With the long coherence time, fixed-frequency transmon qubits are a promising qubit modality for quantum computing. Currently, diverse qubit architectures that utilize fixed-frequency transmon qubits have been demonstrated with high-fidelity gate performance. Nevertheless, the relaxation times of transmon qubits can have large temporal fluctuations, causing instabilities in gate performance. The fluctuations are often believed to be caused by nearly on-resonance couplings with sparse two-level-system (TLS) defects. To mitigate their impact on qubit coherence and gate performance, one direct approach is to tune the qubits away from these TLSs. In this work, to combat the potential TLS-induced performance fluctuations in a tunable-bus architecture unitizing fixed-frequency transmon qubits, we explore the possibility of using an off-resonance microwave drive to effectively tune the qubit frequency through the ac Stark shift while implementing universal gate operations on the microwave-dressed qubit. We show that the qubit frequency can be tuned up to 20 MHz through the ac Stark shift while keeping minimal impacts on the qubit control. Besides passive approaches that aim to remove these TLSs through more careful treatments of device fabrications, this work may offer an active approach towards mitigating the TLS-induced performance fluctuations in fixed-frequency transmon qubit devices.

The classical Keldysh-Rutherford (KR) model of the attoclock [A. W. Bray et al., Phys. Rev. Lett. 121, 123201 (2018)] was introduced to interpret attosecond angular streaking on atomic hydrogen in the low laser intensity regime. In this model, the photoelectron acquires its final velocity immediately after exiting the tunnel and then scatters elastically on the Coulomb field of the residual ion. In the present work, we make a dynamic extension of the KR model and consider a gradual acceleration of the photoelectron by the driving laser field. By doing so, we observe and explain a significant displacement of the peaks of the angular distribution of the mean photoelectron momentum and the angular resolved ionization probability. These peaks should be close within the original KR model.

We nonperturbatively study pairing in the high-temperature regime of polarized unitary two-component Fermi gases by extracting the pair-momentum distribution and shot-noise correlations. Whereas the pair-momentum distribution allows us to analyze the propagation of pairs composed of one spin-up and one spin-down fermion, shot-noise correlations provide us with a tomographic insight into pairing correlations around the Fermi surfaces associated with the two species. Assuming that the dominant pairing patterns right above the superfluid transition also govern the formation of condensates in the low-temperature regime, our analysis suggests that the superfluid ground state is homogeneous and of the Bardeen-Cooper-Schrieffer type over a wide range of polarizations.

We examine the multistability associated with miscibility-immiscibility conditions for a two-component Bose-Einstein condensate coupled to the light field in an optical cavity. For a strongly immiscible condition, the system exhibits a variety of density structures, including a separated state, a stripe state, and their coexistence. The multistability arises from these spatial structures of the two-component condensate, which significantly alter the hysteresis curve with respect to the intensity of cavity pumping. We present a variational approach to confirm our numerical results.

We study the Faraday patterns generated by spin-orbit-coupling induced parametric resonance in a spinor Bose-Einstein condensate with repulsive interaction. The collective elementary excitations of the Bose-Einstein condensate, including density waves and spin waves, are coupled as the result of the Raman-induced spin-orbit coupling and a quench of the relative phase of two Raman lasers without the modulation of any of the system's parameters. We observed several higher parametric resonance tongues at integer multiples of the driving frequency and investigated the interplay between Faraday instabilities and modulation instabilities when we quench the spin-orbit-coupled Bose-Einstein condensate from zero-momentum phase to plane-wave phase. If the detuning is equal to zero, the wave number of combination resonance barely changes as the strength of spin-orbit coupling increases. If the detuning is not equal to zero after a quench, a single combination resonance tongue will split into two parts.

We present a dynamical, multilevel atom-cavity blockade effect and monitor its breakdown transition in time. As in the case of optical bistability, atoms initially impede transmission by detuning a cavity mode from the driving laser. The interacting system, however, eventually transitions into an uncoupled state via a critical runaway process, resulting in maximum transmission. These two extremes of transmission are macroscopic reflections of well-defined atomic states, and thus are interpreted as phases of a dynamical transition. By monitoring the output of the cavity, we make time-resolved measurements of the order parameter and that of the enhanced photon number fluctuations. Considering these results for different cavity driving intensities, we establish finite-size scaling relations that suggest such a runaway effect is in fact a genuine dynamical phase transition.

Quantum illumination can utilize entangled light to detect the low-reflectivity target that is hidden in a bright thermal background. This technique is applied to the detection of an object in the curved spacetime of the Earth, in order to explore how the curvature of spacetime affects quantum illumination. It is found that the spatial quantum illumination with entangled state transmitter outperforms that with coherent-state transmitter in the near-Earth curved spacetime. Moreover, either the quantum illumination system or the coherent-state system is employed, and gravity can enhance the spacetime target detection by reducing the thermal signal at the receiver. Besides, our model in principle can be applied to microwave quantum illumination and thus provides, to some degree, a theoretical foundation for the upcoming spatial quantum radar technologies.

The dynamical role of system-environment correlations will lead to the violation of Landauer's principle and may result in a non-Markovian dynamics of the system. Here, we employ entropic quantities called the telescopic relative entropy, to detect non-Markovianity [N. Megier, Phys. Rev. Lett. 127, 030401 (2021)], and study the relation between information backflow of non-Markovian dynamics and the validity of Landauer's principle. We consider a class of energy-conserving system-environment interactions in qubit systems, and we prove that the violation of Landauer's principle corresponds to information backflow of a thermalizing process without coherence and vice versa. Then we study the effect of quantum coherence of the system on the relationship between non-Markovianity and Landauer's principle, and we find that coherence will break the corresponding relation in the information backflow and violation of Landauer's principle. Based on this, we explain the physical mechanism of non-Markovianity in this irreversible process of heat dissipation and information erasure. We further numerically confirm the above theoretical results by means of a collision model.

We analyze the electric dipole moments and static electric dipole polarizabilities of the alkali-metal–alkaline-earth-metal (Alk-AlkE) dimers by employing finite-field coupled-cluster methods in the frameworks of both nonrelativistic and four-component spin-free relativistic theories. In order to carry out comparative analyses rigorously, we consider those Alk-AlkE molecules made out of the lightest to the medium-heavy constituent atoms (Alk: Li to Rb and AlkE: Be through Sr). We present the behavior of electron correlation effects as well as relativistic effects related to the size of the molecules. Uncertainties in the above quantities of the investigated Alk-AlkE molecules are inferred by analyzing our results from different forms of the Hamiltonian, basis set, and perturbative parameter for a few representative molecules. We also provide empirical relations by connecting the average polarizabilities of the Alk-AlkE molecules with the polarizabilities of the corresponding Alk and AlkE atoms. We finally give our recommended results and compare them with the literature values.

We propose an experimental protocol to directly detect the Andreev-Bashkin effect (entrainment) in the bulk mixture of a bosonic and fermionic superfluid using a ring geometry. Our protocol involves the interferometric detection of the entrainment-induced phase gradient across a superfluid due to the flow of another in which it is immersed. The choice of ring geometry eliminates variations in the stronger mean-field interaction which can thwart the detection of entrainment in other geometries. A significant enhancement of the entrainment phase-shift signal is possible, if the dimer-boson scattering length turns out to be large, which can be measured by tuning the interaction to the limit of miscibility of the two superfluids. With suggested improvements and careful design implementation, one may achieve approximately 67% shift in the interferometer fringes.

We present a detailed study of the effects of imperfect atom-optical manipulation in Bragg-based light-pulse atom interferometers. Off-resonant higher-order diffraction leads to population loss, spurious interferometer paths, and diffraction phases. In a path-dependent formalism, we study numerically various effects and analyze the interference signal caused by an external phase or gravity. We compare first-order single and double Bragg diffraction in retroreflective setups. In double Bragg diffraction, phase imperfections lead to a beating due to three-path interference. Some effects of diffraction phases can be avoided by adding the population of the outer exit ports of double diffraction.

An orthonormal set of optical vortex modes is put forward and identified as the polarized truncated optical Bessel (TOB) set, which is endowed with orbital as well as spin angular momentum. Members of this set of modes can be realized once a circular aperture of radius $R$ is placed centrally in the path of an optical Bessel beam of winding number $\ell$. For a fixed power input $\mathcal{P}$, the properties of the TOB set, namely, its helicity, energy, linear momentum, and spin and orbital angular momenta, are evaluated, and their main features are explored. The similarities and differences between the properties of the TOB mode set and those of the Laguerre-Gaussian set are pointed out and discussed.

The edge-to-edge topological transport of the one-dimensional odd-sized Su-Schrieffer-Heeger model plays a crucial role in robust energy transfer and information processing. However, it has not been verified experimentally. Here we report on an experimental demonstration of this topological transport in a one-dimensional photonic lattice of coupled waveguides. When slowly varying the spacings of the adjacent waveguides, the robust topological transport of light from the leftmost to the rightmost channels has been successfully observed. Moreover, the transport speed and efficiency of light are enhanced considerably by introducing the exponential-type modulations of the coupling strengths. The experimental results agree well with analytical calculations and numerical simulations. This transport of light observed may serve as the basis for photonic devices of unconventional functions.

Quantum computers potentially have an exponential advantage over classical computers for the quantum simulation of many-fermion quantum systems. Nonetheless, fermions are more expensive to simulate than bosons due to the fermionic encoding—a mapping by which the qubits are encoded with fermion statistics. Here we generalize the contracted quantum eigensolver (CQE) to avoid fermionic encoding of the wave function. In contrast to the variational quantum eigensolver, the CQE solves for a many-fermion stationary state by minimizing the contraction (projection) of the Schrödinger equation onto two fermions. We avoid fermionic encoding of the wave function by contracting the Schrödinger equation onto an unencoded pair of particles. Solution of the resulting contracted equation by a series of unencoded two-body exponential transformations generates an unencoded wave function from which the energy and two-fermion reduced density matrix (2-RDM) can be computed. We apply the unencoded and the encoded CQE algorithms to the hydrogen fluoride molecule, the dissociation of oxygen ${\mathrm{O}}_2$, and a series of hydrogen chains. Both algorithms show comparable convergence towards the exact ground-state energies and 2-RDMs, but the unencoded algorithm has computational advantages in terms of state preparation and tomography.

We compute different quantifiers to analyze the spontaneous generation of entanglement for a unidirectional (nonreciprocal) two-two level system. We illustrate this approach by deriving the Markovian master equation in the Lindblad form. The numerical analysis of the unidirectional master equation enabled to obtain the steady-state concurrence. We explore the quantum dense coding between two spatially separated dipoles taking into account the collective damping effect. Our results reveal the sharp decline and then gradual increase in the dense coding capacity to reach a stable value. This procedure opens the possibility to implement this scheme for further applications in the field of quantum information theory.

We report on double and single differential cross section measurements of the $1s2s^2{}^{2}S$, $1s2s2p{}^{2}P_{\pm }$, and $1s2s2p{}^{4}P$ doubly excited states produced by single-electron capture in 6–15 MeV collisions of pre-excited ${\mathrm{C}}^{4+}\left(1s2s{}^{3}S\right)$ ions with ${\mathrm{H}}_2$, He, Ne, and Ar targets. Ab initio cross section calculations based on a three active electrons atomic orbital close-coupling approach are compared to the measurements for collisions with the lighter ${\mathrm{H}}_2$ and He targets, showing an overall good agreement. Theoretical details concerning Auger electron angular distributions, partial cross sections, impact parameter dependencies, and radiative cascade feeding effects are discussed. The populations of the $1s2s2p{}^{4}P$, ${}^{2}P_{-}$, ${}^{2}P_{+}$ multiplets concerning the spin symmetry are finally presented for the four collision systems and discussed following the comparisons between theory and experiment.

We propose that a nonlinear Hall response can be observed in Bloch oscillations of ultracold atoms in optical lattices under the condition of preserved time-reversal symmetry. In the short-time limit of Bloch oscillations driven by a direct current (dc) field, the nonlinear Hall current dominates, being a second-order response to the external field strength. The associated Berry curvature dipole, which is a second-order nonlinear coefficient of the driving field, can be obtained from the oscillation of atoms. In an alternating current (ac) driving field, the nonlinear Hall response has a double frequency of the driving force in the case of time-reversal symmetry.

Matter-wave interferometry with solids is highly susceptible to minute fluctuations of environmental fields, including gravitational effects from distant sources. Hence, experiments require a degree of shielding that is extraordinarily challenging to achieve in realistic terrestrial or even space-based setups. Here, we design protocols that exploit the spatial correlations that are inherent in perturbations due to distant sources to reduce significantly their impact on the visibility of interference patterns. We show that interference patterns that are robust to such type of noise can be encoded in the joint probability distribution of two or more interferometers, provided that these are initialized in suitable states. We develop a general framework that makes use of $N+1$ interferometers, which may differ in their masses, to correct for environmental potential fields up to order $N$ in their multipole expansion. Remarkably, our approach works for fields that fluctuate stochastically in any timescale and does not require the presence of quantum correlations among the different interferometers. Finally, we also show that the same ideas can be extended to the protection of entanglement between pairs of interferometers.

Due to its myriad applications in many fields of science and technology, novel light diffraction from specially designed optical gratings is a subject of great interest. Here, we propose a scheme to realize the linear and nonlinear Bragg diffractions from an electromagnetically induced grating (EIG) with parity-time $\left(\mathcal{PT}\right)$ symmetry in a cold Rydberg atomic gas, where the Rydberg-Rydberg interaction between atoms are mapped to strong and long-range interaction between photons, characterized by a giant nonlocal Kerr nonlinearity. We show that a probe laser beam with very low light intensity incident upon the $\mathcal{PT}$-symmetric EIG can display distinctive asymmetric diffraction patterns, which can be actively manipulated through tuning the gain-absorption coefficient of the EIG and the input power of the laser beam. We also show that the intensity distribution among different diffraction orders depends significantly on the $\mathcal{PT}$-symmetry property of the EIG and on the magnitude and nonlocality degree of the Kerr nonlinearity. In addition, we demonstrate that such Bragg diffraction patterns can be controlled by an external gradient magnetic field, which provides a different way of diffraction control. The research results reported here are not only useful for understanding the unique properties of linear and nonlinear Bragg diffractions by $\mathcal{PT}$-symmetric gratings but may also be promising for designing optical devices applicable in optical information processing and transmission.