Studying the zeros of partition functions in the space of complex control parameters allows one to understand formally how critical behavior of a many-body system can arise in the thermodynamic limit despite various no-go theorems for finite systems. In this work we propose protocols that can be realized in quantum simulators to measure the location of complex-partition-function zeros of classical Ising models. The protocols are solely based on the implementation of simple two-qubit gates, local spin rotations, and projective measurements along two orthogonal quantization axes. Besides presenting numerical simulations of the measurement outcomes for an exemplary classical model, we discuss the effect of projection noise and the feasibility of the implementation on state of the art platforms for quantum simulation.

Ion trap quantum computers are based on modulating the Coulomb interaction between atomic ion qubits using external forces. However, the spectral crowding of collective motional modes could pose a challenge to the control of such interactions for large numbers of qubits. Here, we show that high-fidelity quantum gate operations are still possible with very large trapped ion crystals by using a small and fixed number of motional modes, simplifying the scaling of ion trap quantum computers. We present analytical work that shows that gate operations need not couple to the motion of distant ions, allowing parallel entangling gates with a crosstalk error that falls off as the inverse cube of the distance between the pairs. We also experimentally demonstrate high-fidelity entangling gates on a fully connected set of seventeen ${}^{171}\mathrm{Yb}^{+}$ qubits using simple laser pulse shapes that primarily couple to just a few modes.

Self-calibrating quantum state tomography aims at reconstructing the unknown quantum state and certain properties of the measurement devices from the same data. Since the estimates of the state and device parameters come from the same data, one should employ a joint estimation scheme, including the construction and reporting of joint state-device error regions, to quantify uncertainty. We explain how to do this naturally within the framework of optimal error regions. As an illustrative example, we apply our procedure to the double-crosshair measurement of the BB84 scenario in quantum cryptography and so reconstruct the state and estimate the detection efficiencies simultaneously and reliably. We also discuss the practical situation of a satellite-based quantum key distribution scheme, for which self-calibration and proper treatment of the data are necessities.

In the present article, we explore a suitable coarse-graining channel as a tool to describe effective entanglement spreading in a coarse-grained spin chain with different degrees of resolution. Comparing it with the experimental realizations performed with ultracold atoms, our results suggest that even if we are not able to fully resolve the system, entanglement can still be detected for some coarse-graining levels. Furthermore, we show that it is possible to have some information about the “microscopic” entanglement, even if we only have access to the system's coarse-grained description. We show that the amount of entanglement decays exponentially with the lack of system resolution. The lack of experimental resolution might thus lead to an effective classical description.

We report a precise measurement of the isotope shifts in the $4{}^2{S}_{1/2}\to 3{}^2{D}_{5/2}$ electric quadrupole transition at 729 nm in ${}^{40-42,44,48}\mathrm{Ca}^{+}$. The measurement has been made via high-resolution laser spectroscopy of co-trapped ions, finding measured shifts of 2 771 872 467.6(7.6), 5 340 887 394.6(7.8), and 9 990 381 870.0(6.3) Hz between ${}^{42,44,48}\mathrm{Ca}^{+}$ and ${}^{40}\mathrm{Ca}^{+}$, respectively. By exciting the two isotopes simultaneously, using frequency sidebands derived from a single laser, systematic uncertainties resulting from laser frequency drifts are eliminated. This permits far greater precision than similar previously published measurements in other alkaline-earth-metal systems. The resulting measurement accuracy provides a benchmark for tests of theoretical isotope shift calculations and also offers a step towards probing new physics via isotope shift spectroscopy.

We report on the experimental observation of interatomic Coulombic decay (ICD) in pure ${}^4\mathrm{He}$ nanoclusters of mean sizes between N ∼ 5000 and 30 000 and the subsequent scattering of energetic $\mathrm{H}{\mathrm{e}}^{+}$ fragments inside the neutral cluster by using cold target recoil ion momentum spectroscopy. ICD is induced in He clusters by using vacuum ultraviolet light of $h\nu =67\mathrm{eV}$ from the BESSY II synchrotron. The electronic decay creates two neighboring ions in the cluster at a well-defined distance. The measured fragment energies and angular correlations show that a main energy loss mechanism of these ions inside the cluster is a single hard binary collision with one atom of the cluster.

We investigate how the presence of a localized impurity in a Bose-Einstein condensate of trapped cold atoms that interact with each other weakly and repulsively affects the profile of the condensed and excited components at zero temperature. By solving the Gross-Pitaevskii and Bogoliubov–de Gennes equations, we find that an impurity-boson contact attraction (repulsion) causes both components to change in spatial structure in such a way as to be enhanced (suppressed) around the impurity, while slightly declining (growing) in a region far from the impurity. Such behavior of the quantum depletion of the condensate can be understood by decomposing the impurity-induced change in the profile of the excited component with respect to the radial and azimuthal quantum numbers. The significant role of the centrifugal potential and the hole excitation level is thus clarified.

The behavior of a harmonically trapped dipolar Bose-Einstein condensate with its dipole moments rotating at angular frequencies lower than the transverse harmonic trapping frequency is explored in the co-rotating frame. We obtain semi-analytical solutions for the stationary states in the Thomas-Fermi limit of the corresponding dipolar Gross-Pitaevskii equation and utilize linear stability analysis to elucidate a phase diagram for the dynamical stability of these stationary solutions with respect to collective modes. These results are verified via direct numerical simulations of the dipolar Gross-Pitaevskii equation, which demonstrate that dynamical instabilities of the co-rotating stationary solutions lead to the seeding of vortices that eventually relax into a triangular lattice configuration. Our results illustrate that rotation of the dipole polarization represents a new route to vortex formation in dipolar Bose-Einstein condensates.

We theoretically study radiation pressure induced generation of the frequency components at the sum sideband in an optomechanical system containing an optical parametric amplifier (OPA). It is shown that an OPA inside a cavity can considerably enhance the amplitude of sum sideband even with low power input fields. We find a new matching condition for the upper sum sideband generation. The height and width of the new peak can be adjusted by the nonlinear gain of the OPA. Furthermore, the lower sum sideband generation can be enhanced with several orders of magnitude by tuning the nonlinear gain parameter and the phase of the field pumping the OPA. The enhanced sum sideband may have potential applications to the manipulation of light in an on-chip optomechanical device and the sensitive sensing for precision measurement in the weak optomechanical coupling regime.

In this work, we explore both the internal and external atomic degrees of freedom to demonstrate the observation of giant gain and parametric oscillation in multiple four-wave mixing (FWM) processes in a sample of cold cesium atoms. We employ a standard backward FWM beam configuration to achieve parametric probe-beam gains exceeding 2000. This giant gain is accompanied by the generation of three other beams of equivalent power emitted along the directions satisfying the phase-matching conditions for multiple cascade forward and backward FWM triggered by the incident probe and the counterpropagating pumping beams. Moreover, we have also observed a simultaneous threshold for parametric oscillation of the four coupled optical fields with the pump intensity. These results point to an alternative pathway to generate multipartite correlated optical fields associated with long-lived atomic systems for applications in quantum information.

One of the weaknesses of quantum optical state postselection schemes is the low success probability. Typically there is a trade-off between amplifier properties such as success probability and output state fidelity. However, here we present a state comparison amplifier for optical coherent states, which features an active measurement and feedforward mechanism to correct for errors made during the initial amplification. The simple and relatively low latency mechanism allows us to correct for a binary phase alphabet. We demonstrate a significant simultaneous improvement in the amplifier characteristic parameters: output state fidelity, correct state fraction, and success probability. This demonstrates that nondeterministic quantum amplification can be enhanced significantly by measurement and feedforward.

We present a scheme for generating and manipulating three-mode squeezed states with genuine tripartite entanglement by injecting single-mode squeezed light into an array of coupled optical waveguides. We explore the possibility to selectively generate single-mode squeezing or multimode squeezing at the output of an elliptical waveguide array, determined solely by the input light polarization. We study the effect of losses in the waveguide array and show that quantum correlations and squeezing are preserved for realistic parameters. Our results show that arrays of optical waveguides are suitable platforms for generating multimode quantum light, which could lead to novel applications in quantum metrology.

We study the properties of optical fields created by interfering polychromatic stationary waves that have different spectra and polarizations. Such fields can exhibit both deterministic and random intensity and polarization beatings, where the latter stands for a periodic variation of the field polarization state. For visible light, the beating period enters the femtosecond scale already when the central wavelengths of the waves differ by 10 nm. If the bandwidth of at least one of the waves is also on the order of 10 nm, the periodic variations are accompanied by ultrafast random changes which cannot be measured directly. We propose a set of statistical characteristics for such rapidly varying vector fields and practical methods to determine them in terms of fully time-averaged quantities. Our results may have impact on a variety of fundamental and applied aspects of optical polarimetry and interferometry.

In this Comment we discuss the device-independent conference key agreement (DICKA) protocol [Phys. Rev. A 97, 022307 (2018)]. We show that the suggested honest implementation fails, because perfectly correlated measurement results and the required Bell-inequality violation cannot be achieved simultaneously, in contradiction to what is claimed. We further show via semidefinite programming that there cannot exist any suitable honest implementation in the tripartite setting, rendering the DICKA protocol incomplete.

In this Reply we correct a mistake that we made in the correctness proofs of our protocol. Specifically, the Bell inequality we used ensures security but does not allow us to produce a key. In this Reply we explain and correct this mistake by adjusting the Bell inequality we used in the proof. Incidentally, this correction leads to slightly better asymptotic key rates. Importantly, none of the conclusions of the article are affected.

In the presence of inverse Doppler shift, we verify the superluminal effect in a weak probe field through the medium of our atomic system using classes of the atoms of high average velocity. Additional analysis of analytical results for the advancement of Gaussian pulse in time is presented. The pulse advancement through the single transparency window under the weak control field increases. The pulse advancement through the central (each of the sides) window of the medium under the strong control field increases (decreases) when its intensity is gradually increased.

Production of electron-positron pairs by a relativistic electron beam propagating through an intense laser field is reconsidered by use of the Monte Carlo method within the quantum electrodynamics theory. The scenario of the SLAC experiment is analyzed [D. Burke et al., Phys. Rev. Lett. 79, 1626 (1997)], on the basis of which a scheme for pair production by a high-energy electron beam colliding with a composed laser field is proposed. The laser field consists of two components with different frequencies. We show that this composed laser field can facilitate pair creation by the double-photon Breit-Wheeler process and could significantly amplify the production probability. Our predictions can be tested by utilizing presently available high-energy electron accelerators and high-intensity laser facilities.

The quantum-mechanical decay of two or more overlapped resonances in a common continuum is largely influenced by Fano interference, leading to important phenomena such as the existence of bound states in the continuum, fractional decay and quiescent dynamics for single particle decay, and signature of particle statistics in the many-body quantum decay. An overlooked yet essential requirement to observe Fano interference is time-reversal symmetry of the bath. Here we consider multilevel quantum decay in a bath sustaining unidirectional (chiral) propagating states, such as in quantum Hall or in Floquet topological insulators, and show that the chiral nature of scattering states fully suppresses Fano interference among overlapping resonances. As a result, there are not bound states in the continuum, quantum decay is complete, and there is not any signature of particle statistics in the decay process. Nonetheless, some interesting features are disclosed in the multilevel decay dynamics in a topological bath, such as the appearance of high-order exceptional points, long quiescent dynamics followed by a fast decay, and the possibility to observe damped non-Hermitian Bloch oscillations.

We consider an extended Bose-Hubbard model that includes pair-correlated tunneling. We demonstrate that a minimal four-mode implementation of this model exhibits a pair-correlated regime in addition to Mott insulator and superfluid regimes. We propose a low complexity variational subspace for the ground state of the system in the pair-correlated regime, which we find to be numerically exact in pure pair-tunneling limit. Additionally, we propose a parameter-free high-fidelity model wave function that qualitatively captures the features of the ground state in the pair-correlated regime. Although the operationally accessible entanglement vanishes deep inside the Mott insulator and superfluid regimes due to particle-number conservation, we find that in the pure pair-correlated tunneling limit the accessible entanglement entropy grows logarithmically with the number of particles. Furthermore, we demonstrate that upon application of a unitary beam-splitter operation, the pair-correlated ground state is transformed into a state with completely accessible entanglement that is not limited by superselection rules.

Trapped ions are a versatile platform for the investigation of quantum many-body phenomena, in particular for the study of scenarios where long-range interactions are mediated by phonons. Recent experiments have shown that the trapped ion platform can be augmented by exciting high-lying Rydberg states. This introduces controllable state-dependent interactions that are independent of the phonon structure. However, the many-body physics in this newly accessible regime is largely unexplored. We show that this system grants access to generalized Dicke model physics, where dipolar interactions between ions in Rydberg states drastically alter the collective nonequilibrium behavior. We analyze and classify the emerging dynamical phases and identify a host of nonequilibrium signatures such as multiphase coexistence regions and phonon-lasing regimes. We moreover show how they can be detected and characterized through the fluorescence signal of scattered photons. Our study thus highlights interesting capabilities of trapped Rydberg-ion systems for creating and detecting quantum nonequilibrium phases.