Asynchronous Quantum Distributed Computing: Causality, Snapshots, and Global Operations
This work addresses foundational challenges in quantum distributed computing, offering a formal model and algorithm for global operations, which is incremental by extending classical concepts to quantum systems.
The paper tackles the problem of implementing atomic quantum global operations in asynchronous distributed systems, proposing the QGO Algorithm based on classical snapshot methods and demonstrating that computational causality arguments remain valid in quantum settings despite entanglement.
We initiate the study of asynchronous quantum distributed systems, focusing on the case of implementing atomic quantum global operations that can be decomposed into a collection of local operations on the components of the system. A simple example of such an operation is a quantum snapshot in which the whole system is instantaneously measured. Based on the classical snapshot algorithm of Chandy and Lamport, we design a quantum distributed algorithm to implement such decomposable global operations, which we call the QGO Algorithm. The analysis of our algorithm shows that arguments based on Lamport's computational causality remain valid in the quantum world, even though, due to entanglement, causality is not manifest from the standard description of the system in terms of a (global) quantum state. Our other contributions include a formal model of quantum distributed computing, and a formal specification for the desired behavior of a global operation, which may be of interest even in classical settings (such as in the setting of randomized algorithms).