This work provides experimental evidence of symmetry breaking in electron-impact dissociative ionization of $\mathrm{C}{\mathrm{O}}_2$. Angular correlation between the recoil direction of fragment ions and transferred momentum is measured. It reveals that despite the inversion symmetry of the $\mathrm{C}{\mathrm{O}}_2$ molecule, $\mathrm{C}{\mathrm{O}}^{+}$ is emitted preferentially in the momentum transfer direction near the thresholds of ionization to close-lying ${}^2{\mathrm{\Pi }}_{\mathrm{u}}$ and ${}^2{\mathrm{\Pi }}_{\mathrm{g}}$ states. Similar forward-backward asymmetry is found also for dissociative ionization of ${\mathrm{N}}_2$. The findings cannot be explained if ionization and dissociation are two independent steps, indicating that the Coulomb field of the slow ejected electron induces the mixing of gerade and ungerade states in the molecular ion.

An infinite-dimensional system such as a quantum harmonic oscillator offers a potentially unbounded Hilbert space for computation, but accessing and manipulating the entire state space requires a physically unrealistic amount of energy. When such a quantum harmonic oscillator is coupled to a qubit, for example via a Jaynes-Cummings interaction, it is well known that the total Hilbert space can be separated into independently accessible subspaces of constant energy, but the number of subspaces is still infinite. Nevertheless, a closed four-dimensional Hilbert space can be analytically constructed from the lowest energy states of the qubit-oscillator system. We extend this idea and show how a $d$-dimensional Hilbert space can be analytically constructed, which is closed under a finite set of unitary operations resulting solely from manipulating standard Jaynes-Cummings Hamiltonian terms. Moreover, we prove that the first-order sideband pulses and carrier pulses comprise a universal set for quantum operations on the qubit-oscillator qudit. This work suggests that the combination of a qubit and a bosonic system may serve as hardware-efficient quantum resources for quantum information processing.

We report on the creation of dual-species Bose-Einstein condensates (BECs) of ${}^{23}\mathrm{Na}$ atoms and ${}^{133}\mathrm{Cs}$ atoms. We demonstrate sympathetic cooling of Cs with Na in a magnetic quadrupole trap and a crossed optical dipole trap, leading to Na BECs with $8\left(1\right)\times {10}^5$ atoms and Cs BECs with $3.5\left(1\right)\times {10}^4$ atoms. Investigating cross thermalization and lifetimes of the mixture, we find that the Na and Cs BECs are miscible and overlapping, interacting with a moderate interspecies scattering length of $18\left(4\right)a_0$ at 23 G and $29\left(4\right)a_0$ at 894 G and coexisting for tens of seconds. Overlapping condensates of Na and Cs offer possibilities for many-body physics with ultracold bosonic mixtures and constitute an ideal starting point for the creation of ultracold ensembles of NaCs ground-state molecules.

Classical sensors for spectrum analysis are widely used but lack micro- or nanoscale spatial resolution. On the other hand, quantum sensors, capable of working with nanoscale precision, do not provide precise frequency resolution over a wide range of frequencies. Using a single spin in diamond, we present a measurement protocol for quantum probes which enables full signal reconstruction on a nanoscale spatial resolution up to potentially 100 GHz. We achieve $58\mathrm{nT}/\sqrt{\mathrm{Hz}}$ amplitude and $0.095\mathrm{rad}/\sqrt{\mathrm{Hz}}$ phase sensitivity and a relative frequency uncertainty of ${10}^{-12}$ for a 1.51 GHz signal within 10 s of integration. This technique opens the way to quantum spectrum analysis methods with potential applications in electron spin detection and nanocircuitry in quantum technologies.

Every quantum system is coupled to an environment. Such system-environment interaction leads to temporal correlation between quantum operations at different times, resulting in non-Markovian noise. In principle, a full characterization of non-Markovian noise requires tomography of a multitime processes matrix, which is both computationally and experimentally demanding. In this paper, we propose a more efficient solution. We employ machine learning models to estimate the amount of non-Markovianity, as quantified by an information-theoretic measure, with tomographically incomplete measurement. We test our model on a quantum optical experiment, and we are able to predict the non-Markovianity measure with $90\%$ accuracy. Our experiment paves the way for efficient detection of non-Markovian noise appearing in large scale quantum computers.

Hong-Ou-Mandel interferometry takes advantage of the quantum nature of two-photon interference to increase the resolution of precision measurements of time delays. Relying on few-photon probe states, this approach is applicable also in cases of extremely sensible samples and it achieves attosecond-scale (nanometer path length) resolution, which is relevant to cell biology and two-dimensional materials. Here, we theoretically analyze how the precision of Hong-Ou-Mandel interferometers can be significantly improved by engineering the spectral distribution of two-photon probe states. In particular, we assess the metrological power of different classes of biphoton states with non-Gaussian time-frequency spectral distributions, considering the estimation of both time and frequency shifts. We find that grid states, characterized by a periodic structure of peaks in the chronocyclic Wigner function, can outperform standard biphoton states in sensing applications. The considered states can be feasibly produced with atomic photon sources, bulk nonlinear crystals and integrated photonic waveguide devices.

The subjective Bayesian interpretation of probability asserts that the rules of the probability calculus follow from the normative principle of Dutch-book coherence: A decision-making agent should not assign probabilities such that a series of monetary transactions based on those probabilities would lead them to expect a sure loss. Similarly, the subjective Bayesian interpretation of quantum mechanics (QBism) asserts that the Born rule is a normative rule in analogy to Dutch-book coherence, but with the addition of one or more empirical assumptions characterizing the particularities of the physical world. Here we make this link explicit for a conjectured representation of the Born rule which holds true if symmetric informationally complete quantum measurements (or SIC-POVMs) exist for every finite-dimensional Hilbert space. We prove that an agent who thinks they are gambling on the outcomes of measurements on a sufficiently quantum-like system but refuses to use this form of the Born rule when placing their bets is vulnerable to a Dutch book. The key property for being sufficiently quantum-like is that the system admits a symmetric reference measurement but that this measurement is not sampling any hidden variables.

We study theoretically the nonlinear dynamics induced by shear-flow instability in segregated two-component Bose-Einstein condensates in terms of the Weber number, which is defined by extending the past theory on the Kelvin-Helmholtz instability in classical fluids. Numerical simulations of the Gross-Pitaevskii equations demonstrate that dynamics of pattern formation is well characterized by the Weber number We, clarifying the microscopic aspects unique to the quantum fluid system. For $\text{We}\lesssim 1$, the Kelvin-Helmholtz instability induces flutter-finger patterns of the interface and quantized vortices are generated at the tip of the fingers. The associated nonlinear dynamics exhibits a universal behavior with respect to We. When $\text{We}\gtrsim 1$ in which the interface thickness is larger than the wavelength of the interface mode, the nonlinear dynamics is effectively initiated by the counter-superflow instability. In a strongly segregated regime and a large relative velocity, the instability causes transient zipper pattern formation instead of generating vortices due to the lack of circulation to form a quantized vortex per a finger. In a weakly segregating regime and a small relative velocity, the instability leads to the sealskin pattern in the overlapping region, in which the frictional relaxation of the superflow cannot be explained only by the homogeneous counter-superflow instability. We discuss the details of the linear and nonlinear characteristics of this dynamical crossover from small to large Weber numbers, where microscopic properties of the interface become important for the large Weber number.

Polarization-encoded entanglement remains the simplest platform for the generation, manipulation, and visualization of entangled-photon states. While quantum dots have the potential to emit on-demand polarization entanglement, spontaneous-parametric-down-conversion (SPDC) sources remain the leading method for the generation of polarization-entangled states. SPDC sources suffer from the potential to produce multiple photon pairs in a single pass of an experiment. These multiple pairs have been shown to have negative impacts on quantum experiments involving entanglement. In this work, we now provide a rigorous theoretical model for the loss of entanglement due to additional photon pairs. This is seen as a reduction in a possible measurement of the Clauser-Horne-Shimony-Holt (CHSH) parameter. We perform these calculations for two different methods for the generation of polarization entanglement involving SPDC. The results agree with other observations presented in the literature. We also find that, even for small mean photon numbers, the CHSH parameter is reduced linearly, demonstrating that multiple photon pairs have a critical impact on the entanglement in the system.

We consider a two-dimensional nonlinear waveguide with distributed gain and losses. The optical potential describing the system consists of an unperturbed complex potential depending only on one transverse coordinate, i.e., corresponding to a planar waveguide, and a small nonseparable perturbation depending on both transverse coordinates. It is assumed that the spectrum of the unperturbed planar waveguide features an exceptional point (EP), while the perturbation drives the system into the unbroken phase. Slightly below the EP, the waveguide sustains two-component envelope solitons. We derive one-dimensional equations for the slowly varying envelopes of the components and show their stable propagation. When both traverse directions are taken into account within the framework of the original model, the obtained two-component bright solitons become metastable and persist over remarkably long propagation distances.

QED cascades in a strong electromagnetic field of optical range and arbitrary configuration are considered. A general expression for short-time dependence of the key electron quantum dynamical parameter is derived, allowing one to generalize the effective threshold condition of QED cascade onset. The generalized theory is applied to self-sustained cascades in a single focused laser pulse. According to numerical simulations, if a GeV electron bunch is used as a seed, an ordinary cascade can be converted into a self-sustained one. As an application, it would be also possible to produce this way bright collimated photon beams with up to GeV photon energies.

Reconfigurable distribution of entangled states is essential for operation of quantum networks connecting multiple devices such as quantum memories and quantum computers. We introduce a quantum distribution network architecture enabling control of the entangled-state propagation direction using linear-optical devices and phase shifters and offering reconfigurable connections between multiple quantum nodes. The basic two-photon entanglement distribution scheme is first introduced to illustrate the principle of operation. The scheme is then extended to a network structure with an increased number of spatial modes connecting potential end-users. We present several examples of controllable network configuration modifications using time-dependent phase shifters.

Electromagnetically-induced-transparency (EIT) cooling has established itself as one of the most widely used cooling schemes for trapped ions during the past 20 years. Compared to its alternatives, EIT cooling possesses important advantages such as a tunable effective linewidth, a very low steady-state phonon occupation, and applicability for multiple ions. However, the existing analytic expression for the steady-state phonon occupation of EIT cooling is limited to the zeroth order of the Lamb-Dicke parameter. Here we extend such calculations and present the explicit expression to the second order of the Lamb-Dicke parameter. We discuss several implications of our refined formula and are able to resolve certain difficulties in existing results.

We calculate elastic and positronium (Ps) breakup cross sections for collisions of Ps with ${\mathrm{O}}_2$ and ${\mathrm{CO}}_2$ molecules in the fixed-nuclei approximation. We incorporate electron exchange and correlations for these processes by using the free-electron-gas model developed earlier for Ps scattering by rare-gas atoms and ${\mathrm{N}}_2$ molecules. The results exhibit similarity between electron and Ps scattering when the cross section is plotted as a function of the projectile velocity, confirming experimental observations [S. J. Brawley et al., Science 330, 789 (2010)]. Below the Ps breakup threshold we observe resonance structures similar to those obtained earlier for Ps-${\mathrm{N}}_2$ scattering.

A communication protocol based on a Gaussian modulation of squeezed states in a single quadrature and measured via homodyne detection can completely eliminate information leakage to an eavesdropper in a pure-loss channel. However, the asymmetry of the protocol with respect to the quadratures of light presents security issues and the eavesdropper's information is not necessarily minimized for general asymmetric attacks. Here, we perform asymptotic security analysis of the asymmetric protocol against general asymmetric collective attacks and bound the eavesdropper's information via the Heisenberg uncertainty principle. The bound is not tight, and therefore we symmetrize the protocol in a heralding way, discarding the issues of asymmetry altogether. Our proposed heralding protocol asymptotically eliminates information leakage in a pure-loss channel and minimizes leakage in a noisy channel.

Adaptive filtering is a powerful class of control theoretic concepts useful in extracting information from noisy data sets or performing forward prediction in time for a dynamic system. The broad utilization of the associated algorithms makes them attractive targets for similar problems in the quantum domain. To date, however, the construction of adaptive filters for quantum systems has typically been carried out in terms of stochastic differential equations for weak, continuous quantum measurements, as used in linear quantum systems such as optical cavities. Discretized measurement models are not as easily treated in this framework, but are frequently employed in quantum information systems leveraging projective measurements. This paper presents a detailed analysis of several technical innovations that enable classical filtering of discrete projective measurements, useful for adaptively learning system dynamics, noise properties, or hardware performance variations in classically correlated measurement data from quantum devices. In previous work we studied a specific case of this framework, in which noise and calibration errors on qubit arrays could be efficiently characterized in space; here, we present a generalized analysis of filtering in quantum systems and demonstrate that the traditional convergence properties of nonlinear classical filtering hold using single-shot projective measurements. These results are important early demonstrations indicating that a range of concepts and techniques from classical nonlinear filtering theory may be applied to the characterization of quantum systems involving discretized projective measurements, paving the way for broader adoption of control theoretic techniques in quantum technology.

Symmetry plays a fundamental role in the security analysis of quantum key distribution (QKD). Here we review how symmetry is exploited in continuous-variable (CV) QKD to prove the optimality of Gaussian attacks in the finite-size regime. We then apply these results to improve the feasibility and the key rate of these protocols. First we show how to improve the feasibility of the energy test, which is one important routine aimed at establishing an upper bound on the effective dimensions of the otherwise infinite-dimensional Hilbert space of CV systems. Second, we show how the routine of parameter estimation can be made resource efficient in measurement-device independent QKD. These results show that all the raw data can be used both for key extraction and for the routines of the energy test and parameter estimation. Furthermore, the improved energy test does not require active symmetrization of the measured data, which is computationally demanding.

Introducing precise spatiotemporal structure into a pulsed optical field can lead to remarkable changes with its free propagation. “Space-time” (ST) wave packets, for example, propagate rigidly at a tunable group velocity in free space by inculcating a one-to-one relationship between the axial wave numbers and the temporal frequencies. Here we introduce a class of ST wave packets that we call V-waves (so named because of their characteristically V-shaped spatiotemporal spectrum) in which a linear one-to-one relationship is introduced between the temporal frequencies and the transverse wave numbers (or spatial frequencies). We confirm experimentally that V-waves experience anomalous group velocity dispersion in free space, all the while maintaining the group velocity fixed at the speed of light in vacuum. Extremely large values of group velocity dispersion can be easily realized, which are not accessible with traditional optical materials or photonic structures. Moreover, V-waves are the unique optical wave packets whose diffraction and dispersion lengths are intrinsically equal by virtue of the spatiotemporal structure of the field itself. These results are of interest for optical signal processing and nonlinear optics.

We report on a combined experimental and theoretical study of the low-entropy Mott transition for interacting bosons trapped in a three-dimensional (3D) cubic lattice; namely, the interaction-induced superfluid-to-normal phase transition in the vicinity of the zero-temperature Mott transition. Our analysis relies on the measurement of the 3D momentum distribution, which allows us to extract the momentum-space density $\rho (\mathit{k}=\mathit{0})$ at the center of the Brillouin zone. Upon varying the ratio between the interaction $U$ and the tunneling energy $J$ across the superfluid transition, we observe that $\rho (\mathit{k}=\mathit{0})$ exhibits a sharp transition at a value of $U/J$ consistent with the bulk prediction from quantum Monte Carlo. In addition, the variation of $\rho (\mathit{k}=\mathit{0})$ with $U/J$ exhibits a critical behavior consistent with the expected 3D $XY$ universality class. Our results show that the tomographic reconstruction of the momentum distribution of ultracold bosons can reveal traits of the critical behavior of the superfluid transition even in an inhomogeneous trapped system.

Fractional quantum Hall systems are among the most exciting strongly correlated systems. Accessing them microscopically via quantum simulations with ultracold atoms would be an important achievement toward a better understanding of this strongly correlated state of matter. A promising approach is to confine a small number of bosonic atoms in a quasi-two-dimensional rotating trap, which mimics the magnetic field. For rotation frequencies close to the in-plane trapping frequency, the ground state is predicted to be a bosonic analog of the Laughlin state. Here, we study the problem of the adiabatic preparation of the Laughlin state by ramping the rotation frequency and controlling the ellipticity of the trapping potential. By employing adapted ramping speeds for rotation frequency and ellipticity, and large trap deformations, we improve the preparation time for high-fidelity Laughlin states by a factor of 10 in comparison to previous studies. With this improvement of the adiabatic protocol the Laughlin state can be prepared with current experimental technology.