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arXiv:2004.11705 · Entropy

Clocks without “Time” in Entangled-State Experiments

F. Hadi Madjid, John M. Myers

Published Apr 11, 2020• 2 citations
arXivHTMLPDF

Abstract

Entangled states of light exhibit measurable correlations between light detections at separated locations. These correlations are exploited in entangled-state quantum key distribution. To do so involves setting up and maintaining a rhythm of communication among clocks at separated locations. Here, we try to disentangle our thinking about clocks as used in actual experiments from theories of time, such as special relativity or general relativity, which already differ between each other. Special relativity intertwines the concept of time with a particular definition of the synchronization of clocks, which precludes synchronizing every clock to every other clock. General relativity imposes additional barriers to synchronization, barriers that invite seeking an alternative depending on any global concept of time. To this end, we focus on how clocks are actually used in some experimental situations. We show how working with clocks without worrying about time makes it possible to generalize some designs for quantum key distribution and also clarifies the need for alternatives to the special-relativistic definition of synchronization.

DOI
10.3390/e22040434

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Entangled states of light exhibit measurable correlations between light detections at separated locations. These correlations are exploited in entangled-state quantum key distribution. To do so involves setting up and maintaining a rhythm of communication among clocks at separated locations. Here, we try to disentangle our thinking about clocks as used in actual experiments from theories of time, such as special relativity or general relativity, which already differ between each other. Special relativity intertwines the concept of time with a particular definition of the synchronization of clocks, which precludes synchronizing every clock to every other clock. General relativity imposes additional barriers to synchronization, barriers that invite seeking an alternative depending on any global concept of time. To this end, we focus on how clocks are actually used in some experimental situations. We show how working with clocks without worrying about time makes it possible to generalize some designs for quantum key distribution and also clarifies the need for alternatives to the special-relativistic definition of synchronization.
We present the implementation of a time synchronization protocol as a part of an experimentally deployed entanglement-based quantum key distribution (QKD) link. The system is deployed over 48 km of optical fibers across the Métropole Côte d'Azur and enables secret cryptographic key exchange between two remote users, with an average rate of 7 kbps. We exploit the time correlation of paired photons generated by a high-quality source of energy-time entanglement implemented in the QKD link to synchronize two rubidium clocks located at the end stations. The level of stability achieved guarantees a time offset between the clocks under 12 ps at all time. We also show that this protocol requires less hardware than a typical synchronization protocol for QKD that would distribute a reference clock signal between the users.
The ability to synchronize remote clocks plays an increasingly important role in our infrastructure, from maintaining coherence in the electrical grid to allowing precise positioning and navigation for civilian and military applications. However, many of the techniques to establish and maintain this time synchronization have been shown to be susceptible to interference by malicious parties. Here we propose a protocol that builds on techniques from quantum communication to provide a verified and secure time synchronization protocol. In contrast with classical protocols aimed at increasing the security of time distribution, we need not make any assumptions about the distance or propagation times between the clocks. In order to compromise the security of the protocol, an adversary must be able to perform quantum non-demolition measurements of the presence of a singe photon with high probability. The requirement of such quantum measurements raises a serious technological barrier for any would-be adversary
The security of keys in quantum cryptography is based on fundamental quantum mechanical exclusions (the exclusion of cloning and copying of nonorthogonal quantum states. The physical type of a quantum object that carries information (photon, electron, atom, etc.) is insignificant; only its state vector is important. In relativistic quantum cryptography for open space, both the time of the information carrier (photon that propagates with the extremely allowable velocity in a vacuum) and its quantum state are of fundamental importance. Joint fundamental constraints that are dictated by both special relativity and quantum mechanics on the discrimination of nonorthogonal quantum states allow one to formulate fundamentally new key distribution protocols that are stable against any attacks on a key and guarantee the security of keys for a nonstrictly single-photon source and any losses in the communication channel. Although this protocol is a real-time protocol in the Minkowski space-time, where the attack to the communication channel is detected by the delay of eavesdropper measurement results, the protocol does not require clock synchronization on the transmitter and receiver sides .
This paper proposes a novel distributed time synchronization protocol leveraging quantum entanglement. The core claim is that absolute time synchronization, surpassing the limitations of classical physics, is achievable through the utilization of entangled particles, eliminating the need for a global time reference. The protocol centers on a mechanism for precise time calibration using entangled states, capitalizing on the inherent coherence of entangled states to perform absolute measurements of time differences and achieve synchronization. This approach fundamentally overcomes the reliance on global reference frames inherent in traditional time synchronization methods, offering a quantum mechanical solution to the problem. The protocol details the key steps involved, focusing on the generation, distribution, and measurement of entangled particles to establish a synchronized time base across spatially separated nodes. This research contributes to the emerging field of quantum time synchronization and its potential applications in precision timing, distributed sensing, and fundamental physics experiments.
Time synchronization is a crucial requirement in quantum key distribution (QKD)8 protocols, ensuring accurate key generation via the correct assignment of bits of raw key and9 enabling eavesdropping detection via the precise recording of photon statistics. State-of-the-art10 experiments typically use an extra channel to synchronize the clocks of the transmitter and receiver11 via classical signals. In this work, we study the possibility of performing clock synchronization12 via the signals used for the key generation, which are already present in decoy-state-based BB8413 protocols.14 Without altering the protocol in any way, we use the different mean photon numbers of the15 signal and decoy states for time synchronization without a dedicated physical channel capable of16 clock synchronization. The proposed method relies only on the photons sent and received for17 key generation and does not require any change to the QKD protocol. The only change in the18 experiment is on the software level, thus making it very simple to implement.19 We demonstrate clock synchronization method in a simulation of a specific fiber-based QKD20 experiment. Like other decoy-state-based BB84 protocols, it is based on weak coherent pulses.21 In this simulation, we investigate the parameter space to find limits and optimal choices of our22 proposed method.23 In addition to the non-protocol-altering clock synchronization method, we also discuss an24 approach that significantly improves performance in lossy channels by introducing an additional25 decoy state with a very high mean photon number.26 By eliminating the need for an extra channel capable of clock synchronization, both methods27 proposed potentially reduce the complexity and cost of QKD systems and improve their agility