We demonstrate a diode-laser-pumped system for generation of quadrature squeezing and polarization squeezing. Due to their excess phase noise, diode lasers are challenging to use in phase-sensitive quantum optics experiments such as quadrature squeezing. The system we present overcomes the phase noise of the diode laser through a combination of active stabilization and appropriate delays in the local oscillator beam. The generated light is resonant to the rubidium $D1$ transition at $795\mathrm{nm}$ and thus can be readily used for quantum memory experiments.

We analyze quantum state estimation for finite samples based on symmetry information. The measurement concept used compares an unknown qubit to a reference state. We describe explicitly an adaptive strategy that enhances the estimation fidelity of these measurements.

The quantum electrodynamical vacuum polarization effects arising in the collision of a high-energy proton beam and a strong, linearly polarized laser field are investigated. The probability that laser photons merge into one photon by interacting with the proton’s electromagnetic field is calculated taking into account the laser field exactly. Asymptotics of the probability are then derived according to different experimental setups suitable for detecting perturbative and nonperturbative vacuum polarization effects. The experimentally most feasible setup involves the use of a strong optical laser field. It is shown that in this case measurements of the polarization of the outgoing photon and of its angular distribution provide promising tools to detect these effects for the first time.

The orbital angular momentum (OAM) of photons offers a suitable support to carry the quantum data of multiple users. We present two optical setups that send the information of $n$ quantum communication parties through the same free-space optical link. Those qubits can be sent simultaneously and share path, wavelength, and polarization without interference, increasing the communication capacity of the system. The first solution, a qubit combiner, merges $n$ channels into the same link, which transmits $n$ independent photons. The second solution, the OAM multiplexer, uses controlled-not ($\mathrm{CNOT}$) gates to transfer the information of $n$ optical channels to a single photon. Additional applications of the multiplexer circuits, such as quantum arithmetic, as well as connections to OAM sorting are discussed.

We propose a scheme for implementing single-qubit and nontrivial two-qubit gate operations in a system with superconducting quantum interference devices (SQUIDs) via Raman transitions in circuit QED. The gate operations only involve the two lower flux states of the SQUID, and hence the effect of the excited state can be neglected. By encoding quantum information into the decoherence free subspace of the SQUID system, the evolution of the system results in fault-tolerant quantum gates. The effect of decoherence caused by the decay of the transmission line resonator is investigated.

We examine the stochastic dynamics of entanglement for an uncoupled two-qubit system, undergoing continuous parity measurement. Starting with a fully mixed state, the entanglement is zero for a finite amount of time, when it is suddenly created, which we refer to as entanglement genesis. There can be further entanglement sudden death or birth events culminating in the formation of a fully entangled state. We present numerical investigations of this effect together with statistics of the entanglement genesis time in the weak measurement limit as well as the quantum Zeno limit. An analytic treatment of the physics is presented, aided by the derivation of a simple concurrence equation for Bell basis $X$ states. The probability of entanglement border crossing and mean first passage times are calculated for the case of measurement dynamics alone. We find that states with almost the same probability of entanglement border crossing can have very different average crossing times. Our results provide insight on the optimization of entanglement generation by measurement.

We study the double ionization by electron impact of the ground state of heliumlike atoms and propose a scaling law for fully differential $(e,3e)$ cross sections. Within the first Born approximation, cross sections are calculated with a three-body Coulomb (3C) double-continuum wave function and initial states represented by highly accurate wave functions, which satisfy all two-body Kato cusp conditions. We first consider the helium atom in the kinematical and geometrical conditions of the only absolute, high incident energy, experimental data available: our calculations confirm unambiguously that satisfying or not Kato cusp conditions is not a relevant feature of the ground state. Other heliumlike atoms are then considered. Under similar conditions, cross sections for ${\mathrm{H}}^{-}$ are much larger than for helium while the reverse is true for positive ions; a comparison with the rare other theoretical calculations is performed. Finally, within our theoretical framework, we propose an approximate scaling law for $(e,3e)$ cross sections for heliumlike positive ions, and confirm it by calculations.

Two-photon processes that involve different sublevels of the ground state of an atom, are highly sensitive to depopulation and decoherence within the ground state. For example, the spectral width of electromagnetically induced transparency resonances in a $\Lambda$-type system, are strongly affected by the ground-state depopulation and decoherence rates. We present a direct measurement of decay rates between hyperfine and Zeeman sublevels in the ground state of ${}^{87}\mathrm{Rb}$ vapor. Similar to the relaxation-in-the-dark technique, pumping lasers are used to prealign the atomic vapor in a well-defined quantum state. The free propagation of the atomic state is monitored using a Ramsey-like method. Coherence times in the range $1–10\mathrm{ms}$ were measured for room temperature atomic vapor. In the range of the experimental parameters used in this study, the dominant process inducing Zeeman decoherence is the spin-exchange collisions between rubidium atoms.

We present a method to determine the decay of multiparticle quantum correlations as quantified by the geometric measure of entanglement under the influence of decoherence. With this, we compare the robustness of entanglement in Greenberger-Horne-Zeilinger (GHZ), cluster, $W$, and Dicke states of four qubits and show that the Dicke state is the most robust. Finally, we determine the geometric measure analytically for decaying GHZ and cluster states of an arbitrary number of qubits.

We show the way to speed up the time evolution of a wave function (WF), i.e., to fast-forward the WF in microscopic and macroscopic quantum mechanics, by controlling the driving potential with resultant regulation of the additional phase of the WF, so that a target state is obtained in a shorter time. We first present a general framework of the fast-forwarding of a WF in quantum mechanics and provide an example of the fast-forward of a WF in two-dimensional (2D) free space. Then the framework of the fast-forward is extended to macroscopic quantum mechanics described by the nonlinear Schrödinger equation. We show the fast-forward of (i) transport of Bose-Einstein condensates trapped by a moving 2D harmonic potential and (ii) propagation of a soliton both in free space and through a potential barrier (: macroscopic quantum tunneling).

We investigate quantum repeater protocols based upon atomic qubit-entanglement distribution through optical coherent-state communication. Various measurement schemes for an optical mode entangled with two spatially separated atomic qubits are considered in order to nonlocally prepare conditional two-qubit entangled states. In particular, generalized measurements for unambiguous state discrimination enable one to completely eliminate spin-flip errors in the resulting qubit states, as they would occur in a homodyne-based scheme due to the finite overlap of the optical states in phase space. As a result, by using weaker coherent states, high initial fidelities can still be achieved for larger repeater spacing, at the expense of lower entanglement generation rates. In this regime, the coherent-state-based protocols start resembling single-photon-based repeater schemes.

Fragmentation of $\mathrm{S}\mathrm{F}_6{}^{3+}$, formed from $\mathrm{S}{\mathrm{F}}_6$ by absorption of soft x-ray photons, has been investigated using a momentum imaging time-of-flight spectrometer. We observe triple ion coincidences between two ${\mathrm{F}}^{+}$ ions and $\mathrm{S}\mathrm{F}_m{}^{+}$ where $m=0,\dots ,3$. Strong correlation between the momentum distributions of the two ${\mathrm{F}}^{+}$ ions is observed across all dissociation channels involving triple ion coincidences. We present a method of estimating the contribution of the neutral fragments to the kinematics of ion ejection based on the scalar triple product of the ion momentum vectors. The noncoplanarity of the three ion ejecta inferred in this manner allows us to discern the fragmentation sequence of $\mathrm{S}\mathrm{F}_6{}^{3+}$. The proposed sequence is loss of a neutral F followed by violent ejection of two ${\mathrm{F}}^{+}$ ions and gentle removal of remaining F atoms from $\mathrm{S}\mathrm{F}_3{}^{+}$.

The influence of exchange, correlation, and relativistic effects in the ionization of heavy-target atoms can be sensitively probed by kinematically complete studies involving spin-polarized electrons, in particular when the fine structure of the residual ion is resolved. We present spin asymmetries, triple differential cross sections, and branching ratios for the ionization, by $127.5\text{−}\mathrm{eV}$ and $114.3\text{−}\mathrm{eV}$ electrons, of ground-state krypton atoms leading to the ${\mathrm{Kr}}^{+}$ $4s{}^{2}S_{1∕2}$, $4p^5{}^{2}P_{1∕2}$, and $4p^5{}^{2}P_{3∕2}$ states. In order to untangle contributions from different physical effects, the experimental results are compared to those from distorted-wave Born approximation calculations (non- and semirelativistic) in which bound-state and continuum effects are treated in different ways. Additional insight is gained by comparing the present experimental and theoretical results to recent results on the $e\text{−}\mathrm{Xe}$ system, performed under similar kinematics, for which target relativistic effects play a more significant role.

We present results from two-photon photoassociative spectroscopy of the least-bound vibrational level of the $X{}^1\Sigma _g^{+}$ state of the ${}^{88}\mathrm{Sr}_2$ dimer. Measurement of the binding energy allows us to determine the $s$-wave scattering length $a_{88}=-1.4\left(6\right)a_0$. For the intermediate state, we use a bound level on the metastable ${}^{1}S_0\text{−}{}^{3}P_1$ potential, which provides large Franck-Condon transition factors and narrow one-photon photoassociative lines that are advantageous for observing quantum-optical effects such as Autler-Townes resonance splittings.

A technique is proposed for creating nonground-state Bose-Einstein condensates in a trapping potential by means of the temporal modulation of atomic interactions. Applying a time-dependent spatially homogeneous magnetic field modifies the atomic scattering length. A modulation of the scattering length excites the condensate, which, under special conditions, can be transferred to an excited nonlinear coherent mode. It is shown that a phase-transition-like behavior occurs in the time-averaged population imbalance between the ground and excited states. The application of the technique is analyzed and it is shown that the considered effect can be realized for experimentally available condensates.

Temporal optical soliton molecules were recently demonstrated; they potentially allow a further increase of data rates in optical telecommunication. We present a theoretical study aimed at an explanation of the mechanism responsible for the binding force. To this end we use a perturbation treatment in several variants. We find that the well-known soliton interaction as mediated by the optical Kerr effect, when suitably modified for chirped pulses, captures essential features like the existence of a stable equilibrium separation and small-scale oscillations around this point. Predictions of these models are compared to numerical simulations.

A photon-energy-dependent experimental study of the $L_{2,3}{M}_{2,3}{M}_{2,3}$ Auger transitions in argon clusters is presented, and features without atomic counterparts are observed in the spectra up to photon energies at least $30\mathrm{eV}$ above the $L_{2,3}$ threshold. We propose that these features are due to the process of neighbor-induced recapture, in which the outgoing photoelectron is backscattered by neighboring atoms in the cluster, loses energy in a Bremsstrahlung-like inverse photoemission process, and is recaptured in a Rydberg orbital of the core-ionized atom before Auger decay.

The relation between level crossings, entanglement, and Berry phases is investigated for the Breit-Rabi Hamiltonian of hydrogen and sodium atoms, describing a hyperfine interaction of electron and nuclear spins in a magnetic field. It is shown that the entanglement between nuclear and electron spins is maximum at avoided crossings. An entangled state encircling avoided crossings acquires a marginal Berry phase of a subsystem like an instantaneous eigenstate moving around real crossings accumulates a Berry phase. Especially, the nodal points of a marginal Berry phase correspond to the avoided crossing points.

We establish a rigorous quantitative connection between (i) the interferometric duality relation for which-way information and fringe visibility and (ii) Heisenberg’s uncertainty relation for position and modular momentum. We apply our theory to atom interferometry, wherein spontaneously emitted photons provide which-way information and unambiguously resolve the challenge posed by the metamaterial “perfect lens” to complementarity and to the Heisenberg-Bohr interpretation of the Heisenberg microscope thought experiment.