We describe the three-body loss coefficient of identical fermions with $p$-wave interactions using a set of rate equations in which three-body recombination happens via an indirect process. Our theoretical treatment explains experimental results just above the universal scaling law regime of weak interactions. Furthermore, we theoretically extend and experimentally verify the rate equation model for the case of atoms trapped in two dimensions. Moreover, we find that the three-body loss coefficient in a two-dimensional trap is proportional to $A_p^3$ in the weakly interacting regime, where $A_p$ is the scattering area. Our results are useful in understanding three-body physics with $p$-wave interactions.

We study the ${\mathrm{CN}}^{-}$ production from cyanogen using an experimental setup which combines the trochoidal electron monochromator with the velocity map imaging detection spectrometer. The setup is quantitative and the absolute dissociative electron attachment cross sections are determined using the relative flow technique. The peak value of the dissociative electron attachment cross section is 0.16 ${\AA }^2$ at 5.2 eV. The angular distribution of fragments changes its character considerably with varying electron energies and reveals different symmetries of the involved resonant states.

We extend the two-center wave-packet convergent close-coupling method to proton collisions with helium. The target is treated as a three-body system, where correlations between the electrons are taken into account. We apply a frozen-core approximation, where one of the electrons is described by the ${\mathrm{He}}^{+}1s$ orbital, and obtain the helium singlet wave functions as well as the energy levels using a numerical approach. The wave-packet approach is used to discretize the continuum of the target and the hydrogen atom formed after electron capture by the projectile. Convergence of the results is studied in terms of the included projectile- and target-centered states. We present electron-capture and single- and double-ionization cross sections for protons incident on He in the ground state in the energy range from 15 keV to 1 MeV. We also provide partial cross sections for electron capture and direct excitation into the $n=2$ shell states of hydrogen and helium, respectively. Our results are in good agreement with available experimental data and other calculations, where available.

We precisely investigate the Zeeman and Paschen-Back effects of the hyperfine structure during the rubidium $5{}^2{S}_{1/2}\to 5{}^2{D}_{5/2}$ two-photon transition. Owing to the high sensitivity of the $D$ state of atoms to a magnetic field, the splittings of the spectra of the $D$ state are sufficiently large in a weak magnetic field of $<10$ G. We experimentally observe how in the hyperfine coupling in the $5{}^2{D}_{5/2}$ state is disrupted even in weak magnetic fields around 10 G and the energy levels are arranged according to their total angular momentum $J$ and nuclear spin $I$. The transient of the spectra known as the Paschen-Back effect is observed even in a weak magnetic field with an order of magnitude of several tens of gauss. The results of theoretical calculations for energy shifts and transition intensities under a magnetic field are in quantitative agreement with the experimental results.

Hybrid quantum systems have the potential of mitigating current challenges in developing a scalable quantum computer. Of particular interest is the hybridization between atomic and superconducting qubits. We demonstrate an experimental setup for transferring and trapping ultracold atoms inside a millikelvin cryogenic environment, where interactions between atomic and superconducting qubits may be established, paving the way for hybrid quantum systems. We prepare ${}^{87}\mathrm{Rb}$ atoms in a conventional magneto-optical trap and transport them via a magnetic conveyor belt into a UHV compatible dilution refrigerator with optical access. We store $5\times {10}^8$ atoms with a lifetime of 794 s in the vicinity of the millikelvin stage.

Based on the strong-field approximation, we obtain analytical expressions for the initial momentum at the tunnel exit and instantaneous ionization rate of tunneling ionization in elliptically polarized laser fields with arbitrary ellipticity. The tunneling electron reveals a nonzero offset of the initial momentum at the tunnel exit in the elliptically polarized laser field. We find that the transverse and longitudinal components of this momentum offset with respect to the instantaneous field direction are directly related to the time derivatives of the instantaneous laser electric field along the angular and radial directions, respectively. We further show that the nonzero initial momentum at the tunnel exit has a significant influence on the laser phase dependence of the instantaneous ionization rate in the nonadiabatic tunneling regime.

We present a method for measuring a complete frequency map of laser-dressed states with femtosecond lifetimes. It is based on collecting decay products as a function of the frequency of the excitation pulse and its delay with respect to the linearly chirped laser pulse, centered at the resonant transition frequency. The method has been tested on the case of laser-coupled $2s2p{}^{1}P^o$ and $2p^2{}^{1}S^e$ autoionizing resonances in He and is applicable to other quantum systems with short-lived states. When the laser coupling effects are known, the excitation pulse properties can be extracted from the map.

We investigate pairing in a strongly interacting two-component Fermi gas with positive scattering length. In this regime, pairing occurs at temperatures above the superfluid critical temperature; unbound fermions and pairs coexist in thermal equilibrium. Measuring the total number of these fermion pairs in the gas we systematically investigate the phases in the sectors of pseudogap and preformed pair. Our measurements quantitatively test predictions from two theoretical models. Interestingly, we find that already a model based on classical atom-molecule equilibrium describes our data quite well.

Motivated by recent experiments, we model the dynamics of bright solitons formed by cold gases in quasi-1D traps. A dynamical variational Ansatz captures the far-from-equilibrium excitations of these solitons. Due to a separation of scales, the radial and axial modes decouple, allowing for closed-form approximations for the dynamics. We explore how soliton dynamics influence atom loss and find that the time-averaged loss is largely insensitive to the degree of excitation. The variational approach enables us to perform high precision calculations of the critical atom number (i.e., the maximum number of atoms that can exist in a single soliton before the attractive forces overwhelm quantum pressure, leading to collapse).

We propose a scheme for generating a continuous-variable (CV) quadripartite unlockable bound entangled state from cascaded four-wave mixing processes. Different from the previous method involving a bipartite entangled state and two thermal states, our results show that the CV quadripartite unlockable bound entangled state can be generated via the internal reconfiguration of a four-mode entangled state. Both the nondistillability and superactivation of the generated CV quadripartite unlockable bound entangled state can be demonstrated. We believe that our scheme is experimentally accessible and will contribute to a deeper understanding of CV multipartite unlockable bound entanglement.

Trirefringence is related to the existence of three distinct phase velocity solutions (and polarizations) for light propagation in a same wave-vector direction. This implies that when a trirefringent medium refracts a light ray, it is split into three rays with different velocities and linearly independent polarizations. Here a previous investigation [V. A. De Lorenci and J. P. Pereira, Phys. Rev. A 86, 013801 (2012)] is revisited and its results are generalized to include a broader class of magnetoelectric materials. Moreover, it is argued that trirefringent media could already be achieved using present day technologies. Examples are given to support that focused on built layered media under the influence of applied electromagnetic fields.

In this work we consider the Liouville equation; it describes the dynamics of the photon density matrix in the Schrödinger representation based on the Markov approximation in the channel without dispersion. The equation contains a relaxation superoperator dependent on the phenomenological parameters of the optical fiber. These parameters allow one to take into account the phenomena of birefringence and optical activity, isotropic absorption, and dichroism. We also present in our work that these parameters affect not only the polarization of the states but the length of the Stokes vector. Hence the developed technique describes the decoherence process in the polarization domain in the quantum case and allows one to analyze the dynamics of single-photon states in the quantum (depolarizing) channel more properly. We also present a visual illustration of polarization states' evolution in the polarization-coded quantum key distribution BB84 protocol as an example. We estimate quantum bit error rates' dependence on channel length. Also we examine maximal allowed channel length, dependent on various configurations of channel parameters.

We investigate a tradeoff relation between the internal generation efficiency and the escape efficiency for single-photon generation based on cavity quantum electrodynamics, where cavity internal loss is treated explicitly. Consequently, we analytically derive an upper bound on the overall efficiency. The bound is expressed only with an internal cooperativity, introduced here as the cooperativity parameter with respect to the cavity internal loss rate. This result means that the internal cooperativity is a figure of merit for single-photon generation based on cavity QED. The bound is derived by optimizing the cavity external loss rate, which can be experimentally controlled by designing or tuning the transmissivity of the output coupler. The model here is general enough to treat various cavity-QED effects, such as the Purcell effect, on-resonant or off-resonant cavity-enhanced Raman scattering, and vacuum-stimulated Raman adiabatic passage. For typical optical systems, we additionally take into account a “reexcitation” process, where the atom is reexcited after its decay to the initial ground state.

We put forward a general approach for calculating the energy level shift of a quantum emitter (QE) in arbitrary-shaped nanostructure. It is expressed by the sum of the real part and a simple integral of the imaginary part of the scattered photon Green's function (GF) in a rather narrow real frequency range without a principal value. Compared with the method of direct evaluation of the principal value integral along the positive frequency axis or the method of transferring the principal value into an integration along the imaginary frequency axis, our method avoids the principal value integral and the evaluation of the scattered GF with imaginary frequency. It is numerically demonstrated for a QE located around a nanosphere and in a gap plasmonic nanocavity. The decay dynamics of an excited QE is calculated using the time-domain method through solving the Schrödinger equation in the form of a Volterra integral of the second kind, and using the frequency-domain method based on the Green's function expression for the evolution operator. It is found that the frequency domain method requires much narrower band information of the scattered GF. These findings are instructive in the field of coherent light-matter interactions.

We investigate the effect of breaking the parity of a photonic crystal waveguide designed to have odd and even modes intersecting inside the photonic bandgap. The complete study on the wave field properties of the resulting bonding modes uncovers that the transverse spin and orbital parts of the angular momentum couple in the very slow-light regime by means of their local currents. This leads to a subbandgap opening between bands characterized by distinct organizations of this spin-orbit coupling at the nanoscale.

We show that the electromagnetic wave propagation problem based on a two-level atomic model admits exact traveling wave solutions, of both soliton and cnoidal wave types. These solutions extend the already known solutions of the same type of both the mKdV and sG equations, which have been derived from the same initial model, respectively, in a long-wave and in a short-wave approximation. The continuation is such that no modification of the wave profile is required, but that the wave velocity only has to be corrected.

Photon emission and absorption by an individual qubit are essential elements for the quantum manipulation of light. Here we demonstrate the controllability of spontaneous emission of a qubit in various electromagnetic environments. The parameter regimes that allow for flexible control of the qubit emission routes are comprehensively discussed. By properly tuning the system couplings and decay rates, the spontaneous emission rate of the qubit can undergo Purcell enhancement and inhibition. In particular, when the cavity is prepared in the excited state, the spontaneous emission rate of the qubit can be significantly suppressed. We also demonstrate a spectral filter effect which can be realized by controlling the steady-state emission spectra of qubits.

We analyze certain aspects of the group-theoretical approach to Bell inequalities proposed by Güney and Hillery. The general procedure for constructing the relevant group orbits is described. Using Hall theorem we determine the form of Bell inequality in the single-orbit case. It is shown that in this case the Bell inequality is not violated for a maximally entangled state generating trivial subrepresentation if the representation under consideration is real.

It is well known that observing nonlocal correlations allows us to draw conclusions about the quantum systems under consideration. In some cases this yields a characterisation which is essentially complete, a phenomenon known as self-testing. Self-testing becomes particularly interesting if we can make the statement robust, so that it can be applied to a real experimental setup. For the simplest self-testing scenarios the most robust bounds come from the method based on operator inequalities. In this work we elaborate on this idea and apply it to the family of tilted Clauser-Horne-Shimony-Holt (CHSH) inequalities. These inequalities are maximally violated by partially entangled two-qubit states and our goal is to estimate the quality of the state based only on the observed violation. For these inequalities we have reached a candidate bound and while we have not been able to prove it analytically, we have gathered convincing numerical evidence that it holds. Our final contribution is a proof that in the usual formulation, the CHSH inequality only becomes a self-test when the violation exceeds a certain threshold. This shows that self-testing scenarios fall into two distinct classes depending on whether they exhibit such a threshold or not.

In quantum control theory, a question of fundamental and practical interest is how an arbitrary unitary transformation can be decomposed into a minimum number of elementary rotations for implementation, subject to various physical constraints. Examples include the singlet-triplet (ST) and exchange-only (EO) qubits in quantum-dot systems, and gate construction in the Solovay–Kitaev algorithm. For two important scenarios, we present complete solutions to the problems of optimal decomposition of single-qubit unitary gates with non-orthogonal rotations. For each unitary gate, the criteria for determining the minimal number of pieces is given, the explicit gate construction procedure, as well as a computer code for practical uses. Our results include an analytic explanation to the four-gate decomposition of EO qubits, previously determined numerically by Divincenzo et al. [Nature (London) 408, 339 (2000)]. Furthermore, compared with the approaches of Ramon sequence and its variant [Phys. Rev. Lett. 118, 216802 (2017)], our method can reduce about 50% of gate time for ST qubits. Finally, our approach can be extended to solve the problem of optimal control of topological qubits, where gate construction is achieved through the braiding operations.