Summoning Information in Spacetime, or Where and When Can a Qubit Be?

TL;DR

This paper characterizes the spacetime regions where quantum information can be stored or retrieved, revealing that quantum information can be delocalized and summoned across spacetime without violating causality or no-cloning.

Contribution

It provides a complete characterization of when and where a qubit can be summoned in spacetime using quantum error correction and teleportation techniques.

Findings

Quantum information can be delocalized in spacetime.

Quantum information can be summoned to specific spacetime points.

The paper offers a method to efficiently locate a qubit in spacetime.

Abstract

One of the most important properties of quantum information, and the one ultimately responsible for its cryptographic applications, is that it can't be copied. That statement, however, is not completely accurate. While the no-cloning theorem of quantum mechanics prevents quantum information from being copied in space, the reversibility of microscopic physics actually requires that the information be copied in time. In spacetime as a whole, therefore, quantum information is widely replicated but in a restricted fashion. We fully characterize which regions of spacetime can all hold the same quantum information. Because quantum information can be delocalized through quantum error correction and teleportation, it need not follow well-defined trajectories. Instead, replication of the information in any configuration of spacetime regions not leading to violations of causality or the no-cloning principle is allowed. To demonstrate this, we answer the operational question of exactly when the information can be summoned to a set of spacetime points, showing how to do so efficiently using a combination of teleportation and codeword-stabilized quantum codes. This provides a simple and complete description of where and when a qubit can be located in spacetime, revealing a remarkable variety of possibilities.