Abstract The no-cloning theorem leads to information-theoretic security in various quantum cryptographic protocols. However, this security typically derives from a possibly weaker property that classical information encoded in certain quantum states cannot be broadcast. To formally capture this property, we introduce the study of ‘orthogonality broadcasting.’ When attempting to broadcast the orthogonality of two different qubit bases, we establish that the power of classical and quantum communication is equivalent. However, quantum communication is shown to be strictly more powerful for broadcasting orthogonality in higher dimensions. We then relate orthogonality broadcasting to quantum position verification and provide a new method for establishing error bounds in the no pre-shared entanglement model that can address protocols previous methods could not. Our key technical contribution is an uncertainty relation that uses the geometric relation of the states that undergo broadcasting rather than the non-commutative aspect of the final measurements.
Abstract The no-cloning theorem leads to information-theoretic security in various quantum cryptographic protocols. However, this security typically derives from a possibly weaker property that classical information encoded in certain quantum states cannot be broadcast. To formally capture this property, we introduce the study of ‘orthogonality broadcasting.’ When attempting to broadcast the orthogonality of two different qubit bases, we establish that the power of classical and quantum communication is equivalent. However, quantum communication is shown to be strictly more powerful for broadcasting orthogonality in higher dimensions. We then relate orthogonality broadcasting to quantum position verification and provide a new method for establishing error bounds in the no pre-shared entanglement model that can address protocols previous methods could not. Our key technical contribution is an uncertainty relation that uses the geometric relation of the states that undergo broadcasting rather than the non-commutative aspect of the final measurements.
Quantum cryptography converts structural restrictions on quantum information into operational secrecy. This article develops the chain connecting nonorthogonal-state discrimination, trace distance and fidelity, the Helstrom bound, no-cloning, no-broadcasting, information-disturbance relations, conditional entropy, and privacy amplification. A linearity argument proves that no universal device can copy arbitrary pure states, while the mixed-state no-broadcasting theorem identifies pairwise commutativity as the exact boundary of exact broadcastability. These results explain why an eavesdropper cannot retain a perfect replica of every signal and wait for later basis disclosure. They are not, however, complete security proofs. Modern quantum key distribution must cover coherent attacks, quantum memory, public reconciliation leakage, imperfect sources and detectors, finite samples, authentication, and composition with later applications. The review connects state discrimination to adversarial guessing probability through conditional min-entropy and explains how entropic uncertainty and entanglement-based reductions transform observed statistics into a lower bound on extractable key. Approximate cloning, unambiguous discrimination, channel loss, side channels, and device-independent certification are examined as qualifications to elementary intuition. The central result is a hierarchy: nonorthogonality restricts distinguishability; data processing forbids perfect information amplification; parameter estimation bounds an adversary’s conditional states; and quantum-proof hashing yields a key close in trace distance to an ideal independent key. Cryptographic security therefore depends on quantum limitations embedded within a fully specified protocol and adversarial model.
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.
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.
Abstract Quantum broadcast is a method for distributing known quantum information to multiple recipients at once. Although the no-cloning theorem prohibits copying unknown quantum states, it is still possible to transmit the same known state to different destinations. This research introduces two new approaches. The first approach allows a sender to broadcast a single qubit to n receivers using a specialized entangled state known as a 2 n -qubit cluster state. The paper illustrates this method with a detailed example for four receivers ( n = 4 ) and demonstrates its validity through implementation on the Qiskit platform. The second protocol, builds upon the first protocol and allows for broadcasting a particular class of n-qubit state to n receivers. The effects of different quantum noise types on the protocol are investigated, with the fidelity metric used to quantify the quality of broadcasted states over a noisy quantum channel. These new protocols offer more efficient ways to share quantum information, potentially paving the way for improved quantum communication networks and internet.
We report on our experimental implementation of QKD with orthogonal states. Since, in general, non-orthogonal states cannot be cloned, any eavesdropping attempt in a Quantum Communication scheme using non-orthogonal states as carriers of information introduces some errors in the transmission, leading to the possibility of detecting the spy. Usually, orthogonal states are not used in Quantum Cryptography schemes since they can be faithfully cloned without altering the transmitted data. Nevertheless, L. Goldberg and L. Vaidman proposed a protocol in which, even if the data exchange is realized using two orthogonal states, any attempt to eavesdrop is detectable by the legal users. In this scheme the orthogonal states are superpositions of two localized wave packets travelling along separate channels. Here we describe in detail the first experimental realization of a quantum cryptography system based on orthogonal states and the measures of the main parameters that quantify the quality of the system. Our results demonstrate the possibility of achieving a QKD transmission based on orthogonal state with a Quantum Bit Error Rate comparable with more common secure QKD systems. Therefore, it provides a significant hint to the discussion on the minimal quantum resources necessary for the implementation of quantum tasks overcoming classical limits.
While it is known that unconditionally secure position-based cryptography is impossible both in the classical and the quantum setting, it has been shown that some quantum protocols for position verification are secure against attackers which share a quantum state of bounded dimension. In this work, we consider the security of two protocols for quantum position verification that combine a single qubit with classical strings of total length 2n: The qubit routing protocol, where the classical information prescribes the qubit's destination, and a variant of the BB84-protocol for position verification, where the classical information prescribes in which basis the qubit should be measured. We show that either protocol is secure for a randomly chosen function if each of the attackers holds at most n/2−5 qubits. With this, we show for the first time that there exists a quantum position verification protocol where the ratio between the quantum resources an honest prover needs and the quantum resources the attackers need to break the protocol is unbounded. The verifiers need only increase the amount of classical resources to force the attackers to use more quantum resources. Concrete efficient functions for both protocols are also given -- at the expense of a weaker but still unbounded ratio of quantum resources for successful attackers. Finally, we show that both protocols are robust with respect to noise, making them appealing for applications.