Boosting the communication rate of quantum networks is a central challenge in quantum information science. Recently, an efficient entanglement distribution scheme employing quasi-deterministic Bell-pair sources based on time-frequency multiplexing, referred to as zero-added-loss multiplexing (ZALM), has been proposed. Its implementation, however, requires high-fidelity entanglement swapping across densely multiplexed time-frequency modes, which has remained an experimental challenge. Here we demonstrate entanglement swapping across 16 parallel frequency modes with a high average fidelity of 93.9±1.4%. Notably, polarization-entangled photon pairs in each frequency mode are spectrally single-mode using only off-the-shelf 50-GHz dense wavelength-division multiplexing (DWDM) filters, eliminating the need for additional narrowband filtering. Furthermore, in order to fully exploit the temporal degree of freedom, the pump pulse is operated with a repetition frequency of 3.0GHz. By combining the frequency and time multiplexing, the total swapping rate reaches 5.38±0.17 pairs s^-1, which corresponds to the ZALM Bell-pair rate of 8.2e2pairs s^-1. Our results establish the key experimental capabilities required for ZALM and demonstrate a scalable route toward practical high-rate quantum repeaters and long-haul quantum networks.

This paper proposes a multicast content distribution method over quantum key distribution (QKD) networks that improves delivery latency and resilience to link bandwidth limitations through mesh network design, while also contributing to reduced key consumption. Although QKD ensures information-theoretic security, its limited key generation rate poses challenges for scalable content delivery. To address this, we introduce a mechanism using random linear network coding in mesh topologies, where nodes adaptively collect and forward packets based on network conditions such as bandwidth and link quality. We evaluate the proposed method through numerical simulations on a mid-sized urban-regional network with 51 nodes. The results show that, even on redundant mesh topologies, our method suppresses key consumption by 16% and reduces delivery latency under constrained bandwidth scenarios by 51%, compared to conventional IP multicast over single-tree topologies.

Bright illumination attack (BIA) is considered one of the most serious eavesdropping attacks on quantum key distribution (QKD) systems. Various countermeasures have been proposed to eliminate this threat. Most of them focus on bright light at the telecom wavelength of 1550 nm, often involving power monitors with a sensitivity range from 900 to 1700 nm. However, BIA at other wavelengths-typically shorter than 900 nm-could also pose a threat. The light in this wavelength range has been overlooked because avalanche photodiodes (APDs) in QKD systems have low detection efficiency for communication. However, Indium Phosphide (InP) layers, commonly used as the avalanche-amplification layer and substrate in APDs, can absorb light at these shorter wavelengths, potentially causing blinding due to photocurrent. Such attacks cannot be detected by power monitors designed for telecom wavelengths. In this study, we experimentally investigated the feasibility of BIA on practical QKD systems using the 830-nm continuous-wave light. We found clear evidence of APD blinding, which reduces quantum efficiency. This finding suggests that the security certification of commercial QKD systems should consider potential attacks using near-infrared light.

Quantum key distribution (QKD) is a technology for distributing cryptographic keys between two communication parties based on the quantum physics for the secure communication. A trusted node-based key relay technique is integrated with QKD to overcome the technical limitations of QKD and to enable key distribution between two arbitrary communication parties in the trusted node network, which we refer to as the QKD network. In addition, secret-sharing technology has been integrated with the QKD network to realize secure data storage and secure data communication, which we call a quantum secure cloud. This article presents the development and evaluation of the proof-of-concept (PoC) system for the QKD network and a quantum secure cloud, especially applied to the genome medicine domain. The PoC system was developed at Tohoku University and Toshiba sites to address the “cancer clinical sequencing” use case. We evaluated three practical scenarios with the PoC system: 1) real-time transmission of genome analysis data; 2) “expert panel,” an online video conference for medical experts’ discussion; and 3) distributed backup of genome analysis data. To support these scenarios, we developed three new functions: 1) a function for monitoring output data and pipeline processing of data encryption/decryption and transmission for secure large-scale data transfer; 2) a key management system function to achieve both large-scale data transmission and low-latency data communication; and 3) a function for preemptive key data reading and direct access to storage devices to enable high-speed data transmission and distributed data backup using a secret-sharing scheme. These scenarios and functions were evaluated and demonstrated using real or simulated genome data. The evaluation results reveal that QKD network and quantum secure cloud technologies can be applied to cancer clinical sequencing as a use case of the genome medicine domain.

Multicast for securely sharing confidential data among many users is becoming increasingly important. Currently, it relies on duplicate-and-forward routing and cryptographic methods based on computational security. However, these approaches neither attain multicast capacity of the network, nor ensure long-term security against advances in computing (information-theoretic security: ITS). Existing ITS solutions–quantum key distribution (QKD), physical layer security (PLS), and secure network coding (SNC)–still fail to enable scalable networks, as their underlying assumptions, such as trusted nodes and wiretap thresholds, gradually become invalid as the network grows. Here, we develop an efficient multi-tree multicast path-finding method to address this issue, integrating it with universal strongly ramp SNC. This system, path-controlled universal strongly ramp SNC (PUSNEC), can be overlaid onto QKD/PLS networks, enabling multicast capacity, ITS, and scalability. We derive the maximum leakage information to an eavesdropper under the probabilistic wiretap network assumption and demonstrate secure multicast in multi-hop networks through numerical simulations. Our quantitative analysis of the secrecyreliability tradeoff highlights a practical approach to achieving secure, reliable multicast on a global scale.

We evaluated a two-color two-photon entangled state generated in silicon via spontaneous four-wave mixing (SFWM) as a potential source for bosonic integrated circuits. Spatially entangled photon states were created using a pair of silicon waveguides that produced signal and idler photons through SFWM, allowing us to observe quantum interference between them. Assuming that the frequencies of the two-color photons were nearly identical, we characterized the generated quantum state by performing quantum state tomography on the bosonic system using a linear optical circuit. This study demonstrates the feasibility of using photon-pair sources generated in silicon via SFWM in bosonic optical circuits and highlights their potential for a wide range of applications in silicon-based optical quantum technologies.

We are developing a quantum secure cloud that uses quantum key distribution networks not only for secure communications but also for distributed storage that requires long-term confidentiality and for secure secondary use.

Quantum key distribution networks (QKDNs) offer a promising platform for information-theoretically secure (IT-secure) communication between two or more nodes, representing an increasingly practical communication platform. Despite their promise, these systems continue to encounter significant challenges due to their restricted key-generation rate. Therefore, the efficient and effective utilization of key resources is pivotal to enhancing IT-secure communication. In this study, we propose an in-network coded-cache-aware multipath-transport mechanism called CodCam-TP that exploits information-centric networking (ICN) technologies. CodCam-TP facilitates efficient and effective IT-secure communication through in-network coding and caching, multicast, and multipath communication. Furthermore, by exploiting the pull-based hop-by-hop communication model, CodCam-TP leverages an in-band network telemetry technique to select a path based on the forwarding capacity of each available path, thus improving throughput. Additionally, we conduct several performance evaluations for CodCam-TP using Cefore, an open-source software for driving ICN-based communication, demonstrating improved secure-key-utilization efficiency and data-delivery effectiveness compared with those realized by an existing scheme.

In photonic quantum information processing, quantum operations using nonlinear photon-photon interactions are vital for implementing two-qubit gates and enabling faithful entanglement swapping. However, due to the weak interaction between single photons, the all-photonic realization of such quantum operations has remained out of reach so far. Herein, we demonstrate an entanglement swapping using sum-frequency generation between single photons in a χ(2)-nonlinear optical waveguide. We show that a high signal-to-noise ratio (SNR), stable sum-frequency-generation-based entanglement heralder with an ultralow-dark-count superconducting single-photon detector can satisfy the unprecedented SNR requirement indispensable for the swapping protocol. Furthermore, the system clock is enhanced by utilizing ultrafast telecom entangled photon-pair sources that operate in the GHz range. Our results confirm a lower bound 0.770(76) for the swapped state's fidelity, surpassing the classical limit of 0.5 successfully. Our findings highlight the strong potential of broadband all-single-photonic nonlinear interactions for further sophistication in long-distance quantum communication and photonic quantum computation.

Franson interference can be used to test the nonlocal features of energy-time entanglement and has become a standard in quantum physics. However, most of the previous Franson interference experiments were demonstrated in the time domain, and the spectral properties of Franson interference have not been fully explored. Here, we theoretically and experimentally demonstrate spectrally resolved Franson interference using biphotons with different correlations, including positive correlation, negative correlation, and non-correlation. It is found that the joint spectral intensities of the biphotons can be modulated along both the signal and idler directions, which has potential applications in generating high-dimensional frequency entanglement and time-frequency grid states. This work may provide a new perspective for understanding the spectral-temporal properties of the Franson interferometer.

For information that requires long-term confidentiality (e.g. national security, military security, genomic data etc.), the threat of eavesdropping must be seriously considered. The leakage of such information would not only cause temporary confusion but would also have serious repercussions for future generations. Part of such important data are already being exchanged over the Internet using cryptography that is not resistant to quantum computers. Considering the possibility of harvest attacks on information that must be kept secret for centuries, developing a technology that can immediately eliminate the risk of eavesdropping in principle is desirable. In response to these demands, we previously developed a system called the quantum secure cloud, which realizes an information-theoretically secure data transmission, storage, reconstruction, and authentication with a single password, using an established technology of quantum key distribution network. We now apply this technology to develop an information-theoretically secure long-distance data-relay function and succeed in developing a distributed data-relay simulator that is compatible with current quantum key distribution networks. The throughput of this protocol is more than 10 Mbps for 10 MB data, so it can be applied to practical use.

In this paper, we propose a novel method for interfering frequency-multiplexed photonic quantum states without the use of optical nonlinear effects, and experimentally demonstrate this technique via frequency-domain Hong-Ou-Mandel (HOM) interference. By cascading the generation of quantum states onto arbitrary orthogonal modes, we can induce interference across any desired frequency mode. Following the generation of quantum states onto the frequency modes, performing measurements in independent frequency bands enables the realisation of a frequency-domain linear optical circuit analogous to linear interference in the spatial domain. We successfully demonstrated programmable quantum interference by controlling the spectral mode functions and measurement bases. Our method offers a new approach to harness the full potential of light's temporal-frequency degrees of freedom, providing a path towards scalable and programmable photonic quantum computing architectures without the need for optical nonlinearities or spatial-mode beam splitters.

Modelocked thermal frequency combs (MTCs) are generated by employing spectrally narrowed amplified spontaneous emission (ASE) seeded into an electro-optic frequency comb generator. The MTC emits 2-ps duration ultrashort pulses at a repetition rate of 10 GHz. Autocorrelation of the MTC pulses confirms a reduced coherence time, tau c=213 +/- 16$\tau _{\mathrm{c}} = 213\pm 16$ ps, aligning with the narrowed bandwidth of the ASE seed. Intensity correlations of optically gated MTC pulses at a repetition rate of 250 MHz reveal nearly ideal thermal photon statistics with an experimental gmtc(2)=1.9564 +/- 0.0004$g_{\mathrm{mtc}}<^>{(2)} = 1.9564 \pm 0.0004$, yielding an intrinsic gint(2)=1.9809 +/- 0.0004$g_{\mathrm{int}}<^>{(2)} = 1.9809 \pm 0.0004$ after background noise removal. As a practical application, second harmonic generation (SHG) is performed utilizing the optically gated MTC pulses as a pump and experimental intensity correlations, gsh(2)$g_{\mathrm{sh}}<^>{(2)}$, are examined for the SH photons. An entire transition in gsh(2)$g_{\mathrm{sh}}<^>{(2)}$, continuously changing from six to two by increasing the pump strength, agrees with the single-mode analytical model. Furthermore, time-resolved pulse height correlations allow to simultaneously acquire power variations in SHG and third harmonic generation against the pump. With the maximum peak intensity, I1p approximate to 1.6GWcm-2${\overline{I}_{1}<^>{\mathrm{p}}}\approx 1.6 \ {\text{GW} \ \text{cm}}<^>{{-2}}$, realized in a periodically poled LiNbO3${\rm LiNbO}_3$ waveguide for SHG, the demonstration highlights the potential for various applications in chaotic quantum optics experiments that necessitate ultrashort, high-intensity, single-spatiotemporal-mode thermal pulses. This paper presents the generation and application of modelocked thermal frequency combs (MTCs). MTCs emit 2-ps duration pulses at a 10-GHz repetition rate, showing reduced coherence length. MTC diagnostics reveal nearly ideal, single-mode thermal statistics. The intensity correlation of the second harmonic pumped by MTCs transitions with the analytical prediction. The results showcase the potential for chaotic quantum optics applications that necessitate ultrashort, high-intensity, single-spatiotemporal-mode thermal pulses. image

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.

A challenging issue in Continuous-variable quantum key distribution (CV-QKD) is the improvement in error correction efficiency because random number bits are encoded in a quadrature of faint optical pulses. Herein, we propose an error-control information reconciliation method for CV-QKD based on our recently proposed fixed-bit polar code. In the present scheme, unreliable bits are embedded in the prepared “fixed bits,” enabling the detection and reproduction of errors in these bits without error correction, effectively improving the error-correction efficiency. Numerical simulations demonstrate that the efficiency of the proposed scheme is 10% higher than that of polar-based reconciliation without fixed bits.

Free-space optical (FSO) communications can offer high-capacity transmission owing to the properties of the laser beams. However, performance degradation caused by atmospheric turbulence is an urgent issue. Recently, the application of polar codes, which can provide capacity-achieving error-correcting performance with low computational cost for decoding, to FSO communications has been studied. However, long-distance and real-field experiments have not been conducted in these studies. To the best of our knowledge, this study is the first to present the experimental results of polar-coded transmission over 7.8-km FSO links. Using experimental data, we investigated the performance of polar codes over atmospheric channels, including their superiority to regular low-density parity-check codes. We expect that our results will offer a path toward the application of polar codes in high-speed optical communication networks including satellites.

There are often situations where two remote users each have data, and wish to (i) verify the equality of their data, and (ii) whenever a discrepancy is found afterwards, determine which of the two modified his data. The most common example is where they want to authenticate messages they exchange. Another possible example is where they have a huge database and its mirror in remote places, and whenever a discrepancy is found between their data, they can determine which of the two users is to blame. Of course, if one is allowed to use computational assumptions, this function can be realized readily, e.g., by using digital signatures. However, if one needs information-theoretic security, there is no known method that realizes this function efficiently, i.e., with secret key, communication, and trusted third parties all being sufficiently small. In order to realize this function efficiently with information-theoretic security, we here define the “equality-testing protocol with dispute resolution” as a new framework. The most significant difference between our protocol and the previous methods with similar functions is that we allow the intervention of a trusted third party when checking the equality of the data. In this new framework, we also present an explicit protocol that is information-theoretically secure and efficient.

Free-space optical (FSO) systems are compulsory to realize high capacity and interference-free communication links from low-Earth orbit (LEO) satellite constellations as well as spacecraft and space stations to the Earth. To be integrated with high-capacity ground networks, the collected portion of the incident beam should be coupled into an optical fiber. To accurately evaluate the signal-to-noise ratio (SNR) and bit-error rate (BER) performance metrics, the probability density function (PDF) of fiber coupling efficiency (CE) must be determined. Previous studies have experimentally verified the CE PDF for a single-mode fiber, however, there is no such investigation for the CE PDF of a multi-mode fiber (MMF) in a LEO-to-ground FSO downlink. In this paper, for the first time, the CE PDF for a 200-μm MMF is experimentally investigated using data from an FSO downlink from the Small Optical Link for International Space Station (SOLISS) terminal to a 40-cm sub-aperture optical ground station (OGS) supported by a fine-tracking system. An average CE of 5.45 dB was also achieved given that the alignment between SOLISS and OGS was not optimal. In addition, using the angle-of-arrival (AoA) and received power data, the statistical characteristics such as channel coherence time, power spectral density, spectrogram, and PDFs of AoA, beam misalignments, and atmospheric turbulence-induced fluctuations are revealed and compared with the state-of-the-art theoretical background.

The key relay protocol (KRP) plays an important role in improving the performance and the security of quantum key distribution (QKD) networks. On the other hand, there is also an existing research field called secure network coding (SNC), which has similar goal and structure. We here analyze differences and similarities between the KRP and SNC rigorously. We found, rather surprisingly, that there is a definite gap in security between the KRP and SNC; that is, certain KRPs achieve better security than any SNC schemes on the same graph. We also found that this gap can be closed if we generalize the notion of SNC by adding free public channels; that is, KRPs are equivalent to SNC schemes augmented with free public channels.

In a single qubit system, a universal quantum classifier can be realized using the data reuploading technique. In this study, we propose a new quantum classifier applying this technique to bosonic systems and successfully demonstrate it using a silicon-based photonic integrated circuit. We established a theory of quantum machine learning algorithm applicable to bosonic systems and implemented a programmable optical circuit combined with an interferometer. Learning and classification using part of the implemented optical quantum circuit with uncorrelated two photons resulted in a classification with a success probability of 94±0.8% in the proof of principle experiment. As this method can be applied to an arbitrary two-mode N-photon system, further development of optical quantum classifiers, such as extensions to quantum entangled and multiphoton states, is expected in the future.