We investigate the influence of the environment on the rapid adiabatic passage scheme for optimal population transfer in a two-level system. To cope with strong coupling to an external phonon bath with super-Ohmic spectral density, we are solving the time-dependent Schrödinger equation of the extended system, including a finite number of bath modes using the multi-Davydov D2 ansatz. This allows for the treatment of the non-Markovian reduced dynamics of the two-level subsystem. Surprisingly, it is found that strong system-bath coupling stabilizes the transition probability from the lower to the upper level as a function of the area under the laser pulse. This dissipative engineering effect could only be uncovered by a non-Markovian treatment. For strong coupling, the transition probability then becomes a monotonically increasing function of the pulse area at zero temperature of the heat bath. Finite temperatures break the monotonicity in the range of pulse areas that we studied, but not the stability of the observed effect.

We investigate a one-dimensional Rabi-Hubbard type of model, arranged such that a quantum dot is sandwiched between every cavity. The role of the quantum dot is twofold, to transmit photons between neighboring cavities and simultaneously act as an effective photon nonlinearity. We consider three-level quantum dots in the $\mathrm{\Lambda }$ configuration, where the left and right leg couple exclusively to the left or right cavity. This noncommuting interaction leads to two highly entangled incompressible phases, separated by a second-order quantum phase transition; the degrees of freedom of the quantum dots can be viewed as a dynamical lattice for the photons which spontaneously breaks ${\mathbb{Z}}_2$ symmetry due to a bosonic Peierls instability, leading to a phase with dimerized order. Additionally, we find a normal insulating phase and a superfluid phase that acts as a quantum many-body superradiant phase. In the superradiant phase, a ${\mathbb{Z}}_2$ symmetry is broken and the phase transition falls within the universality class of the transverse-field Ising model. Finally, we show that the model can be interpreted as a ${\mathbb{Z}}_2$ lattice gauge theory in the absence of a dipolar field on the lower qutrit levels.

We quantify the disturbance of a quantum state undergoing a sequence of observations and particularly focus on a weak measurement followed by postselection and compare these results to the projective counterpart. Taking into account the distinguishability of both the system and the device, we obtain the exact trade-off between the system state disturbance and the change in the device pointer state. We show that for particular postselection procedures the coupling strength between the system and the device can be significantly reduced without losing measurement sensitivity, which is directly transferred to a reduced state disturbance of the system. We observe that a weak measurement alone does not provide this advantage, and only in combination with postselection is a significant improvement in terms of increased measurement sensitivity and reduced state disturbance found. We further show that under realistic experimental conditions this state disturbance is small, whereas the exact postselection probability is considerably larger than the approximate value given by the overlap of the initial and final states when neglecting the system state disturbance.

We study a system of two localized detectors (oscillators) interacting through a massless quantum field in a vacuum state via an Unruh-DeWitt coupling. This system admits an exact solution and provides a good model for addressing fundamental issues in particle-field interactions, causality, and locality in quantum field measurements that are relevant to proposed quantum experiments in space. Our analysis of the exact solution leads to the following results. (i) Common approximations used in the study of analogous open quantum systems fail when the distance between the detectors becomes of the order of the relaxation time. In particular, the creation of correlations between remote detectors is not well described by ordinary perturbation theory and the Markov approximation. (ii) There is a unique asymptotic state that is correlated; it is not entangled unless the detector separation is of the order of magnitude of the wavelength of the exchanged quanta. (iii) The evolution of seemingly localized observables is noncausal. The latter is a manifestation of Fermi's two-atom problem, albeit in an exactly solvable system. We argue that the problem of causality requires a reexamination of the notion of entanglement in relativistic systems, in particular, the physical relevance of its extraction from the quantum vacuum.

The molecular anion ${{\mathrm{C}}_2}^{-}$ is currently of interest as a candidate for laser cooling due to its electronic structure and favorable branching ratios to the ground electronic and vibrational states. Helium has been proposed as a buffer gas to cool the molecule's internal motion. We calculate the cross sections and corresponding rates for rovibrational inelastic collisions of ${\mathrm{C}}_2{}^{-}$ with He, and also with Ne and Ar, on three-dimensional ab initio potential energy surfaces using quantum scattering theory. The rates for vibrational quenching with He and Ne are very small and are similar to those for small neutral molecules in collision with helium. The quenching rates for Ar, however, are far larger than those with the other noble gases, suggesting that this may be a more suitable gas for driving vibrational quenching in traps. The implications of these results for laser cooling of ${{\mathrm{C}}_2}^{-}$ are discussed.

We report the design and application of an optically compensated zoom lens supporting a moving optical dipole trap for transporting cold atoms. The proposed zoom lens is designed to have tunable magnification in a compact form factor which can significantly reduce the required long optical path lengths in conventional moving optical dipole trap systems. Without the need of mechanical cams for zooming, the optically compensated zoom lens has the ability to control the position of the trap along the optical axis with high precision while maintaining a constant trap depth by conserving the numerical aperture of the laser beam focus during the transport. We experimentally comprise an optically compensated zoom lens having dimensions of $\lesssim 20$ cm in length, which can be readily configured to suit a wide range of desired transport lengths from 0.15 to 3 m and displacement magnifications from 7 to 100. The performance of our moving optical dipole trap is tested for a target transport length of 600 mm using a 532 nm laser beam on laser-cooled ytterbium atoms. The atoms can be transported to an adjacent vacuum chamber using this optically compensated zoom lens with $\sim 89\%$ efficiency.

We design and generate the bipolar-variant spin angular momentum (SAM) carried by vector optical field and theoretically investigate the evolution of the bipolar-variant SAM in the tight focusing process. It is found that the longitudinal SAM density annihilates and the purely transverse SAM density appears in the focal plane, and the evolution of the complex singularities of SAM into $V$-point singularities is also studied. We further extend the study to general bipolar-variant SAM, and give the theoretical explanation of the annihilation and appearance of different components of SAM density. We believe the presentation of the bipolar-variant SAM can greatly enrich the study of SAM as well as the singular optics. Meanwhile, the evolution of the bipolar-variant SAM in the focusing process is interesting and important in revealing the focal properties of SAM of the photon, and the purely transverse SAM in the focal plane with a spin axis orthogonal to the propagation direction opens up avenues in optical trapping and manipulation, especially for optically induced rotations.

In single-particle or intraparticle entanglement, two degrees of freedom of a single particle, e.g., momentum and polarization of a single photon, are entangled. Single-particle entanglement is a resource that can be exploited both in quantum communication protocols and in experimental tests of noncontextuality based on the Kochen-Specker theorem. Here we show that single-particle entangled states of single photons can be produced from attenuated classical sources of light. To experimentally certify the single-particle entanglement, we perform a Bell test, observing a violation of the Clauser, Horne, Shimony, and Holt inequality. We show that single-particle entanglement can be achieved even in a classical light beam, provided that first-order coherence is maintained between the degrees of freedom involved in the entanglement. This demonstrates that cheap, compact, and low-power photon sources can be used to generate single-particle entangled photons which could be a resource for quantum technology applications.

A single-photon switch is an important element for the building of scalable quantum networks. In this paper, we propose a feasible scheme for efficient single-photon switching. The proposed switch is controlled by a state of a qubit formed by the pair of the lowest levels of a three-level system (qutrit) coupled to a resonator. This resonator-qutrit system comprises a switching unit of the considered setup. For suppression of the Purcell relaxation of the control qubit, the switching unit is embedded into a coupled-resonator array serving as an engineered electromagnetic environment with a band gap on a qubit transition frequency. We discuss the possible implementation of the considered single-photon switch on the microwave circuit QED architecture. We demonstrate that high switching contrasts can be attained for the parameters achievable for the state-of-the-art superconducting circuit QED setups.

The security of measurement device-independent quantum key distribution (MDI QKD) relies on a thorough characterization of one's optical source output, especially any noise in the state preparation process. Here, we provide an extension of the loss-tolerant protocol [Phys. Rev. A 90, 052314 (2014)], a leading proof technique for analyzing the security of QKD, to MDI QKD protocols that employ mixed signal states. We first reframe the core of the proof technique, noting its generalization to treat $d$-dimensional signal encodings. Concentrating on the qubit signal state case, we find that the mixed states can be interpreted as providing Alice and Bob with a virtual shield system they can employ to reduce Eve's knowledge of the secret key. We then introduce a simple semidefinite programming method for optimizing the virtual twisting operations they can perform on the shield system to yield a higher key rate, along with an example calculation of fundamentally achievable key rates in the case of random polarization modulation error.

In this paper we analyze the properties of bound states of an atom interacting with a set of static impurities. We begin with the simplest system of a single atom interacting with two static impurities. We consider two types of atom-impurity interaction: (i) a zero-range potential represented by a regularized $\delta$ interaction and (ii) a more realistic polarization potential, representing the long-range part of the atom-ion interaction. For the former we obtain analytical results for energies of bound states. For the latter we perform numerical calculations based on the application of the finite-element method. We then discuss the case of a single atom interacting with a one-dimensional (1D) infinite chain of static ions. Such a setup resembles the Kronig-Penney model of a 1D crystalline solid, where the energy spectrum exhibits band-structure behavior. For this system we derive analytical results for the band structure of bound states, assuming a regularized $\delta$ interaction, and perform numerical calculations, considering the polarization potential to model an atom-impurity interaction. Both approaches agree quite well when the separation between impurities is much larger than the characteristic range of the interaction potential.

We exactly solve the two-body and three-body problems in the mixture of alkaline-earth-metal(like) (AE) and alkali-metal atomic gases in the presence of three-dimensional isotropic spin-orbit (SO) coupling. The SO coupling is implemented by the Raman scheme that couples the ground state of the AE atom with different nuclear spin. The interaction between AE and alkali-metal atoms is tuned to be near resonant by Feshbach resonance, while the interaction between the two AE atoms is negligible. We present the Skorniakov–Ter-Martirosian (STM) equation for the three-body system composed of two SO coupled AE atoms and one alkali-metal atom. By solving the STM equation in the zero angular momentum sector, we obtain the energy spectrum and find a nonuniversal Efimov trimer and a universal Kartavtsev-Malykh (KM) trimer in different parameter windows of the mass ratio. In the region where the KM trimer is present, strong SO coupling can stabilize the KM trimer to be the ground state, such that experimental realization and observation can be expected.

High-fidelity and controllable optical cloning of high-dimensional (high-D) optical beams is very important for the development of novel techniques for optical imaging, lithography, and communications, etc. Here we propose a scheme to realize the cloning of high-D optical beams with a Rydberg atomic gas via electromagnetically induced transparency. We show that strong atom-atom interaction can map to two probe laser fields, which may acquire giant nonlocal Kerr nonlinearities supporting the formation of stable high-D optical solitons and vortices at very low light power. We also show that such optical solitons and vortices prepared in one probe field can be cloned onto another one with high fidelity, and the cloning may be actively manipulated through the tuning of the nonlocality degree of the Kerr nonlinearities. Moreover, we demonstrate that based on such a cloning scheme multitimes and multicomponents cloning of high-D optical beams are also possible, which allows us to acquire multiple copies of high-D optical beams. The results on the optical cloning reported here are not only of fundamental interest for nonlocal nonlinear optics but also promising for practical applications in optical information processing and transmission.

Driven-dissipative systems have recently attracted great attention due to the existence of novel physical phenomena with no analog in the equilibrium case. The Keldysh path-integral theory is a powerful tool to investigate these systems. However, it has still been a challenge to study strong nonlinear effects implemented by recent experiments, since in this case the photon number is few and quantum fluctuations play a crucial role in the dynamics of the system. Here we develop an approach for deriving exact steady states of driven-dissipative systems by introducing the Keldysh partition function in the Fock-state basis and then mapping the standard saddle-point equations into Keldysh-Heisenberg equations. We take the strong Kerr nonlinear resonators with and without the nonlinear driving as two examples to illustrate our method. It is found that, in the absence of the nonlinear driving, the exact steady state obtained does not exhibit bistability and agrees well with the complex $P$-representation solution. While in the presence of the nonlinear driving, the multiphoton resonance effects are revealed and are consistent with the qualitative analysis. Our method provides an intuitive way to explore a variety of driven-dissipative systems especially with strong correlations.

Hybrid encoding of quantum information is a promising approach towards the realization of optical quantum protocols. It combines advantages of continuous variables encoding, such as high efficiencies, with those of discrete variables, such as high fidelities. In particular, entangled hybrid states were shown to be a valuable resource for quantum information protocols. In this work we present a hybrid entanglement witness that can be implemented on available experiments and is robust to noise currently observed in quantum optical setups. The proposed witness is based on measurements of genuinely hybrid observables. The noise model we consider is general. It is formally characterized with Kraus operators since the considered hybrid system can be expressed in a finite dimension basis. A practical advantage of the witness is that it can be tested by measuring just a few experimentally available observables.

Delayed deprotonation dissociation of dications of two ${\mathrm{C}}_3{\mathrm{H}}_4$ isomers, propyne and allene, induced by 50 keV/u ${\mathrm{Ne}}^{8+}$ ions is investigated employing the reaction microscope technique. The survival time of the precursor ${\mathrm{C}}_3\mathrm{H}{{}_4}^{2+}$ is retrieved according to the time-of-flight difference between the two ionic fragments recorded in coincidence. The survival time spectra of the two isomers exhibit very similar distributions, demonstrating that the initial structure of the isomer does not play a key role in the delayed fragmentation process. In addition, we found that the delayed fragmentation of both isomers could be well described by the power-law decay.

Photoelectron emission from excited states of laser-dressed atomic helium is analyzed with respect to laser intensity-dependent excitation energy shifts and angular distributions. In the two-color extreme ultraviolet (XUV)-infrared (IR) measurement, the XUV photon energy is scanned between 20.4 eV and the ionization threshold at 24.6 eV, revealing electric dipole-forbidden transitions for a temporally overlapping IR pulse $(\approx {10}^{12}\mathrm{W}{\mathrm{cm}}^{-2})$. The interpretation of the experimental results is supported by numerically solving the time-dependent Schrödinger equation in a single-active-electron approximation.