The driven Dicke model, with interesting quantum phases induced by parametrized driving, has been intensively studied in cavities, where permutation symmetry applies due to the atoms' equal coupling to the field and identical interaction. As a result, the system, with proper initialization, can remain in a highly symmetric subset of the state space, where the photon emission of each atom constructively interferes with the other, leading to superradiance at steady state. However, because of the degeneracy of steady states for the driven Dicke model, the steady state can be qualitatively altered by an infinitesimal perturbation. In this work, we simulate superconducting qubits coupled to a one-dimensional waveguide as the extended system and theoretically investigate four kinds of perturbations: local dephasing, individual driving phases, the separation between adjacent qubits, and individual detunings. Using an angular momentum basis, we predict the dimension of the degenerate subspace and study the transition within the subspace due to the perturbation.

Parrondo's paradox, a counterintuitive phenomenon in which two losing strategies combine to produce a winning outcome, has been a subject of interest across various scientific fields, including quantum mechanics. In this study, we investigate the manifestation of Parrondo's paradox in discrete-time quantum walks. We demonstrate the existence of Parrondo's paradox using site- and time-dependent coins without the need for a higher-dimensional coin or adding decoherence to the system. Our results enhance the feasibility of practical implementations and provide deeper insights into the underlying quantum dynamics, specifically the propagation constrained by the interference pattern of quantum walks. The implications of our results suggest the potential for more accessible and efficient designs in quantum transport, broadening the scope and application of Parrondo's paradox beyond conventional frameworks.

Quantum error correction represents a significant milestone in large-scale quantum computing, with the surface code being a prominent strategy due to its high error threshold and experimental feasibility. However, it is challenging to implement non-Clifford logical gates in a fault-tolerant way with low overhead, through the conventional magic-state distillation technique. Here, we enhance the performance of the conventional surface code by incorporating non-Pauli stabilizers and introduce two innovative initialization protocols. Our approach enhances the fidelity of the initialization of the non-Clifford logical state by avoiding unprotected operations before the encoding process. This improved fidelity of the initialization of non-Clifford logical states reduces the necessary number of logical qubits for precise state distillation, ultimately decreasing the overall resource overhead. Furthermore, we demonstrate the ability to entangle logical qubits in non-Pauli and Pauli bases via the lattice surgery technique. This integration enables the use of Pauli-based surface codes for computation while non-Pauli codes are employed for auxiliary qubit initialization, thus making them compatible with the conventional wisdom of logical Clifford operation based on the Pauli-based surface code.

We theoretically investigate the out-of-equilibrium dynamics of irregular one- and two-dimensional arrays of Rydberg dipoles featuring spatially anisotropic interactions. Starting from a collectively polarized initial state, we map out the dynamical phase diagram and identify a crossover between regimes of regular and anomalously slow relaxation of the initial collective order that strongly depends on both the degree of interaction disorder and anisotropy. In addition, we find that the regime of slow relaxation is characterized by a subballistic propagation of correlations that remained confined to short distances even at long times. To explain our findings we develop an analytic model based on decoupled clusters of interacting dipoles that goes beyond prior theoretical works and enables us to identify multiple relaxation timescales. Our findings can be relevant for a wide variety of quantum science platforms naturally featuring disordered dipolar interactions, including polar molecules, frozen Rydberg gases, and nitrogen-vacancy centers.

Ultracold Rydberg-atom arrays are an emerging platform for quantum simulation and computing. However, decoherence in these systems remains incompletely understood. Recent experiments [Guardado-Sanchez et al., Phys. Rev. X 8, 021069 (2018)] observed strong decoherence in the quench and longitudinal-field-sweep dynamics of two-dimensional Ising models realized with ${}^6\mathrm{Li}$ Rydberg atoms in optical lattices. This decoherence was conjectured to arise from the effective spin $\frac{1}{2}$ formed by the ground and Rydberg electronic states coupling to atom motion and therefore inducing an effective interaction noise. Here we show that this spin-motion coupling indeed leads to decoherence in qualitative, and often quantitative, agreement with the experimental data, by treating the difficult spin-motion-coupled problem using the discrete truncated Wigner approximation method. We also show that this decoherence will be an important factor to account for in future experiments with Rydberg atoms in optical lattices and microtrap arrays and discuss methods to mitigate the effect of motion, such as using heavier atoms or deeper traps.

The utilization of orthogonal modes of structured light, such as orbital angular momentum (OAM), in optical communication has led to an increased focus on the self-healing property of beams that contain OAM. In this paper, we theoretically and experimentally investigate the self-healing property in the twin Laguerre-Gaussian (LG) beams generated with a truncated input pump or probe having LG profiles in a four-wave mixing process in a double-$\mathrm{\Lambda }$ configuration. We show that, along with the amplified probe, the newly generated conjugate beam also acquires the truncated profile and exhibits self-healing on propagation. The transverse spatial intensity is reconstructed, and the OAM associated with the topological charge of the beams is restored in the far field. The self-healing in the generated beams is also studied for different topological charges and different obstacle types. The experimental results are supported with a theoretical model based on the semiclassical approach. The results of the present paper hold promise for practical implementation in real-world applications such as quantum communication and information transfer with OAM modes, addressing critical challenges like losses due to obstructions.

The electron spin of a nitrogen-vacancy center in diamond lends itself to the control of proximal ${}^{13}\mathrm{C}$ nuclear spins via dynamical decoupling methods, possibly combined with radio-frequency driving. Long-lived single-qubit states and high-fidelity electron-nuclear gates required for the realization of a multiqubit register have already been demonstrated. Towards the goal of a scalable architecture, linking multiple such registers in a photonic network represents an important step. Multiple pairs of remotely entangled qubits can enable advanced algorithms or error correction protocols. We investigate how a photonic architecture can be extended from the intrinsic nitrogen spin to multiple ${}^{13}\mathrm{C}$ spins per node. Applying decoherence-protected gates sequentially, we simulate the fidelity of creating multiple pairs of remotely entangled qubits. Even though the currently achieved degree of control of ${}^{13}\mathrm{C}$ spins might not be sufficient for large-scale devices, the two schemes are compatible in principle. One requirement is the correction of unconditional phases acquired by unaddressed nuclear spins during a decoupling sequence.

A universal quantum computer of large scale is not available yet, however, intermediate models of quantum computation would still permit demonstrations of a quantum computational advantage over classical computing and could challenge the extended Church-Turing thesis. One of these models based on single photons interacting via linear optics is called boson sampling. Although boson sampling was demonstrated and the threshold to claim quantum computational advantage was achieved, the question of how to scale up boson sampling experiments remains. To make progress with this problem, here we present a practically achievable pathway to scale boson sampling experiments by combining continuous-variable quantum information and temporal encoding. We propose the combination of switchable dual-homodyne and single-photon detections, the temporal loop technique, and scattershot-based boson sampling. We detail the required assumptions for concluding computational hardness for this configuration. Furthermore, this particular combination of techniques moves towards an efficient scaling and certification of boson sampling, all in a single experimental setup.

We derive an exact canonical matrix product state (MPS) representation for Dicke states $|D_k^n\rangle$ with a minimal bond dimension $\chi =k+1$, for general values of $n$ and $k$, for which the $W$ state is the simplest case $k=1$. We use this MPS to formulate a quantum circuit for sequentially preparing Dicke states deterministically, relating it to the recursive algorithm of Bärtschi and Eidenbenz. We also find exact canonical MPS representations with a minimal bond dimension for higher-spin and qudit Dicke states.

Recently, the resource theory of imaginarity was proposed, viewing the imaginary part of a quantum state as a basic resource leading to quantum advantages. As with other quantum resources such as correlations, coherence, and asymmetry, a natural question arises: Can imaginarity resource be broadcast by a real operation which is free in the resource theory of imaginarity? In this work, we introduce two concepts of imaginarity broadcasting. The first pertains to nonreal states, while the second is related to the imaginarity resource. For nonreal states, we demonstrate that a specific type of nonreal states can be broadcast. However, when it comes to broadcasting the imaginarity resource, we get a surprising result that any imaginarity resource can be perfectly broadcast via a real operation. By comparing different (no-)broadcasting theorems, we distinguish the resource theory of imaginarity from the resource theories of correlations, coherence, and asymmetry, and reveal certain distinctive features of the resource theory of imaginarity.

We construct a protocol for extracting distributed single-qubit quantum information encoded in a stabilizer code of multiple qubits, only with single-qubit local operations and classical communication (LOCC) without global operations or entanglement resources. This protocol achieves efficient extraction within a polynomial time in terms of the number of physical qubits. We apply this protocol to a setting of quantum information splitting where a subset of spatially separated parties cooperate by classical communication to extract quantum information shared among all the parties. For this task, our LOCC extraction protocol allows designing hierarchical information access structures among the parties, where the minimum number of parties required to cooperate depends on the location of extracting the shared quantum information. Extraction of quantum information encoded in the stabilizer code appears widely in distributed quantum information processing, such as quantum secret sharing. Our results allow such distributed quantum information processing without entanglement costs.

We study the effect of intercondensate dipole-dipole interactions in a setup consisting of physically disconnected, single-species dipolar Bose-Einstein condensates. In particular, making use of the long-range and anisotropic nature of dipole-dipole interactions, we show that the density of a target dipolar Bose-Einstein condensate can be axially confined and engineered using a trapped control dipolar condensate. Increasing the number of control condensates leads to exotic ground-state structures, including periodic patterns in the target condensate. These periodic patterns display a structural transition between single- and double-peaked structures with coherence between the peaks controlled via the separation between the control condensates.

We investigate the generation and engineering of photonic band gaps in waveguide quantum electrodynamics systems containing periodically arranged atom polymers. We first consider the configuration of a dimer array coupled to a waveguide. The results show that if the intra- and intercell phase delays are properly designed, the center and the width of the band gaps, as well as the dispersion relation of the passbands, can be modified by adjusting the intracell coupling strength. These manipulations provide ways to control the propagating modes in the waveguide, leading to some interesting effects such as slowing or even stopping a single-photon pulse. Finally, we take the case of the tetramer chain as an example to show that, in the case of a larger number of atoms in each unit cell, tunable multigap structures and more sophisticated band-gap engineering can be realized. Our proposal provides efficient ways for photonic band-gap engineering in micro- and nanoquantum systems, which may facilitate the manipulation of photon transport in future quantum networks.

Fully controlling the transformation of an arbitrary quantum state to a predesigned state is a crucial technology in quantum information processing. Despite numerous efforts in pursuit of this objective, a truly viable method remains elusive. Here we successfully devise an experiment capable of engineering any state on the Bloch sphere to a predetermined target state. To validate our findings, we choose different initial states to evolve and compare the purity and fidelity of the evolved state with the target state. Furthermore, we investigate the relationship between the steering speed and the spectrum of the Liouvillian superoperator, proposing an approach to speed up the steering process. Notably, we observe that setting system parameters at the Liouvillian exceptional point (LEP) leads to the fastest evolution to the steady state. In cases without an LEP, a larger Liouvillian gap corresponds to quicker convergence to the steady state. Our method introduces a different avenue for studying Liouvillian dynamics in open quantum systems and provides a practical means for state manipulation.

Although it seems that obtaining the quantum properties of light from classical calculations is a self-contradictory claim, it is shown here that this can be achieved by a unified mechanism which relies on a non-Hermitian Floquet theory. The mechanism governs the three distinct measurements of high-order-harmonic-generation spectra (HGS), above-threshold ionization (ATI), and IR photon number distribution. Here, the conditions that enable the calculations of HGS and ATI spectra for atoms interacting with high-intensity laser fields from photon statistics are first derived. Through a non-Hermitian theoretical simulation, the regimes where there is correspondence between the HHG and ATI spectra and annihilated pump photons (with postselection) are identified. Consequently, the HGS and ATI spectra, as detected by XUV detectors, can be obtained by monitoring the fluctuations of the infrared absorbed photons.

The diagrammatic analysis of interacting particle assemblies harbors a fundamental mismatch between two of its main implementations: $\mathrm{\Phi }$-derivable (conserving) approximations and parquet (crossing-symmetric) models. No termwise expansion, short of the exact theory itself, can be both conserving and crossing symmetric. This work applies the Kraichnan embedded-Hamiltonian formalism for strongly coupled systems to investigate consistency of the interplay between purely pair-mediated correlations and pair-irreducible ones. The approach sheds a different light on the issue of crossing symmetry versus conservation. In the process, the parquet equations acquire a different formulation.

The thermodynamics of quantum phase transitions has long been a rich area of research, providing numerous insights and enhancing our understanding of this important phenomenon. This theoretical framework has been well developed specially because quantum phase transitions occur at equilibrium. However, its dynamical counterpart, known as dynamical quantum phase transitions, takes place out of equilibrium, where conventional thermodynamic tools are inapplicable. In this work we make progress in this area by connecting dynamical criticality to thermodynamics through a geometric perspective on entropy production. Our findings, along with other recent developments, suggest that dynamical criticality can lead the system to highly complex dynamics, indicating a possible pathway to thermalization.

We examine the behavior of quantum discord in a scenario where two identical noninteracting qubits interact with a shared squeezed environment at zero temperature. Our focus lies on how the squeezing of the reservoir impacts the evolution of quantum discord when both qubits are initially in $X$-type states. We observe that there exists a critical time $t_c$ at which quantum discord undergoes a sudden change, a phenomenon crucial for quantum discord amplification. Furthermore, through numerical analysis, we determine the range of parameters where $t_c$ is finite or infinite. In cases where $t_c$ is finite, we demonstrate that increasing the squeezing phase of the bath can extend $t_c$, while increasing the squeezing strength has the opposite effect. Even in situations where $t_c$ is infinite and there is no sudden changes in the correlations, reservoir squeezing still influences quantum discord amplification, allowing adjustment of squeezing parameters to alter the time taken to reach steady-state quantum discord. Furthermore, in the short-time regime, reservoir squeezing significantly impacts the degree of quantum discord amplification. Notably, we explore the quantum speed limit time for a two-qubit system under a squeezed reservoir, finding that it can be decreased by adjusting both squeezing and the initial parameters. Our investigation offers a promising avenue for controlling quantum correlation amplification and the quantum speed limit.

Quantum-enhanced Markov chain Monte Carlo, an algorithm in which configurations are proposed through a measured quantum quench and accepted or rejected by a classical algorithm, has been proposed as a possible method for robust quantum speedup on imperfect quantum devices. While this procedure is resilient to noise and control imperfections, the potential for quantum advantage is unclear. By upper-bounding the gap of the Markov chain, we identify competing factors that limit the algorithm's performance. One needs the quantum dynamics to efficiently delocalize the system over a range of classical states, but it is also detrimental to introduce too much entropy through the quench. Specifically, we show that in the long-time limit, the gap of the Markov chain is bounded by the inverse participation ratio of the classical states in the eigenstate basis, showing there is no advantage when quenching to an ergodic system. For the paradigmatic Sherrington-Kirkpatrick and three-spin model, we identify the regime of optimal spectral gap scaling and link it to the system's eigenstate properties.