Process tensors are quantum combs describing the evolution of open quantum systems through multiple steps of quantum dynamics. While there is more than one way to measure how different two processes are, special care must be taken to ensure quantifiers obey physically desirable conditions such as data-processing inequalities. Here, we analyze two classes of distinguishability measures commonly used in general applications of quantum combs. We show that the first class, called Choi divergences, does not satisfy an important data-processing inequality, while the second one, which we call generalized divergences, does. We also extend to quantum combs some other relevant results of generalized divergences of quantum channels. Finally, given the properties we proved, we argue that generalized divergences may be more adequate than Choi divergences for distinguishing quantum combs in most of their applications. Particularly, this is crucial for defining monotones for resource theories whose states have a comb structure, such as resource theories of quantum processes and resource theories of quantum strategies.

We propose efficient classical algorithms which (strongly) simulate the action of bosonic linear optics circuits applied to superpositions of Gaussian states. Our approach relies on an augmented covariance matrix formalism to keep track of relative phases between individual terms in a linear combination. This yields an exact simulation algorithm whose runtime is polynomial in the number of modes and the size of the circuit and quadratic in the number of terms in the superposition. We also present a faster approximate randomized algorithm whose runtime is linear in this number. Our main building blocks are a formula for the triple overlap of three Gaussian states and a fast algorithm for estimating the norm of a superposition of Gaussian states up to a multiplicative error. Our construction borrows from earlier work on simulating quantum circuits in finite-dimensional settings, including, in particular, fermionic linear optics with non-Gaussian initial states and Clifford computations with nonstabilizer initial states. It provides algorithmic access to a practically relevant family of non-Gaussian bosonic circuits.

Circuit quantum electrodynamics (QED) has emerged as a promising platform for implementing quantum computation and simulation. Typically, junctions in these systems are of a sufficiently small size, such that only the lowest plasma oscillation is relevant. The interplay between the Josephson effect and charging energy renders this mode nonlinear, forming the basis of a qubit. In this work, we introduce a QED architecture based on extended Josephson junctions (JJs), which possess a non-negligible spatial extent. We present a comprehensive microscopic analysis and demonstrate that each extended junction can host multiple nonlinear plasmon modes, effectively functioning as a multiqubit interacting system, in contrast to conventional JJs. Furthermore, the phase modes exhibit distinct spatial profiles, enabling individual addressing through frequency-momentum selective coupling to photons. Our platform has potential applications in quantum computation, specifically in implementing single- and two-qubit gates within a single junction. We also investigate a setup comprising several driven extended junctions interacting via a multimode electromagnetic resonator. This configuration serves as a powerful platform for simulating the generalized Bose-Hubbard model, as the photon-mediated coupling between junctions can create a lattice in both real and synthetic dimensions. This allows for the exploration of quantum phenomena, such as topological phases of interacting many-body systems.

We measure the scalar, vector, and tensor components of the differential dynamic polarizability of the strontium intercombination transition at $1064.7\mathrm{nm}$. We compare the experimental values with the theoretical prediction based on the most recently published spectroscopic data, and find a very good agreement. We also identify a close-to-circular “magic” polarization where the differential polarizability strictly vanishes, and precisely determine its ellipticity. Our work opens new perspectives for laser cooling optically trapped strontium atoms and provides a benchmark for atomic models in the near-infrared spectral range.

Direct measurement of the elastic scattering of real photons on an electromagnetic field would allow the fundamental low-energy constants of quantum electrodynamics to be experimentally determined. We show that scenarios involving the collision of three laser beams have several advantages over conventional two-beam scenarios. The kinematics of a three-beam collision allows for a higher signal-to-background ratio in the detection region, without the need for polarimetry, and separates out contributions from different orders of photon scattering. A planar configuration of colliding a photon beam from an x-ray free-electron laser with two optical beams is studied in detail. We show that measurements of elastic photon scattering and vacuum birefringence are possible with currently available technology.

A fault-tolerant error correction (FTEC) protocol with a high error suppression rate and low overhead is very desirable for the near-term implementation of quantum computers. In this work, we develop a distance-preserving flag FTEC protocol for the $\left[\right[49,1,9\left]\right]$ concatenated Steane code, which requires only two ancilla qubits per generator and can be implemented on a planar layout. We generalize the weight-parity error correction (WPEC) technique from Tansuwannont and Leung [Phys. Rev. A 104, 042410 (2021)] and find a gate ordering of flag circuits for the concatenated Steane code, which makes syndrome extraction with two ancilla qubits per generator possible. The FTEC protocol is constructed using the optimization tools for flag FTEC developed in Pato et al. [PRX Quantum 5, 020336 (2024)] and is simulated under the circuit-level noise model without idling noise. Our simulations give a pseudothreshold of $1.64\times {10}^{-3}$ for the $\left[\right[49,1,9\left]\right]$ concatenated Steane code, which is better than a pseudothreshold of $1.43\times {10}^{-3}$ for the $\left[\right[61,1,9\left]\right]$ 6.6.6 color code simulated under the same settings. This is in contrast to the code capacity model where the $\left[\right[61,1,9\left]\right]$ code performs better.

We closely replicated and extended a recent experiment [Phys. Rev. Lett. 129, 183601 (2022)] that reportedly observed enhancement of two-photon-absorption rates in molecular samples by using time-frequency-entangled photon pairs, and we found that in the low-flux regime, where such enhancement is theoretically predicted in principle, the two-photon fluorescence signal is below the detection threshold using current state-of-the-art methods. The results are important in the context of efforts to enable quantum-enhanced molecular spectroscopy and imaging at ultra-low optical flux. Using an optical parametric down-conversion photon-pair source that can be varied from the low-gain spontaneous regime to the high-gain squeezing regime, we observed two-photon-induced fluorescence in the high-gain regime, but in the low-gain regime any fluorescence was below detection threshold. We supplemented the molecular fluorescence experiments with a study of nonlinear-optical sum-frequency generation, for which we are able to observe the low-to-high-gain crossover, thereby verifying our theoretical models and experimental techniques. The observed rates (or lack thereof) in both experiments are consistent with theoretical predictions and with our previous experiments, and indicate that time-frequency photon entanglement does not provide a practical means to enhance in-solution molecular two-photon fluorescence spectroscopy or imaging with current techniques.

Fault-tolerant quantum computation can be achieved by creating constant-sized, entangled resource states and performing entangling measurements on subsets of their qubits. Linear optical quantum computers can be designed based on this approach, even though entangling operations at the qubit level are nondeterministic in this platform. Probabilistic generation and measurement of entangled states must be pushed beyond the required threshold by some combination of scheme optimization, introduction of redundancy, and auxiliary state assistance. We report progress in each of these areas. We explore multiqubit fusion measurements on dual-rail photonic qubits and their role in measurement-based resource state generation, showing that it is possible to boost the success probability of photonic Greenberger-Horne-Zeilinger state analyzers with single-photon auxiliary states. By incorporating generators of basic entangled “seed” states, we provide a method that simplifies the process of designing and optimizing generators of complex, encoded resource states by establishing links to ZX diagrams.

The quantum repeater cell is a basic building block for a quantum network, as it allows to overcome the distance limitations due to unavoidable fiber loss in direct transmission. We demonstrate the implementation of a quantum repeater cell, based on two free-space coupled ${}^{40}\mathrm{Ca}^{+}$ ions in the same trap that act as quantum memories. We demonstrate the asynchronous generation of atom-photon and photon-photon entanglement by controlled emission of single photons from the individually addressed ions and entanglement swapping. We discuss the fidelity as well as the scaling of the generated rate.

The notions of causality adopted within the quantum information and space-time physics communities are distinct. Although experience tells us that these notions play together in a compatible manner in physical experiments, their general interplay is little understood in theory. Therefore we develop a theoretical framework that connects the two causality notions, while also clearly distinguishing them. The framework describes a composition of quantum operations through feedback loops, and the embedding of the resulting, possibly cyclic information-theoretic structure in an acyclic space-time structure. Relativistic causality (which forbids superluminal communication) then follows as a graph-theoretic compatibility condition between the two structures. Demonstrating that indefinite causal order (ICO) processes, widely studied in the quantum information community, can be formulated within our framework, we shed light on the links between indefinite and cyclic causality, and on questions regarding their physicality. In particular, there are several experiments that claim to implement ICO processes in Minkowski space-time, presenting an apparent theoretical paradox: how can an indefinite information-theoretic causal structure be consistent with a definite space-time structure? We address this through no-go theorems, showing that as a consequence of relativistic causality, (a) realizations of ICO processes necessarily involve the nonlocalization of systems in space-time and (b) will nevertheless admit an explanation in terms of a definite and acyclic causal order process, at a more fine-grained level. These results are made possible by introducing the concept of fine-graining that allows causal structures to be analyzed at different levels of detail. This fully resolves the apparent paradox and bears implications for the physical interpretation of ICO experiments. Our work also sheds light on the limits of quantum information processing in space-time and offers concrete insights on the operational meaning of indefinite causality, both within and beyond the context of a fixed space-time.

We consider an exactly solvable model describing the interaction of a quantized strong laser field with free electrons produced in the ionization of a target gas by the field. The interaction is shown to strongly affect the quantum state of the field. It squeezes and displaces the coherent state of a free laser field, which under realistic experimental conditions can result in the formation of nonclassical and non-Gaussian field states with a ring-shaped Wigner function. Our results demonstrate that the effect of the interaction with free electrons on the quantum state of the ionizing laser field should be taken into account simultaneously with that of harmonic generation and above-threshold ionization studied recently.

We study Thouless pumps, i.e., adiabatic topological transport, in an interacting spin chain described by the dimerized $XXZ$ Hamiltonian. In the noninteracting case, quantized Thouless pumps can only occur when a topological singularity is encircled adiabatically. In contrast, here we show that, in the presence of interactions, such topological transport can even persist for exotic paths in which the system gets arbitrarily close to the noninteracting singularity. We illustrate the robustness of these exotic Thouless pumps through the behavior of the noninteracting singularity, which for sufficiently strong interactions splits into two singularities separated by a spontaneous antiferromagnetic insulator. We perform a numerical benchmark of these phenomena by means of tensor network simulations of ground-state physics and real-time adiabatic dynamics. Finally, we propose an experimental protocol with Floquet-driven Rydberg tweezer arrays.

We have experimentally studied dissociative recombination (DR) of electronically and vibrationally relaxed ${\mathrm{ArH}}^{+}$ in its lowest rotational levels, using an electron-ion merged-beams setup at the Cryogenic Storage Ring. We report measurements for the merged-beams rate coefficient of ${\mathrm{ArH}}^{+}$ and compare it to published experimental and theoretical results. In addition, by measuring the kinetic energy released to the DR fragments, we have determined the internal state of the DR products after dissociation. At low collision energies, we find that the atomic products are in their respective ground states, which are only accessible via nonadiabatic couplings to neutral Rydberg states. Published theoretical results for ${\mathrm{ArH}}^{+}$ have not included this DR pathway. From our measurements, we have also derived a kinetic temperature rate coefficient for use in astrochemical models.

We theoretically investigate nondipole effects in the reconstruction of attosecond beating by interference of two-photon transitions (RABBIT) of helium using linearly polarized extreme ultraviolet and infrared fields. By scanning the time delay between the two fields, we observe modulations in sidebands (SBs) both for angular-integrated photoelectron yield and forward-backward asymmetry in photoelectron distribution along the light-propagation direction. The SB modulations of the forward-backward asymmetry reveal Wigner and continuum-continuum time delays of the electron wave packets ionized via nondipole paths, different from the conventional RABBIT where only the dipole paths are involved. Furthermore, the time delays extracted from the forward-backward asymmetry show an abrupt jump as a function of polar emission angle of photoelectrons, due to the competition among continuum partial waves in nondipole laser-assisted photoionization.

The vibrational branching fractions from the lowest excited electronic state for ${\text{SrOCH}}_3$, ${\text{SrNH}}_2$, and $\text{SrSH}$ are measured at the $<0.1\%$ level. Spectra are obtained by driving the $\tilde{X}\text{−}\tilde{A}$ transitions and dispersing the fluorescence on a grating spectrometer. We also perform ab initio calculations for the energies of vibrational levels relevant for laser cooling, as well as branching fractions to support the interpretations of all molecular spectra. Symmetry-group analysis is applied in conjunction with our data to study rotational closure in these molecules. These analyses indicate favorable prospects for laser cooling ${\text{SrNH}}_2$ and other similar alkaline-earth-metal(-like) amides for future beyond the Standard Model physics searches using polyatomic molecules with long-lived parity doublets.

Optically pumped magnetometers (OPMs) are revolutionizing the task of magnetic-field sensing due to their extremely high sensitivity combined with technological improvements in miniaturization which have led to compact and portable devices. OPMs can be based on spin-oriented or spin-aligned atomic ensembles which are spin polarized through optical pumping with circular or linear polarized light, respectively. Characterization of OPMs and the dynamical properties of their noise is important for applications in real-time sensing tasks. In our work, we experimentally perform spin noise spectroscopy of an alignment-based magnetometer. Moreover, we propose a stochastic model that predicts the noise power spectra exhibited by the device when, apart from the strong magnetic field responsible for the Larmor precession of the spin, white noise is applied in the perpendicular direction aligned with the pumping-probing beam. By varying the strength of the noise applied as well as the linear-polarization angle of incoming light, we verify the model to accurately predict the heights of the Larmor-induced spectral peaks and their corresponding linewidths. Our work paves the way for alignment-based magnetometers to become operational in real-time sensing tasks.

We present measurement schemes that do not rely on photon-number-resolving detectors but are nevertheless optimal for estimating a differential phase shift in interferometry with either an entangled coherent state or a qubit which-path state (where the path taken by a coherent-state wave packet is entangled with the state of a qubit). The homodyning schemes analyzed here achieve optimality (saturate the quantum Cramér-Rao bound) by maximizing the sensitivity of measurement outcomes to phase-dependent interference fringes in a reduced Wigner distribution. In the presence of photon loss, the schemes become suboptimal, but we find that their performance is independent of the phase to be measured. They can therefore be implemented without any prior information about the phase and without adapting the strategy during measurement, unlike strategies based on photon-number parity measurements or direct photon counting.

We study the dynamics of the pump mode in the down-conversion Hamiltonian using the cumulant-expansion method, perturbation theory, and the full numerical simulation of systems with a pump mean photon number of up to $100000$. We particularly focus on the properties of the pump mode such as depletion, entanglement, and squeezing for an experimentally relevant initial state in which the pump mode is initialized in a coherent state. Through this analysis, we obtain the short-time behavior of various quantities and derive timescales at which the above-mentioned features, which cannot be understood through the parametric approximation, originate in the system. We also provide an entanglement witness involving moments of bosonic operators that can capture the entanglement of the pump mode. Finally, we study the photon-number statistics of the pump and the signal and idler modes to understand the general behavior of these modes for experimentally relevant timescales.

Quantum behavior of the electromagnetic field in unbounded macroscopic media displaying absorption is properly described by the Langevin noise formalism (macroscopic quantum electrodynamics), where the field is assumed to be entirely produced by medium fluctuating sources via the dyadic Green's function. On the other hand, such formalism is able to deal with the case of finite-size lossy objects placed in vacuum only as a limiting situation where the permittivity limit $\mathrm{Im}\left(\varepsilon \right)\to 0^{+}$ pertaining the regions filled by vacuum is taken at the end of the calculations. Strictly setting $\mathrm{Im}\left(\varepsilon \right)=0$ is forbidden in the Langevin noise formalism since the field would vanish in the lossless regions, and this is physically due to the fact that the contribution of the scattering modes to the field is not separated from the contribution produced by the medium fluctuating sources. Recently, a modified Langevin noise formalism has been proposed to encompass the scattering modes and accordingly, it is able to describe the structured lossless situations by strictly setting $\mathrm{Im}\left(\varepsilon \right)=0$. However, such modified formalism has been numerically validated only in a few specific geometries. In this paper we analytically derive the modified Langevin noise formalism from the established canonical quantization of the electromagnetic field in macroscopic media, thus proving that it models any possible scenario involving linear, inhomogeneous, and magnetodielectric samples. The derivation starts from quantum Maxwell equations in the Heisenberg picture together with their formal solution as the superposition of the medium assisted field and the scattering modes. We analytically prove that each of the two field parts can be expressed in term of particular bosonic operators, which in turn diagonalize the electromagnetic Hamiltonian and whose associated quasiparticles are medium assisted and scattering polaritons, respectively. The key ingredient underpinning our reasoning is a peculiar integral relation linking the far-field amplitude of the dyadic Green's function and the scattering modes, a relation we rigorously derive and physically explain by identifying the scattering modes as fields generated by infinitely far dipole point sources.

Squeezing is a nonclassical feature of quantum states that is a useful resource, for example, in quantum sensing of mechanical forces. Here, we show how to use optimal control theory to maximize squeezing in an optomechanical setup with two external drives and determine how fast the mechanical mode can be squeezed. For the autonomous drives considered here, we find the inverse cavity decay to lower-bound the protocol duration. At and above this limit, we identify a family of protocols leveraging a two-stage control strategy, where the mechanical mode is cooled before it is squeezed. Identification of the control strategy allows for two important insights: to determine the factors that limit squeezing and to simplify the time dependence of the external drives, making our protocol readily applicable in experiments.