We analyze the efficiency fluctuations of a coherent quantum heat engine coupled to a unimodal cavity using a standard full-counting statistics procedure. The engine's most likely efficiency obtained by computing the large-deviation function corresponds to the quantum efficiency obtained by defining a useful work obtainable from a steady-state fluctuation theorem. The most likely efficiency is independent of the noise-induced coherences. The ratio between coherent and incoherent efficiency cumulants is found to be constant and independent of the stochastic efficiency, which we prove analytically. We also numerically demonstrate that treating the efficiency as a stochastic variable allows the enhancement of constancy only in the presence of coherences. At the engine's most likely efficiency, there is no suppression of the second efficiency cumulant, leading to a robust constancy. We also report the existence of a lower bound on the second efficiency cumulant.

Continuous-time quantum walks (CTQWs) play a crucial role in quantum computing, especially for designing quantum algorithms. However, how to efficiently implement CTQWs is a challenging issue. In this paper, we study the implementation of CTQWs on sparse graphs, i.e., constructing efficient quantum circuits for implementing the unitary operator $e^{-iHt}$, where $H=\gamma A$ ($\gamma$ is a constant and $A$ corresponds to the adjacency matrix of a graph). Our result is, for a $d$-sparse graph with $N$ vertices and evolution time $t$, we can approximate $e^{-iHt}$ by a quantum circuit with gate complexity $(d^3\parallel H\parallel tN{log}_2{\left(N\right))}^{1+o\left(1\right)}$, compared to the general Pauli decomposition, which scales like $(\parallel H\parallel t{N}^4{log}_2{\left(N\right))}^{1+o\left(1\right)}$. For sparse graphs, for instance, $d=O\left(1\right)$, we obtain a noticeable improvement. Interestingly, our technique is related to graph decomposition. More specifically, we decompose the graph into a union of star graphs, and correspondingly, the Hamiltonian $H$ can be represented as the sum of some Hamiltonians $H_j$, where each $e^{-i{H}_{j}t}$ is a CTQW on a star graph which can be implemented efficiently.

In quantum thermodynamics, the noncommutativity of conserved quantities is a key aspect of generalized reservoirs. Squeezed-thermal reservoirs, characterized as non-Abelian thermal reservoirs with noncommuting constant quantities (charges), are redefined in this study using SU(1,1) Lie algebra generators. Our findings suggest that these reservoirs can be more accurately described as generalized Gibbs ensembles featuring three noncommuting operators instead of the generally assumed two. By incorporating three charges and the quantum phase-space method to describe dissipation into such a reservoir, two expressions for Wigner entropy production are derived: one based on the stationary mean value of the exchanged charges and another based on a single operator's stationary mean value—the Casimir operator of the Lie algebra. Analysis of these expressions across two distinct models sheds light on equilibration conditions with generalized reservoirs during the interconversion of noncommuting charges.

A recent study [Ghassemizadeh et al., Phys. Rev. B 106, 174111 (2022)] hypothesized the assembly, along a specific type of one-dimensional defects of diamond, of chains of nitrogen-vacancy (NV) centers, potentially enabling the creation of qubit registers via their dipole-coupled electron spins. Here we investigate the connectivity of chains of up to 10 coupled spins, mediated by the bi- and multipartite entanglement of their eigenstates. Rather conveniently, for regularly spaced spins the vast majority of the eigenstates display strong connectivity, especially towards the center of the spectrum and for longer chains. Furthermore, positional disorder can change, and possibly reduce, the connectivity of the register but seldom suppresses it.

Realizing photonic graph states, crucial in various quantum protocols, is challenging due to the absence of deterministic entangling gates in linear optics. To address this, emitter qubits have been leveraged to establish and transfer the entanglement to photons. We introduce an optimization method for such protocols based on the local Clifford equivalency of states and the graph theoretical correlations of the generation cost parameters. Employing this method, we achieve a 50% reduction in use of the two-qubit gates for generation of the arbitrary large repeater graph states and similar significant reductions in the total gate count for generation of random dense graphs.

We present a theoretical study of the coincidence detection process of molecular fragments arising from the dissociation of an excited molecule. We follow an approach proposed in a previous work in which we computed the probability of coincidence detection by applying two detection operators to the molecular wave function in the asymptotic regime, where the atoms are infinitely separated. That approach led us to predict the existence of interference in the probability distribution for the temporal coincidence spectrum due to the different possible rotational levels during the molecular dissociation. In this work, we investigate the detection of $\text{H}\left(2s\right)$ atoms arising from the dissociation of the hydrogen molecule in the doubly excited state $1Q_2{}^1\mathrm{\Sigma }_g^{+}$ considering a thermal molecular sample excited to the state of interest by electron impact. We refine the previous results, bringing them closer to what is expected experimentally, by computing the occupations of each relevant rotational level from the determination of the generalized oscillator strength with rovibrational resolution.

We theoretically investigate the control of superfluid currents for ultracold gases confined in a ring lattice. The system is assumed to be in the presence of an artificial magnetic flux piercing the ring. By introducing ac-driven local energy shifts, we explore their potential to influence both the intensity and direction of supercurrents. Our analysis reveals that detuning of a resonance frequency associated with a photon-assisted tunneling process can be used to steer the current direction of particles in a ring. In particular, through a theoretical analysis of a three-site ring, we show that a detuning of the resonance frequency can couple the ground state to transporting Floquet states with current directions that depend on the sign of the detuning. This is achieved by ramping the ac-driving amplitude from zero, enabling the coupling of the ground state to the Floquet states. Moreover, our study shows that it is possible to change the direction of the current using either the detuning or the magnetic flux as a control parameter, thus providing extra freedom in the manipulation of the currents. These findings not only contribute to the understanding of transport behavior of matter waves in the presence of local energy shifts but also offer prospects for quantum control techniques in atomtronics and related fields.

Low-dimensional ultracold gases are created in the laboratory by confining three-dimensional (3D) gases inside highly anisotropic trapping potentials. Such trap geometries not only provide access to simulating 1D and 2D physics, but also can be used to study how the character of excitations cross over to three dimensions at higher energy. In this work we study the signature in radio-frequency (rf) spectroscopy for both the 1D-to-3D and the 2D-to-3D crossovers, in spin-polarized Fermi gases. We solve the two-body scattering $T$ matrix in the presence of strong harmonic confinement and use it to evaluate both the high-frequency rf transfer rate and the two-body bound-state energies. These general expressions are then compared both to the low-dimensional limit and to the 3D limit. We find that in order to understand the dimensional crossover for spin-polarized Fermi gases with $p$-wave interactions, one needs to take into account an emergent $s$-wave interaction.

We study the impact of destructive interference on the collective dynamics of two distant three-level V-type systems coupled to a waveguide in a strong-coupling regime in which the rotating-wave approximation (RWA) and Markovian approximation are inapplicable. By employing a polaronlike unitary transformation, we derive a transformed RWA Hamiltonian and combine it with the resolvent operator to study the non-Markovian dynamics and derive the condition under which the bound states exist due to the destructive interference, which can be realized by tuning the distance between the two V systems. We use a time-dependent variational approach with the multiple Davydov ansatz to numerically validate the analytical predictions and reveal the dynamical signature of bound states. We illustrate that the populations of symmetric and antisymmetric single-excitation states can be trapped in the long-time limit due to the bound states or possess a long lifetime due to subradiance. Moreover, the excitation probability can exhibit long-lived oscillation due to the coexistence of a bound state and a subradiant state. Our results offer insights into the destructive interference in distant multilevel emitters in a strong-coupling regime.

In non-Hermitian systems, particularly those adhering to $\mathcal{PT}$ symmetry, exceptional points (EPs) are critical junctures wherein eigenvalues and eigenvectors coalesce. These points induce the convergence of eigenmodes in waveguiding systems, resulting in unique dispersion features and remarkable effects such as slow light. In a configuration comprising two coupled waveguides, EPs can be achieved via mechanisms involving balanced gain-loss modulation or contradirectional modal interference. By leveraging the latter, in homogeneous chirowaveguides without any reliance on periodicity, we demonstrate the signature phase transitions induced by negative refraction due to giant chirality, which does not necessitate the simultaneous negativity of the permittivity and permeability. Our approach offers advantages over traditional $\mathcal{PT}$-symmetric and negative-refractive-index waveguides, as it does not pose the manufacturing difficulties of balancing the creation and absorption of photons while providing opportunities for remarkable light manipulation due to the presence of chirality. Experimental implementations of metamedia with giant and, moreover, controllable chirality indicate that our medium is well within reach of current technology.

We propose a general, fully gate-based quantum algorithm for counterdiabatic driving. The algorithm does not depend on heuristics as in previous variational methods and exploits regularization of the adiabatic gauge potential to suppress only the transitions from the eigenstate of interest. This allows for a rigorous quantum gate complexity upper bound in terms of the minimum gap $\mathrm{\Delta }$ around this eigenstate. We find that, in the worst case, the algorithm requires at most $\tilde{O}\left\{{\mathrm{\Delta }}^{-[3+o(1\left)\right]}{\epsilon }^{-[1+o(1\left)\right]}\right\}$ quantum gates to achieve a target state fidelity of at least $1-{\epsilon }^2$, where $\mathrm{\Delta }$ is the minimum spectral gap. In certain cases, the gap dependence can be improved to quadratic.

The speed limit on entanglement is defined as the maximal rate at which entanglement can be generated or degraded in a physical process. We derive the speed limits on entanglement, using the relative entropy of entanglement and trace-distance entanglement, for unitary and arbitrary quantum dynamics, where we assume that the dynamics of the closest separable state can be approximately described by the closest separable dynamics of the actual dynamics of the system. For the unitary dynamics of isolated bipartite systems which are described by pure states, the rate of entanglement production is bounded by the product of fluctuations of the system's driving Hamiltonian and the surprisal operator, with an additional term reflecting the time-dependent nature of the closest separable state. Removing restrictions on the purity of the input and on the unitarity of the evolution, the two terms in the bound get suitably altered. Furthermore, we find a lower bound on the time required to generate or degrade a certain amount of entanglement by arbitrary quantum dynamics. We demonstrate the tightness of our speed limits on entanglement by considering quantum processes of practical interest.

Here we report the ${}^{4}S_{3/2}\leftrightarrow {}^2{D}_{3/2}$ and ${}^{4}S_{3/2}\leftrightarrow {}^2{D}_{5/2}$ transitions in a series of ions with $n{s}^{2}n{p}^3$ configurations, which could serve as candidates for ionic optical clocks. Using the configuration interaction with the many-body perturbation theory method for calculation, we predict various properties such as lifetime, hyperfine-structure constants, and static electric dipole scalar polarizabilities. Calculation results reveal the differential static electric dipole scalar polarizabilities of two clock transitions are either negative or slightly positive (in the range of $[-0.8,0.2]a_0^3$). An analysis of several systematic effects shows the insensitivity of these clock transitions to external fields. Furthermore, both clock transitions in the ${\mathrm{S}}^{+}$ ion may exhibit negative differential static electric dipole scalar polarizabilities, which may enable the formation of a synthetic frequency which is insensitive to environmental temperature uncertainty and has blackbody-radiation-shift uncertainty below ${10}^{-19}$.

We numerically examine the corotation of two parallel quantized vortices in a self-gravitating Bose-Einstein condensate (BEC) employing the Gross-Pitaevskii-Poisson equations. The long-range gravitationally attractive interaction allows the BEC to self-confine without the need for external potentials, while the density-dependence of the gravitational potential induces intriguing behaviors in the quantized vortices. The aim of this study is to provide a clue for understanding the corotation of two quantized vortices under the influence of gravitational interactions. The corotation of two quantized vortices is coupled with collective modes of the BEC, which markedly differs from the behavior observed in typical BECs confined by an external potential. The rotational period increases linearly with the initial position from the center of the BEC. This deviation from the quadratic increase observed in a uniform BEC suggests that the gravitational interaction exerts a drag effect on the rotating quantized vortices. The two closely positioned quantized vortices rotate along elliptical orbits with radial fluctuations. However, when the quantized vortices are initially positioned beyond a critical radius comparable to their core sizes, their trajectory transitions into an outward spiral, implying the onset of effective dissipation. Our findings demonstrate that the radial fluctuations of the quantized vortex resonate with the quadrupole mode of the BEC, giving rise to a dissipation mechanism.

We report on an experimental platform based on an atom chip encompassing a coplanar waveguide which enables the manipulation of a quantum gas of sodium atoms with strong microwave fields. We describe the production with this setup of a very elongated degenerate quantum gas with typically ${10}^6$ atoms that can be prepared all along the crossover from the three-dimensional to the one-dimensional regime, depending on the atom number and trapping geometry. Using the microwave field radiated by the waveguide, we drive Rabi oscillations between the hyperfine ground states, with the atoms trapped at various distances from the waveguide. At the closest position explored, the field amplitude exceeds 5 G, corresponding to a Rabi frequency on the strongest transition larger than $6\mathrm{MHz}$. This enables fast manipulation of the atomic internal state.

Machine learning for predicting control landscape maps of full quantum molecular dynamics is examined through a case study of the laser-induced three-dimensional (3D) alignment of asymmetric-top molecules, an essential technique for observing and/or manipulating molecular dynamics in a molecule-fixed frame. We consider the “prolate-type” asymmetric-top molecules with the asymmetry parameters $-1<\kappa <0$ and the $C_{2v}$ symmetry in the low-temperature limiting case, which are aligned by using mutually orthogonal linearly polarized double laser pulses. The landscape map for each molecule consists of 6000 pixels, each pixel of which represents the maximum degree of alignment achieved by each set of control parameters. After examining ways to deal with the markedly different molecular parameters in a unified manner for suitably training a convolutional neural network (CNN) model, we train the CNN model by using 55 training sample molecules to predict the control landscape maps of 35 test sample molecules with reasonably high accuracy. As the predicted landscape maps provide a big picture of the alignment control, we examine, for example, the effectiveness of the double-pulse control scheme in terms of molecular parameters.

We demonstrate excitation of metastable krypton and xenon beams using a vacuum ultraviolet lamp and directly compare the performance of this method to metastable excitation based on a radio-frequency-driven plasma discharge. In our apparatus, lamp-based metastable excitation outperforms the plasma discharge across a wide range of beam flux values relevant for atom trap trace analysis (ATTA). Moreover, we do not observe significant degradation in lamp performance after over 160 h of operation. We find that lamp-based excitation is particularly advantageous at the smallest and largest beam fluxes tested, demonstrating the utility of this approach both for improving krypton ATTA and for enabling the detection of radioactive xenon isotopes using ATTA. Finally, we demonstrate an additional enhancement to lamp-based metastable excitation efficiency and stability by applying an external magnetic field.

A daisylike shape of the boundary of hollow optical fibers is proposed, which allows coherent transport of wave beams in an out-of-phase mode. The effective area of the detected mode is proportional to the number of petals, allowing operation with wave beam powers significantly higher than the critical self-focusing power. Numerical simulations of out-of-phase mode propagation show good agreement with analytical estimates. In this case, the mode found remains stable in a nonlinear medium. The existence of this supermode promises unprecedented power scaling capabilities compared to fiber-optic multicore fibers for the same number of cores. This is because the critical power in gases is three orders of magnitude higher than in solids. This advancement will enable the control of terawatt-level laser radiation in waveguide systems.