Symmetric multiport splitters are versatile tools in optical quantum information processing. They can be used for studying multiparticle scattering, for studying distinguishability and mixedness, and also for the generation of multipartite entangled quantum states. Here, we show that $N$-photon $N$-mode Greenberger-Horne-Zeilinger (GHZ) states can be generated using symmetric multiport beam splitters. Varying the input states' internal degrees of freedom and postselecting onto certain photon-number distributions allows the probabilistic generation of GHZ states with arbitrary photon numbers. We present two schemes, one for odd and one for even numbers of photons, to generate GHZ states using symmetric multiport splitters and compare them to a strategy utilizing a $2N$-port network as well as the standard postselection method.

Improving coherence is a fundamental challenge in quantum simulation and sensing experiments with trapped ions. Here we discuss, experimentally demonstrate, and estimate the potential impacts of two different protocols that enhance, through motional parametric excitation, the coherent spin-motion coupling of ions obtained with a spin-dependent force. The experiments are performed on two-dimensional crystal arrays of approximately 100 ${}^9{\mathrm{Be}}^{+}$ ions confined in a Penning trap. By modulating the trapping potential at close to twice the center-of-mass mode frequency, we squeeze the motional mode and enhance the spin-motion coupling while maintaining spin coherence. With a stroboscopic protocol, we measure $5.4\pm 0.9\mathrm{dB}$ of motional squeezing below the ground-state motion, from which theory predicts a 10-dB enhancement in the sensitivity for measuring small displacements using a recently demonstrated protocol [K. A. Gilmore et al., Science 373, 673 (2021)]. With a continuous squeezing protocol, we measure and accurately calibrate the parametric coupling strength. Theory suggests this protocol can be used to improve quantum spin squeezing, limited in our system by off-resonant light scatter. We illustrate numerically the trade-offs between strong parametric amplification and motional dephasing in the form of center-of-mass frequency fluctuations for improving quantum spin squeezing in our setup.

We determine the mass gap of a two-dimensional $\text{O}\left(3\right)$ nonlinear sigma model augmented with a topological $\theta$-term using tensor network and digital quantum algorithms. As proof of principle, we consider the example $\theta =\pi$ and study its critical behavior on a quantum simulator by examining the entanglement entropy of the ground state. We confirm that the quantum theory is massless in the strong-coupling regime, in agreement with analytical results. However, we also highlight the limitations of current quantum algorithms, designed for noisy intermediate-scale quantum devices, in the theory simulation at weak coupling. Finally, we compare the performance of our quantum algorithms to classical tensor network methods.

Quantum communication technologies will play an important role in quantum information processing in the near future as we network devices together. However, their implementation is still a challenging task due to both loss and gate errors. Quantum error-correcting codes are one important technique to address this issue. In particular, the quantum Reed-Solomon codes well suited to communication tasks as photons can naturally carry more than one qubit of information. The high degree of physical resources required, however, makes such a code difficult to use in practice. A recent technique called quantum multiplexing has been shown to reduce resources by using multiple degrees of freedom of a photon. In this work, we propose a method to decompose multicontrolled gates using fewer controlled-x ($\mathrm{CX}$) gates via this quantum multiplexing technique. We show that our method can significantly reduce the required number of $\mathrm{CX}$ gates needed in the encoding circuits for the quantum Reed-Solomon code. Our approach is also applicable to many other quantum error-correcting codes and quantum algorithms, including Grovers and quantum walks.

Optimal field-free molecular orientation for a given number of populated states, achieved by using resonant laser pulses, is theoretically investigated for linear molecules. An optimal phase difference between the rotational states can be obtained, at full revival times, after the laser pulses have decayed and these phase conditions are analytically derived for multilevel systems ($J>2$). The dependence of the phase difference on the duration and absolute area of the laser pulses is analyzed. Finally, the optimal degree of molecular orientation can be obtained by adjusting the ratios of the peak amplitudes of the laser pulses, with the carrier-envelope phases or the delay time between the pulses fulfilling the phase conditions.

We discuss theoretically the non-Hermitian superfluid phase transition in one-dimensional two-component Fermi gases near the $p$-wave Feshbach resonance accompanied by the two-body loss associated with dipolar relaxation. We point out that this system gives us an opportunity to explore the interplay among various nontrivial properties such as universal thermodynamics at divergent $p$-wave scattering lengths, the topological phase transition at vanishing chemical potential, and the non-Hermitian Bardeen-Cooper-Schrieffer (BCS) to Bose-Einstein condensate (BEC) transition, in a unified manner. In the BCS phase, the loss-induced superfluid-normal transition occurs when the exceptional point appears in the effective non-Hermitian Hamiltonian. In the BEC phase, the diffusive gapless mode can be regarded as a precursor of the instability of the superfluid state. Moreover, we show that the superfluid state is fragile against the two-body loss near the topological phase transition point.

Motivated by recent progress in cold-atom experiments, we analyze the $\mathrm{SU}(N$) Fermi-Hubbard model on a $d$-dimensional hypercubic lattice with two-body loss. By focusing on states near the ferromagnetic steady states, we obtain the Liouvillian gap in closed form for any $d$ and $N$. We also investigate the dynamics of a ferromagnetic initial state with a single spin flip both analytically and numerically. In particular, we show that, by decreasing the strength of the interaction and loss, the survival probability of the spin flip exhibits a crossover from the power-law decay to the exponential decay. We expect that our findings can be tested experimentally with ultracold alkaline-earth-like atoms in an optical lattice.

Coherently manipulating single photons in nanophotonic structures with unidirectional propagation is one of the central goals for integrated quantum information processing. Photonic devices constructed by a single atom coupled to a whispering-gallery-mode microresonator (WGMM) with nonideal chiral photon-atom interactions inevitably induce undesired photon scattering. Here, an external scatterer is introduced to the WGMM to cancel the reflection of signal photons by an atom due to nonideal chiral interactions. By properly tuning the positions of the scatterer, destructive and constructive interferences between reflected photons of different pathways are utilized to control the reflection properties of single incident photons. The results show that the reflection probabilities can be suppressed by applying the interplay between chirality and backscattering. Constructing perfect destructive interferences directly leads to unidirectional propagation even when the photon-atom interactions are not ideal. Because the amplitudes of the reflected photons produced by the scatterer can be enhanced to meet requirements of perfect destructive interfering processes, unidirectional propagation is preserved against dissipations.

We study the localization phenomena in a one-dimensional lattice system with a uniformly moving disordered potential. At a low-moving velocity, we find a sliding localized phase in which the initially localized matter wave adiabatically follows the moving potential without diffusion, thus resulting in an initial state memory in the many-body dynamics. Such an intriguing localized phase distinguishes itself from the standard Anderson localization in two aspects: it is not robust against interaction, but persists in the presence of slowly varying perturbations. Such a sliding localized phase can be understood as a consequence of interference between the wave-packet paths under moving quasiperiodic potentials with various periods that are incommensurate with the lattice constant. The experimental realization and detection are also discussed.

We study a three-level Dicke model in V configuration under both closed and open conditions. With independently tunable corotating and counterrotating coupling strength of the interaction Hamiltonian, this model is a generalization of the standard Dicke model that features multiple distinct parameter regimes. Based on a mean-field approach and third quantization analysis, it is found that the system exhibits rich quantum phase behaviors, including distinct superradiant fixed points, multiphase coexistence, and limit-cycle oscillation. In particular, the cavity dissipation stabilizes a family of inverted spin-coherent steady states whose stability region can be enlarged or reduced by properly tuning the imbalance between the corotating and counterrotating interactions. This property provides a conceptually different scenario to prepare the coherent atomic state with high fidelity.

We complement previous studies of an ion coupled with an optical cavity in the dispersive regime, for a model which exhibits bistability of different configurations in the semiclassical description. Our approach is based on a truncated evolution in phase space and is intended to explore an especially interesting parameter region where the fully quantum-mechanical solution becomes hard but the crudest semiclassical approach fails to capture essential phenomena. We compare the results of our techniques with the ones from numerical diagonalization of the quantum evolution and find that although the treatment leads to a smoothening and a slight shift of the transitions in the system, it still provides a clear improvement over localized semiclassical approximations.

Noise-biased qubits are a promising route toward significantly reducing the hardware overhead associated with quantum error correction. The squeezed-cat code, a nonlocal encoding in phase space based on squeezed coherent states, is an example of a noise-biased (bosonic) qubit with exponential error bias. Here we propose and analyze the error correction performance of a dissipatively stabilized squeezed-cat qubit. We find that for moderate squeezing the bit-flip error rate gets significantly reduced in comparison with the ordinary cat qubit while leaving the phase-flip rate unchanged. Additionally, we find that the squeezing enables faster and higher-fidelity gates.

Despite the exponential overhead to describe general multiqubit quantum states and processes, efficient methods for certain state families and operations have been developed and utilized. The stabilizer formalism and the Gottesman-Knill theorem, where pure stabilizer or graph states are manipulated by Clifford operations and Pauli measurements, are prominent examples, and these states play a major role in many applications in quantum technologies. Here we develop a noisy stabilizer formalism, i.e., a method that allows one to efficiently describe and follow not only pure states under Clifford operations and Pauli measurements but also Pauli noise processes acting on such stabilizer states, including uncorrelated and correlated dephasing and single- or multiple-qubit depolarizing noise. The method scales linearly in the number of qubits of the initial state, but exponentially in the size of the target state. Thus, whenever a noisy stabilizer state is manipulated by means of local Pauli measurements such that a multipartite entangled state of a few qubits is generated, one can efficiently describe the resulting state.

We propose a method to construct composite two-qubit gates with narrowband profiles with respect to the spin-spin coupling. The composite sequences are selective to the variations in the amplitude and duration of the spin-spin coupling, and can be used for highly selective qubit addressing with greatly reduced crosstalk, quantum logic spectroscopy, and quantum sensing.

Two-dimensional magneto-optical trapping (2D-MOT) is an efficient tool for generating a high flux of precooled atoms. A 2D-MOT for mercury (Hg) is so far missing despite the potential of this atomic species in several areas. Here, we present the characterization of a 2D-MOT for Hg enabled by addressing the ${}^1{S}_0$- ${}^3{P}_1$ laser cooling transition at 254 nm. The laser source based on an ytterbium-doped fiber amplifier has low-frequency noise and high reliability. Parameters affecting the efficiency of the 2D-MOT are studied, i.e., optical trapping power, push beam power, cooling laser frequency detuning, and magnetic-field gradient. When used with a Hg optical lattice clock, the 2D-MOT increases by a factor of 4.5, the rate of preparation of the atomic samples, yielding an improved clock short term stability of $6.4\times {10}^{-16}$ at 1 s.

Attosecond noncollinear four-wave-mixing spectroscopy with one attosecond extreme ultraviolet (XUV) pulse and two few-cycle near-infrared (NIR) pulses was used to measure the autoionization decay lifetimes of inner valence electronic excitations in neon atoms. After a 43–48-eV XUV photon excites a $2s$ electron into the $2s2p^6\left(np\right)$ Rydberg series, broadband NIR pulses couple the $2s2p^{6}3p$ XUV-bright state to neighboring $2s2p^{6}3s$ and $2s2p^{6}3d$ XUV-dark states. Controllable delays of one or both NIR pulses with respect to the attosecond XUV pulse reveal the temporal evolution of either the dark or bright states, respectively. Experimental lifetimes for the $3s, 3p$, and $3d$ states are measured to be $7\pm 2, 48\pm 8$, and $427\pm 40$ fs, respectively, with 95% confidence. Accompanying calculations with two independent ab initio theoretical methods, newstock and astra, verify the findings. The results support the expected trend that the autoionization lifetime should be longer for states that have a smaller penetration in the radial region of the $2s$ core hole, which in this case is for the higher angular momentum Rydberg orbitals. The underlying theory thus links the lifetime results to electron correlation and provides an assessment of the direct and exchange terms in the autoionization process.

We describe a two-dimensional (2D) optical lattice for ultracold atoms with spatial features below the diffraction limit created by a bichromatic optical standing wave. At every point in space these fields couple the internal atomic states in a three-level $\mathrm{\Lambda }$ coupling configuration. Adiabatically following the local wave function of the resulting dark state yields a spatially uniform Born-Oppenheimer potential augmented by geometric scalar and vector potentials appearing due to spatially rapid changes of the wave function. Depending on the system parameters, we find that the geometric scalar potential can interpolate from a 2D analog of the Kronig-Penney lattice, to an array of tubes with a zig-zag-shaped barrier. The geometric vector potential induces a spatially periodic effective magnetic field (the Berry's curvature) that can be tuned to cause destructive interference between neighboring tubes, thereby decoupling them at a critical point in parameter space. We numerically investigate the energy spectrum including decay from the excited state and find that the adiabatic approximation is sound for strong coupling strengths, leading to negligible loss in the dark-state manifold. Furthermore, the spectrum is well described by a non-Hermitian tight-binding model with on-site losses and hopping characterized by both loss and, surprisingly, gain.

Equal mass fermionic trimers with two different spin components near the unitary limit are shown to possess a universal van der Waals bound or resonance state near $s$-wave unitarity, when $p$-wave interactions are included between the particles with equal spin. Our treatment uses a single-channel Lennard-Jones interaction with long-range two-body van der Waals potentials. While it is well known that there is no true Efimov effect that would produce an infinite number of bound states in the unitary limit, we demonstrate that another type of universality emerges for the symmetry $L^{\mathrm{\Pi }}=1^{-}$. The universality is a remnant of Efimov physics that exists in this system at $p$-wave unitarity, and it leads to modified threshold and scaling laws in that limit. Application of our model to the system of three lithium atoms studied experimentally by Du et al. [Phys. Rev. Lett. 102, 250402 (2009)] provides an interpretation of their measured three-body recombination loss rates.