Hybrid quantum network design against unauthorized secret-key generation, and its memory cost

TL;DR

This paper proposes a quantum network design that prevents unauthorized secret-key generation by using states that contain secure keys but do not allow entanglement swapping, and analyzes the fundamental memory cost of such schemes.

Contribution

It introduces a countermeasure against entanglement swapping attacks in quantum networks and establishes lower bounds on the quantum memory required for secure communication.

Findings

At least twice the quantum memory is needed compared to standard designs.

Tighter lower bounds on memory are derived for PPT and private states.

Upper bounds on the two-way repeater rate are provided for certain states.

Abstract

A significant number of servers that constitute the Internet are to provide private data via private communication channels to mutually anonymous registered users. Such are the servers of banks, hospitals that provide cloud storage and many others. Replacing communication channels by maximally entangled states is a promising idea for the quantum-secured Internet (QI). While it is an important idea for large distances secure communication, for the case of the mentioned class of servers pure entanglement based solution is not only unnecessary but also opens a threat. A crack stimulating a node to generate secure connections via entanglement swapping between two hackers can cause uncontrolled consumption of resources. Turning into positive a recently proven no-go result by S. B\"auml et al. [15], we propose a natural countermeasure against this threat. The solution bases on connections between hub-nodes and end-users realized with states that contain secure key but do not allow for swapping of this key.We then focus on the study of the quantum memory cost of such a scheme and prove a fundamental lower bound on its memory overhead. In particular, we show that to avoid the possibility of entanglement swapping, it is necessary to store at least twice as much memory than it is the case in standard quantum-repeater-based network design. For schemes employing either states with positive partial transposition that approximates certain privates states or private states hardly distinguishable from their attacked versions, we derive much tighter lower bounds on required memory. Our considerations yield upper bounds on a two-way repeater rate for states with positive partial transposition (PPT), which approximates strictly irreducible private states. As a byproduct, we provide a lower bound on the trace distance between PPT and private states, shown previously only for private bits.