The most important law of radioactivity is that of the exponential decay. In the realm of quantum mechanics, however, this decay law is neither rigorous nor fundamental. The deviations from the exponential decay have been observed experimentally at the early stage of a decay process, but there is little evidence for nonexponential behavior at long times. Yet such long-term nonexponentiality is expected theoretically to probe the nonresonant background components of the initial wave function which preserve the structural interference and the memory of how the state was created. In this paper, we propose new observables that can be used for experimental investigations of the post-exponential decay regime, including the decay of threshold resonances, particle correlations in three-body decays, and interference between near-lying resonances. While the specific examples presented in this work pertain to atomic nuclei, the properties of nonexponential decay are generic, i.e., they apply to other many-body open quantum systems, such as hadrons, atoms, molecules, and nanostructures.

We consider a quantum limit-cycle oscillator implemented in a spin system whose quantization axis is slowly rotated. Using a kinematic approach to define geometric phases in nonunitary evolution, we show that the quantum limit-cycle oscillator attains a geometric phase when the rotation is sufficiently slow. In the presence of an external signal, the geometric phase as a function of the signal strength and the detuning between the signal and the natural frequency of oscillation shows a structure that is strikingly similar to the Arnold tongue of synchronization. Surprisingly, this structure vanishes together with the Arnold tongue when the system is in a parameter regime of synchronization blockade. We derive an analytic expression for the geometric phase of this system, valid in the limit of slow rotation of the quantization axis and weak external signal strength, and we provide an intuitive interpretation for this surprising effect.

In this paper, we determine the volume-fraction reduction of sheared fragile glass-forming liquids. We consider a group of hypothetical systems that consist of particles with anisotropic size modulations yet have almost the same average particle configuration as actual systems under shear flow. Our molecular dynamics (MD) simulations demonstrate that one specific hypothetical system can reproduce the relaxation dynamics of an actual sheared system, and we identify the shear-flow effect on the particle size with anisotropic size modulation of this specific system. Then based on the determination of the particle size and the resultant volume fraction, we rationalize how slight decreases in the volume fraction significantly reduce the viscosity and provide a nonlinear constitutive equation. Notably, the obtained rheological predictions, including the crossover shear rate from Newtonian to non-Newtonian behavior, can be expressed only in terms of experimental observables, showing good agreement with the MD simulation results. Our perspective on the volume fraction under shear flow may provide insights into the conventional concept of free volume.

We perform the projector quantum Monte Carlo (QMC) simulation to study the trion formation and quantum phase transition in the half filled attractive SU(3) Hubbard model on a honeycomb lattice. With increasing attractive Hubbard interaction, our simulations demonstrate a continuous quantum phase transition from the semimetal to charge-density wave (CDW) at the critical coupling $U_c/t=-1.52\left(2\right)$. The critical exponents $\nu =0.82\left(3\right)$ and $\eta =0.58\left(4\right)$ determined by the QMC simulation remarkably disagree with those of the $N=3$ chiral Ising universality class suggested by the effective Gross-Neveu-Yukawa (GNY) theory, but coincide with the $N=1$ chiral Ising universality class. In the CDW phase, we show that on-site and off-site trions coexist and the off-site trion forms a local bond state. Our work not only illustrates the formation of off-site trions in two-dimensional Hubbard model but also raises doubts about the extent of applicability of GNY model on attractive SU(3) Dirac fermions.

In the context of quantum integrated photonics, this work investigates the quantum properties of multimode light generated by silicon and silicon nitride microresonators pumped in pulsed regime. The developed theoretical model provides a comprehensive description of the generated quantum states. Remarkably, it shows that a full measurement of states carrying optimal squeezing levels is not accessible to standard homodyne detection, thus leaving hidden part of generated quantum features. By unveiling and discussing this behavior and possible strategies to amend it, this work proves itself essential to future quantum applications exploiting microresonators as sources of multimode states.

${\mathrm{SrTiO}}_3$ is known for its proximity to a ferroelectric phase and for showing an “optimal” doping for superconductivity with a characteristic domelike behavior resembling systems close to a quantum critical point. Several mechanisms have been proposed to link these phenomena, but the abundance of undetermined parameters prevents a definite assessment. Here, we use ab initio computations supplemented with a microscopic model to study the linear coupling between conduction electrons and the ferroelectric soft transverse modes allowed in the presence of spin-orbit coupling. We find a robust Rashba-like coupling, which can become surprisingly strong for particular forms of the polar eigenvector. We characterize this sensitivity for general eigenvectors and, for the particular form deduced by hyper-Raman scattering experiments, we find a Bardeen-Cooper-Schrieffer pairing coupling constant of the right order of magnitude to support superconductivity. The ab initio computations enable us to go beyond the linear-in-momentum conventional Rashba-like interaction and naturally explain the dome behavior including a characteristic asymmetry. The dome is attributed to a momentum-dependent quenching of the angular momentum due to a competition between spin-orbit and hopping energies. The optimum density for having maximum $T_c$ results in rather good agreement with experiments without free parameters. These results make the generalized Rashba dynamic coupling to the ferroelectric soft mode a compelling pairing mechanism to understand bulk superconductivity in doped ${\mathrm{SrTiO}}_3$.

Endocytosis is an essential biological process for the trafficking of macromolecules in cells. In yeast cells, this involves the invagination of a tubular membrane and the formation of endocytic vesicles. The crescent-shaped BAR proteins are generally assumed to squeeze the tubular membrane and pinch off the vesicle. Here, we theoretically investigate how BAR proteins help drive membrane fission via generating anisotropic curvatures. We show that increasing the isotropic spontaneous curvature at a localized region on the side of a tubular membrane cannot induce membrane fission if the coating area is small. However, a tubular membrane coated with proteins that generate anisotropic curvatures are prone to experience an hourglass-shaped or tube-shaped necking process, which leads to membrane fission. In addition, we propose an experimental method to determine the type of anisotropic curvatures of a protein.

Quantum liquidlike states of matter have been realized in an ongoing series of experiments with ultracold Bose gases. Using a combination of analytical and numerical methods we identify the specific criteria for the existence of dark solitons in beyond-mean-field binary condensates, revealing how these excitations exist for both repulsive and attractive interactions, the latter leading to dark quantum droplets with properties intermediate between a dark soliton and a quantum droplet. The phenomenology of these excitations is explored within the full parameter space of the model, revealing a spatial profile of the excitation that differs significantly from the Zakharov-Shabat soliton, leading to a negative effective mass that is enhanced in the presence of the quantum fluctuations. Finally the dynamics of pairs of the excitations are explored, showing nonintegrable dynamics and dark soliton bound states in the attractive regime.

Transcorrelated methods provide an efficient way of partially transferring the description of electronic correlations from the ground-state wave function directly into the underlying Hamiltonian. In particular, Dobrautz et al. [Phys. Rev. B 99, 075119 (2019)] have demonstrated that the use of momentum-space representation, combined with a nonunitary similarity transformation, results in a Hubbard Hamiltonian that possesses a significantly more “compact” ground-state wave function, dominated by a single Slater determinant. This compactness/single-reference character greatly facilitates electronic structure calculations. As a consequence, however, the Hamiltonian becomes non-Hermitian, posing problems for quantum algorithms based on the variational principle. We overcome these limitations with the Ansatz-based quantum imaginary-time evolution algorithm and apply the transcorrelated method in the context of digital quantum computing. We demonstrate that this approach enables up to four orders of magnitude more accurate and compact solutions in various instances of the Hubbard model at intermediate interaction strength ($U/t=4$), enabling the use of shallower quantum circuits for wave-function Ansätzes. In addition, we propose a more efficient implementation of the quantum imaginary-time evolution algorithm in quantum circuits that is tailored to non-Hermitian problems. To validate our approach, we perform hardware experiments on the ibmq_lima quantum computer. Our work paves the way for the use of exact transcorrelated methods for the simulations of ab initio systems on quantum computers.

The simulation of quantum dynamics on a digital quantum computer with parametrized circuits has widespread applications in fundamental and applied physics and chemistry. In this context, using the hybrid quantum-classical algorithm, combining classical optimizers and quantum computers, is a competitive strategy for solving specific problems. We put forward its use for optimal quantum control. We simulate the wave-packet expansion of a trapped quantum particle on a quantum device with a finite number of qubits. We then use circuit learning based on gradient descent to work out the intrinsic connection between the control phase transition and the quantum speed limit imposed by unitary dynamics. We further discuss the robustness of our method against errors and demonstrate the absence of barren plateaus in the circuit. The combination of digital quantum simulation and hybrid circuit learning opens up new prospects for quantum optimal control.

The efficient transfer of a magnetically guided positron beam into a region of closed magnetic field lines is nontrivial. An $\mathbf{E}\times \mathbf{B}$-drift technique has previously been used effectively to inject a low-energy (5- to 20-eV) positron beam into the confinement region of a permanent-magnet-based dipole trap. To complement and extend that strategy, we have investigated an approach in which a high-energy ($\sim 350$-eV) positron beam is remoderated in a SiC crystal immediately outside the confinement region; the reemitted low-energy positrons are then drift injected. Thus we effectively combine positron remoderation and $\mathbf{E}\times \mathbf{B}$-drift injection in the same spatial region. Initial tests with this scheme were shown to have an overall efficiency of up to $15(\pm 1)\%$. Positron trajectory simulations enabled us to account for various loss mechanisms and thereby identify means to improve this in future implementations. This method adds further flexibility to the search for an efficient injection scheme into toroidal magnetic traps with the mission of confining electron-positron pair plasmas.