We discuss a scheme to generate large Fock states. The scheme involves repeatedly applying an experimentally feasible unitary transformation to convert a semiclassical state into a Fock state. The transformation combines Kerr interaction, which is a non-Gaussian operation, and pulsed coherent drives. We identify suitable parameter values (Kerr strength, pulse timings, displacement amplitude) for the physical processes to implement the transformation and generate large Fock states with near-unity fidelity. The feasibility of implementing the scheme in circuit QED architectures is discussed. The method is also suitable for generating Fock states of cavity fields.

Clifford operators play a vital role in quantum computation and information. Conventionally, a single-qudit Clifford operator maps the complete set of generalized Pauli matrices (GPMs) onto itself under conjugation. However, many practical quantum information tasks—such as local discrimination of generalized Bell states (GBSs)—only require mapping a specific subset of GPMs to another, rather than the entire set. To address this we introduce the concept of a local Clifford operator, defined as a unitary operator that conjugates a given set of $n$ GPMs into another such set. We establish necessary and sufficient conditions for such an operator to transform a pair of GPMs and show that these operators admit a generalized symplectic representation, analogous to the symplectic representation of standard Clifford operators. Furthermore, we give a generalized symplectic representation of any local Clifford operator acting on an $n$-GPM set. The generalized symplectic representation of local Clifford operators is directly applicable to determining the local unitary equivalence of sets of GBSs—a fundamental problem in quantum information, with applications in tasks such as local state discrimination. Our results thus bridge algebraic structures and physical applications, offering new tools for classifying and manipulating quantum states and operations.

Clifford gates and transformations, which map products of elementary Pauli or Majorana operators to other such products, are foundational in quantum computing, underpinning the stabilizer formalism, error-correcting codes, magic state distillation, quantum communication and cryptography, and qubit tapering. Moreover, circuits composed entirely of Clifford gates are classically simulatable, highlighting their computational significance. In this work we extend the concept of Clifford transformations to Dirac fermions. We demonstrate that discrete Clifford transformations are generated by half-body and pair operators while continuous Clifford transformations are generated by number operators, providing a systematic framework for their characterization. Additionally, we establish connections with fermionic mean-field theories and applications in qubit tapering, offering insights into their broader implications in quantum computing.

We present a graph-theoretic approach to simplifying generic many-body Hamiltonians. Our primary result introduces a recursive twin-collapse algorithm, leveraging the identification and elimination of symmetric vertex pairs (twins), as well as line-graph modules, within the frustration graph of the Hamiltonian. This method systematically block diagonalizes Hamiltonians, significantly reducing complexity while preserving the energetic spectrum. Importantly, our approach expands the class of models that can be mapped to noninteracting fermionic Hamiltonians (free-fermion solutions), thereby broadening the applicability of classical solvability methods. Through numerical simulations on spin Hamiltonians arranged in periodic lattice configurations and Majorana Hamiltonians, we demonstrate that the twin collapse increases the identification of simplicial and claw-free graph structures, which characterize free-fermion solvability. Finally, we extend our framework by presenting a generalized discrete Stone–von Neumann theorem. This comprehensive framework provides insights into Hamiltonian simplification techniques, free-fermion solutions, and group-theoretical characterizations relevant for quantum chemistry, condensed matter physics, and quantum computation.

Double ionization of small organic molecules followed by roaming-mediated isomerization has been proposed as a potential source of $\mathrm{H}{{}_3}^{+}$ in the interstellar medium. By integrating experimental and theoretical approaches, we report a different mechanism that significantly influences the $\mathrm{H}{{}_3}^{+}$ yields from methanol dication. The experimental investigation employed strong laser-field-induced double ionization of methanol, with fragment ions detected in coincidence using an ion imaging spectrometer. Notably, a non-negligible yield of the ${\mathrm{H}}^{+}+\mathrm{CH}{\mathrm{O}}^{+}$ coincident channel was observed, characterized by low kinetic energy release (KER) that deviates from predictions based on the Coulomb explosion model. Ab initio molecular dynamics simulations reveal that a portion of the generated $\mathrm{H}{{}_3}^{+}$ resides in highly energetic internal states, triggering a sequential fragmentation pathway that leads to the low-KER ${\mathrm{H}}^{+}+\mathrm{CH}{\mathrm{O}}^{+}$ channel. The simulations demonstrate that approximately 27% of the produced $\mathrm{H}{{}_3}^{+}$ dissociates into ${\mathrm{H}}^{+}+{\mathrm{H}}_2$ within a few-hundred femtoseconds. These findings offer insights into both the formation and degradation mechanisms of $\mathrm{H}{{}_3}^{+}$ in the interstellar medium.

We investigate the nonequilibrium dynamics of interacting bosons in a two-leg ring ladder pierced by an artificial magnetic flux, where the particles are initially localized in the central sites of both rings, and the ac-driven local energy shifts are applied to the remaining lattice sites. Within the mean-field approximation, we demonstrate the emergence of nonlinear self-trapping for strong interparticle interactions, and characterize the distinct excitation regimes in the absence of the inter-ring tunneling. The artificial magnetic flux typically introduces the Peierls phase factors, which induces complex-valued hopping amplitudes and leads to directed net particle currents along the chains. By further incorporating the finite inter-ring coupling and biased intra-ring hopping, we reveal that the tuning of the drive frequency and Peierls phase allows the precise control over both the intensity and direction of particle currents, which facilitates the transition between chiral and antichiral dynamics. These findings offer insights into the coherent manipulation of matter-wave transports in closed-loop lattice configurations and the exploration of nonequilibrium synthetic quantum systems in related fields.

In this paper, we introduce the Airyprime plasmon, a surface wave exhibiting unique long focal depth autofocusing along a curved trajectory. This plasmon notably features a highly tunable aspect ratio, which is achieved without altering the structured coupling devices on the metal surface. Furthermore, the plasmon demonstrates a robust self-healing property. Through Fourier-space modulation, we effectively suppress the sidelobe and generate a symmetric Airyprime plasmon. By adjusting a distribution factor, the interference pattern and focusing behavior of symmetric plasmon can be precisely controlled. Theoretical predictions are in good agreement with numerical simulations using the finite-difference time-domain method. Our findings may provide new potential for applications in particle manipulation and super-resolution imaging.

In vortex beams, ‘‘superkicks’’ denote momentum transfer beyond $\hslash k$, with $|{\mathbf{k}}_{\mathrm{loc}}|>k$. Tightly focused vortex beams possess a non-negligible longitudinal electric field, $E_z$. Here we make explicit the role of $E_z$ in enabling superkicks in light-atom interactions. Within a paraxial model augmented by first-order longitudinal corrections, we derive compact analytical expressions for the local wave vector and the scattering force in circularly polarized Laguerre-Gaussian beams, and we isolate the contribution of $E_z$ near the vortex core. We identify parameter regimes in which $E_z$ drives $|{\mathbf{k}}_{\mathrm{loc}}|>k$ and in which the azimuthal component of the scattering force dominates, providing clear conditions for observability. The analysis is supported by numerical evaluations across $\ell$ and $\sigma$, yielding theoretical criteria and parameter maps for the onset of superkicks and the resulting scattering-force enhancement.

Quantum error correction works effectively only if the error rate of gate operations is sufficiently low. However, some rare physical mechanisms can cause a temporary increase in the error rate that affects many qubits; examples include ionizing radiation in superconducting hardware and large deviations in the global control of atomic systems. We refer to such rare transient spikes in the gate error rate as error bursts. In this work, we investigate the resilience of the rotated surface code to generic error bursts. We assume that, after appropriate mitigation strategies, the spike in the error rate lasts for only a single syndrome-extraction cycle; we also assume that the enhanced error rate is uniform across the code block. Under these assumptions, and for a circuit-level depolarizing noise model, we perform Monte Carlo simulations to determine the regime in burst error rate and background error rate for which the memory time becomes arbitrarily long as the code block size grows. Our results indicate that suitable hardware mitigation methods combined with standard decoding methods may suffice to protect against transient error bursts in the rotated surface code.

We analyze the iterative procedure introduced by Ježek et al. [Phys. Rev. A 65, 060301 (2002)] for computing optimal measurements in minimum-error quantum state discrimination. By representing an $n$-outcome positive operator-valued measurements as a block column vector, we identify each update as the partial isometry arising from a polar decomposition, which provides a transparent proof that the corresponding guessing probability increases monotonically along the iteration. Exploiting this monotonicity, we prove the convergence of the algorithm for arbitrary ensembles of pure states under a mild initialization condition on the supports of the states. Finally, we introduce an embedding into a larger $dn$-dimensional space that links minimum-error discrimination to a fidelity optimization over block-diagonal states, and shows that an optimal strategy can be realized by a projective measurement in the embedded space.

Quantum teleportation uses a shared entangled resource, local operations, and a digitally error-corrected classical channel to transfer quantum states between distant parties. We introduce a hybrid teleportation-direct transmission protocol for state transfer that still exploits entanglement, but replaces classical communication and digital error correction with an analog feedforward through a noisy quantum channel. We show that quantum teleportation outperforms this protocol if the communication channel reduces the entanglement of all bipartite states having the same amount of entanglement as the resource; otherwise, the hybrid protocol is optimal. We apply our result to the state transfer of a uniformly distributed coherent-states codebook, highlighting experimentally relevant scenarios where our protocol is most effective. Our findings are directly relevant to both optical and superconducting microwave channels, where analog feedforward techniques have been recently implemented.

We consider the discrimination of quantum sequences under maximum-confidence measurements and show that the optimal discrimination of a quantum sequence ensemble can always be factorized into that of each individual ensemble. In other words, the optimal quantum sequence discrimination under maximum-confidence measurements can be achieved just by performing a maximum-confidence discrimination independently at each step of the quantum sequence. We also show that the maximum confidence of identifying a quantum sequence is to achieve the maximum confidence of identifying each state comprising the quantum sequence. We further provide a necessary and sufficient condition for the optimal quantum state discrimination under maximum-confidence measurements.

Quantum key distribution relies on quantum mechanics to securely distribute cryptographic keys, offering security but necessitating complex infrastructure and significant resources for practical implementation. Quantum keyless private communication ensures information-theoretic security in free-space communication, with simpler setups, and without the need for secret keys by leveraging the wiretap channel model. This model requires assumptions about Eve's ability to interact with the channel, consistent with the framework of physical-layer security. Here, we propose a variant of quantum keyless private communication using polarization encoding and experimentally validate both the original on-off keying method and the polarization-multiplexed approach using time-multiplexed threshold single-photon detectors as photon counting detectors. Our analysis highlights the advantages of polarization-multiplexed schemes for daylight operation. This work paves the way towards practical and scalable quantum communication systems, with potential applications extending to space-based communication.

High-resolution annihilation spectra, acquired with a cryogenic-trap-based positron beam, have revealed discrete resonances corresponding to vibrational excitations that extend significantly beyond features attributed to fundamental vibrations. This systematic analysis isolates and identifies these newly observed features as dipole-active (DA) combination and overtone vibrations. While vibrational Feshbach resonance (VFR) typically proceeds via single-quantum fundamental excitations, these combination and overtone features are shown to support distinct resonances. They exhibit dipole coupling and intramolecular vibrational redistribution (IVR) enhancement factors comparable to the fundamentals. Crucially, this robust IVR enhancement is secured by the absence of positron escape channels, a condition where the individual constituent mode energies are either below the positron binding energy or are symmetry forbidden. These findings provide direct evidence that multimode coupling plays a crucial role in the process of resonant positron annihilation.

We propose a low-pass-filter (LPF) feedback control for cooling a trapped particle with a low-pass filter, which utilizes a shift of the potential caused by the feedback operation. By incorporating this shift in the energy cost function, we show that the LPF control can achieve the minimum phonon occupation number that is lower than cold damping with a bandpass filter, that with delayed feedback, and linear-quadratic-Gaussian (LQG) control, the last two of which are the standard methods of ground-state cooling of a levitated nanoparticle. For a detection efficiency of $90\%$, the achievable phonon occupation number with the LPF control is about one-third, two-fifths, and one-half of that of cold damping with a bandpass filter, that with delayed feedback, and LQG control, respectively. Thus our method has a decisive advantage to reach the absolute ground state.

We study spin-phonon coupled dynamics in the vicinity of a sloped conical intersection created by laser coupling the electronic (spin) and vibrational degrees of freedom of a pair of trapped Rydberg ions. We show that the shape of the potential energy surfaces can be engineered and controlled by exploiting the sideband transitions of the crystal vibration and dipole-dipole interactions between Rydberg ions in the Lamb-Dicke regime. Using the sideband transition, we realize a sloped conical intersection whose cone axis is only tilted along one spatial axis. When the phonon wave packet is located in the minimum of the lower potential surface, the spin and phonon dynamics are largely frozen owing to the geometric phase effect. When starting from the upper potential surface, the electronic and phonon states tunnel to the lower potential surface, leading to a partial revival of the initial state. In contrast, the dynamics drastically change when the initial wave packets are away from the conical intersection. The initial state is revived, and it is almost entirely irrelevant whether it is from the lower or upper potential surface. Complete Rabi oscillations of the adiabatic states are found when the wave packet is initialized on the upper potential surface. The dynamics occur on the microsecond and nanometer scales, implying that Rydberg ions provide a platform for simulating nonadiabatic processes in the vicinity of a sloped conical intersection.

Recent advances in mode-locked fiber lasers have enabled observation of the internal dynamics of soliton molecules, revealing their strong analogy to bound states in nonlinear systems. However, previous studies have mostly focused on passive characterization, limiting access to the rich hierarchy of nonlinear couplings within soliton molecules. Here we extend the frontier by introducing a dual-frequency-driving scheme that actively excites and manipulates the internal degrees of freedom of soliton molecules within the nonlinear Schrödinger equation framework. This approach uncovers complex temporal modulations and cascaded radio-frequency responses of the pulse separation, revealing the multilayered structure of intramolecular nonlinear interactions. Furthermore, these dynamics are further validated through coupling to quantum-limited timing noise, providing insights for controlled manipulation and information-encoding applications of dissipative soliton molecules in ultrafast photonics.

We study the nonlinear, semiclassical (mean-field) dynamics of an open spin-1 (three-level) variant of the traditional Dicke model. In particular, we focus on V-type energy-level configurations with varying degrees of energy-level asymmetry. We also allow for unbalanced coupling, where corotating and counterrotating Hamiltonian terms are independently tunable. We characterize the system with a dynamical systems approach, where different behaviors map to definite dynamical objects, and phase transitions to bifurcations. We find the emergence of both periodic and two-frequency oscillations, as well as multistability and chaotic dynamics.

We develop a systematic projection-operator technique for constructing Gaussian approximations and their perturbative corrections in bosonic nonlinear models. As a case study, we apply it to the driven-dissipative Kerr oscillator. In the absence of external driving, the model can be solved exactly within a low-dimensional Fock subspace, leading to strongly non-Gaussian states. Nevertheless, we demonstrate that the evolution of first- and second-order moments is captured by our Gaussian scheme with high accuracy even in this regime, providing a natural benchmark. For the general case with external driving, our approach reduces the equations of motion to a closed system for means and covariances and allows one to compute systematic corrections beyond the Gaussian level in closed form. We also calculate the dynamics of linear and quadratic combinations of creation and annihilation operators in both weak- and strong-drive regimes.

Spontaneous emission in dipole approximation is studied theoretically using both source-field theory and a Schrödinger-picture approach. Using source-field theory we obtain formal equations for the Poynting vector and energy density without making the rotating-wave approximation (RWA) and the Weisskopf-Wigner approximation (WWA). The initial condition at $t=0$ is one in which the atom is in an excited state and the field in the vacuum state. The field operators are fully retarded in source-field theory. To explore the consequences of not making the RWA and WWA, the Poynting vector and energy density are calculated using perturbation theory. We use three methods to calculate the Poynting vector and energy density of the radiated fields, (1) a calculation based on definitions of the fields in terms of creation and annihilation operators, (2) a traditional source-field calculation, and (3) a hybrid source-field model involving a mixture of fully retarded and unretarded fields. We obtain analytic expressions for the Poynting vector and energy density in perturbation theory that are consistent with the results of Power and Thirunamachandran [Phys. Rev. A 45, 54 (1992)] and Compagno, Passante, and Persico [J. Mod. Opt. 37, 1377 (1990)], obtained using a Heisenberg-picture approach. The expressions for the Poynting vector and energy density that we derive in perturbation theory involve integrals over field mode frequencies, which are divergent unless a convergence factor is used to carry out the integrations. The convergence factor contains an effective cutoff frequency ${\omega }_c$, and results are obtained for both finite ${\omega }_c$ and the limit in which ${\omega }_c\to \infty$. It is also proved that the fields calculated using source-field theory always satisfy Poynting's theorem for arbitrary atom-field interactions and initial conditions.