Indistinguishable quantum emitters confined to length scales smaller than the wavelength of the light become superradiant. Compared to uncorrelated and distinguishable emitters, superradiance results in qualitative modifications of optical signals such as photon coincidences. However, recent experiments revealed that similar signatures can also be obtained in situations where emitters are too far separated to be superradiant if correlations between emitters are induced by the wave-function collapse during an emission-angle-selective photon detection event. Here, we compare two sources for cooperative emission, superradiance and measurement-induced cooperativity, and analyze their impact on time-dependent optical signals. We find that an antidip in photon coincidences at zero time delay is a signature of interemitter correlations in general but does not unambiguously prove the presence of superradiance. This suggests that photon coincidences at zero time delay alone are not sufficient and time-dependent data are necessary to clearly demonstrate a superradiant enhancement of the spontaneous radiative decay rate.

Open-system quantum dynamics described by non-Hermitian effective Hamiltonians have become a subject of considerable interest. Studies of non-Hermitian physics have revealed general principles, including relationships between the topology of the complex eigenvalue space and the breakdown of adiabatic control strategies. We study here the control of a single non-Hermitian qubit, similar to recently realized experimental systems in which the non-Hermiticity arises from an open spontaneous emission channel. We review the topological features of the resulting non-Hermitian Hamiltonian and then present two distinct results. First, we illustrate how to realize any continuous and differentiable pure-state trajectory in the dynamics of a qubit that are conditioned on no emission. This result implicitly provides a work around for the breakdown of standard adiabatic following in such non-Hermitian systems. Second, we use Pontryagin's maximum principle to derive optimal trajectories connecting boundary states on the Bloch sphere, using a cost function which balances the desired dynamics against the controller energy used to realize them. We demonstrate that the latter approach can effectively find trajectories which maintain high state purity, even in the case of inefficient detection.

A spin-polarized triplet metastable helium (He*) beam has been used as a probe for surface magnetism, but changes in the spin state during scattering from a surface remain unclear. In the present study, we explored this issue by constructing an apparatus that allows us to direct a spin-polarized He* beam to a surface and measure the spin polarization of He* scattered from the surface. Magnetic hexapoles were used for both the beam polarization and the spin analysis. The results of the spin-dependent He* scattering experiments on clean ${\mathrm{Fe}}_3{\mathrm{O}}_4$(100), H-terminated ${\mathrm{Fe}}_3{\mathrm{O}}_4$(100), benzene-adsorbed ${\mathrm{Fe}}_3{\mathrm{O}}_4$(100), and nonmagnetic Cu(100) surfaces indicated that, although the spin direction of He* was mostly preserved during scattering from these surfaces, spin-flop scattering of surviving He* occurred with a probability up to approximately 0.1. Our results showed that the survival probability was higher when the spins of He* and the ${\mathrm{Fe}}_3{\mathrm{O}}_4$(100) film were parallel, which can be understood based on the lower He* resonance ionization rate for this spin orientation. Based on our findings, we estimate that a nonpolarized He* gas becomes 10% spin-polarized after a single collision with a clean ${\mathrm{Fe}}_3{\mathrm{O}}_4$(100) surface.

We present a systematic calculation of the electric dipole forbidden transitions in the homonuclear molecular ion ${\mathrm{H}}_2^{+}$. We get that the transition rate from the ground ortho $(v=0,N=1,J=1/2)$ state to the ground para $(v=0,N=0,J=1/2)$ state is $4.9\times {10}^{-14}{\mathrm{s}}^{-1}$, which corresponds to the lifetime of $6.4\times {10}^5$ yr. The Einstein coefficient $A_{n{n}^{\prime }}$ for the $(v=1,N=1,J=1/2)\to (v=0,N=0,J=1/2)$ transition is equal to $0.12\times {10}^{-9}{\mathrm{s}}^{-1}$, which is of comparable order with the values for quadrupole transitions in ${\mathrm{H}}_2^{+}$. It gives hope that the ortho-para transitions in ${\mathrm{H}}_2^{+}$ can be explored by laser spectroscopy.

In our work we study the interatomic Coulombic decay (ICD) in the NeKr dimer, where after $2s$ ionization of the Ne, the system relaxes and the excess energy is utilized to ionize the Kr. The temporal evolution of the ICD process in NeKr has been recently measured and theoretically explained by Trinter et al. [Chem. Sci. 13, 1789 (2022)]. Here we focus on two other main goals. The first goal regards the found interference effects in the photoemission (PE) spectrum, which are unusual phenomena in noble gas dimers. They result from the coherently excited vibrational energy levels and substantial dependence of the large ICD decay width on the internuclear distance. The PE spectrum reacts sensitively to changes in the potential energy curve (PEC) of the $2s$ ionized state, and we modified the available ab initio PEC in such a way that satisfactory agreement between theoretical and experimental data is achieved. The impact of isotope masses on the PE spectrum is briefly discussed and used in the determination of the PEC. Our second main goal concerns the nuclear motion during the ICD process. Here we investigate the impact of different vibrationally excited states of the electronic ground state on the ICD-electron and kinetic energy release (KER) spectra. To transfer our vibrationally selected ICD model to a realizable experiment, we also present the impact of temperature on the ICD-electron spectrum. Finally, our studies are complemented by comparing the directly computed KER spectrum to the mirror image of the ICD-electron spectrum, which coincide under certain conditions.

Recent experimental advances in the field of cold-atom cavity QED provide a powerful tool for exploring nonequilibrium correlated quantum phenomena beyond conventional condensed-matter scenarios. We present the dynamical phase diagram of a driven Bose-Einstein condensate coupled with the light field of a cavity, with a transverse driving field red-detuned from atomic resonance. We identify regions in parameter space showing dynamical instabilities in the form of limit cycles, which evolve into chaotic behavior in the strong driving limit. Such limit cycles originate from the interplay between cavity dissipation and atom-induced resonance frequency shift, which modifies the phase of cavity mode and gives excessive negative feedback on the atomic density modulation, leading to instabilities of the superradiant scattering. We find interesting merging of the limit cycles related by a $Z_2$ symmetry, and identify a limit cycle formed by purely atomic excitations. The effects of quantum fluctuations and atomic interactions are also investigated.

Although crystallization is a ubiquitous phenomenon in nature, crystal formation and melting still remain fascinating processes with several open questions yet to be addressed. In this work we study the emergent crystallization of a laser-driven dipolar Bose-Einstein condensate due to the interplay between long-range magnetic and effectively infinite-range light-induced interactions. The competition between these two interactions results in a collective excitation spectrum with two-roton minima that introduce two different length scales at which crystalline order can emerge. In addition to the formation of regular crystals with simple periodic patterns due to the softening of one of the rotons, we find that both rotons can also soften simultaneously, resulting in the formation of exotic, complex periodic, or aperiodic density patterns. We also demonstrate dynamic state-preparation schemes for achieving all the crystalline ground states found for experimentally relevant and feasible parameter regimes.

A mechanical system can be optimally controlled through continuous measurements of its position followed by feedback. We revisit the complete formalism for predicting the performance of such a system without invoking the standard rotating-wave approximation and the adiabatic approximation. Using this formalism, we deduce both the conditional and unconditional states of a mechanical oscillator using the optimal control and feedback that leads to mechanical cooling and mechanical squeezing. We find large discrepancies between the exact solutions and the approximate solutions, stressing the importance of using the complete model. We also highlight the importance of distinguishing between the conditional and unconditional states by demonstrating that these two cannot coincide in a typical control scheme, even with infinite feedback strength.

Disorder-induced feedback makes random lasers very susceptible to any changes in the scattering medium. The sensitivity of the lasing modes to perturbations in the disordered systems has been utilized to map the regions of perturbation. A tracking parameter that takes into account the cumulative effect of changes in the spatial distribution of the lasing modes of the system has been defined to locate the region in which a scatterer is displaced by a few nanometers. We show numerically that the precision of the method increases with the number of modes. The proposed method opens up the possibility of application of random lasers as a tool for monitoring locations of nanoscale displacement, which can be useful for single-particle detection and monitoring.

The decoherence of a fast quantum particle in a gas is studied by applying the Kramers-Moyal expansion to the quantum master equation for the reduced density matrix of the particle. This expansion leads to a general form of the Caldeira-Leggett master equation accounting for the angular variation of the differential cross section. The equation describes the decoherence in both the longitudinal and transverse directions with respect to the particle motion. It is shown that, when the differential cross section is concentrated in the forward direction, transverse decoherence dominates. The coherence region off the diagonal of the density matrix is characterized by coherence lengths, which can be deduced, for Gaussian states, from the momentum covariance matrix according to a Heisenberg-type uncertainty relation. Finally, the longitudinal-to-transverse ratio of the coherence lengths is estimated for an $\alpha$ particle of a few MeVs. This ratio indicates that the coherence region looks like an ellipsoid elongated in the direction of motion.

We consider quantum batteries given by composite interacting quantum systems in terms of the thermodynamic work cost of local random unitary processes. We characterize quantum correlations by monitoring the average energy change and its fluctuations in the high-dimensional bipartite systems. We derive a hierarchy of bounds on high-dimensional entanglement (the so-called Schmidt number) from the work fluctuations and thereby show that larger work fluctuations can verify the presence of stronger entanglement in the system. Finally, we develop two-point measurement protocols with noisy detectors that can estimate work fluctuations, showing that the dimensionality of entanglement can be probed in this manner.

The nonlocality revealed in a multiparty multisource network Bell experiment is conceptually different than the standard multiparty Bell nonlocality involving a single common source. Here, by introducing variants of asymmetric bilocal as well as trilocal network scenarios, we go beyond the typical bilocal network scenario where both the edge parties have an equal number of measurement settings. We first introduce an asymmetric bilocal network where one of the edge parties (say, Alice) receives $2^{n-1}$ inputs and the other edge party (say, Charlie) receives $n$ inputs. We derive two variants of asymmetric bilocality inequalities and demonstrate their optimal quantum violations. Further, we explore two types of asymmetric trilocal scenarios: (i) when two edge parties receive $2^{n-1}$ inputs each and the other edge party receives $n$ inputs, and (ii) when one edge party receives $2^{n-1}$ inputs and the other two edge parties have $n$ inputs each. We use an elegant sum-of-squares technique that enables us to evaluate the quantum optimal values of the proposed network inequalities without assuming the dimension of the systems for both the asymmetric bilocal as well as the trilocal scenarios. Further, we demonstrate the robustness of the quantum violations of the proposed inequalities in the presence of white noise.

We present a quantum-mechanical model for a four-wave-mixing Josephson traveling-wave parametric amplifier including substrate losses and associated thermal fluctuations. Under the assumption of a strong undepleted classical pump tone, we derive an analytic solution for the bosonic annihilation operator of the weak signal photon field using temporal equations of motion in a reference timeframe, including chromatic dispersion. From this result, we can predict the asymmetric gain spectrum of a Josephson traveling-wave parametric amplifier due to nonzero substrate losses. We also predict the equivalent added input noise including quantum fluctuations as well as thermal noise contributions. Our results are in excellent agreement with recently published experimental data.

Spatial separation restricts the set of locally implementable quantum operations on distributed multipartite quantum systems. We propose that indefinite causal structure arising due to quantum superposition of different space-time geometries can be used as an independent universal resource for local implementation of any quantum operation on spatially distributed quantum systems. Consequently, all such quantum tasks that are not accomplishable by local operations and classical communication (LOCC) only also become locally accomplishable. We show that exploiting indefinite causal structure as the sole resource, it is possible to perfectly teleport the state of one agent's subsystem to the other distant laboratory in such a way that the agent at the distant laboratory can have access to the whole initially shared state in his or her laboratory and can perform any global quantum operation on the joint state locally. We further find that, after the teleportation process, the resource—indefinite causal structure of the space-time does not get consumed. Hence, after implementing the desired quantum operation the state of the first agent's subsystem can be teleported back to its previous laboratory using the same resource. We show that this two-way teleportation is not always necessary for locally executing all nonlocal quantum tasks that are not realizable by LOCC only. Without invoking any kind of teleportation, we present a protocol for perfect local discrimination of the set of four Bell states that exploits indefinite causal structure as the sole resource. As immediate upshots, we present some more examples of such nonlocal tasks as local discrimination of the set of states exhibiting “quantum nonlocality without entanglement” and activation of bound entangled states that are also achievable by our proposed protocol incorporating indefinite causal structure as a resource.

The total cross section of positronium (Ps) scattering from molecular oxygen has been measured in the velocity range 0.27–1.50 a.u. (energy range 2–61 eV) and has been found to be close to the corresponding equivelocity electron cross section above 0.87 a.u. $\left(20\mathrm{eV}\right)$, as previously found by Brawley et al., [Science 330, 789 (2010)]. However, below this value the cross section for positronium is observed to exceed that for electrons by up to a factor of 4 at the lowest energy. Measurements are compared to the predictions of low-energy resonant peaks in the elastic-scattering cross section calculated within a free-electron-gas model refined by applying corrections to the correlation energy for the interaction between Ps and the electron gas. Additionally, cross sections for ${\mathrm{O}}_2^{-}$ formation and positronium breakup have been calculated using a classical-trajectory Monte Carlo approach. Comparisons are made with earlier calculations and discussed in terms of both experimental and theoretical uncertainties.

We demonstrate and characterize Zeeman-Sisyphus (ZS) deceleration of a beam of ytterbium monohydroxide. Our method uses a combination of large magnetic fields ($\sim 2.5$ T) and optical spin-flip transitions to decelerate molecules while scattering only $\sim 10$ photons per molecule. We study the challenges associated with the presence of internal molecular perturbations among the excited electronic states and discuss the methods used to overcome these challenges, including a modified ZS decelerator using microwave and optical transitions.

Strong-field ionization experiments are routinely performed with different atomic targets. While these experiments helped reveal many details about the light-matter interaction in intense laser fields, the target atoms are often modeled in a rather crude manner by neglecting most of their electronic structure. Until the present, therefore, many above-threshold ionization measurements had been understood only qualitatively, and this especially applies to the ionization with elliptically polarized laser pulses, for which the photoelectron angular distributions are known to exhibit a quite strong (Coulomb) asymmetry. This asymmetry arises from the Coulomb and short-range forces between the photoion and the outgoing field-dressed electron in the continuum. We demonstrate here how the strong-field approximation (SFA) can be combined with atomic structure theory for modeling such target-resolved observations. Using a partial-wave representation of the SFA, we show that this combination reproduces the Coulomb asymmetry for argon and xenon targets in good agreement with previous experiments and especially if a distorted-wave Volkov continuum is applied for the active electron. We therefore conclude that a better account of the initial and final electron waves in strong-field theories will enhance our understanding of ionization phenomena in intense fields.

Recent experiments demonstrated deeply subwavelength lattices using atoms with $N$ internal states Raman coupled with lasers of wavelength $\lambda$. The resulting unit cell was $\lambda /2N$ in extent, an $N$-fold reduction compared to the usual $\lambda /2$ periodicity of an optical lattice. For resonant Raman coupling, this lattice consists of $N$ independent sinusoidal potentials (with period $\lambda /2$) displaced by $\lambda /2N$ from each other. We show that detuning from Raman resonance induces tunneling between these potentials. Temporally modulating the detuning couples the $s$ and $p$ bands of the potentials, creating a pair of coupled subwavelength Rice-Mele chains. This operates as a topological charge pump that counterintuitively can give half the displacement per pump cycle of each individual Rice-Mele chain separately. We analytically describe this behavior in terms of infinite-system Chern numbers and numerically identify the associated finite-system edge states.