We theoretically propose and experimentally demonstrate the use of a configurable four-qubit photonic system to generate publicly verifiable quantum random numbers, to perform entanglement verification, and to generate a secure public and private key. Quantum circuits, to generate the desired four-qubit states and its experimental realization in the photonic architecture, are carried out using photon pairs entangled in polarization and path degrees of freedom. By performing measurements on the four-qubit system and accessing partial information of the four-qubit state for public verification, we generate publicly verified and purely secured random bits at the rate of 185 kbps from collective data of 370 kbps. When the system is used for generating public and private keys, an equal number of public and private keys are generated simultaneously. We also record about 97.9% of sampled bits from four-qubit states passing entanglement verification and demonstrate the use of public and private key generated for image encryption-decryption. The theoretical model of noise on the four-qubit state and its effect on the generation rate of verified and secured bits are in perfect agreement with the experimental results. This demonstrates the practical use of the small-scale multiqubit photonic system for quantum-safe applications by providing the option for real-time verification of the security feature of the quantum system.
We theoretically propose and experimentally demonstrate the use of a configurable four-qubit photonic system to generate publicly verifiable quantum random numbers, to perform entanglement verification, and to generate a secure public and private key. Quantum circuits, to generate the desired four-qubit states and its experimental realization in the photonic architecture, are carried out using photon pairs entangled in polarization and path degrees of freedom. By performing measurements on the four-qubit system and accessing partial information of the four-qubit state for public verification, we generate publicly verified and purely secured random bits at the rate of 185 kbps from collective data of 370 kbps. When the system is used for generating public and private keys, an equal number of public and private keys are generated simultaneously. We also record about 97.9% of sampled bits from four-qubit states passing entanglement verification and demonstrate the use of public and private key generated for image encryption-decryption. The theoretical model of noise on the four-qubit state and its effect on the generation rate of verified and secured bits are in perfect agreement with the experimental results. This demonstrates the practical use of the small-scale multiqubit photonic system for quantum-safe applications by providing the option for real-time verification of the security feature of the quantum system.
We propose and experimentally demonstrate an asymmetric key-generation scheme in which public and private keys are derived from a six-qubit photonic entangled state. A six-qubit entangled state is generated from two photons, each carrying one polarization and two path qubits. The state is prepared in an equal superposition of eight of the sixty-four six-qubit basis states, in which each photon's three qubits form a codeword of the classical [3,2,2] even-parity code, and are distributed between the two parties, Alice and Bob. This ensures, each triple is a uniformly random codeword and the two codewords are locked together through the correlation. The parity constraint is a local stabilizer, so each party verifies it on its own data every round, and eliminates the bit flip errors arising from imperfections, detector dark counts and multiple occupancy before it enters the raw key. A single publicly announced bit, chosen at random from either of the path qubits, allows each party to reconstruct the other's full triple while leaking no information to third party. We realize the desired six-qubit state using a Sagnac-based type-II SPDC source, reporting a Bell-CHSH parameter S = 2.798 +/- 0.008, by implementing gate operation on polarization and path degree's of freedom. This approach provides a physics-based alternative to public-key cryptosystems built on computational hardness assumptions. A t-bit string generated from multiple rounds for encryption will be intrinsically random. Without the private key, one needs at least O(2^t) runs on an t-qubit quantum system to find the matching bit to decrypt.
Abstract Measurement outcomes on quantum systems exhibit inherent randomness and are fundamentally nondeterministic. This has enabled quantum physics to set new standards for the generation of true randomness with significant applications in the fields of cryptography, statistical simulations, and modeling of the nondeterministic behavior in various other fields. In this work, we present a scheme for the generation of multi-bit random numbers using path-entangled single photons. For the experimental demonstration, we generate a path-entangled state using single photons from spontaneous parametric down-conversion (SPDC) and assign a multi-qubit state for them in path basis. One-bit and two-bit random numbers are then generated by measuring entangled states in the path basis. In addition to passing the NIST tests for randomness, we also demonstrate the certification of quantumness and self-certification of quantum random number generator (QRNG) using Clauser, Horne, Shimony and Holt (CHSH) inequality violation. We also record the significantly low autocorrelation coefficient from the raw bits generated and this along with CHSH violation rules out multi-photon events and ensure the protection from photon splitting attack. Distribution of photons along multiple paths resulting in multiple bits from one photon extends the limit on bit generation rate imposed by the detection dead time of the individual detector. Thus, the path-entangled states can generate higher bitrates compared to scheme using entangled photon pair which are limited by the coincidence counts. We demonstrate this by generating a high rate of about 80 Mbps when the single photon detector saturates at around 28 Mcps and still show violation of CHSH inequality.
The burgeoning Internet and Internet-of-Things (IoT) sectors necessitate robust cryptographic methods to ensure data security, integrity, and authentication over unsecured networks. Traditional public key cryptography, reliant on computationally hard problems, faces threats from quantum computing advancements. Quantum Key Distribution (QKD) presents a solution through the generation of unconditionally secure cryptographic keys using quantum mechanics. This paper explores the enhancement of QKD protocols to establish secure end-to-end communication in photonic networks. The proposed method involves a QKD system that generates two types of weak quantum signals: one with randomly varied intensity, polarization, or phase, and another with random frequency fluctuations. These signals are used to establish a shared key between a transmitter (Alice) and multiple receivers (Bobs) by measuring the quantum states. This dual signal approach enhances protection against Photon Number Splitting attacks and improves key length. Key Management Agents (KMAs) securely handle QKD-generated keys for data encryption before transmission, ensuring only intended recipients can decrypt the messages. The system leverages optical fiber or free-space optical links to transmit weak quantum signals and synchronization signals, facilitating key distribution even under existing network constraints. The proposed architecture allows for the secure exchange of cryptographic keys between Alice and multiple Bobs, ensuring private and authenticated communication over public channels. The approach mitigates potential eavesdropping by enabling the detection of any interception attempts through Quantum Bit Error Rate (QBER) estimation. This study underscores the promise of QKD as a foundational element of future communication systems, providing uncrackable quantum keys and paving the way for secure photonic networks. Incremental advancements in quantum devices, networking, and infrastructure are essential to fully realize QKD's potential for robust cryptographic security.
Abstract High-dimensional quantum key distribution (QKD) based on entangled qudits has emerged as a promising route for advancing the performance and practicality of quantum communication. By extending beyond the qubit framework, qudit entanglement enables the encoding of multiple bits per particle and offers greater robustness against noise, loss, and eavesdropping. In this review, we present qudit-based entanglement QKD protocols and their experimental demonstrations. We first introduce the basic concepts of qudits and provide an overview of foundational qubit-based entanglement QKD schemes, such as Ekert '91 (E91) and Bennett–Brassard–Mermin '92 (BBM92), which establish the core principles of entanglement-based secure key generation. Subsequently, we examine the generalization of these protocols to entangled qudits, highlighting how higher dimensionality enhances secure key rates and noise tolerance. Finally, we survey experimental demonstrations of qudit-based entanglement QKD across photonic platforms and outline prospects, showing that high-dimensional entanglement can be reliably generated, distributed, and measured for QKD with high stability and scalability. Remarkably, these achievements not only verify the feasibility of practical high-dimensional QKD, but also demonstrate clear advantages over qubit-based systems. Overall, theoretical frameworks and experimental realizations reviewed here position photonic qudit-based entanglement QKD as a promising resource for scalable, high-speed, and secure quantum communication networks, unveiling its potential as a central component in the future of quantum information technologies.
Herewithin we present multiplexed integrated photonics as a realisable route to large-scale quantum technologies. Quantum photonics is one many platforms for experimental demonstrations of quantum information science. The technological advantages afforded by the combination of integrated photonic circuits with multiplexing techniques can enable performance enhancements to make photonics a leading infrastructure for implementation of real-world quantum information systems. For this purpose, we create integrated circuits to solve technical challenges in the fields of quantum cryptography, discrete variable and continuous variable quantum computation. These circuits were fabricated in a facility developed at Griffith University, Centre for Quantum Dynamics, for the production of annealed and reverse proton exchange waveguides in congruent lithium niobate and have found applications across the fields of quantum optics and cryptography. We develop and demonstrate the first integrated many-mode active optical demultiplexing of single photons from a solid-state source. This scheme enables the production of a multiphoton Fock state across multiple spatial modes from a single high brightness solid-state source with temporal indistinguishability. This work addressed a major hurdle in the development of photonic quantum computers, namely generation of a large number of indistinguishable single photons on demand. To perform the demultiplexing of single photons we develop two key technologies; a high-speed 1:4 integrated photonic switch and a many channel arbitrary pulse sequence generator. Cryptography as a field is increasingly reliant on quantum random number generators for added security. With increased demand comes a requirement for higher bitrate random number generators, and as such we demonstrate multiplexing of a random number generation scheme based on measurements of quantum vacuum fluctuations. Furthermore, we show an increased level of security at high bitrates by implementing a new signal processing scheme We demonstrate integrated generation, manipulation and homodyning of squeezed light on a single chip for the first time. This scheme is the first successful demonstration of full integration of all the major components needed for continuous variable quantum computation in a temporally multiplexed architecture. These results represent contributions to several fields of study, demonstrating the advantages of integrated quantum photonic multiplexing, and are of interest to the quantum computing, information security, integrated optics, and electronic control communities.
Quantum entanglement represents an ideal resource to guarantee the security of random numbers employed in many scientific and cryptographic applications. However, entanglement-based certified random number generators are particularly challenging to implement. Here, we demonstrate a certified quantum random number generator based on momentum-polarization entangled single-photon states. The use of single-photon entanglement allows an attenuated laser source to be employed and a simple setup where only linear optical components are utilized. For the latter, a semi-device-independent modeling of the photonic quantum random number generator is developed, which certifies a minimum entropy of $(2.5\ifmmode\pm\else\textpm\fi{}0.5)\mathrm{%}$, corresponding to a generation rate of 4.4 kHz. At the expenses of a higher level of trust in the system, the certified minimum entropy can be increased to $(30.1\ifmmode\pm\else\textpm\fi{}0.5)\mathrm{%}$, implying a generation rate of 52.7 kHz. Our results show that a simple optical implementation combined with an accurate modeling provide an entanglement-based high-security quantum random number generator using imperfect devices.