We report on a conceptually new test of the equivalence principle performed by measuring the acceleration in Earth's gravity field of two isotopes of strontium atoms, namely, the bosonic $^{88}\mathrm{Sr}$ isotope which has no spin versus the fermionic $^{87}\mathrm{Sr}$ isotope which has a half-integer spin. The effect of gravity on the two atomic species has been probed by means of a precision differential measurement of the Bloch frequency for the two atomic matter waves in a vertical optical lattice. We obtain the values $$\eta${}=(0.2\ifmmode\pm\else\textpm\fi{}1.6)\ifmmode\times\else\texttimes\fi{}{10}^{$-${}7}$ for the E\"otv\"os parameter and $k=(0.5\ifmmode\pm\else\textpm\fi{}1.1)\ifmmode\times\else\texttimes\fi{}{10}^{$-${}7}$ for the coupling between nuclear spin and gravity. This is the first reported experimental test of the equivalence principle for bosonic and fermionic particles and opens a new way to the search for the predicted spin-gravity coupling effects.

We report on a precision measurement of gravitational acceleration using ultracold strontium atoms confined in an amplitude-modulated vertical optical lattice. An uncertainty $\Delta${}g/g$\approx${}10-7 is reached by measuring at the 5th harmonic of the Bloch frequency. The value obtained with this microscopic quantum system is consistent with the one measured with a classical gravimeter. Using lattice modulation to prepare the atomic sample, we also achieve high visibility of Bloch oscillations for $\sim${}20\,\,s. These results can be of relevance for testing gravitational redshift and Newtonian law at micrometer scale.

We carry out two searches for periodic gravitational waves using the most sensitive few hours of data from the second LIGO science run. Both searches exploit fully coherent matched filtering and cover wide areas of parameter space, an innovation over previous analyses which requires considerable algorithm development and computational power. The first search is targeted at isolated, previously unknown neutron stars, covers the entire sky in the frequency band 160–728.8 Hz, and assumes a frequency derivative of less than $4\times {10}^{-10}\mathrm{Hz}/\mathrm{s}$. The second search targets the accreting neutron star in the low-mass x-ray binary Scorpius X-1 and covers the frequency bands 464–484 Hz and 604–624 Hz as well as the two relevant binary orbit parameters. Because of the high computational cost of these searches we limit the analyses to the most sensitive 10 hours and 6 hours of data, respectively. Given the limited sensitivity and duration of the analyzed data set, we do not attempt deep follow-up studies. Rather we concentrate on demonstrating the data analysis method on a real data set and present our results as upper limits over large volumes of the parameter space. In order to achieve this, we look for coincidences in parameter space between the Livingston and Hanford 4-km interferometers. For isolated neutron stars our 95% confidence level upper limits on the gravitational wave strain amplitude range from $6.6\times {10}^{-23}$ to $1\times {10}^{-21}$ across the frequency band; for Scorpius X-1 they range from $1.7\times {10}^{-22}$ to $1.3\times {10}^{-21}$ across the two 20-Hz frequency bands. The upper limits presented in this paper are the first broadband wide parameter space upper limits on periodic gravitational waves from coherent search techniques. The methods developed here lay the foundations for upcoming hierarchical searches of more sensitive data which may detect astrophysical signals.

We report on a search for gravitational waves from the coalescence of compact binaries during the third and fourth LIGO science runs. The search focused on gravitational waves generated during the inspiral phase of the binary evolution. In our analysis, we considered three categories of compact binary systems, ordered by mass: (i) primordial black hole binaries with masses in the range $0.35M_{\odot }<m_1$, $m_2<1.0M_{\odot }$, (ii) binary neutron stars with masses in the range $1.0M_{\odot }<m_1$, $m_2<3.0M_{\odot }$, and (iii) binary black holes with masses in the range $3.0M_{\odot }<m_1$, $m_2<m_{\max }$ with the additional constraint $m_1+m_2<m_{\max }$, where $m_{\max }$ was set to $40.0M_{\odot }$ and $80.0M_{\odot }$ in the third and fourth science runs, respectively. Although the detectors could probe to distances as far as tens of Mpc, no gravitational-wave signals were identified in the 1364 hours of data we analyzed. Assuming a binary population with a Gaussian distribution around $0.75-0.75M_{\odot }$, $1.4-1.4M_{\odot }$, and $5.0-5.0M_{\odot }$, we derived 90%-confidence upper limit rates of $4.9{\mathrm{yr}}^{-1}{L}_{10}^{-1}$ for primordial black hole binaries, $1.2{\mathrm{yr}}^{-1}{L}_{10}^{-1}$ for binary neutron stars, and $0.5{\mathrm{yr}}^{-1}{L}_{10}^{-1}$ for stellar mass binary black holes, where $L_{10}$ is ${10}^{10}$ times the blue-light luminosity of the Sun.

An experimental study characterizes subradiance—inhibited emission due to destructive interference—in ultracold molecules close to the dissociation limit and shows that it could be used for precision molecular spectroscopy. Weakly bound molecules have physical properties without atomic analogues, even as the bond length approaches dissociation. For instance, the internal symmetries of homonuclear diatomic molecules result in the formation of two-body superradiant and subradiant excited states. Whereas superradiance1,2,3 has been demonstrated in a variety of systems, subradiance4,5,6 is more elusive owing to the inherently weak interaction with the environment. Here we characterize the properties of deeply subradiant molecular states with intrinsic quality factors exceeding 1013 via precise optical spectroscopy with the longest molecule–light coherent interaction times to date. We find that two competing effects limit the lifetimes of the subradiant molecules, with different asymptotic behaviours. The first is radiative decay via weak magnetic-dipole and electric-quadrupole interactions. We prove that its rate increases quadratically with the bond length, confirming quantum mechanical predictions. The second is non-radiative decay through weak gyroscopic predissociation, with a rate proportional to the vibrational mode spacing and sensitive to short-range physics. This work bridges the gap between atomic and molecular metrology based on lattice-clock techniques7, enhancing our understanding of long-range interatomic interactions.