In this paper we use time-dependent Josephson coupling to enhance unconventional photon blockade in a system of two coupled nonlinear bosonic modes which are initially loaded with weakly populated coherent states, so the evolution is restricted to the manifold of up to two field quanta. Using numerical optimal control, we find the optimal coupling which minimizes the two-photon occupation of one mode, which is actually transferred to the other, while maintaining a nonzero one-photon occupation in the same mode. Moreover, we choose for the continuous coupling to vanish after the transfer between the modes such that they are decoupled and one of them is left only with some one-photon population which can be observed upon its decay. We numerically find lower values of the second-order correlation function obtained at earlier times than with constant coupling, with larger one-photon populations and for longer time windows, corresponding thus to higher emission efficiency and easier detection. The presented methodology is not restricted to the system under study, but it can also be transferred to other related frameworks, to find the optimal driving fields which can improve the single-photon emission statistics from these systems.

We study the scattering of a single photon in the surface plasmon polariton mode propagating on a two-dimensional surface, by a two-level quantum system in the regime where the photon wavelength becomes comparable or smaller than the size of the electron wave function. In this interesting regime, the scattering spectral signature becomes highly dependent on the shape of the ground and excited state electron wave functions and shows a significant departure from the Lorentzian response obtained from the usual perturbative method of multipole expansion. We interpret the observed multipeaked line shape by considering the two-level atom as a combination of a quantum system with macroscopic current distribution which adds structural resonances to the radiation spectrum.

Quantum simulation promises to address many challenges in fields ranging from quantum chemistry to material science and high-energy physics, and could be implemented in noisy intermediate-scale quantum devices. A challenge in building good digital quantum simulators is the fidelity of the engineered dynamics given a finite set of elementary operations. Here we present a framework for optimizing the order of operations based on a geometric picture, thus abstracting from the operation details and achieving computational efficiency. Based on this geometric framework, we provide two alternative second-order Trotter expansions: one with optimal fidelity at a short timescale, and the second robust at a long timescale. Thanks to the improved fidelity at different timescales, the two expansions we introduce can form the basis for experimental-constrained digital quantum simulation.

Bose-Einstein condensation (BEC) is a powerful tool for a wide range of research activities, a large fraction of which is related to quantum simulations. Various problems may benefit from different atomic species, but cooling down novel species interesting for quantum simulations to BEC temperatures requires a substantial amount of optimization and is usually considered to be a difficult experimental task. In this work, we implemented the Bayesian machine learning technique to optimize the evaporative cooling of thulium atoms and achieved BEC in an optical dipole trap operating near 532 nm. The developed approach could be used to cool down other novel atomic species to quantum degeneracy without additional studies of their properties.

We extend non-Hermitian topological quantum walks on a Su-Schrieffer-Heeger (SSH) lattice [Phys. Rev. Lett. 102, 065703 (2009)] to the case of non-Markovian evolution. This non-Markovian model is established by coupling each unit cell in the SSH lattice to a reservoir formed by a quasicontinuum of levels. We find a topological transition in this model even in the case of non-Markovian evolution where the walker may visit the reservoir and return to the SSH lattice at a later time. The existence of a topological transition does, however, depend on the low-frequency properties of the reservoir, characterized by a spectral density $J\left(\epsilon \right)\propto {\left|\epsilon \right|}^{\alpha }$. In particular, we find a robust topological transition for a sub-Ohmic ($\alpha <1$) and Ohmic ($\alpha =1$) reservoir, but no topological transition for a super-Ohmic ($\alpha >1$) reservoir. This behavior is directly related to the well-known localization transition for the spin-boson model. We confirm the presence of non-Markovian dynamics by explicitly evaluating a measure of Markovianity for this model.

In two dimensions, a system of self-gravitating particles collapses and forms a singularity in finite time below a critical temperature $T_c$. We investigate experimentally a quasi-two-dimensional cloud of cold neutral atoms in interaction with two pairs of perpendicular counterpropagating quasiresonant laser beams, in order to look for a signature of this ideal phase transition: indeed, the radiation pressure forces exerted by the laser beams can be viewed as an anisotropic, and nonpotential, generalization of two-dimensional self-gravity. We first show that our experiment operates in a parameter range which should be suitable to observe the collapse transition. However, the experiment unveils only a moderate compression instead of a phase transition between the two phases. A three-dimensional numerical simulation shows that both the finite small thickness of the cloud, which induces a competition between the effective gravity force and the repulsive force due to multiple scattering, and the atomic losses due to heating in the third dimension contribute to smearing the transition.

We discuss theoretically the optical binding of one-dimensional chains of cold atoms shone by a transverse pump, where particles self-organize to a distance close to an optical wavelength. As the number of particles is increased, the trapping potential increases logarithmically as the contributions from all atoms add up constructively. We identify a cooperative cooling mechanism, due to the mutual exchange of photons between atoms, which can beat the spontaneous emission for chains that are long enough. Surprisingly, the cooling is optimal very close to the resonance. This peculiar cooling mechanism thus gives new insights into the cooperative physics of low-dimensional cold atom systems.

To date, the concept of topological order relies heavily on the properties of single-particle bands. Only recently it has been realized that interactions can have a dramatic impact on topological properties, not only modifying the topology of the bands but also creating a topological order in an otherwise trivial system. Applying an extended version of the Bose-Hubbard model, we investigate a system which, being topologically trivial in the single-particle regime, harbors topologically nontrivial edge and interface states of repulsively bound photon pairs. Whereas binding of the photons in this model is captured by the standard local interaction term, an additional direct two-photon hopping renders the system topologically nontrivial. Besides their interaction-induced origin, predicted two-photon edge states exhibit a range of other unexpected features, including the robustness to collapse of the corresponding bulk band and the ability to coexist with the continuum of two-photon scattering states forming a bound state in the continuum. Performing rigorous calculation of the Zak phase for bound photon pairs, we prove the topological origin of the two-photon edge states.

We present a rigorous justification for the applicability of the recently developed methods of generalized autobalanced Ramsey spectroscopy (GABRS) [V. I. Yudin et al., Phys. Rev. Appl. 9, 054034 (2018)] and combined error signal in Ramsey spectroscopy (CESRS) [ , New J. Phys. 20, 123016 (2018)] for resonances of coherent population trapping (CPT). A protocol of modified combined error signal for CPT resonances is also proposed, in which two successive interrogations of atoms under different times of free evolution are integrated in one cycle. The implementation of the GABRS and CESRS methods in a compact atomic clock based on the CPT effect allows us to significantly suppress the light shift and its fluctuation, which will lead to an improvement in the long-term stability and accuracy of these devices. Moreover, these spectroscopic schemes are insensitive to various distortions of the pulse shape, relaxation processes, errors in the generation of phase jumps, etc., which provides their high reliability and immunity to perturbations.

The entanglement of quantum systems can produce a variety of nonclassical effects that have practical applications in quantum information science. One example of this is nonlocal dispersion cancellation, in which the effects of dispersion on one photon can be canceled out by the dispersion experienced by a second photon at a distant location. In this paper, we extend the analysis of nonlocal dispersion cancellation to three or more photons. We find that energy-time entanglement of three or more photons can lead to a complete or partial cancellation of dispersion depending on the experimental conditions. These results may be useful in implementing quantum key distribution in networks with three or more nodes.

We consider an ensemble of cavity-coupled two-level emitters interacting via full (coherent and dissipative) dipole-dipole interactions. We detail an adiabatic elimination procedure to derive effective equations of motion for a subsystem consisting of the cavity and a single emitter and analyze limitations of effective subsystem parameters. We study how joint dissipative decay processes in the subsystem affect cavity-coupling properties of the single emitter and cavity transmission spectra.

The simultaneous ground-state cooling of multiple degenerate or near-degenerate mechanical modes coupled to a common cavity-field mode has become an outstanding challenge in cavity optomechanics. This is because the dark modes formed by these mechanical modes decouple from the cavity mode and prevent extracting energy from the dark modes through the cooling channel of the cavity mode. Here we propose a universal and reliable dark-mode-breaking method to realize the simultaneous ground-state cooling of two degenerate or nondegenerate mechanical modes by introducing a phase-dependent phonon-exchange interaction, which is used to form a loop-coupled configuration. We find an asymmetrical cooling performance for the two mechanical modes and expound this phenomenon based on the nonreciprocal energy transfer mechanism, which leads to the directional flow of phonons between the two mechanical modes. We also generalize this method to cool multiple mechanical modes. The physical mechanism in this cooling scheme has general validity and this method can be extended to break other dark-mode and dark-state effects in physics.

A method is proposed that allows one to infer the sum of the values of an observable taken during contacts with a pointer state. Hereby, the state of the pointer is updated while contacted with the system and remains unchanged between contacts while the system evolves in time. After a prescribed number of such contacts, the position of the pointer is determined by means of a projective measurement. The outcome is specified in terms of a probability distribution function for unitary and Markovian dissipative dynamics and compared with the results of the same number of generalized Gaussian measurements of the considered observable. As a particular example, a qubit is considered with an observable contacting to the pointer that does not commute with the system Hamiltonian.

Non-Markovian effects in an open-system dynamics are usually associated to information backflows from the environment to the system. However, the way these backflows manifest and how to detect them is unclear. A natural approach is to study the backflow in terms of the correlations the evolving system displays with another unperturbed system during the dynamics. In this work, we study the power of this approach to witness non-Markovian dynamics using different correlation measures. We identify simple dynamics where the failure of completely positive divisibility is in one-to-one correspondence with a correlation backflow. We then focus on specific correlation measures, such as those based on entanglement and the mutual information, and identify their strengths and limitations. We conclude with a study of a recently introduced correlation measures based on state distinguishability and see how, for these measures, adding an extra auxiliary system enlarges the set of detectable non-Markovian dynamics.

In quantum key distribution, measurement-device-independent and decoy-state techniques enable the two cooperative agents to establish a shared secret key using imperfect measurement devices and weak Poissonian sources, respectively. Investigations so far are not comprehensive as they restrict to less than or equal to four decoy states. Moreover, many of them involve pure numerical studies. Here I report a general security proof that works for any fixed number of decoy states and any fixed raw key length. There are two key ideas involved here. The first one is the repeated application of the inversion formula for the Vandermonde matrix to obtain various bounds on certain yields and error rates. The second one is the use of a recently proven generalization of the McDiarmid inequality. These techniques raise the best provably secure key rate of the measurement-device-independent version of the Bennett-Brassard 1984 scheme by at least 1.25 times and increase the workable distance between the two cooperative agents from slightly less than 60 km to slightly greater than 130 km in case where there are ${10}^{10}$ photon pulse pairs sent without a quantum repeater.

Quantum electrodynamical (QED) calculations of ionization energies of the $1sndD$ states are performed for the helium atom. We reproduce the previously known relativistic and QED effects up to order $m{\alpha }^5$ and extend the theory by calculating the complete $m{\alpha }^6$ correction. The total contribution of the $m{\alpha }^6$ effects is shown to be much smaller than previously estimated, due to a large cancellation between the radiative and nonradiative parts of this correction. As a result of our calculations, we confirm the previously reported deviations between measured transition energies and theoretical predictions for the $nD\text{−}2S$ and $nD\text{−}2P$ transitions. Possible reasons for this discrepancy are analyzed.

Starting from an ultracold ensemble of ${}^6{\mathrm{Li}}_2$ Feshbach molecules, we produce deeply bound triplet $a\left(1{}^3{\mathrm{\Sigma }}_u{}^{+}\right)$ molecules in a single quantum state by stimulated Raman adiabatic passage. The ensemble lifetime is found to be limited by two-body molecule-molecule collisions that depend on both the vibrational and rotational states. The loss rate observed is near the universal rate for the $|v=0,5,8;N=0,2\rangle$ states and, remarkably, below universality for the $|v=9,N=0\rangle$ state. We also observe a strong rotational state dependence of the $v=9$ reaction rate. Consistent with theoretical predictions, we observe that molecules in the absolute lowest triplet level are unstable and likely decay due to trimer formation.

We present a computational framework for canonical quantization in arbitrary inhomogeneous dielectric media by incorporating quantum electromagnetic effects into complex solutions of quantum Maxwell's equations. To do so, the proposed algorithm integrates and performs (1) numerical computation of normal modes and (2) evaluation of arbitrary products of ladder operators acting on multimode Fock states. The former is associated with Hermitian-Helmholtz linear systems using finite-element or finite-difference methods; consequently, the complete set of numerical normal modes diagonalizes the Hamiltonian operators up to floating-point precision. Its Hermiticity is retained, allowing its quantization. Then, we perform quantum numerical simulations of two-photon interference occurring in a 50:50 beam splitter to observe the Hong-Ou-Mandel effect. Our prototype model is useful for numerical analyses on various narrow-band quantum-optical multiphoton systems such as quantum metasurfaces, quantum-optical filters, and quantum electrodynamics in open optical cavities.

We show that the quantum Cramér-Rao bound on the error of measurement of the optical phase gradient with a beam of finite width (or the wavefront tilt within a finite aperture) is consistent with the Fourier-transform uncertainty principle for the single-photon state, and is a factor of $N$ lower for the maximally entangled $N$-photon state. This fundamental bound therefore governs the tradeoff between quantum sensitivity and spatial resolution. Error bounds for a structured configuration using binary projective-field measurements implemented by an image-inversion (II) interferometer are higher, and the factor of $N$ advantage attained by the $N$-photon maximally entangled state is reduced and eventually washed out as the beam width or the phase gradient increases. This reduction is more rapid for larger $N$, so that the quantum advantage is more vulnerable. The precision of the II interferometer is greater than that based on a split detector placed in the focal plane of a lens.

We analyze the scattering dynamics and spectrum of a quantum particle on a tight-binding lattice subject to a non-Hermitian (purely imaginary) local potential. The reflection, transmission, and absorption coefficients are studied as a function of the strength of this absorbing potential. The system is found to have an exceptional point at a certain strength of the potential. Unusually, all (or nearly all) of the spectrum pairs up into mutually coalescing eigenstate pairs at this exceptional point. At large potential strengths, the absorption coefficient decreases and the effect of the imaginary potential is similar to that of a real potential. We quantify this similarity by utilizing properties of a localized eigenstate.