Quantum Networks Research Articles

Indistinguishable photons from an artificial atom in silicon photonics

Silicon is the ideal material for building electronic and photonic circuits at scale. Integrated photonic quantum technologies in silicon offer a promising path to scaling by leveraging advanced semiconductor manufacturing and integration capabilities. However, the lack of deterministic quantum light sources and strong photon-photon interactions in silicon poses a challenge to scalability. In this work, we demonstrate an indistinguishable photon source in silicon photonics based on an artificial atom. We show that a G center in a silicon waveguide can generate high-purity telecom-band single photons. We perform high-resolution spectroscopy and time-delayed two-photon interference to demonstrate the indistinguishability of single photons emitted from a G center in a silicon waveguide. Our results show that artificial atoms in silicon photonics can source single photons suitable for photonic quantum networks and processors.

Multi-participant quantum anonymous communication based on high-dimensional entangled states

Anonymous quantum communication (AQC) enables the secure transmission of quantum or classical messages between a sender and a receiver while ensuring the anonymity of their identities. Existing AQC protocols primarily address one-to-one anonymous communication. When there are multiple senders, they must send their messages sequentially to avoid collision problems. In this paper, we propose a multi-participant quantum anonymous communication (MQAC) by employing high-dimensional entangled states. In this protocol, multiple senders establish anonymous entanglement with multiple receivers of their choice and send quantum messages anonymously and simultaneously via quantum teleportation. To propose MQAC, we first present the toolbox we developed, which includes several sub-protocols such as the verification protocol, collision detection protocol, address distribution protocol, and notification protocol. These sub-protocols, which we propose, not only are important parts of MQAC but also serve as the foundation for solving many more complex tasks, such as anonymous commodity transaction. Additionally, the analysis of MQAC demonstrates that neither internal nor external attackers can steal the identities of the communication parties or the transmitted messages by employing any attack. Our work represents an original exploration of anonymous message transmission between multiple senders and receivers in quantum networks.

Enhancement of Nonreciprocal Entanglement and Transmission via Phase‐Dependent Unidirectional Coupling

AbstractThe nonreciprocal property of quantum systems is crucial for chiral quantum networks. In this study, a phase‐controlled scheme is proposed to enhance the nonreciprocal quantum entanglement and optical transmission in a cavity optomechanical system by introducing a phase‐dependent unidirectional coupling between two whispering‐gallery‐mode (WGM) resonators. The Sagnac effect induced by the spinning resonator generates nonreciprocity, while the interference effect between direct evanescent coupling and indirect phase‐dependent unidirectional coupling significantly enhances entanglement under the anti‐Stokes sideband. For specific coupling phases, ideal nonreciprocity can be achieved in both entanglement and transmission. The proposed phase‐controlled nonreciprocity presented offers an effective strategy for constructing a unidirectional quantum channel and implementing an optical diode in chiral quantum networks, thereby opening up new possibilities for applications in quantum technologies.

Guiding charged particles in vacuum via Lagrange points

We propose a method for guiding charged particles such as electrons and protons, in vacuum, by employing the exotic properties of Lagrange points. This leap is made possible by the dynamics unfolding around these equilibrium points, which stably capture such particles, akin to the way Trojan asteroids are held in Jupiter’s orbit. Unlike traditional methodologies that allow for either focusing or three-dimensional storage of charged particles, the proposed scheme can guide both non-relativistic and relativistic electrons and protons in small cross-sectional areas in an invariant fashion over long distances without any appreciable loss in energy – in a manner analogous to photon transport in optical fibers. Here, particle guiding is achieved by employing twisted electrostatic potentials that in turn induce stable Lagrange points in vacuum. In principle, guidance can be realized within the fundamental mode of the resulting waveguide, thereby presenting a prospect for manipulating these particles in the quantum domain. Our findings may be useful in a wide range of applications in both scientific and technological pursuits. These applications could encompass electron microscopies and lithographies, particle accelerators, quantum and classical communication/sensing systems, as well as methods for shuttling entangled qubits between nodes within a quantum network.

Automated Distribution of Polarization-Entangled Photons Using Deployed New York City Fibers

The distribution of high-fidelity high-rate entanglement over telecommunication infrastructure is one of the main paths toward large-scale quantum networks, enabling applications such as quantum encryption and network protection, blind quantum computing, distributed quantum computing, and distributed quantum sensing. However, the fragile nature of entangled photons operating in real-world fiber infrastructure has historically limited continuous operation of such networks. Here, we present a fully automated system capable of distributing polarization-entangled photons over a 34-km deployed fiber in New York City, achieving high rates of nearly 5×105 pairs/s. Separately, we demonstrate a high fidelity of approximately 99% for rates up to 2×104 pairs/s. Lastly, we achieve 15 days of continuous distribution, with a network up-time of 99.84%. Our work paves the way for practical deployment of always-on entanglement-based networks with rates and fidelity adequate for many current and future use cases. Published by the American Physical Society 2024

Algorithmic optimization of quantum optical storage in solids

Quantum memory devices with high storage efficiency and bandwidth are essential elements for future quantum networks. Solid-state quantum memories can provide broadband storage, but they primarily suffer from low storage efficiency. We use passive optimization and algorithmic optimization techniques to demonstrate nearly a sixfold enhancement in quantum memory efficiency. In this regime, we demonstrate coherent and single-photon-level storage with a high signal-to-noise ratio. The optimization technique presented here can be applied to most solid-state quantum memories to significantly improve the storage efficiency without compromising the memory bandwidth. Published by the American Physical Society 2024

Scalable determination of multipartite entanglement in quantum networks

Quantum networks comprised of entangled end nodes serve stronger than the classical correlation for unparalleled quantum internet applications. However, practical quantum networking is affected by noise, which at its worst, causes end nodes to be described by pre-existing classical data. In such untrusted networks, determining quantum network fidelity and genuine multi-node entanglement becomes crucial. Here, we show that determining quantum network fidelity and genuine N-node entanglement in an untrusted star network requires only N + 1 measurement settings. This method establishes a semi-trusted framework, allowing some nodes to relax their assumptions. Our network determination method is enabled by detecting genuine N-node Einstein-Podolsky-Rosen steerability. Experimentally, using spontaneous parametric down-conversion entanglement sources, we demonstrate the determinations of genuine 3-photon and 4-photon quantum networks and the false positives of the widely used entanglement witness, the fidelity criterion of 1/2. Our results provide a scalable method for the determination of multipartite entanglement in realistic quantum networks.

Quantum state transfer and distribution of past–future correlations in a quantum network

Quantum state transfer and distribution of past–future correlations in a quantum network

Demonstration of teleportation across a quantum network code

Demonstration of teleportation across a quantum network code

A variational approach to quantum gated recurrent units

Quantum Recurrent Neural Networks are receiving an increased attention thanks to their enhanced generalization capabilities in time series analysis. However, their performances were bottlenecked by long training times and unscalable architectures. In this paper, we propose a novel Quantum Recurrent Neural Network model based on Quantum Gated Recurrent Units. It uses a learnable Variational Quantum Layer to process temporal data, interspersed with two classical layers to properly match the dimensionality of the input and output vectors. Such an architecture has fewer quantum parameters than existing Quantum Long Short-Term Memory models. Both the quantum networks were evaluated on periodic and real-world time series datasets, together with the classical counterparts. The quantum models exhibited superior performances compared to the classical ones in all the test cases. The Quantum Gated Recurrent Units outperformed the Quantum Long Short-Term Memory network despite having a simpler internal configuration. Moreover, the Quantum Gated Recurrent Units network demonstrated to be about 25% faster during the training and inference procedure over the Quantum Long Short-Term Memory. This improvement in speed comes with one less quantum circuit to be executed, suggesting that our model may offer a more efficient alternative for implementing Quantum Recurrent Neural Networks on both simulated and real quantum hardware.

On the possibility of implementing a quantum entanglement distribution in a biosystem: Microtubules

The paper considers the possibility of implementing a quantum entanglement distribution in the cell microtubule. It has been shown that a quantum entanglement distribution proposed in the paper determines the process of quantum state teleportation through microtubule tryptophan chain. The work shows that the system of tryptophans in a microtubule essentially is a quantum network that consists of: spatially spaced nodes — tryptophans, quantum communication channels connecting tryptophans and qubits transmitted through these communication channels. The connection between the process of quantum teleportation in living nature and its classical analogue is discussed. The quantum protocol established in the work determines the possible principle of quantum information transmission in biosystems and also in the similar nanostructures.

Theoretical concept of error-rejecting entanglement purification of quantum-dots electronic spins in single-sided optical microcavity

We use the quantum-dots (QD) coupled with single-sided microcavities system to construct the error-rejecting controlled not (CNOT) gate of electronic spins system in QD; We have designed an error-rejecting entanglement purification protocol (EPP) of electronic spins system in QD by using the error-rejecting CNOT gate and unitary operation. It can extract the high entanglement from the mixed entanglement states of the electronic spins with low entangled states. This EPP can eliminate the operation errors caused by the nonideal interaction between photons and QD coupled with optical microcavities system, and improve the fidelity of the EPP through iteration. Our scheme is more practical in future long-distance quantum communications, especially providing significant benefits for solving decoherence problems in quantum networks and quantum repeaters.

Unconventional Light‐Matter Interactions Between Giant Atoms and Structured Baths with Next‐Nearest‐Neighbor Couplings

AbstractIn this paper, the unconventional light‐matter interactions between giant atoms and structured baths (i.e., lattices) are studied with either Hermitian or non‐Hermitian next‐nearest‐neighbor coupling terms. Essentially different dynamics of the atoms and the propagating field in the Hermitian and non‐Hermitian cases is revealed, which can be further engineered by tuning parameters such as the atomic transition frequency and the (synthetic) magnetic field associated to the coupling terms. The next‐nearest‐neighbor couplings play an important role in controlling the emission direction and the field distribution in the lattice, thus providing opportunities for tailoring exotic dipole–dipole interactions. The results in this paper have potential applications in, e.g., engineering unconventional quantum networks and simulating quantum many‐body systems.

Narrowband stimulated Raman scattering and molecular modulation in anti-resonant hollow-core fibres

Raman scattering is the inelastic process where photons bounce off molecules, losing energy and becoming red-shifted. This weak effect is unique to each molecular species, making it an essential tool in, e.g., spectroscopy and label-free microscopy. The invention of the laser enabled a regime of stimulated Raman scattering (SRS), where the efficiency is greatly increased by inducing coherent molecular oscillations. However, this phenomenon required high intensities due to the limited interaction volumes, and this limitation was overcome by the emergence of anti-resonant fibres (ARFs) guiding light in a small hollow channel over long distances. Based on their unique properties, this Perspective reviews the transformative impact of ARFs on modern SRS-based applications ranging from development of light sources and convertors for spectroscopy and materials science, to quantum technologies for the future quantum networks, providing insights into future trends and the expanding horizons of the field.

Fiber loop quantum buffer for photonic qubits

We report a fiber loop quantum buffer based on a low-loss 2 × 2 switch and a unit delay made of a fiber delay line. We characterize the device by using a two-photon polarization entangled state in which one photon of the entangled photon pair is stored and retrieved at a repetition rate up to 78 kHz. The device, which enables integer multiples of a unit delay, can store the qubit state in a unit of fiber delay line up to 5.4 km and the number of loop round-trips up to 3. Furthermore, we configure the device with other active elements to realize integer multiplier and divider of a unit delay of a qubit. The quantum state tomography is performed on the retrieved photon and its entangled photon. We obtain a state fidelity > 94 % with a maximum storage time of 52 s with an insertion loss of 5.56 dB. To further characterize the storing and retrieving processes of the device, we perform entanglement-assisted quantum process tomography on the buffered qubit state. The process fidelity of the device is > 0.98. Our result implies that the device preserves the superposition and entanglement of a qubit state from a two-photon polarization-entangled state. This is a significant step towards facilitating applications in optical asynchronous transfer mode based quantum networks.

Security analysis of satellite-to-ground reference-frame-independent quantum key distribution with beam wandering

Abstract The reference-frame-independent (RFI) quantum key distribution (QKD) is suitable for satellite-based links by removing the active alignment on the reference frames. However, how the beam wandering influences the performance of RFI-QKD remains a pending issue in satellite-to-ground links. In this paper, based on the mathematical model for characterizing beam wandering, we present the security analysis for satellite-to-ground RFI-QKD and analytically derive formulas for calculating the secret key rate with beam wandering. Our simulation results show that the performance of RFI-QKD is better than the Bennett–Brassard 1984 (BB84) QKD with beam wandering in asymptotic case. Furthermore, the degree of influences of beam wandering is specifically presented for satellite-to-ground RFI-QKD when statistical fluctuations are taken into account. Our work can provide theoretical support for the realization of RFI-QKD using satellite-to-ground links and have implications for the construction of large-scale satellite-based quantum networks.

Research on polarization compensation for practical satellite-based quantum key distribution

Satellite-based quantum key distribution (QKD) is one of the effective solutions for building a global-scale quantum network. For practical satellite-based polarization-encoding QKD systems, the polarization degradation introduced by the single-mode fiber and the relative rotation between satellites and ground stations will lead to a decrease in the polarization performance and need to be compensated. In this work, we propose two novel on-orbit satellite-to-ground joint polarization measurement and compensation methods, which can ensure high polarization fidelity while effectively saving hardware resources and reducing costs. By establishing a polarization compensation model and performing theoretical analysis, combined with desktop experimental verification, we demonstrated the effectiveness of the satellite-to-ground joint polarization compensation and achieved a low quantum bit error rate below 0.3% after background removal. The presented polarization compensation laid the foundation for the subsequent applications in satellite-based QKD and satellite-based quantum networks.

Maximal diameter of integral circulant graphs

Integral circulant graphs are proposed as models for quantum spin networks enabling perfect state transfer. Understanding the potential information transfer between nodes in such networks involves calculating the maximal graph diameter. The integral circulant graph ICGn(D) has vertex set Zn={0,1,2,…,n−1}, with vertices a and b adjacent if gcd⁡(a−b,n)∈D, where D⊆{d:d|n,1≤d<n}. Building on the upper bound 2|D|+1 for the diameter provided by Saxena, Severini, and Shparlinski, we prove that the maximal diameter of ICGn(D) for a given order n with prime factorization p1α1⋯pkαk is r(n) or r(n)+1, where r(n)=k+|{i|αi>1,1≤i≤k}|. We show that a divisor set D with |D|≤k achieves this bound. We calculate the maximal diameter for graphs of order n and divisor set cardinality t≤k, identifying all extremal graphs and improving the previous upper bound.

Quantum Physics-Informed Neural Networks.

In this study, the PennyLane quantum device simulator was used to investigate quantum and hybrid, quantum/classical physics-informed neural networks (PINNs) for solutions to both transient and steady-state, 1D and 2D partial differential equations. The comparative expressibility of the purely quantum, hybrid and classical neural networks is discussed, and hybrid configurations are explored. The results show that (1) for some applications, quantum PINNs can obtain comparable accuracy with less neural network parameters than classical PINNs, and (2) adding quantum nodes in classical PINNs can increase model accuracy with less total network parameters for noiseless models.

Prospects for Legal Regulation of Quantum Communication

The leading countries across the world have entered the race to develop quantum technologies that will enable them to ensure their continued economic prosperity. Among these technologies, a special place is occupied by quantum communication, which is designed to ensure information security in an era where a quantum computer is capable of compromising a number of cryptography algorithms. In this article, this new digital technology includes quantum key distribution and encryption methods that are cryptographically resistant to a quantum computer. The study does not consider the regulation of the quantum communication sub-technology, the so-called “quantum internet,” due to the technical limitations of the existing equipment. The authors note that their predictions about the cryptographic strength of encryption algorithms are based solely on modern knowledge about the capabilities of quantum computing and do not take into account its hidden potential, for example, in terms of cryptanalysis information systems based on a machine learning model generated by a quantum computer. Currently, the only data protection system that is not subject to quantum threats is the technology of quantum key distribution. In today’s information and digital age, information security systems are an important element of critical infrastructure. Given the importance of these technologies, different states use different methods to regulate this field. This article puts forward and substantiates the hypothesis that the implementation of a combination of regulatory legal acts could have a greater positive impact on the development of quantum communication and ensure an acceptable level of information security in the post-quantum era. The analysis showed that a significant number of states and interstate associations are conducting research in this area, relying only on investment growth. This strategy has prevented any country from achieving the competencies of the People’s Republic of China. The authors also analyzed the methods of legal support used by China, Russia, and other countries in the field of quantum communication, which made it possible to identify a model of legal regulation of quantum communication that stimulates this technology’s development.