We propose a protocol in quantum illumination (QI) leveraging entanglement in discrete-variable states. Our investigation shows that, as $M\to \infty$, the $M$-mode Bell state matches the 6 dB advantage of the two-mode squeezed vacuum in high noise. It also excels in low- and mid-noise conditions, demonstrating that QI's benefits are not restricted to high background noise. Moreover, the protocol benefits from a sequential decision rule, increasing the advantage beyond 6 dB. These findings present an intriguing alternative to continuous-variable states and open different applications for QI using discrete-variable states.

In the framework of non-Hermitian photonics, we investigate the interplay between disorder and non-Hermiticity in a one-dimensional Hatano-Nelson lattice. While Anderson localization dictates the wave's evolution in conservative random systems, the introduction of non-Hermiticity tends to force the beam to unidirectionally propagate towards one edge of the potential due to the existence of skin modes. As we show, the antagonism between these effects results in qualitatively distinct phases of wave diffraction, including counterintuitive characteristics regarding the relationship between the strength of disorder and the wave packet's velocity.

We investigate a quantum thermometry scheme-based collision model with Gaussian systems. A key open question of these schemes concerns the scaling of the quantum Fisher information (QFI) with the number of ancillas. In qubit-based implementations this question is difficult to assess, due to the exponentially growing size of the Hilbert space. Here we focus on Gaussian collision models, which allow for the scaling of the QFI to be evaluated for arbitrarily large sizes. This numerical flexibility enables us to explore the thermometric properties of the model for a wide range of configurations. Despite the infinite Markov order of the stochastic process of the model, we provide a simple phenomenological analysis for the behavior of the QFI, estimating the asymptotic Fisher information density and how the transient effects of correlations for an increasing number of ancillas depend on the physical parameters of the model.

On the millisecond to second time scale, stored beams of diatomic carbon anions ${\mathrm{C}}_2{}^{-}$ from a sputter ion source feature unimolecular decay of yet unexplained origin by electron emission and fragmentation. To account for the magnitude and time dependence of the experimental rates, levels with high rotational and vibrational excitation are modeled for the lowest electronic states of ${\mathrm{C}}_2{}^{-}$, also including the lowest quartet potential. Energies, spontaneous radiative decay rates (including spin-forbidden quartet-level decay), and tunneling dissociation rates are determined for a large number of highly excited ${\mathrm{C}}_2{}^{-}$ levels and their population in sputter-type ion sources is considered. For the quartet levels, the stability against autodetachment is addressed and recently calculated rates of rotationally assisted autodetachment are applied. Nonadiabatic vibrational autodetachment rates of high vibrational levels in the doublet ${\mathrm{C}}_2{}^{-}$ ground potential are also calculated. The results are combined to model the experimental unimolecular decay signals. Comparison of the modeled to the experimental rates measured at the cryogenic storage ring facility CSR gives strong evidence that ${\mathrm{C}}_2{}^{-}$ ions in quasistable levels of the quartet electronic states are the so far unidentified source of unimolecular decay.

It has been a long-standing goal to bring massive objects into a superposition of different locations in real space, not only to confirm quantum theory in new regimes, but also to explore the interface with gravity. The main challenge is usually thought to arise from forces or scattering due to environmental fields and particles that decohere the large object's wave function into a statistical mixture. We unveil a decoherence channel which cannot be eliminated by improved isolation from the environment. It originates from sound waves within the object, which are excited as part of any splitting process and carry partial Welcher Weg information. This puts stringent constraints on future spatial superpositions of large objects.

Cooling atoms to the ground state of optical tweezers is becoming increasingly important for high-fidelity imaging, cooling, and molecular assembly. While extensive theoretical work has been conducted on cooling in free space, fewer studies have focused on cooling in bound states. In this work, we present a unified formalism for optical cooling mechanisms in neutral atom tweezers, including resolved and unresolved sideband cooling with different trapping potentials, polarization gradient cooling, gray molasses cooling, $\mathrm{\Lambda }$-enhanced gray molasses cooling, and Raman sideband cooling. We perform simulations with hyperfine levels and demonstrate good agreement with a simplified spin model. We derive and discuss the fundamental limits of each cooling mechanism and propose new strategies for achieving ground state cooling in optical tweezers. Our findings provide valuable insights into optimizing cooling schemes for neutral atoms in optical tweezers, paving the way for minimizing thermal decoherence in Rydberg and molecular gates and improving efficiencies of molecular assembly.

Recent work has studied fermion transport through a finite one-dimensional lattice of quantum dots, with localized particle loss from the central lattice site. The dots at each end of the lattice are connected to macroscopic leads, represented as zero-temperature reservoirs of free fermions with a given potential difference. Here we show how this model represents one limiting case of a larger class of models that can be realized with cold quantum gases in optical lattices. While quantum gas realizations allow many system parameters to be varied, we note limitations from finite-size effects and conclude that quantum dots and quantum gases offer complementary views on transport through lossy finite lattices.

Low-frequency $1/f^{\alpha }$ charge noise significantly hinders the performance of voltage-controlled spin qubits in quantum dots. Here, we utilize fractional calculus to design voltage control pulses yielding the highest average fidelities for noisy quantum gate operations. We focus specifically on the exponential voltage control of the exchange interaction generating two-spin swap${}^k$ gates. When stationary charge noise is the dominant source of gate infidelity, we derive that the optimal exchange pulse is long and weak, with the broad shape of the symmetric $\mathrm{beta}$ distribution function with parameter $1-\alpha /2$. The common practice of making exchange pulses fast and high in amplitude still remains beneficial in the case of strongly nonstationary noise dynamics, modeled as fractional Brownian motion. The proposed methods are applicable to the characterization and optimization of quantum gate operations in various voltage-controlled qubit architectures.

Magic-state resource theory is a powerful tool with applications in quantum error correction, many-body physics, and classical simulation of quantum dynamics. Despite its broad scope, finding tractable resource monotones has been challenging. Stabilizer entropies have recently emerged as promising candidates (being easily computable and experimentally measurable detectors of nonstabilizerness) though their status as true resource monotones has been an open question ever since. In this Letter, we establish the monotonicity of stabilizer entropies for $\alpha \ge 2$ within the context of magic-state resource theory restricted to pure states. Additionally, we show that linear stabilizer entropies serve as strong monotones. Furthermore, we extend stabilizer entropies to mixed states as monotones via convex roof constructions whose computational evaluation significantly outperforms optimization over stabilizer decompositions for low-rank density matrices. As a direct corollary, we provide improved conversion bounds between resource states, revealing a preferred direction of conversion between magic states. These results conclusively validate the use of stabilizer entropies within magic-state resource theory and establish them as the only known family of monotones that are experimentally measurable and computationally tractable.

In recent experiments, a novel type of cascaded quantum system has been realized using nanofiber-coupled cold atomic ensembles. This setup has enabled the study of superradiant decay of highly excited collective spin states of up to a thousand atoms, featuring unidirectional coupling mediated by the waveguide mode. The complexity arising from the large, multi-excited ensemble and the cascaded interactions between atoms makes conventional simulation methods unsuitable for predicting the correlations of superradiant emission beyond the first order. To address this challenge, we developed a simulation technique based on the truncated Wigner approximation for spins. Our stochastic simulation tool can predict the second-order quantum coherence function, $g^{\left(2\right)}$, along with other correlators of the light field emitted by a strongly excited cascaded system of two-level emitters. This approach thus provides an effective and scalable method for analyzing cascaded quantum systems with large numbers of particles.

In this paper, we build upon our previous study on super spontaneous four-wave mixing (SFWM) in single-channel side-coupled integrated spaced sequence of resonators (SCISSOR) by investigating the impact of loss. Our results show that, in the presence of loss, superradiant photon-pair generation in single-channel sequences is achievable only when the resonators are either deeply over or undercoupled. We then consider super-SFWM in double-channel sequences composed of ring resonators in the add-drop configuration. We show that this configuration can also exhibit superradiant emission once the ring resonators are strongly over-coupled. We study the spectral correlations of the generated pairs in the regime of pulsed pump excitation and find that without being affected by loss, double-channel configuration can generate uncorrelated photon pairs. Our calculations suggest that in the presence of undesirable nonlinear loss mechanisms ring resonator sequences can be employed to achieve a higher generation rate.

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.