The exact microscopic structure of the environments that produce $1/f$ noise in superconducting qubits remains largely unknown, hindering our ability to create robust simulations and harness the noise. In this paper we show how it is possible to infer information about such an environment based on a single measurement of the qubit coherence, circumventing any need for separate spectroscopy experiments. Similarly to other spectroscopic techniques, the qubit is used as a probe which interacts with its environment. The complexity of the relationship between the observed qubit dynamics and the impurities in the environment makes this problem ideal for machine learning methods—more specifically neural networks. With our algorithm we are able to reconstruct the parameters of the most prominent impurities in the environment, as well as differentiate between different environment models, paving the way towards a better understanding of $1/f$ noise in superconducting circuits.

We investigate the interplay between two mechanisms for magnetic self-organization in a cloud of cold rubidium atoms subjected to a retroreflected laser beam. The transition between two different phases, one linked to a spontaneous spatial modulation of the $\mathrm{\Delta }m=2$ ground-state coherence and the other to that of the magnetic orientation (spin), can be induced by tuning either a weak transverse magnetic field or the laser intensity. We observe both first- and second-order transitions depending on the presence of the magnetic field. The experimental observations are successfully compared to extended numerical simulations based on a spin-1 model.

We experimentally and theoretically investigate the influence of the dipole-dipole interactions (DDIs) on the total interspecies interaction in an erbium-dysprosium mixture. By rotating the dipole orientation we are able to tune the effect of the long-range and anisotropic DDI, and therefore the in-trap displacements of the erbium and dysprosium clouds. We present a theoretical description for our binary system based on an extended Gross-Pitaevskii theory, including the single-species beyond mean-field terms, and we predict a lower and an upper bound for the interspecies scattering length $a_{12}=105[-65,+162]a_0$. Our work is a step towards the investigation of the experimentally unexplored dipolar miscibility-immiscibility phase diagram and the realization of quantum droplets and supersolid states with heteronuclear dipolar mixtures.

Various experimental platforms have proven to be valid testbeds for the implementation of nondipolar light-matter interactions, where atomic systems and confined modes interact via two-photon couplings. Here, we study a damped quantum harmonic oscillator interacting with $N$ two-level systems via a two-photon coupling in the so-called bad-cavity limit, in the presence of finite-temperature baths and coherent and incoherent drivings. We have succeeded in applying a recently developed adiabatic elimination technique to derive an effective master equation for the two-level systems, presenting two fundamental differences compared to the case of a dipolar interaction: an enhancement of the two-level systems spontaneouslike emission rate, including a thermal contribution and a quadratic term in the coherent driving, and an increment of the effective temperature perceived by the two-level systems. These differences give rise to striking effects in the two-level systems dynamics, including a faster generation of steady-state coherence and a richer dependence on temperature of the collective effects, which can be made stronger at higher temperature.

The ability to distribute high-quality entanglement between remote parties is a necessary primitive for many quantum communication applications. A large range of schemes for realizing the long-distance delivery of remote entanglement has been proposed, for both bipartite and multipartite entanglement. For assessing the viability of these schemes, knowledge of the time at which entanglement is delivered is crucial. Specifically, if the communication task requires multiple remote-entangled quantum states and these states are generated at different times by the scheme, the earlier states will need to wait and thus their quality will decrease while being stored in an (imperfect) memory. For the remote-entanglement delivery schemes which are closest to experimental reach, this time assessment is challenging, as they consist of nondeterministic components such as probabilistic entanglement swaps. For many such protocols even the average time at which entanglement can be distributed is not known exactly, in particular when they consist of feedback loops and forced restarts. In this work, we provide improved analytical bounds on the average and on the quantiles of the completion time of entanglement distribution protocols in the case that all network components have success probabilities lower bounded by a constant. A canonical example of such a protocol is a nested quantum repeater scheme which consists of heralded entanglement generation and entanglement swaps. For this scheme specifically, our results imply that a common approximation to the mean entanglement distribution time, the 3-over-2 formula, is in essence an upper bound to the real time. Our results rely on a novel connection with reliability theory.

Ghost imaging is a developing imaging technique that employs random masks to image a sample. Ghost projection utilizes ghost-imaging concepts to perform the complementary procedure of projection of a desired image. The key idea underpinning ghost projection is that any desired spatial distribution of radiant exposure may be produced, up to an additive constant, by spatially uniformly illuminating a set of random masks in succession. We explore three means of achieving ghost projection: (i) weighting each random mask, namely, selecting its exposure time, according to its correlation with a desired image, (ii) selecting a subset of random masks according to their correlation with a desired image, and (iii) numerically optimizing a projection for a given set of random masks and desired image. The first two protocols are analytically tractable and conceptually transparent. The third is more efficient but less amenable to closed-form analytical expressions. A comparison with existing image-projection techniques is drawn and possible applications are discussed. These potential applications include: (i) a data projector for matter and radiation fields for which no current data projectors exist, (ii) a universal-mask approach to lithography, (iii) tomographic volumetric additive manufacturing, and (iv) a ghost-projection photocopier.

The decoy-state method is a standard enhancement to quantum key distribution (QKD) protocols that has enabled countless QKD experiments with inexpensive light sources. However, new technological advancements might require further theoretical study of this technique. In particular, the decoy-state method is typically described under the assumption of a Poisson statistical distribution for the number of photons in each QKD pulse. This is a practical choice, because prepare-and-measure QKD is often implemented with attenuated lasers, which produce exactly this distribution. However, sources that do not meet this assumption are not guaranteed to be compatible with decoy states. In this work, we provide security bounds for decoy-state QKD using a source with an arbitrary photon emission statistic. We consider both the asymptotic limit of infinite key and the finite-size scenario, and evaluate two common decoy-state schemes: the $\text{vacuum}+\text{weak}$ and one-decoy protocols. We numerically evaluate the performance of the bounds, comparing three realistic statistical distributions (Poisson, thermal, and binomial), showing that they are all viable options for QKD.

The paper explores a theoretical possibility of resonant amplification of electromagnetic modes generated by a nonlinear effect in Euler-Heisenberg electrodynamics. Precisely, we examine the possibility of the amplification for the third harmonics induced by a single electromagnetic mode in a radio-frequency cavity as well as the generation of the signal mode of combined frequencies induced by two pump modes $({\omega }_1 \mathrm{and} {\omega }_2)$ in the cavity. Solving inhomogeneous wave equations for the signal mode, we formulate two resonant conditions for a cavity of arbitrary shape and apply the obtained formalism to linear and rectangular cavities. We explicitly show that the third harmonics as well as the mode of combined frequency $2{\omega }_1+{\omega }_2$ are not resonantly amplified whereas the signal mode with frequency $2{\omega }_1-{\omega }_2$ is amplified for a certain cavity geometry.

In this work, we show the characterization of quantum iterations that would generally construct quantum amplitude-amplification algorithms with a quadratic speedup, namely, quantum amplitude-amplification operators (QAAOs). Exact quantum search algorithms that find a target with certainty and with a quadratic speedup can be composed of sequential applications of QAAOs: existing quantum amplitude-amplification algorithms thus turn out to be sequences of QAAOs. We show that an optimal and exact quantum amplitude-amplification algorithm corresponds to the Grover algorithm together with a single iteration of QAAO. We then realize three-qubit QAAOs with current quantum technologies via cloud-based quantum computing services, IBMQ and IonQ. Finally, our results show that the fixed-point quantum search algorithms known so far are not a sequence of QAAOs; for example, the amplitude of a target state may decrease during quantum iterations.

Quantum variational optimization has been posed as an alternative to solve optimization problems faster and at a larger scale than what classical methods allow. In this paper we study systematically the role of entanglement, the structure of the variational quantum circuit, and the structure of the optimization problem, in the success and efficiency of these algorithms. For this purpose, our study focuses on the variational quantum eigensolver (VQE) algorithm, as applied to quadratic unconstrained binary optimization (QUBO) problems on random graphs with tunable density. Our numerical results indicate an advantage in adapting the distribution of entangling gates to the problem's topology, specially for problems defined on low-dimensional graphs. Furthermore, we find evidence that applying conditional value at risk type cost functions improves the optimization, increasing the probability of overlap with the optimal solutions. However, these techniques also improve the performance of Ansätze based on product states (no entanglement), suggesting that a new classical optimization method based on these could outperform existing NISQ architectures in certain regimes. Finally, our study also reveals a correlation between the hardness of a problem and the Hamming distance between the ground- and first-excited state, an idea that can be used to engineer benchmarks and understand the performance bottlenecks of optimization methods.

We demonstrate the direct loading of a ${\mathrm{D}}_2$ magneto-optical trap (MOT) of potassium-39 atoms from a cryogenic buffer gas beam source. We load ${10}^8$ atoms in a 10 ms pulse, with no degradation in performance up to a 10 Hz repetition rate. Observed densities reach $\approx {10}^{11}$ atoms/${\mathrm{cm}}^3$ in a single pulse, achieved with a modest Zeeman slower but no sub-Doppler cooling or transverse compression. This system produces an ideal starting point for ultracold atom experiments where high experimental repetition rates are desirable and initial high densities are critical. Extension to other atomic species (e.g., refractory metals) that present technical challenges to high-yield production using oven-based sources is straightforward.

We give strong analytic and numerical evidence that, under mild measurement assumptions, two qubits cannot both be recycled to generate Bell nonlocality between multiple independent observers on each side. This is surprising, as under the same assumptions it is possible to recycle just one of the qubits an arbitrarily large number of times [P. J. Brown and R. Colbeck, Phys. Rev. Lett. 125, 090401 (2020)]. We derive corresponding one-sided monogamy relations that rule out two-sided recycling for a wide range of parameters, based on a general tradeoff relation between the strengths and maximum reversibilities of qubit measurements. We also show if the assumptions are relaxed to allow sufficiently biased measurement selections, then there is a narrow range of measurement strengths that allows two-sided recycling for two observers on each side, and propose an experimental test. Our methods may be readily applied to other types of quantum correlations, such as steering and entanglement, and hence to general information protocols involving sequential measurements.

Quantum measurement is ultimately a physical process, resulting from an interaction between the measured system and a measuring apparatus. Considering the physical process of measurement within a thermodynamic context naturally raises the following question: How can the work and heat be interpreted? In the present paper we model the measurement process for an arbitrary discrete observable as a measurement scheme. Here the system to be measured is first unitarily coupled with an apparatus and subsequently the compound system is objectified with respect to a pointer observable, thus producing definite measurement outcomes. The work can therefore be interpreted as the change in internal energy of the compound system due to the unitary coupling. By the first law of thermodynamics, the heat is the subsequent change in internal energy of this compound due to pointer objectification. We argue that the apparatus serves as a stable record for the measurement outcomes only if the pointer observable commutes with the Hamiltonian and show that such commutativity implies that the uncertainty of heat will necessarily be classical.

Quantum gates are typically vulnerable to imperfections in the classical control fields applied to physical qubits to drive the gates. One approach to reduce this source of error is to break the gate into parts, known as composite pulses, that typically leverage the constancy of the error over time to mitigate its impact on gate fidelity. Here we extend this technique to suppress secular drifts in Rabi frequency by regarding them as sums of power-law drifts the first-order effects of which on over- or under-rotation of the state vector add linearly. Power-law drifts have the form $t^p$ where $t$ is time and the constant $p$ is its power. We show that composite pulses that suppress all power-law drifts with $p\le n$ are also high-pass filters of filter order $n+1$ [H. Ball and M. J. Biercuk, Walsh-synthesized noise filters for quantum logic, EPJ Quantum Technol. 2, 11 (2015)]. We present sequences that satisfy our proposed power-law amplitude criteria, $\text{PLA}\left(n\right)$, obtained with this technique, and compare their simulated performance under time-dependent amplitude errors to some traditional composite pulse sequences. We find that there is a range of noise frequencies for which the $\text{PLA}\left(n\right)$ sequences provide more error suppression than the traditional sequences, but in the low-frequency limit nonlinear effects become more important for gate fidelity than frequency roll-off. As a result, the previously known $F_1$ sequence, which is one of the two solutions to the $\text{PLA}\left(1\right)$ criteria and furnishes suppression of both linear secular drift and the first-order nonlinear effects, is a sharper noise filter than any of the other $\text{PLA}\left(n\right)$ sequences in the low-frequency limit.

Modern experiments using nanoscale devices come ever closer to bridging the divide between the quantum and classical realms, bringing experimental tests of objective collapse theories that propose alterations to Schrödinger's equation within reach. Such objective collapse theories aim to explain the emergence of classical dynamics in the thermodynamic limit and hence resolve the inconsistency that exists within the axioms of quantum mechanics when assuming measurement can be described by quantum mechanics as well. Here, we show that requiring the emergence of Born's rule for relative frequencies of measurement outcomes without imposing them as part of any axiom implies that such objective collapse theories cannot be linear. Previous suggestions for proof of the emergence of Born's rule in classes of problems that include linear objective collapse theories are analyzed and shown to include hidden assumptions.

Optical beams with periodic lattice structures have broadened the study of structured waves. In the present work, we generate spin-orbit entangled photon states with a lattice structure and use them in a remote state preparation protocol. We sequentially measure spatially dependent correlation rates with an electron-multiplying intensified CCD camera and verify the successful remote preparation of spin-orbit states by performing pixel-wise quantum state tomography. Control of these structured waves in the quantum regime provides a method for quantum sensing and manipulation of periodic structures.

We characterize a class of environmental noises that decrease the coherent properties of quantum channels by introducing and analyzing the properties of dephasing superchannels. These are defined as superchannels that affect only nonclassical properties of a quantum channel $\mathcal{E}$, i.e., they leave invariant the transition probabilities induced by $\mathcal{E}$ in the distinguished basis. We prove that such superchannels ${\mathrm{\Xi }}_C$ form a particular subclass of Schur-product supermaps that act on the Jamiołkowski state $J\left(\mathcal{E}\right)$ of a channel $\mathcal{E}$ via a Schur product, $J^{\prime }=J\circ C$. We also find physical realizations of general ${\mathrm{\Xi }}_C$ through pre- and postprocessing employing dephasing channels with memory, and we show that memory plays a nontrivial role for quantum systems of dimension $d>2$. Moreover, we prove that the coherence-generating power of a general quantum channel is a monotone under dephasing superchannels. Finally, we analyze the effect that dephasing noise can have on a quantum channel $\mathcal{E}$ by investigating the number of distinguishable channels that $\mathcal{E}$ can be mapped to by a family of dephasing superchannels. More precisely, we upper-bound this number in terms of hypothesis-testing channel divergence between $\mathcal{E}$ and its fully dephased version, and we also relate it to the robustness of coherence of $\mathcal{E}$.

Employing a coincidence three-dimensional momentum imaging technique, we investigate the ultrafast, intense laser-induced dissociation of a metastable ${\mathrm{NO}}^{2+}$ ion beam into ${\mathrm{N}}^{+}+{\mathrm{O}}^{+}$. Based on the kinetic energy release and angular distributions, measured using both 774-nm and second-order-harmonic 387-nm pulses, we show that the main processes driving dissociation in pulses of about ${10}^{14}\mathrm{W}/{\mathrm{cm}}^2$ peak intensity are one- and two-photon transitions from the $X{}^2\mathrm{\Sigma }^{+}$ ground state to the $A{}^2\mathrm{\Pi }$ first-excited state. First-order perturbation theory calculations also corroborate these findings.

We theoretically explore the possibility to detect weak localization of light in a hot atomic vapor, where one usually expects the fast thermal motion of the atoms to destroy any interference in multiple scattering. To this end, we compute the coherent backscattering peak, assuming high temperature and taking into account the quantum level structure of the atomic scatterers. It is found that the decoherence due to thermal motion can be partially counterbalanced by working at large laser detuning and using small atomic cells with an elongated geometry. Under these conditions, our estimates suggest that weak localization in a hot vapor should be within reach of experimental detection.

We show that the performance and functionality of atom chips can be transformed by using graphene-based van der Waals heterostructures to overcome present limitations on the lifetime of the trapped atom cloud and on its proximity to the chip surface. Our analysis involves Green's-function calculations of the thermal (Johnson) noise and Casimir-Polder atom-surface attraction produced by the atom chip. This enables us to determine the lifetime limitations produced by spin flip, tunneling, and three-body collisional losses. Compared with atom chips that use thick metallic conductors and substrates, atom-chip structures based on two-dimensional materials reduce the minimum attainable atom-surface separation to a few hundred nanometers and increase the lifetimes of the trapped atom clouds by orders of magnitude so that they are limited only by the quality of the background vacuum. We predict that atom chips with two-dimensional conductors will also reduce spatial fluctuations in the trapping potential originating from imperfections in the conductor patterns. These advantages will enhance the performance of atom chips for quantum sensing applications and for fundamental studies of complex quantum systems.