We introduce and study the disordered Dicke model in which the spin-boson couplings are drawn from a random distribution with some finite width. Regarding the quantum phase transition, we show that, when the standard deviation $\sigma$ of the coupling strength gradually increases, the critical value of the mean coupling strength $\mu$ gradually decreases, and after a certain $\sigma$ there is no quantum phase transition at all; the system always lies in the superradiant phase. We derive an approximate expression for the quantum phase transition in the presence of disorder in terms of $\mu$ and $\sigma$, which we numerically verify. Studying the thermal phase transition in the disordered Dicke model, we obtain an analytical expression for the critical temperature in terms of the mean and standard deviation of the coupling strength. We observe that, even when the mean of the coupling strength is zero, there is a finite temperature transition if the standard deviation of the coupling is sufficiently high. Disordered couplings in the Dicke model will exist in quantum dot superlattices, and we also sketch how they can be engineered and controlled with ultracold atoms or molecules in a cavity.

Giant atoms, where the dipole approximation ceases to be valid, allow us to observe unconventional quantum optical phenomena arising from interference and time-delay effects. Most previous studies consider giant atoms coupling to conventional materials with right-handed dispersion. In this study, we first investigate the quantum dynamics of a giant atom interacting with left-handed superlattice metamaterials. Different from those right-handed counterparts, the left-handed superlattices exhibit an asymmetric band gap generated by anomalous dispersive bands and Bragg scattering bands. With the assumption that the giant atom is in resonance with the continuous dispersive energy band, spontaneous emission will undergo periodic enhancement or suppression due to the interference effect. At the resonant position, there is a significant discrepancy in the spontaneous decay rates between the upper and lower bands which arises from the differences in group velocity. Second, we explore the non-Markovian dynamics of the giant atom by considering the emitter's frequency outside the energy band, where bound states will be induced by the interference between two coupling points. By employing both analytical and numerical methods, we demonstrate that the steady atomic population will be periodically modulated, driven by variations in the size of the giant atom. The presence of asymmetric band edges leads to diverse interference dynamics. Finally, we consider the case of two identical emitters coupled to the waveguide and find that the energy within the two emitters undergoes exchange through the mechanism of Rabi oscillations.

The Rydberg dipole blockade has emerged as the standard mechanism to induce entanglement between neutral-atom qubits. In these protocols, laser fields that couple qubit states to Rydberg states are modulated to implement entangling gates. Here we present an alternative protocol to implement entangling gates via Rydberg dressing and a microwave-field-driven spin-flip blockade [Y.-Y. Jau et al., Nat. Phys. 12, 71 (2016)]. We consider the specific example of qubits encoded in the clock states of cesium. An auxiliary hyperfine state is optically dressed so that it acquires partial Rydberg character. It thus acts as a proxy Rydberg state, with a nonlinear light shift that plays the role of blockade strength. A microwave-frequency field coupling a qubit state to this dressed auxiliary state can be modulated to implement entangling gates. Logic gate protocols designed for the optical regime can be imported to this microwave regime, for which experimental control methods are more robust. We show that unlike the strong dipole-blockade regime usually employed in Rydberg experiments, going to a moderate-spin-flip-blockade regime results in faster gates and smaller Rydberg decay. We study various regimes of operations that can yield high-fidelity two-qubit entangling gates and characterize their analytical behavior. In addition to the inherent robustness of microwave control, we can design these gates to be more robust to laser amplitude and frequency noises at the cost of a small increase in Rydberg decay.

Quasicrystals, a fascinating class of materials with long-range but nonperiodic order, have revolutionized our understanding of solid-state physics due to their unique properties at the crossroads of long-range-ordered and disordered systems. Since their discovery, they continue to spark broad interest for their structural and electronic properties. The quantum simulation of quasicrystals in synthetic quantum matter systems offers a unique playground to investigate these systems with unprecedented control parameters. Here, we investigate the localization properties and spectral structure of quantum particles in two-dimensional quasicrystalline optical potentials. While states are generally localized at low energy and extended at high energy, we find alternating localized and critical states at intermediate energies. Moreover, we identify a complex succession of gaps in the energy spectrum. We show that the most prominent gap arises from strongly localized ring states, with the gap width determined by the energy splitting between states with different quantized winding numbers. In addition, we find that these gaps are stable for quasicrystals with different rotational symmetries and potential depths, provided that other localized states do not enter the gap generated by the ring states. Our findings shed light on the unique properties of quantum quasicrystals and have implications for their many-body counterparts.

We propose a theoretical approach for entangling two Dicke states in a periodic modulated quantum system. By considering two qubit ensembles that are nonuniformly coupled to a common resonator, we can derive an effective Hamiltonian whose energy levels depend nonlinearly on the excitation number of each qubit ensemble. A more simplified effective Hamiltonian can be obtained by selecting the appropriate driving parameters and initial state. Based on the dynamic evolution of the effective Hamiltonian, we can selectively achieve Dicke state transitions and generate entangled Dicke states controllably. For a special case, we can obtain ensemble-ensemble entangled states by performing a projective even-odd cat-state measurement. By implementing Gaussian soft temporal modulation, we can effectively suppress off-resonant contributions in the interaction and enhance the fidelity of target states. Furthermore, by utilizing the Holstein-Primakoff transformation, we study the resonator-ensemble coupling system in the thermodynamic limit and investigate the generation of entangled magnon states. Additionally, we propose a scheme of creating magnon NOON states through frequency modulation and study the influence of decoherence on the fidelity of target states.

We utilize multilevel atoms trapped in a driven resonant optical cavity to produce scalable multimode squeezed states for quantum sensing and metrology. While superradiance or collective dissipative emission by itself has been typically a detrimental effect for entanglement generation in optical cavities, in the presence of additional drives it can also be used as an entanglement resource. In a recent work [B. Sundar, D. Barberena, A. M. Rey, and A. Piñeiro Orioli, companion paper, Phys. Rev. Lett. 132, 033601 (2024)], we described a protocol for the dissipative generation of two-mode squeezing in the dark state of a six-level system with only one relevant polarization. There we showed that up to two quadratures can be squeezed. Here, we develop a generalized analytic treatment to calculate the squeezing in any multilevel system where atoms can collectively decay by emitting light into two polarization modes in a cavity. We show that in this more general system up to four spin squeezed quadratures can be obtained. We study how finite-size effects constrain the reachable squeezing, and analytically compute the scaling with $N$. Our findings are readily testable in current optical cavity experiments with alkaline-earth-like atoms.

We develop a microscopic calculation scheme for the excitation spectrum of a single-electron atom localized near a dielectric nanostructure. The atom originally has an arbitrary degenerate structure of its Zeeman sublevels on its closed optical transition and we follow how the excitation spectrum would be modified by its radiative coupling with a mesoscopically small dielectric sample of arbitrary shape. The dielectric medium is modeled by a dense ensemble of $V$-type atoms having the same dielectric permittivity near the transition frequency of the reference atom. Our numerical simulations predict strong coupling for some specific configurations and then suggest promising options for quantum interface and quantum information processing at the level of single photons and atoms. In particular, the strong resonance interaction between atom(s) and light, propagating through a photonic crystal waveguide, justifies as realistic the scenario of a signal light coupling with a small atomic array consisting of a few atoms. As a potential implication, the directional one-dimensional resonance scattering, expected in such systems, could provide a quantum bus by entangling distant atoms integrated into a quantum register.

In the last few years, the $l_1$ norm has been used as a quantum coherence measure by many authors. In this work, we introduce a variable that mediates the complementarity relation between the $l_1$ norm of coherence, linear entropy, and the predictability of a quantum state of dimension greater than two. We show that this variable indicates the coherence distribution among the pairs of base states expanding the Hilbert space. We also show that a uniform quantum coherence distribution creates a direct complementarity between the $l_1$ norm, predictability, and linear entropy.

The question of whether complex numbers play a fundamental role in quantum theory has been debated since the inception of quantum mechanics. Recently, a feasible proposal to differentiate between real and complex quantum theories based on the technique of testing Bell nonlocalities has emerged [Renou et al., Nature (London) 600, 625 (2021)]. Based on this method, the real quantum theory has been falsified experimentally in both photonic and superconducting quantum systems [Li et al., Phys. Rev. Lett. 128, 040402 (2022); Chen, et al., Phys. Rev. Lett. 128, 040403 (2022)]. The quantum networks with multiple independent sources which are not causally connected have gained significant interest as they offer a new perspective on studying the nonlocalities. The independence of these sources imposes additional constraints on observable covariances and leads to new bounds for classical and quantum correlations. In this study, we examine the discrimination between the real and complex quantum theories with an entanglement swapping scenario under a stronger assumption that the two sources are causally independent, which was not made in previous works. Using a revised Navascúes-Pironio-Acín method and Bayesian optimization, we find a proposal with optimal coefficients of the correlation function which could give a larger discrimination between the real and quantum theories compared with the existing proposals. This work opens up avenues for further exploration of the discrimination between real and complex quantum theories within intricate quantum networks featuring causally independent parties.

We demonstrate that spin-dependent electron diffraction is possible for a smooth range of transverse electron momenta in a two-photon Bragg scattering scenario of the Kapitza-Dirac effect. Our analysis is rendered possible by introducing a generalized specification for quantifying spin-dependent diffraction, yielding an optimization problem which is solved by making use of a Newton gradient iteration scheme. With this procedure, we investigate the spin-dependent effect for different transverse electron momenta and different laser polarizations of the standing-light-wave Kapitza-Dirac scattering. The possibility of using arbitrarily low transverse electron momenta when setting up a spin-dependent Kapitza-Dirac experiment allows longer interaction times of the electron with the laser and therefore enables less constraining parameters for an implementation of the effect.

We investigate parametrized multipartite entanglement measures from the perspective of $k$ nonseparability in this paper. We present two types of entanglement measures in $n$-partite systems, $q\text{−}k\text{−}\mathrm{ME}$ concurrence $(q>1,2\le k\le n)$ and $\alpha \text{−}k\text{−}\mathrm{ME}$ concurrence $(0\le \alpha <1,2\le k\le n)$, which unambiguously detect all $k$-nonseparable states in arbitrary $n$-partite systems. Rigorous proofs show that the proposed $k$-nonseparable measures satisfy all the requirements for being an entanglement measure including the entanglement monotone, strong monotone, convexity, vanishing on all $k$-separable states, and being strictly greater than zero for all $k$-nonseparable states. In particular, the $q\text{−}\mathrm{2}\text{−}\mathrm{ME}$ concurrence and $\alpha \text{−}2\text{−}\mathrm{ME}$ concurrence, renamed as $q\text{−}\mathrm{GME}$ concurrence and $\alpha \text{−}\mathrm{GME}$ concurrence, respectively, are two kinds of genuine entanglement measures corresponding to the case where the systems are divided into bipartition $(k=2)$. The lower bounds of the two classes $k$-nonseparable measures are obtained by employing the approach that takes into account the permutationally invariant part of a quantum state. Furthermore, the relations between $q\text{−}n\text{−}\mathrm{ME}$ concurrence $(\alpha \text{−}n\text{−}\mathrm{ME}$ concurrence) and global negativity are established. In addition, we discuss the degree of separability and elaborate on an effective detection method with concrete examples. Moreover, we compare the $q\text{−}\mathrm{GME}$ concurrence defined by us to other genuine entanglement measures.

Fabrication errors on qubits can render data and syndrome qubits faulty, impacting quantum-processor reliability. Our proposed method focuses on the recovery of faulty syndrome qubits, preserving the functionality of neighboring qubits and minimizing the loss of data qubits. Through experimental simulation, we demonstrate that our approach significantly improves percolation rates and reduces logical error rates while enhancing the robustness of the surface code against fabrication errors. Moreover, our approach raises the critical point for syndrome-fabrication errors by 29.4% on average and 33.33% when the percolation rate reaches 100%. Furthermore, it reduces logical error rates by 18.42% on average and achieves a 50% reduction in low computational Pauli error rates.

It is shown that the total amount of correlation stored in $N$-qubit systems, as characterized by the discrete Rényi entropy, can be effectively employed to detect the presence of $N$-partite correlations within the framework of deterministic measurements. An associated optimization procedure can be analytically performed for a broad range of $N$-qubit states, encompassing both symmetric and nonsymmetric ones. This analytical approach enables the analysis of the asymptotic limit $N\gg 1$. It is proved that the appropriately normalized quadratic discrete Rényi entropy always decreases in the process of deterministic measurements. This allows us to introduce a robustness parameter for assessing the stability of pure multipartite states under the protocol of measurement-induced optimal correlation transfer.

We study the remote entanglement generation between macroscopic microwave magnon modes in a coupled cavity system. The cavities are connected via an optical fiber, which necessitates the use of a frequency conversion inside the cavity. The converter may be implemented via a rare-earth doped crystal acting like an effective three-level system. The entanglement dynamics of the system is analytically studied, and an active optimal control method is also proposed where one may generate maximally entangled Bell states on demand with a given evolution time. The system dynamics and its control have also been studied in a generic non-Markovian open system framework, and the generated entanglement is found to be robust against environmental noises.

In previous work [Phys. Rev. A 107, 052420 (2023)] we showed that the quantum teleportation protocol can be used to detect quantum critical points (QCPs) associated with a couple of different classes of quantum phase transitions, even when the system is away from the absolute zero temperature ($T=0$). Here, working in the thermodynamic limit (infinite chains), we extend the previous analysis for several other spin-$\frac{1}{2}$ models. We investigate the usefulness of the quantum teleportation protocol to detect the QCPs of those models when the temperature is either zero or greater than zero. The spin chains we investigate here are described by the $XXZ$ model, the $XY$ model, and the Ising model, all of them subjected to an external magnetic field. Specifically, we use a pair of nearest-neighbor qubits from an infinite spin chain at thermal equilibrium with a reservoir at temperature $T$ as the resource to execute the quantum teleportation protocol. We show that the ability of this pair of qubits to faithfully teleport an external qubit from the chain is dramatically affected as we cross the QCPs related to the aforementioned models. The results here presented together with the ones of aforementioned work suggest that the quantum teleportation protocol is a robust and quite universal tool to detect QCPs even when the system of interest is far from the absolute zero temperature.

Photon-mediated interactions in subwavelength atomic arrays have numerous applications in quantum science. In this paper, we explore the potential of three-level quantum emitters, or “impurities” embedded in a two-dimensional atomic array to serve as a platform for quantum computation. By exploiting the altered behavior of impurities as a result of the induced dipole-dipole interactions mediated by subwavelength arrays, we design and simulate a set of universal quantum gates consisting of the $\sqrt{\text{i}\mathrm{SWAP}}$ and single-qubit rotations. We demonstrate that these gates have very high fidelities due to the long atomic dipole-dipole coherence times, as long as the atoms remain within a proximal range. Finally, we design and simulate quantum circuits leading to the generation of the maximally entangled two-qubit Bell states, as well as the entangled three-qubit Greenberger-Horne-Zeilinger state. These findings establish subwavelength emitter arrays as an alternative platform for quantum computation and quantum simulation.

We present an adaptive procedure for aligning quantum nonlocality experiments without any knowledge of the two-qudit state shared by the participating parties. The quantum state produced by the source, its unitary evolution, as well as the actual measurement bases remain unknown to both parties at all times. The entanglement of the quantum state helps establish desired correlations between individual measurement bases of the two distant parties. We implement the procedure in a fiber-based quantum key distribution (QKD) setup with polarization-entangled photons, where we do not rely on any additional alignment tools such as lasers or polarizers. In a QKD scenario, the procedure can be done without any additional measurements beyond those that are performed regardless.

Hyperfine splittings of the ${}^{2}P_{3/2}$ manifold in ${}^9\mathrm{Be}^{+}$ were measured directly using a single laser-cooled ion stored in a radiofrequency Paul trap. As the hyperfine structure is unresolved beneath the natural linewidth of the transition, manipulation of the initial state, polarization, and final-state populations was used to preferentially detect scattering events through specific intermediate excited states. While quantum interference effects typically complicate the modeling of unresolved measurement lineshapes, in this work quantum interference helped to suppress extraneous scattering components. The hyperfine splittings between the $|{}^{2}P_{3/2},F=3\rangle$ state and the $|{}^{2}P_{3/2},F=2\rangle$ and $|{}^{2}P_{3/2},F=1\rangle$ states were measured to be $\mathrm{\Delta }{\nu }_{32}=0.801\left(56\right)$ MHz and $\mathrm{\Delta }{\nu }_{31}=5.050\left(83\right)$ MHz, respectively.

Achiral molecules can be made temporarily chiral by excitation with electric fields, in the sense that an average over molecular orientations displays a net chiral signal [D. S. Tikhonov et al., Sci. Adv. 8, eade0311 (2022)]. Here, we go beyond the assumption of molecular orientations to remain fixed during the excitation process. Treating both rotations and vibrations quantum mechanically, we identify conditions for the creation of chiral vibrational wave packets, with net chiral signals, in ensembles of achiral molecules which are initially randomly oriented. Based on the analysis of symmetry and controllability, we derive excitation schemes for the creation of chiral wave packets using a combination of (a) microwave and IR pulses and (b) a static field and a sequence of IR pulses. These protocols leverage quantum rotational dynamics for pump-probe spectroscopy of chiral vibrational dynamics, extending the latter to regions of the electromagnetic spectrum other than the UV.