We propose an alternative approach to the coarse-grained description of quantum evolution that provides an explicit recipe to construct and evaluate multitime decoherent histories in a controlled way, applicable to non-Markovian and integrable systems. Specifically, we study the local interaction quench of a local degree of freedom (an open quantum system) within a noninteracting integrable environment. This setting allows us to identify the environmental degrees of freedom that irreversibly store records of the system's past. These modes emerge sequentially in time and define the projectors required for decoherent histories. We show numerically that the off-diagonal elements of the decoherence functional are exponentially suppressed relative to a significance threshold.

We formulate an exact Green's function theory for non-Hermitian single-microwave-photon scattering by finite superconducting circuit subsystems embedded in an Su-Schrieffer-Heeger (SSH) waveguide. The structured SSH environment is integrated out exactly and enters the local scattering problem as an energy-dependent matrix self-energy, reducing the full open system to a finite-dimensional effective non-Hermitian Hamiltonian. This reduction places scattering amplitudes, exceptional-point diagnostics, coherent-perfect-absorption conditions, and lasing thresholds within one unified framework. Within this approach we analyze two superconducting devices. A flux-controlled two-qubit interferometric scatterer exhibits a broad bright branch and a narrow quasidark branch whose interference is reshaped by the SSH environment and changes qualitatively across the two dimerizations. A mediator-assisted two-qubit scatterer generates an additional energy-dependent complex coupling, reorganizes the dressed spectrum, and produces clearer dimerization-sensitive transparency-versus-absorption windows together with a pronounced separation between zerolike and polelike scattering branches. In the active regime, near-exceptional-point hybridization enhances the pole-dominated response while deepening the singular-value valley associated with near-coherent perfect absorption. These results show how structured topological waveguides can be used not only to host scattering but also to design non-Hermitian superconducting microwave functionalities.

We demonstrate a trapped-ion protocol in which a nearby, dedicated monitor qubit tracks magnetic field drifts in real time without interrupting data-qubit operations. Using two ${}^{40}\mathrm{Ca}^{+}$ ions and the optical-metastable-ground architecture, we encode the data qubit in the ground-state manifold and the monitor qubit in a metastable-state manifold to achieve spectral separation. The monitor qubit senses common magnetic fluctuations during data-qubit experiments, enabling feedforward corrections to the qubit-control drives. Under applied magnetic noise with a realistic spectrum ($1/f^2$), the protocol maintains coherence and, when compared with interleaved calibration, it extends usable data-qubit probe times by up to a factor of approximately $\sqrt{2}$ and doubles the experimental duty cycle. These results establish monitor qubits as a scalable tool for real-time recalibration in quantum information processors.

Measurement-only models offer an ideal platform for exploring entanglement dynamics in the absence of unitary evolution. Despite extensive numerical evidence for entanglement phase transitions in measurement-only dynamics, the underlying mechanism attributed to noncommutativity among multisite projective measurements has remained qualitative and coarse grained. In this work, we identify a quantitative noncommutative index for spatially and temporally homogeneous Pauli stabilizer measurement-only circuits. By applying this index to three representative measurement-only models, we find that the emergence of a volume-law phase is governed by the noncommutative structure of the measurement ensemble, while the transition point is quantitatively determined by the amount of critical noncommutativity. More strikingly, the critical noncommutativity exhibits a linear scaling with the measurement range, independent of the microscopic details of the measurement ensembles. Our findings deepen the understanding of the fundamental mechanism behind the measurement-only entanglement phase transition.

Typical dynamical quantum chaos probes are initial-state dependent (e.g., local observables or purities) and thus may fail to capture typical decoherent behavior one expects of a subsystem. Quantum channels fully capture the reduced dynamics of a subsystem. Here, we investigate the purity of the Choi state of a single spin in a chain, which acts as the state representation of the channel encoding the reduced dynamics. Operationally, we show this quantity functions as an echo protocol, termed the Choi echo. It measures the environment's recovery fidelity when subjected to a forward evolution, a completely depolarizing operation on the local subsystem, and a subsequent backward evolution. We investigate the equilibration value of the Choi echo across the integrability-to-chaos transition in three paradigmatic spin-$1/2$ chains. We show that average single-spin decoherence does not uniquely correspond to spectral chaos. Specifically, coherent transport in integrable systems can mimic the mean relaxation of a single spin typically induced by chaotic scrambling, generating false positives for spectral chaos. This work offers a perspective bridging quantum information tools with many-body quantum chaos.

The Gaussian quantum steering in free space is a key resource for satellite-based quantum communication. In this work, we model the free space transmission of Gaussian states as a fading and noisy quantum channel, incorporating key loss mechanisms including diffraction, atmospheric attenuation, turbulence, and receiver-side background photons. Our study encompasses a broad range of practical conditions, including uplink and downlink transmission, and daytime and nighttime operation. We show that Gaussian steerability decreases monotonically with satellite altitude, transitioning from two-way to one-way steering and ultimately to nonsteerability. Furthermore, we find that the nighttime downlink channel experiences the lowest overall loss and exhibits the greatest robustness, thereby providing the most favorable conditions for achieving Gaussian quantum steerability. Our results quantitatively characterize channel-induced steering asymmetry and provide guidelines for optimizing continuous-variable satellite quantum communication.

We show that odd order Rényi entropies $S^{(2q+1)}$ of a system of interacting scalar fields can be calculated as the free energy of $2q+1$ replicas of the system with additional quadratic inter-replica couplings in the subsystem at the time of measurement of the entropy. These couplings replace boundary field matching conditions. This formalism works both in and out of thermal equilibrium, for closed as well as open quantum systems, and provides a general dictionary between measurable correlation functions and entanglement entropy. $S^{(2q+1)}$ can be analytically continued to calculate the von Neumann entropy $S^{\mathrm{vN}}$. We provide an exact formula relating $S^{(2q+1)}$ and $S^{\mathrm{vN}}$ with correlation functions in a noninteracting theory. For interacting theories, we provide rules for constructing all possible Feynman diagrams for $S^{(2q+1)}$. We show that the boundary matching conditions cannot be completely eliminated while calculating Rényi entropies of even order due to the presence of zero modes in replica space.

We investigate the performance of two quantum error-correcting codes—the surface code and the Bacon-Shor code—for implementation with spin qubits in silicon. In each case, we construct a logical qubit using a planar array of quantum dots, exploring two encoding schemes: one based solely on single-electron Zeeman qubits (Loss-DiVincenzo qubits) and a hybrid approach combining Zeeman and singlet-triplet qubits. For both codes, we evaluate key performance metrics, including logical state preparation fidelity and cycle-level error correction performance, using state-of-the-art experimental parameters. Our results show that the hybrid encoding consistently outperforms the pure Zeeman qubit implementation. By identifying the dominant error mechanisms that limit quantum error correction performance, our study highlights concrete targets for improving spin-qubit hardware and provides a path toward scalable fault-tolerant architectures. In particular, we find that the logical error rate is not limited by memory errors but rather by gate errors, especially one- and two-qubit gate errors.

Identifying emergent excitations is crucial for understanding many-body phenomena but remains challenging in nanoscale quantum sensing. Conventional methods based on second-order correlations are often unable to distinguish different excitation types due to their similar spectral signatures, whereas higher-order correlations contain richer information that reveals their distinct nature. Here, we propose a fourth-order cumulant spectroscopy that diagnoses realistic spin and pseudospin excitations emerging from interacting spin systems based on their different correlation structure. By employing a quantum sensor, we extract both second- and fourth-order time-ordered correlations and construct their cumulant spectra, enabling unambiguous identification of different excitation types. This approach is robust against environmental interference and may offer a powerful tool for probing emergent excitations in quantum materials at the nanoscale.

We theoretically study strong-field ionization dynamics under a two-color laser field composed of a strong 800-nm circularly polarized field and a weak 400-nm linearly polarized field. By numerically solving the three-dimensional time-dependent Schrödinger equation, we obtain the photoelectron momentum distributions at different two-color relative phases. These distributions are analyzed using two-color phase-of-the-phase (POP) spectroscopy, yielding both the Relative-Phase Contrast (RPC) and POP spectra, which exhibit pronounced angular and energy dependencies. We demonstrate that when the relative phase of the two-color field takes the value of the POP, the synthesized field effectively mimics an 800-nm elliptical field with the ellipticity dictated by the intensity ratio of the two colors. This mapping enables the angle-dependent POP spectrum to encode the temporal dynamics of ionization induced by a strong elliptical field, from which we reconstruct the ionization time in such a field. Furthermore, we derive an analytic expression for the RPC and show that the RPC spectrum carries information about the imaginary phase of electrons accumulated during strong-field tunneling by elliptical fields. Our work provides an approach to probe ultrafast ionization dynamics in strong elliptical pulses.

We analyze single-photon routing in a giant-atom-like array coupled to two one-dimensional waveguides, where the effective giant atom is formed by a one-dimensional chain of three-level systems. In the regime of strong atom-waveguide coupling and weak interatomic interactions, the device functions as an efficient and directionally controllable router. Routing is governed by interference controlled by an effective phase accumulated between distant coupling points, and can be tuned by the number of coupling sites $N$, the photon energy $E$, and the interatomic coupling strength $J$. We identify regimes with near-unity transfer (up to $100\%$) sustained over a broad energy window, enabling broadband operation and dynamic control of the output channel. We further assess robustness against parameter variations and discuss scalability with $N$. These results highlight the potential of this platform for reconfigurable and integrated quantum photonic networks.

A major challenge in photonic quantum technologies is developing strategies to prepare suitable discrete-variable quantum states using simple input states, linear optics, and auxiliary photon measurements to identify successful outcomes. Fundamentally, this challenge arises from the lack of strong nonlinearities on the single-photon level, meaning that photonic state preparation based on linear optics cannot benefit from the deterministic gate-based approach available to other physical platforms. Instead, the preparation of quantum states can be probabilistically implemented using single photons, linear-optical networks, and photon detection. However, determining whether an input state can be transformed into a target state using a specific measurement pattern—a problem that can be mapped to deciding the feasibility of a system of polynomial equations—is a complex problem in general. To solve it, we apply the Nullstellensatz Linear Algebra algorithm from algebraic geometry to quantum state generation; this can provide definitive no-go results by proving infeasibility when the state preparation task in question has no solution. We demonstrate this capability to validate and establish lower bounds on the physical resource requirements for the realization of several ubiquitous optical states and gates.

Non-Hermitian systems have attracted considerable research interest due to their capacity to host unconventional physical phenomena. However, research on pseudo-Hermitian symmetry and re-entrant localization in one-dimensional non-Hermitian settings remain relatively limited. Here, we propose a one-dimensional non-Hermitian chain model composed of two coupled sublattices and investigate the effects of nonreciprocal hopping and quasiperiodic modulation on its physical properties. As the nonreciprocal hopping increases, the system undergoes a transition from a pseudo-Hermitian phase with entirely real energy spectra to a non-Hermitian phase characterized by the emergence of complex eigenenergies. Under a quasiperiodic potential, the system exhibits a rich phase diagram that includes extended, localized, and intermediate phases with mobility edges. Remarkably, we observe re-entrant localization, where the system switches nonmonotonically between extended and localized states as the potential strength or nonreciprocal hopping varies. This behavior originates from the synergy between quasiperiodicity-induced interference and the non-Hermitian skin effect. Dynamical analyses, such as wave-packet evolution, mean square displacement, and diffusion coefficients, further confirm these localization transitions. Our results underscore the essential role of nonreciprocal hopping in modulating topological and transport properties, offering valuable insights for the design of non-Hermitian quantum devices and sensors operating near exceptional points.

We propose a unifying framework for nonequilibrium relaxation dynamics in ensembles of positionally disordered interacting quantum spins based on the statistical properties, such as mean and variance, of the underlying disorder distribution. Our framework is validated through extensive exact numerical calculations and we use it to disentangle and understand the importance of dimensionality and interaction range for the observation of glassy (i.e., sub-xponential) decay dynamics. Leveraging the deterministic control of qubit positioning enabled by modern tweezer array architectures, we also introduce a method (“$J$-mapping”) that can be used to emulate the relaxation dynamics of a disordered system with arbitrary dimensionality and interaction range in bespoke one-dimensional arrays. Our approach paves the way towards tunable relaxation dynamics that can be explored in quantum simulators based on arrays of neutral atoms and molecules.

Quantum image processing relies on the efficient encoding of classical image data into quantum states, yet the practical performance of existing encoding schemes on noisy intermediate-scale quantum (NISQ) hardware remains insufficiently understood. Most prior studies have emphasized asymptotic resource scaling or idealized circuit constructions, leaving a gap between theoretical proposals and realistic device performance. In this work, we present a systematic, hardware-realistic benchmark of six representative quantum image encoding schemes—Basis, Angle, Amplitude, Flexible Representation of Quantum Images (FRQI), Novel Enhanced Quantum Representation (NEQR), and Order-Encoded Quantum Image Model (OQIM)—using image retrieval as a test task. Our analysis explicitly incorporates transpilation overhead, device connectivity constraints, and calibrated noise models derived from multiple IBM quantum processors. Reconstruction fidelity is evaluated across different image sizes, circuit depths, and measurement budgets. We find that qubit efficiency alone is a poor predictor of practical performance on NISQ devices. Instead, post-transpilation circuit depth emerges as the dominant factor governing noise resilience and information preservation. Encoding schemes with favorable asymptotic qubit scaling but deep circuit implementations suffer severe degradation under realistic noise, while shallower constructions retain meaningful image structure. Among the considered schemes, amplitude encoding consistently achieves the highest reconstruction fidelity under hardware-calibrated simulation; angle encoding, validated on physical hardware, demonstrates high fidelity. These results clarify the practical performance hierarchy of quantum image encodings under NISQ conditions and provide experimentally grounded guidance for encoding selection and circuit design on near-term quantum platforms.

Entanglement is one of the fundamental properties of a quantum state and is a crucial differentiator between classical and quantum computation. There are many ways to define entanglement and its measure, depending on the problem or application under consideration. Each of these measures may be computed or approximated by multiple methods. However, hardly any of these methods can be run on near-term quantum hardware. This work presents a quantum adaptation of the iterative higher-order power method for estimating the geometric measure of entanglement of multiqubit pure states using rank-1 tensor approximation. This method is executable on early fault-tolerant (hybrid) quantum hardware and does not depend on quantum memory. We simulate this algorithm and mitigate the effects of noise on the results of the computation using a theoretical model based on a known mitigation approach, which assumes a global depolarizing noise channel.

In this work, we investigate quantum correlations, the quantum-memory-assisted entropic uncertainty relation (QMA-EUR), and its lower bound in a system with two coupled double quantum dots, where each coupled double quantum dot interacts independently with its own infinite-dimensional bosonic reservoir. We contrast Markovian and non-Markovian dynamics' impacts on the QMA-EUR, demonstrating that trace-distance revivals (a hallmark of information backflow) accompany accelerated dynamics induced by non-Markovian effects. Quantum discord and conditional entropy govern QMA-EUR tightness, with stronger nonclassical correlations yielding a tighter bound. This bound depends on the two-qubit system's intrinsic quantum correlations and the synergy between quantum discord and the minimal von Neumann conditional entropy. Tuning electron temperature, tunneling coupling, and interelectron Coulomb interaction can effectively suppress the entropic uncertainty bound and enhance quantum correlations, thereby providing valuable insights into the manipulation of quantum uncertainty in this solid-state quantum system.

Hidden Markov models (HMMs) are ubiquitous in time-series modeling, with applications ranging from chemical reaction modeling to speech recognition. These HMMs are often large, with high-dimensional memories. A recently proposed application of quantum technologies is to execute quantum analogs of HMMs. Such quantum HMMs (QHMMs) are strictly more expressive than their classical counterparts, enabling the construction of more parsimonious models of stochastic processes. However, state-of-the-art techniques for QHMM compression, based on tensor networks, are only applicable for a restricted subset of HMMs, where the transitions are deterministic. In this work we introduce a pipeline by which any finite, ergodic HMM can be compressed in this manner, providing a route for effective quantum dimension reduction of general HMMs. We demonstrate the method on both a simple toy model, and on a speech-derived HMM trained from data, obtaining favorable memory-accuracy tradeoffs in the examples studied, relative to a simple classical state-merging baseline.