We report the experimental determination of the interspecies $s$-wave scattering length ($a_{34}$) between the $2{}^3{S}_1(F=3/2,m_F=3/2)$ state of ${}^3\mathrm{He}^{*}$ and the $2{}^3{S}_1(m_J=1)$ state of ${}^4\mathrm{He}^{*}$. We determine $a_{34}$ by inducing oscillations in a trapped Bose-Einstein condensate of ${}^4\mathrm{He}^{*}$ and measuring the damping rate of these oscillations due to the presence of ${}^3\mathrm{He}^{*}$ atoms. Our measured value of $a_{34}=30\pm 4$ nm is in good agreement with theoretical predictions. The knowledge of this scattering length is important for many fundamental experiments between these helium isotopes.

A widely used method for mitigating errors in noisy quantum computers is Richardson extrapolation, a technique in which the overall effect of noise on the estimation of quantum expectation values is captured by a single parameter that, after being scaled to larger values, is eventually extrapolated to the zero-noise limit. We generalize this approach by introducing layerwise Richardson extrapolation (LRE), an error mitigation protocol in which the noise of different individual layers (or larger chunks of the circuit) is amplified and the associated expectation values are linearly combined to estimate the zero-noise limit. The coefficients of the linear combination are analytically obtained from the theory of multivariate Lagrange interpolation. LRE leverages the flexible configurational space of layerwise unitary folding, allowing for a more nuanced mitigation of errors by treating the noise level of each layer of the quantum circuit as an independent variable. We provide numerical simulations demonstrating scenarios where LRE achieves superior performance compared to traditional (single-variable) Richardson extrapolation.

We engineer angular momentum eigenstates of a single atom by using an all-optical approach based on the interference of Laguerre-Gaussian beams. We confirm the imprint of angular momentum by measuring the two-dimensional density distribution and by performing Ramsey spectroscopy in a slightly anisotropic trap, which additionally reveals the sense of rotation. This article provides the experimental details on the quantum state control of angular momentum eigenstates reported in P. Lunt et al., Phys. Rev. Lett. 133, 253401 (2024).

Thermal rate coefficients for some atomic collisions have been observed to be remarkably independent of the details of interatomic interactions at short range. This makes these rate coefficients universal functions of the long-range interaction parameters and masses, which was previously exploited to develop a self-defining atomic sensor for ambient pressure. Here we employ rigorous quantum scattering calculations to examine the response of thermally averaged rate coefficients for atom-atom collisions to changes in the interaction potentials. We perform a comprehensive analysis of the universality, and the boundaries thereof, by treating the quantum scattering observables as probabilistic predictions determined by a distribution of interaction potentials. We show that there is a characteristic change of the resulting distributions of rate coefficients, separating light, few-electron atoms, and heavy polarizable atoms. We produce diagrams that illustrate the boundaries of the thermal collision universality at different temperatures and provide guidance for future experiments seeking to exploit the universality.

There is a growing interest in high-precision spectroscopy and frequency metrology for fundamental studies using sympathetically cooled highly charged ions (HCIs) embedded in Coulomb crystals of laser-cooled ions. In order to understand how their strong repulsion affects the crystal structure and dynamics, we study the thermal motion and rearrangement of small mixed linear and homogeneous crystals by both measurements and simulations. Cocrystallized HCIs form superlattices and divide the crystal into domains, where different reordering rates, melting points, and localized phase transitions are observed due to decoupling of motional modes across boundaries. These results improve our understanding of homogeneous and inhomogeneous ion strings over a wide range of charge-to-mass ratios. This allows us to test our own simulations of the dynamic behavior of ion strings and gives us confidence in their suitability for applications related to quantum simulation as well as computing and the search for new physics.

Entangled photon pairs form the foundation for many applications in the realm of quantum communication. For fiber-optic transfer of entangled photon pairs, time-bin encoding offers improved stability compared to polarization-encoded qubits. Here, we lay the theoretical foundations to describe the measurement of time-bin-entangled photons. We derive multitime correlation functions of the time-bin-encoded photon pairs, corresponding to quantum state tomographic measurements. Our theory provides a starting point to extend the simulations to include all kinds of loss or decoherence effects that apply in a specific quantum system for realistic simulation for time-bin entanglement from quantum emitters.

A continuous-variable (CV) quantum position verification (QPV) protocol was recently studied and proven to be secure if and only if attackers do not preshare any entanglement. In the discrete-variable (DV) analog of that protocol, it had been shown that modifying how the classical challenge is sent from the verifiers to the prover leads to a lower bound on the amount of entanglement needed by the attackers. In this work we show that similar conclusions can be drawn for CV QPV. We design a CV QPV protocol in which the quantum part of the challenge is a coherent state and the classical part consists of $n$-bit strings coming from both verifiers. We show that the protocol is secure against attackers who have fewer than $cn$ preshared entangled qubits, where $c$ is a constant. The security proof is given for a certain range of attenuation and excess noise and holds even if the quantum information travels arbitrarily slowly.

In this paper we measure all the motional-mode frequencies (common and stretch) of a balanced and an unbalanced two-ion Coulomb crystal in a 7-tesla Penning trap using an electric-dipole transition in ${}^{40}\mathrm{Ca}^{+}$. The three common modes of the balanced crystal were obtained using a laser-pulse measurement technique developed in a previous work for a single ion, whereas the stretch mode frequencies were measured in a stationary state while applying laser cooling. Due to the Coulomb interaction, and the heating of the crystal, nonlinearities appear in the oscillators, which are treated analytically and numerically to obtain the experimental frequency values. The generalized invariance theorem has been tested yielding a relative shift of $-1.0\times {10}^{-6}$ with a relative uncertainty of $4\times {10}^{-6}$. Single-ion frequency measurements are used to assess its validity.

We present a general framework for the computation of structure-agnostic bounds on the performance of passive cloaks over a nonzero bandwidth. We apply this framework in two dimensions to the canonical scenario of cloaking a circular object. We find that perfect cloaking using a finite-sized isotropic cloak is impossible over any bandwidth, with the bounds scaling linearly with the bandwidth before saturating due to the finite size of the cloak and the presence of material loss. The bounds also exhibit linear scaling with material loss in the cloak and linear scaling with the inverse of the radial thickness of the design region before saturation due to finite-size effects or the presence of material loss. The formulation could readily find applications in the development of cloaking devices, setting expectations, and benchmarks for optimal performance.

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.