In recent twenty years, the research on nuclear clocks based on Th nuclear transition has become increasingly intensive. Among them, solid-state nuclear clocks based on thorium-doped crystals are considered to have unique advantages in the detections of accurate energy and lifetime of nuclear isomers and the stability and miniaturization of optical clocks. In this work, The feasibility and potential of using Th:$\mathrm{BaLi}{\mathrm{F}}_3$ single crystal as a solid-state nuclear clock material was calculated and predicted by using density functional theory (DFT). The theoretical results showed that Th ions could be effectively doped in $\mathrm{BaLi}{\mathrm{F}}_3$ crystal through a suitable charge compensation mechanism, and only slightly decreased the transmittance of $\mathrm{BaLi}{\mathrm{F}}_3$ crystal in the nuclear transition band. On this basis, a Th:$\mathrm{BaLi}{\mathrm{F}}_3$ single crystal was successfully grown through experiments. Th concentration of the as-grown crystal can reach $3.13\times {10}^{18}\mathrm{c}{\mathrm{m}}^{-3}$, and the transmittance can reach $T$ ∼ 50% at ∼150 nm. Compared with the pure $\mathrm{BaLi}{\mathrm{F}}_3$ crystal grown under similar conditions, it only slightly decreased. The Th:$\mathrm{BaLi}{\mathrm{F}}_3$ single crystal is expected to be a potential candidate for solid-state nuclear clock material.

We study the effect of electron-electron correlations on the quantum state of the light emitted from high-harmonic generation (HHG). The quantum state of the emitted light is obtained by using a fully quantum-mechanical description of both the optical modes as well as the electronic system. This is different from the usual semiclassical description of HHG, which only treats the electronic target system quantum mechanically. Using the generic Fermi-Hubbard model, the strength of the electron-electron correlation can be treated as a parameter enabling us to investigate the two limiting cases of a completely uncorrelated phase and a correlated Mott-insulating phase. In the completely uncorrelated phase, the model reduces to a single-band tight-binding model in which only intraband currents contribute to the spectrum. In this limit, we analytically find that the emitted light is in a classical coherent state. In the Mott-insulating phase, a consideration of the photon statistics and squeezing of the emitted photonic state shows that the inter-Hubbard-subband current generates nonclassical light. In this sense, we show that electron-electron correlation can induce the generation of nonclassical states of light.

We analyze the formation of three-dimensional spatiotemporal solitons in waveguides with a parabolic refractive index profile and pure quartic chromatic dispersion. We show, by applying both variational approaches and full three-dimensional numerical simulations, that fourth-order dispersion has a positive impact on soliton stabilization against spatiotemporal wave collapse. Specifically, pure quartic spatiotemporal solitons remain stable within a significantly larger energy range with respect to their second-order dispersion counterparts.

We demonstrate a method for determining the three-dimensional location of single atoms in a quantum gas microscopy system using a phase-only spatial light modulator to modify the point-spread function of the high-resolution imaging system. Here, the typical diffracted spot generated by a single atom as a point source is modified to a double spot that rotates as a function of the atom's distance from the focal plane of the imaging system. We present and numerically validate a simple model linking the rotation angle of the point-spread function with the distance to the focal plane. We show that, when aberrations in the system are carefully calibrated and compensated for, this method can be used to determine an atom's position to within a single lattice site in a single experimental image, extending quantum simulation with microscopy systems further into the regime of three dimensions.

We introduce a family of quantized field states that can perform exact (entanglement- and error-free) rotations of a two-level atom starting from a specific state on the Bloch sphere. We discuss the similarities and differences between these states and the recently introduced transcoherent states. Our field states have the property that they are left unchanged after the rotation, and we find they are the asymptotic states obtained when a field interacts with a succession of identically prepared ancillary atoms. Such a scheme was recently proposed [npj Quantum Inf. 3, 17 (2017)] as a way to “restore” a field state after its interaction with a two-level atom, so as to reuse it afterward, thus reducing the energy requirements for successive quantum logical operations. We generalize this scheme to find optimal pulses for arbitrary rotations and also study analytically what happens if the ancillas are in a mixed rather than a pure state. Consistent with the numerical results in the original proposal, we find that as long as the ancilla preparation error is small (of the order of $1/\overline{n}$, where $\overline{n}$ is the average number of atoms in the pulses considered) it will introduce only higher-order errors in the performance of the restored pulse.

Dipolar gases like erbium and dysprosium have a dense spectrum of resonant loss features associated with their strong anisotropic interaction potential. These resonances display various behaviors with density and temperature, implying diverse microscopic properties. Here we quantitatively investigate the low-field ($B<6\mathrm{G}$) loss features in ultracold thermal samples of ${}^{162}\mathrm{Dy}$. The atoms are spin polarized in their lowest internal state so that pure two-body losses due to spin relaxation are forbidden. However, our analysis reveals that some resonances lead to a two-body-like decay law, while others show the expected three-body decay. We present microscopic one-step and two-step models for these losses, investigate their temperature dependence, and detect a feature compatible with a $d$-wave Fano-Feshbach resonance that has not been observed before. We also report the variation of the scattering length around these resonances, inferred from the time-of-flight expansion of the condensate.

We demonstrate that the efficiency of effective-negative-temperature-based quantum Otto engines, already known to outperform their traditional counterparts operating with positive-temperature thermal reservoirs, can be further improved by terminating the isochoric strokes before the working substance reaches perfect equilibrium with its environment. Our investigation encompasses both Markovian and non-Markovian dynamics during these finite-time isochoric processes while considering a weak coupling between the working substance and the reservoirs. We assess the performance of these engines as they undergo a transition from the Markovian to the non-Markovian regime using two figures of merit: maximum achievable efficiency at a certain finite time during the isochoric heating stroke, and overall performance of the engine over an extended period during the transient phase of this stroke. We show that the maximum efficiency increases with the increase of non-Markovianity. However, the overall engine performance decreases as non-Markovianity increases. Additionally, we discover the existence of effective-negative-temperature-based necessarily transient quantum Otto engines. These engines operate within an extended operational domain, reaching into temperature ranges where conventional effective-negative-temperature-based quantum Otto engines, which rely on perfect thermalization during the isochoric strokes, are unable to function. Furthermore, this extended operational domain of an effective-negative- temperature-based necessarily transient quantum Otto engine increases as non-Markovianity becomes more pronounced.

When He is ionized by a chirped xuv pulse, additional peaks are identified in the joint energy spectra besides ordinary ionization peaks. Our results are based on the solution of the full-dimensional time-dependent Schrödinger equation of helium. Further numerical studies show that these additional structures are sensitive to both the pulse duration and the chirp parameter. To reveal the underlying mechanism, we have developed an analytical model based on the symmetric ionization assumption and the second-order time-dependent perturbation theory, which can qualitatively reproduce these additional structures. By systematic comparison studies, one can attribute the origin of these structures to the interference of two indistinguishable electrons and the diffraction in the time domain. In addition, we find that the interference effect is absent when one uses a negatively chirped laser pulse, while the diffraction in the time domain always persists.

Causal networks beyond that in the paradigmatic Bell's theorem can lead to new kinds and applications of nonclassical behavior. Their study, however, has been hindered by the fact that they define a nonconvex set of correlations and only very incomplete or approximated descriptions have been obtained so far, even for the simplest scenarios. Here we take a different stance on the problem and consider the relative volume of classical or nonclassical correlations a given network gives rise to, considering distances to sets of interest and how they distribute too. Among other results, we show instances where the inflation technique, arguably the most disseminated tool in the community, is unable to detect a significant portion of the nonclassical behaviors, up to three copies of each source, and that a concentration phenomenon of distances happens in one of them. Interestingly, we also show that the use of interventions, a central tool in causal inference, can enhance substantially our ability to witness nonclassicality.

We experimentally demonstrate control of molecular rotation in bulk superfluid ${}^4\mathrm{He}$. Metastable helium dimers, ${\mathrm{He}}_2{}^{*}$, are rotationally excited by a periodic train of linearly polarized femtosecond laser pulses. We show that the degree of rotational excitation of ${\mathrm{He}}_2{}^{*}$ can be enhanced or suppressed by varying the period of the pulse train, whereas the directionality of molecular rotation can be controlled by the relative angle between the polarization vectors of pulses in the train. The experimental results are in agreement with numerical calculations, based on a simple model, in which ${\mathrm{He}}_2{}^{*}$ molecules do not interact with the superfluid.

Recently the photonic golden rule, which predicts that the spontaneous emission rate of an atom depends on the projected local density of states, was shown to fail in an optical medium with a linear gain amplifier. We present a classical light-matter theory to fix this widely used spontaneous emission rate, fully recovering the quantum-mechanical rate reported in Franke et al. [S. Franke, J. Ren, M. Richter, A. Knorr, and S. Hughes, Phys. Rev. Lett. 127, 013602 (2021)]. The corrected classical Purcell factor, for media containing linear amplifiers, is obtained in two different forms, both of which can easily be calculated in any standard classical Maxwell equation solver. We also derive explicit analytical results in terms of quasinormal modes, which are useful for studying practical cavity structures in an efficient way, including the presence of local-field effects for finite-size dipole emitters embedded inside lossy or gain materials (using a real cavity model). Finally, we derive a full classical correspondence from the viewpoint of quantized quasinormal modes in the bad cavity limit. Example numerical calculations are shown for coupled loss-gain microdisk resonators, showing excellent agreement between few-mode expansions and full numerical dipole simulations.

The experimental realization of Fermi-Hubbard tweezer arrays opens a new stage for engineering fermionic matter, where programmable lattice geometries and Hubbard model parameters are combined with single-site imaging. In order to use these versatile experimental Fermi-Hubbard models as quantum simulators, it is crucial to know the Hubbard parameters describing them. Here we develop methods to calculate the Hubbard model parameters of arbitrary two-dimensional lattice geometries: The tunneling $t$, on-site potential $V$, and interaction $U$ for multiple bands and for both fermions and bosons. We show several examples. One notable finding is that a finite array of equally strong and separated individual tweezer potentials actually sums to give a nonperiodic total potential and thus spatially nonuniform Hubbard parameters. We demonstrate procedures to find trap configurations that equalize these parameters. More generally, these procedures solve the inverse problem of calculating Hubbard parameters: Given desired Hubbard parameters, find trap configurations to realize them. These methods will be critical tools for using tunnel-coupled tweezer arrays.

It is now well understood that a Lorentz boost of a spatially coherent monochromatic optical beam yields a so-called space-time wave packet (STWP): A propagation-invariant pulsed beam whose group velocity is determined by the relative velocity between the source and observer. Moreover, the Lorentz boost of an STWP is another STWP, whose group velocities are related by the relativistic law for addition of velocities typically associated with massive particles. We present an experimental procedure for testing this prediction in both the subluminal and superluminal regimes that makes use of spatiotemporal Fourier synthesis via a spatial light modulator. Our approach enables realizing the change in temporal bandwidth, invariance of the spatial bandwidth, concomitant change in the spatiotemporal wave-packet envelope, and change in group velocity that all accompany a Lorentz boost of a monochromatic optical beam. The only consequence of the Lorentz boost not captured by this methodology is the Doppler shift in the optical carrier. This work may provide an avenue for further table-top demonstration of relativistic transformations of optical fields.

A test of quantumness is a protocol where a classical user issues challenges to a quantum device to determine if it exhibits nonclassical behavior, under certain cryptographic assumptions. Recent attempts to implement such tests on current quantum computers rely on either interactive challenges with efficient verification or noninteractive challenges with inefficient (exponential time) verification. In this paper, we execute an efficient noninteractive test of quantumness on an ion-trap quantum computer. Our results significantly exceed the bound for a classical device's success.

Variational quantum algorithms offer fascinating prospects for the solution of combinatorial optimization problems using digital quantum computers. However, the achievable performance in such algorithms and the role of quantum correlations therein remain unclear. Here, we shed light on this open issue by establishing a tight connection to the seemingly unrelated field of quantum metrology: Metrological applications employ quantum states of spin ensembles with a reduced variance to achieve an increased sensitivity, and we cast the generation of such squeezed states in the form of finding optimal solutions to a combinatorial MaxCut problem with an increased precision. By solving this optimization problem with a quantum approximate optimization algorithm (QAOA), we show numerically as well as on an IBM Quantum chip how highly squeezed states are generated in a systematic procedure that can be adapted to a wide variety of quantum machines. Moreover, squeezing tailored for the QAOA of the MaxCut permits us to propose a figure of merit for future hardware benchmarks. Exploiting the connection, we show how the performance can be improved by warm-starting the optimization algorithm with the squeezed state.

We introduce a classical analog of quantum matter in ultracold molecule-synthetic or Rydberg atom-synthetic dimensions, by extending the Potts model to include interactions $J_1$ between atoms adjacent in both real and synthetic space and studying its finite-temperature properties. For intermediate values of $J_1$, the resulting phases and phase diagrams are similar to those of the clock and Villain models, in which three phases emerge. There exists a sheet phase analogous to that found in quantum synthetic dimension models between the high-temperature disordered phase and the low-temperature ferromagnetic phase. We also employ machine learning to uncover nontrivial features of the phase diagram using the learning by confusion approach, which is able to discern several successive phase transitions.

Signatures of superfluid-like behavior have recently been observed experimentally in a nonlinear optical mesh lattice, where the arrival time of optical pulses propagating in a pair of coupled optical fiber loops is interpreted as a synthetic spatial dimension. Here we develop a general theory of the fluid of light in such optical mesh lattices. On the one hand, this theory provides a solid framework for an analytical and numerical interpretation of the experimental observations. On the other hand, it anticipates new physical effects stemming from the specific spatiotemporally periodic geometry of our setup. Our work opens the way towards the full exploitation of optical mesh lattices system as a promising platform for studies of hydrodynamics phenomena in fluids of light in novel configurations.

The building blocks of quantum algorithms and software are quantum gates, with the appropriate combination of quantum gates leading to a desired quantum circuit. Deep expert knowledge is necessary to discover effective combinations of quantum gates to achieve a desired quantum algorithm for solving a specific task. This is especially challenging for quantum machine learning and signal processing. For example, it is not trivial to design a quantum Fourier transform from scratch. This work proposes a quantum architecture search algorithm which is based on a Monte Carlo graph search and measures of importance sampling. It is applicable to the optimization of gate order for both discrete gates and gates containing continuous variables. Several numerical experiments demonstrate the applicability of the proposed method for the automatic discovery of quantum circuits.

We discuss the role of nonideal measurements within the context of measurement engines by contrasting examples of measurement engines which have the same work output but with varying amounts of entanglement. Accounting for the cost of resetting, correlating the engine to a pointer state, and cooling the pointer state, we show that for a given work output, thermally correlated engines can outperform corresponding entanglement engines. We also show that the optimal efficiency of the thermally correlated measurement engine is achieved with a higher-temperature pointer than the pointer temperature of the optimal entanglement engine.

We investigate fermionic Mott insulators in a geometrically frustrated triangular lattice, a paradigm model system for studying spin liquids and spontaneous time-reversal symmetry breaking. Our study demonstrates the preparation of triangular Mott insulators and reveals antiferromagnetic spin-spin correlations among all nearest neighbors. We employ a real-space triangular-geometry quantum gas microscope to measure density and spin observables. Comparing experimental results with calculations based on numerical linked cluster expansions and quantum Monte Carlo techniques, we demonstrate thermometry in the frustrated system. Our experimental platform introduces an alternative approach to frustrated lattices which paves the way for future investigations of exotic quantum magnetism which may lead to a direct detection of quantum spin liquids in Hubbard systems.