Bogoliubov excitations of Bose-Einstein condensates in optical lattices may possess band topology in analogous to topological insulators in class AII of fermions. Using the language of the Krein-space theory, this topological property is shown to be protected by a pseudo-time-reversal symmetry that is pseudo-antiunitary and squares to $-1$, with the associated bulk topological invariant also being a ${\mathbb{Z}}_2$ index. We construct three equivalent expressions for it, relating to the Pfaffian, the pseudo-time-reversal polarization, and most practically the Wannier center flow, all adopted from the fermionic case, defined here with respect to the pseudo inner product. In the presence of an additional pseudo-unitary and pseudo-Hermitian inversion symmetry, a simpler expression is derived. We then study two toy models feasible on cold atom platforms to numerically confirm the bulk-boundary correspondence. The Krein-space approach developed in this work is a universal formalism to study all kinds of symmetry-protected topological bosonic Bogoliubov bands.

We perform an analysis of the supercooled state in an analog to an early universe phase transition based on a one-dimensional, two-component Bose gas. We demonstrate that the thermal fluctuations in the relative phase between the components are characteristic of a relativistic thermal system. Furthermore, we demonstrate the equivalence of two different approaches to the decay of the metastable state: specifically a nonperturbative thermal instanton calculation and a stochastic Gross-Pitaevskii simulation.

Cholesky factorization provides photonic lattices that are the isosectral partners or the square root of other arrays of coupled waveguides. The procedure is similar to that used in supersymmetric quantum mechanics. However, Cholesky decomposition requires initial positive-definite mode-coupling matrices and the resulting supersymmetry is always broken. That is, the isospectral partner has the same range as that of the initial mode-coupling matrix. It is possible to force a decomposition where the range of the partner is reduced but the characteristic supersymmetric intertwining is lost. As an example, we construct a Cholesky isospectral partner and the square root of a waveguide necklace with cyclic symmetry. We use experimental parameters from the telecommunication $C$ band to construct a finite-element model of these Cholesky photonic lattices to good agreement with our analytic prediction.

We demonstrate the state-of-the-art technique of an active clock to provide a continuous superradiant lasing signal using an ensemble of trapped Cs atoms in an optical lattice. A magic wavelength of the proposed $|7S_{1/2};F=4,M_F=0\rangle \to |6P_{3/2};F=3,M_F=0\rangle$ clock transition in Cs atoms is identified at 1181 nm for constructing the optical lattice. Pertinent optical lines are also found for pumping and repumping atoms from their ground states. A fractional uncertainty about ${10}^{-15}$ in the clock frequency has been predicted by carrying out rigorous calculations of several atomic properties. The bad-cavity operational mode of the active clock is anticipated to improve remarkably its short-term stability by suppressing intrinsic thermal fluctuations. Thus, a composite clock system with better short-term and long-term stabilities can be built by combining the proposed active clock with another high-accuracy passive optical clock.

We demonstrate a cryogenic buffer gas-cooled molecular beam source capable of producing bright, continuous beams of cold and slow free radicals via laser ablation over durations of up to 60 seconds. The source design uses a closed liquid helium reservoir as a large thermal mass to minimize heating and ensure reproducible beam properties during operation. Under typical conditions, the source produces beams of our test species SrF, containing $5\times {10}^{12}$ molecules per steradian per second in the $X{}^2\mathrm{\Sigma }(v=0,N=1)$ state with a rotational temperature of 1.0(2) K and a forward velocity of $140\mathrm{m}/\mathrm{s}$. The beam properties are robust and unchanged for multiple cell exit geometries, but depend critically on the helium buffer gas flow rate, which must be $\ge 10$ standard cubic centimeters per minute to produce bright, continuous beams of molecules for an ablation repetition rate of 55 Hz.

The entropy production in dissipative processes is the essence of the arrow of time and the second law of thermodynamics. For dissipation of quantum systems, it was recently shown that the entropy production contains indeed two contributions: a classical one and a quantum one. Here we show that for degenerate (or near-degenerate) quantum systems there are additional quantum contributions which, remarkably, can become negative. Furthermore, such negative contributions are related to significant changes in the ongoing thermodynamics. This includes phenomena such as generation of coherences between degenerate energy levels (called horizontal coherences), alteration of energy exchanges, and, last but not least, reversal of the natural convergence of the populations toward the thermal equilibrium state. Going further, we establish a complementarity relation between horizontal coherences and population convergence, which is particularly enlightening for understanding heat flow reversals. Conservation laws of the different types of coherences are derived. Some consequences for thermal machines and resource theory of coherence are suggested.

Central to the AdS/CFT correspondence is a precise relationship between the curvature of an anti–de Sitter (AdS) space-time and the central charge of the dual conformal field theory (CFT) on its boundary. Our work shows that such a relationship can also be established for tensor network models of AdS/CFT based on regular bulk geometries, leading to an analytical form of the maximal central charges exhibited by the boundary states. We identify a class of tensors based on Majorana dimer states that saturate these bounds in the large curvature limit, while also realizing perfect and block-perfect holographic quantum error correcting codes. Furthermore, the renormalization group description of the resulting model is shown to be analogous to the strong disorder renormalization group, thus giving an example of an exact quantum error correcting code that gives rise to a well-understood critical system. These systems exhibit a large range of fractional central charges, tunable by the choice of bulk tiling. Our approach thus provides a precise physical interpretation of tensor network models on regular hyperbolic geometries and establishes quantitative connections to a wide range of existing models.

Performing efficient quantum computer tune-up and calibration is essential for growth in system complexity. In this work we explore the link between facilitating such capabilities and the underlying architecture of the physical hardware. We focus on the specific challenge of measuring (“mapping”) spatially inhomogeneous quasistatic calibration errors using spectator qubits dedicated to the task of sensing and calibration. We introduce an architectural concept for such spectator qubits: arranging them spatially according to prescriptions from optimal two-dimensional approximation theory. We show that this insight allows for efficient reconstruction of inhomogeneities in qubit calibration, focusing on the specific example of frequency errors which may arise from fabrication variances or ambient magnetic fields. Our results demonstrate that optimal interpolation techniques display near optimal error scaling in cases where the measured characteristic (here the qubit frequency) varies smoothly, and we probe the limits of these benefits as a function of measurement uncertainty. For more complex spatial variations, we demonstrate that the noise mapping for quantum architectures formalism for adaptive measurement and noise filtering outperforms optimal interpolation techniques in isolation and, crucially, can be combined with insights from optimal interpolation theory to produce a general purpose protocol.

Efficient optimization of quantum systems is a necessity for reaching fault-tolerant thresholds. A standard tool for optimizing simulated quantum dynamics is the gradient-based grape algorithm, which has been successfully applied in a wide range of different branches of quantum physics. In this work, we derive and implement exact second-order analytical derivatives of the coherent dynamics and find improvements compared to the standard of optimizing with the approximate second-order Broyden-Fletcher-Goldfarb-Shanno algorithm. We demonstrate performance improvements for both the best and the average errors of constrained unitary gate synthesis on a circuit-QED system over a broad range of different gate durations.

Using electron-beam ion traps (EBITs), we have made direct measurements for the fine-structure splitting $2p^5$ of S viii and Cl ix, which are important to the test of ab initio theory in F-like ions. Their wavelength are redetermined in the laboratory with an accuracy of 23 ppm, i.e., $10085.40\pm 0.23{\mathrm{cm}}^{-1}$ for S viii, where the early value observed in the solar spectrum is questionable, and $13643.39\pm 0.32{\mathrm{cm}}^{-1}$ for Cl ix, where only indirect measurements had been performed. High-precision calculations, including ab initio treatment of QED effects, are accomplished for $10\le Z\le 18$ and compared with the experimental results. Good agreement is found between our experimental and theoretical values of the fine-structure energy among these ions except for Al v, which calls for further investigations. Besides, we have succeeded in pushing EBIT operation towards the infrared spectral region relevant to astrophysics.

Drive-induced shifts, such as ac Stark shift and Bloch-Siegert shift, are routinely used in various spectroscopies. These shifts are experimentally known to show dispersive Lorentzian behavior as a function of its characteristic frequencies in optical pumping experiments. However, the drive-induced Stark shifts, as calculated using Floquet or dressed atom approaches, do not show the above nonlinear behavior. To address this, we theoretically investigated the drive-induced shifts using a previously reported fluctuation-regulated quantum master equation [A. Chakrabarti and R. Bhattacharyya, Phys. Rev. A 97, 063837 (2018)]. The shifts are obtained as closed-form expressions over the entire detuning range of the drive. The predicted shifts match satisfactorily with the known experimental data of the light shifts. We show that the calculated shifts are a Kramers-Kronig pair of the drive-induced dissipation in conformity with experimental findings. Moreover, we show that at low temperatures, i.e., for less thermal fluctuations, our results asymptotically match with the known theoretical form of the shifts. In the high-temperature regime, we predict that the shifts decrease in magnitude and are inversely proportional to the square of the temperature.

We perform high-resolution measurements of momentum distribution on ${\mathrm{Rb}}^{n+}$ recoil ions up to charge state $n=4$, where laser-cooled rubidium atoms are ionized by femtosecond elliptically polarized lasers with the pulse duration of 35 fs and the intensity of $3.3\times {10}^{15}\mathrm{W}/\mathrm{c}{\mathrm{m}}^2$ in the over-barrier ionization (OBI) regime. The momentum distributions of the recoil ions are found to exhibit multiband structures as the ellipticity varies from the linear to circular polarizations. The origin of these band structures can be explained quantitatively by the simple man model based on the OBI mechanism and dedicated classical trajectory Monte Carlo simulations with Heisenberg potential. Specifically, with back analysis of the classical trajectories, we reveal the ionization time and the OBI geometry of the sequentially released electrons, disentangling the mechanisms behind the tilted angle of the band structures. These results indicate that the classical treatment can describe the strong-field multiple ionization processes of alkali atoms.

We aim to create deterministic collisions between orbiting bodies by applying a time-dependent external force to one or both bodies, whether the bodies are mutually repulsive, as in the two- or multielectron atomic case, or mutually attractive, as in the planetary-orbit case. Specifically, we have devised a mathematical framework for causing deterministic collisions by launching an inner orbiting body to a higher energy such that this inner body is guaranteed to collide with the outer body. Our method first expresses the problem mathematically as coupled nonlinear differential equations with a time-dependent driving force and solves to find a feasible solution for the force function. Although our calculation is based strictly on classical physics, our approach is suitable for the case of helium with two highly excited electrons and is also valid for creating collisions in the gravitational case, such as for our solar system.

We study two-component bosons on the Harper-Hofstadter model with two legs. The synthetic magnetic fields for the two types of bosons point to either the same direction or opposite directions. The bosons have hardcore intraspecies interaction such that there can be no more than one boson of the same type on each lattice site. For certain filling factors in the absence of interspecies interaction, each component realizes a vortex Mott insulator with rung current or a Meissner superfluid without rung current. The system undergoes phase transitions to other phases as interspecies interaction is turned on, which are characterized numerically using the density matrix renormalization group method and supplemented with analytical studies when possible. The vortex Mott insulator transits to a gapped Meissner phase without rung current and the Meissner superfluid transits to a gapped vortex phase with rung current. In both cases, we observe gapped spin density wave states that break certain ${\mathbb{Z}}_2$ symmetries.

In non-Hermitian systems, the eigenfrequency of resonance modes coalesces at the exceptional points (EPs) in the frequency domain, leading to extraordinary parametric sensitivity. In this study, the controllable generation of high-order sidebands and frequency combs is investigated in the optomagnonical system, which comprises a single optomagnonical resonator coupled with two nanotips. Exploiting the scattering induced by the nanotips, the EPs periodically emerge as the relative angle of the two nanoscatterers in the system is tuned, and the sidebands' spectrum could be modified significantly. Moreover, we uncover that the driving field plays an essential role in controlling the orders and amplitudes of the sidebands. In particular, the generation of the frequency combs can be controlled by the external magnetic field, which determines the precession frequency of the spin. This study provides a potential method to manipulate the optical nonlinearity and to apply this method in the fields of optical communications and information processing.

We study quantum population transfer via a common intermediate state initially in thermal equilibrium with a finite temperature $T$, exhibiting a multilevel stimulated Raman adiabatic passage structure. We consider two situations for the common intermediate state, namely a discrete two-level spin and a bosonic continuum. In both cases we show that the finite temperature strongly affects the efficiency of the population transfer. We also show in the discrete case that strong coupling with the intermediate state, or a longer duration of the controlled pulse, would suppress the effect of finite temperature. In the continuous case, we adapt the thermofield-based chain-mapping matrix product states algorithm to study the time evolution of the system plus the continuum under time-dependent controlled pulses, which shows great potential to be used to solve open quantum system problems in quantum optics.

We demonstrate the superresolution localization of the nitrogen-vacancy centers in diamond by a fluorescence photoswitching technique based on coherent quantum control. The photoswitching is realized by the quantum phase encoding based on pulsed magnetic field gradient. Then we perform superresolution imaging and achieve a localizing accuracy better than $1.4\mathrm{nm}$ under a scanning confocal microscope. Finally, we show that the quantum phase encoding plays a dominant role on the resolution, and a resolution of $0.15\mathrm{nm}$ is achievable under our current experimental condition. This method can be applied in subnanometer scale addressing and control of qubits based on multiple coupled defect spins.

We investigate inelastic collision dynamics of a single cold ion in a Bose-Einstein condensate. We observe rapid ion-atom-atom three-body recombination leading to the formation of weakly bound molecular ions followed by secondary two-body molecule-atom collisions quenching the rovibrational states towards deeper binding energies. In contrast to previous studies exploiting hybrid ion traps, we work in an effectively field-free environment and generate a free low-energy ionic impurity directly from the atomic ensemble via Rydberg excitation and ionization. This allows us to implement an energy-resolved field-dissociation technique to trace the relaxation dynamics of the recombination products. Our observations are in good agreement with numerical simulations based on Langevin capture dynamics and provide a complementary means to study stability and reaction dynamics of ionic impurities in ultracold quantum gases.

We investigate the feasibility of using an effective pulse control method to realize an almost exact state transmission through a uniform chain where the communication channel is immersed in a quantum environment. This pulse control is treated in a nonperturbative fashion, allowing for a more precise view. Using the non-Markovian quantum state diffusion equation, we calculate the dynamics of the transmission fidelity. The results show that the memory effects of the environment can support an effective transmission control that can be boosted under a non-Markovian environment. Our scheme provides another way to realize almost exact state transmission in a spin chain system.