Since the Efimov effect was introduced, a detailed theoretical understanding of Efimov physics has been developed in the few-body context. However, it has proven challenging to describe the role Efimov correlations play in many-body systems such as quenched or collapsing Bose-Einstein condensates (BECs). To study the impact the Efimov effect has in such scenarios, we consider a light impurity immersed in a weakly interacting BEC, forming a Bose polaron. In this case, correlations are localized around the impurity, making it more feasible to develop a theoretical description. Specifically, we employ a variational Gaussian state Ansatz in the reference frame of the impurity, capable of capturing both the Efimov effect and the formation of a polaron cloud consisting of a macroscopic number of particles. We find that the Efimov effect entails cooperative binding of bosons to the impurity, leading to the formation of large clusters. These many-particle Efimov states exist for a wide range of scattering lengths, with energies significantly below the polaron energy. As a result, the polaron is not the ground state, but rendered a metastable excited state which can decay into these clusters. While this decay is slow for small interaction strengths, it becomes more prominent as the attraction increases, up to a point where the polaron becomes completely unstable. We show that the critical scattering length where this happens can be interpreted as a many-body shifted Efimov resonance, where the scattering of two excitations of the bath with the polaron can lead to polaron-cloud assisted bound-state formation. Compared to the few-body case, the resonance is shifted to weaker attraction due to the participation of the polaron cloud in the cooperative binding process. This represents an intriguing example of chemistry in a quantum medium [A. Christianen et al., Phys. Rev. Lett. 128, 183401 (2022)], where many-body effects lead to a shift in the resonances of the chemical recombination, which can be directly probed in state-of-the-art experiments.

Bell inequalities follow from a set of seemingly natural assumptions about how to provide a causal model of a Bell experiment. In the face of their violation, two types of causal models that modify some of these assumptions have been proposed: (1) those that are parametrically conservative and structurally radical, such as models where the parameters are conditional probability distributions (termed “classical causal models”) but where one posits interlab causal influences or superdeterminism, and (2) those that are parametrically radical and structurally conservative, such as models where the labs are taken to be connected only by a common cause but where conditional probabilities are replaced by conditional density operators (these are termed “quantum causal models”). We here seek to adjudicate between these alternatives based on their predictive power. The data from a Bell experiment are divided into a training set and a test set, and for each causal model, the parameters that yield the best fit for the training set are estimated and then used to make predictions about the test set. Our main result is that the structurally radical classical causal models are disfavored relative to the structurally conservative quantum causal model. Their lower predictive power seems to be due to their tendency to mistake statistical fluctuations away from the no-signaling condition for real features and thereby overfit the data. Our technique shows that it is possible to witness quantumness even in a Bell experiment that does not close the locality loophole. It also overturns the notion that it is impossible to experimentally test the plausibility of superdeterminist models of Bell inequality violations.

We propose two schemes to obtain Gottesman-Kitaev-Preskill (GKP) error syndromes by means of linear-optical operations, homodyne measurements, and GKP ancillas. This includes showing that for a concatenation of GKP codes with an $[n,k,d]$ stabilizer code only $2n$ measurements are needed in order to obtain the complete syndrome information, significantly reducing the number of measurements in comparison to the canonical concatenated measurement scheme and at the same time generalizing linear-optics-based syndrome detections to higher GKP codes. Furthermore, we analyze the possibility of building the required ancilla states from single-mode states and linear optics. We find that for simple GKP codes this is possible, whereas for concatenations with qubit Calderbank-Shor-Steane codes of distance $d\ge 3$ it is not. We also consider the canonical concatenated syndrome measurements and propose methods for avoiding crosstalk between ancillas. In addition, we make use of the observation that the concatenation of a GKP code with a stabilizer code forms a lattice in order to see the analog information decoding of such codes from a different perspective allowing for semianalytic calculations of the logical error rates.

The strong short-range repulsion, characteristic to realistic interatomic potentials, complicates the description of weakly bound few-body systems such as those of ${}^4\mathrm{He}$ atoms. The present work proposes an approach for solving this problem and applies it to a realistic system of four ${}^4\mathrm{He}$ atoms. The potential is gradually softened such that rigorous four-body equations for bound and scattering states can be accurately solved in the momentum-space framework, and the results are extrapolated back to the limit of the original potential. Linear correlations between three- and four-body quantities are observed, and the accuracy of the procedure is improved by extrapolating in one of the three-body quantities. Results for the ${}^4\mathrm{He}$ tetramer ground-state and excited-state binding energies and atom-trimer scattering agree well with at least some of earlier determinations and shed light on the existing disagreements. An additional case of the Phillips correlation line is established for the dimer-dimer scattering length. The trimer production rate via the ultracold two-dimer collisions is estimated; it exhibits significant finite-range effects despite the weak binding of the dimer.

The toric code is a simple and exactly solvable example of topological order realizing Abelian anyons. However, it was shown to support nonlocal lattice defects, namely twists, which exhibit non-Abelian anyonic behavior [Phys. Rev. Lett. 105, 030403 (2010)]. Motivated by this result, we investigated the potential of having non-Abelian statistics from puncture defects on the toric code. We demonstrate that an encoding with mixed-boundary punctures reproduces Ising fusion, and a logical Pauli-$X$ upon their braiding. Our construction paves the way for local lattice defects to exhibit non-Abelian properties that can be employed for quantum information tasks.

Hyperfine atomic states are among the most promising candidates for qubit encoding in quantum information processing. In atomic systems, hyperfine transitions are typically driven through a two-photon Raman process by a laser field which is amplitude modulated at the hyperfine qubit frequency. Here we introduce a method for generating amplitude modulation by phase modulating a laser and reflecting it from a highly dispersive optical element known as a chirped Bragg grating. This approach is passively stable, offers high efficiency, and is compatible with high-power laser sources, enabling large Rabi frequencies and improved quantum coherence. We benchmark this approach by globally driving an array of approximately 300 neutral ${}^{87}\mathrm{Rb}$ atomic qubits trapped in optical tweezers and obtain Rabi frequencies of 2 MHz with photon-scattering error rates of less than $2\times {10}^{-4}$ per $\pi$ pulse. This robust approach can be directly integrated with local addressing optics in both neutral atom and trapped ion systems to facilitate high-fidelity single-qubit operations for quantum information processing.

We investigate the strong out-of-equilibrium dynamics occurring when a harmonically trapped ultracold bosonic gas is evaporatively cooled across the Bose-Einstein condensation transition. By imaging the cloud after free expansion, we study how the cooling rate affects the timescales for the growth of the condensate order parameter and the relaxation dynamics of its spatial fluctuations. We find evidence of a delay on the condensate formation related to the collisional properties and a universal condensate growth following the cooling rate. Finally, we measure an exponential relaxation of the spatial fluctuations of the order parameter that also shows a universal scaling. Notably, the scaling for the condensate growth and for the relaxation of its fluctuations follow different power laws.

Recently, intensely driven laser-matter interactions have been used to connect the fields of strong laser field physics with quantum optics by generating nonclassical states of light. Here, we take a further key step and show the potential of strong laser fields for generating controllable high-photon-number coherent-state superpositions. This has been achieved by using two of the most prominent strong-laser induced processes: high-harmonic generation and above-threshold ionization. We show how the obtained coherent-state superpositions change from an optical Schrödinger “cat” state to a “kitten” state by changing the atomic density in the laser-atom interaction region, and we demonstrate the generation of a nine-photon shifted optical “cat” state, which, to our knowledge, is the highest photon number optical “cat” state experimentally reported. Our findings anticipate the development of new methods that naturally lead to the creation of high-photon-number controllable coherent-state superpositions, advancing investigations in quantum technology.

X-ray free-electron-laser facilities generate nowadays ultrashort pulses with intensities as high as ${10}^{20}\mathrm{W}/{\mathrm{cm}}^2$. The interaction of these light sources with atoms and molecules thus opens the way to still unexplored phenomena, where the commonly employed dipole approximation breaks down, and nonlinear excitation and ionization, e.g., Raman and Compton-like processes, come into play. We have developed theoretical tools to describe the interaction of intense attosecond soft-x-ray pulses with the hydrogen molecule in the photon energy ranging from 0.5 to 1.6 keV. We solve the time-dependent Schrödinger equation including both dipole and nondipole terms to obtain single-ionization photoelectron spectra differential in both energy and angle of the ejected electron. The signature of stimulated Compton scattering is clearly seen in the photoelectron spectra just above the ionization threshold, where nondipole effects manifest. Remarkably, the interference between dipole- and nondipole-induced quantum paths leads to a significant asymmetry of the photoelectron angular distributions. We show that this asymmetry strongly depends on the relative direction of the light polarization and propagation vectors with respect to the molecular orientation.

If one seeks to test quantum theory against many alternatives in a landscape of possible physical theories, then it is crucial to be able to analyze experimental data in a theory-agnostic way. This can be achieved using the framework of generalized probabilistic theories (GPTs). Here we implement GPT tomography on a three-level system corresponding to a single photon shared among three modes. This scheme achieves a GPT characterization of each of the preparations and measurements implemented in the experiment without requiring any prior characterization of either. Assuming that the sets of realized preparations and measurements are tomographically complete, our analysis identifies the most likely dimension of the GPT vector space describing the three-level system to be nine, in agreement with the value predicted by quantum theory. Relative to this dimension, we infer the scope of GPTs that are consistent with our experimental data by identifying polytopes that provide inner and outer bounds for the state and effect spaces of the true GPT. From these, we are able to determine quantitative bounds on possible deviations from quantum theory. In particular, we bound the degree to which the no-restriction hypothesis might be violated for our three-level system.

Entangled quantum states share properties that do not have classical analogs; in particular, they show correlations that can violate Bell inequalities. It is, therefore, an interesting question to see what happens to entanglement measures—such as the entanglement entropy for a pure state—taking the semiclassical limit, where the naive expectation is that they may become singular or zero. This conclusion is, however, incorrect. In this paper, we determine the $\hslash \to 0$ limit of the bipartite entanglement entropy for a one-dimensional system of $N$ quantum particles in an external potential and we explicitly show that this limit is finite. Moreover, if the particles are fermionic, we show that the $\hslash \to 0$ limit of the bipartite entanglement entropy coincides with the Shannon entropy of $N$ bits.

We provide a comprehensive study of ultracold-atom diffraction by an optical lattice. We focus on an intermediate regime between the Raman-Nath and Bragg regimes, the so-called quasi-Bragg regime. The experimental results are in good agreement with a full numerical integration of the Schrödinger equation. We investigate the long-pulse regime limited by a strong velocity selection and the short-pulse regime limited by “nonadiabatic losses.” For each of these regimes, we estimate the multiport features of the Bragg interferometers. Finally, we discuss the best compromise between these two regimes, considering the diffraction phase shift and the existence of parasitic interferometers.

The process of single-photon subtraction (SPS) is known to dramatically alter the properties of certain quantum optical states. Somewhat surprisingly, subtracting zero photons can also modify quantum states and has practical applications in quantum communication. Here we experimentally investigate zero-photon subtraction (ZPS) using a wide variety of input states and conditional measurements based on actively detecting zero photons in one output port of a variable beamsplitter. We find that SPS and ZPS can exhibit complementary behavior depending on the photon statistics of the input states and highlight deeper connections with Mandel's $Q$ parameter for classifying quantum states.

Trapped ions form an advanced technology platform for quantum information processing with long qubit coherence times, high-fidelity quantum logic gates, optically active qubits, and a potential to scale up in size while preserving a high level of connectivity between qubits. These traits make them attractive not only for quantum computing, but also for quantum networking. Dedicated, special-purpose trapped-ion processors in conjunction with suitable interconnecting hardware can be used to form quantum repeaters that enable high-rate quantum communications between distant trapped-ion quantum computers in a network. In this regard, hybrid traps with two distinct species of ions, where one ion species can generate ion-photon entanglement that is useful for optically interfacing with the network and the other has long memory lifetimes, useful for qubit storage, have been proposed for entanglement distribution. We consider an architecture for a repeater based on such dual-species trapped-ion systems. We propose and analyze a protocol based on spatial and temporal mode multiplexing for entanglement distribution across a line network of such repeaters. Our protocol offers enhanced rates compared to rates previously reported for such repeaters. We determine the ion resources required at the repeaters to attain the enhanced rates, and the best rates attainable when constraints are placed on the number of repeaters and the number of ions per repeater. Our results bolster the case for near-term trapped-ion systems as quantum repeaters for long distance quantum communications.

An excellent candidate molecule for the measurement of the electron's electric dipole moment (eEDM) is thorium monofluoride (${\mathrm{ThF}}^{+}$) because the eEDM–sensitive state, ${}^3{\mathrm{\Delta }}_1$, is the electronic ground state, and thus is immune to decoherence from spontaneous decay. We perform spectroscopy on $X{}^3{\mathrm{\Delta }}_1$ to extract three spectroscopic constants crucial to the eEDM experiment: the hyperfine coupling constant, the molecular frame electric dipole moment, and the magnetic $g$ factor. To understand the impact of thermal blackbody radiation on the vibrational ground state, we study the lifetime of the first excited vibrational manifold of $X{}^3{\mathrm{\Delta }}_1$. We perform ab initio calculations, compare them to our results, and discuss prospects for using ${\mathrm{ThF}}^{+}$ in an eEDM experiment at JILA.

We study the interaction of a two-level atom with a single-mode nonlinear electromagnetic resonator, considering coupling strengths ranging from zero to the so-called deep strong coupling regime. When the qubit-resonator coupling is very strong, the standard Kerr model for the resonator becomes questionable. Moreover, recently, it has been shown that extra care is needed when constructing gauge-independent theories in the presence of approximations as the truncation of the Hilbert space of the matter system. Such a truncation can ruin gauge invariance leading to nonphysical results, especially when the light-matter interactions strength is very high. Here we face and solve these issues to provide a consistent nonlinear-resonator quantum Rabi model satisfying the gauge principle.

Experimental measurements and theoretical calculations of state-to-state rate coefficients for rotational energy transfer of CO in collision with ${\mathrm{H}}_2$ are reported at the very low temperatures prevailing in dense interstellar clouds ($5–20$ K). Detailed agreement between quantum state-selected experiments performed in cold supersonic flows using time-resolved infrared–vacuum-ultraviolet double-resonance spectroscopy and close-coupling quantum scattering calculations confirms the validity of the calculations for collisions between the two most abundant molecules in the interstellar medium.

Synthetic chiral light fields were recently introduced as a novel source of chirality [Ayuso et al., Nat. Photonics 13, 866 (2019)]. This locally chiral light spans a three-dimensional polarization that plots a chiral trajectory in space-time, leading to huge nonlinear chiral signals upon interactions with chiral media. The degree of chirality of this new form of light was defined, characterized, and shown to be proportional to the chiral signal in high harmonic generation and photo-emission. However, the sign of the light's chirality—its “handedness”—has not yet been defined. Standard definitions of helicity are inapplicable for locally chiral light due to its complex three-dimensional structure. Here, we define an unambiguous handedness for locally chiral fields and employ it in practical calculations.

High-Q microwave cavity modes coupled to transmon ancillas provide a hardware-efficient platform for quantum computing. Due to their coupling, the cavity modes inherit finite nonlinearity from the transmons. In this paper, we theoretically and experimentally investigate how an off-resonant drive on the transmon ancilla modifies the nonlinearities of the cavity modes in qualitatively different ways, depending on the interrelation among cavity-transmon detuning, drive-transmon detuning, and transmon anharmonicity. For a cavity-transmon detuning that is smaller than or comparable to the drive-transmon detuning and transmon anharmonicity, the off-resonant transmon drive can induce multiphoton resonances among cavity and transmon excitations that strongly modify cavity nonlinearities as drive parameters vary. For a large cavity-transmon detuning, the drive induces cavity-photon-number-dependent ac Stark shifts of transmon levels that translate into effective cavity nonlinearities. In the regime of weak transmon-cavity coupling, the cavity Kerr nonlinearity relates to the third-order nonlinear susceptibility function ${\chi }^{\left(3\right)}$ of the driven ancilla. This susceptibility function provides a numerically efficient way of computing the cavity Kerr, particularly for systems with many cavity modes controlled by a single transmon. It also serves as a diagnostic tool for identifying undesired drive-induced multiphoton resonance processes. Lastly, we show that by judiciously choosing the drive amplitude, a single off-resonant transmon drive can be used to cancel the cavity self-Kerr nonlinearity or the intercavity cross-Kerr. This provides a way of dynamically correcting the cavity Kerr nonlinearity during bosonic operations and quantum error correction protocols that rely on the cavity modes being linear.

Pairs of photons entangled in their time-frequency degree of freedom are of great interest in quantum optics research and applications, due to their relative ease of generation and their high capacity for encoding information. Here we analyze, both theoretically and experimentally, the behavior of phase-insensitive spectrally resolved interferences arising from two pairs of time-frequency-entangled photons. At its core, this is a multimode entanglement swapping experiment, whereby a spectrally resolved joint measurement on the idler photons from both pairs results in projecting the signal photons onto a Bell state the form of which depends on the measurement outcome. Using this scheme we herald a large, though nonorthogonal, set of pulse-mode Bell states, which we characterize through spectral measurements and through two-photon quantum interference. In principle, the number of mutually orthogonal states which can be generated with our scheme is limited only by the amount of entanglement available in the sources, and thus our setup demonstrates a versatile and scalable multimode entanglement swapping scheme requiring no source engineering.