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This paper presents the analytical inquisition of photonic entanglement generation circuit (PEGC) for non local Qubits designed using quantum dot within the optical micro-cavity, considering practical environment. It is established that the concurrence of the PEGC is considerably contingent on the interplay between spin of quantum dot and photon within the optical micro-cavity. The maximum concurrence obtained is 61.24 at \(g/k=.3\) and \({k}_{s}/k=0.1\) and 90.11 at \(g/k=4\) and \({k}_{s}/k=0.1\) with and without noisy environment, respectively. Different quantum cryptography-based protocols may be implemented using reported work.

Motivated by the Bose et al.-Matletto-Vedral (BMV) proposal for detecting quantum superposition of spacetime geometries, we study a toy model of a quantum entanglement generation between two spins (qubits) mediated by a relativistic free scalar field. After time evolution, spin correlation is generated through the interactions with the field. Because of the associated particle creation into an open system, the quantum state of spins is partially decohered. In this paper, we give a comprehensive study of the model based on the closed-time path formalism, focussing on relativistic causality and quantum mechanical complementarity. We calculate various quantities such as spin correlations, entanglement entropies, mutual information and negativity, and study their behaviors in various limiting situations. In particular, we calculate the mutual information of the two spins and compare it with spin correlation functions. We also discuss why no quantum entanglement can be generated unless both spins are causally affected by one another while spin correlations are generated., 45 pages, 19 figures

We study maximal coin-position entanglement generation via a discrete-time quantum walk, in which the coin operation is randomly selected from one of two coin operators set at each step. We solve maximal entanglement generation as an optimization problem with quantum process fidelity as the cost function. Then we determine the maximal entanglement that can be rigorously generated for any step beyond the second regardless of initial condition with appropriate coin sequences. The simplest coin sequence comprising Hadamard and identity operations is equivalent to the generalized elephant quantum walk, which exhibits an increasingly faster spreading in terms of probability distribution. Experimentally, we demonstrate a ten-step quantum walk driven by such coin sequences with linear optics, and thereby show the desired high-dimensional bipartite entanglement as well as the transport behavior of faster spreading., 13 pages, 6 figures. Comments are welcome

Quantum network applications such as distributed quantum computing and quantum secret sharing represent a promising future network equipped with quantum resources. Entanglement generation and distribution over long distances are critical and unavoidable when utilizing quantum technology in a fully connected network. The distribution of bipartite entanglement over long distances has seen some progress, while the distribution of multipartite entanglement over long distances remains unsolved. Here we report a two-dimensional quantum repeater protocol for the generation of multipartite entanglement over long distances with an all-photonic framework to fill this gap. The entanglement generation yield remains proportional to the transmission efficiency regardless of the number of network users and shows long transmission distance under various numbers of network users. With the improved efficiency and flexibility of extending the number of users, we anticipate that our protocol can work as a significant building block for quantum networks in the future.

We propose a method of generating and detecting entanglement in two spatially separated excitonpolariton Bose-Einstein condensates (BECs) at steady-state. In our scheme we first create a spinor polariton BEC, such that steady-state squeezing is obtained under a one-axis twisting interaction. Then the condensate is split either physically or virtually, which results in entanglement generated between the two parts. A virtual split means that the condensate is not physically split, but its near-field image is divided into two parts and the spin correlations are deduced from polarization measurements in each half. We theoretically model and examine logarithmic negativity criterion and several correlation-based criteria to show that entanglement exists under experimentally achievable parameters., 10 pages, 5 figures

The heralded generation of entangled states underpins many photonic quantum technologies. As quantum error correction thresholds are determined by underlying physical noise mechanisms, a detailed and faithful characterization of resource states is required. Non-computational leakage, e.g. more than one photon occupying a dual-rail encoded qubit, is an error not captured by standard forms of state tomography, which postselect on photons remaining in the computational subspace. Here we use the continuous-variable (CV) formalism and first quantized state representation to develop a simulation framework that reconstructs photonic quantum states in the presence of partial distinguishability and resulting non-computational leakage errors. Using these tools, we analyze a variety of Bell state generation circuits and find that the five photon discrete Fourier transform (DFT) Bell state generation scheme [Phys Rev. Lett. 126 23054 (2021)] is most robust to such errors for near-ideal photons. Through characterization of a photonic entangling gate, we demonstrate how leakage errors prevent a modular characterization of concatenated gates using current tomographical procedures. Our work is a necessary step in revealing the true noise models that must be addressed in fault-tolerant photonic quantum computing architectures., 24 pages, 16 figures

We investigate the dynamics of the driven open double two-level system by deriving a driven Markovian master equation based on the Lewis-Riesenfeld invariant theory. The transitions induced by coupling to the heat reservoir occur between the instantaneous eigenstates of the Lewis-Riesenfeld invariant. Therefore, different driving protocols associated with corresponding Lewis-Riesenfeld invariants result in different open system dynamics and symmetries. In particular, we show that since the instantaneous steady state of the driven double two-level system is one of eigenstates of the Lewis-Riesenfeld invariant at ultralow reservoir temperature, the inverse engineering method based on the Lewis-Riesenfeld invariants has a good performance in rapidly preparing the quantum state of open quantum systems. As an example, a perfect entangled state is generated by means of the inverse engineering method., 11 pages, 2 figures

Strong light–matter interaction plays a central role in realizing quantum photonic technologies. The entanglement state, which results from the hybridization of excitons and cavity photons, forms the foundation of quantum information science. In this work, an entanglement state is achieved by manipulating the mode coupling between surface lattice resonance and quantum emitter into the strong coupling regime. At the same time, a Rabi splitting of 40 meV is observed. A full quantum model based on the Heisenberg picture is used to describe this unclassical phenomenon, and it perfectly explains the interaction and dissipation process. In addition, the observed concurrency degree of the entanglement state is 0.5, presenting the quantum nonlocality. This work effectively contributes to the understanding of nonclassical quantum effects arising from strong coupling and will intrigue more interesting potential applications in quantum optics.

Inspired by a recent Letter [S. R. Beane et al., Phys. Rev. Lett. 122, 102001(2019)], the entanglement generated in the elastic $S$-wave scattering of $p+{}^{3}\text{He}$ and $n+{}^3\text{H}$ is studied, where the proton, neutron, ${}^{3}$He, and ${}^3$H are all regarded as qubits. To deal with the Coulomb interaction between the proton and ${}^{3}$He, we derive the entanglement power, a physical quantity that measures the average entanglement generated by a scattering process, for charged qubits within the screening method. The entanglement power in the aforementioned two few-nucleon scatterings is found to be generally much smaller than that in the $S$-wave $n+p$ scattering at low energies, with the corresponding cluster effective field theories possessing an enhanced approximate $\text{SU}(2)_1\otimes\text{SU}(2)_{2}$ symmetry at leading order. Our study suggests that the entanglement generation capacities of effective interactions between nucleons and light nuclei could be more suppressed than realistic nucleon-nucleon interactions at low energies., 7 pages, 4 figures; published version

We present protocols to generate quantum entanglement on nonlocal magnons in hybrid systems composed of yttrium iron garnet (YIG) spheres, microwave cavities and a superconducting (SC) qubit. In the schemes, the YIGs are coupled to respective microwave cavities in resonant way, and the SC qubit is placed at the center of the cavities, which interacts with the cavities simultaneously. By exchanging the virtual photon, the cavities can indirectly interact in the far-detuning regime. Detailed protocols are presented to establish entanglement for two, three and arbitrary $N$ magnons with reasonable fidelities., Comment: 10 pages, 7 figures

Multi-photon entanglement plays a central role in optical quantum technologies. One way to entangle two photons is to prepare them in orthogonal internal states, for example, in two polarisations, and then send them through a balanced beam splitter. Post-selecting on the cases where there is one photon in each output port results in a maximally entangled state. This idea can be extended to schemes for the post-selected generation of larger entangled states. Typically, switching between different types of entangled states require different arrangements of beam splitters and so a new experimental setup. Here, we demonstrate a simple and versatile scheme to generate different types of genuine tripartite entangled states with only one experimental setup. We send three photons through a three-port splitter and vary their internal states before post-selecting on certain output distributions. This results in the generation of tripartite W, G and GHZ states. We obtain fidelities of up to $(87.3 \pm 1.1)\%$ with regard to the respective ideal states, confirming a successful generation of genuine tripartite entanglement.

The generation and preservation of entanglement is a central goal in quantum techonology. Traditionally, dissipation in quantum systems is thought to be detrimental to entanglement, however dissipation can also be utilised as a means of generating entanglement between quantum spins that are not directly interacting. In particular entanglement can be generated between two qubits, or multi qubit systems via a collective coupling to a reservoir. In this work, we explore multiple spin domains pairwise coupled to different reservoirs and show that entanglement can be generated between spins which are not coupled to each other, or even coupled to the same reservoir., Comment: 9 pages, 7 figures

Supplementary Materials

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Discrete frequency-bin entanglement is an essential resource for applications in quantum information processing. In this Letter, we propose and demonstrate a scheme to generate discrete frequency-bin entanglement with a single piece of periodically poled lithium niobate waveguide in a modified Sagnac interferometer. Correlated two-photon states in both directions of the Sagnac interferometer are generated through cascaded second-order optical nonlinear processes. A relative phase difference between the two states is introduced by changing the polarization state of pump light, thus manipulating the two-photon state at the output of the Sagnac interferometer. The generated two-photon state is sent into a fiber polarization splitter, then a pure discrete frequency-bin entangled two-photon state is obtained by setting the pump light. The frequency entanglement property is measured by a spatial quantum beating with a visibility of $96.0 \pm 6.1\%$. The density matrix is further obtained with a fidelity of $98.0 \pm 3.0\%$ to the ideal state. Our demonstration provides a promising method for the generation of pure discrete frequency-bin entanglement at telecom band, which is desired in quantum photonics., Comment: 4 pages. 3 figures

This work presents a theoretical study of a protocol for dynamical generation and storage of the durable, highly entangled Greenberger-Horne-Zeilinger (GHZ) state in a system composed of bosonic atoms loaded into a one-dimensional optical lattice potential. A method of indicating entanglement in the system is also presented. The system ground-state can be either in the superfluid phase or in the Mott insulator phase. The wave functions of atoms in the superfluid phase are delocalised over the whole lattice and overlap. In the Mott phase, the wave functions are localised around lattice sites. The GHZ state is being generated in the superfluid phase and stored in the Mott insulator phase. It is achieved by a linear change of the potential depth in an optical lattice filled with atoms of two species. The numerical method used to describe the system is based on the exact diagonalisation of the Bose-Hubbard Hamiltonian. A quantum correlator indicating the level of multi-mode entanglement is introduced. Finally, it is shown that the value of the correlator indicates generation of the GHZ state. The appearance of the GHZ state is confirmed by the numerical representation of the state.

We study the remote entanglement generation between macroscopic microwave magnon modes in a coupled cavity system. The cavities are connected via an optical fiber, which necessitates the use of a frequency conversion inside the cavity. The converter may be implemented via a rare-earth doped crystal acting like an effective three-level system. The entanglement dynamics of the system is analytically studied, and an active optimal control method is also proposed where one may generate maximally entangled Bell states on demand with a given evolution time. The system dynamics and its control have also been studied in a generic non-Markovian open system framework, and the generated entanglement is found to be robust against environmental noises., 9 pages, 7 figures

In this paper, we investigate the entanglement generation of n-qubit states in a model consisting of n independent qubits, each coupled to a harmonic oscillator which is in turn coupled to a bath of N additional harmonic oscillators with nearest-neighbor coupling. With analysis, we can find that the steady multipartite entanglement with different values can be generated after a long-time evolution for different sizes of the quantum system. Under weak coupling between the system and the harmonic oscillator, multipartite entanglement can monotonically increase from zero to a stable value. Under strong coupling, multipartite entanglement generation shows a speed-up increase accompanied by some oscillations as non-Markovian behavior. Our results imply that the strong coupling between the harmonic oscillator and the N additional harmonic oscillators, and the large size of the additional oscillators will enhance non-Markovian dynamics and make it take a very long time for the entanglement to reach a stable value. Meanwhile, the couplings between the additional harmonic oscillators and the decay rate of additional harmonic oscillators have almost no effect on the multipartite entanglement generation. Finally, the entanglement generation of the additional harmonic oscillators is also discussed.

Strong light-matter interaction plays a central role in realizing quantum photonic technologies. The entanglement state, which results from the hybridization of excitons and cavity photons, forms the foundation of quantum information science. In this work, an entanglement state is achieved by manipulating the mode coupling between surface lattice resonance and quantum emitter into the strong coupling regime. At the same time, a Rabi splitting of 40 meV is observed. A full quantum model based on the Heisenberg picture is used to describe this unclassical phenomenon, and it perfectly explains the interaction and dissipation process. In addition, the observed concurrency degree of the entanglement state is 0.5, presenting the quantum non-locality. This work effectively contributes to the understanding of non-classical quantum effects arising from strong coupling and will intrigue more interesting potential applications in quantum optics.

On this PhD thesis we cover the results contained in arXiv:2001.07050, arXiv:2111.10096 and arXiv:2011.02822, while providing further details about their derivations. In the first two papers, we study the generation and detection of entangled non-Gaussian states of microwave radiation. These states are produced in a new parametric oscillator, built recently within the field of cQED, capable of down-converting a microwave tone into three different tones at once. These three photons share among their magnitudes quantum correlations, in particular genuine entanglement. In this text we refer to it as non-Gaussian because of its manifestation on statistical moments higher than covariances, and we propose a simple and practical criterion for the design of witnesses capable of detecting it: they must be built from higher statistical moments that change through time. Additionally, we speculate on the theoretical implications of the criterion and find suggestive connections to other entanglement classes, such as the paradigmatic nonequivalent GHZ and W three qubit states. In the third paper, we explore one of the possible applications of quantum technologies: analog simulation of quantum systems. The literature prior to this thesis showcases multiple examples of superconducting circuits capable of mimicking systems in which one must consider both quantum and relativistic phenomena, such as the dynamical Casimir and Unruh effects. This work explores the information that can be obtained through analog simulation, proposing a circuit capable of featuring the internal dynamics of a mirror experiencing a relativistic trajectory, that is, a mirror producing the dynamical Casimir effect., 88 pages, PhD thesis

openaire: EC/H2020/862644/EU//QUARTET | openaire: EC/H2020/820505/EU//QMiCS | openaire: EC/H2020/820363/EU//OpenSuperQ | openaire: EC/H2020/824109/EU//EMP | openaire: EC/H2020/670743/EU//QuDeT Funding Information: We thank Alpo Ahonen, Paula Holmlund, and Harri Pohjonen for technical assistance and Terra Quantum AG for scientific support. K.V.P. is funded by the European Union’s Horizon 2020 research and innovation programme under Grant Agreement No. 862644 (FET-Open project: Quantum readout techniques and technologies, QUARTET). The work at VTT is funded from the EU Flagship on Quantum Technology Grant No. H2020-FETFLAG-2018-03 Project Nos. 820363 OpenSuperQ and 820505 QMiCS. The contribution of M.R.P., I.L., M.W., and A.S. is supported by Grant Agreement No. 824109 (European Microkelvin Platform project, EMP), and ERC Grant Agreement No. 670743 (QuDeT). P.J.H. and V.V. acknowledge financial support from the Academy of Finland through Grants No. 314448 and No. 321700, respectively. The work of S.H.R. and P.J.H. is supported by a MATINE research grant. G.S.P. and K.V.P. thank Saab for scientific collaboration under a research agreement with Aalto University. This work is done under the “Finnish Center of Excellence in Quantum Technology QTF” of the Academy of Finland, Projects No. 312059, No. 312294, No. 312295, No. 336810, and No. 312296. Publisher Copyright: © 2022 American Physical Society. Entangled microwave photons form a fundamental resource for quantum information processing and sensing with continuous variables. We use a low-loss Josephson metamaterial comprising superconducting, nonlinear, asymmetric inductive elements to generate frequency-entangled photons from vacuum fluctuations at a rate of 2 giga entangled bits per second spanning over the 4-GHz bandwidth. The device is operated as a traveling-wave parametric amplifier under Kerr-relieving biasing conditions. Furthermore, we demonstrate single-mode squeezing in such devices - 3.1±0.7dB below the zero-point level at half of modulation frequency.

A first quantum revolution has already brought quantum technologies into our everyday life for decades: in fact, electronics and optics are based on the quantum mechanical principles. Today, a second quantum revolution is underway, leveraging the quantum principles of superposition, entanglement and measurement, which were not fully exploited yet. International innovation activities and standardization bodies have identified four main application areas for quantum technologies and services: quantum secure communications, quantum computing, quantum simulation, and quantum sensing and metrology. This paper focuses on quantum secure communications by addressing the evolution of Quantum Key Distribution (QKD) networks (under early exploitation today) towards the Quantum-ready networks and the Quantum Internet based also on entanglement distribution. Assuming that management and control of quantum nodes is a key challenge under definition, today, a main obstacle in exploiting long-range QKD and Quantum-ready networks concerns the inherent losses due to the optical transmission channels. Currently, it is assumed that a most promising way for overcoming this limitation, while avoiding the presence of costly trusted nodes, it is to distribute entangled states by means of Quantum Repeaters. In this respect, the paper provides an overview of current methods and systems for end-to-end entanglement generation, with some simulations and a discussion of capacity upper bounds and their impact of secret key rate in QKD systems.

In contrast to classical systems, actual implementation of non-Hermitian Hamiltonian dynamics for quantum systems is a challenge because the processes of energy gain and dissipation are based on the underlying Hermitian system&ndash, environment dynamics, which are trace preserving. Recently, a scheme for engineering non-Hermitian Hamiltonians as a result of repetitive measurements on an ancillary qubit has been proposed. The induced conditional dynamics of the main system is described by the effective non-Hermitian Hamiltonian arising from the procedure. In this paper, we demonstrate the effectiveness of such a protocol by applying it to physically relevant multi-spin models, showing that the effective non-Hermitian Hamiltonian drives the system to a maximally entangled stationary state. In addition, we report a new recipe to construct a physical scenario where the quantum dynamics of a physical system represented by a given non-Hermitian Hamiltonian model may be simulated. The physical implications and the broad scope potential applications of such a scheme are highlighted.

Quantum entanglement is an essential resource for quantum networks. However, the generation of entanglement between physical devices at remote network nodes is a challenging task towards practical implementation of quantum networks. In this work, we use the open-source Simulator of QUantum Network Communication (SeQUeNCe), developed by our team, to simulate entanglement generation between two atomic frequency comb (AFC) absorptive quantum memories to be deployed on the Argonne-Chicago quantum network. We realize the representation of photonic quantum states within truncated Fock spaces in SeQUeNCe and build models for a spontaneous parametric down-conversion (SPDC) source, AFC absorptive quantum memories, and measurement devices with non-number-resolving photon detectors. Based on these developments, we observe varying fidelity with SPDC source mean photon number, and varying entanglement generation rate with both mean photon number and memory mode number. We also simulate tomographic reconstruction of the effective density matrix for the bipartite photonic states retrieved from quantum memories. Our work extends the usability of the SeQUeNCe simulator with new hardware modules and Fock state representation that will improve the simulation of near term quantum network hardware and protocols., Comment: 7 pages, 5 figures, published version

The Bose-Marletto-Vedral (BMV) experiment [3, 4] is a proposal for testing the quantum nature of gravity with entanglement due to Newtonian gravity. This proposal has stimulated controversy on how the entanglement due to Newtonian gravity is related to the essence of quantum gravity and the existence of gravitons. Motivated by this, we analyze the entanglement generation between two charged particles coupled to a photon field. We assume that each particle is in a superposition of two trajectories and that the photon field is initially in a coherent state. Based on covariant quantum electrodynamics, the formula for the entanglement negativity of the charged particles is derived for the first time. Adopting simple analytic trajectories of the particles, we demonstrate the entanglement between them. It is observed that the entanglement is suppressed by the decoherence due to the vacuum fluctuations of the photon field. We also find that the effect of quantum superposition of bremsstrahlung appears in the entanglement negativity formula. The similar structures between the gravity theory and electromagnetic theory suggests that a similar feature may be observed in the entanglement generation by quantum gravitational radiation., Comment: 34 pages, 16 figures

Entanglement is a key resource for quantum information technologies ranging from quantum sensing to quantum computing. Conventionally, the entanglement between two coupled qubits is established at the time scale of the inverse of the coupling strength. In this work, we study two weakly coupled non-Hermitian qubits and observe entanglement generation at a significantly shorter time scale by proximity to a higher-order exceptional point. We establish a non-Hermitian perturbation theory based on constructing a biorthogonal complete basis and further identify the optimal condition to obtain the maximally entangled state. Our study of speeding up entanglement generation in non-Hermitian quantum systems opens new avenues for harnessing coherent nonunitary dissipation for quantum technologies., Comment: 6+10 pages, 4+5 figures. Zeng-Zhao Li and Weijian Chen contributed equally to this work

We develop a Floquet protocol for long-range entanglement generation in the one-dimensional quantum Potts model, which generalizes the transverse-filed Ising model by allowing each spin to have $n>2$ states. We focus on the case of $n=3$, so that the model describes a chain of qutrits. The suggested protocol creates qutrit Bell-like pairs with non-local long-range entanglement that spans over the entire chain. We then conjecture that the proposed Floquet protocol is integrable and explicitly construct a few first non-trivial conserved quantities that commute with the stroboscopic evolution operator. Our analysis of the Floquet integrability relies on the deep connection between the quantum Potts model and a much broader class of models described by the Temperley-Lieb algebra. We work at the purely algebraic level and our results on Floquet integrability are valid for any representation of the Temperley-Lieb algebra. We expect that our findings can be probed with present experimental facilities using Rydberg programmable quantum simulators and can find various applications in quantum technologies., 12+10 pages, 2 figures

We consider a system of multiple qubits without any quantum control. We show that one can mediate entanglement between different subsystems in a controlled way by adding a (locally) controlled auxiliary system of the same size that couples via an always-on, distant dependent interaction to the system qubits. Solely by changing the internal state of the control system, one can selectively couple it to selected qubits, and ultimately generate different kinds of entanglement within the system. This provides an alternative way for quantum control and quantum gates that does not rely on the ability to switch interactions on and off at will, and can serve as a locally controlled quantum switch where all entanglement patterns can be created. We demonstrate that such an approach also offers an increased error tolerance w.r.t. position fluctuations., 15 + 7 pages, 5 figures, 3 tables

In this paper, we analyze the macroscopic entanglement generation between two movable mirrors in an optomechanical system beyond the Markov approximation. We derive a non-Markovian master equation for the optomechanical system and analyze the entanglement generation based on the numerical simulation. It is found that the maximum entanglement can be achieved by utilizing both the optomechanical coupling and the non-Markovian properties of the bath. In particular, we show the contribution from the system-bath coupling has a positive impact only if the bath is non-Markovian. Besides, the symmetric properties of the optomechanical system as well as the initial state are crucial to the entanglement generation. The results presented in the paper illustrate that the impact of the non-Markovian bath is an important factor in the entanglement generation in the optomechanical system.

We consider two qubits interacting with a common bosonic bath, but not directly between themselves. We derive the (bipartite) entanglement generation conditions for Gaussian non-Markovian dynamical maps and show that they are similar as in the Markovian regime. However, they depend on different physical coefficients and hold on different time scales. Indeed, for small times, in the non-Markovian regime entanglement is possibly generated on a shorter time scale (α t2 ) than in the Markovian regime (α t). Moreover, although the singular coupling limit of non-Markovian dynamics yields Markovian ones, we show that the same limit does not lead from non-Markovian entanglement generation conditions to Markovian ones. Also, entanglement generation conditions do not depend on the initial time for non-Markovian open dynamics resulting from couplings to bosonic Gaussian baths, while they may depend on time for open dynamics originated by couplings to classical, stochastic Gaussian environments.

We develop a measurement operator formalism to handle quantum nondemolition (QND) measurement induced entanglement generation between two atomic gases. We first derive how the QND entangling scheme reduces to a positive operator-valued measure, and consider its limiting case when it can be used to construct a projection operator that collapses the state to a total spin projection state. We then analyze how a stroboscopic sequence of such projections made in the x and z basis evolves the initial wave function. Such a sequence of QND projections can enhance the entanglement between the atomic ensembles and makes the state converge towards a highly entangled state. We show several mathematical identities which greatly simplify the state evolution in the projection sequence and allow one to derive the exact state in a highly efficient manner. Our formalism does not use the Holstein-Primakoff approximation as is conventionally done, and treats the spins of the atomic gases in an exact way., 18 pages, 10 figures

Entanglement is a fundamental concept at the core of quantum information science, both in terms of its theoretical underpinnings and practical applications. A key priority in studying and utilizing entanglement is to find reliable procedures for the generation of entangled states. In this research, we propose a graph-based method for systematically searching for schemes that can produce genuine entanglement in arbitrary N-partite linear bosonic systems, without postselection. While the entanglement generation without postselection renders more tolerable schemes for quantum tasks, it is in general more challenging to find appropriate circuits for systems with a large number of parties. We present a straightforward strategy to mitigate the limitation through the implementation of our graph technique. Our physical setup is based on the sculpting protocol, which utilizes an $ N$ indeterministic subtractions of single bosons to convert Fock states of evenly distributed bosons into entanglement. We have identified general schemes for qubit N-partite GHZ and W states. These schemes are significantly more efficient than previous schemes. Furthermore, we have found an N-partite GHZ state generation scheme for qudits, which was never proposed before to our knowledge because of their complicated structures. These results demonstrate the power of our approach in discovering simple solutions for the generation of intricate entangled states. Our schemes can be directly realizable in many-boson systems. As a proof of concept, we propose a linear optical scheme for the generation of the Bell state by heralding detections. Our method has yet to exhaust its potential, and is expected to serve as a promising tool in generating diverse entanglement., Comment: REVTeX 4.2, 16 pages, comments are welcome; (v2) 19 pages

Quantum networks, which enable the transfer of quantum information across long distances, promise to provide exciting benefits and new possibilities in many areas including communication, computation, security, and metrology. These networks rely on entanglement between qubits at distant nodes to transmit information; however, creation of these quantum links is not dependent on the information to be transmitted. Researchers have explored schemes for continuous generation of entanglement, where network nodes may generate entanglement links before receiving user requests. In this paper we present an adaptive scheme that uses information from previous requests to better guide the choice of randomly generated quantum links before future requests are received. We analyze parameter spaces where such a scheme may provide benefit and observe an increase in performance of up to 75% over other continuous schemes on single-bottleneck and autonomous systems networks. We also test the scheme for other parameter choices and observe continued benefits of up to 95%. The power of our adaptive scheme on a randomized request queue is demonstrated on a single-bottleneck topology. We also explore quantum memory allocation scenarios, where a difference in latency performance implies the necessity of optimal allocation of resources for quantum networks., 6 pages, 7 figures, 1 table, published version

We discuss the generation and the long-time persistence of entanglement in open two-qubit systems whose reduced dissipative dynamics is not a priori engineered but is instead subjected to filtering and Markovian feedback. In particular, we analytically study (1) whether the latter operations may enhance the environment capability of generating entanglement at short times and (2) whether the generated entanglement survives in the long-time regime. We show that, in the case of particularly symmetric Gorini–Kossakowski–Sudarshan–Lindblad it is possible to fully control the convex set of stationary states of the two-qubit reduced dynamics, therefore the asymptotic behaviour of any initial two-qubit state. We then study the impact of a suitable class of feed-back operations on the considered dynamics.