Recently it has been shown how the use of quantum entanglement can lead to the creation of real-time communication channels whose viability can be made location dependent. Such functionality leads to new security paradigms that are not possible in classical communication networks. Key to these new security paradigms are quantum protocols that can unconditionally determine that a receiver is in fact at an a priori assigned location. A limiting factor of such quantum protocols will be the decoherence of states held in quantum memory. Here we investigate the performance of quantum location verification protocols under decoherence effects. More specifically, we address the issue of how decoherence impacts the verification using N = 2 qubits entangled as Bell states, as compared to N >; 2 qubits entangled as GHZ states. We study the original quantum location verification protocol, as well as a variant protocol, introduced here, which utilizes teleportation. We find that the performance of quantum location verification is in fact similar for Bell states and some N >; 2 GHZ states, even though quantum decoherence degrades larger-qubit entanglements faster. Our results are important for the design and implementation of location-dependent communications in emerging quantum networks.
In this work, we study position-based cryptography in the quantum setting. The aim is to use the geographical position of a party as its only credential. On the negative side, we show that if adversaries are allowed to share an arbitrarily large entangled quantum state, the task of secure position-verification is impossible. To this end, we prove the following very general result. Assume that Alice and Bob hold respectively subsystems
A and
B of a (possibly) unknown quantum state
∣ψ⟩∈HA⊗HB. Their goal is to calculate and share a new state
∣φ⟩=U∣ψ⟩, where
U is a fixed unitary operation. The question that we ask is how many rounds of mutual communication are needed. It is easy to achieve such a task using two rounds of classical communication, whereas, in general, it is impossible with no communication at all. Surprisingly, in case Alice and Bob share enough entanglement to start with and we allow an arbitrarily small failure probability, we show that the same task can be done using a single round of classical communication in which Alice and Bob exchange two classical messages. Actually, we prove that a relaxed version of the task can be done with no communication at all, where the task is to compute instead a state
∣φ′⟩ that coincides with
∣φ⟩=U∣ψ⟩ up to local operations on
A and on
B, which are determined by classical information held by Alice and Bob. The one-round scheme for the original task then follows as a simple corollary. We also show that these results generalize to more players. As a consequence, we show a generic attack that breaks any position-verification scheme. On the positive side, we show that if adversaries do not share any entangled quantum state but can compute arbitrary quantum operations, secure position-verification is achievable. Jointly, these results suggest the interesting question whether secure position-verification is possible in case of a bounded amount of entanglement. Our positive result can be interpreted as resolving this question in the simplest case, where the bound is set to zero. In models where secure position-verification is achievable, it has a number of interesting applications. For example, it enables secure communication over an insecure channel without having any preshared key, with the guarantee that only a party at a specific location can learn the content of the conversation. More generally, we show that in settings where secure position-verification is achievable, other position-based cryptographic schemes are possible as well, such as secure position-based authentication and position-based key agreement.
Position information underpins many modern technologies, from navigation and timing to authentication and critical infrastructure. However, classical methods of proving that information originates from a particular position are vulnerable to spoofing. This limitation can be overcome with quantum technologies but current protocols rely on trust in quantum hardware that can be undermined, or require quantum computers and bounds on adversarial computation. Nevertheless, there has been significant interest in experimental demonstrations, and aspects of these protocols have been implemented. Here we introduce and experimentally demonstrate the Bell-test quantum position verification protocol for device-independent quantum position verification that guarantees security with only observed correlations from a loophole-free Bell test across a quantum network. We experimentally implement a version of this device-independent protocol against adversaries who, before each trial, are weakly entangled. Our demonstration achieves a one-dimensional localization 2.47(2) times smaller than the best, necessarily non-remote, classical localization protocol. Compared to classical protocols with identical latencies, the localization volume is 4.53(5) times smaller, and represents a certifiable quantum advantage. The general Bell-test protocol is loss tolerant and secure against adversaries with significant quantum resources. This work allows digital security to be anchored to physically trusted locations, enabling new position-based authentication protocols for applications such as financial transactions, legal agreements, and securing critical infrastructure.
The possibility of attaining current position of the message sender without person's consent seriously compromises the secrecy of correspondence. Classical communication systems cannot guarantee the security of communication against unwanted location tracking, because the sender must broadcast a signal at the moment of sending the message. The source of the signal could be always located, at least in principle. We show that quantum communication enables sending the message with pre-distributed entangled quantum systems, local weak measurements, time synchronization between the sender and the receiver and classical communication by the message receiver alone, therefore, physically securing the sender's location.
While quantum position verification aims to certify a prover's location using quantum information, existing security definitions only guarantee that part of the successful adversarial party is in the claimed location. This leaves open the possibility that a distributed team of adversaries can jointly simulate a prover in a way that defeats the intended meaning of `being at a location' in position-based cryptography. We introduce stronger notions of position verification that we call quantum localization, which requires that there is a specified, unclonable state at the verified spacetime point-and that this state can be found nowhere else. We show that quantum localization leads naturally to a meaningful notion of trajectory verification, in which quantum information is verifiably tracked through space and time. We construct quantum localization and trajectory verification protocols using quantum anchor states, which generalize coset states from unclonable cryptography. The security of our schemes is proven in the classical oracle (i.e. ideal obfuscation) model, which can be heuristically instantiated in the plain model using post-quantum indistinguishability obfuscation. We also introduce and instantiate the concept of functionality localization, which guarantees that the adversary has the ability to compute a secret function at the verified spacetime point, and this function cannot be computed anywhere else. This raises the intriguing possibility of localizing computational capabilities in space and time. More broadly, we believe our notions of quantum localization and subsequent feasibility results provide stronger foundations for position-based cryptography. At a conceptual level, our results also explore the tight link between two fundamental notions in physics-location and information-thus serving as a natural foundation for cryptographic capabilities tied to physical spacetime.
Abstract Quantum position verification (QPV) enables multiple verifiers to certify a prover’s location using quantum communication and physical assumptions. With experimental demonstrations of QPV becoming increasingly feasible, enhancing the practicality and security of QPV protocols is more important than ever. In this work, we make three key contributions toward this goal. First, we introduce a robust QPV protocol in which the verifier’s state preparation is independent of channel loss, improving reliability in real-world conditions. Second, we refine existing security analysis techniques to bolster protocol resilience against experimental imperfections. Third, we identify and address some implicit assumptions present in existing security analyses, providing a framework to eliminate such assumptions. Additionally, as an example of QPV application beyond location verification, we illustrate how QPV can be leveraged for authenticating classical communication in quantum key distribution.