Quantum token protocols enable unforgeable quantum tokens promising unconditional security beyond classical cryptographic assumptions. We show here that the three stages of the Quantum token protocols involving the preparation, storage and verification can be made more secure when involving spin-photon interfaces that leverage high fidelity hybrid tripartite (spin-photon-spin) entanglement, Bell state measurements and highly coherent spin quantum memories. Further we describe the physical implementation of various stages of the protocol using the hybrid electron and nuclear spins in diamond interfaced with time-bin photons.
Quantum token protocols enable unforgeable quantum tokens promising unconditional security beyond classical cryptographic assumptions. We show here that the three stages of the Quantum token protocols involving the preparation, storage and verification can be made more secure when involving spin-photon interfaces that leverage high fidelity hybrid tripartite (spin-photon-spin) entanglement, Bell state measurements and highly coherent spin quantum memories. Further we describe the physical implementation of various stages of the protocol using the hybrid electron and nuclear spins in diamond interfaced with time-bin photons.
The demanding requirements for a controllable large scale quantum system made of only one type of physical qubit, such as photons, can lead to complex quantum computational architectures which are challenging to realise in practise. This has driven interest towards hybrid quantum systems which can combine the relative strengths of different physical qubits to vastly simplify architecture. Here we study the key building block for a hybrid spin-photon system: the spin-photon interface (SPI). The negatively charge nitrogen-vacancy centre (NV- of research attention in recent years due to the ability to optically polarise and measure its ground state electron spin. Its long electronic spin coherence times and optical readout, at room temperature, makes the NVcentre a strong candidate for demonstrating a flexible SPI. Furthermore, coherent electronic interaction with nearby long-lived 13C nuclear spins provide a powerful platform for performing multi-qubit manipulations whilst offering extremely long coherence and storage times. However, engineering strong optical interaction with NVspins has been limited by the inability to either fabricate high quality cavities on demand, or to efficiently couple NVcentres to those cavities. Here we will describe progress towards realising a useful SPI. We detail a novel approach to constructing microcavities which will allow high-fidelity fabrication and efficient coupling of NVcentres in nanodiamond through a two-photon lithography fabrication process. We will present our lithographically defined microcavities and other developments we have made towards a SPI, such as Solid Immersion Lenses (SIL’s) and Distributed Bragg Reflectors (DBR’s). Additionally, we outline progress towards coherent electronic and nuclear spin control, a key requirement of a scalable SPI. The ability to combine the high fidelity fabrication of optical cavities with coherent spin manipulation will allow the creation of a flexible SPI that can form a core component of proposed quantum technologies.
Quantum money is the first invention in quantum information science, promising advantages over classical money by simultaneously achieving unforgeability, user privacy, and instant validation. However, standard quantum money relies on quantum memories and long-distance quantum communication, which are technologically extremely challenging. Quantum "S-money" tokens eliminate these technological requirements while preserving unforgeability, user privacy, and instant validation. Here, we report the first full experimental demonstration of quantum S-tokens, proven secure despite errors, losses and experimental imperfections. The heralded single-photon source with a high system efficiency of 88.24% protects against arbitrary multi-photon attacks arising from losses in the quantum token generation. Following short-range quantum communication, the token is stored, transacted, and verified using classical bits. We demonstrate a transaction time advantage over intra-city 2.77 km and inter-city 60.54 km optical fibre networks, compared with optimal classical cross-checking schemes. Our implementation demonstrates the practicality of quantum S-tokens for applications requiring high security, privacy and minimal transaction times, like financial trading and network control. It is also the first demonstration of a quantitative quantum time advantage in relativistic cryptography, showing the enhanced cryptographic power of simultaneously considering quantum and relativistic physics.
Blockchain is a decentralized system that allows transaction transmission and storage according to the roles of the Consensus algorithm and Smart contracts. Non-fungible tokens (NFTs) consolidate the best characteristics of blockchain technology to deliver unique and bona fide tokens, each with distinctive attributes with non-fungible resources. Unfortunately, current classical NFTs are suffering from high costs regarding the consumed power of mining and lack of security. Therefore, this paper presents a new protocol for preparing quantum non-fungible tokens where a quantum state representing NFT is mounted on a blockchain instead of physically giving it to the owner. The proposed scheme is simulated and analyzed against various attacks and proves its ability to secure against them. Furthermore, the presented protocol provides reliable and cheaper NFTs than the classical one.
Spins in a solid-state environment and photons are both promising physical systems regarding the implementation of qubits. Whereas electron and nuclear spins act very well as localized qubits that are able to store and process quantum information, photons are the primary candidates to transmit quantum information between distant locations. Entanglement, on the other hand, is a fundamental resource of quantum information processing enabling, e.g., secure communication over long distances. The generation and manipulation of entanglement are thus fundamental steps in order to leverage the benefits of quantum information processing. The aim of this dissertation is therefore to theoretically investigate entangling mechanisms for various physical systems. A functioning source of entangled photons is a basic hardware component required for quantum communication. To this end, we study a system composed of semiconductor quantum wells inside a microcavity, known as intersubband cavity system, in which the interaction of the electronic system with the cavity field can reach the ultrastrong coupling regime. The ground state contains a finite number of photons, and we find that these photons are entangled. The amount of entanglement is quantified analytically, and maximal entanglement is found to be possible. The technique of entanglement purification allows to restore maximal entanglement that has decreased, e.g. due to decoherence. Purification requires quantum memory, a role for which electron spins in electrically-defined quantum dots are well suited. However, existing purification protocols are rather unpractical in this case. Here, the concept of asymmetric bilateral two-qubit operations is introduced to purify spin entanglement by harnessing the typical interaction between electrons in neighboring quantum dots. As it turns out, this concept can be applied to a variety of qubit systems, e.g. superconducting qubits or spins in nitrogen-vacancy centers in diamond. The latter example offers several possibilities to implement a qubit, of which the intrinsic nitro- gen nuclear spin has proven its viability in many ways. We develop a scheme to deterministically couple two nuclear spin qubits, in which the interaction is mediated by an optical cavity. It is found that an entangling two-qubit gate, also required for universal quantum computation, can be implemented with operation times below 100 nanoseconds, i.e. several orders of magnitude faster compared to the decoherence time of the nuclear spin. The verification of entanglement, which is required e.g. in entanglement-based quantum com- munication to detect an eavesdropping attack, typically involves a measurement of qubit states. Using the input-output formalism, we derive a fully quantum-mechanical model of an optical readout scheme to measure the spin state of an electron in a self-assembled quantum dot.
Abstract Unforgeable quantum money tokens were the first invention of quantum information science, but remain technologically challenging as they require quantum memories and/or long-distance quantum communication. More recently, virtual “S-money” tokens were introduced. These are generated by quantum cryptography, do not require quantum memories or long-distance quantum communication, and yet in principle guarantee many of the security advantages of quantum money. Here, we describe implementations of S-money schemes with off-the-shelf quantum key distribution technology, and analyse security in the presence of noise, losses, and experimental imperfection. Our schemes satisfy near-instant validation without cross-checking. We show that, given standard assumptions in mistrustful quantum cryptographic implementations, unforgeability and user privacy could be guaranteed with attainable refinements of our off-the-shelf setup. We discuss the possibilities for unconditionally secure (assumption-free) implementations.
ABSTRACT The rapid advancement of quantum computing poses a serious threat to classical authentication mechanisms that rely on computational hardness assumptions and software-based randomness. Traditional authentication systems such as passwords, one-time passwords (OTPs), and public-key cryptography depend heavily on pseudorandom number generators (PRNGs) and classical cryptographic algorithms, many of which are vulnerable to future quantum attacks. This paper proposes and analyzes a Hybrid Post-Quantum Secure Authentication system using Quantum-Generated One-Time Tokens (Q-OTT). The proposed approach combines true quantum randomness obtained from Quantum Random Number Generators (QRNGs) with post-quantum cryptographic (PQC) algorithms for token protection and verification. A detailed comparison between classical authentication systems and the hybrid Q-OTT approach is presented in terms of randomness, security, entropy, and quantum resistance. The study demonstrates that hybrid Q-OTT systems offer significantly enhanced security against both classical and quantum adversaries, at the cost of moderate infrastructure requirements. Keywords Post-Quantum Cryptography, Quantum Random Number Generator, One-Time Token, Authentication Systems, Quantum Security, Hybrid Authentication