We present a thorough experimental investigation of the loading process of laser-cooled atoms from a magneto-optical trap into an optical dipole trap located inside a hollow-core photonic bandgap fiber, followed by propagation of the atoms therein. This, e.g., serves to identify limits to the loading efficiency and thus optical depth which is a key parameter for applications in quantum information technology. Although only limited access in 1D is available to probe atoms inside such a fiber, we demonstrate that a detailed spatially resolved characterization of the loading and trapping process along the fiber axis is possible by appropriate modification of probing techniques combined with theoretical analysis. Specifically, we demonstrate the loading of up to $2.1\times {10}^5$ atoms with a transfer efficiency of $2.1\%$ during the course of 50 ms and a peak loading rate of $4.7\times {10}^3$ atoms ${\mathrm{ms}}^{-1}$ resulting in a peak atomic number density on the order of ${10}^{12}{\mathrm{cm}}^{-3}$. Furthermore, we determine the evolution of the spatial density (profile) and ensemble temperature as it approaches its steady-state value of $T=1400\mu \mathrm{K}$, as well as loss rates, axial velocity and acceleration. The spatial resolution along the fiber axis reaches a few millimeters, which is much smaller than the typical fiber length in experiments. We compare our results to other fiber-based as well as free-space optical dipole traps and discuss the potential for further improvements.

Utilizing an adiabatic approximation method, a bipartite qudit-oscillator Hamiltonian is studied for low-spin values in both strong and ultrastrong coupling regimes. The quasiprobability densities on the hybrid factorized phase space are introduced. Integrating over a sector of the composite phase space, the quasiprobability distributions of its complementary subsystem are recovered. In the strong coupling regime, the qudit entropy displays a pattern of quasiperiodic collapses and revivals, where the latter coincide with locally minimum entropy configurations. Starting with a bipartite factorizable initial state, we observe that almost pure spin kitten type states dynamically develop at near-null values of entropy. The Hilbert-Schmidt distance measure of these states puts them metrically far away from the initial state. Other localized spin states form at locally minimum but significantly large values of entropy. The evolution to the nonclassical transitory spin states is displayed via the diagonal spin ${\mathrm{P}}_{\mathcal{Q}}$ representation. As another manifestation of nonclassicality the emergence of the spin-squeezed states during the bipartite evolution is observed. In the ultrastrong coupling domain, a large number of interaction-dependent modes and their harmonics are generated. The consequent randomization of the phases eliminates the quasiperiodicity of the system which is now driven towards a stabilization of the entropy that also undergoes stochastic fluctuations around a suitably stabilized value. In both the strong and ultrastrong coupling realms, antibunching of the photoemission events is realized particularly for the small spin values.

Quantum nonlocality offers a secure way to produce random numbers: Their unpredictability is intrinsic and can be certified just by observing the statistic of the measurement outcomes, without assumptions on how they are produced. To do this, entangled pairs are generated and measured to violate a Bell inequality with the outcome statistics. However, after a projective quantum measurement, entanglement is entirely destroyed and cannot be used again. This fact poses an upper bound to the amount of randomness that can be produced from each quantum state when projective measurements are employed. Instead, by using weak measurements, some entanglement can be maintained and reutilized, and a sequence of weak measurements can extract an unbounded amount of randomness from a single state as predicted in [Phys. Rev. A 95, 020102(R) (2017)]. We study the feasibility of these weak measurements, analyze the robustness to imperfections in the quantum state they are applied to, and then test them using an optical setup based on polarization-entangled photon pairs. We show that the weak measurements are realizable, but can improve the performance of randomness generation only in close-to-ideal conditions.

Quantum error correction protocols have been developed to offset the high sensitivity to noise inherent in quantum systems. However, much is still unknown about the behavior of a quantum error-correcting code under general noise, including noisy measurements. This lack of knowledge is largely due to the computational cost of simulating quantum systems large enough to perform nontrivial encodings. In this paper, we develop general methods for incorporating noisy measurement operations into simulations of quantum error-correcting codes and show that measurement errors on readout qubits manifest as a renormalization on the effective logical noise. We also derive general methods for reducing the computational complexity of calculating the exact effective logical noise by many orders of magnitude. This reduction is achieved by determining when different recovery operations produce equivalent logical noise. These methods could also be used to better approximate soft decoding schemes for concatenated codes or to reduce the size of a lookup table to speed up the error correction step in implementations of quantum error-correcting codes. We give examples of such reductions for the three-qubit, five-qubit, Steane, concatenated, and toric codes.

We investigate superpositions of two-mode squeezed states (TMSSs), which have potential applications in quantum information processing and quantum sensing. We study some properties of these nonclassical states such as the statistics of each mode and the degree of entanglement between the two modes, which can be higher than that of a TMSS with the same degree of squeezing. The states we consider can be prepared by inducing two-mode Jaynes-Cummings and anti-Jaynes-Cummings interactions in a system of two modes and a spin-$\frac{1}{2}$ particle, for instance, in the trapped ion domain, as described here. We show that when two harmonic oscillators are prepared in a superposition of two TMSSs, each reduced single-mode state can be advantageously employed to sense arbitrary displacements of the mode in phase space. The Wigner function of this reduced state exhibits a symmetrical peak centered at the phase-space origin, which has the convenient peculiarity of getting narrower in both quadratures simultaneously as the average photon number increases. This narrow peak can be used as the pointer of our quantum sensor, with its position in phase space indicating the displacement undergone by the oscillator.

Phenomena involved in magnons have attracted extensive attention for several decades because of their challenging physical properties, powerful capabilities, and potential applications in signal processing in the microwave frequency range. In this work study we study the steady-state dynamic processes of a hybrid quantum system based on magnonics, where the magnon mode couples dispersively to a superconducting qubit. We find that the dispersive coupling between the qubit and magnons causes the system to be bistable and we discuss in detail an interesting phenomenon of dispersive-coupling-induced absorption of magnons. Furthermore, a feasible scheme to measure the qubit-magnon dispersive shift by employing the absorption curves of the probe field is proposed. The results presented here are of great significance to the investigation of the steady-state dynamic behaviors of quantum magnonics in a strong-dispersive regime and may provide a theoretical reference for the realization of quantum sensing of magnons.

The generation of photons from the vacuum by means of the movement of a mirror is known as the dynamical Casimir effect (DCE). In general, this phenomenon is effectively described by a field with time-dependent boundary conditions. Alternatively, we introduce a microscopic model of the DCE capable of capturing the essential features of the effect with no time-dependent boundary conditions. Besides the field, such a model comprises a subsystem representing the mirror's internal structure. In this work, we study one of the most straightforward mirror systems: a qubit moving in a cavity and coupled to one of the bosonic modes. We find that under certain conditions on the qubit's movement that do not depend on its physical properties, a large number of photons may be generated without changing the qubit state, as should be expected for a microscopic model of the mirror.

The quantum walk is a powerful tool to develop quantum algorithms, which usually are based on searching for a vertex in a graph with multiple marked vertices, with Ambainis's quantum algorithm for solving the element distinctness problem being the most shining example. In this work, we address the problem of calculating analytical expressions of the time complexity of finding a marked vertex using quantum-walk-based search algorithms with multiple marked vertices on arbitrary graphs, extending previous analytical methods based on Szegedy's quantum walk, which can be applied only to bipartite graphs. Two examples based on the coined quantum walk on two-dimensional lattices and hypercubes show the details of our method.

Quantum nonlocality, one of the most important features of quantum mechanics, is normally connected in experiments with the violation of Bell-Clauser-Horne (Bell-CH) inequalities. We propose effective methods for the rearrangement and linear inequality to prove a large variety of Bell-CH inequalities. We also derive a set of Bell-CH inequalities using these methods which can be violated in some quantum entangled states.

Quantum speed limits are rigorous estimates on how fast a state of a quantum system can depart from the initial state in the course of quantum evolution. Most known quantum speed limits, including the celebrated Mandelstam-Tamm and Margolus-Levitin ones, are general bounds applicable to arbitrary initial states. However, when applied to mixed states of many-body systems, they, as a rule, dramatically overestimate the speed of quantum evolution and fail to provide meaningful bounds in the thermodynamic limit. Here we derive a quantum speed limit for a closed system initially prepared in a thermal state and evolving under a time-dependent Hamiltonian. This quantum speed limit exploits the structure of the thermal state and, in particular, explicitly depends on the temperature. In a broad class of many-body setups it proves to be drastically stronger than general quantum speed limits.

All metal objects support fluctuating currents that are responsible for evanescent-wave Johnson noise in their vicinity due to both thermal and quantum effects. The noise fields can decohere qubits. It is quantified by the average value of $\mathbf{B}(\mathit{x},t)\mathbf{B}({\mathit{x}}^{\prime },t^{\prime })$ and its time Fourier transform. We develop the formalism particularly for objects whose dimensions are small compared with the skin depth, which is the appropriate regime for nanoscale devices. This leads to a general and surprisingly simple formula for the two-point noise correlation function of an object of arbitrary shape. This formula has a clear physical interpretation in terms of induced currents in the object, and it can be the basis for straightforward numerical evaluation. We discuss its experimental implications. For a sphere, a solution is given in closed form in terms of a generalized multipole expansion. Plots of the solution illustrate the physical principles involved. We give examples of how the spatial pattern of noise can affect quantum information processing in nearby qubits. The theory implies that if the qubit system is miniaturized to a scale $D$, then decoherence rates of qubits scale as $1/D$.

Quantum technologies exploiting bipartite entanglement could be made more efficient by using states having the minimum amount of energy for a given entanglement degree. Here, we study how to generate these states in the case of a bipartite system of arbitrary finite dimension either by applying a unitary transformation to its ground state or through a zero-temperature thermalization protocol based on turning on and off a suitable interaction term between the subsystems. In particular, we explicitly identify three possible unitary operators and five possible interaction terms. On one hand, two of the three unitary transformations turn out to be easily decomposable in terms of local elementary operations and a single nonlocal one, making their implementation easier. On the other hand, since the thermalization procedures can be easily adapted to generate many different states, we numerically show that, for each degree of entanglement, generating minimum-energy entangled states costs, in general, less than generating the vast majority of the other states.

Magnetic Johnson-Nyquist noise (JNN) originating from metal electrodes, used to create a static electric field in neutron electric-dipole-moment (nEDM) experiments, may limit the sensitivity of measurements. We present here a dedicated study on JNN applied to a large-scale long-measurement-time experiment with the implementation of a comagnetometry. In this study, we derive surface- and volume-averaged root-mean-square normal noise amplitudes at a certain frequency bandwidth for a cylindrical geometry. In addition, we model the source of noise as a finite number of current dipoles and demonstrate a method to simulate temporal and three-dimensional spatial dependencies of JNN. The calculations are applied to estimate the impact of JNN on measurements with the new apparatus, n2EDM, at the Paul Scherrer Institute. We demonstrate that the performances of the optically pumped ${}^{133}\mathrm{Cs}$ magnetometers and ${}^{199}\mathrm{Hg}$ comagnetometers, which will be used in the apparatus, are not limited by JNN. Further, we find that, in measurements deploying a comagnetometer system, the impact of JNN is negligible for nEDM searches down to a sensitivity of $4\times {10}^{-28}e\mathrm{cm}$ in a single measurement; therefore, the use of economically and mechanically favored solid aluminum electrodes is possible.

Based on the exact relativistic multipole calculations using pure Coulomb and Debye-Hückel potentials, discussions of bremsstrahlung cross sections and corresponding emitted photon angular distributions from fully stripped tungsten (${\mathrm{W}}^{74+}$) are given for electron scattering at energies of 10–150 keV. Nonrelativistic dipole calculations are also presented, showing that for the above energy region, the nonrelativistic results are completely invalid and the relativistic and multipole effects must be taken into account. The reduction of bremsstrahlung cross sections due to screening of the nuclear charge is studied using the relativistic multipole method at different values of the screening length and photon energy. With an increase of electron scattering energy, the effect of the screening potential on both the dynamics of fast free electrons and the cross sections is reduced. The peak of the angular distribution of bremsstrahlung would shift towards smaller emission angles when the screening strength is reduced.

We employ a relativistic coupled-cluster theory to compute the ground-state electric dipole polarizability $\alpha$ and the electron correlation energy of the superheavy elements Cn, ${\mathrm{Nh}}^{+}$, and Og. To assess the electron correlation trends with $Z$, we also compute the correlation energies of the three lighter homologs for each of the elements. In the computations, we use the Dirac-Coulomb-Breit Hamiltonian and incorporate the quantum electrodynamical corrections from the Uehling potential and the self-energy. The effects of triple excitations are considered perturbatively in the theory. Our recommended values of $\alpha$ are in good agreement with previous theoretical results. As expected, the dominant contribution is from the valence electrons. Except for Cn and Og, the contribution from the Breit interaction decreases with $Z$. For the vacuum polarization and self-energy corrections, the contributions increase with $Z$. To understand the correlation energy trends better, we also compute the correlation energy with the relativistic many-body perturbation theory.

We present a comprehensive study of photoionization of OH radicals from the ground and first excited electronic states. Cross sections and asymmetry parameters for the valence orbitals are reported for photon energies from near the threshold to 60 eV. The calculations have been performed with the $R$-matrix method. Two different continuum basis types, namely, the Gaussian and $B$-spline bases, are used to assess the quality of the description of the continuum. The results show both types of continuum bases provide a similar quality description in the near threshold resonant region, but the latter can predict more reliable photoionization observables in higher-energy regions. Our calculations are compared with available experimental and theoretical results, and the discussions are provided.

Combining two Bose-Einstein condensates (BECs) may result in a miscible or immiscible mixture, or even a violent implosion. We theoretically demonstrate that dipolar two-component BECs produce far richer physics than their nondipolar counterparts. Intriguingly, when both components have equivalent dipoles, the transition to immiscibility is largely unaffected by dipolar physics, yet the dipoles maximally affect stability. Conversely, antiparallel dipoles strongly affect miscibility but have little effect on stability. By performing three-dimensional calculations of the ground states and their excitations, we find strong dependencies on the confinement geometry. We explore and elucidate the various phononic and rotonic phase transitions, as well as symmetry-preserving crossovers.

In this paper we formulate the quantum theory for longitudinal (rotational free) collective electron modes, the so-called plasmons. Starting from the jellium dispersion relation, a first-quantized wave-mechanical theory is established. We show that plasmon quantum (quasi)particles are charged bosons described by complex Klein-Gordon fields satisfying a “relativistic” scalar Klein-Gordon equation in which the speed of light $c$ is replaced by a velocity $a$ on the order of the Fermi velocity. Based on a formally ($c\to a$) covariant description, the first-quantized theory is extended to the second-quantized level via a Lagrangian formalism. We show how the second-quantized theory enables the study of the plasmon quantum particles interaction with an electromagnetic gauge field via a modification of the free plasmon Lagrangian density by the minimal coupling principle. Utilizing the Weyl expansion, first- and second-quantized theories for surface-plasmon quantum particles are established in close analogy to those of the bulk plasmon quantum particle. This work opens up for the study of, e.g., squeezed, entangled, and coherent plasmon states in line with what has been studied theoretically and experimentally for photons for many years.