Towards experimental demonstration of quantum position verification using single photons
Kirsten N. Kanneworff, Mio Poortvliet, Dirk Bouwmeester, Rene Allerstorfer, Philip Verduyn Lunel, Florian Speelman, Harry Buhrman, Petr Steindl, W. Löffler
Abstract The geographical position can be a good credential for authentication of a party. This is the basis of position-based cryptography—but classically this cannot be done securely without physical exchange of a private key. Recently it has been shown that by combining quantum mechanics with the speed-of-light limit of special relativity, this might be possible: quantum position verification (QPV). Here we demonstrate experimentally a protocol that uses two-photon Hong–Ou–Mandel interference at a beamsplitter, which, in combination with two additional beam splitters and four detectors is rendering the protocol resilient to loss. With this, we are able to show first results towards an experimental demonstration of QPV.
Abstract The geographical position can be a good credential for authentication of a party. This is the basis of position-based cryptography—but classically this cannot be done securely without physical exchange of a private key. Recently it has been shown that by combining quantum mechanics with the speed-of-light limit of special relativity, this might be possible: quantum position verification (QPV). Here we demonstrate experimentally a protocol that uses two-photon Hong–Ou–Mandel interference at a beamsplitter, which, in combination with two additional beam splitters and four detectors is rendering the protocol resilient to loss. With this, we are able to show first results towards an experimental demonstration of QPV.
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.
In this thesis we explore position-based quantum cryptography zooming in on the task of position verification. In position verification, the idea is to use an individual's geographical location as a cryptographic credential. In practice, such protocols can authenticate that a message originated from a specific location or ensure that messages can only be read at a certain location. In a position-verification protocol, the limitations imposed by the speed of light, as described by special relativity, are used to verify that a party is at their claimed location. It has been shown that any QPV protocol can be broken by attackers that use an amount of entanglement exponential in the input size. However, from a practical point of view, not all is lost. We can still hope for QPV protocols that are secure in practice. This raises the problem of designing protocols that are secure in a practical setting. An important problem that arises is that of signal loss. Signal loss can be detrimental as it allows attackers to only answer on a subset of rounds. We investigate several questions related to the practical implementation of such protocols. We propose a new experimentally feasible protocol, which is fully loss-tolerant against attackers restricted to classical communication. Furthermore, we consider the role of allowing quantum communication with no pre-shared entanglement for attackers and show that this setting is strictly stronger than the classical one, indicating that allowing for quantum communication may break security. Conversely, we then show that any protocol secure against classical communication can be transformed into a protocol secure against quantum communication. Finally, we invert the picture, and consider the task of non-local quantum computation (NLQC), which corresponds to the operations of the attackers in a QPV protocol. We connect NLQC to the wider context of information-theoretic cryptography by relating it to a number of other cryptographic primitives.
Quantum position verification (QPV) - the subject of this thesis - is a novel way to authenticate unknown nodes in a quantum network without the need to physically meet up. In this thesis, we show our work towards an experimental demonstration of QPV using a temporally demultiplexed quantum dot single-photon source. First, we give a more in-depth overview of the novel field of quantum position verification by describing different well-known protocols and possible adversarial attacks under certain conditions. Next, we investigated the operation and stability of several optical devices concerning the polarization state of light. Then we investigated the quantum dot single-photon source in more detail by measuring the indistinguishability of the single photons. To measure the indistinguishability, the single-photon stream needs to be distributed over two arms of a Mach-Zehnder interferometer. In this thesis, we investigated both probabilistic distribution through the use of a beamsplitter and deterministic distribution by using an optical switch.
This doctoral thesis explores quantum position verification (QPV)---a cryptographic task where one attempts to confirm someone’s geographical location. The core question that QPV aims to answer is: Are you truly at the location you claim to be? To achieve this, QPV combines two pillars of the fundamental laws of nature: (i) special relativity, which limits the speed at which information can travel to the speed of light in vacuum, and (ii) quantum mechanics, which governs the behavior of quantum particles in ways that defy intuition. QPV protocols rely on (i) timing communication between the entities involved in the protocol, and (ii) transmitting information encoded in quantum particles. However, quantum hackers may attempt to pretend to be at the claimed location while actually being elsewhere. This raises the critical question: Can one be certain that the claimed location is genuine and not forged by hackers? This thesis presents new advances toward a fundamental understanding of QPV, its security against powerful quantum hackers, and the feasibility of secure QPV protocols despite experimental challenges that, if exploited by hackers, can severely compromise their security. Furthermore, related to attacks on QPV protocols, this thesis analyzes quantum correlations that emerge in broader cryptographic primitives.
This thesis deals with position-based quantum cryptography (PBQC), a type of quantum cryptography. More concretely, we focus on the design and study of quantum position verification (QPV) protocols, which act as a building block for PBQC. We prioritise the development of QPV protocols that are loss-tolerant, error-robust, and feasible with current or near-future technology, concentrating on photon-based protocols and linear-optical equipment. We also theoretically study different attack settings on QPV in the context of state discrimination, dealing with the interplay between non-locality and interaction. We introduce a new QPV protocol based on the SWAP test and Hong-Ou-Mandel interference and study it in detail regarding attacks and practical feasibility. We further provide a solution to a major practical issue of QPV -- signal loss. A minor modification of the standard structure of QPV, namely introducing a small time delay and a commitment to play from the honest prover, allows us to provably make the transmission loss between the verifiers and the honest party irrelevant for security for a class of QPV protocols. Moreover, we generalise and study QPV protocols in the continuous-variable setting. Exploring theoretical aspects of QPV attacks, we focus on the difference between quantum and classical communication. We show new bounds on existing protocols, construct protocols that can be attacked with quantum communication but not locally, show a separation between quantum and classical communication attacks on QPV, and derive a necessary condition on the attack error based on the structure of the QPV input ensemble.
Quantum position verification (QPV) aims to certify the location of an untrusted prover, but faces two major obstacles: fundamentally, entanglement-based attacks and, experimentally, photon loss. A commitment-based modification introduced in Phys. Rev. Lett. 135, 260801 addresses both in sequentially repeated protocols. Its security analysis, however, relies on the sequential structure and does not extend to parallel repetition. We use a different proof approach to establish security of the commitment modification for a parallel BB84-based QPV protocol. Against bounded-entanglement adversaries, the acceptance probability decays exponentially in the minimum number k of successfully committed qubits. The protocol tolerates arbitrary transmission loss and noise rates up to 3.7%, while allowing arbitrarily slow quantum communication. This yields a fully loss-tolerant, single-execution QPV protocol secure against bounded-entanglement attacks, removing transmission loss as a fundamental limitation on verification distance. We also revisit sequential repetition, correcting the treatment of conditioning on commitment and replacing the earlier conditional guarantee with an unconditional security bound, without the finite-size correction to the correctness gap. The resulting bound provides improved quantitative security parameters for experimental implementations.