Manipulating the structure of single photons in the ultrafast domain is enabling new quantum information processing technologies. At the picosecond timescale, quantum information can be processed before decoherence can occur. In this work, we study the capabilities of few-mode cross-phase modulation via the optical Kerr effect using ultrafast pulses. We observe a significant modulation in the spatial mode of structured photons on timescales less than 2 ps.

The traditional quantum speed limits are not attainable for many physical processes, as they tend to be loose and fail to determine the exact time taken by quantum systems to evolve. To address this we derive exact quantum speed limits for the unitary dynamics of pure-state quantum systems that outperform the existing quantum speed limits. Using these exact quantum speed limits, we can precisely estimate the evolution time for two- and higher-dimensional quantum systems. Additionally, for both finite- and infinite-dimensional quantum systems, we derive an improved Mandelstam-Tamm bound for pure states and show that this bound always saturates for any unitary generated by self-inverse Hamiltonians. Furthermore, we show that our speed limits establish an upper bound on the quantum computational circuit complexity and yield a tighter quantum Cramér-Rao bound. These results will have a significant impact on our understanding of quantum physics as well as rapidly developing quantum technologies, such as quantum computing, quantum control, and quantum batteries.

Quantum memory plays a critical role in quantum communication, sensing, and computation. However, studies on quantum memory under a unified benchmarking framework remain scarce. In this paper we propose a weight-based quantifier as a benchmarking method to evaluate the performance advantage of quantum memory in nonlocal exclusion tasks. We establish a general lower bound for the weight-based measure of quantum memory. Moreover, this measure provides fundamental theoretical bounds for transforming a general channel into an ideal quantum memory. Finally, we present explicit calculations of the weight-based quantifier for various channels, including unitary channels, depolarizing channels, maximal replacement channels, stochastic damping channels, and erasure channels.

Quantum solutions to differential equations represent quantum data—states that contain relevant information about the system's behavior, yet are difficult to analyze. We propose an algorithm for reading out information from such data, where customized quantum circuits enable efficient extraction of flow properties. We concentrate on the process referred to as quantum vortex detection, where specialized operators are developed for pooling relevant features related to vorticity. Specifically, we propose approaches based on sliding windows and quantum Fourier analysis that provide a separation between patches of the flow field with vortex-type profiles. First, we show how contour-shaped windows can be applied, trained, and analyzed sequentially, providing a clear signal to flag the location of vortices in the flow. Second, we develop a parallel window extraction technique, such that signals from different contour positions are coherently processed to avoid looping over the entire solution mesh. We show that Fourier features can be extracted from the flow field, leading to classification of datasets with vortex-free solutions against those exhibiting Lamb-Oseen vortices. Our work exemplifies a successful case of efficiently extracting value from quantum data, and it points to the need for developing appropriate models for quantum data analysis that can be trained on them.

Machine learning on quantum computers has attracted attention for its potential to deliver computational speedups in different tasks. However, deep variational quantum circuits require a large number of trainable parameters that grows with both qubit count and circuit depth, often rendering training infeasible. In this study we introduce the adaptive interpolating quantum transform (AIQT), a quantum-native framework for flexible and efficient learning. The AIQT defines a trainable unitary that interpolates between quantum transforms, such as the Hadamard and quantum Fourier transforms. This approach enables expressive quantum state manipulation while controlling parameter overhead. When built upon efficient quantum circuits such as the quantum Fourier transform, the AIQT achieves polynomial scaling of the number of gates with respect to the number of qubits. Our results show that the AIQT achieves high performance with minimal parameter count, offering a scalable and interpretable alternative to deep variational circuits.

Multiqubit entangled photonic graph states are an important ingredient for all-photonic quantum computing, repeaters, and networking. Preparing them using probabilistic stitching of single photons using linear optics presents a formidable resource challenge due to multiplexing needs. Quantum emitters provide a viable solution to prepare photonic graph states, as they enable deterministic production of photons entangled with emitter qubits and deterministic two-qubit interactions among emitters. A handful of emitters often suffice to generate useful-size graph states that would otherwise require millions of emitters used as single photon sources, using the linear-optics method. Photon loss, however, impedes the emitter method due to a large circuit depth and hence loss accrual on the photons of the graph state produced, given the typically large number of slow two-qubit controlled-not (cnot) gates between emitters. We propose an algorithm that can trade the number of emitters with the graph-state depth, while minimizing the number of emitter cnot gates. We apply our algorithm to generate a repeater graph state (RGS) for an alternative all-photonic repeater protocol, which achieves a far superior rate-distance trade-off compared to using the least number of emitters needed to generate the RGS. Yet it needs five orders of magnitude fewer emitters than the multiplexed linear-optics method—with each emitter used as a photon source—to achieve a desired rate-distance performance.

Having multiple Bell pairs shared by distant quantum registers provides a key resource for both quantum networks and distributed quantum computing. In this paper, we present a protocol for parallelized Bell-pair generation that uses the phase of a coherent light pulse to encode a qudit, enabling the simultaneous generation of multiple Bell pairs. By encoding a qudit in a basis of light-matter Schrödinger's cat states, the loss of a photon in transit can be detected through an $XX$ parity syndrome, allowing the backaction due to the lost photon to be deterministically corrected through single-qubit rotations. The protocol presented here is compatible with existing technologies in both optical and microwave (circuit QED) architectures, supporting near-term implementation across diverse quantum platforms.

We experimentally demonstrate the control of high-harmonic spectrum discreteness in the vicinity of 100 eV under modulation of multi-cycle driving laser field (1.24 µm, 100 fs) by intense mid-IR radiation (4.5 µm, 200 fs) and a strong terahertz field (100 µm, 530 fs). The observed transition between the discrete harmonic spectrum and broadband quasicontinuum is explained by the modification of attosecond electron dynamics, which can be controlled by the strength and polarization direction of copropagating long-wavelength field. The presented research reveals the underlying physical mechanisms and demonstrates the prospectives of long-wavelength radiation for the generation of quasicontinuous spectrum in the extreme ultraviolet and soft-x-ray regions.

The non-Markovian effects of open quantum systems subjected to external environments are deemed to be valuable resources in quantum optics and quantum information processing. In this work, we construct the model Hamiltonian of three-level giant atom with multiple coupling points in the semi-infinite photonic waveguide via complex coupling coefficients and investigate the non-Markovian dynamics of the three-level giant atom driven by a classical laser field. In the non-Markovian regime, the traveling time of a photon between different coupling points is sufficiently large compared to the inverse of the bare relaxation rate of the giant atom. We derive the analytical expressions for the probability amplitudes of the driven three-level giant atom by solving a set of delay differential equations and obtain two independent conditions of bound states' formation that can inhibit the dissipation. We find two different types of bound states (including the static bound states and the periodic equal-amplitude oscillating bound states) and discuss the physical origins of the bound states' formation. Moreover, we discuss the case of the driven three-level giant atom interacting with the infinite photonic waveguide, where there is only one purely imaginary solution (i.e., only one bound-state condition exists) for its complex frequency (coming from the absence of mirror at one end of the waveguide) compared to that of a driven three-level giant atom coupling with a semi-infinite photonic waveguide. With this, we also find two different types of bound states, including the static bound state and the periodic equal-amplitude oscillating bound states. We study the influences of the phase in the coupling strength on the bound states in semi-infinite and infinite photonic waveguides. The presented protocol might open a way to better understand exactly the non-Markovian dynamics of driven multilevel giant atoms interacting with semi-infinite (or infinite) photonic waveguides.

In recent years, the study of non-Hermitian systems has attracted significant attention, particularly for their unique topological phenomena such as the non-Hermitian skin effect (NHSE). Here we investigate the NHSE in a non-Hermitian Su-Schrieffer-Heeger atomic chain and its coupling with a cavity mode, focusing on how NHSE modifies the cavity's transmission spectrum. We theoretically model the atomic chain which possesses a specific symmetry under open boundary conditions. Our results show that the eigenstates localize at the chain edges for certain parameters, indicating the existence of the NHSE. This effect produces distinct peaks in the cavity transmission spectrum at frequencies corresponding to eigenstates in the nontrivial phase; these peaks vanish in the trivial phase. By tuning atomic gain and loss, we identify sharp spectral minima at the topological phase transition point, which enables precise detection of the transition. These results demonstrate that the cavity transmission spectrum serves as a sensitive probe for non-Hermitian topological effects, offering a scalable experimental framework to explore the NHSE without calculating topological invariants.

The Addition of photons to coherent states is shown to produce effects that display remarkable similarities with cubic phase shifts acting on the vacuum state, with fidelities in excess of 90%. The strength of the cubic interaction is found to vary inversely with the displacement of the coherent state and the strongest interactions were one order of magnitude greater than previous observations. Although these results demonstrate an interaction that is nonperturbative, the current experimental implementation relies on postselection.

Light-matter systems that exhibit two-photon interactions have emerged as powerful platforms for exploring quantum applications. In this work, we focus on the two-photon Dicke model, a system of significant experimental relevance that displays spectral collapse and undergoes a phase transition from a normal to a superradiant phase. We analyze the normal phase, where a classical limit with two degrees of freedom can be derived using a mean-field approximation. Our study presents a detailed investigation of the loss of integrability in the two-photon Dicke model, employing both quantum and classical diagnostics. These results allow us to explore various dynamical features of the system, including the onset of chaos and the existence of mixed phase-space behavior.

Genovese and Piacentini [Phys. Rev. A 111, 022204 (2025)] claim that, in a recent experiment [S. Virzì et al., Quantum Sci. Technol. 9, 045027 (2024)], the Bell parameter was measured on a single pair of photons, thus challenging several conclusions and discussions on the meaning of Bell inequalities as well as certain quantum mechanical interpretations. We explain that the parameter measured in the experiment of Virzì et al. is not the Bell parameter $S$, which has been discussed and estimated in many loophole-free tests. Therefore, this experiment neither challenges our understanding of Bell tests nor allows having doubts about Bohr complementarity and contextuality.

In a recent article [Phys. Rev. A 111, 022204 (2025)], Genovese and Piacentini analyzed recent experiments measuring what they call “the entire Bell-Clauser-Horne-Shimony-Holt (Bell-CHSH) parameter.” They claimed those experiments may have implications for interpreting loophole-free tests of the Bell-CHSH inequality. We explain that the Bell-CHSH inequality is not based on the entire Bell parameter, so these experiments are unrelated to their empirical tests and cannot close any eventual loopholes that might still persist. We point out that the physical meaning of these new experiments measuring the entire Bell parameter could be interpreted differently.

We reply to the Comments by Kupczynski and Lambare on our recent paper [Phys. Rev. A 111, 022204 (2025)], questioning our claims on the implications on quantum mechanics interpretations (and alternative theories) generated by the results of a recent experiment [Quantum Sci. Technol. 9, 045027 (2024)] demonstrating the possibility of measuring the entire Bell parameter on a single entangled pair.

We establish a necessary and sufficient condition for the existence of a quantum state that reproduces given correlation values in the Clauser–Horne–Shimony–Holt setup for any fixed normalized observables. This result addresses a fundamental question shared by both local realism and quantum mechanics: under what conditions a given set of observed data can be reproduced by a physical model. While previous studies have mainly addressed conditions for correlations achievable without specifying the measurement settings, our result gives a finer characterization by treating the observables as fixed in advance. The resulting quantum condition strengthens previously known constraints, such as Tsirel'son's inequalities and the Tsirel'son–Landau inequality, by characterizing statistical constraints explicitly for each specified set of observables. In particular, we show that our condition applies to Bell's original scenario and reveals that whether Bell's original inequality is violated depends sensitively on the chosen observables. More broadly, this perspective offers new insights into how quantum violations of local realism depend on the measurement settings.

Non-Gaussian states are essential for achieving a quantum advantage in continuous-variable (CV) information processing. Among these, coherent superpositions of squeezed states are a foundational resource. While exact higher-order statistics are available in the undisplaced case, a complete and analytically tractable treatment with a common displacement has been missing. We introduce and solve the displaced Janus state—a coherent superposition of two squeezed coherent states that share the same displacement—and develop an analytical framework, based on a family of generalized squeezing polynomials, that yields closed-form expressions for arbitrary-order factorial moments and coherence functions $g^{\left(k\right)}\left(0\right)$, the full Wigner function, and the quantum Fisher information. The analysis shows how interference at a fixed mean, driven by a mismatch of the component covariances rather than by mean separation, can be precisely engineered to transform the extreme photon bunching of the constituents into strong sub-Poissonian and even perfect multiphoton suppression. We further provide a rigorous quantum Fisher information analysis, proving that parameters encoded by linear generators (for example, the number operator) are bounded by the standard quantum limit, whereas parameters encoded by quadratic generators (for example, squeezing transformations) achieve Heisenberg-limited scaling. Together, these results furnish a complete analytical toolkit for a versatile class of non-Gaussian states, establishing the displaced Janus state as a key primitive for hybrid quantum protocols, quantum metrology, and fault-tolerant continuous-variable computation.

The confined variational method is employed to study low-energy electron collisions with a lithium atom, for energies below the $2p$ excitation threshold. The accuracy of the phase shifts has been markedly improved in our calculations. From these calculations, we obtain an electron affinity of 4985.6124 ${\mathrm{cm}}^{-1}$, in excellent agreement with experiment, validating the accuracy of the approach. A least-squares fit of the phase shifts to effective-range theory yields an $S$-wave scattering length of $A=3.02a_0$, where $a_0$ is the Bohr radius. These results provide a stringent benchmark for future theoretical and experimental investigations. In this study, the sum of all electron spins is considered to be zero.