Entanglement Generation Research Articles

Generation of genuine all-way entanglement in defect-nuclear spin systems through dynamical decoupling sequences

Multipartite entangled states are an essential resource for sensing, quantum error correction, and cryptography. Color centers in solids are one of the leading platforms for quantum networking due to the availability of a nuclear spin memory that can be entangled with the optically active electronic spin through dynamical decoupling sequences. Creating electron-nuclear entangled states in these systems is a difficult task as the always-on hyperfine interactions prohibit complete isolation of the target dynamics from the unwanted spin bath. While this emergent cross-talk can be alleviated by prolonging the entanglement generation, the gate durations quickly exceed coherence times. Here we show how to prepare high-quality GHZM-like states with minimal cross-talk. We introduce the M-tangling power of an evolution operator, which allows us to verify genuine all-way correlations. Using experimentally measured hyperfine parameters of an NV center spin in diamond coupled to carbon-13 lattice spins, we show how to use sequential or single-shot entangling operations to prepare GHZM-like states of up to M=10 qubits within time constraints that saturate bounds on M-way correlations. We study the entanglement of mixed electron-nuclear states and develop a non-unitary M-tangling power which additionally captures correlations arising from all unwanted nuclear spins. We further derive a non-unitary M-tangling power which incorporates the impact of electronic dephasing errors on the M-way correlations. Finally, we inspect the performance of our protocols in the presence of experimentally reported pulse errors, finding that XY decoupling sequences can lead to high-fidelity GHZ state preparation.

Selective active resonance tuning for multi-mode nonlinear photonic cavities

Resonant enhancement of nonlinear photonic processes is critical for the scalability of applications such as long-distance entanglement generation. To implement nonlinear resonant enhancement, multiple resonator modes must be individually tuned onto a precise set of process wavelengths, which requires multiple linearly-independent tuning methods. Using coupled auxiliary resonators to indirectly tune modes in a multi-resonant nonlinear cavity is particularly attractive because it allows the extension of a single physical tuning mechanism, such as thermal tuning, to provide the required independent controls. Here we model and simulate the performance and tradeoffs of a coupled-resonator tuning scheme which uses auxiliary resonators to tune specific modes of a multi-resonant nonlinear process. Our analysis determines the tuning bandwidth for steady-state mode field intensity can significantly exceed the inter-cavity coupling rate g if the total quality factor of the auxiliary resonator is higher than the multi-mode main resonator. Consequently, over-coupling a nonlinear resonator mode to improve the maximum efficiency of a frequency conversion process will simultaneously expand the auxiliary resonator tuning bandwidth for that mode, indicating a natural compatibility with this tuning scheme. We apply the model to an existing small-diameter triply-resonant ring resonator design and find that a tuning bandwidth of 136 GHz ≈ 1.1 nm can be attained for a mode in the telecom band while limiting excess scattering losses to a quality factor of 106. Such range would span the distribution of inhomogeneously broadened quantum emitter ensembles as well as resonator fabrication variations, indicating the potential for the auxiliary resonators to enable not only low-loss telecom conversion but also the generation of indistinguishable photons in a quantum network.

A versatile single-photon-based quantum computing platform

Quantum computing aims at exploiting quantum phenomena to efficiently perform computations that are unfeasible even for the most powerful classical supercomputers. Among the promising technological approaches, photonic quantum computing offers the advantages of low decoherence, information processing with modest cryogenic requirements, and native integration with classical and quantum networks. So far, quantum computing demonstrations with light have implemented specific tasks with specialized hardware, notably Gaussian boson sampling, which permits the quantum computational advantage to be realized. Here we report a cloud-accessible versatile quantum computing prototype based on single photons. The device comprises a high-efficiency quantum-dot single-photon source feeding a universal linear optical network on a reconfigurable chip for which hardware errors are compensated by a machine-learned transpilation process. Our full software stack allows remote control of the device to perform computations via logic gates or direct photonic operations. For gate-based computation, we benchmark one-, two- and three-qubit gates with state-of-the art fidelities of 99.6 ± 0.1%, 93.8 ± 0.6% and 86 ± 1.2%, respectively. We also implement a variational quantum eigensolver, which we use to calculate the energy levels of the hydrogen molecule with chemical accuracy. For photon native computation, we implement a classifier algorithm using a three-photon-based quantum neural network and report a six-photon boson sampling demonstration on a universal reconfigurable integrated circuit. Finally, we report on a heralded three-photon entanglement generation, a key milestone toward measurement-based quantum computing.

Heralded Entanglement Distribution Between Two Spin‐Wave Memories Using Temporally Multiplexed Scheme

AbstractThe elementary link, which is formed by entanglement between two remote atomic ensembles, is a fundamental component of Duan–Lukin–Cirac–Zoller (DLCZ) quantum repeater. For practical realization of quantum repeaters, it is required that entanglement between two atomic‐ensemble‐based memories can be generated with high rates and retrieved with high efficiencies. However, so far, such quantum‐repeater links remain challenge in experiments. Here, heralded entanglement distribution between atomic‐ensemble‐based memories is demonstrated via temporally multiplexed scheme. A train of 12 write pulses in time is applied to a cloud of cold atoms along different directions, which generates temporally multiplexed pairs of Stokes photons and spin waves via DLCZ processes. The Stokes fields propagating in the two spatial modes, which are paired with two spin waves are combined to perform a single‐photon Bell‐state measurement. A successful detection projects the two spin waves into a single‐excitation entanglement, which represents entanglement distribution in an elementary link. Compared with single‐mode storage scheme, the temporally multiplexed scheme gives rise to a 11.8‐fold increase in entanglement generation rate. By using cavity‐enhanced scheme, spin waves stored in the atoms are retrieved on demand and the retrieval efficiencies are up to 70%, which is beneficial for the subsequent entanglement swapping between adjacent links.

Ion trap with in-vacuum high numerical aperture imaging for a dual-species modular quantum computer.

Photonic interconnects between quantum systems will play a central role in both scalable quantum computing and quantum networking. Entanglement of remote qubits via photons has been demonstrated in many platforms; however, improving the rate of entanglement generation will be instrumental for integrating photonic links into modular quantum computers. We present an ion trap system that has the highest reported free-space photon collection efficiency for quantum networking. We use a pair of in-vacuum aspheric lenses, each with a numerical aperture of 0.8, to couple 10(1)% of the 493nm photons emitted from a 138Ba+ ion into single-mode fibers. We also demonstrate that proximal effects of the lenses on the ion position and motion can be mitigated.

Nonreciprocal microwave-optical entanglement in a magnon-based hybrid system

We propose a theoretical scheme for the generation of nonreciprocal microwave-optical entanglement in a magnon-based hybrid system where an optical photon in a spinning resonator is coupled to a phonon representing the quantum of the mechanical deformation of a yttrium iron garnet crystal via radiation pressure. Meanwhile, the magnon interacts with the phonon and the microwave through the magnetostrictive and magnetic dipole–dipole interactions, respectively. By spinning the resonator, the light driven in opposite directions propagates irreversibly, which leads to the classical nonreciprocity of mean optical photon numbers. Strikingly, the nonreciprocal entanglement between microwave and optical photons can be generated owing to the Fizeau light-dragging effect. Physically, the magnon–phonon entanglement induced by the magnomechanical parametric downconversion interaction is partially transferred to the microwave-optical subsystem through magnon-microwave and optomechanical state-exchange interactions. Moreover, the nonreciprocity of entanglement can be manipulated by properly choosing various system parameters and the ideal nonreciprocal microwave–optical entanglement could be achieved, in which the entanglement depending on the effective optical detuning is present in a chosen direction but disappears in the other direction. Our work could be applied in the multi-task quantum information processing and construction of chiral quantum networks.

Transform-Limited Photon Emission from a Lead-Vacancy Center in Diamond above 10K.

Transform-limited photon emission from quantum emitters is essential for high-fidelity entanglement generation. In this Letter, we report the coherent optical property of a single negatively charged lead-vacancy (PbV) center in diamond. Photoluminescence excitation measurements reveal stable fluorescence with a linewidth of 39MHz at 6K, close to the transform limit estimated from the lifetime measurement. We observe 4 orders of magnitude different linewidths of the two zero-phonon lines, and find that the phonon-induced relaxation in the ground state contributes to this huge difference in the linewidth. Because of the suppressed phonon absorption in the PbV center, we observe nearly transform-limited photon emission up to 16K, demonstrating its high temperature robustness compared to other color centers in diamond.

Cavity-Mediated Molecular Entanglement and Generation of Non-classical States of Light.

The generation and control of entanglement in a quantum mechanical system are critical elements of nearly all quantum applications. Molecular systems are promising candidates, with numerous degrees of freedom able to be targeted. However, knowledge of intersystem entanglement mechanisms in such systems is limited. In this work, we demonstrate the generation of entanglement between vibrational degrees of freedom in molecules via strong coupling to a cavity mode driven by a weak coherent field. In a bimolecular system, we show that entanglement can be generated not only between the cavity and molecular system but also between molecules. This process also results in the generation of nonclassical states of light, providing potential pathways for harnessing entanglement in molecular systems.

Decoherence-induced cross-Kerr nonlinearity for quantum entanglement

In this paper, we show that the cross-Kerr nonlinearity is induced by the ground-state decoherence in a Λ system even under the exact one- and two-photon resonance conditions, which is attributed to the fact that the dark-state resonance is broken slightly. By adiabatically eliminating the atomic variables, we find that the decoherence results in a frequency shift of cavity detuning and the two cavity fields experience cross-Kerr nonlinearity by choosing appropriate parameters. As a consequence, the internal nonlinear effects are responsible for the generation of light entanglement, which is verified by our numerical results. The present scheme demonstrates that the decoherence plays a positive role in controlling quantum entanglement, which may be useful for quantum information processing.

Asynchronous entanglement routing for the quantum internet

With the emergence of the Quantum Internet, the need for advanced quantum networking techniques has significantly risen. Various models of quantum repeaters have been presented, each delineating a unique strategy to ensure quantum communication over long distances. We focus on repeaters that employ entanglement generation and swapping. This revolves around establishing remote end-to-end entanglement through repeaters, a concept we denote as the “quantum-native” repeaters (also called “first-generation” repeaters in some literature). The challenges in routing with quantum-native repeaters arise from probabilistic entanglement generation and restricted coherence time. Current approaches use synchronized time slots to search for entanglement-swapping paths, resulting in inefficiencies. Here, we propose a new set of asynchronous routing protocols for quantum networks by incorporating the idea of maintaining a dynamic topology in a distributed manner, which has been extensively studied in classical routing for lossy networks, such as using a destination-oriented directed acyclic graph or a spanning tree. The protocols update the entanglement-link topology asynchronously, identify optimal entanglement-swapping paths, and preserve unused direct-link entanglements. Our results indicate that asynchronous protocols achieve a larger upper bound with an appropriate setting and significantly higher entanglement rate than existing synchronous approaches, and the rate increases with coherence time, suggesting that it will have a much more profound impact on quantum networks as technology advances.

Gate‐Based Protocol Simulations for Quantum Repeaters using Quantum‐Dot Molecules in Switchable Electric Fields

AbstractElectrically controllable quantum‐dot molecules (QDMs) are a promising platform for deterministic entanglement generation and, as such, a resource for quantum‐repeater networks. A microscopic open‐quantum‐systems approach based on a time‐dependent Bloch–Redfield equation is developed to model the generation of entangled spin states with high fidelity. The state preparation is a crucial step in a protocol for deterministic entangled‐photon‐pair generation that is proposed for quantum repeater applications. The theory takes into account the quantum‐dot molecules' electronic properties that are controlled by time‐dependent electric fields as well as dissipation due to electron–phonon interaction. The transition between adiabatic and non‐adiabatic regimes is quantified, which provides insights into the dynamics of adiabatic control of QDM charge states in the presence of dissipative processes. From this, the maximum speed of entangled‐state preparation is inferred under different experimental conditions, which serves as a first step toward simulation of attainable entangled photon‐pair generation rates. The developed formalism opens the possibility for device‐realistic descriptions of repeater protocol implementations.

Optically mediated remote entanglement generation in magnon-cavity systems

Optically mediated remote entanglement generation in magnon-cavity systems

Quantum interference of pulsed time-bin entanglement generated from silicon ring resonator

We demonstrate a pulsed operation of an entangled photon pair source that is based on a silicon ring resonator. Time-bin entangled photon pairs at telecommunication wavelengths are generated via spontaneous four-wave mixing, which is excited by a pulsed pump laser. The entanglement between the generated photon pair is analyzed by using asymmetric Mach–Zehnder interferometers followed by single-photon detectors, resulting in non-classical interference with a visibility exceeding a classical limit. The reason for the degradation of the interference visibility is discussed using the theoretical model with experimental parameters. Our experimental results show successful pulsed generation of entanglement, which represents an important step towards a synchronized quantum network based on silicon photonics.

Evaluating quantum entanglement generation in two-dimensional graphene systems through lithium ion interactions: A DFT-based study.

In this article, we employed concepts from Density Functional Theory to investigate the interaction energy behavior between the fragments of two-dimensional systems composed of graphene-based materials and lithium ions. Specifically, the proposed system consists of two graphene sheets separated by a controlled distance (face-to-face), with a lithium ion positioned at the center of this separation. Additionally, we examined potential electronic transitions within these systems and assessed the feasibility of quantum entanglement generation and manipulation. Our findings revealed that the interaction energies within the analyzed systems exhibited behavior akin to that described by the Lennard-Jones potential, which characterizes systems with favorable energy for their formation. The results further yielded estimates for the constants and , with values of kcal/mol and 1.63 , respectively. Specific electronic transitions were identified, suggesting the potential for quantum entanglement generation and manipulation among the two-dimensional graphene system mediated by the lithium ion interactions.

Entanglement Generation in Capacitively Coupled Transmon–Cavity System

AbstractIn this paper, the higher energy levels of the transmon qubit are taken into consideration to investigate the continuous variable entanglement generation between the transmon qubit and the single‐mode cavity. Based on the framework of cavity Quantum Electrodynamics (QED), the authors show the entanglement generation depends on the driving field intensity, coupling strength, cavity field frequency, and qubit frequency. The numerical results show that strong entanglement can be generated by properly tuning these parameters. It is hoped that the results presented in this paper may lead to a better understanding of quantum entanglement generation in cavity QED systems and provide new perspectives for further research in quantum information processing.

Realization of a crosstalk-avoided quantum network node using dual-type qubits of the same ion species

Generating ion-photon entanglement is a crucial step for scalable trapped-ion quantum networks. To avoid the crosstalk on memory qubits carrying quantum information, it is common to use a different ion species for ion-photon entanglement generation such that the scattered photons are far off-resonant for the memory qubits. However, such a dual-species scheme can be subject to inefficient sympathetic cooling due to the mass mismatch of the ions. Here we demonstrate a trapped-ion quantum network node in the dual-type qubit scheme where two types of qubits are encoded in the S and F hyperfine structure levels of 171Yb+ ions. We generate ion photon entanglement for the S-qubit in a typical timescale of hundreds of milliseconds, and verify its small crosstalk on a nearby F-qubit with coherence time above seconds. Our work demonstrates an enabling function of the dual-type qubit scheme for scalable quantum networks.

Quantum Network Communication Resource Optimization Scheme Based on Multi-Scale Entanglement Renormalization Ansatz

Quantum Key Distribution (QKD) is a pivotal technology in the field of secure communications, leveraging the principles of quantum mechanics to enable theoretically unbreakable encryption. However, despite its promise, QKD faces significant challenges in achieving large-scale deployment. The primary hurdle lies in the scarcity of quantum resources, especially entangled photon pairs, which are fundamental to protocols such as Ekert91. In traditional QKD implementations, only a fraction of the entangled pairs generated contribute to raw key production, leading to substantial inefficiencies and resource wastage. Addressing this limitation is crucial to the advancement and scalability of QKD networks.<br>This paper introduces an innovative approach to QKD by integrating the Multiscale Entanglement Renormalization Ansatz (MERA), a technique originally developed for many-body quantum systems. By utilizing MERA's hierarchical structure, the proposed method not only improves the efficiency of entanglement distribution but also reduces the consumption of quantum resources. Specifically, MERA compresses many-body quantum states into lower-dimensional representations, allowing for the transmission and storage of entanglement in a more efficient manner. This compression significantly reduces the number of qubits required, optimizing both entanglement utilization and storage capacity in quantum networks.<br>To evaluate the performance of this method, we conducted simulations under standardized conditions. The simulations assumed a 1024-bit encryption request, an 8\% error rate, an average path length of 4 hops in the quantum network, and a 95\% success rate for both link entanglement generation and entanglement swapping operations. These parameters mirror realistic physical conditions found in contemporary QKD networks. The results demonstrate that the MERA-based approach saves an impressive 124,151 entangled pairs compared to traditional QKD protocols. This substantial reduction in resource consumption underscores the potential of MERA to revolutionize the efficiency of QKD systems without compromising security. Importantly, the security of the key exchange process remains intact, as the method inherently adheres to the principles of quantum mechanics, particularly the no-cloning theorem and the use of randomness in decompression layers.<br>The paper concludes that MERA not only enhances the scalability of QKD by optimizing quantum resource allocation but also maintains the security guarantees essential for practical cryptographic applications. By integrating MERA into existing QKD frameworks, we can significantly lower the resource overhead, making large-scale, secure quantum communication more feasible. These findings contribute a new dimension to the field of quantum cryptography, suggesting that advanced quantum many-body techniques like MERA hold the potential to unlock the full potential of quantum networks in real-world scenarios.

DFT-based investigation of graphene-mediated manipulation for efficient potassium ion entanglement

In this paper we address the potential entanglement of potassium ions mediated by interaction with a graphene sheet using DFT. The system consists of a neutral graphene sheet (GQD) positioned equidistant between two positively charged potassium ions (K+). The entanglement is strongly influenced by the K+@GQD@K+ separation distance, in which it is possible to observe specific electronic transitions originating exclusively from the GQD to the potassium ions, enabling the acquisition of entangled states between the two ions with fidelity close to unity, demonstrating the potential of the present system for a robust generation of quantum entanglement between alkali metal ions.

Entanglement generation between distant spins via quasilocal reservoir engineering

Entanglement generation between distant spins via quasilocal reservoir engineering

Multipartite entanglement generation and dynamics of three giant atoms coupling to a one-dimensional waveguide

Multipartite entanglement generation and dynamics of three giant atoms coupling to a one-dimensional waveguide