. To experimentally observe and constrain these shifts in an ^{171}Yb optical lattice clock, we construct a lattice enhancement cavity that exaggerates the light shifts. We observe an atomic temperature that is proportional to the optical trap depth, fundamentally altering the scaling of trap-induced light shifts and simplifying their parametrization. We identify an "operational" magic wavelength where frequency shifts are insensitive to changes in trap depth. These measurements and scaling analysis constitute an essential systematic characterization for clock operation at the 10^{-18} level and beyond.
Atomic clocks based on optical transitions are the most stable, and therefore precise, timekeepers available. These clocks operate by alternating intervals of atomic interrogation with the 'dead' ...time required for quantum state preparation and readout. This non-continuous interrogation of the atom system results in the Dick effect, an aliasing of frequency noise from the laser interrogating the atomic transition. Despite recent advances in optical clock stability that have been achieved by improving laser coherence, the Dick effect has continually limited the performance of optical clocks. Here we implement a robust solution to overcome this limitation: a zero-dead-time optical clock that is based on the interleaved interrogation of two cold-atom ensembles. This clock exhibits vanishingly small Dick noise, thereby achieving an unprecedented fractional frequency instability assessed to be for an averaging time tau in seconds. We also consider alternate dual-atom-ensemble schemes to extend laser coherence and reduce the standard quantum limit of clock stability, achieving a spectroscopy line quality factor of Q>410 super(15).
We report on the first Earth-scale quantum sensor network based on optical atomic clocks aimed at dark matter (DM) detection. Exploiting differences in the susceptibilities to the fine-structure ...constant of essential parts of an optical atomic clock, i.e., the cold atoms and the optical reference cavity, we can perform sensitive searches for DM signatures without the need for real-time comparisons of the clocks. We report a two orders of magnitude improvement in constraints on transient variations of the fine-structure constant, which considerably improves the detection limit for the standard model (SM)-DM coupling. We use Yb and Sr optical atomic clocks at four laboratories on three continents to search for both topological defect and massive scalar field candidates. No signal consistent with a DM coupling is identified, leading to considerably improved constraints on the DM-SM couplings.
We demonstrate the absence of a dc Stark shift in an ytterbium optical lattice clock. Stray electric fields are suppressed through the introduction of an in-vacuum Faraday shield. Still, the ...effectiveness of the shielding must be experimentally assessed. Such diagnostics are accomplished by applying high voltage to six electrodes, which are grounded in normal operation to form part of the Faraday shield. Our measurements place a constraint on the dc Stark shift at the 10^{-20} level, in units of the clock frequency. Moreover, we discuss a potential source of error in strategies to precisely measure or cancel nonzero dc Stark shifts, attributed to field gradients coupled with the finite spatial extent of the lattice-trapped atoms. With this consideration, we find that Faraday shielding, complemented with experimental validation, provides both a practically appealing and effective solution to the problem of dc Stark shifts in optical lattice clocks.
Red blood cell (RBC) transfusion therapy is a key component in the comprehensive management of patients with sickle cell disease (SCD). Consequently, most adult SCD patients will receive at least ...one, and many will receive more than a hundred RBC transfusions in their lifetime. SCD patients develop RBC alloantibodies much more frequently than non-SCD transfused patients, which often make the selection of compatible RBCs extremely difficult, in addition to placing patients at significantly higher risk of suffering from delayed hemolytic transfusion reactions (DHTRs). Similar to alloimunization, DHTRs are much more common in patients with SCD compared to other heavily transfused populations, and are particularly consequential due to their propensity to cause hyperhemolysis, a life-threatening phenomenon in which both transfused RBCs in addition to the patient's own sickle-erythrocytes are destroyed. In this review, we highlight the incidence and pathophysiology of DHTRs; illustrate common presentations, appropriate evaluations and outcomes of DHTRs in patients with SCD; and discuss strategies for preventing or reducing the likelihood of DHTRs from occurring.
La transfusion de globules rouges est un traitement majeur au cours de la drépanocytose. Ainsi la plupart des patients reçoivent au cours de leur vie de 1 à plusieurs centaines de concentrés de globules rouges. Ces patients développent beaucoup plus fréquemment des anticorps anti-érythrocytaires que les patients non drépanocytaires, entraînant une difficulté particulière à trouver des CGR compatibles, mais surtout les plaçant à risque de développer des hémolyses post-transfusionnelles. Parallèlement, ces réactions sont aussi beaucoup plus fréquentes chez ces patients que dans les autres populations de patients transfusés. Au cours de la drépanocytose, cette réaction hémolytique peut être particulièrement sévère, mettant en jeu le pronostic vital avec notamment une destruction concomitante des propres globules rouges du patient. Dans cette revue, nous faisons un focus sur l’incidence et la physiopathologie de cette réaction, avec la description de la présentation clinicobiologique, l’évolution, et les stratégies de prévention pour diminuer l’incidence de ces réactions.
Audio-visual recording and location tracking produce enormous quantities of digital data with which researchers can document children's everyday interactions in naturalistic settings and assessment ...contexts. Machine learning and other computational approaches can produce replicable, automated measurements of these big behavioral data. The economies of scale afforded by repeated automated measurements offer a potent approach to investigating linkages between real-time behavior and developmental change. In our work, automated measurement of audio from child-worn recorders-which quantify the frequency of child and adult speech and index its phonemic complexity-are paired with ultrawide radio tracking of children's location and interpersonal orientation. Applications of objective measurement indicate the influence of adult behavior in both expert ratings of attachment behavior and ratings of autism severity, suggesting the role of dyadic factors in these "child" assessments. In the preschool classroom, location/orientation measures provide data-driven measures of children's social contact, fertile ground for vocal interactions. Both the velocity of children's movement toward one another and their social contact with one another evidence homophily: children with autism spectrum disorder, other developmental disabilities, and typically developing children were more likely to interact with children in the same group even in inclusive preschool classrooms designed to promote interchange between all children. In the vocal domain, the frequency of peer speech and the phonemic complexity of teacher speech predict the frequency and phonemic complexity of children's own speech over multiple timescales. Moreover, children's own speech predicts their assessed language abilities across disability groups, suggesting how everyday interactions facilitate development.
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