We investigated both spatial and temporal profiles of the 420-nm yoked superfluorescence (YSF) from the atomic vapor of rubidium by driving the $5S-5D$ two-photon transition with an ultrashort laser pulse. By increasing the pump-pulse power beyond the saturation intensity, the spatial profile of the 420-nm YSF periodically changed between a central bright spot and a ring-shaped radial profile. The temporal profile of the 420-nm YSF was measured with a sampling oscilloscope with a time resolution of 50 ps. The experimental results were successfully reproduced by simulations using the Maxwell-Bloch equations. The simulated results confirmed that, in the strong excitation regime, the 420-nm emission from the lower transition component of the YSF is significantly delayed with respect to the 5.2-$\mu \mathrm{m}$ emission from the upper transition, while they are overlapped in the weak excitation regime.

The implementation of nonclassical light sources is becoming increasingly important for various quantum applications. A particularly interesting approach is to integrate such functionalities on a single chip as this could pave the way towards fully scalable quantum photonic devices. Several approaches using dielectric systems have been investigated in the past. However, it is still not understood how on-chip nanoplasmonic antennas, interacting with a single quantum emitter, affect the quantum statistics of photons reflected or transmitted in the guided mode of a waveguide. Here we investigate a quantum photonic platform consisting of an evanescently coupled nanoplasmonic cavity-emitter system and discuss the requirements for nonclassical light generation. We develop an analytical model that incorporates quenching due to the nanoplasmonic cavity to predict the quantum statistics of the transmitted and reflected guided waveguide light under weak coherent pumping. The analytical predictions match numerical simulations based on a master equation approach. It is moreover shown that for resonant excitation the degree of antibunching in transmission is maximized for an optimal cavity modal volume $V_c$ and cavity-emitter distance $s$. In reflection, perfectly antibunched light can only be obtained for specific $(V_c,s)$ combinations. Finally, our model also applies to dielectric cavities and as such can guide future efforts in the design and development of on-chip nonclassical light sources using dielectric and nanoplasmonic cavity-emitter systems.

We study optical Bloch oscillations in the one- and two-dimensional arrays of helical waveguides with transverse refractive index gradient. Longitudinal rotation of waveguides may lead to notable variations of the width of the band of quasienergies and even its complete collapse for certain radii of the helix. This drastically affects the amplitude and direction of Bloch oscillations. Thus they can be completely arrested for certain helix radii or their direction can be reversed. If the array of helical waveguides is truncated and the near-surface waveguide is excited, the helix radius determines whether periodic Bloch oscillations persist or are replaced by the irregular near-surface oscillations.

We derive an exact computationally-efficient formula for the angular Schmidt spectrum of orbital angular momentum (OAM) entangled states produced by parametric down-conversion (PDC). Our formula yields the true spectrum and does not suffer from convergence issues arising due to infinite summations, as has been the case with previously derived formulas. We use this formula to experimentally characterize the angular Schmidt spectrum of entangled photons produced by PDC with noncollinear phase matching. We report measurements of very broad angular Schmidt spectra, corresponding to the angular Schmidt numbers up to 229. Our work could have important implications for OAM-based quantum information applications.

The entangled quantum state spanned in a spatial domain is of special importance for fundamental research in quantum information. Previous experiments on spatial entanglement were individually studied on continuous variables of momentum and position bases and discrete variables of orbital-angular-momentum (OAM) space. Momentum and position entanglement in more complex scenarios with structured wavefronts, such as vortex phases, have not been investigated, which is promising for studying the entanglement from continuous to discrete variables in infinite Hilbert space, thus providing much needed benchmarks for studying spatial high-dimensional entanglement. Here, by measuring the transverse position or momentum of entanglement, we detect the arbitrary phase in the spatial domain, including the vortex phase dependence on the parity of OAM superposition and the relative phase in OAM superposition. Our demonstrations on spatial entanglement show a very promising way to explore high-dimensional space and different degrees of freedom of entanglement in quantum information.

Four-photon stimulated Raman adiabatic passage–like (STIRAP-like) processes in the frame of the $D_1$ optical transition line of atomic ${}^{39}\mathrm{K}$ are investigated theoretically by means of a rigorous model that takes into account all the atomic states involved in the interaction with the pump and Stokes light fields and is valid for any field intensities. The same sequential up-down-up-down transition involving two pump (up) and two Stokes (down) photons, going from the initial ground state $|F=2,M=+2\rangle$ to the final ground state $|F=1,M=0\rangle$, which was proven in the past to be very convenient for two-photon amplification and lasing, is considered here. The fact that two atomic states, one with $F=1$ and another with $F=2$, participate at each intermediate step of the multiphoton process makes the existence of an ideal unique adiabatic transfer state impossible, and at exact four-photon resonance the population-transfer efficiency to the final atomic state is low (below 60% in general, 80% in the best conditions). Nevertheless, it is shown that by choosing appropriate static detunings for the pump and Stokes fields with respect to the atomic transitions the system follows, sequentially, two adiabatic states, connected by an efficient diabatic passage, so that population-transfer efficiency asymptotically approaching 100% for increasing field amplitudes can be reached. These results are quite robust and suggest the possibility of using STIRAP (or STIRAP-like) pumping to, among other applications, increase the (presently very low) efficiency of two-photon amplification and lasing, a so far unfulfilled goal in quantum and nonlinear optics.

The goal of entanglement distillation is to turn a large number of weakly entangled states into a smaller number of highly entangled ones. Practical entanglement distillation schemes offer a trade-off between the fidelity to the target state and the probability of successful distillation. Exploiting such trade-offs is of interest in the design of quantum repeater protocols. Here, we present a number of methods to assess and optimize entanglement distillation schemes. We start by giving a numerical method to compute upper bounds on the maximum achievable fidelity for a desired probability of success. We show that this method performs well for many known examples by comparing it to well-known distillation protocols. This allows us to show optimality for many well-known distillation protocols for specific states of interest. As an example, we analytically prove optimality of the distillation protocol utilized within the Extreme Photon Loss entanglement generation scheme, even in the asymptotic limit. We proceed to present a numerical method that can improve an existing distillation scheme for a given input state, and we present an example for which this method finds an optimal distillation protocol. An implementation of our numerical methods is available as a Julia package.

The clustering and drift properties of barium ions in xenon gas are explored theoretically, using density-functional theory and computational ion mobility theory, with the goal of better understanding barium ion transport for neutrinoless double beta decay. We derive the equilibrium conformations, energies, and entropies of molecular ions in the ${\mathrm{Ba}}^{+}$-Xe and ${\mathrm{Ba}}^{2+}$-Xe systems, which yield a predictive model of cluster formation in high-pressure gas. We calculate ion-neutral interaction potential curves for these species and use them to predict effective molecular ion mobilities. Our calculation consistently reproduces experimental data on effective mobility and molecular ion formation for the ${\mathrm{Ba}}^{+}$ system, and predicts strong cluster formation in the ${\mathrm{Ba}}^{2+}$ system, dominated by stable ${\left[{\mathrm{BaXe}}_6\right]}^{2+},{\left[{\mathrm{BaXe}}_7\right]}^{2+}$, ${\left[{\mathrm{BaXe}}_8\right]}^{2+}$, and ${\left[{\mathrm{BaXe}}_9\right]}^{2+}$ complexes in the range of interest. Some implications for barium tagging in gas-phase neutrinoless double beta decay experiments are discussed, and the first predictions of pressure-dependent mobility of the doubly charged ${\mathrm{Ba}}^{2+}$ species are presented.

Potential realization of a quantum thermometer operating in the nanokelvin regime, formed by a few-fermionic mixture confined in a one-dimensional harmonic trap, is proposed. Thermal states of the system are studied theoretically from the point of view of fundamental sensitivity to temperature changes. It is pointed out that the ability to control the interaction strength in such systems allows obtaining high-temperature sensitivity in the regime where the temperature is much lower than the characteristic temperature scale determined by a harmonic confinement. This sensitivity is very close to the fundamental bound that involves optimal engineering of level separations. The performance of practical measurement schemes and the possible experimental coupling of the thermometer to the probe are discussed.

A strict mathematical framework combining perturbation theory and temporal coupled-mode theory is developed to model graphene-induced saturable absorption in graphene-enhanced nanophotonic resonators. To allow for loss saturation in graphene, a power-dependent model of its surface conductivity is carefully introduced, based on the underlying physics. The framework is then cautiously unfolded to capture the two-dimensional nature of graphene and its interaction with the electromagnetic mode, additionally allowing one to incorporate any bulk or sheet material that is subject to saturable loss, together with other nonlinear effects. Both exact and approximative approaches are introduced, revealing the capabilities of the proposed framework to address the effect of saturable absorption. A graphene-enhanced silicon slab ring resonator is examined using the developed framework, uncovering its excellent accuracy and its capability to downgrade the computational complexity of a full-wave nonlinear simulation to a phenomenological but physically definitive differential equation. The potential of the resonant structure to act as an optically addressed switching element is being demonstrated, exposing high extinction ratio and low power requirements. Finally, it is illustrated how the framework is capable of capturing the rich dynamics of a resonant system that may additionally exhibit Kerr- and/or free-carrier-induced optical bistability and self-pulsation.

The possibility of observing nonclassical features in a physical system comprised of a cavity with two ensembles of two-level atoms has been investigated by considering different configurations of the ensembles with respect to the node and antinode of the cavity field under the framework of open quantum systems. The study reveals the strong presence of nonclassical characters in the physical system by establishing the existence of many facets of nonclassicality, such as the sub-Poissonian boson statistics and squeezing in single modes, intermodal squeezing, intermodal entanglement, antibunching, and steering. The effect of a number of parameters, characterizing the physical system, on the different aspects of nonclassicality is also investigated. Specifically, it is observed that the depth of the nonclassicality witnessing parameters can be enhanced by externally driving one of the ensembles with an optical field. The work is done in the presence of open system effects, in particular, use is made of Langevin equations along with a suitable perturbative technique.

Nonlinear THz photonics is probably the last frontier of nonlinear optics. The strength of both the electric and the magnetic fields of these ultrashort low-frequency light bunches opens the way to exciting science and applications. Progress in the field though is slow because of the deficiency in suitable sources. Here we show that two-color filamentation of midinfrared $3.9\mu \mathrm{m}$ laser pulses allows one to generate single-cycle THz pulses with multimillijoule energies and record conversion efficiencies. Moreover, the focused THz peak electric and magnetic fields reach values of GV/cm and kT, respectively, exceeding by far any available quasi-dc field source today. These fields enable extreme field science, including into other, relativistic phenomena. In addition, we elucidate the origin of this high efficiency, which is made up of several factors, including a mechanism where the harmonics produced by the midinfrared pulses strongly contribute to the field symmetry breaking and enhance the THz generation.

The dispersive behavior of materials with atomic charge numbers varying from $Z=4$ (beryllium, Be) to $Z=82$ (lead, Pb) was investigated experimentally and theoretically at $\gamma$-ray energies up to 2 MeV. The experiment was performed at the double-crystal $\gamma$ spectrometer GAMS6 of the Institut Laue-Langevin in Grenoble. The experimental results were compared with theoretical calculations which account for all major elastic processes involved. Overall, we found a good agreement between theory and experiment. We find that, for the development of refractive optics at $\gamma$-ray energies beyond those currently in use, high-$Z$ materials become increasingly attractive compared to the beryllium lens-stacks used at x-ray energies.

For ${}^{229m}\mathrm{Th}$ isomer nucleus with an anomalously low nuclear excitation energy, $E^{★}$ (currently accepted value of $7.8\pm 0.5\mathrm{eV}$), the bound internal conversion (BIC) decay process is caused by the excitation of a valence electron that is sensitive to the electronic structure of the atomic-sized neighborhood. We analyze an experiment where an impacting Th ion is neutralized by a negatively charged ion-receiving cesium iodide (CsI) surface, to obtain the minimal nuclear excitation energy $E_{\min }^{★}$ necessary for BIC promotion of a Th-impurity valence electron in a CsI matrix. We analyze two cases: CsI with Th deposited into the bulk and with Th deposited on the CsI surface. In the bulk we consider band gap effects while on the surface we consider the work function. The energy to pull the electron to the surface is $1.5–2.2\mathrm{eV}$ depending on the surface plane, while to promote to the conduction band in the bulk it is $1.5\mathrm{eV}$. Therefore we conclude that the Th-surface interaction can significantly reduce the lower $E^{★}$ bound of 6.3 eV, as estimated from the direct observation of the Th-clock transition [von der Wense et al., Nature (London) 533, 47 (2016)]. We suggest coating the multichannel plate with differing materials leading to different Th-impurity gaps and work functions can further narrow the $E^{★}$ uncertainty interval.

The operation of autonomous finite-time quantum heat engines relies on the existence of a stable limit cycle in which the dynamics becomes periodic. The two main questions that naturally arise are therefore whether such a limit cycle will eventually be reached and, once it has, what the state of the system is within the limit cycle. In this paper we show that the application of Floquet's theory to Lindblad dynamics offers clear answers to both questions. By moving to a generalized rotating frame, we show that it is possible to identify a single object, the Floquet Liouvillian, which encompasses all operating properties of the engine. First, its spectrum dictates the convergence to a limit cycle. Second, the state within the limit cycle is precisely its zero eigenstate, therefore reducing the problem to that of determining the steady state of a time-independent master equation. To illustrate the usefulness of this theory, we apply it to a harmonic oscillator subject to a time-periodic work protocol and time-periodic dissipation, an open-system generalization of the Ermakov-Lewis theory. The use of this theory to implement a finite-time Carnot engine subject to continuous frequency modulations is also discussed.

Quantum network coding is an effective solution for alleviating bottlenecks in quantum networks. We introduce a measurement-based quantum network coding scheme for quantum repeater networks (MQNC), and analyze its behavior based on results acquired from Monte Carlo simulation that includes various error sources over a butterfly network. By exploiting measurement-based quantum computing, operation on qubits for completing network coding proceeds in parallel. We show that such an approach offers advantages over other schemes in terms of the quantum circuit depth and therefore improves the communication fidelity without disturbing the aggregate throughput. The circuit depth of our protocol has been reduced by 56.5% compared to the quantum network coding scheme (QNC) introduced in 2012 by Satoh et al. For MQNC, we have found that the resulting entangled pairs' joint fidelity drops below 50% when the accuracy of local operations is lower than 98.9%, assuming that all initial Bell pairs across quantum repeaters have a fixed fidelity of 98%. Overall, MQNC showed substantially higher error tolerance compared to QNC and slightly better than buffer space multiplexing using step-by-step entanglement swapping, but not quite as strong as simultaneous entanglement swapping operations.

Boson Sampling is the problem of sampling from the same output probability distribution as a collection of indistinguishable single photons input into a linear interferometer. It has been shown that, subject to certain computational complexity conjectures, in general the problem is difficult to solve classically, motivating optical experiments aimed at demonstrating quantum computational “supremacy.” There are a number of challenges faced by such experiments, including the generation of indistinguishable single photons. We provide a quantum circuit that simulates Boson Sampling with arbitrarily distinguishable particles. This makes clear how distinguishabililty leads to decoherence in the standard quantum circuit model, allowing insight to be gained. At the heart of the circuit is the quantum Schur transform, which follows from a representation theoretic approach to the physics of distinguishable particles in first quantization. The techniques are quite general and have application beyond Boson Sampling.

We probe dephasing mechanisms within a quantum network node consisting of a single nitrogen-vacancy center electron spin that is hyperfine coupled to surrounding ${}^{13}\mathrm{C}$ nuclear-spin quantum memories. Previous studies have analyzed memory dephasing caused by the stochastic electron-spin reset process, which is a component of optical internode entangling protocols. Here, we find, by using dynamical decoupling techniques and exploiting phase matching conditions in the electron-nuclear dynamics, that control infidelities and quasistatic noise are the major contributors to memory dephasing induced by the entangling sequence. These insights enable us to demonstrate a 19-fold improved memory performance which is still not limited by the electron reinitialization process. We further perform pump-probe studies to investigate the spin-flip channels during the optical electron spin reset. We find that spin flips occur via decay from the metastable singlet states with a branching ratio of 8(1):1:1, in contrast with previous work. These results allow us to formulate straightforward improvements to diamond-based quantum networks and similar architectures.

The Deutsch-Jozsa algorithm is one of the first examples of a quantum algorithm that is exponentially faster than any possible deterministic classical algorithm. It was proposed by Deutsch and Jozsa in 1992 with improvements by Cleve, Ekert, Macchiavello, and Mosca in 1998. The Deutsch-Jozsa problem is a promise problem and we can equivalently describe it as a partial function ${\text{DJ}}_n^0:{\{0,1\}}^n\to \{0,1\}$ defined as ${\text{DJ}}_n^0\left(x\right)=1$ for $\left|x\right|=n/2, {\text{DJ}}_n^0\left(x\right)=0$ for $\left|x\right|=0,n$, and it is undefined for the rest of the cases, where $n$ is even, and $\left|x\right|$ is the Hamming weight of $x$. The optimal quantum algorithm needs only one query to compute ${\text{DJ}}_n^0$ but the classical deterministic algorithm requires $2^{n-1}+1$ queries to compute it in the worse case. In this article, we generalize the Deutsch-Jozsa problem as ${\text{DJ}}_n^k\left(x\right)=1$ for $\left|x\right|=n/2, {\text{DJ}}_n^k\left(x\right)=0$ for $\left|x\right|$ in the set $\{0,1,...,k,n-k,n-k+1,...,n\}$, and it is undefined for the rest of the cases, where $0\le k<n/2$. In particular, we give and prove an optimal exact quantum query algorithm with complexity $k+1$ for computing the generalized Deutsch-Jozsa problem ${\text{DJ}}_n^k$. It is clear that the case of $k=0$ is in accordance with the Deutsch-Jozsa problem. Also, we give a method for finding the approximate and exact degrees of symmetric partial Boolean functions.

Magic states can be used as a resource to circumvent the restrictions due to stabilizer-preserving operations, and magic-state conversion has not been studied in the single-copy regime thus far. Here we solve the question of whether a stabilizer-preserving quantum operation exists that can convert between two given magic states in the single-shot regime. We first phrase this question as a feasibility problem for a semidefinite program, which provides a procedure for constructing a stabilizer-preserving quantum operation (free channel) if it exists. Then we employ a variant of the Farkas lemma to derive necessary and sufficient conditions for existence, and this method is used to construct a complete set of magic monotones.