Apart from the Bell nonlocality, which deals with the correlations generated from the local input-output statistics, quantum theory exhibits another kind of nonlocality that involves the indistinguishability of the locally preparable set of multipartite states. While Bell-type nonlocality cannot be activated from a given local correlation via local operations and shared randomness, it is already reported that the latter kind of nonlocality can be activated from a “local,” i.e., locally distinguishable set of states. However, recently it is shown that a stronger notion of such a nonlocality, which deals with elimination instead of discrimination, can be activated from locally preparable bipartite states of dimension $7\times 8$. The present work observes that the same notion can be demonstrated even in lower dimensional multipartite systems. Importantly, the possibly strongest version of such an activation is further depicted here, where none of the transformed product states can be eliminated, even if all but one of the parties come together.

Can classical systems grasp quantum dynamics executed in an untrusted quantum device? Metger and Vidick answered this question affirmatively by proposing a computational self-testing protocol for Bell states that certifies generation of Bell states and measurements on them. Since their protocol relies on the fact that the target states are stabilizer states, it is highly nontrivial to reveal whether the other class of quantum states, nonstabilizer states, can be self-tested. Among nonstabilizer states, magic states are indispensable resources for universal quantum computation. Here, we show that a magic state for the $CCZ$ gate can be self-tested while that for the $T$ gate cannot. Our result is applicable to a proof of quantumness, where we can classically verify whether a quantum device generates a quantum state having nonzero magic.

We relate observables in open quantum systems with the topology of non-Hermitian models using the Keldysh path-integral method. This allows to extract an effective Hamiltonian from the Green's function which contains all the relevant topological information and produces $\omega$-dependent topological invariants, linked to the response functions at a given frequency. Then, we show how to detect a transition between different topological phases by measuring the response to local perturbations. Our formalism is exemplified in a one-dimensional Hatano-Nelson model, highlighting the difference between the bosonic and the fermionic case.

We study a one-dimensional array of bosons with infinite-range interactions mediated by a laser-driven dissipative optical cavity. The cavity-mediated infinite-range interactions open up an alternative pathway to crystallization, hitherto only known for dipolar bosons due to their long-range interactions. In parameter ranges attainable in state-of-the-art experiments, we systematically compare observables for bosons and fermions with infinite-range interactions. At sufficiently large laser pump powers, many observables, including density distributions in real and momentum space, correlation functions, eigenvalues of the one-body density matrix, and superradiance order parameters, become identical for bosons and fermions. We map out the emergence of this cavity-induced crystallization as a function of pump power and contact interactions. We discover that cavity-mediated interactions can compensate a reduction by several orders of magnitude in the strength of the contact interactions needed to trigger crystallization.

We consider the problem of $N$ identical fermions interacting via a zero-range attractive potential with a lighter atom in one dimension. Using the few-body approach based on the Skorniakov and Ter-Martirosian equation, we determine the energies and the critical mass ratios for the emergence of the tetramer, pentamer, and hexamer. For large $N$, we solve the problem analytically by using the mean-field theory based on the Thomas-Fermi approximation. The system becomes bound when the heavy-to-light mass ratio exceeds a critical value, which grows as $N^3$ at large $N$. We also employ a more sophisticated Hartree-Fock approach, which turns out to be equivalent to the Thomas-Fermi approximation for determining the energies, but provides a better description of the microscopic structure of the clusters.

We study a model of a quantum collective spin weakly coupled to a spin-polarized Markovian environment and find that the spectrum is divided into two regions that we name normal and exceptional Liouvillian spectral phases. In the thermodynamic limit, the exceptional spectral phase displays the unique property of being made up exclusively of second-order exceptional points. As a consequence, the evolution of any initial density matrix populating this region is slowed down and cannot be described by a linear combination of exponential decays. This phase is separated from the normal one by a critical line in which the density of Liouvillian eigenvalues diverges, a phenomenon analogous to that of excited-state quantum phase transitions observed in some closed quantum systems. In the limit of no bath polarization, this criticality is transferred onto the steady state, implying a dissipative quantum phase transition and the formation of a boundary time crystal.

We consider statistical methods based on finite samples of locally randomized measurements in order to certify different degrees of multiparticle entanglement in intermediate-scale quantum systems. We first introduce hierarchies of multiqubit criteria, satisfied by states which are separable with respect to partitions of different size, involving only second moments of the underlying probability distribution. Then, we analyze in detail the statistical error of the estimation in experiments and present several approaches for estimating the statistical significance based on large deviation bounds. The latter allows us to characterize the measurement resources required for the certification of multiparticle correlations, as well as to analyze given experimental data in detail.

Quantum communication research has in recent years shifted to include multipartite networks for which questions of quantum network routing naturally emerge. To understand the potential for multipartite routing, we focus on the most promising architectures for future quantum networks—those connecting nodes close to each other. Nearest-neighbor networks, such as rings, lines, and grids, have been studied under different communication scenarios to facilitate the sharing of quantum resources especially in the presence of bottlenecks. We analyze the potential of nearest-neighbor entangling gate quantum networks and identify some serious limitations by demonstrating that rings and lines cannot overcome common bottleneck communication problems.

Optomechanical systems have attracted considerable attention due to their wide applications in hybrid transduction systems, sensors, and quantum measurements. Classical optomechanical systems have various dynamical regimes that depend on the parameters, and the situation can be even more complicated when incorporated with other nonlinearities, such as the thermal nonlinearity that is common in optical resonators. Here, we experimentally and theoretically study the thermal bistability induced by optomechanical effect in a whispering-gallery mode microresonator. The mechanical oscillation modulates the intracavity photon number and modifies the thermal stability of states under a blue-detuned pump laser. Utilizing a silica microtip to suppress the mechanical motion, we identify the regions where the optical cavity temperature and detuning change due to the onset of oscillation and characterize the shifts in the detuning quantitatively. Hysteretic response in the mechanical frequency shifts (optical spring effect) to the pump wavelength is also observed to confirm the presence of bistability. The thermal dynamics discussed here is essentially different from the optomechanical heating and cooling of the effective temperature of the mechanical oscillator. This discovery shows that the thermal nonlinearity, which has been used for locking the pump frequency in the optomechanical systems, may also make some detuning states thermally unstable. Our study demonstrates an approach to characterize the detuning of the pump laser in an optomechanical resonator with thermal nonlinearity and also lays the groundwork for further understanding of dynamical effects induced by optomechanical effects.

Nonlocality consists in the existence of nonclassical correlations between local measurements. So far, it has been investigated mostly in isolated quantum systems. Here we show that nonlocal correlations are present, can be detected, and might be robust against noise also in many-body open quantum systems, both in the steady-state and in the transient regime. We further discuss the robustness of nonlocal correlations when the open quantum system undergoes repeated measurements, a relevant scenario for quantum cryptography.

We address the problem of determining the physically correct definition of the momentum and angular momentum densities in a spatially structured electromagnetic field, given that the expressions are not the same when one uses the canonical energy-momentum tensor instead of the symmetric Belinfante energy-momentum tensor in electrodynamics. This has important consequences for the interaction of matter with structured light, for example, twisted photons, and would give drastically different results for forces and angular momenta induced on small test objects. We show, with numerical estimates of the size of the effects, situations where the canonical and symmetrized forms induce very different torques or (superkick) recoil momenta on small objects or atomic rotors, over a broad range of circumstances.

We consider a quantum system with a time-independent Hamiltonian parametrized by a set of unknown parameters $\alpha$. The system is prepared in a general quantum state by an evolution operator that depends on a set of unknown parameters $P$. After the preparation, the system evolves in time, and it is characterized by a time-dependent observable $\mathcal{O}\left(t\right)$. We show that it is possible to obtain closed-form expressions for the gradients of the distance between $\mathcal{O}\left(t\right)$ and a calculated observable with respect to $\alpha , P$, and all elements of the system density matrix, whether for pure or mixed states. These gradients can be used in projected gradient descent to infer $\alpha , P$, and the relevant density matrix from dynamical observables. We combine this approach with random phase wave function approximation to obtain closed-form expressions for gradients that can be used to infer population distributions from averaged time-dependent observables in problems with a large number of quantum states participating in dynamics. The approach is illustrated by determining the temperature of molecular gas (initially, in thermal equilibrium at room temperature) from laser-induced time-dependent molecular alignment.

The edge-to-edge topological transport of the one-dimensional odd-sized Su-Schrieffer-Heeger model plays a crucial role in robust energy transfer and information processing. However, it has not been verified experimentally. Here we report on an experimental demonstration of this topological transport in a one-dimensional photonic lattice of coupled waveguides. When slowly varying the spacings of the adjacent waveguides, the robust topological transport of light from the leftmost to the rightmost channels has been successfully observed. Moreover, the transport speed and efficiency of light are enhanced considerably by introducing the exponential-type modulations of the coupling strengths. The experimental results agree well with analytical calculations and numerical simulations. This transport of light observed may serve as the basis for photonic devices of unconventional functions.

Recent studies on supersolidity in a single-component Bose-Einstein condensate (BEC) have relied on the Lee-Huang-Yang (LHY) correction for stabilization of self-bound droplets, which, however, involves a high density inside the droplets, limiting the lifetime of the supersolid. Here we propose a two-component mixture of dipolar and nondipolar BECs, such as an ${}^{166}\mathrm{Er}\text{−}{}^{87}\mathrm{Rb}$ mixture, to create and stabilize a supersolid without the LHY correction, which can suppress the atomic loss and may allow observation of the long-time dynamics of the supersolid. In such a system, supersolidity can be controlled by the difference in the trap centers between the two components.

The emerging quantum technologies rely on our ability to establish and control quantum systems in nonclassical states, exhibiting entanglement and quantum coherence. It is thus crucial to understand how entanglement and coherence can be created in the most efficient way. In this Letter we study optimal ways to create a large amount of quantum coherence via quantum channels. For this, we compare different scenarios, where the channel is acting on an incoherent state, on states which have coherence, and also on subsystems of multipartite quantum states. We show that correlations in multipartite systems do not enhance the ability of a quantum channel to create coherence. We also study the ability of quantum channels to destroy coherence, proving that a channel can destroy more coherence when acting on a subsystem of a bipartite state. Crucially, we also show that the destroyed coherence on a multipartite system can exceed the upper bound of those on the single system when the total state is entangled. Our results significantly simplify the evaluation of the coherence generating capacity of quantum channels, which we also discuss.